Inclusion Criteria:

1. Aged ≥ 18 years at the time of signing the ICF.
2. Willing and able to give written informed consent and adhere to protocol requirements.
3. Patient has a history of multiple myeloma with relapsed and refractory disease as defined by the protocol.
4. Patients must have measurable disease (as determined by the local laboratory) as defined by the protocol.
5. Performance status of 0 to 2 based on the Eastern Cooperative Oncology Group (ECOG) performance scale.
6. Estimated life expectancy of 12 weeks or longer.
7. Prior palliative radiotherapy must have been completed at least 14 days prior to dosing on Cycle 1 Day 1.
8. Toxicities related to prior study therapy should have resolved to Grade 1 or less according to criteria of NCI CTCAE v5.0, except for alopecia. Patients with chronic but stable Grade 2 toxicities may be allowed to enroll after an agreement between the Investigator and Sponsor.
9. Have adequate liver and kidney function and hematological parameters within a normal range as defined by the protocol.

Exclusion Criteria:

1. Patient has symptomatic central nervous system involvement of MM.
2. Patient has nonsecretory MM, plasma cell leukemia, Waldenstrom's macroglobulinemia, POEMS syndrome (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, and skin changes), or amyloidosis.
3. Patient had a prior autologous stem cell transplant ≤ 3 months prior to first dose of study drug on Cycle 1 Day 1.
4. Patient had a prior allogeneic stem cell transplant with either standard or reduced intensity conditioning ≤ 6 months prior first dose of study drug on Cycle 1 Day 1 or is on systemic immunosuppression for graft-versus-host disease.
5. Patients with concomitant second malignancies (Except adequately treated non-melanomatous skin cancers, ductal carcinoma in situ, superficial bladder cancer, prostate cancer, Grade 1 stage 1A/1B endometrioid endometrial cancer or cervical cancer in situ) are excluded unless in complete remission three years prior to study entry, and no additional therapy is required or anticipated to be required during study participation.
6. Patients with any active autoimmune disease or a history of known or suspected autoimmune disease, or history of a syndrome that requires systemic corticosteroids treatment or immunosuppressive medications, except for patients with vitiligo, resolved childhood asthma/atopy or autoimmune thyroid disorders on stable thyroid hormone supplementation.
7. A serious uncontrolled medical disorder that would impair the ability of the patient to receive protocol therapy or whose control may be jeopardized by the complications of this therapy.
8. Treatment with systemic antiviral, antibacterial or antifungal agents for acute infection within ≤ 7 days of first dose of study drug on Cycle 1 Day 1.
9. Patient has active peripheral neuropathy or neuropathic pain Grade 2 or higher, as defined by the NCI-CTCAE v5.0.
10. Diagnosed with HIV, Hepatitis B, or Hepatitis C infection.
11. Treatment with non-oncology vaccines for the control of infectious diseases (i.e., HPV vaccine) within 28 days of first dose of study drug on Cycle 1 Day 1.
12. Active SARS-CoV-2 infection based on positive SARS-CoV-2 test within 4 weeks prior to enrollment or patients with suspected active infection based on clinical features or pending results.
13. Has received immunosuppressive medications including but not limited to CellCept, methotrexate, infliximab, anakinra, tocilizumab, cyclosporine, or corticosteroids (≥ 10 mg/day of prednisone or equivalent), within 28 days of first dose of study drug on Cycle 1 Day 1.
14. Patient has history of drug-related anaphylactic reactions to any components of CLN-619. History of Grade 4 anaphylactic reaction to any monoclonal antibody therapy.
15. Certain treatment with investigational agents and other anti-neoplastic therapy as defined by the protocol
16. Female of child-bearing potential (FOCBP) who is pregnant or breast-feeding, plans to become pregnant within 120 days of last study drug administration or declines to use an acceptable method to prevent pregnancy during study treatment and for 120 days after the last dose of study drug administration.
17. Male patients who plans to father a child or donate sperm within 120 days or 5 half-lives of CLN-619, whichever comes later, of last study drug administration, or who has a partner who is a FOCBP, and declines to use an acceptable method to prevent pregnancy during study treatment and for 120 days or 5 half-lives of CLN-619, whichever comes later, after the last dose of study drug administration.