Degradation of rapamycin and its ring-opened isomer: role of base catalysis

Rapamycin is a natural macrolide immunosuppressant with a distinct mechanism of action. Quantitative analysis of rapamycin poses many challenges associated with facile degradation and the multitude of isomeric forms. The primary goal of this study was to compare degradation of rapamycin and its ring-opened isomer, secorapamycin, in aqueous solution under identical conditions. Reaction kinetics and mechanisms were studied in 30/70 vol/vol acetonitrile-water mixtures containing either MeCOONH4 (apparent pH 7.3) or NaOH (apparent pH 12.2). Degradation kinetics was well described by the first-order rate law. For rapamycin in 237 and 23.7 mM solutions of MeCOONH4, apparent half-lives of 200 h and 890 h were obtained. When compared to the latter value, the rapamycin half-life was reduced by 3 orders of magnitude in the pH 12.2 solution. Under all conditions studied, secorapamycin degradation was significantly slower than that of the parent compound. Both specific and general base catalysis was observed for reactions of rapamycin and secorapamycin. Two primary products of rapamycin degradation were identified as individual isomers of secorapamycin and a hydroxy acid formed via lactone hydrolysis. No evidence for the interconversion between the products was obtained. In highly basic solutions, both products undergo fragmentation and water addition reactions.