Supramolecular approach to the design of nanocarriers for antidiabetic drugs: targeted patient-friendly therapy

Diabetes mellitus (DM) is a group of serious chronic diseases associated with endocrine disorder and characterized by high blood glucose levels (hyperglycemia) caused by a combination of genetic, environmental, and behavioral factors, including genetic predisposition, ethnicity, sedentary lifestyle, obesity and others.[1-7] In turn, for the patients suffering from hyperglycemia over prolonged period, macro- and microangiopathy are developed, which can lead to a number of serious and even fatal complications affecting the cardiovascular system, different tissues and organs (nerves, kidneys, foot, eyes, etc.). Among the latter are diabetic neuropathy, diabetic nephropathy, diabetic retinopathy, impaired wound healing, etc.[8-13] According to the International Diabetes Federation (IDF), more than 500 million adults between the ages of 20 and 79 are affected by DM. This number is expected to increase, as the disease is becoming more widespread.[1, 2, 4, 14] According to the IDF data, DM is among the top 10 causes of death worldwide; 4.2 million patients in 2019 and 6.7 million patient in 2021 died of DM.[1, 2, 4, 14] Importantly, in addition to the social and human effects, DM is associated with serious economic burdens. These depressing statistics have led to an increased research attention on the management of DM and its complications.

Generally, two types of diabetes, i.e., type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM), are classified depending on various pathogenesis. T1DM is insulin-dependent diabetes characterized by absolute insulin deficiency due to destruction of pancreatic β-cells. It is typical for young population and mainly caused by genetic predisposition. Patients with T1DM need insulin administration throughout their life. T2DM is characterized by reduced insulin sensitivity with a progressive deficiency in insulin secretion by pancreatic β-cells,[15] thereby resulting in a lack of the hormone in the body. According to statistical data, about 90% of patients with DM have T2DM.[15] Meanwhile, more complex classification may be accepted in some cases, including gestational diabetes.[7] In other cases, authors emphasize the role of insulin deficiency in the etiology and pathogenesis of the Alzheimer’s disease (AD), with insulin resistance in the brain being considered as type 3 diabetes (T3DM).[5]

Diabetes management domain involves a combination of research and clinical strategies focusing on modelling, diagnosis, differentiation, monitoring, drug administration, etc. The diagnosis of DM usually involves the monitoring of glucose levels in body fluids, such as blood (> 125 mg dL^–1) and urine. Additional glycemic control may be used by measuring glycated hemoglobin (HbA1c) levels, with an acceptable level of less than 6.5%. Nanomedicine tools for the diagnosis, prediction, prevention and monitoring of DM present a separate line of research, which is mainly remained beyond this review, with only principal points briefly outlined.

Both types of DM lead to hyperglycemia and require administration of exogenic insulin to keep the metabolic glucose balance. This is conjugated with a number of problems, since (1) conventional treatment of DM occurs through multiple subcutaneous injections of exogenous insulin, which is painful, discomforting and may be the source of infection; (2) there are limited ways of insulin administration, and each has its own weaknesses; (3) unlike low molecular weight drugs insulin is a protein consisting of 51 amino acids (21 and 30 amino acids in chain A and B, respectively) (Fig. 1), which makes it even more difficult to deliver. Despite these challenges numerous efforts are made to develop noninvasive routes of administration, including oral, intranasal, pulmonary, transdermal and others.[3, 16, 17] A variety of antidiabetic drugs are used to treat T2DM, with small molecules showing clear advantages. Classical drugs for these cases are metformin, sulfonylureas, glinides, thiazolidinediones, etc., with metformin being of special importance.[4] Although mechanism of metformin therapeutic effect is not clear, it is suggested that the drug affect liver insulin sensitivity due to controlling glucose production. It is noteworthy that administration of metformin allows both producing hypoglycemic therapy and protection from impaired fasting glucose.

Fig. 1

Amino acid sequence of human insulin and a schematic representation of the monomer, dimer and hexamer of insulin

Structure of metformin

Rapid progress in medicinal chemistry is leading to new generations of drugs with novel mechanisms of action are developed. Much attention is being paid to naturally derived medications[18, 19] due to their affinity to therapeutic targets, safety and availability. First of all, the following antidiabetics should be mentioned: dipeptidyl-peptidase-4 (DPP-4) inhibitors; sodium-glucose cotransporter-2 (SGLT2) inhibitors, glucagon-like peptide-1 (GLP-1) receptors agonists, that belong to a new class of medicines for the treatment of T2DM and its complications, including those with cardioprotective effects.[20] Despite this encouraging information, the occurrence of drug resistance, multitargeted character of the DM, long-term and complicated progression of the disease, existence of additional diseases, and individual sensitivity of diabetic patients to the drug should not be forgotten. These dictate the need to intensify the research activity towards the discovery of new targets and new multitarget therapeutics.[21] Meanwhile, the design of new therapeutic molecules and their bench-to-bedside translation are very expensive and time-consuming processes. Alternatively, essential increase in efficacy of drug therapy can be provided by the use of nanosized carriers, or nanocontainers, that can dramatically modulate the effect of traditional drugs. This is achieved by tuning stability, side effect, targeting properties of drug formulations, as well as their adapting to different administration routes. Since the 1980s, nanotechnology approach finds wide application in medicine, with many generations of drug delivery systems developed for different diseases including oncology, neurodegenerative disorders, inflammatory processes, tuberculosis, DM, etc.[2, 11, 13, 22-24] Typically, these nanocarriers are categorized into several domains, including inorganic, organic (lipid, polymeric) and hybrid.[25, 26] Meanwhile, this is a challenging task to adapt, modify and optimize these nanocontainers to the specific disease, drug and administration route. In view of these facts, effective management of DM and DM-related complications is impracticable outside the nanotechnological approach, which plays an important role in the diagnosis, monitoring, and treatment processes.[2] Application of nanocarriers makes it possible to modify pharmacokinetics and biodistribution of drugs, control their release profile, improve stability, biocompatibility and bioavailability, create depot-like nanoformulations.

Nanomedicine is a dynamically developing field, so to provide the most breakthrough information this review focuses on the most recent publications, primarily covering a period of last one to five years, with the pioneer ideas and approaches highlighted.

Despite the enormous number of publications in the field of diabetes therapy, the structure and content of this review allow it to take its unique place among other recent review publications on this topic. According to the analysis of recent literature, reviews are becoming highly specialized both in terms of medical aspects and in drug formulation development. A significant number of works are devoted to the delivery of a specific drug, primarily insulin, the description of particular administration route, and the use of a certain class of nanocarriers or materials.[3, 6, 9, 17] This review, authored by chemists specializing in drug delivery, includes a classification of systems by both carrier type (lipid, polymer nanocontainers) and delivery route (e.g., oral, transdermal, intranasal, etc.), which allows us to discuss various aspects of modern nanomedicine in the field of diabetes treatment. For a more detailed analysis and understanding of observed trends, forecasting the direction of research and assessing the effectiveness of various systems, it is important to take into account medical aspects, including pathogenesis, etiology of the disease, information on risk factors, stages of disease development, biological targets, involved signalling pathways, etc. Therefore, in addition to chemical, supramolecular and nanotechnological aspects, this review will pay attention to a balanced discussion of biochemical and medical issues necessary to justify the choice of medicinal compositions and nanomaterials.

As described above, DM can be categorized into two main types based on their underlying causes and pathogenesis: type 1 and type 2. The terms used to denote age of onset (juvenile or adult) or treatment type (insulin-dependent or non-insulin-dependent) are considered outdated because these indicators for different types of DM may be similar in some cases.[27] The main reason for the development of T1DM is genetic predisposition,[28, 29] however, viruses (Coxsackievirus) and some poisons (streptozocin (STZ) and rat poison) can have a destructive effect on the pancreatic β-cells. The causes of T2DM are primarily related to metabolic disorders, age, ethnicity, family history, sedentary lifestyle, and unhealthy diet may contribute. Diagnosis of T2DM is complicated by the fact that its symptoms are often mild, and the disease is diagnosed when complications arise several years after its onset.

