Nuclear receptor cofactor interactions as targets for new drug discovery
The classical models of nuclear receptor pharmacology held that agonists functioned by binding to their cognate receptors facilitating their conversion from an inactive form to one that was capable of activating transcription. By extrapolation, it was considered that antagonists functioned by competitively inhibiting agonist binding, freezing the receptor in an inactive state. However, as early as 1967 when the biological actions of the “antiestrogen” tamoxifen were first described, it was clear that this simple model did not adequately describe estrogen receptor pharmacology. Indeed, tamoxifen is now classified as a Selective Estrogen Receptor Modulator (SERM), one of a group of compounds whose relative agonist/antagonist activity differs between cells. Similarly, tissue selective progesterone, androgen and glucococorticoid receptor modulators have also been identified. Significant progress has been made in defining the molecular mechanism(s) by which cells distinguish between agonists and antagonists and how some receptor modulators can manifest their actions in a cell-selective manner. The most important of these are (1) differences in the relative expression level of receptor isoforms or subtypes, (2) the impact which the bound ligand has on the structure of its cognate receptor, and (3) the complement of coactivators and corepressors in a target cell which can interact with the activated receptor. Exploitation of this complexity will lead to the development of novel classes of nuclear receptor modulators with useful therapeutic activities.