Although reports of synergistic interactions between proteasome and histone deacetylase (HDAC) inhibitors in acute leukemias have been limited, they are well described in B-cell malignancies (e.g., myeloma and lymphoma). Nevertheless, preclinical findings have shown striking synergism between the HDAC inhibitor belinostat (previously PXD-101) and the proteasome inhibitor bortezomib, administered at low (sub-micromolar) concentrations, in cultured and primary acute myeloid leukemia (AML) and acute lymphocytic leukemia cells (Dai Y et al. Br J Haematol. 2011). These findings prompted initiation of a phase I trial, using a 3+3 design, with the primary objective of determining the maximum tolerated dose (MTD) for the combination of bortezomib and belinostat in patients with relapsed or refractory acute leukemia, myelodysplastic syndrome (MDS), or chronic myelogenous leukemia in blast crisis (CML-BC).
To date, 25 patients with the following disease types have been treated: acute leukemia (n=19), MDS (n=4), and CML-BC (n=2). The male:female ratio was n=11 (44%):14 (56%); the median age was 62 (range 27–83) years; ECOG performance scores ranged from 0–2; and the median number of prior therapies was 2 (range 1–5). The schedule of administration was belinostat, 30 minutes intravenous (IV) infusion, on days 1–5 and 8–12; and bortezomib, IV bolus, preceding belinostat on days 1, 4, 8, 11; on a 21-day cycle. Dose levels were, in mg/m2(bortezomib/belinostat): 1.0/500 (n=6); 1.3/500 (n=6); 1.3/650 (n=4); 1.3/850 (n=3); 1.3/1000 (n=4); 1.3/1200 (n=2). The study is currently enrolling to dose level 6 (1.3/1200).
No dose-limiting toxicities (DLTs) have been observed to date. Non-DLT ≥ grade 2 (CTCAE version 4) treatment-related adverse events have included: fatigue (grade 2, 36%), leukopenia (grade 4, 12%), nausea (grade 2, 12%), peripheral sensory neuropathy (grade 2, 12%), and thrombocytopenia (grade 3, 20%). No serious adverse events have occurred at an unexpected frequency or severity. Two deaths have occurred due to disease progression, and one death has occurred due to a cerebrovascular accident that was related to pre-existing comorbidities and not to study-therapy.
Of the 25 patients treated, 22 have been evaluable for response, 2 are too early to evaluate, and 1 patient was not evaluable for response. There have been 2 partial responses (PRs) and 1 complete response (CR) in this heavily pretreated population. The CR was achieved at dose level 1 in a patient with biphenotypic acute leukemia refractory to 7+3 and Flag-IDA. The patient proceeded to allogeneic hematopoietic stem cell transplantation (SCT) after 4 cycles of treatment. In addition, 1 patient with CML-BC had stable disease (SD) by protocol criteria but a CR with incomplete blood count recovery (CRi) by standard criteria, and is undergoing evaluation for allogeneic hematopoietic SCT. The patient is currently in cycle 8 at dose level 4. One of the PRs was achieved in a patient with AML transformed from MDS (2 prior regimens); after 4 cycles of treatment at dose level 5, the patient proceeded to allogeneic hematopoietic SCT. The second PR was achieved in an AML patient after cycle 2; a bone marrow biopsy revealed chronic myelomonocytic leukemia, and the response was deemed sufficient to proceed to allogeneic hematopoietic SCT. Also of note, a patient with AML transformed from MDS is currently on treatment in cycle 5 at dose level 5 with SD. An additional 6 patients have had SD, and 11 patients have had progressive disease. Correlative studies examining pre- and post-treatment leukemic blast expression of nuclear RelA, Bim, Bcl-xL, and XIAP are ongoing.
Collectively, these findings indicate that a regimen combining belinostat and bortezomib is well tolerated in patients with relapsed or refractory acute leukemia, MDS, or CML-BC and shows evidence of activity. The MTD has not yet been reached. Pending identification of the MTD, phase II evaluation of this therapeutic strategy, should determine its activity more definitively.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.