Insulin, a peptide hormone, is secreted by the β-cells within the islets of Langerhans. Its primary role is facilitating glucose enter into cells of insulin-dependent tissues such as liver, muscles, and adipose.[2, 30] In the pancreas, insulin is predominantly stored as hexamers — structures made up of six insulin molecules that form a stable complex with zinc ions. In this form, insulin remains inactive and cannot interact with cell receptors. Upon release into the bloodstream, these hexamers gradually dissociate into dimers and monomers, which are then able to carry out their biological function. Upon binding to two monomers of the insulin receptor on the cell membrane, insulin triggers their combination into a dimer, initiating the formation of a channel for glucose transport. The stabilization of insulin as hexamers, or conversely, the acceleration of its dissociation into monomers, determines the rate of insulin action. This process is characterized by key parameters such as maximum concentration in the body, time of onset, and action time (Fig. 2). Insulins are categorized into four main types based on their action time: rapid-acting, short-acting, intermediate-acting, and long-acting.[31] Regular human insulin typically needs to be administered 0.5 to 1 hour before meals, which can be inconvenient. To address this, rapid-acting insulins were developed with slight modifications in the amino acid sequence to accelerate the dissociation of dimers into monomers. For instance, Insulin lispro, a human insulin analogue, differs by the reversed sequence of proline and lysine residues at positions 28 and 29 on the B-chain (see Fig. 1).[32] On the contrary, NPH (neutral protamine Hagedorn) insulin includes protamine, which interacts with insulin hexamers, promoting their crystallization and precipitation. This allows to delay the onset of action, but to achieve a more prolonged effect. In addition, to achieve the maximum therapeutic effect, it is possible to use a mixture of different insulins.[33]

Fig. 2

Classification of insulins by time of action

Insulin is primarily injected subcutaneously. Recently, there has been a growing focus on noninvasive insulin delivery methods, including the development of systems for oral, intranasal, buccal, pulmonary, transdermal, rectal, vaginal, and ocular delivery.[34-36] Oral insulin delivery remains a promising field of research with significant potential for clinical application,[37, 38] but low bioavailability is a major obstacle to this route of administration. Inhaled insulin Afrezza® [39] is administered through the respiratory tract and is absorbed into the lungs through the pulmonary alveoli. This method is considered convenient and express for patients, but there may be problems in selecting the required dose. Vaginal and rectal insulin delivery involves the use of specialized forms, such as suppositories and gels, which are administered into the vagina or rectum, allowing insulin to be absorbed through the mucous membranes. Despite the advantage of bypassing first-pass hepatic metabolism, the challenges of uncontrolled insulin absorption and potential irritation of the mucous membranes have limited the widespread adoption of these delivery routes.[40] Other noninvasive methods of insulin administration include intranasal and transdermal delivery. These methods could help patients overcome the psychological challenges and risk of infections linked to frequent injections. These routes of insulin administration will be discussed in more detail in this review.

The nasal mucosa is highly vascularized, allowing drugs to be rapidly absorbed into the systemic circulation through transcellular and paracellular transport. The olfactory region in the nasal cavity provides a direct route to the central nervous system, bypassing the blood-brain barrier. For diabetic patients requiring daily insulin, the nasal route offers a viable alternative to subcutaneous injections.[41-43] In addition, the intranasal route of insulin administration is often considered in the context of the neurodegenerative diseases treatment.[44, 45] The intranasal delivery of insulin using various nanocontainers is of utmost interest.

Chitosan and its derivatives are most commonly used in the design of nanocontainers for intranasal delivery. The optimized thermosensitive hydrogel based on 2 wt.% chitosan, 1 wt.% chitosan quaternary ammonium salt, and 0.5 wt.% gelatin showed short gelation time, uniform pore structure, and the ideal swelling rate. These properties facilitated the prolonged release of 65% of encapsulated insulin over 24 hours after intranasal administration.[46] The non-toxic NPs were characterized by a hydrodynamic diameter of 165.3 nm, polydispersity index (PdI) of 0.24, and zeta potential of +21.6 mV.[47] The Krebs – Henseleit buffer was used as a novel diffusion medium in a Franz diffusion cells because of its ability to maintain tissue viability, thereby allowing for more accurate monitoring of insulin penetration efficacy. Chitosan was also commonly used to coat the surface of other nanocontainers. For instance, chitosan-coated insulin-loaded solid lipid nanoparticles (SLN) and poly(lactic-co-glycolic acid) NPs were obtained.[48] Encapsulating insulin in chitosan-coated NP improved structural stability and enhanced nasal absorption. Transfersomes obtained at phospholipids/Tween 80 weight ratios of 10 : 1, 7 : 1, 5 : 1, 3 : 1, and 1 : 1 were modified during hydration of the lipid film with acetate buffer containing chitosan.[49] For example, the shift of the zeta potential from a negative value (–3.5 ± 0.1 mV) to a positive value (+23 ± 4 mV) for the composition with ratio of 3 : 1 indicated the successful coating of the transfersome surface with chitosan. The nasal uptake of insulin encapsulated in transfersomes was validated through fluorescence imaging.

Insulin-loaded fast-dissolving films were developed using the casting method with hydroxypropyl methylcellulose and polyvinyl alcohol.[50] The clinical study of the selected film in post-COVID-19 anosmic patients demonstrated a significant improvement in olfactory detection scores (lower olfactory threshold) in the group of 20 patients.

The insulin-loaded self-emulsifying nanoemulsion (NE) increased insulin permeation three times compared to the insulin solution.[51] In vivo results demonstrated that the C_max (maximum plasma concentration) and the bioavailability (relative to the subcutaneous delivery) was 255.9 mU mL^–1 and 68% (insulin-loaded NE), 5.8 mU mL^–1 and 5% (insulin solution). Intranasal delivery of 3.6 IU kg^–1 insulin-loaded self-emulsified nanoemulsion significantly reduced plasma glucose level up to 70% with the effect lasting for 4 hours of the experiment.

To prepare arginine-coated self-emulsifying nanoglobule system, lauroyl ethyl arginate was added to a preconcentrate consisting of a medium chain monoglyceride Capmul, a medium chain triglyceride Captex 8000, and a surfactant Kolliphor® RH 40 at a weight ratio of 2 : 3 : 4 (Fig. 3).[52] Insulin loading was achieved by electrostatic binding between the negatively charged α-helix of insulin and positively charged arginine nanoglobules. Following intranasal administration of the insulin-loaded nanoglobule system to diabetic rats at a dose of 2 IU kg^–1, a rapid insulin absorption was observed with a significantly higher C_max = 14.3 mU L^–1 and a relative bioavailability of 23.3%. The optimized protamine/insulin formulation after intranasal administration demonstrated blood glucose-lowering activity comparable to subcutaneous insulin.[53] Tissue histology and confocal microscopy revealed that protamine induced rearrangement of adherens junctions in the nasal epithelium, thereby enhancing insulin penetration to the lamina propria for systemic absorption.

Fig. 3

Scheme depicting fabrication of insulin-loaded arginine-coated self-emulsifying nanoglobules system (INS-LANano): medium chain triglyceride (MCT), medium chain monoglyceride (MCM), lauroyl ethyl arginate (LAE), phosphate buffered saline (PBS).[52] This figure is published under a Creative Commons Attribution license

Intranasal insulin delivery has been extensively studied and tested due to its ability to achieve the necessary therapeutic levels for DM treatment. This method offers several advantages, including increased bioavailability, painless administration, and usability. Moreover, intranasal insulin delivery is not only beneficial for diabetic patients but has also shown promising results in the treatment of cognitive disorders, such as AD. Drugs administered intranasally can be rapidly absorbed through the nasal mucosa and enter the systemic circulation, which is crucial for quickly achieving therapeutic concentrations in the blood. In addition, intranasal administration allows certain substances to bypass the blood-brain barrier and reach the brain directly. While insulin can be delivered intranasally, this method is not universally applicable for systemic insulin administration. Inhaled insulin, such as Afrezza®, is delivered through the respiratory tract, but not through the nose, to facilitate faster entry into the systemic circulation.[39] Insulin, being a large molecule, has limited absorption through the nasal mucosa into the bloodstream, making intranasal insulin less effective for glucose control compared to traditional methods like subcutaneous injections. At present, noninvasive insulin administration through the nose is still the subject of fundamental research. But it is of great interest in the context of the connection between DM and AD. Intranasal administration may be more suitable for the delivery of small molecules for the treatment of T2DM.

