A toxicology study in rhesus monkeys resulted in the death of all rhesus fetuses exposed to 1 and 10 times the human dose equivalent of OHPC. As of 2008, OHPC was a category D progestin according to the FDA (that is, there is evidence of fetal harm). There is speculation that the castor oil in the OHPC formulation may not be beneficial for pregnancy. Of note, the above-mentioned NEJM study by Meirs et al. compares the effect of OHPC (with the castor oil component) to castor oil injection as the placebo. A study published in February 2016 in The Lancet stated the below, amongst other findings:
OPPTIMUM strongly suggests that the efficacy of progesterone in improving outcomes is either non-existent or weak. Given the heterogeneity of the preterm labour syndrome we cannot exclude benefit in specific phenotypic or genotypic subgroups of women at risk. However, the subgroups of women who might benefit do not appear to be easily identifiable by current selection strategies, including cervical length measurement and fibronectin testing. Reassuringly, our study suggests that progesterone is safe for those who wish to take it for preterm birth prophylaxis. The overall rate of maternal or child adverse events was similar in the progesterone and placebo groups. There were few differences in the incidence of adverse secondary outcomes in the two groups, with the exception of a higher rate of renal, gastrointestinal, and respiratory complications in childhood in the progesterone groups. Importantly, the absolute rates of these complications was low.
However, in women pregnant with twins rather than a singlet, the elimination half-life of OHPC was found to be shorter than this, at 10 days. OHPC has been detected in pregnant women up to 44 days after the last dose. Elimination
OHPC is eliminated 50% in feces and 30% in urine when given by intramuscular injection to pregnant women. Both the free steroid and conjugates are excreted by these routes, with the conjugates more prominent in feces. Veterinary
The pharmacokinetics of OHPC in various ungulates including cattle, buffalo, sheep, and goat have been studied. Time–concentration curves
Chemistry
OHPC, also known as 17α-hydroxyprogesterone caproate or as 17α-hydroxypregn-4-ene-3,20-dione 17α-hexanoate, is a synthetic pregnane steroid and a derivative of progesterone. It is specifically a derivative of 17α-hydroxyprogesterone with a hexanoate (caproate) ester at the C17α position. Analogues of OHPC include other 17α-hydroxyprogesterone derivatives such as algestone acetophenide (dihydroxyprogesterone acetophenide), chlormadinone acetate, cyproterone acetate, hydroxyprogesterone acetate, hydroxyprogesterone heptanoate, medroxyprogesterone acetate, and megestrol acetate, as well as the caproate esters chlormadinone caproate, gestonorone caproate (norhydroxyprogesterone caproate), medroxyprogesterone caproate, megestrol caproate, and methenmadinone caproate. Synthesis
Chemical syntheses of OHPC have been described. : 6

History
Along with hydroxyprogesterone acetate, OHPC was developed by Karl Junkmann of Schering AG in 1953 and was first reported by him in the medical literature in 1954. It was reportedly first marketed in Japan in 1954 or 1955, and was subsequently introduced as Delalutin in the United States in 1956. Due to its much longer duration than parenteral progesterone, OHPC had largely replaced progesterone in clinical practice by 1975.