Hydrolytic stability of selected pharmaceuticals and their transformation products

Abstract

The fact that pharmaceuticals are present in the environment has been proven in numerous publications. Nevertheless, their transformation products (mainly metabolites) are detected significantly less often, mainly because they are not included in the detecting methods, even though many of them are excreted from organisms at high rates and may be biologically active or have other properties that make them a potential threat to the environment. One of the most common processes that occur in the aqueous environment is hydrolysis, which may be one of the most important factors influencing the persistency of pharmaceuticals. Therefore four pharmaceuticals (carbamazepine, ibuprofen, tramadol and naproxen) as well as ten metabolites (10,11-dihydro-10-hydroxy carbamazepine, 10,11-dihydro-2-hydroxy carbamazepine, carbamazepine epoxide, 2-hydroxy ibuprofen, ibuprofen carboxylic acid, O-desmethyltramadol, hydroxy metronidazole, N-acetylsulfamethoxazole, 4'-hydroxy diclofenac, and O-desmethylnaproxen) were selected for the hydrolytic stability tests in accordance to OECD 111 Guideline. The preliminary test showed that only carbamazepine epoxide at pH 4, hydroxy metronidazole at pH 9 and 4'-hydroxy diclofenac at pH 4 and 9 were unstable and were included in the extended tests, which resulted in calculation of rate constants and half-lives at the temperature of 20, 50 and 70 °C as well as the activation energy at the pH values in which these compounds were unstable. The obtained results show that carbamazepine epoxide is quite unstable and the half-life at 20 °C was a little more than 8 days. Nevertheless, hydroxy-metronidazole and 4'-hydroxy diclofenac did not degrade at 20 °C for 30 days.

Keywords: Environment; Hydrolysis; Metabolites; OECD 111; Stability.