T-cell immune checkpoint inhibition plus hypomethylation for locally advanced HER2-negative breast cancer: a phase 2 neoadjuvant window trial of decitabine and pembrolizumab followed by standard neoadjuvant chemotherapy
Authors: Harry D Bear, Xiaoyan Deng, Dipankar Bandyopadhyay, Michael Idowu, Taylor M Jenkins, Maciej Kmieciak, Monique Williams, Giovanni Archer, Lindsey Gwaltney, Patrick Dillon, Daniel Flora, Daniel Stover, Andrew S Poklepovic, Mary Hackney, Masey Ross, Hetal Vachhani, Raphael Louie, Kandace P McGuire, Amelia Grover, Tasnim Rahman, Amber Hendrix
Journal: Journal for ImmunoTherapy of Cancer (2025)

Background Higher levels of tumor-infiltrating lymphocytes (TILs) in breast cancers are associated with increased likelihood of pathologic complete response (pCR) to chemotherapy. DNA methyltransferase inhibitors (DNMTi) can augment immune responses to cancers, decreasing myeloid-derived suppressor cells (MDSCs) and increasing T lymphocyte responsiveness. We have shown that the DNMTi decitabine augments the effectiveness of immunotherapy using murine triple-negative breast cancer (TNBC) models. The primary objective was to determine whether DNMTi+immune checkpoint blockade would increase stromal TIL (sTIL) in primary breast cancers before neoadjuvant chemotherapy (NCT). Methods In a phase 2 study ( NCT02957968 ), patients with human epidermal growth factor receptor 2-negative breast cancer received window immunotherapy—decitabine (15 mg/m 2 ×4 doses over 5 days) followed by 2 doses of pembrolizumab (200 mg, 2 weeks apart)—before starting NCT. Biopsies before and after window immunotherapy quantified TILs and programmed death-ligand 1 (PD-L1) expression. Patients proceeded to NCT and tumor resection per standard of care. Mid-study, results of the KEYNOTE 522 trial led to patients with TNBC receiving additional pembrolizumab concurrently with standard NCT and in the adjuvant setting. Results 46 patients (median age 54.5 years, range 28–72; 71.7% white, 28.3% black; 100% female) were treated. 21 patients had TNBC and received neither neoadjuvant pembrolizumab concurrently with NCT nor adjuvant pembrolizumab (Cohort A), 7 patients had TNBC and did receive concurrent and/or adjuvant pembrolizumab (Cohort A2), and 18 patients were estrogen receptor positive and/or progesterone receptor positive and received neither concurrent nor adjuvant pembrolizumab (Cohort B). Blood samples collected after decitabine administration before pembrolizumab showed a 59% decrease (p&lt;0.01) in monocytic MDSCs compared with baseline. 38 patients had paired biopsies for sTIL and 37 for PD-L1 evaluation. Cohorts A/A2 experienced an sTIL increase of 6.1% (p&lt;0.008); Cohort B experienced an sTIL increase of 8.3% (p=0.006). PD-L1 expression increased by 73.9% (p&lt;0.01). 14 of 43 patients (32.6%) who proceeded to resection achieved pCR (n=11 of 27 (40.1%) in Cohorts A/A2 and n=3 of 16 (18.8%) in Cohort B). The most frequently reported immune-related adverse events were adrenal insufficiency (AI) (n=6, 13.0%), maculopapular rash (n=3, 6.5%), and hypothyroidism (n=3, 6.5%). Five of the six AI instances were at least partially attributable to hypophysitis/pituitary dysfunction, and one remains uncertain. Conclusions Treatment in the pre-neoadjuvant window with decitabine and pembrolizumab could sensitize breast cancers to standard NCT by recruitment of TILs to the tumor tissue. The treatment was well-tolerated. Trial registration number NCT02957968 .