Transdermal delivery offers numerous advantages, including sustained drug release, the maintenance of steady plasma drug level, and enhanced patient compliance.[54] Needle-free injectors, patch, microneedle devices, chemical enhancers, ionophoresis, laser therapy are new techniques to improve the efficiency of insulin delivery through the skin (Fig. 4).[16, 54-56] Moreover, 3D printing technologies are being actively researched to obtain microneedles, also using available raw materials.[57-59]

Fig. 4

A schematic illustration of the different strategies for the delivery of insulin via the transdermal route.[54] This figure is published under a Creative Commons Attribution license

Innovative patch with heating elements on polyimide substrates for electrothermal transdermal therapy was presented by Pagneux et al.[60] The results demonstrate that adjusting the electrical resistivity of a gold nanohole array patterned on polyimide enables a fast-responding electrothermal skin patch, while a subsequent coating with reduced graphene oxide provides insulin encapsulation capabilities.

Microneedles are needles from 50 to 1000 μm, which can be either soluble or insoluble depending on their purpose and design. Soluble microneedles are made of biocompatible materials (e.g. hyaluronic acid, polyvinylpyrrolidone (PVP), etc.) that dissolve in the skin after injection, releasing active substances. Hollow insoluble microneedles serve as channels for the delivery of drugs. They can be made of metals, silicone, or other solid materials. Drugs are injected through the cavity of the microneedle into the deeper layers of the skin. Zhang et al. proposed polymeric nanocarriers based on carboxymethyl chitosan for insulin delivery, whose penetration through the stratum corneum is facilitated by the use of microneedles.[61] Compared to the transdermal rate of passive diffusion (2.77 ± 0.64 μg cm⁻² h⁻¹), the transdermal rate of the insulin-loaded NP is increased by 4.2-fold (10.24 ± 1.06 μg cm⁻² h⁻¹) when administered with microneedles.

The water-soluble polysaccharide pullulan was used to fabricate microneedles,[62] which dissolved within 2 hours releasing up to 87% of the insulin. After 4 weeks of storage at 4, 20, and 40°C, the insulin successfully retained its secondary structure. Microneedles can be collected on a substrate, such as a patch or roller. A roller with microneedles made of polyvinyl alcohol has been developed for rapid drug delivery across large surface areas.[63] The insulin-loaded roller demonstrated an immediate hypoglycemic effect: lowering blood glucose levels to less than 50% of their original value within one hour of application.

Structure of pullulan

Insoluble microneedles formed by crosslinking gelatin methacrylate with polyethylene glycol diacrylate have been developed.[64] Molybdenum disulfide is added to the microneedles for insulin delivery under near-infrared irradiation. Transdermal administration of insulin using these microneedles significantly lowers blood glucose levels in C57BL/6 mice and mini-pigs achieving effects comparable to subcutaneous insulin injections.

The use of microneedles in combination with ionophoresis significantly enhanced controlled insulin delivery.[65] In this study, the smartphone powered the ionophoresis-driven circuit through its charging port. The natural polymer material (silk fibroin) was often used as a raw material for forming hydrogels for transdermal insulin delivery.[66-68] Poly(3,4-ethylenedioxy­thiophene):polystyrene sulfonate as a drug carrier and silk fibroin as a hydrogel matrix were used for the transdermal delivery of insulin via ionophoresis allowing precise control of release rate, dosage, and duration.[68] Porous thermoplastic polyurethane was used as a matrix for transdermal insulin delivery.[69] In addition, the ionophoresis technique achieved a twofold increase in insulin penetration through pig skin.

Various types of stimuli-responsive systems are of particular interest. Microneedles that respond to external stimuli have been developed based on metal-organic framework (MOF) made of Co²⁺ and Zn²⁺.[70] The MOF in these systems has been used as a storage reservoir for insulin. An increase in glucose levels can trigger the destruction of MOF and the release of insulin. Mesoporous silica NPs with insulin have been incorporated into hyaluronic acid-based dissolving microneedle patch.[71] This smart and elegantly built system consists of the following parts: mesoporous silica NPs as the insulin reservoir, Wulff-type phenylboronic acid (PBA-2) as a sensitive ‘linker’, zinc oxide NPs as gatekeepers. At high blood glucose levels, the stable boronic acid ester formed between ZnO-PBA-2 and diols on the surface of mesoporous silica NPs is completely destroyed, followed by insulin release. In another study,[72] phenylboronic acid (PBA)-modified methacrylate hyaluronic acid was proposed as a base material for microneedle fabrication. In addition, the formation of a covalent bond between PBA and gluconic acid-modified insulin, degrading at high glucose concentrations, was suggested as a stimulus-responsive element. Lin et al. modified multivesicular liposomes with PBA derivatives to confer glucose-sensitive properties.[73] In vitro studies demonstrated the sensitivity of these multivesicular liposomes to pH and H₂O₂ , attributed to the presence of glucose oxidase (GOx) within the liposomes. Additionally, in vivo experiments showed that when blood glucose levels were reduced to normal, insulin release was decreased.

Microemulsion formulations were successfully obtained using ionic liquids of choline-fatty acids composition as surfactant, Span 20 as a co-surfactant, choline propionate ionic liquid as an internal polar phase, and isopropyl myristate as a continuous oil phase.[74] In vivo transdermal administration of low insulin doses (50 IU kg^–1) to diabetic mice revealed that these microemulsion formulations maintained insulin level for a significantly longer period (half-life > 24 hours) compared to subcutaneous injection (half-life = 1.32 hours). In work,[75] optimized insulin-loaded niosome emulgel combination was investigated. The transdermal flux of insulin in the niosome emulgel was ten times higher than that of the insulin solution. In vivo data demonstrated that the insulin niosome emulgel reduced plasma glucose level more effectively than other tested formulations with insulin, including niosome gel, emulgel, gel, and niosomes. Phospholipid derivative 1,2-dimyristoyl-sn-glycero-3-ethyl-phosphatidylcholine as a cation and unsaturated linoleic acid as an anion were used to form ethosomes for transdermal insulin delivery.[76] The formulation with 35% ethanol was identified as the lead candidate. These ethosomes effectively encapsulated insulin with an encapsulation efficiency of 99% and showed long-term stability at both 4°C and 25°C.

Three different deep eutectic solvents were investigated as chemical penetration enhancers for transdermal administration of insulin across rat skin: choline chloride/urea, choline chloride/glycerol, and choline chloride/ethylene glycol, each in a 1 : 2 molar ratio.[77] The cumulative insulin permeation over 24 hours was 131.0, 89.4, and 29.6 mg cm⁻² in the presence of choline chloride/ethylene glycol, choline chloride/glycerol, and choline chloride/urea, respectively.

A water-in-oil NE system was developed for percutaneous insulin delivery, consisting of 5 wt.% oleic acid as an oil phase, 60 wt.% Tween 80 as a surfactant, and isopropyl alcohol as a co-surfactant (in a 3 : 1 weight ratio), with 35 wt.% water as an aqueous phase.[78] The optimized formulation was characterized by a small droplet size (41.05 ± 8 nm), a high entrapment efficiency (93.08%), and a notable flux (1.75 μg cm^–2 h^–1). Сircular dichroism spectra indicated minor conformational changes in the insulin structure due to its entrapment within the NE, indicating that the hormone retained its biological functions.

