Abstract PS1-11-12: Rac/cdc42 inhibitor MBQ-167 in the estrogen receptor positive breast cancer tumor microenvironment

Abstract MBQ-167 is a dual Rac and Cdc42 inhibitor with significant anti-tumor growth and metastasis blocking activities in metastatic breast cancer cell and mouse models. MBQ-167 reduces both Rac and Cdc42 activation by GTP exchange and consequently, decreases phosphorylation of the common Rac and Cdc42 downstream effector p21-activated kinase (PAK). MBQ-167 is currently in Phase 1 clinical trials in advanced breast cancer patients, and so far, has shown safety and superior bioavailability up to 120 mg/kg BID (NCT06075810). Immune cell phenotyping from mouse tissue (tumor and spleen) and plasma from mice with tumors established from TNBC tumors demonstrated significant changes in myeloid cells and T lymphocytes indicating immune modulatory anticancer effects of MBQ-167 in the TME via inhibition of Rac and Cdc42. Immunohistochemistry (IHC) of breast cancer patient tissues demonstrate that Rac and Cdc42 are expressed in basal, HER2+ and (or) ER/PR+ breast cancers regardless of grade or type. However, the levels of their active downstream effector PAK, identified via IHC using a phospho (P) PAK antibody, is elevated in invasive breast cancer. Since the majority of breast cancers (∼70%) are ER+, the objective in this study is to determine the effect of MBQ-167 in ER+ breast cancer, using an ex vivo drug testing platform. ER+ breast cancer patient tissues (>2 cm lesion) were collected immediately following surgical resection. Tissue slices (5 mm) were incubated in 0-1000 nM MBQ-167 for 24 h in ex vivo culture and fixed for H&E staining and IHC for PAK or p-PAK. A set of treated tissue slices were also snap frozen in liquid N2 and subjected to single cell RNA Seq to determine the effect of MBQ-167 on cellular heterogeneity in the TME. Results (N=5) show that when ER+ tissue slices following vehicle or 1000 nM MBQ-167 treatment were fixed and subjected to IHC using antibodies to p-PAKT423 or PAK, p-PAK levels were significantly reduced by ∼70% without affecting total PAK levels. From the frozen samples treated with vehicle or 1000 nM MBQ-167 for 24 hrs, a total of 10 libraries was created for single cell RNA seq for 5 ER+ tumors. These data allowed us to systematically annotate the changes in gene expression in response to MBQ-167 in the different cell types within these tumor specimens. In conclusion, MBQ-167 reduces active p-PAK levels in ER+ breast cancer, and p-PAK levels can be used to demonstrate target engagement for the Rac/Cdc42 inhibitor MBQ-167 in ER+ breast cancer. These results inform the dynamics of Rac/Cdc42 inhibitor therapy in breast cancer patients and forward Phase 2 trials for this novel therapeutic in all breast cancer patients regardless of the clinical subtype. This study was supported by the DoD/BCRP HT94252310166, BC220526 and HT94252410094, BC230070 (to SD), Women’s Health Supplement from NIH/NIGMS 5P20GM103475 (to SD, RP, and HF), and Susan Komen for the Cure Aspire supplement (to ACC). Citation Format: A. Cruz Collazo, N. Zaragoza-Rodriguez, H. Fogle, A. Torres-Sanchez, N. Grafals, V. Negron, V. Carlo, R. Papa, S. Dorta-Estremera, H. Franco, S. Dharmawardhane. Rac/cdc42 inhibitor MBQ-167 in the estrogen receptor positive breast cancer tumor microenvironment [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-11-12.