Voxelotor Polymorphs: Crystalline Diversity of a Highly Potent Sickle Cell Anaemia Drug

The solid-state landscape of highly flexible organic molecules often presents profound challenges and opportunities in materials chemistry, critically dictating macroscopic, physicochemical, and biopharmaceutical properties. Herein, we report voxelotor, a BCS Class II drug historically developed for sickle cell disease, as a noteworthy model system to elucidate nucleation pathways and a non-classical structure-performance relationship, with bioavailability that is critically dependent on its solid form. We report the preferential crystallization, multigram scale-up, and comprehensive structural elucidation of seven distinct polymorphic forms (Forms I-VII), including four previously undiscovered solid phases. Single-crystal X-ray diffraction and computational lattice energy calculations, including Hirshfeld surface analysis, show that its rich polymorphic diversity is rooted in extreme torsional adaptability (variations in τ2 from -160.0° to +110.0°), which drives parallel displaced π···π stacking and hydrogen-bonded assemblies. Thermal and humidity stress profiling discloses a complex relationship between thermodynamic stability and kinetic trapping. Notably, we identified paradoxical stability behavior in which seemingly robust, high lattice energy metastable states (e.g., Forms III, IV, V, and VI) undergo temperature- and moisture-promoted structural transformation and solid-state conversion to Form I crystals, which exhibit greater stability under ambient conditions. Crucially, biopharmaceutical and pharmacokinetic evaluation showed a divergence between drug solubility and biological efficacy. The kinetically trapped Form VI exhibited the highest equilibrium aqueous solubility. Form II exhibited superior systemic exposure in rats and the highest ex vivo anti-sickling efficacy (74% inhibition at 200 μM). Collectively, these findings offer fundamental insight into how subtle solid-state energetic balance and specific molecular conformations can influence solubility behavior. By demonstrating how structural nuances dictate superior macroscopic performance, this study provides a robust, predictive framework for rationalizing solid-form selection in highly flexible functional drugs.