Research paperRandomized phase II trial of cytosine arabinoside with and without the CHK1 inhibitor MK-8776 in relapsed and refractory acute myeloid leukemia
Introduction
Cytosine arabinoside (AraC) is the backbone of both first-line induction and salvage regimens for acute myeloid leukemia (AML) [1]. In newly diagnosed AML, AraC-based regimens lead to complete remission (CR) in up to 80% of patients, yet more than half will ultimately relapse [2]. Outcomes of salvage therapy for relapsed and refractory AML patients are worse, with only 23–32% achieving remission with high-dose AraC (HiDAc) alone, and there is scant evidence that adding a second agent improves outcomes [3], [4], [5], [6]. The low cure rate among newly diagnosed patients and the limited effectiveness of AraC-based salvage regimens suggest that many patients harbor undetectable residual disease with either de novo or acquired AraC resistance. Thus treatments designed to overcome AraC resistance might help to eradicate minimal residual disease (MRD), thereby increasing progression free survival among newly diagnosed patients and improving the rates of remission for relapsed/refractory AML.
AraC exerts its cytotoxic effect on leukemic cells through its incorporation into DNA, which is dependent on the intracellular concentration of and duration of exposure to AraC triphosphate, an active metabolite of AraC [7]. Numerous studies have demonstrated that cellular retention of AraC triphosphate correlates with the likelihood of achieving CR [8], [9], [10]. Consistent with this view, changes that reduce AraC triphosphate formation and retention have been reported to cause AraC resistance [11]. Upon incorporation into cellular DNA, AraC activates ataxia telangiectasia and Rad3-related protein (ATR), and checkpoint kinase 1 (Chk1) [12], [13]. The ATR-Chk1 checkpoint signaling pathway causes cell cycle arrest and diminishes AraC cytotoxicity by stabilizing stalled replication forks, activating DNA repair, and suppressing apoptosis [12], [14], [15], [16], [17], [18]. Indeed, a recent study found that high levels of Chk1 are associated with AraC resistance during induction chemotherapy [19]. Conversely, disruption of Chk1 in vitro abrogates these protective effects and yields increased sensitivity to antimetabolites such as AraC [16], [20], [21], [22].
A number of clinical studies have investigated the safety and efficacy of Chk1 inhibition to overcome AraC resistance in AML. In a phase I study, the combination of AraC and tanespimycin, an inhibitor of heat shock protein 90 (Hsp90) that downregulates Chk1 ex vivo, led to modest Chk1 downregulation in vivo at clinically intolerable tanespimycin doses without any discernible impact on patient outcomes [23]. Similarly, in a trial of AraC with UCN-01, an inhibitor of Chk1 and other kinases, only 1/13 AML patients achieved a CR despite modest decreases in Chk1 activation [24], [25]. More recently, high content screening identified MK-8776 as a potent and selective Chk1 inhibitor that sensitized AML cells to AraC in vitro [13], [26]. Additionally, Chk1 was pulled out as a major hit in an unbiased AML RNAi screen of the human kinome with AraC, validating Chk1 as a high priority target in AML [27]. A phase I study of the AraC/MK-8776 combination in relapsed or refractory acute leukemias established a maximum tolerated dose (2 g/m2 of AraC over 72 h on days 1 and 10 with a 100-mg flat dose of MK-8776 on days 2, 3, 11, and 12) and suggested a possible dose response curve with a response rate of 39% at the highest dose levels [28]. Administration of MK-8776 to patients receiving AraC also led to increased γ − H2AX, an established biomarker of DNA damage [29], [30], [31], [32], in bone marrow blasts from 3 of 5 patients [28]. These observations provided the basis for the current randomized phase II trial of AraC versus AraC plus MK-8776 in relapsed or refractory AML.
The AraC dosing strategy chosen for this study differs from conventional re-induction approaches using HiDAc [3] and is based on the principles of timed sequential therapy (TST). In TST, an initial dose of chemotherapy stimulates humoral factors that increase cell cycling, thereby increasing the incorporation of a subsequent dose of cell-cycle specific agents, such as AraC, into DNA, a key event in cytotoxicity [33]. An in vivo model of TST using consecutive cycles of AraC has shown that the timing of the second cycle of AraC is key to extending survival [34]. Based on these findings, TST-based approaches have been used both as initial induction and salvage AML therapy with promising results [35], [36], [37]. Thus a salvage regimen based on TST was chosen in combination with MK-8776 for the initial phase I trial [3], [28].
Section snippets
Patient eligibility and selection
Between June 2013 and September 2014, patients aged 18–75 years with a pathologically confirmed diagnosis of relapsed or primary refractory AML were enrolled in a multi-institution study with a planned enrollment of 52 patients. Patients were eligible if they received ≤2 prior cytotoxic induction regimens and were >2 weeks beyond previous cytotoxic chemotherapy or radiation. The study was conducted in accordance with the Declaration of Helsinki after approval by the ethics committee of each
Patient characteristics
Thirty-two patients (Arm A: n = 14, Arm B: n = 18) from 4 institutions were randomized, treated, and included in the analysis. An interim analysis for futility after 20 patients showed the predicted probability of rejecting the null hypothesis was 0.27, which did not meet the stopping criteria. The second interim analysis was not performed and accrual was truncated after 32 patients due to the discontinued development of MK-8776.
Clinical demographics and disease biological features of all patients
Discussion
Increasing the cytotoxicity of AraC and improving responses seen with AraC-based salvage regimens for refractory and relapsed AML remains a challenge. Chk1 inhibition has been shown to augment AraC-induced DNA damage and cytotoxicity in AML ex vivo. Accordingly, the ability of the Chk1 inhibitor MK-8776 to improve the response rate or progression-free survival in relapsed/refractory AML was assessed in the present study. Accrual to this randomized study was stopped due to the termination of the
Conflict of interest statement
The authors declare no potential conflicts of interest.
Financial support
Larry Karnitz received support from R01 CA190473 for the correlative studies. The study also received support from UO1 CA70095, UM1 CA186691, and P30 CA006973.
Acknowledgements
We are grateful for the tireless work and dedication of the nursing staff, residents, fellows and faculty on the inpatient units at each center and most of all, the patients and their families for participating in this research. The study was sponsored by the Cancer Therapy Evaluation Program (CTEP) in the Division of Cancer Treatment and Diagnosis at the National Cancer Institute (NCI). Funding was provided through NCI grantsR01 CA190473, UO1 CA70095, UM1 CA186691, and P30 CA006973.
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