The purpose of this study is to compare the pharmacokinetic (PK) similarity, safety, tolerability, immunogenicity, and efficacy of HLX15-SC versus US-DARZALEX FASPRO® following single and multiple subcutaneous (SC) injections in newly diagnosed MM patients ineligible for transplant.

Participants who meet all inclusion criteria and none of the exclusion criteria will receive either the HLX15-SC-Rd regimen or the D-Rd regimen for 4 cycles (one cycle = 4 weeks). After 4 cycles of treatment, based on clinical benefit and participant preference, participants may continue to receive the locally marketed daratumumab subcutaneous formulation (Dara-SC) in combination with Rd according to clinical practice, up to 32 weeks or until loss of clinical benefit, death, unacceptable toxicity, withdrawal of informed consent, or any other protocol-specified reason, whichever occurs first. After 32 weeks of dosing, participants will continue to receive appropriate standard of care according to local guidelines (including marketed Dara-SC).

Inclusion Criteria:

1. Age ≥ 18 years at the time of signing the informed consent form (ICF).

2. Body mass index (BMI): 18.5 kg/m2 ≤ BMI < 28 kg/m2.

3. Subjects must participate voluntarily, understand the study, and sign the ICF.

4. Patients must have a documented diagnosis of multiple myeloma (MM) according to the International Myeloma Working Group (IMWG) criteria, with measurable lesion .

5. Serum albumin ≥ 35 g/L.

6. Newly diagnosed, untreated, and considered ineligible for high-dose chemotherapy with autologous stem cell transplantation (ASCT) by the investigator.

7. The patient's ECOG performance status must be 0 or 1 .

8. Patient must have clinical laboratory values meeting the following criteria during the screening period:

   1. Hemoglobin ≥ 7.5 g/dL (≥ 5 mmol/L; red blood cell [RBC] transfusion or use of recombinant human erythropoietin at least 1 week prior to randomization is allowed).
   2. Absolute neutrophil count (ANC) ≥ 1.0 × 109/L (use of granulocyte-colony stimulating factor [G-CSF] is allowed).
   3. Alanine aminotransferase (ALT) ≤ 2.5 × upper limit of normal (ULN).
   4. Without the following evidence of impaired liver function, including mild impairment (total bilirubin ≤ ULN and AST > ULN or ULN < total bilirubin ≤ 1.5 x ULN), moderate impairment (1.5 x ULN < total bilirubin ≤ 3 x ULN), and severe impairment (total bilirubin > 3 x ULN).
   5. Measured creatinine clearance ≥ 40 mL/min .
   6. Corrected serum calcium < 14 mg/dL (< 3.5 mmol/L); or free ionized calcium < 6.5 mg/dL (< 1.6 mmol/L) .
   7. Platelet count ≥ 70 × 109/L for patients with plasma cells < 50% of bone marrow nucleated cells; platelet count > 50 × 109/L for all other patients (transfusion within 3 days prior to randomization to achieve the minimum platelet count is not permitted).

9. Contraceptive criteria: Contraceptive use by men or women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.

   1. Female patients: a female patient is eligible to participate if she is not pregnant or breastfeeding, and at least one of the following conditions applies:

      Not a woman of childbearing potential (WOCBP) or WOCBP: must commit to either abstain continuously from heterosexual sexual intercourse or to use 2 methods of reliable birth control simultaneously from signed ICF to at least 140 days following the last dose of study products. This includes one highly effective contraceptive method with a failure rate of < 1% per year (tubal ligation, intrauterine device, hormonal [birth control pills, injections, hormonal patches, vaginal rings or implants] or partner's vasectomy) and one additional effective contraceptive method (male latex or synthetic condom, diaphragm, or cervical cap). Reliable contraception is indicated even where there has been a history of infertility, unless due to hysterectomy or bilateral oophorectomy.

      The subjects also need to agree not to donate or cryopreservation eggs (ova, oocytes) from signed ICF to at least 140 days following the last dose of study products.

   2. Male patients: male patients are eligible to participate if they agree to the following during the intervention period and for at least 140 days following the last dose of study products:

   Be abstinent from heterosexual intercourse as their preferred and usual lifestyle (abstinent on a long-term and persistent basis) and agree to remain abstinent.

   or Agree to use a male condom and female partner to use an additional highly effective contraceptive method with a failure rate of <1% per year as when having sexual intercourse with a woman of childbearing potential who is not currently pregnant.

   Agree not to donate or cryopreservation sperm.

10. A WOCBP must have a negative serum pregnancy test at screening within 72 h prior to randomization. The investigator is responsible for review of medical history, menstrual history, and recent sexual activity to decrease the risk for inclusion of a woman with an early undetected pregnancy

Exclusion Criteria:

1. Patient has a diagnosis of primary amyloidosis, monoclonal gammopathy of undetermined significance (MGUS), smoldering multiple myeloma (SMM), Waldenström's disease, or other conditions in which IgM M-protein is present in the absence of a clonal plasma cell infiltration with lytic bone lesions.

