Abstract: E881

Type: Eposter Presentation

Background
Treatment with alvocidib has shown significant improvements in the complete remission rates in newly diagnosed acute myeloid leukemia (AML) patients when administered before cytarabine and mitoxantrone (ACM regimen) in a randomized Phase 2 study compared to 7+3. Although the mechanism of alvocidib action as a single agent is documented, the mechanism underlying synergy found in the ACM regimen is not fully understood. The ACM regimen was originally developed based on the perceived benefit of a time-sequential regimen starting with cell-cycle arrest (alvocidib), followed by release of the cells from arrest and inhibition of DNA replication (cytarabine/mitoxantrone) during S-phase. However, recent reports suggest that the transcriptional repression of key anti-apoptotic proteins (eg., MCL-1) mediated by alvocidib’s CDK9 inhibition, may contribute to the activity in the ACM regimen.

Aims
We hypothesized that MCL-1 transcriptional repression constitutes the primary mechanism for the synergism observed with the ACM treatment regimen.

Methods
Following treatment, cell viability and caspase activation, an indicator of apoptosis, were assessed using CellTiter-Glo and Caspase-Glo assays, according to manufacturer protocol. mRNA levels were assessed using RT-PCR. Protein levels were assessed using standard immunoblotting technique.

Results
In this study, we demonstrate that treatment with alvocidib, followed by treatment with cytarabine and mitoxantrone, synergized in vitro and correlated with the downregulation of MCL-1 protein and mRNA expression. Indeed, the ACM regimen resulted in a 2.4 or 3.4-fold increase in caspase activity relative to any single agent within the combination in MV4-11 or OCI-AML3 cells, respectively. As has been previously reported, we also observed that increased activity of cytarabine in alvocidib-treated cells corresponded with progression into the S-phase of the cell cycle, following the washout of alvocidib. However, this observation accounted for only a small portion of the inhibition of cell proliferation. This was further confirmed by the observation that CDK4/6 (cell cycle) specific inhibitors, such as palbociclib, did not show synergistic increases in caspase activity following treatment in the same setting. In various AML cell lines treated with MCL-1 siRNA, followed by cytarabine and mitoxantrone treatment, we also observed a synergistic increase in the inhibition of cell proliferation.

Conclusion
Considering our earlier work showing that MCL-1 dependence predicts AML patient response to the ACM regimen, we propose that MCL-1 repression is the primary mechanism of alvocidib’s clinical activity. As MCL-1 also confers resistance to cytarabine, the current study provides additional rationale for the inclusion of alvocidib in the treatment of AML, and in the ACM regimen specifically. Taken together, this data suggests that the ACM regimen may be an effective regimen in treating patients with high-risk AML, because of alvocidib’s inhibition MCL-1.

Session topic: 3. Acute myeloid leukemia - Biology

Keyword(s): Mcl-1, AML