The Mineralocorticoid Activity of Progesterone Derivatives Depends on the Nature of the C₁₈ Substituent

To investigate the role of the C₁₈ substituents in the agonist/antagonist properties of mineralocorticoids, the activities of certain C₁₈-substituted progesterone (P) derivatives were examined. These compounds were characterized by an unsaturated side-chain in the case of 18-vinylprogesterone (18VP) and 18-ethynylprogesterone (18EP) and by an enone group in the case of 18-oxo-18-vinylprogesterone (18OVP). P and its 18-substituted derivatives bind to the recombinant human MR (hMR) overexpressed in Sf9 cells with the following hierarchy of affinity: P > aldosterone > 18VP > 18EP ≫ 18OVP. Functional cotransfection assays in CV-1 cells, using mouse mammary tumor virus promoter as a steroid receptor-inducible DNA target sequence, indicated that the mineralocorticoid activity depends on the nature of the C₁₈ substituent. 18VP and 18EP retained the antimineralocorticoid feature of P, with the following order of activity: P = 18VP > 18EP. The antagonist potency of 18VP was higher (IC₅₀, ∼10^-8 M) than that of spironolactone (IC₅₀, ∼7 × 10^-8 M), the most widely used aldosterone antagonist. Interestingly, introducing an oxo function at C₁₈ conferred agonist mineralocorticoid properties; 18OVP behaves as a full agonist (ED₅₀, ∼10^-7 M) with no antagonist activity. In contrast to what was observed when the three 18-substituted P derivatives acted through hMR, they retained the agonist feature of P through the human P receptor, with the following order of potency: P > 18VP = 18OVP > 18EP. The activity of the 18-substituted P derivatives through the human glucocorticoid receptor was only detected at concentrations higher than 10^-6 M; P and 18VP displayed a partial antagonist activity, whereas 18OVP had a full agonist activity (ED₅₀, ∼2 × 10^-6 M). Thus, the presence of an oxo group at C₁₈ (18OVP) does not change the agonist feature of P through human P receptor, but confers to the ligand an agonist activity through hMR, suggesting that the C₁₈ carbonyl group of aldosterone plays a crucial role in its agonist activity.