Nestorone® (NES) a novel progestin for non-oral contraception: Structure-activity relationships and brain metabolism studies

Nestorone® (NES) is a highly potent non-androgenic progestin being developed for contraception. NES is a synthetic progestin that may possess neuroprotective and myelin regenerative potential as an added health benefits. In receptor transactivation experiments, NES displayed greater potency than progesterone to transactivate the human progesterone receptor (hPR). This was confirmed by docking experiments which revealed that NES adopts the same docking position within the PR ligand-binding domain (LBD) as progesterone and forms additional stabilizing contacts between 17α-acetoxy and 16-methylene groups and PR LBD supporting its higher potency than progesterone. The analogue 13-ethyl NES also establishes similar contacts as NES with Met909, leading to comparable potency as NES. In contrast, NES is not stabilized within the human androgen receptor (hAR)-LBD leading to negligible AR transactivation. Since progesterone acts in the brain by both PR-binding and indirectly via the metabolite allopregnanolone binding to GABA_A receptor (GABA_A_R), we investigated if NES is metabolized to 3α, 5α-tetrahydronestorone (3α, 5α-THNES) in the brain and if this metabolite could interact with GABA_A_R. In female mice, low concentrations of reduced NES metabolites were identified by Gas Chromatography-Mass Spectrometry in both plasma and brain. However, electrophysiological studies showed that 3α, 5α-THNES exhibited only limited activity to enhance GABAAR-evoked responses with WSS-1 cells and did not modulate synaptic GABA_A_Rs of mouse cortical neurons. Thus the inability of reduced metabolite of NES (3α, 5α-THNES) to activate GABA_A_R suggests that the neuroprotective and myelin regenerative effects of NES are mediated via PR binding and not via its interaction with the GABA_A_R