Background: Immunotherapy has improved clinical outcomes in esophageal carcinoma; however, patients with advanced esophageal cancer that progresses after first-line chemo continue to have limited treatment options and poor prognosis. KEYMAKER-U06 substudy 06A (NCT05342636) is evaluating pembro (anti–PD-1) + investigator choice of chemo (irinotecan or paclitaxel), MK-4280A, a coformulation of pembro and favezelimab (anti-LAG3) + chemo, pembro + MK-4830 (anti-ILT4) + chemo, and pembro + MK-4830 + lenvatinib (multitargeted RTK inhibitor) for the second-line treatment of patients with PD-1/L1 treatment-naive advanced esophageal squamous cell carcinoma (ESCC). Methods: KEYMAKER-U06 substudy 06A is a phase 1/2, multicenter, open-label study comprising a safety lead-in phase and an efficacy phase. Adults with histologically or cytologically confirmed metastatic or locally advanced ESCC, who experienced disease progression on 1 prior line of therapy, have not received anti-PD-1/L1 therapy, any immune-modulating therapy, and/or VEGF targeted therapy, have measurable disease per RECIST v1.1 confirmed by blinded independent central review (BICR), and have an Eastern Cooperative Oncology Group performance status of 0 or 1 are eligible. Patients are being allocated to 1 of 4 treatment arms: pembro 200 mg IV Q3W + investigator choice of chemo (paclitaxel 80-100 mg/m² IV days 1, 8, and 15 of every 28-day cycle or irinotecan 180 mg/m² day 1 of every 14-day cycle) (arm 1), MK-4280A (pembro 200 mg/favezelimab 800 mg IV day 1 then Q3W) + investigator choice of chemo (arm 2), pembro 200 mg IV Q3W + MK-4830 800 mg IV Q3W + investigator choice of chemo (arm 3), or pembro 200 mg IV Q3W + MK-4830 800 mg IV Q3W + lenvatinib 20 mg orally once daily (arm 4). Arms 2-4 will have a safety lead-in phase; arm 1 will not have a safety lead-in phase because the safety and tolerability of pembro + chemo has been established in multiple phase 3 studies in the first-line treatment setting. The safety lead-in phase includes 10 patients who will be closely monitored for dose-limiting toxicities (DLTs) for 21 days after the first dose of study intervention. If ≥4 patients experience DLTs in any treatment arm, then enrollment in the efficacy phase may be delayed to allow for examination of safety data and to consider design changes. If ≤3 patients experience DLTs, up to 30 patients per arm will be enrolled (inclusive of the 10 patients from the safety lead-in) for efficacy assessment. In the safety lead-in phase, the primary end point is safety/tolerability assessed by DLTs, adverse events (AEs), and discontinuation of treatment due to AEs. In the efficacy phase, the primary end point is ORR per RECIST v1.1 by BICR, and secondary end points are PFS and DOR, per RECIST v1.1 by BICR, and OS and safety/tolerability. Enrollment in this study is ongoing. Clinical trial information: NCT05342636.