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e13062 Background: Metastatic triple negative breast cancer (mTNBC) is a highly invasive BC subtype with limited treatment options and poor clinical outcomes. Leronlimab, a humanized IgG4κ antibody, competitively inhibits CCR5, a cancer motility receptor and target for cancer inhibition. We report on a pooled analysis of n = 28 mTNBC patients (pts) showing that leronlimab has potent antitumor activity with improved 1 year progression free (PFS) & overall survival (OS) with few treatment emergent adverse events (TEAEs). Further, we explored the effect of leronlimab on circulating tumor associated cells (TACs) from peripheral blood as a surrogate and early predictor of drug response. Methods: mTNBC pts results from 3 blinded prospective clinical drug studies, Phase 1b/2 dose escalation (NCT03838367), Compassionate Use (NCT04313075), and Basket Study (NCT04504942) were pooled to evaluate leronlimab’s safety & efficacy at 12 months (mos). Pts received ≥1 dose of leronlimab alone (n = 2), with carboplatin (n = 11) or with physician’s choice (n = 15). Pts received 1-33 doses, ranging from 350mg (n = 9), 525mg (n = 16) or 700mg (n = 3). In addition, anonymized pt peripheral blood was procured before and after (̃30 days) induction as an exploratory biomarker, to evaluate TACs in predicting efficacy. Progressive disease, stable disease or partial response was determined by RESICT v1.1, and univariate analysis was used evaluate PFS & OS. Results: mTNBC pts were pooled from Phase 1b/2 (n = 10), Compassionate Use (n = 16), and Basket Study (n = 2) treated with 350-700mg doses, 4 pts escalating 350 to 525mg. Pts had 1-5+ prior systemic therapies for mTNBC (median = 2), median age 52 (range 33-84), ECOG 0 (n = 18) or ECOG 1 (n = 10), n = 17 had visceral mets, and n = 6 had brain mets. A total of n = 68 TEAEs were reported, with n = 7 grade I/II & n = 1 grade III related to leronlimab. At 12 mos, pts had a mPFS = 3.8 mos (95%CI 2.3-6.2) and mOS = 6.6 mos (CI95% 4.9-12+). However, pts treated with 525-700 mg doses (n = 19) had a > 75% improved mPFS = 6.1 mos (95%CI 2.3-7.5) and mOS 12+ mos (95%CI 5.5-12+). Further, a drop in circulating TACs was identified in 75% (n = 21/28) pts and predicted for significantly better clinical outcomes, mPFS = 6.2 and mOS > 12 mos. Conclusions: These studies suggest that mTNBC pts dosed with leronlimab had high clinical benefit, i.e. longer PFS & OS with few TEAEs, and leronlimab resulted in a drop in circulating TACs in the majority of pts correlating with early therapy response. Clinical trial information: NCT03838367, NCT04313075, NCT04504942.