You can participate in this study if you:

1. Willing and able to provide informed consent
2. 18 years of age at the time of signing the informed consent form
3. Pathologically documented histological or cytological evidence of NSCLC, HNSCC, or melanoma.
4. Has at least 1 measurable target lesion per RECIST v1.1 that has not been resected/biopsied/or irradiated before enrollment in the study
5. Diagnosis of locally advanced unresectable or metastatic NSCLC, HNSCC, or melanoma in subjects who are ICI treatment-naive or relapsed/refractory, including PD-1/PD-L1 inhibitors

1. ICI treatment-naive subjects must meet the following criteria:

a. NSCLC: Subjects with metastatic or with unresectable, recurrent NSCLC whose tumors must have no EGFR or ALK genomic aberrations and express PD-L1

b. HNSCC: Subjects with metastatic or with unresectable, recurrent HNSCC whose tumors express PD-L1

c. Melanoma: Irrespective of PD-L1 result and BRAF V600 mutation

d. Subjects has not received prior systemic treatment for their metastatic tumor. Subjects who received adjuvant or neoadjuvant therapy are eligible if the adjuvant/neoadjuvant therapy was completed at least 6 months before the development of metastatic disease.

1. ICI treatment-refractory subjects as defined by the following criteria:

a. Has received at least 2 cycles of anti-PD-(L)1 therapy either as monotherapy or in combination

b. Has demonstrated disease progression after ICI treatment by RECIST v1.1

c. Has received less than three lines of systemic therapy for metastatic tumor

1. ECOG performance status of 0 or 1
2. Be willing to provide archival tissue or fresh biopsy
3. Have adequate organ function
4. All Grade 3 or greater AEs resolved earlier to Grade 2 or less

You cannot participate in this study if you:

1. Cancer type and genomic tumor aberrations:

a. NSCLC subjects with EGFR or ALK genomic tumor aberrations

b. HNSCC subjects with nasopharyngeal cancer

1. For ICI refractory/relapsed subjects: Immune related AEs greater than or equal to Grade 3 that led to discontinuation of prior immune-modulatory agents including PD-1/PD-L1 inhibitors
2. With uncontrolled or untreated brain metastasis or leptomeningeal disease
3. Active autoimmune disease that has required systemic treatment in the past 2 years
4. Received a fecal transplant
5. Concurrent participation in another interventional clinical study or use of another investigational agent within 30 days of study consent
6. Contraindication to IV contrast that cannot be managed with pre-medication
7. Female subjects who are pregnant or breastfeeding
8. Male subjects who are unwilling or unable to use an acceptable method of birth control to avoid pregnancy
9. Has a known inability for oral intake of capsules
10. Has received a live vaccine within 4 weeks of start of the study treatment
11. Diagnosis of prior immunodeficiency or organ transplant requiring immunosuppressive therapy
12. Has received whole blood transfusion, blood component transfusion, or colony stimulating factors within 1 week prior to the 1st dose of study treatment
13. In the judgment of the investigator, subjects unlikely to comply with study procedures, restrictions and requirements
14. Has active interstitial lung disease (ILD)/pneumonitis or a history of ILD/pneumonitis requiring treatment with systemic steroids
15. Have allergy to clindamycin, erythromycin, and ampicillin
16. Has signs and symptoms of colitis at screening