Abstract PO2-20-10: Exceptional long-term disease control on pembrolizumab monotherapy in a patient with TMB-high PMS2-mutated MMR-deficient triple negative breast cancer brain metastasis
Authors: Cha Jaeyoon , Grinshpun Albert , Buckley Lauren , Alexander Brian , Tolaney Sara , Lin Nancy 
Published: May 2024

Abstract
Introduction The brain is among the most common sites of breast cancer (BC) spread. Prognosis of BC brain metastasis (BCBM) differs by receptor subtype, with poorest median overall survival (OS) for triple-negative (TN) BCBM at under 5 months. Though treatment guidelines have evolved in recent years, most strategies have focused on local therapies including surgery and stereotactic radiotherapy (SRT), rather than on systemic agents. The 2020 FDA approval of pembrolizumab (pembro) for patients (pts) with metastatic high-tumor mutational burden (TMB-h, ≥ 10 mutations/megabase) solid tumors, agnostic of primary site, combined with clinical trials in melanoma reporting efficacy of immune checkpoint inhibitors (ICI) against BM, suggest a potential use of ICI for pts with TMB-h BCBM. Results of the TAPUR study supported modest therapeutic benefit of pembro for 28 pts with metastatic BC selected for TMB-h, with median OS of 30.6 weeks, but did not include pts with active BCBM. TMB-h is uncommon in BC, and further genomic signatures remain to be elucidated to identify pts with BCBM responsive to ICI. We present a remarkable case of a pt with Lynch Syndrome (LS) due to a germline PMS2 mutation, who developed TMB-h, mismatch repair (MMR)-deficient TN BCBM, and has maintained stable disease for over 6 years on pembro monotherapy. Case description A 62-y.o. woman underwent screening mammography that identified a right breast mass. Pt has a family history of a mother with uterine and pancreatic cancer, and father with bladder cancer. Biopsy confirmed invasive ductal carcinoma (IDC), grade 1 (G1), ER/PR-positive, HER2 score 0. She underwent partial mastectomy, axillary dissection, radiation, and received adjuvant anastrozole. Three years later, while still on anastrozole, she presented with a 6 cm right breast mass. Mastectomy disclosed G3, ER/PR-negative, HER2-positive (3+, Her2:CEP17 ratio 5.6) IDC. She received adjuvant chemotherapy (AC-T) and 1 year of trastuzumab. Seven years later, pt presented with headaches and word finding difficulties. MRI identified a 3.6 cm frontal mass, which biopsy revealed as TN BCBM. Systemic imaging ruled out distant metastases. Pt proceeded with SRT and exemestane (despite receptor staining ER- in case of a possible false negative result). Despite radiotherapy, pt had BCBM progression within the same year with the largest mass diameter grown to 4.8 cm. Resection (followed by SRT) identified the dominant histology as G3 TNBC. Following transition of care to our institution, germline testing revealed a pathogenic mutation in PMS2 (c943C &gt;T). Tumor sequencing of the resected brain metastasis showed TMB of 48 and MMR deficiency. Therefore, systemic therapy with pembro monotherapy was initiated and maintained with good tolerability. Two years later, concern for progression from rising tumor markers led to addition of concurrent carboplatin/gemcitabine for 5 cycles. Imaging and tumor markers stabilized, and pt returned to pembro monotherapy. Today, 3 years later, and over 6.5 years after initial TN BCBM diagnosis, pt continues to have stable disease on pembro with an excellent performance status (ECOG 0) and no evidence of intracranial or extracranial progression. Discussion We document an exceptional case of prolonged benefit to pembro in a pt with TMB-h PMS2-mutated MMR-deficient TN BCBM. This case highlights two major pragmatic points to clinical practice; (i) the significance of germline testing in all pts with metastatic BC, and (ii) the impact of molecular testing of tumor tissue to identify rare alterations which may have dramatic effects on pt outcomes. Altogether, this case is a remarkable addition to growing evidence that TMB could be a biomarker for predicting response to ICI therapy and provides an example of ICI efficacy in the setting of BCBM. Further research on additional genomic, clinical, and pathologic features associated with ICI responsiveness is warranted for greater precision in BCBM management.
Citation Format: Jaeyoon Cha, Albert Grinshpun, Lauren Buckley, Brian Alexander, Sara Tolaney, Nancy Lin. Exceptional long-term disease control on pembrolizumab monotherapy in a patient with TMB-high PMS2-mutated MMR-deficient triple negative breast cancer brain metastasis [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-20-10.