Project Summary The long-term objective of this project is to identify new, more effective, and less toxic therapeutic approaches for triple-negative breast cancer (TNBC), the most aggressive and poor-prognosis breast cancer subtype. Our team recently led the clinical development of the first antibody-drug conjugate (ADC) for metastatic TNBC (mTNBC), Sacituzumab Govitecan (SG, aka Trodelvy), achieving dramatically improved objective response rates (ORRs), progression-free survival (PFS), and overall survival (OS), and resulting in accelerated FDA approval in 2020. SG comprises the topoisomerase 1 (TOP1) inhibitor SN-38 (the active metabolite of irinotecan) coupled to a humanized monoclonal antibody targeting Trop-2, a tumor antigen expressed in >90% of mTNBC. While it represents a paradigm-changing therapy, only approximately 30% of mTNBC patients experience a therapeutic response to SG, highlighting the need to identify combination therapies with SG that will uniquely complement and enhance its efficacy. Our preliminary data lead to the hypothesis that PARP inhibition (PARPi) is synergistic with SG in mTNBC. Accordingly, we are carrying out a funded investigator- initiated phase 1b/2 clinical trial (NCT04039230) of SG and PARPi (talazoparib) for mTNBC, notably delivered via a sequential dosing schedule to minimize toxicity and improve the therapeutic window. Here, our team of clinical, translational, and basic science investigators seeks to move forward the rational therapeutic use of SG and SG/PARPi, and to discover new combinatorial therapies incorporating SG. Our aims are: i) to establish the association of therapeutic response with pre-treatment and pharmacodynamic markers of DNA damage and repair with SG/talazoparib versus SG alone for mTNBC through analysis of clinical trial and other patient samples; ii) to determine mechanisms of resistance to SG monotherapy and SG/PARPi through CRISPR screens and analysis of post-progression patient samples, and to test select druggable targets to overcome them; and iii) to optimize drug scheduling and in vivo efficacy for novel combinations to overcome SG/PARPi resistance. Collectively, these studies will enable and inform the next generation of mechanism-based therapeutic trials investigating SG-based combinatorial therapy for patients with mTNBC.