Abstract B82: Pinometostat (EPZ-5676) enhances the antiproliferative activity of MAP kinase pathway inhibitors in MLL-rearranged leukemia cell lines

Despite recent advances in identifying treatment options for acute leukemias, those bearing rearrangements of MLL  ( MLL ‑r) remain a population with significant unmet medical need. Phase 1 clinical trials are underway to investigate the single agent activity of pinometostat (EPZ‑5676), a first‑in‑class inhibitor of the histone methyltransferase DOT1L, for the treatment of patients with MLL ‑r acute leukemias. Preliminary results of the adult Phase 1 clinical trial indicate evidence of clinical activity including complete responses in a subset of patients (Blood Dec 2014, 124 (21) 387). While this activity is encouraging, given that the backbone of effective leukemia therapies are combination regimens, investigations into potential pinometostat combinations are of interest. We have previously shown in vitro synergistic anti‑proliferative activity when pinometostat was combined with the current standard of care drugs for acute leukemias, cytarabine and daunorubicin (Klaus, Iwanowicz et al., 2014); the greatest combination benefits with these drugs were observed in cell lines with nanomolar pinometostat IC 50 values. Here we report results of an expanded effort, across a leukemia cell line panel to identify novel pinometostat combinations. A high‑throughput screening platform consisting of a library of approved oncology drugs, emerging therapies and tool compounds (n = 250), was utilized to identify synergistic anti‑proliferative activity with pinometostat against a panel of cell lines with a range of reported sensitivities to pinometostat alone (IC 50 : 0.075 to >3 μM). The study paradigm for this effort consisted of pretreating each cell line for 7 days ( MLL ‑r: OCI‑AML‑4, ML2, THP‑1, RS4‑11, MOLM‑13, and non‑ MLL ‑r SKM‑1) with pinometostat prior to the addition of the enhancer agent for 3 days. Combination benefit was calculated by measuring the growth inhibition (GI), and the GI 100 potency change of each enhancer compound in the presence of pinometostat or vehicle control. Of the 250 compounds, 12 target classes were observed to have a GI 100 shift ≥ 2-fold in at least two cell lines. Among the most compelling findings was synergistic activity of pinometostat with several modulators of the MAP kinase pathway (e.g., trametinib, an approved MEK inhibitor) in multiple MLL ‑r cell lines. Study of dosing schedule of the combination of pinometostat with trametinib revealed that all schedules, no matter the order of compound addition, demonstrated combination benefit. Pretreatment with the DOT1L inhibitor, however, elicited dramatic cell killing at physiologically achievable concentrations. When compared to monotherapy, our results indicate that combinatorial treatment of pinometostat with trametinib boosts the inhibitory effect on cell lines sensitive and resistant to DOT1L inhibition. Taken together, these findings suggest that suppression of DOT1L activity prior to MEK inhibition may have advantages over monotherapy of either agent. Additional preclinical investigations into the mechanistic basis of the observed synergy between trametinib and pinometostat are ongoing in support of future potential clinical trials. Citation Format: Alejandra Raimondi, Christine R. Klaus, Jeffrey A. Keats, Scott R. Daigle, Robert A. Copeland, Jorge DiMartino, Jesse J. Smith, Stephen J. Blakemore. Pinometostat (EPZ-5676) enhances the antiproliferative activity of MAP kinase pathway inhibitors in MLL -rearranged leukemia cell lines. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B82.