Ligand

Ligand binding changes the receptor's shape and starts a signal inside the cell; the specificity of the fit is why one growth factor activates one family of receptors. Cancers exploit ligands by making their own (autocrine signalling), by persuading neighbouring cells to make them, or by mutating receptors so they no longer need the ligand at all. Several drugs act on ligands rather than receptors: bevacizumab soaks up VEGF before it can reach its receptor, and 'radioligands' are small molecules or peptides that bind a receptor such as PSMA to deliver radiation.