Abstract 3016: Enhancing CX-5461 therapy in TNBC through liposomal delivery and ATR inhibitor combination

Abstract Background: Breast cancer (BC) is the second leading cause of cancer death in women. Triple-negative breast cancers (TNBCs), an aggressive subtype, account for 15-20% of cases and disproportionately contribute to BC mortality. For patients with metastatic TNBC, the current standard of care is chemotherapy; although initially effective, overall survival remains poor. Thus, novel therapeutic strategies are urgently needed to improve TNBC outcomes. Aims: We aim to evaluate the novel DNA-damaging agent CX-5461 (Pidnarulex), which has shown promising preclinical activity in TNBC patient-derived xenograft (PDX) models with homologous recombination deficiency (HRD), derived from metastatic patients resistant to standard-of-care chemotherapy. CX-5461 is currently in clinical trial and holds FDA Fast Track designation for breast and ovarian cancers with HRD. To enhance efficacy and improve survival, we are exploring liposomal delivery and rational combination with the ATR inhibitor, Berzosertib. As CX-5461 is most effective in DNA repair-deficient backgrounds, we focused on models with HR or other DNA repair pathway deficiencies. Methods: Human TNBC cell lines and PDX-derived cultures were screened in vitro for single-agent CX-5461 activity. PDX models with IC50 ≤ 5 µM were prioritized for in vivo testing to determine sensitivity (defined as ≥50% tumor volume reduction compared to control). One PDX model (BCM-3887) has been tested in vivo to date. CX-5461 liposomes were prepared using thin-film hydration method with copper complexation-based gradient loading. In vitro responsive cell lines and BCM-3887 were screened for single-agent activity of Berzosertib. Cell lines and BCM-3887 with IC50 ≤ 10 µM for Berzosertib were selected for in vitro combination studies to evaluate synergistic interactions with CX-5461. Results: CX-5461 has demonstrated potent in vitro single-agent activity. In a pilot in vivo study using BCM-3887 model, strong tumor growth inhibition was observed. Liposomes were prepared with favorable physical characteristics. In vitro combination studies with Berzosertib showed synergy. Conclusion: CX-5461 is a promising agent for DNA repair-deficient TNBC, with strong single-agent activity demonstrated in vitro and in vivo. Preliminary combination studies with Berzosertib revealed synergistic potential. Future in vivo studies will evaluate whether liposomal delivery and combination therapy can improve survival in TNBC. Citation Format: Henos S. Negash, Rachel K. Myrick, Sandro R.P. da Rocha, J. Chuck Harrell. Enhancing CX-5461 therapy in TNBC through liposomal delivery and ATR inhibitor combination [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3016.