Hydrolysis kinetics and stability predictions for a mixture of R and S temocillin isomers

The hydrolysis kinetics of a temocillin isomer mixture (similar to 68% R and similar to 32% S) were studied as a function of pH, temperature and buffers. Temocillin concentrations were determined from HPLC analyses of the individual isomers where [TEM] = [R] + [S]. Rate constants (k(R), k(S) and k(SUM)) were determined from linear first-order plots for [R], [S] and [TEM], respectively. The true time for 10% loss of temocillin (T-true) was determined by examining the time-course for the sum, [R] + [S]. Although the k(R) and k(S) values were generally similar, they differed by 30-40% under some conditions. Nevertheless, the calculated time for 10% loss (T-90 = -ln 0.9/k(SUM)) agreed with T-true for all experimental conditions. This was explained using computer simulations that indicated epimerization was faster than hydrolysis. Under these circumstances, reliable predictions of temocillin stability were achieved with k(SUM) = k(H1)(a(H+))f(1) + k(H2)(a(H+))f(2) + k(H3)(a(H+))f(3) + kH(2)O + k(OH)(a(OH-)) + k(B4O7) [B4O72-], where the catalytic constants were k(H1), k(H2) and k(H3) for hydrogen ion activity, k(OH) for hydroxyl ion activity, kB(4)O(7) for B4O72- (in berate buffer only); kH(2)O was the first-order rate constant for spontaneous hydrolysis and f(1), f(2), f(3) were the fractions of temocillin in three stages of dissociation. The Arrhenius expression, k = Ae(-E/RT), and the experimentally determined A and E values were substituted for k(H1), k(H2) k(H3), k(OH) and k(B4O7). This equation successfully calculated 92% of the observed k(SUM) values with < 10% error. A shelf-life of similar to 1.8 days was predicted and experimentally verified at 30 degrees C in the region of maximum stability, pH 6.5-7.5. (C) 1998 Elsevier Science B.V. All rights reserved.