Sanofi has stopped a phase 3 autoimmune clinical trial early after an interim analysis found the therapy was “unlikely to provide sufficient efficacy.” The setback eliminates one opportunity for a key late-phase program and continues the “bumpy ride” for the French pharma.
Paris-based Sanofi started two phase 3 studies of riliprubart, an antibody that blocks C1s in the classical complement pathway, in 2024 based on midphase data suggesting the drug candidate could become a subcutaneous alternative to intravenous immunoglobulin. The trials enrolled two different populations of chronic inflammatory demyelinating polyneuropathy (CIDP) patients.
The drugmaker had planned to compare riliprubart to placebo over 24 weeks in about 140 patients who had been failed by immunoglobulin or corticosteroid therapy. Instead, Sanofi announced June 10 that it had ended the trial early in response to the independent data monitoring committee’s analysis of interim data.
The company said it is assessing whether to continue the other phase 3 trial, which is comparing riliprubart to intravenous immunoglobulin in people who responded to the standard of care and are receiving a stable maintenance dose.
At Week 24 of Sanofi’s phase 2 study, 87% of standard-of-care patients improved or remained stable after switching to riliprubart. But the drug appeared similarly effective in patients failed by standard of care in the midphase study, only for the phase 3 trial in that population to fail to generate evidence to support further development.
The update is a blow for an asset that Sanofi lists among its key mid- and late-stage programs. Looking past a provisionally quiet 2026 for Sanofi’s pipeline, Guggenheim Securities analysts named riliprubart as one of the assets with readouts that could add value in 2027. The analysts, who made the comment in a note to investors last year, estimated 2033 riliprubart sales of 480 million euros ($555 million).
Sanofi’s phase 3 riliprubart trials were initially due to deliver data this year, but the drugmaker pushed back the readouts in response to slow enrollment. Having adapted the screening process, Sanofi saw the enrollment rate pick up and was “optimistic about the impact riliprubart can have on patient outcomes,” Houman Ashrafian, Ph.D., head of R&D at the company, said on a first-quarter earnings call in April.
The trouble with riliprubart continues a run of clinical setbacks for Sanofi that ultimately led to CEO Paul Hudson’s ouster from the pharma in February, weeks after admitting that 2025 had been a “bumpy ride” for the company. That ride included the anti-OX40L-ligand antibody amlitelimab flunking a phase 2 asthma trial, the oral TNF inhibitor balinatunfib missing the goal of a midstage psoriasis study and the Regeneron-partnered IL-33 candidate itepekimab failing one of a pair of phase 3 studies in chronic obstructive pulmonary disease.
The implications of the setback to riliprubart extend beyond Sanofi. Immunoglobulin manufacturers such as CSL Behring, Grifols and Pfizer could benefit if riliprubart flops. Argenx, which has already won FDA approval for its FcRn inhibitor Vyvgart Hytrulo in CIDP, is running phase 3 trials of a C2 inhibitor in the indication.
Dianthus Therapeutics has a C1 inhibitor, called claseprubart, in phase 3. The biotech has cited Sanofi’s phase 2 data as validation of the potential of C1 inhibitors in CIDP, while also arguing that the response rates seen in an open-label claseprubart trial support the hypothesis that its asset is more efficacious than riliprubart.
