The purpose of the study is to evaluate efficacy of riliprubart compared to placebo in adult participants with CIDP whose disease is refractory to standard of care. The study duration will be for a maximum of 111 weeks including screening, treatment phases, and follow-up.

Inclusion Criteria:

Participants are eligible to be included in the study only if all of the following criteria apply:

• Participant must have CIDP or possible CIDP criteria, based on European Academy of Neurology (EAN)/ Peripheral Nerve Society (PNS) Task Force CIDP guidelines, second revision (2021).

• Participant must have either typical CIDP, or one of the following two CIDP variants: motor CIDP, multifocal CIDP (also known as Lewis Sumner Syndrome). Diagnosis must be confirmed by the adjudication committee.

• Participant must be refractory to either immunoglobulin therapy or corticosteroid therapy, as defined below.

  • Immunoglobulin-refractory subgroup: Historic evidence of failure or inadequate response to immunoglobulin therapy prior to screening
  • Corticosteroid-refractory subgroup: Historic evidence of failure or inadequate response to corticosteroid therapy prior to screening

• Participant has an INCAT score of 2 to 9

• Any allowed immunosuppressant drugs (azathioprine, cyclosporine, or mycophenolate mofetil) have been taken for ≥6 months

• Participant may be receiving low-dose oral corticosteroids (≤20 mg/day of prednisone or equivalent)

• Participant must have active disease, defined by a CIDP disease activity score (CDAS) of ≥ 2 points at Screening

• Participant must have documented vaccinations against encapsulated bacterial pathogens given within 5 years prior to Day 1 or initiated a minimum of 14 days prior to first dose of study intervention

• Contraception for sexually active male or female participants; not pregnant or breastfeeding; no sperm donating for male participant

• A body weight at Screening of 35 kg to 154 kg (77 to 340 lbs), inclusive

Exclusion Criteria:

Participants are excluded from the study if any of the following criteria apply:

• Polyneuropathy of other causes, including but not limited to: acute demyelinating polyneuropathies (eg, Guillain-Barré syndrome), hereditary demyelinating neuropathies, neuropathies secondary to infection or systemic disease, diabetic neuropathy, drug- or toxin-induced neuropathies, multifocal motor neuropathy, polyneuropathy related to Immunoglobulin M (IgM) monoclonal gammopathy, POEMS syndrome, and lumbosacral radiculoplexus neuropathy.

• Sensory CIDP, Distal CIDP and focal CIDP variants.

• Any other neurological or systemic disease that can cause symptoms and signs interfering with treatment or outcome assessments

• Poorly controlled diabetes

• Serious infections requiring hospitalization within 30 days prior to Screening and any active infection requiring antimicrobial treatment during screening or presence of a condition that may predispose the participant to increased risk of infection (eg, medical history such as known immunodeficiency or history of recurrent infections)

• Clinical diagnosis of Systemic Lupus Erythematosus (SLE) or family history of SLE. For a participant with an antinuclear antibody (ANA) titer ≥1:160 and a positive anti-double-stranded DNA (anti-dsDNA) at Screening, SLE diagnosis must be ruled out prior to enrollment.

• Sensitivity to any of the study interventions, or components thereof, or drug or other allergy that, in the opinion of the Investigator, contraindicates participation in the study. Specifically, history of any hypersensitivity reaction to riliprubart or its components or of a severe allergic or anaphylactic reaction to any humanized or murine monoclonal antibody.

• Any other clinically meaningful medical history or ongoing medical condition (as determined by the Investigator at Screening) that might impact benefit-risk assessment, jeopardize the safety of the participant, or compromise the quality of the data collected in this study; or history or presence of other significant concomitant illness that would adversely affect participation in this study, per Investigator's judgment.

• Documented history of attempted suicide over the 6 months prior to the Screening visit, presence of suicidal ideation of category 4 or 5 on C-SSRS during screening, OR if in the Investigator's judgment, the participant is at risk for a suicide attempt.

• Evidence of CIDP worsening within the 6 weeks following a prior vaccination that, in the opinion of the Investigator, constituted a relapse

• Recent or planned major surgery that could confound the results of the trial or put the participant at undue risk

• Participant has recently received immunoglobulins (IVIg or SCIg)

• Recent treatment with plasma exchange

• Prior treatment with riliprubart

• Prior treatment with (any time) with highly immunosuppressive/chemotherapeutic medications with sustained effects, eg, mitoxantrone, alemtuzumab, cladribine

• Prior treatment (any time) with total lymphoid irradiation or bone marrow transplantation

• Prior treatment with B-cell-depleting agents such as rituximab within 6 months

• Use of any specific complement system inhibitor (eg, eculizumab) within 12 weeks or 5 times the half-life of the product, whichever is longer

• Treatment within 6 months prior to dosing with immunosuppressive/ chemotherapeutic medications, such as cyclophosphamide, methotrexate, tacrolimus, interferon, or tumor necrosis factor (TNF)-α inhibitors. Certain immunosuppressants commonly used in CIDP (azathioprine, cyclosporine, or mycophenolate mofetil) are allowed, as indicated under inclusion criterion.

• Any vaccination received within 28 days prior to dosing (with few exceptions to be confirmed at screening)

• Participation in another clinical trial with an investigational drug or receipt of an investigational product within 12 weeks or 5 times the half-life of the product, whichever is longer, prior to Screening

• Any screening laboratory values outside normal limits or abnormal ECG considered in the Investigator's judgment to be clinically significant in the context of this trial

• Positive result of any of the following tests:

  • Hepatitis B surface antigen (HBsAg).
  • Anti-hepatitis B core antibodies (anti-HBc Ab) (unless anti-hepatitis B surface antibodies [anti-HBs Ab] are also positive, indicating natural immunity).
  • Anti-hepatitis C virus (anti-HCV) antibodies (participants with positive hepatitis C antibody due to prior resolved disease can be enrolled, only if a confirmatory negative Hepatitis RNA test is obtained).
  • Anti-human immunodeficiency virus 1 and 2 (anti-HIV1 and anti-HIV2) antibodies.

• Pregnancy, defined as a positive result of a highly sensitive urine or serum pregnancy test, or lactation

• Accommodation in an institution because of regulatory or legal order; eg, imprisoned or legally institutionalized

• Participant not suitable for participation, whatever the reason, as judged by the Investigator, including medical or clinical conditions, or potential risk for noncompliance to study procedures

• Participants are employees at the clinical study site or other individuals directly involved in the conduct of the study, or immediate family member of such individuals

• Any country related specific regulation that would prevent the participant from entering the study

• Recent treatment with efgartigimod

The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.

Qualified researchers may request access to patient level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi's data sharing criteria, eligible studies, and process for requesting access can be found at: https://vivli.org