Injectable in situ crosslinkable depots for ultra-long-acting delivery of hydrophilic therapeutics

Introduction: Ultra-long-term delivery of hydrophilic drugs via injection remains a major hurdle in treating chronic diseases. To address this, we introduce an innovative solvent-free, injectable, biodegradable, and in situ crosslinking depot (ISCD) platform. This platform utilizes a low molecular weight, liquid methacrylated biopolymer to carry both hydrophilic and hydrophobic drugs. Upon injection, this polymer solidifies in situ, forming a degradable depot, effectively controlling drug release and preventing initial bursts.

Our comprehensive research on the physicochemical properties of the ISCD, adjusted through formulation parameters, reveals its impact on drug release and degradation. This research has led to the successful sustained release of eight drugs with diverse hydrophilicities, lasting between 5 and 12 months in vitro. In vivo studies in rats further demonstrated ultra-long-term drug release and depot degradation over seven months without any adverse effects.

Pharmacokinetic modeling, based on thorough studies simulating ISCD behavior in rats, was then used to predict its behavior in humans. To our knowledge, this is the first successful demonstration of an injectable formulation capable of achieving ultra-long-term delivery of hydrophilic drugs. This pioneering platform presents a promising avenue for developing transformative ultra-long-acting therapies for a broad spectrum of diseases.

Materials and

Methods: We developted an injectable and biodegradable ISCD from ultra-low-molecular-weight (Mn~600), liquid polymer – PCLDMA. Hydrophilic drugs can be suspended in the polymer, and the polymer undergoes crosslinking in situ after injection with radical initiator and accelerator, physically encapsulating drugs. In vitro release kinetics of the drug-loaded ISCD were quantified by high-performance liquid chromatography (HPLC). The microstructures of the ISCDs were evaluated by scanning electron microscopy (SEM). For in vivo pharmacokinetics (PK), the drug loaded ISCD formulations were administered subcutaneously in rats, and plasma drug levels were analyzed by liquid chromatography–mass spectrometry (LC-MS). The PK analysis of ISCD formulations utilized both non-compartmental and compartmental methods and convolution analysis was utilized to predict the human PK of drug release from ISCD by integrating human disposition kinetics. Histological and immunohistochemical analysis were performed on cryosections of the explanted implant samples in order to characterize the inflammatory response elicited by the implanted material. We used magnetic resonance imaging (MRI) to evaluate morphological changes and biodegradation of LAIs in vivo.

Results, Conclusions, and Discussions: We've developed an innovative injectable depot platform capable of delivering a wide range of therapeutics with varying hydrophilicity over 6 months in vivo. Our platform utilizes low molecular weight PCLDMA, a polymer that remains in a liquid state, eliminating the need for solvents and minimizing initial burst release. The hydrophobic dense mesh network of the PCLDMA depots effectively prevents water influx/efflux, leading to prolonged therapeutic release. Our ISCD platform demonstrated versatility in therapeutic delivery, encapsulating and releasing drugsincluding tenofovir alafenamide (TAF), lamivudine (LAM), abacavir (ABC), emtricitabine (FTC), naltrexone (NAL), vancomycin (VAN), and tacrolimus (TAC), for up to 5-12 months in vitro and TAF, NAL, and TAC for up to 7 months in vivo.

We've established that the release profile of ISCD can be customized by modifying its polymer network composition. Our in vitro release studies, combined with analysis using the Flory-Rehner equation, identified key factors influencing the release rate, such as the molecular weight of PCLDMA, concentrations of crosslinking agents (BPO and DMT), and density of functional groups within the polymer network. Additionally, incorporating more hydrophilic polymers led to faster drug release.

Our studies revealed a positive correlation between a drug's water solubility and its release profile from the ISCD platform. In vivo experiments with SC-injected rats demonstrated that drugs with higher water solubility exhibited a faster initial burst and a higher continuous release rate. Importantly, all drugs, regardless of hydrophilicity, were successfully delivered for extended periods of 6-7 months.

We used convolution analysis to predict the human PK of ISCD for NAL and TAC, indicating 2-3 months of extended release for TAC and 6 months for NAL, offering substantial improvements over existing dosing regimens.

Thorough pre-clinical evaluation confirmed the biocompatibility and retrievability of the ISCD platform. Minimal foreign body response was observed in H&E and IHC staining for immune cells (T lymphocytes and macrophages), with significant reduction by week 4, indicating good biocompatibility. Furthermore, upon retrieval of the implant, plasma TAF concentrations exhibited a rapid decline, demonstrating that sustained drug release can be effectively halted by implant removal, promoting the flexibility and safety of our long-acting drug delivery system.

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