Melanoma Checkpoint and Gut Microbiome Alteration With Microbiome Intervention

This is a Phase 1b, multicenter, randomized, placebo-controlled, blinded study in adult
participants with anti-PD-1 therapy naïve, unresectable or metastatic melanoma to evaluate
the safety and tolerability of SER-401, or matching placebo in combination with anti-PD-1
therapy (nivolumab). Prior to initiating microbiome study intervention and nivolumab,
participants will undergo an antibiotic or antibiotic placebo treatment lead-in to prime the
gut microbiome for engraftment of the oral microbiome study intervention. Study intervention
groups will be assessed for safety, changes in the microbiome, changes in the percentage of
tumoral CD8 T cells, and antitumor activity. Participants must have measurable disease that
can be biopsied and consent to baseline and on-treatment biopsies, as well as stool and blood
biomarker collection throughout the study.

Inclusion Criteria:

1. Participant must be willing to provide a baseline stool sample.

2. Histologically-confirmed Stage IV cutaneous melanoma or Stage III cutaneous, acral or
mucosal melanoma that is judged inoperable. Participants with a history of uveal
melanoma are not eligible.

3. Measurable disease as defined by Response Evaluation Criteria in Solid Tumors version
1.1 (RECIST v1.1; ie, defined as at least 1 lesion that can be accurately measured in
at least 1 dimension [longest diameter to be recorded] with a minimum size of ≥ 10 mm
by computerized tomography [CT] scan or caliper measurement on clinical exam or ≥ 20
mm by chest X-ray).

1. Malignant lymph nodes must be ≥ 15 mm in short axis when assessed by CT scan to
be considered pathologically enlarged and measurable.

2. Participants must have at least one measurable lesion by RECIST and a separate
lesion amenable to biopsy that has not been previously irradiated.

i. Participants must be willing to undergo a newly-obtained core needle or incisional
biopsy at baseline (prior to antibiotic or antibiotic placebo administration). Fine
needle aspiration is not acceptable.

4. Participants must be willing to undergo tumor biopsy on treatment.

5. Prior adjuvant or neoadjuvant melanoma therapy is permitted if completed at least 6
weeks prior to randomization and all related AEs have either returned to baseline or
stabilized.

1. Prior anti-CTLA-4 therapy in the adjuvant setting is allowed if completed at
least 12 weeks prior to the first dose of anti-PD-1.

2. Prior BRAF-targeted therapy in the neoadjuvant, adjuvant, or metastatic setting
is allowed if completed at least 4 weeks prior to the first dose of anti-PD-1.

Exclusion Criteria:

1. Participants who require hemodialysis.

2. Participants with a history of another cancer in the last 5 years, except for: a)
curatively resected non-melanoma skin cancer; b) curatively treated cervical carcinoma
in situ; c) localized prostate cancer not requiring systemic therapy; and c) other
primary tumors with no known active disease present that, in the opinion of the
Investigator and the Sponsor, will not affect participant outcome in the setting of
the current diagnosis.

3. Any known, untreated brain metastases. Participants with brain metastases are eligible
if these have been treated, and provided:

1. Brain metastases must be stable (image-documented) 4 weeks after completion of
treatment for brain metastases and require treatment with less than 10 mg/day
prednisone equivalent for at least 2 weeks prior to study intervention
administration.

2. Neurological symptoms should be absent or returned to baseline.

4. Prior checkpoint inhibitor therapy with anti-PD-1 or anti-PD-L1 in the adjuvant
setting.

5. Prior systemic treatment (ie, anticancer chemotherapy, immunotherapy, or
investigational agents) for unresectable or metastatic melanoma.

a. Prior BRAF-targeted therapy (ie, BRAF or BRAF-MEK) in the metastatic setting is
allowed if completed at least 4 weeks prior to the first dose of anti-PD-1.

6. History of active inflammatory bowel disease (eg, active Crohn's disease or ulcerative
colitis) with diarrhea OR major gastrointestinal surgery (not including appendectomy
or cholecystectomy) within 3 months of enrollment (ie, signed informed consent for the
study), OR any history of total colectomy or bariatric surgery (bariatric surgery
which does not disrupt the gastrointestinal lumen, ie, restrictive procedures such as
banding, are permitted).

7. Any diagnosis of autoimmune disease. Participants with Type I diabetes mellitus,
hypothyroidism only requiring hormone replacement, adrenal insufficiency on
replacement dose steroids, skin disorders (such as vitiligo, psoriasis or alopecia)
not requiring systemic treatment, or conditions not expected to recur in the absence
of an external trigger are permitted to enroll.

a. Participants with controlled Type 1 diabetes mellitus on a stable insulin regimen
may be eligible.

8. Has a condition requiring systemic treatment with either corticosteroids (> 10 mg
daily prednisone equivalents) or other immunosuppressive medications within 14 days of
study intervention administration or has a contrast allergy requiring premedication
with corticosteroids. Inhaled or topical steroids, and adrenal replacement doses > 10
mg daily prednisone equivalents are permitted in the absence of active autoimmune
disease.

9. Has a transplanted organ or has undergone allogeneic bone marrow transplant.