Theory and practical impact of binding of antimicrobials to serum proteins and tissue

Binding of antimicrobials to serum proteins and tissue affects their distribution, elimination and antimicrobial activity. Penetration of drugs into most tissues and interstitial and inflammatory fluids correlates with level of free drug in serum. Serum protein binding can increase or decrease the rate of drug elimination depending on whether total or free drug is available to the excretory or metabolic routes of elimination. Binding to soluble intracellular proteins (ligandin and fatty acid binding protein) appears to be important in the cellular transport and elimination of antimicrobials by renal tubular secretion and hepatic extraction. Although only free, unbound drug is antimicrobially active, the lack of well designed studies has prevented precise quantitation of the influence of binding on therapeutic efficacy in vivo. Pharmacokinetic considerations predict that serum binding greater than 80% would be necessary to significantly reduce free levels of drug in the body. However, extensive protein binding is often compensated for by greater intrinsic activity of lipophilic antimicrobials.