Degradation kinetics of aplidine, a new marine antitumoural cyclic peptide, in aqueous solution

The degradation kinetics of aplidine were investigated using reversed-phase high-performance liquid chromatography combined with UV detection. Aplidine consists of at least two isomers that undergo interconversion at a low rate. Influences of pH, temperature, buffer ions and ionic strength on the degradation kinetics were studied. The log kobs–pH profile can be divided into three parts, a proton, a solvent and a hydroxyl-catalysed section. The stability-indicating properties of the used analysis technique as well as the identities of the main degradation products were checked using gradient liquid chromatography and mass spectrometric detection. The overall degradation rate constant as a function of the temperature under acidic and alkaline conditions obeys the Arrhenius equation. No catalytic influences were observed with phosphate and carbonate buffers and, in addition, the ionic strength showed no substantial effect on the stability, as expected. Results from gradient LC–MS indicated that hydrolysis of the ester groups present in the ring structure was the main degradation route. There is no difference in degradation rate constants for the individual isomers.