A Study of XmAb®23104 in Subjects With Selected Advanced Solid Tumors (DUET-3)

Inclusion Criteria:

1. Subjects in Part A (dose escalation) must have a diagnosis of any of the following:

1. Histologically or cytologically confirmed advanced solid tumors, including the
following:

2. Melanoma (excluding uveal melanoma)

3. Cervical carcinoma

4. Pancreatic carcinoma

5. Breast carcinoma that is estrogen receptor, progesterone receptor, and Her2
negative (TNBC)

6. Hepatocellular carcinoma

7. Urothelial carcinoma

8. Squamous cell carcinoma of the head and neck (HNSCC)

9. Nasopharyngeal carcinoma (NPC)

10. Renal cell carcinoma

11. Colorectal carcinoma

12. Endometrial carcinoma

13. NSCLC

14. Small cell lung cancer

15. Gastric or gastroesophageal junction adenocarcinoma

2. Subjects in Part B (expansion) must have a diagnosis of any of the following:

Histologically or cytologically confirmed advanced solid tumors of the following
types:

1. Non-squamous NSCLC

2. TNBC

3. HNSCC

4. NPC

3. All subjects' cancer must have progressed after treatment with standard/approved
therapies or have no appropriate available therapies.

4. Subjects must have measurable disease by RECIST 1.1.

5. All subjects in Part B (dose expansion) must have a tumor lesion that can be biopsied
at acceptable risk (in the judgment of the Investigator) and must agree to both a
fresh biopsy during screening and a second biopsy following treatment.

6. Subjects have an ECOG performance status of 0-1.

Exclusion Criteria:

1. Treatment with any PDL1 or PDL2-directed therapy within 4 weeks of the start of study
drug

2. Prior treatment with an investigational anti-ICOS therapy

3. Treatment with nivolumab within 4 weeks of the start of study drug

4. Treatment with pembrolizumab within < 6 - 24 weeks prior to enrollment (cohort
dependent)

5. Treatment with any other anticancer therapy within 2 weeks of the start of study drug
(ie, other immunotherapy, chemotherapy, radiation therapy, etc.)

6. A life-threatening (Grade 4) IRAE related to prior immunotherapy

7. Failure to recover from any IRAE from prior cancer therapy to Grade ≤ 1

8. Failure to recover from any other toxicity (other than immune-related toxicity)
related to previous anticancer treatment to Grade ≤ 2

9. Active known or suspected autoimmune disease (except that subjects are permitted to
enroll if they have vitiligo; type 1 diabetes mellitus or residual hypothyroidism due
to an autoimmune condition that is treatable with hormone replacement therapy only;
psoriasis, atopic dermatitis, or another autoimmune skin condition that is managed
without systemic therapy; or arthritis that is managed without systemic therapy beyond
oral acetaminophen and non-steroidal anti-inflammatory drugs)

10. Receipt of an organ allograft

11. History or evidence of any other clinically unstable/uncontrolled disorder, condition,
or disease (including, but not limited to, cardiopulmonary, renal, metabolic,
hematologic or psychiatric) other than their primary malignancy, that in the opinion
of the Investigator would pose a risk to patient safety or interfere with study
evaluations, procedures, or completion

12. Treatment with antibiotics within 14 days prior to first dose of study drug