Covalent cross-linking approaches for all-<i>trans</i> retinoic acid-loaded thermo-responsive hydrogels

All-<i>trans</i> 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 <i>in vivo</i> hinder its use. The development of injectable controlled release systems for ATRA delivery can potentially address these challenges. Building on a poly(caprolactone-<i>co</i>-lactide)-poly(ethylene glycol)-poly(caprolactone-<i>co</i>-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.