A Phase 1 Dose-Escalation Study of Adding Venetoclax to a Reduced Intensity Conditioning (RIC) Regimen Prior to Allogeneic Hematopoietic Cell Transplantation for Patients with High Risk Myeloid Malignancies

Background: Patients (pts) with myeloid malignancies characterized by high risk mutations or cytogenetics who undergo allogeneic hematopoietic cell transplantation (HCT) have poor outcome primarily due to risk of relapse. Strategies to limit relapse include post-transplant maintenance or novel conditioning regimens. The selective BCL-2 inhibitor venetoclax (VEN), a BH3 mimetic, promotes mitochondrial apoptosis in myeloblasts in the presence of cytotoxic stress and is associated with high response rates when combined with hypomethylating agents or low dose cytarabine in untreated elderly AML. We found that VEN could induce apoptotic priming in myeloblasts regardless of poor risk mutations such as TP53 by BH3 profiling, which measures the mitochondrial response to perturbation by a panel of BH3 domain peptides. We thus hypothesized that VEN would promote the anti-leukemic effect of conditioning chemotherapy and therefore reduce the risk of relapse without undue toxicity.

Methods: The primary objective of this phase 1 study is to determine the safety of adding VEN to the RIC regimen of fludarabine and busulfex (FluBu2). This study uses a 3+3 design with dose expansion. Eligible pts included: AML in CR (adverse risk per ELN or secondary AML) or MDS (t-MDS; Int-2 or higher IPSS; TP53 mutation; or RAS pathway gene mutation) or MDS/MPN (including +8; chr 7 abnl; complex karyotype; or ASXL1 mutation) with up to 10% blasts at the time of transplant. Dose-limiting toxicity (DLT) was defined as any treatment-related death, failure to engraft (defined as absolute neutrophil count (ANC) ≥ 500/μL on 2 consecutive measurements), or any gr 4 non-heme toxicity or tumor lysis syndrome from day -8 until day +28. Conditioning chemotherapy consisted of fludarabine 30 mg/m2/d, days -5 to -2 and busulfex 0.8 mg/kg bid, days -5 to -2, followed by PBSC infusion on day 0. VEN is scheduled to maximize overlap with FluBu2. VEN dose levels (DL) are 200 mg on days -8 through -3 (DL1); 200 mg on days -8 through -2 (DL2); and 400 mg on days -8 through -2 (DL3). Flow cytometry-based BH3 profiling was performed on pre-VEN treatment bone marrow in pts with measurable residual myeloblasts. NGS using a 95-gene TruSeq panel was performed on pre-VEN treatment and day +100 marrow samples.

Results: Nine pts (67% male, median age of 65 y (range 41-71)) have been treated including 3 AML (1 with -17 and 2 with mutations in TP53, RUNX1, and/or ASXL1), 5 MDS (all mutant TP53), and 1 CMML (with mutant ASXL1) in DL1-DL3. 8 donors were matched unrelated and 1 was a matched related. Donors were 44% male. No DLTs were observed at DL1-DL3; dose expansion at DL3 is ongoing. VEN-related toxicities included gr 1 diarrhea (n=3), gr 1 fatigue (n=1), and gr 1 nausea (n=2). Median time to ANC ≥ 500/μL was 16 days (range 13, 25; 1 pt did not nadir) and median donor-derived myeloid chimerism was 100% (range 97, 100) at day +28. Median time to platelet ≥ 20K/μL was 14 days (range 13, 18; 3 did not nadir). 3 developed gr 1 acute graft-versus host disease (GVHD) involving skin and gastric sites. In the 4 followed beyond day +100, 3 developed chronic GVHD (2 moderate, 1 severe) and 1 died from complications of acute GVHD of the gut (stage 3) on day +183. With a median follow-up time of 5 months (range 1.3, 7.8), 2 of 9 pts had disease progression and 1 died.

Five out of 6 pts with available paired day +100 samples had no detectable mutation by NGS with >200X mean coverage at the later time point (Fig A). Pt 3 had persistent mutations in TP53 at day +100. Myeloblasts that are more primed require lower doses of [BIM] or [PUMA] peptide to induce cytochrome c release. BH3 profiling of residual myeloblasts on pre-VEN treatment marrow using pro-apoptotic BIM and PUMA peptides (Fig B-C) showed differential individual baseline priming status, which we will correlate with response.

Conclusions: VEN at doses up to 400 mg can be safely added to RIC conditioning with FluBu2 resulting in a high molecular negativity rate at day +100 without impairing neutrophil engraftment. Additional correlative studies include targeted deep sequencing to better evaluate measurable residual disease for individual genes and BH3 profiling analysis of myeloblasts collected pre-VEN and post-VEN/pre-FluBu2 to evaluate for a VEN-induced increase in apoptotic priming. To further minimize disease relapse in these high risk pts, we amended the trial to include maintenance therapy with a combination of VEN and azacitidine.

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