Of these women, 9 had infants with congenital malformations (2%), but there was no consistent pattern and none involved internal organs. OHPC is currently (as of June 2014) pregnancy category B, meaning there is no evidence of fetal risk with use of this medication during pregnancy. Although this is now the recommendation, this has not always been the case. A review by Marc Keirse of Flinders University concluded that information about the potential harms was lacking. Three clinical studies in singleton pregnancies of 250 mg/week of intramuscular OHPC have all shown a trend for an increase in pregnancy loss due to miscarriage compared to placebo. One of them, a large National Institutes of Health (NIH) study in 2003, looked at the effect of OHPC injections in women at risk for repeat premature birth and found that the treated group experienced premature birth in 37% versus 55% in the controls. A follow-up study of the offspring showed no evidence that OHPC affected the children in the first years of life. Based on these NIH data, OHPC was approved by the Food and Drug Administration (FDA) in 2011 as a medication to reduce the risk of premature birth in selected women at risk. (v.i.) The FDA expressed concern about miscarriage at the 2006 advisory committee meeting; the committee voted unanimously that further study was needed to evaluate the potential association of OHPC with increased risk of second trimester miscarriage and stillbirth.
OHPC has been studied in humans at doses as high as 5,000 mg per week by intramuscular injection, with safety and without glucocorticoid effects observed. The medication does interact with the glucocorticoid receptor however; it has about 4% of the affinity of dexamethasone for the rabbit glucocorticoid receptor. But it acts as a partial agonist of the receptor and has no greater efficacy than progesterone in activating the receptor and eliciting associated gene expression in vitro. Other activities
As a pure progestogen, OHPC has no androgenic, antiandrogenic, estrogenic, or glucocorticoid activity. The absence of androgenic and antiandrogenic activity with OHPC is in contrast to most other 17α-hydroxyprogesterone-derivative progestins. Due to its lack of androgenic properties, similarly to progesterone, OHPC does not have any teratogenic effects on the fetus, making it safe for use during pregnancy. Although OHPC has been described as a pure progestogen, there is evidence that it possesses some antimineralocorticoid activity, similarly to progesterone and 17α-hydroxyprogesterone. This includes clinically important diuretic effects and reversal of estrogen-induced fluid retention and edema. Unlike progesterone, OHPC and its metabolites are not anticipated to interact with non-genomic receptors such as membrane progesterone receptors or the GABAA receptor. In accordance, OHPC is not thought to possess the neurosteroid activities of progesterone or its associated sedative effects.In relation to cytochrome P450 enzymes, OHPC has no effect on CYP1A, CYP2D6, CYP2C9, or CYP3A4, but is a modest inducer of CYP2C19. Differences from progesterone
There are pharmacodynamic differences between progesterone and OHPC, which may have implications for obstetrical use.