A selective progesterone receptor modulator (SPRM) is an agent that acts on the progesterone receptor (PR), the biological target of progestogens like progesterone. A characteristic that distinguishes such substances from full receptor agonists (e.g., progesterone, progestins) and full antagonists (e.g., aglepristone) is that their action differs in different tissues, i.e. agonist in some tissues while antagonist in others. This mixed profile of action leads to stimulation or inhibition in tissue-specific manner, which further raises the possibility of dissociating undesirable adverse effects from the development of synthetic PR-modulator drug candidates.
History
Ever since the discovery of the progesterone hormone in the mid-1930s. and especially after the discovery of its receptor in 1970 there has been a significant interest in developing an antagonistic agent for therapeutic use. Various progesterone analogs, known as progestins, were synthesized and in 1981 the first progesterone receptor antagonist was introduced by the name RU 38486 (RU 486, mifepristone). However, the clinical limitation of mifepristone due to its relatively high binding affinity for glucocorticoid receptor compared to the progesterone receptor has sparked the demand for more selective progesterone antagonist to minimize risk of adverse effects. As a contribution, so-called Selective Progesterone Receptor Modulators (SPRMs) have been developed. They have been described as agents with mixed antagonistic and agonistic effects on progesterone receptors in a tissue specific manner, while minimizing interactions with other steroidal receptors. Opposed to progesterone antagonists, the mixed agonist-antagonist SPRM, due to their intrinsic progesterone agonistic activity, have an absent or only a minimal effect on pregnancy termination and are thus ideal for treating gynecological conditions without eliminating the potential of pregnancy. Both steroidal and non steroidal SPRMs have been described and the most notable examples are asoprisnil, which failed phase 3 clinical trial in 2008, and ulipristal acetate, the first SPRM on the market (2009 in Europe).
Progesterone receptor
Receptor
As a protein, the progesterone receptor (Fig. 1) is a member of the ligand-dependent nuclear hormone receptor family. Two major progesterone receptor isoforms, A and B, as well as some other less common splice variants have been identified and they are all encoded by the same 8 exons gene. Like other steroid nuclear receptors, the full-length protein, isoform B, can be divided into 4 functional regions, namely a variable N-terminal region followed by a highly conserved DNA-binding domain, variable hinge region and moderately conserved ligand binding domain. The ligand binding site, known as AF2 domain , is expressed by exons 4-8, corresponding to 253 amino acids, and its structure is of great interest to SPRM development. It consists of 10 α-helices (H1, H3-H12) forming 3 layered bundle entwined with 4 β-sheets . H12 is a condensed contiguous unit composed of helices 10 and 11, which has been suggested to participate in the process of co-activator binding. The ligand binding domain of the receptor is in equilibrium between two different conformations. The first is an agonist conformation which favors the binding of coactivator proteins which in turn favors upregulation of gene transcription. The second is an antagonistic conformation which in contrast favors the binding of corepressors and as a consequence down regulation of gene expression. Full agonists such as progesterone, which display agonist properties in all tissues, strongly shift the conformational equilibrium in the agonist direction. Conversely full antagonists such as aglepristone strongly shift the equilibrium in the antagonist direction. Finally, the overall ratio of concentrations of coactivator to corepressor may differ in different cell types.
G protein-coupled receptor
At the turn of the millennium it was apparent that progesterone activity was not mediated only via transcription factor, but also by a membrane-bound G protein-coupled receptor designated as 7TMPR. When the receptor is activated it blocks adenylyl cyclase, leading to decreased biosynthesis of the intracellular second-messenger cAMP.