The DOT1L inhibitor pinometostat reduces H3K79 methylation and has modest clinical activity in adult acute leukemia

Abstract

Pinometostat (EPZ-5676) is a first-in-class small-molecule inhibitor of the histone methyltransferase disrupter of telomeric silencing 1-like (DOT1L). In this phase 1 study, pinometostat was evaluated for safety and efficacy in adult patients with advanced acute leukemias, particularly those involving mixed lineage leukemia (MLL) gene rearrangements (MLL-r) resulting from 11q23 translocations. Fifty-one patients were enrolled into 6 dose-escalation cohorts (n = 26) and 2 expansion cohorts (n = 25) at pinometostat doses of 54 and 90 mg/m² per day by continuous intravenous infusion in 28-day cycles. Because a maximum tolerated dose was not established in the dose-escalation phase, the expansion doses were selected based on safety and clinical response data combined with pharmacodynamic evidence of reduction in H3K79 methylation during dose escalation. Across all dose levels, plasma pinometostat concentrations increased in an approximately dose-proportional fashion, reaching an apparent steady-state by 4-8 hours after infusion, and rapidly decreased following treatment cessation. The most common adverse events, of any cause, were fatigue (39%), nausea (39%), constipation (35%), and febrile neutropenia (35%). Overall, 2 patients, both with t(11;19), experienced complete remission at 54 mg/m² per day by continuous intravenous infusion, demonstrating proof of concept for delivering clinically meaningful responses through targeting DOT1L using the single agent pinometostat in MLL-r leukemia patients. Administration of pinometostat was generally safe, with the maximum tolerated dose not being reached, although efficacy as a single agent was modest. This study demonstrates the therapeutic potential for targeting DOT1L in MLL-r leukemia and lays the groundwork for future combination approaches in this patient population. This clinical trial is registered at www.clinicaltrials.gov as NCT01684150.

Graphical abstract

Figure 1.

Summary of plasma pinometostat PK. (A) Mean (SD) plasma pinometostat concentration–time profile after administration of 24, 36, or 54 mg/m² per day with 21-day CIV infusion. Pinometostat concentrations in plasma rose quickly, reaching an apparent steady-state within 4-8 hours postinfusion, followed by a biphasic decline. A pinometostat concentration–time profile for the 80 mg/m² per day group (n = 3) is not displayed, because only a median profile was available. (B) Mean (SD) plasma pinometostat concentration–time profiles after administration of 54 or 90 mg/m² per day with 28-day CIV infusion. (C) Summary of PK data for each treatment cohort.

Figure 2.

Sustained global H3K79me2 inhibition is dependent on uninterrupted DOT1L inhibition. (A) Global H3K79me2 inhibition in bulk PBMCs representing patients from all dosing cohorts vs the mean C_ss. Percent inhibition was calculated by taking the cycle 1 time point with maximum H3K79me2 inhibition relative to a pretreatment baseline sample. Decreases in H3K79me2 levels were exhibited by all patients who could be reliably measured, but the magnitude of inhibition was only weakly proportional to C_ss. (B) Inhibition of H3K79me2 levels in bulk PBMCs collected from patients at days 15 and 28 relative to preinfusion baseline controls. Inhibition levels from patients treated continuously for 21 days, followed by a 7-day-off period, were compared with those from patients treated for the full 28-day treatment cycle. After initial decreases with both regimens, global H3K79me2 levels recovered toward baseline in patients receiving the 7-day break in treatment compared with those patients treated continuously for 28 days (P = .005, Mann-Whitney 2-tailed Student t test). Consistent with rapid drug clearance, continuous treatment with pinometostat was needed to sustain H3K79me2 inhibition. Data are mean ± SEM.

Figure 3.

Resolution of leukemia cutis and cytogenetic changes following pinometostat treatment. Cutaneous leukemia cutis in an 81-year-old patient presenting with MLL-r CMML that was treated with 54 mg/m² per day of pinometostat by 21-day CIV infusion. Leukemia cutis neck lesions that were apparent on day 1 (D1) at the start of treatment (A) progressively resolved over the course of subsequent treatment cycles (B-D). (E) Translocation-positive cell in a peripheral blood sample detected by FISH from the same patient showing the t(11:19) MLL-r product colocalizing with the MLLT1 fusion partner (arrows). (F) The number of translocation-positive cells (arrow) decreased from 90% pretreatment to 0.2% at the start of the fourth pinometostat treatment cycle, with most cells demonstrating normal segregation of MLL and MLLT1 signals (arrowheads).