Development, characterization, and in vitro evaluation of poly(ethylene oxide)-block-poly(ε-caprolactone)-α-tocopheryl succinate micelles as a novel nanocarrier for rapamycin delivery

Abstract

Rapamycin holds significant therapeutic potential for various diseases; however, its clinical application is limited by several formulation challenges, primarily its extremely low aqueous solubility (2.6 μg/mL). To address this, nanoparticle-based delivery systems have emerged as a promising strategy to enhance solubility and enable sustained drug release. Currently, Fyarro® (Aadi Bioscience, Inc.), an albumin-bound nanoparticle formulation, is the only FDA-approved injectable rapamycin product. In this study, we aimed to develop and evaluate novel poly(ethylene oxide)-block-poly(ε-caprolactone)-α-tocopheryl succinate (PEO-b-PCL-α-TS) micelles and assess their potential as a delivery system for rapamycin. PEO-b-PCL copolymers with varying PCL/PEO ratios were prepared via ring-opening polymerization and modified by α-TS conjugation, as confirmed by ¹H NMR, GPC, XRD, DSC analyses. The optimum rapamycin-loaded micelles (PEO₂₀₀₀-b-PCL₄₀₀₀-α-TS) exhibited nano-sized particles (< 22 nm) with a narrow polydispersity index (<0.29), high drug encapsulation efficiency (≥92 %), and enhanced solubility (>1.3 mg/mL). Stability studies demonstrated that encapsulation protected rapamycin from degradation, maintaining over 90 % drug retention for three months at 4 °C, while in vitro release studies showed sustained release, with 50 % of rapamycin released from PEO₂₀₀₀-b-PCL₄₀₀₀-α-TS micelles over 72 h. In vitro cytotoxicity assays revealed anticancer activity against lung carcinoma epithelial cells (A549), and the human colon adenocarcinoma cell line (HCT116). Minimal toxicity (≥70 % viability) was observed in normal human fibroblast cells (HFF1). These results point to the potential of PEO-b-PCL-α-TS micelles as a promising nanocarrier system, offering improved rapamycin solubility, enhanced stability, sustained release, and effective anticancer activity.