DEGRADATION KINETICS OF ANTAGONIST [ARGG(6), D-TRP(7,9), MEPHE(8)]-SUBSTANCE-P (6-11) IN AQUEOUS-SOLUTIONS

Antagonist [Arg(6), D-Trp(7,9), MePhe(8)]-substance P {6-11} was subje cted to a systematic stability study in which kinetic parameters were obtained for the degradation of this hexapeptide under several well-de fined conditions. The influences of pH, temperature, ionic strength, b uffer concentration, and initial concentration of the peptide on the r eaction rate constant, k(obs), were investigated with a stability-indi cating reversed-phase high-performance liquid chromatographic system. From the pH-log k(obs) degradation profile, obtained at 63 degrees 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 thi s pH and temperature is 150 days. In both the hydroxyl- and proton-cat alyzed parts of the pH-log k(obs) degradation profile, the influence o f temperature was investigated and Arrhenius plots were constructed. T he activation energies in both parts were comparable; however, the fre quency factor in the hydroxyl-catalyzed part was 3.3 x 10(4) times hig her than in the proton-catalyzed part. Eyring analysis of the data rev eals that in both acidic and alkaline media the overall degradation wa s endotherm (Delta H-double dagger as well as Delta G(double dagger) p ositive between 273 and 373 degrees K) and the entropy was negative. I ncreasing ionic strengths in acidic media causes an increase in k(obs) , while in alkaline media the k(obs) decreases with increasing ionic s trength. Increasing buffer concentrations of acetate, phosphate, and c arbonate led to an increase of k(obs) values. Drug concentrations up t o 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 resu lts are obtained. (C) 1995 Academic Press, Inc.