Kinetics and mechanism of degradation of epothilone-D: an experimental anticancer agent.

The objective of this study was to investigate the stability and the degradation pathway of epothilone-D (Epo-D), an experimental anticancer agent. In pH range 4-9, Epo-D displayed pH-independent stability and the highest stability was observed at pH 1.5-2 where its thiazole group is protonated. Increasing the pH >9 or <1.5 resulted in an increase in the degradation rate. Epo-D contains an ester group that can be hydrolyzed. The formation of the hydrolytic product was confirmed by the nuclear magnetic resonance (NMR), fast atom bombardment mass spectroscopy and liquid chromatography/mass spectroscopy/mass spectroscopy techniques. The largely sigmoidal pH-rate profile is not consistent with the normal pH dependency of ester hydrolysis involving an addition/elimination mechanism. Hence, a hydrolysis mechanism through a carbonium ion was suggested. At pH 4 and 7.4, no buffer catalysis was observed (0.01, 0.02, and 0.05 M buffers) and no significant deuterium kinetic solvent isotope effect was noted. The degradation was very sensitive to changes in the dielectric constant of the solvents as significant enhancement in the stability was observed in buffer-acetonitrile and 0.1 M (SBE)7m-beta-cyclodextrin solutions compared with just buffer, suggesting that the rate-determining step in the degradation pathway involved formation of a polar transition state. Mass spectral analysis of the reaction run in 18O water was consistent with incorporation of the 18O in the alcohol hydroxyl rather than the carboxylate group. These observations strongly support the carbonium ion mechanism for the hydrolysis of Epo-D in the pH range 4-9. A pKa value of 2.86 for Epo-D was estimated from the fit of the pH-rate profile. This number was confirmed independently by the changes in ultraviolet absorbance of Epo-D as a function of pH (pKa 3.1) determined at 25 degrees C and the same ionic strength.

Degradation of NSC-281612 (4-[bis[2-[(methylsulfonyl)oxy]ethyl]amino]-2-methyl-benzaldehyde), an experimental antineoplastic agent: effects of pH, solvent composition, (SBE)7m-beta-CD, and HP-beta-CD on stability.

NSC-281612 (4-[bis[2-[(methylsulfonyl)oxy]ethyl]amino]-2-methyl-benzaldehyde, 1), is a chemically unstable, poorly water soluble, experimental antineoplastic agent. The saturated solubility in water at 25 degrees C was determined as approximately 30 microg/mL. In the pH range 2-11, 1 displayed pH-independent stability (t(50) was around 24 hr). However, an increase in the degradation rate was observed at pH 12. The hydrolysis of the methane sulfonate groups to the corresponding hydroxyl groups was the major degradation pathway in water in the absence of buffers and added halide ions. In phosphate buffer solutions without sodium chloride, phosphate degradants appear to be formed in addition to the mono- and dihydroxy degradants. Additional degradants, the mono- and dichloro degradation products, were formed when the ionic strength of the solution was adjusted with sodium chloride. When bromide and iodide ions were added, the corresponding mono- and dihalides were formed. The chloro compounds subsequently underwent further degradation to the hydroxy products. A deuterium kinetic solvent isotope effect study showed that water was minimally involved in the rate-determining step. The addition of either (SBE)(7m)-beta-cyclodextrin (CD) or HP-beta-CD resulted in a significant enhancement in drug solubility and stability. The apparent binding constants for HP-beta-CD and (SBE)(7m)-beta-CD were 1,486 and 2,740 M(-1), respectively. The stability of 1 in the presence of 0.1 M HP-beta-CD and (SBE)(7m)-beta-CD was enhanced 9- and 15-fold, respectively. Thus, (SBE)(7m)-beta-CD displayed better solubilization and stabilization efficacy than HP-beta-CD.