DETERMINATION OF THE IONIZATION CONSTANT OF FAVIPIRAVIR API

Abstract

One of the aspects of studying the physical and chemical properties of compounds is the establishment of their ionization constants. These indicators are extremely important from both a theoretical and a practical point of view. Theoretical analysis of proteolytic equilibria and potentiometric titration of an aqueous solution of an oral antiviral drug approved for the treatment of influenza in Japan, AFI favipiravir, were carried out in the paper. The presence of a hydroxy group in the third position of the pyrazine cycle in the molecule determines the ability of favipiravir to exhibit acid-base properties. According to acid-base titration, the ionization constant of Favipiravir API was determined: pKa = 5.05 ± 0.02. On the basis of the found pK value, the degree of formation of protolytic forms was calculated depending on the pH of the solution. It was established that favipiravir API in solutions exists in two protonated (non-ionized), tautomeric forms A1H and A2H and one deprotonated (ionized) form A-. Based on the calculations, the following conclusions can be drawn: - at a pH value of the solution from 0 to 3.5, approximately 100% of the substance will be in two protonated (non-ionized), tautomeric forms A1H and A2H; - at pH = 5, approximately 50% of the substance will be in two protonated (non-ionized), tautomeric forms, A1H and A2H; - at a pH value > 6.5, approximately 100% of the substance will be in one deprotonated (ionized) form of A-. This is confirmed by the nature of the electronic absorption spectra of aqueous solutions of favipiravir. In the spectrum of favipiravir in a 0.1 M hydrochloric acid solution, there are two bands at the wavelengths of 322 nm and 365 nm, which correspond to the two tautomeric forms A1H and A2H. In the spectrum of favipiravir in 0.1 M sodium hydroxide solution, there is one band at a wavelength of 364 nm, which corresponds to one ionized form of A-. At the stage of pharmaceutical development, these data on the acid-base properties of the Favipiravir API can be used in the development of production technology, the creation of quality control methods and their validation, as well as in the study of the solubility of the API and the dissolution profiles of the finished medicinal product.