This is an open-label, multicenter phase 1 clinical trial to evaluate the safety and tolerability, efficacy, and pharmacokinetics of SIM0500 in adult participants with Relapsed or Refractory Multiple Myeloma(RRMM). The trial is consisted of two parts, Part 1 (dose escalation) and Part 2 (dose optimization). In both parts, SIM0500 will be administered until disease progression, intolerable toxicity, withdraw of consent or end of trial.

Inclusion Criteria:

1. Voluntary participation and signature of informed consent form.
2. ≥18 years of age.
3. Have documented diagnosis of relapsed or refractory multiple myeloma according to Criteria for Response to Multiple Myeloma Treatment(IMWG)diagnostic criteria who have failed all established standard of care.
4. Life expectancy ≥12 weeks.
5. Eastern Cooperative Oncology Group (ECOG) score of 0 or 1.
6. Adequate hematologic, hepatic, and renal function.

Exclusion Criteria:

1. Toxicities from previous anticancer therapies should have resolved to baseline levels or to Grade 1 or less except for alopecia or peripheral neuropathy.
2. Active hepatitis B (HBsAg positive and HBV DNA ≥ 1×104 copies/mL or ≥ 2,000 international unit [IU]/mL) or hepatitis C (HCV antibody positive and HCV RNA ≥ ULN) infection; participant with HBsAg positive or detective HBV-DNA at screening should receive antiviral treatment as per local practice during the trial.
3. Known human immunodeficiency virus (HIV) infection or acquired immunodeficiency syndrome (AIDS).
4. Participant is pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the trial.
5. Active known or suspected autoimmune disease. Participants with vitiligo, residual hypothyroidism only requiring hormone replacement, psoriasis not requiring systemic treatment or conditions not expected to recur in the absence of an external trigger, type 1 diabetes mellitus (blood glucose can be controlled by insulin therapy) can be included.
6. Current or previous other malignancy within 3 years of study entry, except basal or squamous cell skin cancer, superficial bladder cancer, or carcinoma in situ of the prostate, cervix or breast.
7. Known active central nervous system (CNS) involvement or exhibits clinical signs of meningeal involvement of multiple myeloma.
8. Participants with known active infection within 14 days prior to the first SIM0500.