The purpose of this Phase 3 study is to demonstrate the efficacy of claseprubart (DNTH103) as compared to placebo in participants with chronic inflammatory demyelinating polyneuropathy (CIDP).

The study includes the following periods:

• Part A: An open-label period (up to 13 weeks)
• Part B: A randomized, placebo-controlled, double-blind treatment period (up to 52 weeks) for participants who respond to DNTH103 in Part A
• Optional open-label extension (OLE) for eligible participants (up to 104 weeks)
• Safety follow-up (40 weeks)

Inclusion Criteria:

1. Must have given written informed consent before any study-related activities are carried out.

2. Weight range between 40 kilograms (kg) and 120 kg.

3. Confirmed diagnosis of CIDP or possible CIDP. Participants must have either typical CIDP or one of the following variants: motor or multifocal CIDP. Diagnosis must be confirmed by the Independent CIDP Review Panel.

4. CIDP Disease Activity Status (CDAS) score ≥ 3 at screening.

5. Must be neurologically stable.

6. Must have an INCAT score between 2 and 9 inclusive.

7. Must fulfill one of the following treatment conditions for CIDP:

   1. Currently treated with and responded to immunoglobulin (Ig) (intravenous immunoglobulin [IVIg] or subcutaneous immunoglobulin [SCIg]) alone or Ig (IVIg or SCIg) plus oral corticosteroids, or previously treated with and responded to, but are no longer being treated with (eg, lost access to), a maintenance regimen of Ig (IVIg or SCIg) alone or Ig (IVIg or SCIg) plus oral corticosteroids.
   2. Currently treated with and responded to oral corticosteroids alone or oral corticosteroids in combination with azathioprine or mycophenolate mofetil.
   3. Refractory participants who have had treatment failure (worsening) or an inadequate response to Ig and/or oral corticosteroids (defined as no clinically meaningful improvement after a period of a minimum of 12 weeks, which may include both active treatment and observation to assess response), or who at any time were unable to tolerate these treatments, experienced adverse effects, or have documented contraindications.
   4. Treatment naïve with no history of prior treatment for CIDP.

8. Documented vaccinations against encapsulated bacteria in accordance with local requirements and vaccine availability.

9. Female participants must be of nonchildbearing potential or if of childbearing potential, must agree not to donate ova, not to attempt to become pregnant and, if engaging in sexual intercourse with a male partner, must agree to use a highly effective method of contraception.

10. Male participants must agree not to donate sperm and, if engaging in sexual intercourse with a female partner who could become pregnant, must agree to use an acceptable method of contraception or be surgically sterile for at least 90 days prior to Screening.

Exclusion Criteria:

1. Clinical signs or symptoms suggestive of polyneuropathy of causes other than CIDP.
2. Known evidence of central demyelination or known history of myelopathy.
3. History or presence of significant medical/surgical condition including any acute illness or major surgery considered to be clinically significant or that could have a potential impact on safety/efficacy or study procedures.
4. Any other condition, including mental illness or prior therapy that would make the participant unsuitable for this study.
5. Known complement deficiency or history of positive titer for anti-C1 antibodies.
6. Diagnosis of systemic lupus erythematosus (SLE) or family history of SLE (defined as a parent, sibling, or child).
7. Participants with an autoimmune disease affecting joints, muscle or nervous system.
8. Any coexisting or overlapping condition, which may interfere with outcome assessments, such as severe diabetic neuropathy, fibromyalgia, inflammatory arthritis or osteoarthritis affecting the hands and feet.
9. Prior history of N. meningitidis infection.
10. History of active malignancy within 5 years prior to screening, except basal cell carcinoma of the skin, curatively resected squamous cell carcinoma of the skin, cervical carcinoma in situ curatively treated or low-grade prostate adenocarcinoma for which appropriate management is observation alone.
11. Positive test results for active human immunodeficiency virus (HIV-1 or HIV-2), hepatitis B surface antigen (HBsAg), or hepatitis C virus (HCV) antibodies.