Covalent cross-linking approaches for all-trans retinoic acid-loaded thermo-responsive hydrogels

Abstract

All-trans retinoic acid (ATRA) is a promising therapeutic for the treatment of a wide range of cancers. However, its short half-life, poor water-solubility, and low stability in vivo hinder its use. The development of injectable controlled release systems for ATRA delivery can potentially address these challenges. Building on a poly(caprolactone-co-lactide)–poly(ethylene glycol)–poly(caprolactone-co-lactide) (PCLA–PEG–PCLA) triblock copolymer system that undergoes thermo-responsive gelation at 37 °C, we explore and compare different approaches to stabilize the gels through covalent bonding. The attempted cross-linking of methacrylate end-capped PCLA–PEG–PCLA through thiol-Michael addition reactions using small molecule and 4-arm-PEG thiols led to precipitation rather than gelation. However, azide end-capped PCLA–PEG–PCLA was gelled using 5 kg mol⁻¹ 4-arm-PEG with terminal dibenzocyclooctyne (DIBAC) groups by strain-promoted azide–alkyne cycloaddition. This hydrogel was then compared with previously reported methacrylate end-capped PCLA–PEG–PCLA hydrogels cross-linked by free radical chemistry, as well as non-covalently cross-linked hydrogels. The azide–alkyne hydrogels exhibited properties intermediate between the free radical and non-covalently cross-linked gels. Incorporation of ATRA substantially disrupted the free radical cross-linking, but imparted only modest changes in the azide–alkyne gels. ATRA was released over about two weeks. The proliferation of MDA-MB-468 cells in the presence of ATRA-loaded and control azide–alkyne gels was investigated. The ATRA-loaded gel released active drug, while the unloaded gel did not affect proliferation.