Drug interactions and clinical pharmacokinetics

Some drugs influence the gastro-intestinal absorption, distribution , metabolism or renal excretion of other drugs, i.e., processes involved in pharmacokinetic interactions. The clinical consequences of pharmacokinetic drug-drug interactions will be either an increase or a decrease in known therapeutic or toxic effects of the interacting drug. In order to evaluate the importance of drug interaction affecting gastro-intestinal absorption, it is necessary to distinguish between interactions which alter the rate of absorption of another drug and those which alter the amount of drug absorbed. Many drugs displace other drugs from their protein binding sites in vitro. This may cause an increase in the pharmacological effect of the displaced drug. However, much discrepancy exists between in vitro findings. In some cases, the enhanced effect only seems to be a temporary phenomenon. The degree of protein binding and the size of apparent volume of distribution (Vd) must also be taken into consideration. Perhaps the importance of interaction involving protein binding has been overemphasized. Barbiturates, glutethimide, rifampicin and phenytoin increase the rate of drug metabolism in man. The most important interactions reported are between oral anti-coagulants and barbiturates. After withdrawal of these hepatic microsomal enzyme inducing drugs, it takes 2 to 3 weeks before the rate of drug metabolism reaches the pretreatment level. In this period, risk of haemorrhage exists. Induction seems to be dose-dependent, but not all persons are inducible. Many drugs compete for the drug metabolising enzyme system in the liver and consequently some drugs inhibit the biotransformation of other drugs. The time course of these interactions depends on the pharmacokinetic properties of the drug involved, and these interactions also seem to be dose-dependent. The most important of such interactions, clinically involved the oral sulphonylurea hypoglycaemic drugs and the antiepilepic drug phenytoin. Drugs are eliminated by urinary excretion through three mechanisms: glomerular filtration, tubular reabsorption, and active tubular secretion. The most important interactions seem to be those involving competition for tubular secretion.