Mechanisms of renal excretion of drugs (with special reference to drugs used by anaesthetists).

Contrary to popular belief, the elimination of active drug by renal excretion is a relatively unimportant mechanism for the termination of drug action. Most drugs are weak electrolytes which are lipid-soluble in the un-ionized state, and as such they cannot be excreted by the kidney because they are extensively reabsorbed. These lipid-soluble drugs are invariably metabolized to more water-soluble derivatives which usually have little or no pharmacological activity and can readily be excreted in the urine. The duration of action is therefore determined largely by redistribution, metabolism, and in the case of the anaesthetic gases, by elimination through the lungs. On the other hand, some drugs have limited lipid solubility or are highly ionized in the physiological range of pH. They tend to be eliminated largely unchanged by renal excretion and their duration of action may be prolonged in the presence of impaired renal function. Familiar drugs which fall into this category include quaternary ammonium bases, ganglion blocking drugs, non-depolarizing muscle relaxants, many antibiotics, thiazide diuretics, methotrexate, digoxin and barbitone (table I). From a practical point of view it is obviously important to know whether the major route of elimination of a drug is through metabolism or renal excretion.