Clinical pharmacokinetics of enzyme inhibitors in antimicrobial chemotherapy

The effectiveness of some antimicrobial agents can be enhanced by using them in combination; such combinations are termed synergistic. Where one compound potentiates the effect of a second drug they may be coformulated. Inhibition of the bacterial degradation of an active antimicrobial is the basis of clavulanate and sulbactam-potentiated penicillin combinations, and inhibition of degradative pathways in the host is the rationale behind imipenem/cilastatin therapy. Trimethoprim/sulphonamide combinations depend on the maintenance of an effective ratio for synergistic action. In order to achieve potentiation the coformulated drugs should have similar pharmacokinetics. Trimethoprim was originally matched with sulphamethoxazole, since these two drugs have similar elimination half-lives, but the significantly poorer penetration of sulphonamides, their greater non-renal clearance, the emergence of resistance, and the adverse reactions attributable to them argue against the rationale that underlies their coformulation. Time-dependent inhibition of bacterial beta-lactamases by clavulanic acid and sulbactam has extended the use of penicillins which are highly susceptible to beta-lactamase inactivation. The beta-lactamase inhibitors must penetrate to the same extent as the penicillin used with them, and be present long enough to effect inhibition; thus, rapid penetration, similar or slower elimination and equivalent volume of distribution are necessary. These requirements are met for amoxycillin/clavulanic acid, ticarcillin/clavulanic acid and ampicillin/sulbactam combinations. Clavulanic acid is absorbed orally and is given with amoxycillin. However, since sulbactam is labile by this route, the combination of sulbactam with ampicillin to form the prodrug sultamicillin has been necessary to enable an oral form to be developed. Imipenem is metabolised by renal brush-border dehydropeptidases, and may cause proximal tubular necrosis. Cilastatin was designed to inhibit this metabolism, which it effectively does, thereby both potentiating the effect of imipenem and avoiding toxicity. Appropriate matching of the kinetics of coformulated drugs is intended to maximise potentiation and minimise the risk of emergent resistance. The kinetics of the above combinations are discussed in the light of these requirements and the effects of age and disease.