Effects of Polymer Blending on the Performance of a Subcutaneous Biodegradable Implant for HIV Pre-Exposure Prophylaxis (PrEP)

Safety and efficacy of a biodegradable implant releasing tenofovir alafenamide for vaginal protection in a macaque model

OBJECTIVES

To advance the initiative of ending the global epidemic, long-lasting HIV protection is needed through sustained release of antiretroviral drugs for months to years. We investigated in macaques the safety and efficacy of biodegradable polycaprolactone implants releasing tenofovir alafenamide for HIV pre-exposure prophylaxis (PrEP).

METHODS

Implants were administered subcutaneously in the arm using a contraceptive trocar. Efficacy against vaginal simian-HIV (SHIV) infection was investigated in six pigtailed macaques that received two tenofovir alafenamide implants (0.35 mg/day), one in each arm, for a total release rate of tenofovir alafenamide at 0.7 mg/day. Macaques were exposed to SHIV twice weekly for 6 weeks. Statistical analyses were used to compare outcome with eight untreated controls. Histological assessments were performed on skin biopsies collected near implantation sites.

RESULTS

Median (range) tenofovir diphosphate level in PBMCs was 1519 (1068-1898) fmol/106 cells. All macaques with tenofovir alafenamide implants were protected against vaginal SHIV infection. In contrast, 7/8 controls were infected after a median of 4 SHIV exposures (P = 0.0047). Histological assessment of tissues near tenofovir alafenamide implant sites showed inflammation and necrosis in 5/6 animals, which were not evident by visual inspection.

CONCLUSIONS

We demonstrated complete protection against vaginal SHIV infection with two implants releasing a total of 0.7 mg of tenofovir alafenamide per day. We also identified tenofovir diphosphate concentrations in PBMCs associated with complete vaginal protection. Consistent with previous findings, we observed adverse local toxicity and necrosis near the tenofovir alafenamide implant site. Improved tenofovir alafenamide implants that are safe and maintain high efficacy have the potential to provide long-lasting protection against vaginal HIV infection.

Reservoir-Style Polymeric Drug Delivery Systems: Empirical and Predictive Models for Implant Design

Controlled drug delivery systems can provide sustained release profiles, favorable pharmacokinetics, and improved patient adherence. Here, a reservoir-style implant comprising a biodegradable polymer, poly(ε-caprolactone) (PCL), was developed to deliver drugs subcutaneously. This work addresses a key challenge when designing these implantable drug delivery systems, namely the accurate prediction of drug release profiles when using different formulations or form factors of the implant. The ability to model and predict the release behavior of drugs from an implant based on their physicochemical properties enables rational design and optimization without extensive and laborious in vitro testing. By leveraging experimental observations, we propose a mathematical model that predicts the empirical parameters describing the drug diffusion and partitioning processes based on the physicochemical properties of the drug. We demonstrate that the model enables an adequate fit predicting empirical parameters close to experimental values for various drugs. The model was further used to predict the release performance of new drug formulations from the implant, which aligned with experimental results for implants exhibiting zero-order release kinetics. Thus, the proposed empirical models provide useful tools to inform the implant design to achieve a target release profile.

Comparative Pharmacokinetics and Local Tolerance of Tenofovir Alafenamide (TAF) From Subcutaneous Implant in Rabbits, Dogs, and Macaques

The administration of antiretrovirals (ARVs) for HIV pre-exposure prophylaxis (PrEP) is highly efficacious and may benefit from new long-acting (LA) drug delivery approaches. This paper describes a subcutaneous, reservoir-style implant for the LA delivery of tenofovir alafenamide (TAF) and documents the preclinical assessment of implant safety and pharmacokinetics (PK) in New Zealand White (NZW) rabbits (3 groups of n = 5), beagle dogs (2 groups of n = 6), and rhesus macaques (2 groups of n = 3). Placebo implants were placed in rabbits (n = 10) and dogs (n = 12). Implant parameters, including selection of the TAF form, choice of excipient, and PCL formulation were tuned to achieve targeted concentrations of the active anabolite of TAF, tenofovir diphosphate (TFV-DP), within peripheral blood mononuclear cells (PBMCs) and mucosal tissues relevant to HIV transmission. Sustained concentrations of TFV-DP in PBMCs over 100 fmol/10⁶ cells were achieved in all animal species indicating that the implants effectively delivered TAF for 3-6 months. Unlike placebo implants without TAF, all active implants resulted in local adverse events (AEs) proximal to the implant ranging in severity from mild to moderate and included dermal inflammation and necrosis across all species. Despite these AEs, the implant performed as designed and achieved a constant drug release profile, supporting the continued development of this drug delivery platform.