Nestorone^® as a novel progestin for nonoral contraception: Structure-activity relationships and brain metabolism studies

Nestorone^® (NES) is a potent nonandrogenic progestin being developed for contraception. NES is a synthetic progestin that may possess neuroprotective and myelin regenerative potential as added health benefits. In receptor transactivation experiments, NES displayed greater potency than progesterone to transactivate the human progesterone receptor (PR). This was confirmed by docking experiments where NES adopts the same docking position within the PR ligand-binding domain (LBD) as progesterone and forms additional stabilizing contacts between 17a-acetoxy and 16-methylene groups and PR LBD, supporting its higher potency than progesterone. The analog 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 LBD, leading to negligible androgen receptor transactivation. Because progesterone acts in the brain by both PR binding and indirectly via binding of the metabolite allopregnanolone to γ-aminobutyric acid type A receptor (GABA_AR), we investigated if NES is metabolized to 3α, 5α-tetrahydronestorone (3α, 5α-THNES) in the brain and if this metabolite could interact with GABA_AR. In female mice, low concentrations of reduced NESmetabolites were identified by gas chromatography/mass spectrometry in both plasma and brain. Electrophysiological studies showed that 3α, 5α-THNES exhibited only limited activity to enhance GABA_AR-evoked responses with WSS-1 cells and did not modulate synaptic GABA_ARs of mouse cortical neurons. Thus, the inability of reducedmetabolite of NES (3a, 5a-THNES) to activate GABA_AR suggests that the neuroprotective and myelin regenerative effects of NES are mediated via PR binding and not via its interaction with the GABA_AR.