Advances in transdermal technologies, such as microneedles, ionophoresis, and chemical enhancers, have shown their effectiveness in insulin delivery through the skin. Although further research is needed to optimize these systems and confirm their safety and efficacy, recent trends highlight the potential of transdermal drug delivery for DM treatment. A combination of different methods, for example, NPs with microneedles, microneedles with ionophoresis, to enhance transdermal delivery is particularly promising.

T2DM is much more common than other forms of the disease.[79] Its peculiarity is that the body’s cells become less sensitive to insulin, which means that insulin loses its ability to transport glucose to the tissues. The pancreas perceives this as a signal of insulin deficiency and begins to produce it with increased intensity. As a result, the β-cells of the pancreas, responsible for producing insulin, become depleted over time, and the person is forced to resort to insulin injections.

There are several classes of drugs used to treat T2DM, each with its own mechanism of action and route of administration (Fig. 5). Many medications for T2DM treatment are designed for oral administration, which, despite its advantages, has several drawbacks: first-pass metabolism, ‘peak and valley’ pharmacokinetic profile, and low degree of absorption in the gastrointestinal tract.

Fig. 5

Main classes of antidiabetic drugs for treatment of T2DM

Metformin (1,1-dimethylbiguanide) is the first-line treatment for T2DM because of its strong glucose-lowering effects, proven safety profile, and affordability. In the form of hydrochloride, it is easily soluble in water. It is administered orally in tablet or liquid form. Metformin treatment is associated with moderate weight loss in patients and, with long-term use, a reduction in cardiovascular risk, as well as antioxidant and anti-inflammatory effects.[80, 81] At this stage, metformin is considered as a polyfunctional drug that can be used to treat various diseases. In addition to the treatment of DM, metformin is also used to treat glucose intolerance, obesity, and polycystic ovary syndrome.[82] Metformin is attracting attention as a potential anticancer agent. In DM management, metformin decreases hepatic glucose production through activation of adenosine monophosphate-activated protein kinase and increases insulin sensitivity in peripheral tissues. Metformin also reduces the rate of glucose absorption in the intestine, which helps to reduce postprandial (after a meal) hyperglycemia. However, metformin is not without negative side effects, including digestive upset, vitamin B12 deficiency, lactic acidosis, and kidney problems. To minimize side effects and unlock the potential of metformin, active fundamental and applied research with this drug is ongoing.

Sulfonylureas (e.g., Glipizide, Glyburide, Gliclazide) and meglitinides (e.g., Repaglinide, Nateglinide) are administered orally in tablet form. Sulfonylureas bind to β-cells of the pancreas and inhibit the adenosine triphosphate-sensitive potassium channels, reducing potassium efflux and causing β-cell membrane depolarization. This depolarization opens calcium channels, allowing calcium influx and increasing intracellular calcium levels, which in turn stimulates insulin secretion from the pancreatic β-cells.[83, 84] Meglitinides (glinides) also bind to β-cells as sulfonylureas, but with less affinity, which reduces the risk of hypoglycemia.

Thiazolidinediones (e.g., Pioglitazone, Rosiglitazone) are also used orally. Through the activation of peroxisome proliferator-activated receptors, thiazolidinediones modify the expression of genes responsible for glucose and lipid metabolism, leading to enhanced blood sugar regulation and reduced triglyceride levels.[85, 86] Thiazolidinediones also increase the sensitivity of tissues to insulin. However, these medications are associated with a wider range of side effects (weight gain, edema, and an increased risk of heart failure) and are expensive.

DPP-4 inhibitors (e.g., Sitagliptin, Linagliptin, Gemigliptin) are a class of medications known as gliptins. They function by elevating levels of incretins, namely GLP-1 and glucose-dependent insulinotropic peptide, thereby enhancing insulin secretion in the blood.[87-89] They are available in tablet form for oral administration. DPP-4 inhibitors are weight neutral and well tolerated.

SGLT2 inhibitors (e.g., Canagliflozin, Dapagliflozin, Empagliflozin) prevent the reabsorption of glucose in the kidneys, which leads to the excretion of glucose in the urine (glycosuria).[90] In addition to their glucose-lowering effects, SGLT2 inhibitors also help to prevent the onset of chronic kidney disease and reduce the incidence of cardiovascular complications in patients with T2DM.[91] This helps to lower blood glucose levels. Medications of this class are used in oral tablet form either as monotherapy or in combination with other antidiabetic drugs.

GLP-1 receptor agonists (e.g., Exenatide, Liraglutide, Semaglutide) enhance the secretion of insulin by b-cells of the pancreas in response to increased blood glucose level and inhibit the secretion of glucagon by a-cells of the pancreas, which reduces the production of glucose by the liver.[92] Route of administration of these large molecules is subcutaneous injection.

Therefore, the primary classes of drugs for treating T2DM include metformin, sulfonylureas, DPP-4 inhibitors, SGLT2 inhibitors, and GLP-1 receptors agonists, each providing unique mechanisms of action and benefits. These medications are typically administered in tablet form, although injectable options are available for some, such as GLP-1 receptors agonists and insulin. Personalized treatment plans that consider the patient’s health profile are crucial for effective management of T2DM, aiming not only to regulate blood glucose levels but also to reduce associated risks such as cardiovascular events and chronic kidney disease.

The continuous and intensive development of supramolecular chemistry has brought medicine, in particular the field of drug delivery, to a new quality level. The demand for nanoscale delivery systems and the increasing publication activity can be attributed to (1) the simplicity of supramolecular structure assembling, (2) the possibility of fine-tuning of NP properties, and (3) the enhanced bioavailability, improved biocompatibility, and drug targeting facilitated by encapsulation in nanocarriers, which in turn leads to reduced toxicity of pharmaceutical substances.[93, 94] In addition, the use of nanoscale drug delivery systems makes it possible to reduce the drug dosage, control the onset of its action, and regulate the release rate, which is especially relevant in the context of DM therapy.[95] Currently, a multitude of nanocontainers with diverse nature (organic, inorganic, and hybrid) are being developed for the treatment of various diseases, including DM. Supramolecular structures for DM therapy can be constructed through electrostatic interactions,[96] host-guest interactions,[97] hydrophobic effect,[98] and intra- and/or intermolecular π – π stacking interactions.[99] The nanocontainers required for the drug delivery routes used have now been developed. In DM therapy, research has focused primarily on improving the efficacy of drugs for oral administration.[100-102] Nonetheless, there are also studies exploring transdermal [103] and intranasal [104, 105] drug administration (Table 1). The search for alternatives to intravenous administration is driven mainly by the inconvenience and pain associated with it, which leads to poor patient adherence to treatment regimens. At first sight, alternative routes of drug administration offer significant advantages over intravenous route, but they also have disadvantages, which means that researchers need to carefully select drug delivery systems and their components.

