Degradation Kinetics of Antagonist [Arg 6, D-Trp 7,9, MePhe 8]-Substance-P {6-11} in Aqueous Solutions

Abstract

Antagonist [Arg 6, D-Trp 7,9, MePhe 8]-substance P {6-11} was subjected to a systematic stability study in which kinetic parameters were obtained for the degradation of this hexapeptide under several well-defined conditions. The influences of pH, temperature, ionic strength, buffer concentration, and initial concentration of the peptide on the reaction rate constant, k obs, were investigated with a stability-indicating reversed-phase high-performance liquid chromatographic system. From the pH-log k obs degradation profile, obtained at 63°C, it appears that antagonist [Arg 6, D-Trp 7,9, MePhe 8]-substance P {6-11} shows its maximum stability around pH 4.2. The half-life at this pH and temperature is 150 days. In both the hydroxyl- and proton-catalyzed parts of the pH-log k obs degradation profile, the influence of temperature was investigated and Arrhenius plots were constructed. The activation energies in both parts were comparable; however, the frequency factor in the hydroxyl-catalyzed part was 3.3 × 10 4 times higher than in the proton-catalyzed part. Eyring analysis of the data reveals that in both acidic and alkaline media the overall degradation was endotherm (Δ H‡ as well as Δ G‡ positive between 273 and 373°K) and the entropy was negative. Increasing ionic strengths in acidic media causes an increase in k obs, while in alkaline media the k obs decreases with increasing ionic strength. Increasing buffer concentrations of acetate, phosphate, and carbonate led to an increase of k obs values. Drug concentrations up to 1 mg/ml at pH 10.8 and constant temperature and ionic strength have no influence on the overall degradation rate. At higher concentrations, above 1 mg/ml, k obs decreases. In acidic media (pH 2) similar results are obtained.