D P 2d 4 P

When a reaction pathway involves toth pseudo-fist and pseudo-second-order pathways, Eqs. (2.41) and (2.42) adequately describe the kinetics.

Figure 17. Time course of degradation of ampicillin (pH 8.50, 35°C). P2 was determined by two assay methods (o, •). (Reproduced from Ref. 260 with permission.)

Under neutral-to-alkaline pH conditions, the degradation of ampicillin (Fig. 17),260 amoxicillin,261262 cefaclor,263 and cefatrizine263 can be reasonably described by this model. Equilibrium, Pseudo-First-Order Parallel Reactions ki K ¿2

This case obtains when a drug, D, forms a complex (DA) with A, which is defined by the equilibrium constant, K, and both D and DA are capable of undergoing independent pseudo-first-order reactions. When the concentration of A is significantly higher than that of D, the kinetics can be described by Eqs. (2.43) and (2.44).264

where or

The degradation of carbenicillin in the presence of human serum albumin165 conforms to this model, as does the degradation of drugs in the presence of cyclodextrins (see Section 2.3.3 for a more complete discussion). Product Catalysis k

Equation (2.45) describes the degradation rate of drug D catalyzed by its product P, which is continuously changing as the reaction proceeds.

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