Table 1

\[ \]

Examples of nanocarriers used for the DM treatment [105-114]

(1)

Another important challenge in DM therapy is the controlled release and targeted drug delivery. For example, systems responsive to blood glucose levels,[115] stomach pH [116] or both stimuli [117] are known. It is worth noting that a substantial amount of research is focused on the development of an oral form of hypoglycemic drugs, and responsiveness to environmental pH is one of the key factors in the effectiveness of the therapy. pH-Sensitive systems include polymeric nanocontainers, which can undergo protonation and ionization in response to pH changes. This property can be utilized both for protecting the nanocontainer and for the controlled release of drugs. For instance, nanocarriers based on poly((2-dimethylamino)ethyl methacrylate) [109] and the natural biopolymer alginate [110] released only 15 – 20% of drug at acidic pH. In contrast, at pH 7.4, the release of the drugs was significantly higher due to the NP swelling.[118, 119] Similar results were obtained for MOF based on FeCl₃ · 6 H₂O and 2-aminoteraphtalic acid with zwitterionic polymethacrylate Eudragit® L100-55. Additionally, this system included a mixture of sodium bicarbonate and citric acid as a pH-controlled generator of CO₂ bubbles, which accelerated the unpacking of the capsules at neutral pH.[120] Another promising approach is to form NPs with vitamin B12, which at low pH values can form a strong bond with the negatively charged components of NPs and prevent premature drug release.[121, 122] To create glucose-responsive NPs, their surface can be conjugated with phenylboronic acid,[123-125] glucose oxidase [126] or concanavalin A (Con A).[117, 127] GOx and Con A are proteins with lower specificity and stability compared to PBA.[128] As shown in Figure 6, glucose-sensitive fragments can be conjugated to various nanocontainers for the delivery of insulin and other hypoglycemic drugs. GOx is capable of oxidizing β-D-glucose to glucono-1,5-lactone, which spontaneously hydrolyzes to gluconic acid with the formation of hydrogen peroxide (Fig. 6a). In this case, nanocontainers, mainly polymeric, respond to changes in the pH of the environment, swelling and releasing the drug into the blood in response to high glucose levels.[129] The mechanism of action of Con A involves specific binding to glucose through noncovalent interactions, which leads to changes in aggregate morphology and the subsequent release of the drug (Fig. 6b).[130, 131] PBA and its derivatives have attracted considerable attention due to their stability and ease of modification. PBA exists in water in a dynamic equilibrium between a neutral and a negatively charged form (Fig. 6c). The negatively charged form of PBA is capable of binding glucose, whereas the uncharged form can convert to the charged form. This reversible transition between water-soluble and insoluble forms in response to glucose level has led to the development of various glucose-responsive drug delivery systems, including polymer carriers such as hydrogels, microgels, microcapsules, etc. However, due to the high pK_a values of PBA (8 – 9), its glucose responsiveness under physiological conditions (pH 7.4) is limited. Consequently, researchers have focused on lowering its pK_a values by introducing electron-withdrawing substituents into the benzene ring (such as carboxyl groups, halogens, and nitriles).[128, 132, 133] In addition to conjugating PBA to various types of nanocarriers, another noteworthy strategy involves crosslinking PBA and its derivatives directly to insulin, creating ‘smart insulins’. For instance, Chou et al. synthesized a class of insulin derivatives that contain both an aliphatic segment, designed to bind to serum albumin or other hydrophobic components in the bloodstream to prolong the half-life, and a PBA moiety.[134] These novel insulin conjugates demonstrated a more rapid restoration of normal blood glucose levels in mice compared to standard insulin and clinically used long-acting insulin derivatives in vivo. In another study, PBA, specifically 3-fluorophenylboronic acid, was conjugated with long-acting insulin (insulin glargine, marketed as Lantus®).[135] It was found that attaching the hydrophobic residues of 3-fluorophenylboronic acid to the C-terminus of the A-chain of insulin increased its solubility without compromising its efficacy. Ohno et al. also successfully synthesized a modified PBA-insulin conjugate to evaluate its glucose-lowering activity and cellular adhesion properties.[136] It was demonstrated that subcutaneous injection of the conjugate, in contrast to intravenous injection, exhibited low activity in reducing glucose level. It is worth noting that there is currently significant interest in modifying various nanocarriers with the aforementioned glucose-sensitive compounds. Both in vitro and in vivo studies have shown that NP can not only protect the drug but also regulate the drug release rate according to the glucose concentration in the body. It is important to note that while the primary focus in DM therapy is on lowering glucose level, it is equally crucial for the drug formulation to respond to hypoglycemia. This concept was proposed by Dong et al. and includes the development of a painless and flexible system for regulating blood glucose levels consisting of glucose-loaded pressure-responsive nanovesicles, insulin-loaded black phosphorus nanosheets, as well as hydrogel applied via patch.[137]

Fig. 6

Glucose-responsive compounds used in NP development for hypoglycemic drug delivery

Currently, there are various methods for drug delivery, numerous types of nanosystems, and ways to modify them for specific requirements, but how can the appropriate type of NP be selected for the delivery of antidiabetic drugs? Despite the diversity of nanocontainers, there are requirements that all systems must meet, namely biocompatibility and low toxicity. Failure to meet these requirements can often be a limiting factor in the approval of drugs by the Food and Drug Administration (FDA). Nowadays, the majority of NPs among FDA-approved drugs (since 2016) are represented by liposomal (22% or 43% if including lipid-based NP) and polymeric (29%) nanocarriers.[138] This is due to their low toxicity, as well as the relative ease of modifying the NP properties through covalent and noncovalent interactions. Although the number of FDA-approved lipid and polymer nanocarriers for DM therapy is limited, innovative delivery systems for insulin and other hypoglycaemic agents are being researched to improve the efficacy and safety of DM treatment. Research and development in this area is ongoing, and future advances may lead to the approval of new drugs for DM therapy utilizing nano­technologies.

The first mention of lipid vesicles dates back to the 1960s, marking an important milestone in the development of new drug formulations with improved properties. The second important step was the FDA approval of Doxil®, the first liposomal anticancer drug doxorubicin, for clinical use. Due to low toxicity, improved biocompatibility, ability to encapsulate hydrophobic and/or hydrophilic substrates, etc., lipid-based drug delivery systems are increasingly reaching clinical applications (more than 20 to date) compared to other types of nanocontainers.[139] In the context of DM therapy, the main challenges are (1) protection of hypoglycemic drugs from premature chemical and enzymatic degradation, (2) overcoming biological barriers, (3) controlled drug release (stimulus-responsive systems), (4) high encapsulation efficiency.[140]

The DM pathogenesis can be slowed down by using natural ingredients obtained from medicinal plants. An example of such compounds is quercetin, which has antioxidant and anti-inflammatory activities. Incorporation of quercetin into soy phosphatidylcholine and cholesterol liposomes has been shown to reduce oxidative stress, inflammation, endoplasmic reticulum stress, and autophagy in pancreatic tissue in a rat model of T1DM.[141] It is worth noting that the efficacy of quercetin encapsulated in conventional liposomes, i.e., phospholipids (or a mixture of lipids), cholesterol and sometimes polyethylene glycol (PEG), has been repeatedly demonstrated, making this therapeutic strategy promising for addressing both insulin resistance and the associated inflammation.[142] Another example of a natural therapeutic agent with similar properties is silibinin (or silybin), which has been shown to normalize blood glucose levels, enhance insulin sensitivity in cells, and improve pancreatic β-cell function, as confirmed by in vivo experiments.[98, 143] It is worth noting that the classical liposome composition is safe and non-toxic for both oral and transdermal delivery of antidiabetic drugs.[144]

Structures of quercetin, silibinin and hyodeoxycholic acid

Liposomes are very ‘flexible’ type of nanocarrier, since they can be used in their classical composition, modified with various ligands to confer specificity of action, and loaded with both hydrophilic and hydrophobic substrates. Additionally, their size, charge, and the number of bilayers can be varied. This versatility explains their continued popularity as one of the most sought-after delivery systems.[145] While monolayer liposomes of classical composition have been described above, Tian et al. developed multilamellar liposomes for intramuscular injection of exendin-4. Exendin-4 has a short half-life in the body, which is around 2.4 h, however, encapsulating it in liposomes modified with palmitic acid (with encapsulation efficiency of > 99%) extended its half-life to 77.28 ± 12.92 hours.[146]

One of the main properties of liposomes is to increase the bioavailability of drugs. At the same time, through various modifications of the vesicle composition, their therapeutic effect can also be enhanced. Such modifying agents include hyodeoxycholic acid, which can induce insulin secretion. It has been shown that the content of hyodeoxycholic acid directly affects the physicochemical characteristics of the vesicles. Increasing the concentration of hyodeoxycholic acid increases the hydrodynamic diameter of the liposomes from 196 nm to 332 nm and decreases the encapsulation efficiency of metformin from 67% to 59%. Furthermore, in vivo experiments have established that the most optimal system is liposomes with a 1 : 1 mass ratio of hyodeoxycholic acid to metformin (compared to ratios of 0.5 : 1 and 2 : 1). Oral administration of this composition significantly reduces glucose levels, bringing it close to the level of healthy animals and even slightly lower than in the group receiving free metformin.[147]

As noted earlier, liposomes as drug delivery systems occupy a leading position both in the market of nanosized dosage forms and in the field of scientific research. Most liposomal drugs currently available on the market have a classical composition.[22] However, despite their advantages, conventional liposomes have several limitations, such as low stability, rapid elimination from the body, non-targeted drug delivery, etc. As a result, the current trend in the literature is increasingly focused on the development of nanocontainers with improved properties through the incorporation of specific ligands. The primary goals of these modifications are to increase the circulation time of liposomes in the body, achieve controlled drug release, enhance targeting, improve the stability of liposomes during storage, and adapt them to various routes of administration.[148] This research has led to the development of next-generation lipid nanocontainers that have been successfully tested for the DM treatment.

