Comparison of progesterone and glucocorticoid receptor binding and stimulation of gene expression by progesterone, 17-alpha hydroxyprogesterone caproate, and related progestins.

Objective: This study was undertaken to determine whether the reduction in premature birth attributable to 17-alpha hydroxyprogesterone caproate occurs because of a greater affinity for progesterone or glucocorticoid receptors or by enhanced stimulation of progestogen responsive genes when compared with progesterone.

Study Design: We performed competitive steroid hormone receptor binding assays using cytosols expressing either recombinant human progesterone receptor-A or -B or rabbit uterine or thymic cytosols. We used 4 different carcinoma cell lines to assess transactivation of reporter genes or induction of alkaline phosphatase.

Results: Relative binding affinity of 17-alpha hydroxyprogesterone caproate for recombinant human progesterone receptor-B, recombinant human progesterone receptor-A, and rabbit progesterone receptors was 26-30% that of progesterone. Binding of progesterone to rabbit thymic glucocorticoid receptors was weak. 17-alpha hydroxyprogesterone caproate was comparable to progesterone in eliciting gene expression in all cell lines studied.

Conclusion: Binding to progesterone receptors, glucocorticoid receptors, or expression of progesterone-responsive genes is no greater with 17-alpha hydroxyprogesterone caproate than with progesterone. Other mechanisms must account for the beneficial effect of 17-alpha hydroxyprogesterone caproate on preterm birth rates.