Precision Immunomodulation in Pregnancy — Lessons from Nipocalimab

The development of neonatal Fc receptor (FcRn)–blocking therapies represents a profound shift in the management of maternal alloimmune disease, offering a mechanism-based strategy to prevent the transplacental transfer of pathogenic immunoglobulin G (IgG).¹ Nipocalimab, a fully human IgG1 monoclonal antibody that inhibits FcRn-mediated IgG transport,² is the first of such therapies to be evaluated in pregnancy. In the phase 2 UNITY study,³ antenatal nipocalimab was associated with decreased fetal anemia, and fewer intrauterine transfusions, as compared with a historical benchmark, among women at high risk of early-onset severe hemolytic disease of the fetus and newborn, a condition traditionally associated with substantial fetal morbidity and procedural risk.³

In this follow-up article by de Winter and colleagues of the UNITY Study Group⁴ published in this issue of NEJM Evidence, the investigators characterize nipocalimab exposure in fetal and cord blood, colostrum, and breast milk, and describe infant IgG kinetics, infection patterns, and vaccine responses through 96 weeks postdelivery. Although maternal serum concentrations of nipocalimab were consistently within the pharmacologically active range, fetal and neonatal concentrations were minimal — only 1 of 4 cordocentesis samples and 1 of 11 cord blood samples had detectable nipocalimab, and both values were far below the threshold for FcRn saturation. Even among women treated up to the day of delivery, cord-to-maternal nipocalimab concentration ratios were orders of magnitude lower than those observed with endogenous IgG or other therapeutic monoclonal antibodies. These observations suggest that placental transfer of nipocalimab may be restricted, an unexpected but reassuring finding given nipocalimab’s Fc-based structure.

Infant IgG concentrations at birth were observed to be lower than normal, suggestive of the anticipated effect of impaired maternal IgG transport. IgG levels appeared to nadir between 4 and 24 weeks, reflecting the physiologic clearance of residual maternal IgG and the early period of immature endogenous IgG synthesis. Nonetheless, by 24 weeks, the reported IgG values were within or near age-appropriate reference ranges, and all but one normalized by 48–96 weeks. Importantly, infection patterns remained typical for early childhood. Fourteen of the fifteen infections were mild to moderate and self-limited, including a single severe case of respiratory syncytial virus (RSV) infection that occurred during a severe RSV season. Routine infant vaccinations elicited protective antibody responses in six of seven infants tested, supporting the possibility of immune competence in the setting of transient early IgG suppression.

The maternal–fetal transfer findings deserve particular attention. Although FcRn blockade is postulated to attenuate the transplacental transfer of both beneficial and pathogenic IgG,^5,6 the strikingly low fetal and neonatal nipocalimab concentrations observed in this study may suggest a more nuanced pharmacobiologic phenomenon. Specifically, nipocalimab’s biophysical attributes, most notably its high-affinity interaction with FcRn and its atypical pH-independent binding behavior,² may paradoxically impede its own FcRn-mediated transcytosis across the placenta. This feature differentiates nipocalimab from both endogenous IgG and other Fc-containing therapeutics that traverse the placenta at higher rates. Whether this potentially limited transfer reflects saturable maternal receptor interaction, rapid endosomal sequestration, or competition with endogenous IgG, remains an important area for further investigation.

Breast milk pharmacokinetics provides an additional layer of insight. Nipocalimab was quantifiable only when maternal dosing extended into the immediate peripartum interval, approximately within 3 weeks of delivery, and, even then, at concentrations far below thresholds expected to confer pharmacodynamic activity. The relative infant dose (RID), defined as the estimated percentage of the maternal weight–based dose received by the infant through breast milk,⁷ provides a practical benchmark for contextualizing lactational exposure. Using the median detectable milk concentration (<4 μg/ml), the calculated RID is approximately 9%, a value consistent with a safety margin that is likely sufficient and in line with the absence of detectable serum nipocalimab in all breastfed infants at 4 weeks postpartum. Moreover, the exceptionally low oral bioavailability of IgG-based therapeutics, driven by their large molecular size, structural complexity, and susceptibility to proteolytic degradation within the infant gastrointestinal tract, further supports the conclusion that systemic infant exposure is trivial. These observations align with a well-established pharmacologic paradigm where IgG-based therapies exhibit minimal enteral absorption due to their molecular size, structural complexity, and susceptibility to luminal catabolism.^8,9

Taken together, these early-phase safety data support the overall tolerability of antenatal nipocalimab and suggest that its pharmacodynamic potency, reflected in its potentially robust maternal FcRn inhibition, may occur without clinically meaningful fetal or neonatal drug exposure. Nonetheless, important limitations remain. The study cohort was small, open-label, and devoid of a comparator group. Moreover, rare infections or more nuanced immunologic effects could not be definitely excluded. Infant vaccine responses, though reassuring, were assessed in only a subset of participants. Larger phase 3 trials now under way will be critical to confirming these observations and guiding clinical decision-making.

The UNITY program marks a pivotal advancement in maternal–fetal therapeutics, as use of nipocalimab has proven to be a targeted immunomodulatory strategy capable of altering the natural history of a devastating autoimmune condition. As the field of obstetric therapeutics continues to evolve, mechanistically informed therapies such as nipocalimab offer a blueprint for how precision medicine may expand to include and prioritize pregnant women. The current findings reinforce that such innovation must be paired with rigorous, prospective safety evaluation across the maternal–infant dyad. The path forward requires both scientific ambition and humility, ensuring that treatments designed to protect the fetus do so without compromising the infant’s emerging immune system.