In addition to the degradation of nanocarriers in the gastrointestinal tract, low oral bioavailability of drugs may be due to first-pass metabolism, limited drug solubility and permeability.[149, 150] As a result, conventional NPs are not always suitable (or insufficiently effective) for oral drug delivery, which motivates researchers to develop new, adapted systems. Surfactants are a vast class of compounds and serve as versatile tools for creating a wide range of NPs with enhanced properties due to their amphiphilic nature and structural diversity. Among the most well-known nanocontainers obtained using surfactants are transfersomes and transethosomes,[151-153] cationic liposomes,[154-156] micro- and nanoemulsions,[157-159] niosomes,[160-162] bilosomes,[163, 164] etc. (Fig. 7).

Fig. 7

Schematic representation of nanocontainers obtained using surfactants

In the context of oral drug delivery, a relatively new type of nanocontainers — bilosomes — is of particular interest. The main components of bilosomes are nonionic surfactants (Span, Tween), bile salts (more commonly sodium deoxycholate), and lipids. Bile salts also act as biosurfactants, repelling external bile salts present in the intestinal lumen, thereby protecting the aggregates and the drug from degradation.[165] Since their first mention by Conacher et al. in 2001,[166] bilosomes have been tested for the therapy of various diseases, including oncological and neurodegenerative. They have also been adapted for transdermal and ocular drug delivery.[167] In DM management, Soliman et al. developed lactoferrin-coated bilosomes for oral delivery of the flavonoid quercetin.[168] Based on the results of in vitro, ex vivo, and in vivo experiments, the authors demonstrated a significant enhancement of the antidiabetic effect of bilosomes due to the synergistic effect of lactoferrin and quercetin, specifically owing to their hypoglycemic, anti-inflammatory, and antioxidant properties. In another study, the effectiveness of bilosomes in enhancing the oral bioavailability of berberine (an isoquinoline alkaloid) was evaluated for the first time. The authors found that bilosomes exhibited optimal characteristics with a concentration of phosphatidylcholine at 0.06 mol L^–1, 25 mg of sodium deoxycholate, and 15.2 mg of cholesterol. It is important to note that the size of bilosomes and their encapsulation efficiency are primarily influenced by the cholesterol content, while the zeta potential is affected by the concentration of nonionic surfactants and bile salts.[169, 170] Pharmacokinetic experiments revealed a 6.4-fold increase in the relative bioavailability of berberine when loaded into nanocontainers compared to the free form of the alkaloid. Although the authors found a 99.3% and 31.7% pharmacological effect in reducing blood glucose levels for the formulated and free berberine, respectively, subcutaneous insulin injection remains more effective.[170] It is worth noting that the literature includes studies on loading bilosomes with first-line DM medications, i.e., insulin for oral delivery and metformin hydrochloride for transdermal delivery.[113, 171] However, bilosomes can be used not only for lowering blood glucose levels but also for addressing the complications and consequences of DM, such as diabetic nephropathy. In vivo studies have demonstrated the nephroprotective effects of eprosartan mesylate and losartan-loaded bilosomes upon oral administration.[172, 173]

Structures of berberine, losartan, and eprosartan mesylate

Another example of an effective dosage form containing surfactants is niosomes. Due to their composition (nonionic surfactants and cholesterol), niosomes exhibit lower toxicity compared to other types of NP and can be administered via various routes, including oral, transdermal, ophthalmic, and pulmonary.[174] According to years of research in the field of drug delivery systems, niosomes are promising vehicles for dermal and transdermal drug delivery.[175] For instance, the classic drug metformin hydrochloride was encapsulated in niosomes for transdermal administration to extend its antidiabetic effect and to explore its ability to enhance wound healing in diabetic patients.[176] The authors of the study claim that transdermal administration of metformin hydrochloride every 2 days provides a more sustained antidiabetic effect compared to daily oral administration of metformin. Additionally, the niosomal formulation demonstrated wound-healing activity by improving wound contraction and collagen deposition, and by stimulating serum transforming growth factor beta (TGF-β) level, which is also a significant outcome in the context of DM therapy.

One of the advantages of niosomes (as well as other lipid-based nanocontainers) is their ability to encapsulate drugs over a wide range of solubility. Due to their bilayer structure, niosomes can encapsulate both hydrophobic (within the bilayer) and hydrophilic compounds (within the aqueous core). Dual loading of substrates into nanocontainers has been and remains a highly attractive strategy, as it not only enhances therapeutic efficacy but also targets multiple sites and produces a synergistic effect. This approach has also been applied in DM therapy for one of the most widely used drugs – hydrophobic glipizide and hydrophilic metformin.[177] The main idea of this work is to develop a drug formulation based on a suspension, which could make oral administration of antidiabetic medications more convenient, especially for patients with dysphagia. Pursuing the same goal, the authors of the [178] developed chewable tablets containing niosomes with repaglinide. It was found that encapsulating metformin and glipizide in niosomes results in a more prolonged release of the drugs, which potentially addresses the issue of their short half-life (less than 6 hours) and, consequently, reduces the frequency of medication administration. In another study, Eissa et al. formulated proniosomal compositions loaded with amitriptyline and liraglutide for the treatment of diabetic peripheral neuropathy.[179] In this study, the strategy was different: amitriptyline, used for treating diabetic neuropathy, can lead to hyperglycemia, which is a major drawback. Combination therapy with amitriptyline and an antihyperglycemic agent (liraglutide) can provide effective treatment for neuropathic pain while also managing blood glucose level. Moreover, the developed formulation enables the replacement of the currently available injectable form of liraglutide with an oral one. Interestingly, not only drugs can be combined, but also drug delivery systems. For example, novel deformable liponiosomal hybrids have been developed, combining components of both liposomes and niosomes (phospholipids, cholesterol, nonionic amphiphiles Span 60 and Tween 80), for oral delivery of repaglinide.[180] Researchers are also highly interested in the antidiabetic properties of plant-based medicinal compounds encapsulated in niosomes.[181-183]

Structures of amitriptyline and liraglutide

Another well-known example of modifying the properties of lipid nanocontainers with amphiphilic molecules is the creation of ultra-deformable liposomes – transfersomes – by incorporating edge activators, most of which are nonionic surfactants, into the lipid bilayer. Transfersomes were first proposed in 1992 as transdermal drug delivery systems by Cevc et al.[184] Eleven years later, in 2003, Cevc published a review on the potential of ultra-deformable carriers for transdermal insulin delivery.[185] It is noteworthy that there are references to clinical studies conducted with transfersomes loaded with insulin and ketoprofen.[186, 187] Over the years, transfersomes have been tested for transdermal delivery of classic hypoglycemic drugs such as insulin[188] and metformin hydrochloride,[111] as well as pioglitazone and eprosartan mesylate for combined therapy against hypertension and DM.[189] For all compositions tested in the STZ-induced model of DM, an improved glucose-lowering effect was observed in rats compared to the free and/or oral forms of the drugs. It is worth noting that transfersomes have ‘siblings’ – transethosomes – which have also been successfully used for loading antidiabetic drugs.[190, 191] The only difference between transethosomes and transfersomes is that transethosomes contain ethanol to enhance the penetration of aggregates through the skin. Transfersomes and transethosomes have also been successfully used as delivery systems to address complications caused by DM, particularly for promoting the diabetic wound healing.[192, 193]

Thus, the strategy of modifying liposomes with surfactants is quite promising. Special attention should be paid to other lipid nanocarriers containing surfactants, such as SLN, nanostructured lipid carriers (NLC), and NE, as they offer certain advantages over liposomes, including improved stability, controlled drug release, higher drug loading capacity, and easier production and scalability.

