Augmenting Therapeutic Performance of CDK4/6 Inhibitor through a Nanocrystal-Integrated Thermoresponsive In Situ Gel Platform

Palbociclib (PLB), a US FDA-approved CDK4/6 inhibitor, is limited by poor aqueous solubility (BCS II), impairing its local therapeutic potential. We developed and optimized palbociclib nanocrystals (PLB NCs), which were then loaded into a thermoresponsive poloxamer-based in situ gel to achieve localized, sustained intratumoral delivery and enhanced anticancer activity against breast cancer cells. PLB NCs were developed using a combined bottom-up/top-down approach (antisolvent precipitation followed by microfluidization). OFAT optimization found THF as the best solvent and Tween-80/HPC-M as stabilizers. NCs were characterized by DLS, zeta potential, FTIR, PXRD, DSC, SEM, BET, residual solvent analysis, saturation solubility, and in vitro dissolution. Optimized PLB NCs showed a mean hydrodynamic diameter of 185.7 ± 12.9 nm, PDI 0.213 ± 0.033, and zeta potential −18.3 ± 1.6 mV; microfluidization at 24,000 psi for four cycles produced the smallest particle 177.8 ± 0.8 nm (PDI 0.195). At physiological pH, the PLB NC demonstrated a solubility increase of 3.31 times. In comparison to free PLB, PLB NCs exhibited 0.96- and 0.76-fold higher cytotoxicity and 0.7- and 1.15-fold greater quantitative uptake in MCF-7 and MDA-MB-231 cells, respectively. Furthermore, PLB NCs displayed marked morphological/apoptotic changes, reduced migration, and a 5.3-fold rise in ROS generation. The optimized thermogel, gelled at 36.8 ± 0.5 °C, showed suitable rheology and gel strength, and provided sustained PLB release for 72 h with reduced initial systemic release. This CDK4/6 inhibitor-based intratumoral platform has promising potential to enhance local therapeutic outcomes and reduce systemic toxicity in breast cancer treatment.