Mcl-1 as a therapeutic target in acute myelogenous leukemia (AML)

2.1. Simultaneously targeting multiple arms of the apoptotic regulatory machinery (Fig. 1)

As ABT-737 does not inhibit Mcl-1, combining such agents with drugs capable of down-regulating Mcl-1 is a particularly appealing therapeutic strategy in AML, analogous to Bad and Noxa, which synergistically induce apoptosis. In this context, cyclin dependent kinase (CDK) inhibitors that inhibit the cyclin T-CDK9 complex (positive transcription elongation factor b, P-TEFb), such as flavopiridol (alvocidib), roscovitine (seliciclib) or SCH727965 (dinaciclib), down-regulate Mcl-1 by acting as transcriptional repressors. Thus, roscovitine dramatically increases ABT-737 lethality in human leukemia cells.³ Coadministration of roscovitine and ABT-737 untethered Bak from Mcl-1 and Bcl-X_L, respectively, triggering Bak activation and Bax translocation, with resultant induction of MOMP.

Another mechanism of down-regulating Mcl-1 involves inhibition of translation. It has been demonstrated that the multi-targeted tyrosine kinase inhibitor sorafenib mediates apoptotic cell death in human leukemia cells, at least in part, through down-regulation of Mcl-1 via inhibition of translation.⁴ Accordingly, sorafenib synergizes with ABT-737 to induce apoptosis in AML cell lines.⁵

ABT-737 may induce activation of extracellular signal regulated kinase and up-regulation of Mcl-1 in AML cells, arguing for a combination strategy involving inhibitors of mitogen activated protein kinase (MEK), which have been used successfully to suppress Mcl-1. Indeed, the MEK inhibitor PD0325901 potently synergizes with ABT-737 to promote killing of AML-derived cell lines, primary AML blasts, and CD34⁺38^–123⁺ AML progenitor/stem cells, both in vitro and in vivo.⁶

Navitoclax (ABT-263) is an orally available analog of ABT-737. Based on the promising laboratory findings noted above, clinical trials of navitoclax in combination with sorafenib, transcriptional CDK inhibitors, or MEK inhibitors in patients with AML are under consideration.

2.2. Combining down-regulation of Mcl-1 with antagonism of Mcl-1 function

Obatoclax (GX15-070) is another small-molecule BH3-mimetic that inhibits Mcl-1 in addition to Bcl-2, Bcl-xL and Bcl-w through a Noxa-dependent mechanism. Obatoclax potently interferes with the sequestration of Bak by Mcl-1, untethering Bak and overcoming resistance to apoptosis conferred by Mcl-1.⁷ In leukemia cell lines and primary AML cells, obatoclax both triggers apoptosis and induces autophagy.⁸ Apoptosis in this setting was preceded by the release of Bak from Mcl-1, liberation of Bim from both Bcl-2 and Mcl-1, and the formation of an active Bak/Bax complex. A recent study has demonstrated marked potentiation by obatoclax of sorafenib-induced apoptosis in multiple AML cell lines and primary AML cells but not normal CD34⁺ cells.⁹ Indeed, exposure to comparable or significantly higher concentrations of sorafenib and obatoclax alone or in combination exhibited minimal lethality toward normal CD34⁺ cells. Up-regulation of Bcl-2 family anti-apoptotic proteins represents an important mechanism of therapeutic resistance to sorafenib, and can be abolished by the concomitant administration of obatoclax. In line with the known FLT3-inhibitory action of sorafenib, synergism occurred at much lower sorafenib concentrations in FLT3-mutated cells. In FLT3-ITD AML, up-regulation of Mcl-1 is dependent on FLT3 signaling and an essential effector of FLT3-ITD-mediated drug resistance, and down-regulation of Mcl-1 sensitizes FLT3-ITD hematopoietic cells to cytotoxic and targeted therapies.¹⁰ In light of these observations, a National Cancer Institute-sponsored phase I/Ib trial (with a limited expansion cohort in FLT3-mutated patients) of sorafenib and obatoclax in patients with relapsed/refractory and selected poor prognosis types of AML or MDS is planned by the Southeast Phase II consortium. This strategy of simultaneously down-regulating Mcl-1 with sorafenib and inhibiting its function (and that of Bcl-2 and Bcl-X_L) with obatoclax to trigger apoptosis in AML cells is depicted schematically in Fig. 1.

2.3. Combinations of agents capable of down-regulating Mcl-1 with cytotoxic chemotherapy

Preclinical studies have shown that in addition to triggering apoptosis, flavopiridol may recruit surviving leukemia cells to a proliferative state, thereby priming such cells for the S-phase-related cytotoxicity of cytarabine. Pretreatment with flavopiridol enhanced the pro-apoptotic and cytotoxic effects of cytarabine on primary bone marrow blasts from patients with refractory acute leukemias.¹¹ Based on this concept, several clinical trials of timed sequential therapy with flavopiridol, cytarabine and mitoxantrone (FLAM) have been conducted. In a phase II study in 45 newly diagnosed patients with poor-risk AML, 67% achieved complete remission (CR).¹² Accordingly, a randomized phase II study comparing FLAM to cytarabine and daunorubicin in newly diagnosed patients with AML (NCT01413880) is currently ongoing.

Several groups have combined sorafenib with chemotherapy in AML. In a phase II study, 51 adults with previously untreated AML received sorafenib, cytarabine and idarubicin. While the overall CR rate was 75%, it was significantly higher (p=0.033) in FLT3-mutated patients (93%) than in FLT3-wild type (FLT3-WT) patients (66%).¹³ While responses to the combination of sorafenib and low dose cytarabine in unselected older patients with AML or high risk MDS were disappointing,¹⁴ encouraging activity was observed in both FLT3-mutated and FLT3-WT pediatric patients with relapsed/refractory AML receiving sorafenib in combination with clofarabine and cytarabine on a phase I trial.¹⁵ Whether Mcl-1 down-regulation by sorafenib (or flavopiridol) contributes to the anti-leukemic actions of genotoxic agents remains to be determined.