Despite the advancements achieved by researchers in the development of liposomal delivery systems for antidiabetic drugs, several significant challenges remain, such as premature drug leakage, limited stability, low targeting ability, nonspecific clearance, and difficulties in scaling up production.[194] To address these issues, the next generation of lipid nanocarriers has been developed, specifically solid lipid nanoparticles and nanostructured lipid carriers. The primary components of SLN are biodegradable and biocompatible solid lipids (e.g., 1-tetradecanol, cetyl palmitate, various triglycerides, etc.) and surfactants (such as sodium lauryl sulfate, Tween, Precirol® ATO-5, etc.). In contrast, NLC include these components along with liquid lipids or oils (castor oil, Labrafac™ CC, oleic acid, etc.).[195] Another example of such systems is NE, which are fine water-in-oil (w/o) and oil-in-water (o/w) dispersions of two immiscible fluids. Nanoemulsions — emulsions with droplet sizes ranging from 20 to 100 nm — are kinetically stabilized dispersions formed by combining and stabilizing two immiscible phases using a surfactant. Due to their small droplet size, they possess characteristics such as high surface area per unit volume, high stability, optically transparent appearance, and improved bioavailability of drugs.[196, 197] The primary components of these systems include various semisynthetic oily esters, triglycerides, partial glycerides, and nonionic ester emulsifiers. It is important to note that emulsifiers with high hydrophile-lipophile balance (HLB) values (8 – 18) are used to produce o/w NEs, while emulsifiers with low HLB values (3 – 6) are more suitable for producing w/o NE.[198] Numerous studies have confirmed that SLN, NLC, and NE have low toxicity, high stability and have been extensively studied for the treatment of a wide range of diseases, including DM.

Like any drug delivery system, the composition and physicochemical characteristics of NPs must be thoroughly optimized before testing on animal models. This stage is crucial and requires the preliminary statistical and mathematical data processing. Priyanka et al. optimized the composition of SLN using a central composite design with Design Expert software by varying four factors during NP preparation: homogenization speed and time, as well as sonication time and intensity.[199] The results were monitored by changes in particle size, PdI, and zeta potential of the NPs. The same design method was applied for the development of curcumin-loaded SLN.[200] The authors found that increasing the homogenization speed (up to 15 000 rpm) and time (up to 30 min) reduced particle size and PdI while increasing the zeta potential. Increasing sonication time up to 5 min demonstrated similar results, whereas sonication intensity had minimal impact on the SLN physicochemical characteristics. In another study, the Box-Behnken Design (using Design Expert software) was employed to examine the effects of the drug-to-lipid ratio, surfactant and co-surfactant concentration on SLN size, entrapment efficiency, and PdI.[201] Statistical analysis revealed that the most optimal values for particle size (147.1 nm), entrapment efficiency (83.6 ± 0.8%), and PdI (0.265), which were close to the predicted values, were obtained with a drug-to-lipid molar ratio of 1 : 4.16, 1.21% surfactant, and 0.4775% co-surfactant (Fig. 8). Subsequently, the optimized formulations were loaded with extracts of Ficus religiosa L.[199] and tetrahydrocurcumin,[201] both of which possess antioxidant properties. In vivo, these formulations demonstrated an enhanced ability to lower blood glucose levels compared to the free drugs. In general, compounds with antioxidant properties that are encapsulated in SLN are highly effective for DM therapy and exhibit significant potential.

Fig. 8

3-D response surface plots of selected responses: (A) particle size, (B) encapsulation efficiency, (C) and PdI.[201] This figure is published under a Creative Commons Attribution NC-ND license

SLN have been successfully tested for the delivery of glibenclamide,[202] tetrahydrobiopterin,[203] repaglinide,[204] myricetin,[205] and many other therapeutic compounds. Evidently, SLN possess excellent properties, and in addition, like conventional liposomes, they can be modified with various ligands. For example, SLNs loaded with the hydrophilic drug saxagliptin were further coated with the methacrylic acid copolymer Eudragit RS100 to enhance the oral bioavailability of the drug.[206] A similar approach was applied for the transdermal delivery of glibenclamide using NLC coated with Eudragit L.[207] Polymethacrylates can be used to enhance drug penetration through the skin, intestinal epithelium, and cornea, improve bioavailability, serve as enterosoluble coatings, prolong drug release, enable pH-dependent release, etc.[208] In another study, SLNs were adapted for endosomal escape.[209] The authors prepared nanocarriers with pH-sensitive HA2 fusion peptides from influenza virus hemagglutinin, which can respond to the acidic environment in endosomes and initiate escape. It was found that loading HA2 peptides into the outer shell of SLN results in more efficient release of nanocarriers from early endosomes compared to peptides loaded into the aqueous core, leading to increased biological activity of insulin.

NLCs are also being successfully developed for the delivery of hypoglycemic drugs for both oral [210] and transdermal [211] administration. As noted earlier, structurally, NLCs are very similar to their predecessors, but they differ in the inclusion of liquid lipids, resulting in greater stability of NPs. In DM treatment, NLCs have found widespread use both as carriers of hypoglycemic drugs and for healing ulcers and wounds associated with DM. For successful vesicle production, it is crucial to select the right components. The authors of the [212] tested a range of solid lipids at various concentrations (Imwitor® 900 K, Dynasan® 116, Kollivax® GMS II, cetostearyl alcohol), as well as several surfactants (Tween 80, Poloxamer 188, Lecithin, and TPGS) to obtain stable NLCs and load sucupira oil. Using a 2² factorial design, it was established that an optimal sucupira oil-loaded NLCs should consist of 0.5 wt.-vol.% sucupira essential oil, 4.5 wt.-vol.% Kollivax® GMS II, and 1.425 wt.-vol.% TPGS, resulting in a mean particle size ranging from 100 to 200 nm and PdI < 0.3. Overall, plant-based medicines are gaining increasing interest due to their lower toxicity compared to synthetic drugs. For example, silymarin inhibits gluconeogenesis, reduces glucose-6-phosphatase activity, and suppresses the overexpression of the cytochrome CYP 3A2 enzymatic system, contributing to its antidiabetic properties. However, its low water solubility has prompted researchers to create NLCs and investigate the impact of various solid lipids (stearic acid (C₁₈) and cetyl palmitate (C₃₂)) on their efficiency.[213] It was found that both NLC systems reduced glucose and triglyceride levels better than free silymarin in vivo, but the effect of NLCs with stearic acid was more pronounced and lasted longer. The authors argue that as the fatty acid chain length increases, the degradation rate decreases. Additionally, both formulations demonstrated a significant antihyperalgesic effect in diabetic neuropathy in mice. NLCs have also been adapted for the oral delivery of hypoglycemic drugs such as pioglitazone,[214] piperine [215] and glargine insulin,[216] as well as transdermal delivery of repaglinide.[211]

