Biological activity and ligand binding mode to the progesterone receptor of A-homo analogues of progesterone.

The biological activity of two seven-membered A-ring (A-homo) analogues of progesterone was evaluated by transactivation assays in Cos-1 cells and by determination of Bcl-x(L) expression levels in T47D cells. The results show that both compounds act as selective progesterone receptor (PR) agonists but lack mineralocorticoid receptor (MR) activity. Molecular modelling using semiempirical AM1 and ab initio HF/6-31G** calculations, showed that the A-ring of the A-homo steroids may adopt five different conformations, although only three correspond to low energy conformers. The low energy conformers of each analogue were introduced into the ligand binding pocket of the PR ligand binding domain (LBD) obtained from the PR LBD-progesterone crystal structure. The steroid binding mode was then analyzed using 10 ns of molecular dynamics (MD) simulation. The PR LBD-progesterone complex was also simulated as a control system. The MD results showed that both A-homo steroids have one conformer that may be properly recognized by the PR, in agreement with the observed progestagen activity. Moreover, the simulation revealed the importance of a water molecule in the formation of a hydrogen bonding network among specific receptor residues and the steroid A-ring carbonyl.