Pre-clinical investigation of liquid sirolimus for local drug delivery

Peripheral artery disease (PAD) is often treated with percutaneous interventional therapies like drug-eluting stents (DESs) and drug-coated balloons (DCBs). Paclitaxel has been the primary drug, but sirolimus is being explored as a safer alternative due to its reversible binding to mTOR, preventing cell cycle progression (cytostatic) unlike paclitaxel's cytotoxic mechanism. Concerns regarding paclitaxel's safety, including potential for distal embolization and increased mortality rates, have fueled the search for safer alternatives. While liquid paclitaxel delivery has been studied, liquid sirolimus delivery remains largely unexplored. This study aimed to investigate the feasibility of delivering liquid sirolimus to arterial tissue using various liquid drug delivery (LDD) devices and assess drug retention in a clinically relevant ex vivo model.

Existing literature highlights the use of paclitaxel in DESs and DCBs for PAD treatment. However, recent meta-analyses have raised concerns about the safety of paclitaxel, linking its use to increased mortality and amputation rates. These findings are attributed to the crystalline nature of paclitaxel in current DCBs, leading to reduced solubility and the risk of distal embolization. Sirolimus, with its reversible mTOR inhibition and demonstrated anti-inflammatory effects, offers a potentially safer alternative. Previous studies have shown the successful delivery of liquid paclitaxel to the arterial medial layer using LDD devices, providing a foundation for exploring similar approaches with sirolimus.

An ex vivo bioreactor system mimicking physiological flow and pressure conditions was used. Porcine carotid arteries were harvested and prepared. Four LDD devices – Bullfrog, ClearWay RX, OPC, and TAPAS – were initially compared using a fluorescently tagged paclitaxel analog (Flutax-1). Drug penetration depth was measured via fluorescence microscopy. Based on penetration depth, the OPC was selected for further investigation with liquid sirolimus and paclitaxel. Liquid sirolimus and paclitaxel (both dissolved and commercially available liquid form) were delivered using the OPC into the ex vivo model. Arterial drug retention was analyzed at 1 and 24 hours post-delivery using liquid chromatography-tandem mass spectrometry (LC-MS/MS). For comparison, sirolimus-coated balloons (SELUTION SLR) were also deployed and analyzed. Statistical analysis was performed using ANOVA and Tukey post hoc tests.

Fluorescence microscopy revealed that the OPC demonstrated significantly greater Flutax-1 penetration into the arterial medial wall compared to the other three devices (OPC: 234 ± 161 µm; TAPAS: 127 ± 68 µm; ClearWay: 118 ± 77 µm; Bullfrog: 2.12 ± 3.78 µm; p = 0.098). Pharmacokinetic analysis showed successful delivery and retention of liquid sirolimus using the OPC. At 1 hour post-delivery, liquid sirolimus showed a concentration of 5.17 ± 4.48 ng/mg, decreasing to 0.78 ± 0.55 ng/mg at 24 hours (84.86% decrease). Liquid paclitaxel showed similar levels at 1 hour (9.87 ± 6.28 ng/mg) and 24 hours (3.06 ± 2.50 ng/mg), while dissolved paclitaxel had higher initial levels but a similar 24-hour level. There was no statistically significant difference between the drug groups at either time point. Importantly, liquid sirolimus levels were significantly higher than those observed with sirolimus-coated balloons at 1 hour (liquid sirolimus: 5.17 ± 4.48 ng/mg vs. sirolimus coated balloon: 0.0106 ± 0.002 ng/mg, p = 0.1096).

The findings demonstrate the feasibility of delivering liquid sirolimus directly to the arterial medial layer, a significant advancement over current sirolimus-coated balloon technology which relies on drug diffusion from a surface coating. The deeper penetration achieved with the OPC and the superior drug retention compared to sirolimus-coated balloons suggest a potential advantage for treating PAD. The comparable drug levels of liquid sirolimus and paclitaxel at both time points are noteworthy, particularly considering the differences in lipophilicity and drug release mechanisms. However, the lower retention of sirolimus compared to paclitaxel over 24 hours warrants further investigation, potentially through the use of excipients to improve tissue retention. The study’s use of healthy porcine arteries presents a limitation; further research should use diseased arteries to better mimic clinical conditions.

This study successfully demonstrated, for the first time, the feasibility of delivering liquid sirolimus to the arterial wall using an LDD device. The OPC was identified as the most effective device, achieving superior drug penetration and higher drug levels compared to sirolimus-coated balloons. While further research is needed to optimize sirolimus retention and assess in vivo efficacy, these findings offer a promising new approach to PAD treatment, potentially mitigating the risks associated with paclitaxel-coated balloons. Future studies should focus on the use of excipients to enhance drug retention, in vivo studies assessing neointimal growth inhibition and re-occlusion, and investigation in more clinically relevant diseased models.

The study used healthy porcine arteries, which may not perfectly replicate the complex anatomy and pathology of human diseased arteries, including the presence of branching, bifurcations, intimal thickening, fibrosis and calcification. The ex vivo model lacked blood, which could influence drug delivery and retention, particularly over longer time periods. The relatively small sample size in some analyses might limit the statistical power to detect significant differences.