Abstract A012: SV-BR-1-GM, a whole-cell targeted immunotherapy for advanced breast cancer: Pharmacodynamic markers of response

Background: SV-BR-1-GM is a GM-CSF-engineered breast cancer cell line employed – after irradiation – as a targeted immunotherapy for advanced breast cancer. Tumor regressions at metastatic sites have been observed, most notably in patients with HLA allele matches to the cell line. We are assessing SV-BR-1-GM in the phase IIa portion of a phase I/IIa clinical trial in metastatic and locally recurrent breast cancer (ClinicalTrials.gov identifier NCT03066947). Additionally, we are co-developing a companion diagnostic (BriaDX™) to identify patients likely to respond to SV-BR-1-GM. Currently, BriaDX™ consists of HLA typing; however, we have begun assessing biomarkers in sera, lymphocyte characteristics, circulating tumor cells (CTCs), and cancer-associated macrophage-like cells (CAMLs) from patients collected at baseline and after inoculation of SV-BR-1-GM to layer in additional components to improve accuracy, with the number/subtyping of CTCs and CAMLs being prognostic indicators. Methods: Subjects are pretreated with low-dose cyclophosphamide to reduce immune suppression. SV-BR-1-GM is inoculated intradermally with follow-up local injections of IFNα2. Cycles are every 2 weeks x 3, then monthly. HLA typing was conducted via LabType R-SSO Kits (One Lambda). Cytokines were measured via single- or multiplex assays. Anti-SV-BR-1 antibodies were determined by incubation of SV-BR-1 cells with diluted patient sera followed by staining with a fluorescently-labeled anti-IgG antibody and detection by flow cytometry. CTCs and CAMLs were evaluated by CellSieve™ at Creatv MicroTech. Results: To date, 16 clinical trial subjects have been inoculated with the SV-BR-1-GM regimen as rescue immunotherapy. All were treatment refractory and had received a median of 4.5 prior chemo/biologic therapy regimens (range 1-13). Two of the 16 patients remained on study for ≥3 months (5 cycles) with 4 patients currently on study not having reached the 3-month evaluation time point. Objective regression of tumor was seen in 2 subjects. One subject had virtually complete regression of 20 of 20 lung metastases noted at 3 and 6 months (but with progressive bone and liver metastases). Another subject had improvement of chest wall metastases and quality of life but expired due to nontreatment-related causes. Response appeared to correlate with HLA allele-matching to SV-BR-1-GM. Anti-SV-BR-1 antibody titers increased in several patients. Among the cytokines assessed, interleukin (IL)-8 levels increased in HLA-DRB3 allele-matched subjects after SV-BR-1-GM inoculation. Of 15 patients evaluated, CTCs were present in 6 patients at baseline while CAMLs were present in all 15. Five of 5 patients evaluated for PD-L1 expression had mostly low-to-medium expression of PD-L1 on their CTCs/CAMLs. In the patient who had regression of lung metastases but progression of liver metastases, PD-L1 expression and maximum CAML size increased, but the number of CAMLs decreased during treatment. CAML number also decreased in a patient who reached the 3-month evaluation visit without progression and in a patient with inflammatory breast cancer who dropped out due to worsening inflammation. Conclusions: In addition to the patients’ HLA types, several pharmacodynamic parameters correlated with tumor regression and/or HLA matching status. CTCs or CAMLs are frequently detectable in this population, and PD-L1 expression appears common on these cells. Both CAML number and size appear to correlate with response, though larger studies are needed. Future steps include evaluation of SV-BR-1-GM with checkpoint inhibitors. Citation Format: Markus D. Lacher, Sanne Graeve, Vivekananda (Vivek) Sunkari, Daniel L. Adams, Cha-Mei Tang, Pete Amstutz, Charles L. Wiseman, George E. Peoples, William V. Williams. SV-BR-1-GM, a whole-cell targeted immunotherapy for advanced breast cancer: Pharmacodynamic markers of response [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr A012.