5β Dihydroprogesterone

5β-Dihydroprogesterone (5β-DHP), also known as pregnanedione, as well as 5β-pregnane-3,20-dione, is an endogenous neurosteroid and an intermediate in the biosynthesis of pregnanolone and epipregnanolone from progesterone. It is synthesized from progesterone by the enzyme 5β-reductase. 5β-DHP has been found to act as a positive allosteric modulator of the GABA_A receptor (albeit with an affinity for this receptor regarded as relatively low (in comparison to 3α-hydroxylated progesterone metabolites such as pregnanolone and allopregnanolone)) and also as a negative allosteric modulator of the GABA_A-rho receptor. In accordance with the former action, it is reported to possess anesthetic, anxiolytic, and antinociceptive effects. 5β-DHP has been found to act as an agonist of the pregnane X receptor (PXR) as well (albeit weakly (EC₅₀ >10,000 µM)), and has been found to regulate uterine contractility through activation of this receptor. Unlike 5α-dihydroprogesterone, 5β-DHP does not possess affinity for the progesterone receptor, and hence, is not a progestogen.

A study found that 5β-DHP, but not progesterone, directly bound to and antagonized the oxytocin receptor at nanomolar concentrations, and it was suggested that this may be one of the mechanisms by which progesterone maintains pregnancy. However, a subsequent study was unable to replicate this finding, and there has been no further investigation since. In any case, 5β-DHP has nonetheless been shown to possess tocolytic effects in animals, and this may alternatively be mediated by activation of the PXR.