Sanofi has halted a late-stage trial of its experimental complement inhibitor riliprubart in chronic inflammatory demyelinating polyneuropathy (CIDP), a setback that analysts say could create volatility across the wider complement inhibitor field. The company said in a statement on 10 June 2026 that it would discontinue the Phase 3 MOBILIZE study after an independent data monitoring committee judged the trial unlikely to provide sufficient efficacy in patients whose disease had not responded to standard care. The committee identified no safety concerns linked to the drug.
What Sanofi announced
MOBILIZE enrolled around 140 adults with CIDP who had been failed by, or had responded poorly to, established therapies such as intravenous immunoglobulin (IVIg) and corticosteroids. Launched in 2024, the study compared intravenous and subcutaneous formulations of riliprubart against placebo over 24 weeks, with longer follow-up planned. At an interim review, the monitoring committee concluded the study was unlikely to meet its efficacy goals, prompting Sanofi to wind it down and move enrolled patients onto other care.
Sanofi stressed that the decision carried no safety concerns and would not incur any significant financial cost, adding that its 2026 financial guidance remained unchanged. The company said it would analyse the MOBILIZE data in full and decide separately whether to continue VITALIZE, a second Phase 3 trial testing riliprubart against IVIg in patients already stable on maintenance treatment.
How riliprubart fits the complement theory of CIDP
Riliprubart, also known as SAR445088, is a humanised IgG4 monoclonal antibody that selectively blocks activated C1s, an early protein in the classical complement pathway. By acting at that point, it is designed to dampen complement-driven inflammation while leaving the lectin and alternative pathways intact. Sanofi had positioned it as a potential first-in-class therapy and a possible subcutaneous alternative to IVIg.
The rationale rests on growing evidence that complement activation contributes to the nerve damage and demyelination seen in CIDP, a rare autoimmune disorder that causes progressive weakness, numbness and fatigue and can leave patients reliant on a wheelchair. The disappointment is sharper because earlier results had looked encouraging. In open-label Phase 2 testing, 88 percent of patients who switched from standard care remained stable or improved, and roughly half of those who had not previously responded showed a benefit. Those data were strong enough to justify two separate Phase 3 trials.
What the failure means for the complement inhibitor class
The immediate question for the sector is whether MOBILIZE points to a problem for complement inhibition in CIDP as a whole, or a problem specific to this drug, this patient group or this trial design. Early reaction leaned towards caution rather than alarm. William Blair analyst Myles Minter described the result as “disappointing for the complement inhibitor class in CIDP,” while noting that trial design could be a factor and that rival programmes use different protocols. Industry analysts also flagged that the implications reach beyond Sanofi.
Two competitors are watched most closely. Argenx is developing empasiprubart, a C2 inhibitor that blocks both the classical and lectin complement pathways, and is running two Phase 3 CIDP studies, EMVIGORATE against IVIg and EMNERGIZE against placebo, with readouts expected in the second half of 2027. Because empasiprubart acts at a different point in the cascade, and because argenx is using its own trial designs, several analysts argued the read-across from riliprubart is limited. Investors appeared to agree, and argenx shares were little changed after the news.
Dianthus Therapeutics felt more of the impact. The biotech is advancing claseprubart, an antibody that, like riliprubart, targets C1s, through the Phase 3 CAPTIVATE programme. Dianthus had pointed to Sanofi’s Phase 2 data as validation of the approach, so concern that MOBILIZE might foreshadow trouble sent its shares down sharply on the day. Some analysts pushed back, noting that CAPTIVATE uses a randomised-withdrawal design that enriches for responders, in contrast to MOBILIZE’s harder-to-treat, all-comers refractory population.
The competitive and commercial picture
For now, the clearest beneficiary is argenx’s approved CIDP therapy. Vyvgart Hytrulo, an FcRn blocker rather than a complement inhibitor, became the first new targeted treatment for CIDP in more than 30 years when the US Food and Drug Administration cleared it in 2024. Analysts suggested that setbacks among experimental rivals could reinforce its commercial standing, which has been one of the sector’s strongest recent launches. Established IVIg suppliers such as CSL Behring, Grifols and Pfizer could also benefit if a subcutaneous complement-based alternative fails to reach the market.
The episode is a reminder of how difficult CIDP trials remain. The condition is heterogeneous, diagnosis can be uncertain, and placebo-controlled studies in refractory patients are notoriously hard to interpret. Minter noted that parallel-group studies in CIDP carry particular enrolment and diagnostic challenges, and that argenx’s record in running successful CIDP trials should not be underestimated.
Sanofi’s wider pipeline pressure
The setback lands at a sensitive moment for Sanofi. The company endured a run of clinical disappointments through 2025 that weighed on its shares and raised questions about its ability to offset the looming patent expiry of its best-selling medicine Dupixent. In February 2026, Sanofi announced the departure of Chief Executive Paul Hudson, whose mandate ended on 17 February, and named former Merck KGaA Chief Belén Garijo as his successor. She took up the role on 29 April 2026, at the close of the company’s Annual General Meeting, with Olivier Charmeil serving as Interim Chief Executive during the intervening period.
The riliprubart decision arrived several months after that leadership change rather than before it, so it did not feature in the board’s reasoning at the time. It does, however, add to the pipeline pressure facing the new management as it works to restore confidence in Sanofi’s research engine. The drug had come into the company through its 2018 buyout of Bioverativ.
What happens next
Attention now turns to VITALIZE, which tests riliprubart in a different and arguably more favourable population of patients who are stable on IVIg maintenance. Its fate, and the broader future of Sanofi’s C1s programme, will be decided once the company has reviewed the MOBILIZE data in detail. For the complement inhibitor class as a whole, the more meaningful tests lie further ahead, when argenx and Dianthus report their own Phase 3 results and reveal whether a different target, a different design or a different patient population can succeed where MOBILIZE did not.
