(W1030-02-11) Injectable and Sustained Release Delivery System of Capsaicin-Loaded PLGA-Based In Situ Forming Implant for Treating Obesity: Characterization, Drug Release, and Stability Studies

Purpose: Capsaicin, a TRPV1 agonist, has been established as an anti-obesity by enhancing metabolism, energy expenditure, and thermogenesis to counter HFD-induced obesity. However, its low solubility in an aqueous environment, the short biological half-life and bioavailability, and adverse side effects of oral administration hampered the clinical development of capsaicin. The most effective strategy for addressing these issues is by entrapping capsaicin within polymeric carriers to generate a sustained-release delivery system, ensuring only a small amount of drug is present in the blood, reducing adverse side effects, and preventing intracellular enzyme degradations, thus, enhancing bioavailability. Additionally, obesity necessitates drug delivery over a prolonged period, whereas oral or transmucosal drug delivery is limited to 24 hours to seven days. Therefore, it is necessary to develop an extended delivery system for capsaicin, for instance, 1 to 6 months. In situ forming implants (ISFIs) overcome some drawbacks of other controlled delivery systems like microspheres, liposomes, and surgical implants. ISFI has a simple manufacturing and high loading efficiency, suitability for heat-sensitive drugs, generating a site-specific action and extended drug release, diminishing injection frequency, and enhancing patient compliance. ISFI is a liquid dosage form that transforms into a solid implant upon injection and contact with the aqueous environment in the body. Therefore, it is less complicated to administer and less invasive than a surgical implant since ISFI utilizes a biodegradable and biocompatible polymer, like Polylactic-co-glycolic acid (PLGA), an FDA-approved polymer for parenteral dosage forms. Moreover, drug-release retarders should be incorporated within ISFI to minimize the burst effect generally exhibited by polymeric-based formulations. Therefore, this research aimed to develop capsaicin-loaded PLGA-based ISFI as a sustained-release drug delivery system that is stable and injectable with a minimum burst effect.

Methods: Capsaicin-loaded ISFIs were prepared by dispersing capsaicin into PLGA-DMSO solution, with three different liquid lipids as the drug release retarders, as shown in Figure 1, then compared their visual appearances, viscosities, solidifying times, the initial burst, and drug release profiles. Viscosity was measured using a Brookfield viscometer at 25°C. The solidifying time was tested in a thermostatically controlled water bath at 37 °C. In-vitro release study was performed using Sample and Separate method by collecting samples hourly, daily, then weekly for up to 5 months, and analyzed using a validated HPLC method. The initial burst release was investigated for the cumulative drug release after 24 hours. The drug release kinetics were studied using zero-order, first-order, Higuchi’s, Hixon-Crowell, and Korsmeyer-Peppas models. The selected ISFI was characterized for its surface morphological features, thermal analysis, rheological behavior, and injectability parameters, then studied its stability. The morphological features of the solidified implant were observed using Scanning Electron Microscopy. Thermal analysis of Capsaicin, PLGA, physical mixture of PLGA-Capsaicin (1:1), Maisine, liquid ISFI, and solidified ISFI were performed using DSC. The rheological behavior was determined using a Bohlin rheometer at 25°C then calculated with Furrow's equation. Injectability parameters were predicted by a mathematical approach of the Hagen–Poiseuille law for six different syringe and needle dimensions. The stability study was carried out for six months at room temperature.

Results: The results showed that all formulations were clear and transparent to yellowish and solidified spontaneously upon contact with the release medium. As shown in Fig. 2A, the assays of 8 ISFIs were greater than 95% of drug recovery. Increasing drug concentration after adding liquid lipids as release retarders increased the viscosity, where CM75 showed 134.97 ± 0.12 mPa.s of viscosity at 20 rpm. The Capsaicin-loaded ISFIs showed a triphasic pattern of drug release, as shown in Fig. 2B, where CM75 was entirely sustained released after 5 months and generated the lowest initial burst release of 7.97% ± 1.29%. The drug release kinetics, as shown in Fig. 2C, described that most ISFIs, including CM75, followed a Korsmeyer-Peppes kinetics model. The SEM micrographs in Fig. 3A revealed that the implant formed a “hollow and sponge-like” structure, confirming the formation of various pore-size features on the surface morphology of the solidified implant, allowing the drug release in a sustained and prolonged manner. The DSC thermograms in Fig. 3B confirmed the crystallinity of pure Capsaicin with a single endothermic peak, a degradation temperature of the CM75 liquid implant, and performed the glass transition and degradation temperatures of the solidified implant. The Furrow constant from the plot, as shown in Fig. 3C and 3D, confirmed that CM75 followed Newtonian fluid behavior and the injection force values met the acceptance criterion of less than 40 N. The stability study in Fig. 3E showed that CM75 has remained stable with no color changes with a recovery percentage of 96.35 ± 0.58% after 6 months.

Conclusion: Capsaicin has been successfully loaded in a PLGA-based in-situ forming implant and generated a stable, injectable, and sustainable drug release with a very minimum burst effect, where CM75 is the best formulation.

References: 1. Baskaran, P. et al., 2017. TRPV1 Activation Counters Diet-Induced Obesity Through Sirtuin-1 Activation and PRDM-16 Deacetylation in Brown Adipose Tissue. Int J Obes (Lond) 41, 739–749.
2. Ibrahim, T.M.et al., 2021. An overview of PLGA in-situ forming implants based on solvent exchange technique: effect of formulation components and characterization. Pharmaceutical Development and Technology 26, 709–728.
3. Justin C. M. et al., 2022, Anti-cancer activity of sustained-release capsaicin formulations, Pharmacology & Therapeutics, Vol. 238, 108-177, ISSN 0163-7258.

Figure 1. ISFI's composition and methodology

Figures (2A). Characterization of ISFIs; (2B). Drug release profiles of ISFIs; (2C). Drug release kinetics of ISFIs

Figures (3A). SEM micrographs of CM75 (7.5% Capsaicin-loaded ISFI with maisine); (3B). Thermograms of Capsaicin, physical mixture, liquid implant, and solidified ISFI; (3C). Rheogram of CM75; (3D). Injectability parameters of CM75; (3E). Stability study (assay) of CM75 for 6 months