The stimuli-responsive polymers exhibit pseudo-plastic behaviour, which is highly desirable to formulate sol–gels as they offer better distribution on the mucosal tissue [
162]. For the drug formulation exhibiting such behaviour, viscosity is reduced during application on the mucosa due to the change in alignment of tangled polymeric chains in the shear direction. However, the original rheological profile is regained immediately after application on the vaginal mucosa in response to the physiological stimuli [
6,
162]. This leads to the formation of a viscous gel in the vaginal cavity, which is essential for prolonged drug residence time in the vaginal cavity and ultimately results in sustained and controlled drug release [
158]. The sustained and controlled drug release profiles exhibited in the examples of sol–gel systems included in
Table 4 are attributed to the increased vaginal residence time of the drug formulations. Further, drug distribution in the mucosal tissue is also assisted by the ability of the gel to permeate the mucosa [
6]. For instance, a significant increase in drug permeation was observed with a vaginal in situ gel formulation of fluconazole, which could be attributed to both the improved drug release profile of the drug in the formulation as well as the interaction between chitosan used in the formulation and mucin present in the vaginal mucosal layer [
163]. Studies on VDDS report an increase in drug permeability by the use of polyethylene glycol [
164].
Mucoadhesive in situ gelling systems are those (containing natural or synthetic mucoadhesive polymers) that interact with and adhere to the mucosal epithelial surface components, particularly mucin, via hydrogen bonding, electrostatic interaction, and van der Waals forces, and, once administered in soluble form, they rapidly undergo in situ gelation (
Figure 3) [
8,
47,
50]. Published results suggest that at least 6 h of vaginal drug residence time is desired to represent the mucosal clearance turnover rate for the drug to cross the mucosal barrier (30–100 µm thick), epithelium layer (200–300 µm), and other adjacent layers before reaching the blood vessels in the vagina. This target can be met by a gel using mucoadhesive polymers containing suitable surface characteristics, charge, and functional groups, such as hydroxyl, sulphate, carboxyl, and amine groups—for example, polymers such as polyacrylic acid, cellulose, chitosan, hyaluronic acid, carrageenan, alginate, gums, and sulphated polysaccharides [
6,
7,
31,
50]. Use of these polymers results in optimum mucoadhesive strength, sustained drug release, and increased drug uptake by the vaginal mucosa [
7]. Mucoadhesive systems not only improve bioavailability through localised action but also alter tissue permeability and enhance the absorption of protein and peptide-based drugs [
165]. This makes mucoadhesion an essential parameter to consider and optimise as per the requirements to obtain formulations for controlled and sustained deliveries [
20,
166]. For instance, in a study comprising a thermosensitive formulation, mucoadhesive formulation, and thermosensitive–mucoadhesive system, the in situ mucoadhesive gel was considered the optimum formulation, with a longer vaginal residence time compared to the two other systems (>8 h). The study suggests that both gelation and mucoadhesiveness together result in a robust vaginal formulation [
164].
Figure 3. Stages of mucoadhesion and drug release from stimuli-responsive sol–gel formulations.
Mucoadhesion occurs in two stages: (i) contact stage involving hydration, wetting, and spreading; (ii) consolidation stage involving strengthening polymer and mucin interactions through hydrogen bonds, hydrophobic interactions, van der Waals forces, electrostatic interactions driven by negatively charged mucin, and/or mucoadhesive or polymer chain interpenetration into the cervicovaginal mucus gel (
Figure 3) [
6,
7,
166]. Mucoadhesive intravaginal formulations should be engineered considering the nature and physico-chemical characteristics of drugs and their transportation route to overcome the associated challenges and improve their therapeutic efficacy [
167]. In addition, for vaginal application, these polymers should be non-toxic, non-irritating, flexible, comfortable, and ideally remain unabsorbed in the vaginal epithelium [
37]. The various polymers used in VDDS are represented. Natural polymers used in these systems typically respond to single or multiple stimuli, while synthetic polymers respond to specific stimuli. However, a major issue with the use of synthetic polymers is that they can result in irritation and toxicity to the underlying tissue [
158]. Hence, diligent polymer and excipient selection is necessary before formulation into sol–gel systems, for either localised or systemic action.
