Modulation of Solid-State Chemical Stability of Gabapentin by Pyridinecarboxylic Acid

Gabapentin (GBP) is an anticonvulsant drug with poor chemical stability that is particularly sensitive to heat and mechanical stress, which can lead to intramolecular lactamization. The purpose of this study was to enhance the chemical stability of GBP by cocrystallization with organic acids.

Two novel multicomponent crystals, GBP-2,6-pyridinedicarboxylic acid salt (GBP-2,6PDA salt) and GBP-2,5-pyridinedicarboxylic acid cocrystal (GBP-2,5PDA cocrystal) were synthesized and characterized by various solid-state analytical techniques. The degradation behavior of GBP, GBP-2,6PDA salt and GBP-2,5PDA cocrystals were evaluated under thermal and mechanical stresses.

Under thermal and mechanical stresses, GBP-2,5PDA cocrystals were found to undergo severer degradation than GBP-2,6PDA salt and neat GBP. GBP-2,6PDA salt exhibited superior chemical stability compared to the others. Furthermore, the crystal structure revealed that the order of atomic distance between the carboxyl group (C7) and amino group (N12) of GBP is as follows: GBP-2,5PDA cocrystal < GBP < GBP-2,6PDA salt, which is consistent with the chemical stability of GBP in different solid forms. Therefore, we believe that the distance between C7 and N12, the reaction active sites leading to dehydrative condensation of GBP, is a key factor determining the chemical stability of GBP in the solid state.

These results provide a potential method to improve the chemical stability of GBP during the manufacturing process and storage.