Acyclovir chemical kinetics with the discovery and identification of newly reported degradants and degradation pathways involving formaldehyde as a degradant and reactant intermediate.

The purpose of this research was to determine acyclovir (ACV) acidic degradation kinetics which is relevant to gastric retentive device product design. A stability-indicating method revealed two unknown degradation products which have been identified by mass spectrometry as ACV and guanine formaldehyde adducts. In addition to the formation of these adducts, a proposed degradation scheme identifies the formation of methyl acetal ethylene glycol, formaldehyde, ethylene glycol, and guanine as additional ACV degradation products. pH-rate profiles were explained by using a rate law which assumed acid-catalyzed hydrolysis of protonated and unprotonated ACV. The predicted and observed rate constants were in good agreement. Data-driven excipient selection recommendations were based on the chemical kinetic study results, degradation scheme, and pH-rate profiles. The average activation energy for the degradation reaction was determined to be 31.3±1.6kcal/mol. The predicted ACV t90% at 37°C and pH 1.2 was calculated to be 7.2days. As a first approximation, this suggests that ACV gastric retentive devices designed to deliver drug for 7days should have acceptable drug product stability in the stomach.