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.

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.

New York Heart Association Class III or IV cardiac disease, myocardial infarction within the past 6 months, unstable arrhythmia, a history of risk factors for Torsades de Pointes (TdP), including heart failure, hypokalemia, and family history of long QTc syndrome, or evidence of ischemia on ECG.

Symptomatic ischemic heart disease or unstable angina pectoris; or history of cardiac angioplasty, cardiac stenting, or coronary artery bypass graft. A clinically significant baseline prolongation of QT/QTcF interval at screening.

The use of concomitant medications that may significantly prolong the QT/QTc interval.

Active, uncontrolled bacterial, viral, or fungal infections requiring systemic therapy.

Known hypersensitivity to the investigational product or components (anti-CD138 IgG1 antibody, Interferon A2a and/or the formulation excipients: histidine, sucrose, arginine, polysorbate 80).

Female participant is lactating.

Any other clinically significant comorbidities.

Received prior anticancer therapy within 28 days or 5x the half-life (whichever is shorter) prior to the first dose of investigational product.

Participants who received wide-field radiation therapy within 4 weeks prior to first dose of investigational product, (2 weeks for limited field radiation therapy)

Major surgery within 30 days before first dose of investigational product

Chronic use of systemic corticosteroids of more than 20 mg/day of prednisone or equivalent.

Active, clinically significant liver disease such as Hepatitis B or C, autoimmune hepatitis, or cirrhosis (Child Hugh Stage B or C).

Current or history of mood disorder such as major depression per DSM-5 within past two years not controlled with current therapy.

Active autoimmune disorders not controlled with current therapy.

Active endocrine disorders including hypothyroidism, hyperthyroidism, hypoglycemia, hyperglycemia, and diabetes mellitus not controlled with current therapy.