Abstract P3-09-12: Peripheral T cell clonality and exhaustion as novel biomarkers for anti-PD-1 (pembrolizumab) maintenance therapy in patients with metastatic inflammatory breast cancer (mIBC) and non-IBC triple negative breast cancer (mTNBC)

Background: Inflammatory breast cancer (IBC) and triple negative breast cancer (TNBC) are more aggressive than other breast cancer subtypes, and up until recently lacked therapy options that maintain acceptable quality of life. While chemotherapy is a treatment option for all subtypes of breast cancer, it is used long-term as maintenance therapy for metastatic TNBC (mTNBC) and often produces cumulative toxicity resulting in discontinuation of treatment. Moreover, responses are rarely durable after discontinuation of chemotherapy. Immune checkpoint blockade has the potential to maintain therapy. To determine factors associated with response to anti-PD-1 therapy (pembrolizumab), we performed minimally invasive blood-based analyses of T-cell repertoire and phenotype to evaluate their association with progression-free survival (PFS) in 15 patients with metastatic IBC (mIBC) or mTNBC. Materials and Methods: Fifteen patients with mIBC (n=6) or mTNBC (n=9) were enrolled on an ongoing phase II study to receive pembrolizumab as maintenance therapy after achieving a clinical response or stable disease to systemic chemotherapy for metastatic disease. We performed analyses of T-cell repertoire and phenotype on peripheral blood mononuclear cells (PBMC) from samples obtained post induction therapy but before initiation of pembrolizumab (baseline) therapy. Expression of T-cell exhaustion markers 2B4, CTLA4, BTLA, Lag3, PD-1, and Tim3 was evaluated on PBMC by flow cytometry. T-cell receptor (TCR) beta chain CDR3 DNA sequencing by ImmunoSEQ and richness (diversity) and clonality (reactivity) were evaluated. Patients were followed through 5 months of treatment with pembrolizumab to evaluate the association between T-cell repertoire and phenotype with PFS. Results: Seven patients had stable disease (SD) and 8 had progressive disease (PD) by 5 months. The median follow-up was 14.2 months (range: 4.5 to 27.7 months). Among the patients who progressed within 5 months, the earliest and the latest time to progression was at 1.38 months and 4.82 months, respectively. CTLA4 expression in CD4+ T cells at baseline was significantly higher in patients with PD than in patients with SD (p = 0.040). Patients with a low percentage of CD4+ T cells expressing exhaustion markers (CTLA4, Tim3, and 2B4) at baseline were more likely to have SD (chi-square p = 0.041) and significantly longer median PFS than patients with PD (median time to progression was not reached in SD vs. 4.1 months in PD, p = 0.018). Additionally, patients with high clonality and low CD4+ T exhaustion markers were more likely to have SD (chi-square p = 0.041) and a longer median time to progression (median time to progression was not reached in SD vs. 4.1 months in PD, p = 0.015). Conclusions: Baseline T-cell clonality and T-cell exhaustion markers have significant translational relevance and can help to explain variable responses to immune checkpoint blockade. Our data suggest that T-cell reactivity at baseline, and a lower percentage of CD4+ T cells expressing CTLA4/Tim3/2B4 were favorable prognostic factors for pembrolizumab maintenance therapy. This proof-of-concept provides compelling data that T-cell clonality and phenotyping of T-cell exhaustion markers can be useful and should be applied to future trials with immune checkpoint inhibitors. Citation Format: Hui Gao, Kumiko Kida, Evan N Cohen, Angela Alexander, Bora Lim, Charla Parker, Sanda Tin, Vicente Valero, Debu Tripathy, Alexandre Reuben, Naoto T Ueno, James M Reuben. Peripheral T cell clonality and exhaustion as novel biomarkers for anti-PD-1 (pembrolizumab) maintenance therapy in patients with metastatic inflammatory breast cancer (mIBC) and non-IBC triple negative breast cancer (mTNBC) [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-09-12.