Page "Selective progesterone receptor modulator" Paragraph 11

When SPRMs bind to the progesterone receptor, the equilibrium between the two conformational states is more closely balanced and hence more easily perturbed by differences in the cellular environment.

In tissues where the concentration of coactivators is higher than corepressors, the excess coactivators drive the equilibrium in the agonist direction.

Conversely in tissues where corepressor concentration is higher the equilibrium is driven in the antagonist direction.

Hence SPRMs display agonist activity in tissues where coactivators predominate and antagonist activity where corepressors are in excess.