Molecular interactions in remdesivir-cyclodextrin systems

Remdesivir (REM) is the first antiviral drug (Veklury™) approved by the Food and Drug Administration for the therapy of COVID-19. Due to its poor water solubility, the preparation of Veklury™ requires a suitable solubilizing excipient at pH 2 conditions. For this purpose, the final formulation contains the randomly substituted sulfobutylether-β-cyclodextrin (SBEβCD) as a complexing agent. Herein, extensive NMR spectroscopic study with various cyclodextrin (CD) derivatives were conducted to understand the interactions in SBEβCD - REM systems at the molecular level. The pK_a value of REM has been determined experimentally for the first time, as the protonation state of the aminopyrrolo-triazine moiety can play a key role in CD-REM inclusion complex formation as SBEβCD has permanent negative charges. The UV-pH titration experiments yielded a pK_a of 3.56, thus the majority of REM bears a positive charge at pH 2.0. NMR experiments were performed on β- and γCD derivatives to determine complex stabilities, stoichiometries and structures. The stability constants were determined by nonlinear curve fitting based on ¹H NMR titrations at pH 2.0, while Job's method was used to determine the stoichiometries. βCD complexes were one order of magnitude more stable than their γCD counterparts. Sulfobutylation resulted in a significant increase in stability and the single isomer derivatives showed unexpectedly high stability values (logK = 4.35 for REM - per-6-SBEβCD). In the case of βCDs, the ethylbutyl-moiety plays a key role in complexation immersing into the βCD cavity, while the phenoxy-moiety overtakes and drives the inclusion of REM in the case of γCDs. This is the first comprehensive study of REM-CD complexation, allowing the design of new CD derivatives with tailored stabilities, thereby aiding the formulation or production and even the analytical characterization of REM.