⁃ Participants with Solid Tumors

∙ Histopathologically confirmed diagnosis of an advanced, unresectable, or metastatic solid tumor (ovarian, pancreatic, urothelial, renal, hepatocellular, gastrointestinal (GI), lung, prostate, and breast cancer).

‣ Have progressed despite standard therapies, or for whom conventional therapy is not effective or tolerable, as judged by the Investigator. Patients must have no available therapeutic options known to confer clinical benefit for their tumor type.

⁃ Participants with Multiple Myeloma

∙ Have progressed despite standard therapies, or for whom conventional therapy is not effective or tolerable, as judged by the Investigator.

‣ Patients must have failed at least 3 prior therapies for myeloma and should have had prior exposure to a proteosome inhibitor, an IMiD, and an anti-CD38-directed therapy.

• 2\. Male or female participants ≥18 years of age at the time of informed consent 3. An Eastern Cooperative Oncology Group (ECOG) performance status scale of 0, 1, or 2 at Screening 4. Must have at least 1 measurable lesion by RECIST version 1.1 (solid tumors only), or evaluable disease by IMWG Uniform Response Criteria (multiple myeloma only) 5. Adequate organ function and bone marrow reserve 6. Adequate cardiac function as estimated by left ventricular ejection fraction 7. Female participants of child-bearing potential must:

• Have a negative serum pregnancy test at screening and a negative pregnancy test at Week 1 Day 1 prior to first dose of QXL138AM, AND

• Agree to use at least 1 highly effective method of contraception for the duration of study participation, and for 120 days after last dose of QXL138AM.

• 8\. Male participants of child-bearing potential must:

• Agree to use at least 1 highly effective method of contraception for the duration of study participation, and for 120 days after last dose of QXL138AM, AND

• Refrain from sperm donation prior to the first dose of investigational product through 120 days following the last dose of QXL138AM.