Oral GnRH Antagonist (Linzagolix) Versus Injectable GnRH Antagonist and Progestin-primed Ovarian Stimulation (PPOS) in Controlled Ovarian Stimulation for in Vitro Fertilization: a Prospective Comparative Observational Study

Prevention of a premature luteinizing hormone (LH) surge is essential during controlled ovarian stimulation (COS) for IVF/ICSI, since an unopposed surge compromises oocyte retrieval. The established approaches are the injectable GnRH antagonist (ganirelix, cetrorelix), administered subcutaneously and considered standard of care, and progestin-primed ovarian stimulation (PPOS), which suppresses the LH surge with an oral progestin but mandates a freeze-all strategy. Non-peptide oral GnRH antagonists (relugolix, elagolix, linzagolix) are approved for uterine fibroids and endometriosis but have not previously been used in controlled ovarian stimulation; this study constitutes their first documented use in this indication.

Linzagolix is a non-peptide oral GnRH receptor antagonist producing dose-dependent suppression of the pituitary-ovarian axis; at 200 mg/day it fully suppresses endogenous estradiol secretion. In COS, however, follicular growth and estradiol production are driven by exogenous FSH acting directly on the follicle, independent of pituitary function, so linzagolix's role is limited to preventing the endogenous LH surge. Linzagolix has a shorter half-life (approximately 15-20 hours) than relugolix (approximately 37-42 hours), for which reduced oocyte yield has been reported when final maturation is triggered with a GnRH agonist. The faster elimination is expected to leave lower residual receptor occupancy at the time of trigger, allowing an agonist bolus to evoke an adequate maturation flare despite competitive antagonism - relevant to OHSS prevention in high responders. Preliminary center experience with linzagolix 200 mg is consistent with this expectation and informed the sample size assumptions.

This is a single-center, prospective, comparative observational study with three concurrent groups, conducted at CENTRO AMBRA. Patients receive one of three LH-surge suppression protocols - oral linzagolix, injectable GnRH antagonist, or PPOS - according to clinical judgment and center practice; treatment is not assigned by the study protocol and there is no randomization. Because assignment reflects clinical and organizational criteria (for example, an indication for a freeze-all strategy in the PPOS group), the three groups may differ systematically in baseline and prognostic characteristics - confounding by indication affecting all three groups, not PPOS alone. This is addressed through pre-specified multivariable adjustment rather than eliminated; results are interpreted as associations, not causal effects. For each patient, the clinical reason for treatment assignment is prospectively documented to support confounding assessment. As treatment is known to investigators and patients, there is no masking; oocyte and embryo assessment follows standardized, predefined laboratory procedures to limit subjectivity.

The primary objective is to estimate, with adjustment for confounders, the difference in mature (MII) oocytes retrieved between linzagolix and the injectable antagonist; this is an estimation study, not a hypothesis test. A clinically relevant reference value of 2.0 MII oocytes is pre-specified solely to aid interpretation of the confidence interval, not as a non-inferiority margin. Secondary objectives include an exploratory comparison with PPOS; efficacy of LH suppression; feasibility of GnRH agonist trigger under linzagolix; safety (OHSS and adverse events); stimulation duration and gonadotropin consumption; and embryological parameters including fertilization, blastulation, and euploidy in the PGT-A subgroup. Exploratory objectives include effect modification by responder category, the relationship between the last-dose-to-trigger interval and oocyte yield (linzagolix group), and the role of progestin type within the PPOS group.

The primary analysis uses a multivariable model (ANCOVA or negative binomial regression, as dictated by dispersion) with linzagolix as the reference category, adjusting for ovarian reserve, age, BMI, gonadotropin type and dose, and trigger type. Sensitivity analyses include restriction to comparable patients and, where sample size permits, propensity-score methods. As linzagolix is used off-label in this indication, eligibility for the observational-study regulatory pathway (per AIFA Determination 425/2024) is being confirmed with the Ethics Committee prior to final submission; should off-label use require interventional classification, the corresponding regulatory pathway will apply. Reporting will follow STROBE recommendations for observational studies.