This preclinical study evaluated whether Nestorone (segesterone acetate), a potent progestin, could prevent inflammation-triggered preterm birth in pregnant mice at a much lower dose (0.02 mg/day) than progesterone (2 mg/day), and characterized its progesterone receptor activity and anti-inflammatory effects in human cell lines.
In a systemic inflammation model, Nestorone reduced preterm birth from 100% (12/12) to 8% (1/12), a ~92% reduction — matching progesterone at 100× lower dose. In an intrauterine inflammation model, both Nestorone and progesterone eliminated preterm birth entirely (100% to 0%; n=10/group). Nestorone also showed the highest progesterone receptor transcriptional activity among progestins tested, with only mild glucocorticoid receptor activity.
All efficacy data come from murine models; translation to human pregnancy outcomes is unproven. The study does not address populations where vaginal progesterone has already failed (e.g., prior spontaneous preterm birth without cervical shortening). No safety or pharmacokinetic data in pregnancy are reported.
Nestorone is not yet indicated for preterm birth prevention, but these preclinical results justify designing translational and clinical trials — particularly for higher-risk patients who do not respond to standard vaginal progesterone. Clinicians should watch for future phase I/II studies in this population.
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