2. Patient has plasma cell leukemia (according to IMWG criterion: ≥ 5% of plasma cells in the peripheral blood and/or an absolute plasma cell count of ≥ 2 x 109/L) or POEMS syndrome (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, and skin changes).

3. Patient has prior or current systemic therapy or ASCT for MM, with the exception of an emergency use of a short course (equivalent of dexamethasone 40 mg/day for a maximum 4 days) of corticosteroids before randomization.

4. Patient has peripheral neuropathy or neuropathic pain Grade 2 or higher, as defined by the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI-CTCAE) Version 6.

5. Patient has a history of malignancy (other than MM) within 3 years before the date of randomization (exceptions are squamous and basal cell carcinomas of the skin and carcinoma in situ of the cervix, or malignancy that in the opinion of the investigator is considered cured with minimal risk of recurrence within 3 years).

6. Patient has clinical signs of meningeal involvement of MM.

7. Patient has known chronic obstructive pulmonary disease (COPD) (defined as a forced expiratory volume in 1 second [FEV1] < 50% of predicted normal), persistent asthma, or a history of asthma within the last 2 years. Patient with known or suspected COPD or asthma must have a FEV1 test during screening.

8. Patient is known to be seropositive for history of human immunodeficiency virus (HIV) or known to have treponema pallidum antibodies (Anti-TP).

9. Patient is known to have active hepatitis B or C.

   1. Patient is seropositive for hepatitis B (defined by a positive test for hepatitis B surface antigen [HBsAg]). Patients with resolved infection (that is, patients who are HBsAg negative but positive for antibodies to hepatitis B core antigen [Anti-HBc] and/or antibodies to hepatitis B surface antigen [Anti-HBs]) must be screened using real-time polymerase chain reaction (PCR) measurement of hepatitis B virus (HBV) deoxyribonucleic acid (DNA) levels. Those who are PCR positive will be excluded.

      EXCEPTION: Patients with serologic findings suggestive of HBV vaccination (Anti-HBs positivity as the only serologic marker) AND a known history of prior HBV vaccination, do not need to be tested for HBV DNA by PCR.

   2. Patient is seropositive for hepatitis C must be screened using PCR measurement of hepatitis C virus (HCV) ribonucleic acid (RNA) levels. Those who are PCR positive will be excluded.

10. Patient has any concurrent medical or psychiatric condition or disease (e.g., active systemic infection, uncontrolled diabetes, acute diffuse infiltrative pulmonary disease) that is likely to interfere with the study procedures or results, or that in the opinion of the investigator, would constitute a hazard for participating in this study.

11. Patient has clinically significant cardiac disease, including:

    1. Myocardial infarction within 1 year before randomization, or an unstable or uncontrolled disease/condition related to or affecting cardiac function (e.g., unstable angina, congestive heart failure, New York Heart Association [NYHA] Class III-IV ).
    2. Cardiac arrhythmia (NCI-CTCAE Version 6 Grade ≥ 2) or clinically significant ECG abnormalities.
    3. Screening 12-lead ECG showing a baseline QT interval as corrected by Fridericia's formula (QTcF) (See Appendix 6) > 470 ms.

12. Patient has known allergies, hypersensitivity, or intolerance to lenalidomide, corticosteroids, monoclonal antibodies or human proteins, or their excipients or known sensitivity to mammalian-derived products.

13. Patient has history of drug abuse or substance abuse one year prior to randomization. Patient is known or suspected of not being able to comply with the study protocol (e.g., because of alcoholism, drug dependency, or psychological disorder).

14. Patient is a woman who is pregnant, or breast-feeding, or planning to become pregnant or donate eggs (ova, oocytes) while enrolled in this study or within 140 days after the last dose of study products.

    Or patient is a man who plans to father a child and/or donate sperm while enrolled in this study or within 140 days after the last dose of study products.

    Patient does not agree to abstain completely from sexual intercourse, or plan to use a contraceptive method that is not acceptable to the investigator (unacceptable methods of contraception include: i. periodic abstinence [such as calendar method, ovulation method, basal body temperature method, post-ovulation safety period method, etc.], withdrawal, etc.; ii. medical contraceptive measures such as oral contraceptives, contraceptive injections, contraceptive patches, subcutaneous implantation, intrauterine hormone contraceptive devices, local contraceptives such as spermicides, etc.).

15. Patient had radiation therapy within 14 days of randomization.

16. Patient had plasmapheresis within 28 days of randomization.

17. Patient had a history of blood donation or total blood loss of 200 mL or more within 3 months before randomization.

18. Patient had major surgery within 28 days before randomization or has not fully recovered from surgery, or has surgery planned during the time the patient is expected to participate in the study or within 28 days after the last dose of study treatment. Kyphoplasty is not considered major surgery.

19. Patient in clinical trials of any other drug or device within 3 months (or 5 half-lives of the corresponding investigational product if the half-life of the drug is long [5 half-lives > 3 months]) before randomization.

20. Patient has any condition for which, in the opinion of the investigator, participation would not be in the best interest of the patient (e.g., compromise the well-being) or that could prevent, limit, or confound the protocol-specified assessments