Conformation-Dependent Complexation Interactions for a SMARCA2 PROTAC and Cyclodextrin

Bivalent protein degraders, or proteolysis targeting chimeras (PROTACs), are an emerging therapeutic modality that can be used to drug challenging targets and drive differentiated pharmacology. However, the atypical physicochemical and structural properties of PROTACs can contribute to poor biopharmaceutical properties (e.g., solubility), and complicate drug research and development. To date, there are limited publications on modality-specific formulation strategies to mitigate these liabilities. Herein, we use multiple NMR spectroscopy techniques, isothermal titration calorimetry, and quantitative solubility measurements to study solubilization of a VHL-based PROTAC for SMARCA2, A515. In aqueous solution without solubilizing excipients, we found that A515 exists in two distinct and measurable populations that differ in the rotation of the amide group─a <i>trans</i>-proline rotational isomer that comprises a set of relatively open conformations with larger hydrodynamic size, and a <i>cis</i>-proline rotational isomer with relatively condensed conformations with smaller hydrodynamic size. Upon addition of the solubilizing excipient 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) to A515, we show that the efficiency of ternary host-guest complexation (i.e., the mechanism of solubilization) in each population is distinct and modulated by conformation (i.e., the accessibility of the terminal regions of A515 that are involved in complexation). These observations highlight the unreported and important role of stereochemistry and conformation in the rational formulation design for PROTACs (including solubility enhancement), and suggest specific considerations for VHL-based PROTACs.