However, when the concentration of the drug in the extracellular fluid falls, the
equilibrium will move to the left and the drug–receptor complex will dissociate.
Consequently, drugs and endogenous ligands become ineffective as soon as their
concentrations fall below a certain limit as an insufficient number of receptors
are being activated by these ligands. Endogenous reduction of drug concen-
tration is brought about by metabolism and excretion. Consequently, both these
processes will have a direct bearing on the duration of action of a drug.
Drugs that form strong bonds with their receptors do not readily dissociate
from the receptor when their concentrations in the extracellular fluid fall.
Consequently, drugs that act in this manner will often have a long duration of
action. This is a useful attribute for drugs used in the treatment of cancers,
where it is particularly desirable that the drug forms irreversible bonds to the
receptors of tumour cells.
248 APPENDIX 4 RECEPTORS