Background: Identifying new therapeutic options for patients (pts) with high grade serous ovarian cancer (HGSC) and triple negative breast cancer (TNBC) is a priority. For pts with pathogenic variants in BRCA, the PARP inhibitor (PARPi) olaparib is used to treat TNBC and as maintenance for HGSC. PARPi can induce a senescence-like phenotype in cancer cells with cell survival dependent on anti-apoptotic proteins. The intrinsic apoptotic pathway is regulated by the Bcl-2 family composed of anti-apoptotic proteins (including Bcl-2, Bcl-xL), pro-apoptotic effectors and the pro-apoptotic BH3-only proteins. Increased expression of Bcl-xL occurs in 88% of HGSC. Olaparib and navitoclax, a Bcl-2/Bcl-xL inhibitor, are synergistic in pre clinical models of HGSC and TNBC. Exactis- 03 investigates targeting, olaparib-induced sensecence as a treatment strategy for pts with HGSC and TNBC. Methods: Exactis-03 is a multi-centre phase I trial determining if olaparib can be safely combined with navitoclax in pts with TNBC who have somatic or germline mutations in BRCA1/2 or PALB2 and pts with recurrent HGSC who have progressed ≥ 6 months since their last platinum based chemotherapy. Additional eligibility criteria include : ≥3 prior lines of treatment for TNBC (no limit for HGSC); for pts with HGSC prior PARPi is allowed provided there was no progression on or ≤6 months since discontinuation of the PARPi; ECOG PS ≤2; ability to absorb study medication and pts must be willing and able to undergo study related procedures. The primary endpoint is identification of the recommended phase II dose (RP2D) of olaparib combined with navitoclax. Olaparib 200mg bid is administered alone for the first 2 weeks. Tumor biopsies will be performed at baseline and on day 7-12 (prior to navitoclax) to evaluate senescence and apoptosis biomarkers and to create 3D organoids and ex vivo microdissected tumor (MDT) models for functional assessment of drug response. Navitoclax is dose escalated with a fixed dose of olaparib in 28-day cycles with 3 pts initially treated at each dose level. If no dose limiting toxicity (DLT) is observed, the dose level will be escalated until ≥ 1/3 or ≥ 2/6 patients experience DLT. The RP2D will be defined as the dose level below the one where ≥ 1/3 or ≥ 2/6 patients experience DLT. The 2-week lead-in with olaparib alone and one full cycle of the combination (navitoclax/olaparib) must be completed before dose escalation is permitted. Blood samples are collected for pharmacokinetics and serial evaluation of plasma biomarkers. There is an optional tumor biopsy on progression. Exploratory objectives include determining levels of pro- and anti- apoptotic proteins in biopsy samples and evaluation of cell fate decision biomarkers in biopsy tissue and blood (senescence secretome). Patient-derived organoids and ex vivo MDT will be used to explore drug response and sequencing (NCT05358639). Clinical trial information: NCT05358639.