PH_II_L: Pharmaceutical Sciences II. Lectures
Bianka Várnai, Semmelweis University Department of Pharmacognosy, Budapest
Remdesivir (REM) was the first antiviral agent approved by the FDA for the treatment of SARS-CoV-2 virus. Initially, this compound was designed and developed to fight Ebola and Marburg viruses. Since January 2020, REM has been administered to treat SARS-CoV-2 virus disease worldwide. Due to its poor water solubility, the preparation of Veklury™ requires pH 2 conditions and a suitable solubilizing excipient. For this purpose, the final formulation contains the randomly substituted sulfobutyl-ether-beta-cyclodextrin (SBEβCD) as a complexing agent. Unfortunately, there is no published experimental data on the pKa value(s) of REM, only predicted values are available in the literature, which makes it difficult to interpret the pH-dependent behavior of REM.
We aimed to determine the pKa value of the REM as the protonation state of the aminopyrrolo-triazine moiety can play a key role in cyclodextrin (CD) – REM inclusion complex formation. Thereafter, we aimed to characterize the molecular interactions between various (randomly- and regioselectively-sulfobutylated) CD-derivatives and REM by NMR spectroscopic methods.
Based on our UV-pH titration experiment the unusual heterocyclic moiety possesses a pKa value of 3.56, thus the majority of REM bears a positive charge at pH 2.0. To explore the contribution of the CD structure to the stability of host-guest complexes native-, randomly substituted- and the per6-SBEβCDs were investigated. In order to gain detailed structural information about the inclusion behavior of REM, a βCD derivative was used bearing pH-dependent anionic sidechains having an extended cavity compared to SBEβCD. In all cases, 1H NMR experiments and Job’s method were used to determine the stoichiometry of the complexes. The stability constants were determined by nonlinear curve fitting based on 1H NMR-CD titrations of REM at pH 2.0. It has been revealed that the anionic sidechains of the host play a major role in increasing the complex stability indicated by one order of magnitude increase in stability values when comparing βCD (logK 3.06) to SBEβCD (logK 3.99). The 2D ROESY experiments revealed that the ethylbutyl-moiety is involved in the molecular encapsulation. Our results also highlighted, that the charge state of the host and the guest may remarkably contribute to the inclusion complex formation of REM.
Semmelweis University, Doctoral School of Pharmaceutical Sciences