In this context, the World Health Organization (WHO) has issued guidance for vaginal preparations, such as lubricants, recommending that they be mildly acidic (pH 4.5), with an upper limit of osmolality not exceeding 1200 mOsm/kg, to minimise any risk of mucosal/epithelial damage [
20,
75]. In the context of tonicity, while hypotonic vaginal products enhance muco-penetration, hyperosmolar vaginal products raise safety concerns with respect to vaginal tissue health and sperm viability and mobility [
72]. This was corroborated in a phase 1 clinical trial for a vaginal microbicide developed against HIV, which was discontinued, with the sponsor citing unacceptable side effects resulting from the high osmolality of the gel, reinforcing the need to factor in osmolality when developing vaginal semi-solid formulations [
168]. In addition, the size of the carriers/particles infused in semi-solid formulations has a direct impact on cervicovaginal mucosal and epithelium penetration, with a 200–500 nm particle size range recommended for VDD mucus [
20]. Surprisingly, lowering the particle size range to 100–150 nm results in them becoming trapped and immobilised in the numerous tiny pores/pockets of the cervicovaginal mucosa, rendering them appropriate for localised, deep mucosal drug delivery. In contrast, larger-sized carriers >1000 nm are unable to diffuse into such pores/pockets and so remain on the outer mucosal surface, where they are susceptible to more rapid clearance by ciliated mucosa [
20,
169].
Considering the biological and physicochemical challenges, nanotechnology-based delivery systems have proven a promising means of improving drug distribution, retention, and therapeutic efficacy in VDD [
170]. Nanoparticulate systems can enhance the solubility, bioavailability, and targeting of drugs, while increasing the rates of dissolution and surface area that can be reached. This can be achieved through the design of particulates including, but not limited to, micelles, carbon nanotubes, polymeric lipid nanoparticles, nanocapsules, nanogels, nanofibers, dendrimers, quantum dots, and nanocomposites, which are extensively reviewed elsewhere [
157,
169,
171]. The literature suggests that these nanocarriers have demonstrable drug solubility enhancement properties, while also protecting against rapid drug degradation and enhancing drug concentrations in target tissues, further masking them from the harsh conditions of the FRT and addressing the many shortfalls of conventional VDDS [
62,
150,
151]. For instance, the aqueous solubility, stability, and mucosal permeability of antifungal drugs have been addressed using inclusion complexes and gel flakes [
172,
173]. Antifungal drugs prepared using hydroxypropyl β-cyclodextrin (HPβ-CD) have been readily incorporated into sol–gel formulations, and exhibited sustained drug release without any detrimental effect on the vaginal tissue [
172]. Similarly, enhanced drug permeation/bioavailability was achieved through the vaginal epithelium, alongside improved epithelial drug viability, when in situ nanoparticles of acyclovir, a highly water-insoluble drug, were formed, in comparison to the pure drug [
174]. Likewise, atorvastatin, a BCS class II drug, when formulated into nanoparticles exhibited significant improvements in solubility and efficacy, compared to the native powdered atorvastatin [
175]. Hence, the use of vaginal sol–gel formulations as vehicles for such tailored and innovative micro- and nano-encapsulated drug forms is a highly promising proposition to address the shortfalls of conventional dosage forms for a range of conditions, as elaborated in
Table 4 [
1,
9,
176,
177].
The range of gynaecological conditions receiving significant attention through innovative formulation development include vaginal infection and atrophy, neoplasia, labour induction, prophylaxis of HIV, and contraception [
72,
78,
84,
106]. Vaginal infections in particular are of growing concern, and, in this context, several studies have focused on the development of sol–gels to treat a range of STIs of bacterial and fungal origin [
3,
177]. A few studies on vaginal sol–gel systems are represented in
Table 4. These studies suggest that the sol–gel formulations, when used in vaginal infections, not only have better efficacy compared to their conventional formulations but also are associated with reduced toxicity towards the underlying tissues of the FRT. Hence, they present strong potential to solve the problems of high reinfection rate and incomplete treatment with the current treatment regimen of vaginal infections. For instance, in a pilot, randomised, controlled trial with confirmed bacterial vaginosis, which has a very high rate of reoccurrence, the initial treatment rate for an in situ gel and gel was 85% and 71.24%, respectively. However, the difference was more prominent with treatment for 4 weeks, with values of 80% with the in situ gel and 47.4% with gel application, indicating the higher long-term efficacy of in situ gels, which can be ascribed to the increased mucoadhesiveness, increased vaginal residence time, and sustained release nature exhibited by the use of poloxamers in in situ gel formulation [
177].