It is necessary to highlight the studies, in which NLCs were used as carriers for the healing of wounds and ulcers caused by DM. Diabetic wound healing is known to be a complex process that involves multiple stages. However, in many cases, severe inflammation leads to improper and consequently prolonged wound healing. For this purpose, the authors of the [217, 218] developed NLC-based systems with various auxiliary components to deliver anti-inflammatory agents 20(S)-protopanaxadiol and pioglitazone. For 20(S)-protopanaxadiol, the authors designed a three-dimensional mesh structure of silicone elastomer loaded with drug-bearing NLCs. In in vivo experiments, it was shown that due to the synergistic action of the wound-healing elastomer and the anti-inflammatory 20(S)-protopanaxadiol, the authors managed to achieve suppression of inflammatory infiltration, stimulation of angiogenesis, and regulation of collagen level during the inflammatory, proliferative, and remodelling phases of diabetic wound healing, respectively.[218] Pioglitazone was loaded into NLCs within a framework of collagen (a structural stabilizer) and chitosan (a controlled drug delivery system). The authors demonstrated a similar synergistic effect in diabetic wound healing in vivo: pioglitazone suppressed the inflammatory phase, while collagen and chitosan promoted cell proliferation and migration, leading to 99% wound healing within 21 days.[217]

Over the past decade, there has been a significant increase in the use of NE, especially in the pharmaceutical and cosmetic industries, and DM therapy is no exception. The well-known antioxidant quercetin was loaded into NE for DM therapy. Mahadev et al. investigated the NE form of quercetin and evaluated its combinatory effect with the β-carotene.[219, 220] The process of obtaining NE in both cases was optimized using the Box-Behnken design. It was demonstrated that NE-formulated quercetin, both alone and in combination with β-carotene, exhibited excellent oral bioavailability and significant protective and therapeutic activity against STZ-induced DM over 21 days. This composition allowed control of body weight and blood glucose levels while suppressing high serum lipid levels. To enhance the antidiabetic potential, quercetin was loaded into self-nanoemulsifying drug delivery systems (SNEDDS) along with curcumin, Ganoderma lucidum powder extract, and probiotics. An in vivo pharmacodynamic study demonstrated improved recovery of blood glucose levels, body weight and lipid profile in rats over 42 days of treatment.[221] It is worth noting that quercetin loaded into NE has been investigated in vivo as a cardioprotective agent to mitigate cardiac toxicity and deoxyribonucleic acid (DNA) damage induced by DM.[222] In DM therapy, SNEDDS have been successfully developed for the oral delivery of exenatide-4 [223] and mahogany seed extract (Swietenia mahagoni).[224] Since exenatide-4 has low oral bioavailability due to enzymatic degradation in the gastrointestinal tract, the authors combined it with the chymotrypsin inhibitor chymostatin. It was shown that the pharmacodynamic response after oral administration of the NE was comparable to that of the commercially available oral form of exenatide-4, Byetta®. In another study, SNEDDS based on black seed oil (which contains thymoquinone), Capmul MCM, and Cremophor EL were loaded with sitagliptin and dapagliflozin, encapsulated in hard and/or soft gelatin capsules for the effective treatment of T2DM (Fig. 9).[225] In turn, NE have been used for the delivery of both synthetic drugs (pioglitazone hydrochloride,[226] as well as metformin and repaglinide individually or in combination [227]) and plant-derived substances (glycyrrhizin,[228] 2,4,6-triphenylaniline,[229] berberine,[230] essential oil from Alpinia zerumbet Fructus [231]), whose efficacy has also been demonstrated in vivo. Much of the current research is focused on creating gel forms of NE for better retention on the skin and diabetic wound healing. The pharmaceutical substances chosen for these formulations typically include drugs with antidiabetic, anti-inflammatory, antimicrobial, and wound-healing properties.[232-235] As a gel matrix, substances such as chitosan, hyaluronic acid, alginate, agarose, gelatin, cyclodextrin, carrageenan, fibrin, collagen, dextran, etc. can be used.[236-238]

Fig. 9

(a) Appearance of the SNEDDS preconcentrate (in glass tubes) and SNEDDS after dilution with water (in glass beaker) that were developed with black seed oil and soybean oil for the model drugs, sitagliptin (SN) and dapagliflozin (DN), as combined dosage form. (b) The combined F1-SNEDDS dosage form filled in a hard gelatin capsule and the appearance of the diluted SNEDDS (10 mg anhydrous formulation was diluted in 10 mL water) and (c) formation of nanodroplets/micelles of F1-SNEDDS in the presence of bile salt (BS): phospholipid (PL) and pancreatic lipase during lipid digestion in the small intestine.[225] This figure is published under a Creative Commons Attribution license

Thus, the use of lipid nanocarriers encompasses a wide range of applications, from delivering various types of drugs for DM therapy to addressing disease complications. Researchers have learned techniques for controlled drug release, achieving high loading efficiency, and prolonging drug action by modifying nanocarriers with specific ligands, leading to the development of new generations of lipid vesicles. Today, there is a growing global trend to develop new drug delivery systems for DM treatment, to adapt existing systems, and to incorporate drugs of natural origin. This surge in interest is driven by the ease of manipulation and low toxicity of lipid nanocarriers. Polymer nanocarriers, which will be discussed in the following section, also meet these criteria.

In general, oral administration of antidiabetic drugs is preferred, as the dosing and frequency of medication make invasive methods challenging. The gastrointestinal tract, however, presents a wide pH range, a multi-layered mucosal structure, and enzymes reducing the bioavailability of peptides and proteins. Polymer NPs made of various natural biodegradable polymers, as well as synthetic ones are pH-sensitive (Fig. 10). This, combined with their modifiability, high biodegradability, biocompatibility, and stability, provides a broad platform for selecting the appropriate composition for antidiabetic drug delivery. Nanocarriers can either be composed entirely of polymers (polymeric micelles, vesicles, and dendrimers) or polymers can act as modifiers for other NPs, like liposomes, silicone nanospheres, etc.[239]

Fig. 10

Examples of synthetic and natural polymers used to create and modify nanocontainers

Polymers occupy a leading position in the field of biomedicine in terms of applicability and scientific research.[240] Review articles emphasize the advantages of polymeric nanocontainers, including precise size control (ranging from 1 to 1000 nm), diverse morphologies (rods, worms, disks, etc.), the capacity to load a wide variety of substances (hydrophobic, hydrophilic, large and small biomolecules), stimulus responsiveness (to pH, redox environment, enzymatic environment, and temperature), as well as their stability, modifiability, etc.[241, 242] Due to these numerous advantages, polymeric nanocontainers are extensively studied for the treatment of some of the most pressing diseases, including cancer,[243, 244] neurodegenerative,[245] cardiovascular,[246] DM and its complications,[247, 248] rheumatoid arthritis [249] and many others. As described above, the majority of FDA-approved NPs are represented by lipid and polymeric nanocarriers.[138] The choice between polymer and lipid nanocontainers depends on the specific goals and conditions of treatment. Polymer nanocontainers may be preferable for sustained, controlled drug release and where high stability is required. Lipid nanocontainers, on the other hand, may be better in cases where high biocompatibility and ease of production are required. With each passing year, more and more research is being conducted in the field of polymer-based drug delivery systems to address the existing limitations. For instance, a common method of increasing the stability of drug carriers is to use polyacids and polybases, which are resistant to pH changes but capable of swelling and degrading. Another example of a responsive system is a nanocarrier with dual sensitivity to both glucose level and pH value. The creation of multifunctional systems that are immediately responsive to several stimuli was pursued in the work of Lin.[73] The authors investigated polymeric vesicles based on glucose-sensitive polyphenylboronic acid and pillar[5]­arene, containing GOx for transcutaneous delivery of insulin. The selected formulation demonstrated a rapid response to hyperglycemia and the ability to restore blood glucose concentration to normal level.

For more effective therapy, additional requirements for polymeric NPs, such as the ability to be adsorbed onto mucosal surfaces or to avoid exposure to the immune system, must be taken into account. These factors are particularly important given that transdermal and oral routes are the primary methods for delivering hypoglycemic drugs. These aspects, which will be discussed in following sections, have the potential to significantly enhance the therapeutic efficacy of nanocarriers in DM treatment.