Background: Immune checkpoint inhibitors (ICIs) have not yet benefited most patients with breast cancer. In mTNBC, ICIs combined with chemotherapy have improved survival only in PD-L1+ mTNBC. The optimal ICI combination agent to overcome primary resistance in PD-L1– mTNBC is unknown. One promising agent is the anti-Trop-2-SN-38 antibody drug conjugate (ADC) SG, which led to median progression-free survival (PFS) of 5.6 months in mTNBC with ≥2 prior lines of chemotherapy.¹ This ADC may boost anticancer immunity by binding immune cell receptors to promote antibody-dependent cellular cytotoxicity.² In addition, the SN-38 payload of SG is the active metabolite of irinotecan, which depletes regulatory T cells, upregulates MHC class I and PD-L1 expression, and augments the antitumor activity of anti-PD-1/L1 antibodies in murine tumor models.³ The irinotecan analogue camptothecin also enhances CD8+ cytotoxic T cell effector functions and antitumor immune responses by inhibiting NR4A transcription factors,⁴ which have recently been shown to play a central role in inducing the T cell dysfunction associated with chronic antigen stimulation in solid tumors. Methods: This is a multi-center 1:1 randomized phase II trial to investigate whether the addition of pembrolizumab (200 mg IV every 3 weeks) to SG (10 mg/kg IV days 1+8 every 21 days) improves PFS compared to SG alone in mTNBC that is PD-L1– by standard of care testing with 22C3 CPS < 10 or SP142 immune cells < 1% (NCT04468061). Key eligibility criteria include no prior systemic therapy for mTNBC and evaluable disease. Previously treated brain metastases are permitted. Exclusion criteria include prior treatment with SG, irinotecan, and PD-1/L1 inhibitors. Based on a sample size of 110 patients per arm, the trial has 80% power to detect a 3-month difference in median PFS from 5.5 months in the SG-alone cohort to 8.5 months in the SG + pembrolizumab cohort with a one-sided alpha of 0.1. Participants undergo mandatory baseline and on-treatment research biopsies if their disease is safely accessible. Tumor biopsies will be evaluated for Trop-2, immune cells, inhibitory checkpoints, transcriptomic signatures, and genomic alterations. Stool specimens will be submitted for microbiome analyses, and health-related quality of life will be assessed. The trial is currently open and enrolling patients. References: 1) Bardia A et al. Ann Oncol 31, S1142-S1215 (2020). 2) Cardillo TM et al. Bioconjug Chem 26, 919-931 (2015). 3) Iwai T et al. Oncotarget 9, 31411-31421 (2018). 4) Hibino S et al. Cancer Res 78, 3027-3040 (2018). Clinical trial information: NCT04468061.