A phase Ib trial of the cyclin-dependent kinase inhibitor dinaciclib (dina) in combination with pembrolizumab (P) in patients with advanced triple-negative breast cancer (TNBC) and response correlation with MYC-overexpression.

Abstract

1076 Background: Increased expression of the MYC transcriptional oncogene is found in about 70% of TNBC and is associated with poor prognosis. In MYC-overexpressing TN xenografts, CDK1 inhibition with dina results in synthetic lethality, and attenuates distant metastasis. In syngeneic models, the combination of dina with anti-PD1 therapy is synergistic and increases immune cell tumor infiltration and activation. Methods: Patients (pts) with advanced TNBC received dina IV day 1 and 8 in combination with fixed dose P 200 mg once every 21 days. Dina was dose escalated using a toxicity probability interval design targeting a dose limiting toxicity (DLT) rate of 25%. Evaluable disease and pre-treatment metastatic biopsies were required. After 17 pts were enrolled, eligibility was amended to require ≤2 lines of prior chemotherapy, and LDH ≤1.5x normal. MYC IHC was performed on baseline tumor biopsies and correlated with clinical response using Welch’s unequal variances t-test. Results: 32 pts were enrolled (median age 53, median 2 prior lines of therapy, 13 pts (41%) had disease which was previously ER+). 18 pts were enrolled in the dose escalation phase with no observed DLTs; 33 mg/m2 was determined to be the recommended phase 2 dose (RP2D). 14 additional patients were enrolled in dose expansion at 33 mg/m2, completing trial accrual. Grade ≥3 adverse events (AEs) in all pts included neutropenia (ntp) (37.5%), febrile ntp (12.5%), fatigue (12.5%), transaminitis (3.2%), and neuromuscular weakness (3.2%). Most common all grade AEs included fatigue (63%), diarrhea (63%), nausea (63%), and ntp (53%). Possible immune-related AEs included sinusitis (4 pts), hemolytic anemia (1 pt), pneumonitis (1 pt), rash (2 pts), neuromuscular weakness (1 pt), and transaminitis (1 pt). Of the 29 patients evaluable for response, 1 pt (3.4%) had a CR, 4 pts (13.8%) had a PR, and 6 pts (20.6%) had SD. MYC IHC staining on baseline metastatic tumor biopsies in 19 pts correlated significantly with clinical response. Additional biomarker testing will be reported. Conclusions: The RP2D of dina given in combination with standard dose P is 33 mg/m2 on D1, 8 of a 21-day cycle. Toxicities were generally manageable and non-overlapping. In exploratory analysis, greater MYC IHC staining correlated significantly with response to study therapy. Clinical trial information: NCT01676753 .