Current strategies for HIV prophylaxis utilise vaginal microbicides that act specifically on the critical steps of HIV transmission, and several such formulations are in different stages of development [
4,
178]. In this regard, vaginal sol–gels provide an opportunity for early intervention to the sexual transmission of HIV in females, with pH-induced gelation triggered by exposure to semen, effectively shielding underlying epithelial cells, and restricting the entry of virions into the systemic circulation [
106]. For instance, a pH-responsive polymeric network comprising phenylboronic acid, salicylhydroxamic acid, and 2-hydroxypropyl methacrylamide impeded the migration of HIV at pH ≥ 4.8 [
108]. This mechanism gives rise to the concept of “molecular condoms”, where the temperature and pH responsiveness of formulations can be applied to the vaginal mucosa setting, with the gel form effectively covering the mucosal tissue and releasing microbicides [
179]. Moreover, this concept of shielding the mucosal tissue helps drug concentration and retention at the vaginal mucosa surface to facilitate mucopenetration and enhance pharmacokinetics at the target tissue [
14]. In cases where the microbiome and/or vaginal mucosal tissue integrity is compromised, susceptibility to HIV infection is elevated. Therefore, co-administration of anti-HIV drugs alongside the localised delivery of mucosal barrier formulations is a combination approach to the prophylaxis of HIV infection that warrants widespread use [
14]. For instance, thermosensitive nanoparticles of the combination of hydrophilic drug Raltegravir, a HIV integrase inhibitor, and lipophilic drug efaviren (non-nucleoside reverse transcriptase inhibitor), prepared using a poloxamer (
Table 5), resulted in thermogelation at 32.5 °C and exhibited anti-HIV activity at a concentration lower than that exhibited by the solution of the combination of these drugs. Moreover, nanoparticles were taken up rapidly by HeLa cells (within 30 min) and exhibited sustained drug release without exhibiting cytotoxicity for a period of 14 days, which indicates that the formulation is a suitable candidate for the prevention of prolonged vaginal pre-exposure of HIV. Furthermore, the incorporation of (RAL + EFV) nanoparticles did not result in the aggregation of nanoparticles, suggesting that the thermosensitive gel is an effective drug delivery vehicle for these anti-HIV drug-loaded nanoparticles [
13].
Similarly, intravaginal dendrimer-based sol–gels have also earned considerable attention in recent years for the treatment of the highly challenging HPV infection, particularly in pregnant women, where systemic drug exposure is not desirable for either the mother or the growing foetus [
62]. An in situ hydrogel infused with amoxicillin using a generation 4 poly(amidoamine) dendrimer with polyethylene glycol provided in vitro drug release for 240 h and a sustained antibiotic effect through a dual mechanism, i.e., the antibiotic effect of the dendrimer itself and the sustained release of the drug. Moreover, the dendrimer complex targeted the inflammatory cells and reduced cytotoxicity and hence no change in vaginal pH or tissue necrosis was observed while the formulation was retained in the vaginal mucosa (72 h), after which the hydrogel started to become degraded [
180].
With these established benefits of VDDS, and the many limitations/unwanted effects associated with current modes of hormonal contraceptive administration, safer and more patient-friendly intravaginal hormonal contraceptives—specifically, stimuli-responsive in situ hydrogels—are gaining attention and interest [
78,
181]. It has been demonstrated that formulations containing multiple drugs are more efficient contraceptives compared to single-drug formulations [
122,
181]. In this context, an in situ pH-responsive hydrogel containing indomethacin, gestodene, and ethinyl estradiol prevented pregnancy completely compared to a control group, which presented 100% pregnancy. However, surprisingly, another group receiving hydrogels without any drugs had a 60% pregnancy rate, indicating that the hydrogel components also play a role in contraception [
181]. Likewise, in situ hydrogels of non-hormonal agents have also been explored for vaginal contraceptives [
10,
84]. For instance, a chitosan-based in situ hydrogel of iron (II) gluconate dihydrate, used prior to sexual intercourse, releases iron rapidly in the vagina and exhibits spermicidal properties [
10]. In another study, poloxamer-based temperature-responsive in situ hydrogels of nonoxynol-9 resulted in up to 10 h of vaginal residence time [
182]. With the rising interest in using multi-drug treatment regimens for more comprehensive therapeutic coverage, drugs with complementary modes of action have been proposed (e.g., anti-HIV + anti- HPV + spermicide), and so the development of sol–gels in this context would be an important milestone in advancing VDD and women’s health and well-being more broadly [
14,
183]. For instance, vaginal administration of a nanoparticle formulation containing an antimicrobial and spermicidal agent, curcumin, and the anti-HIV agent efavirenz exhibited better encapsulation efficiency compared to single-drug nanoparticles and exhibited better efficacy compared to their solution form, without affecting lactobacilli viability or vaginal tissue, hence indicating the formulation as an efficient example of multiple prevention technology (MPT)-based VDDS [
184]. In this context, the development of an efficient delivery vehicle such as sol–gels would enhance the therapeutic benefits [
185]. Additionally, the intravaginal administration of hormones infused in sol–gels is also being investigated for hormonal replacement therapy and fertility treatment, while its application in cervical cancer has shown promise (
Table 5 and
Table 6) [
186,
187].
Table 5. Sol–gel formulations designed for gynaecological indications.
Table 6. Types of polymers used in VDD.