Over the past 4 decades since effective chemotherapy regimens for AML were introduced, several common observations have dominated thinking about AML therapy. These observations include: 1)some cases have excellent responses to chemotherapy while others do not; 2) patients who relapse after initial response are unlikely to be cured by chemotherapy alone; 3) there is an unexplained therapeutic index for induction chemotherapy; and 4)allogeneic transplant is required to rescue relapsed and poor risk patients. Our results suggest that differences in pre-treatment mitochondrial priming, as measured by BH3 profiling, provide a novel biological explanation for these heretofore unexplained clinical observations.
The observation that some patients achieve a complete remission and maintain it after chemotherapy alone, while others do not achieve remission, or relapse, has lacked a biological explanation. While cytogenetics have proven valuable as prognostic indicators of this behavior, it should be noted that cytogenetics have behaved mainly as empirically derived markers, with little power to explain the biological mechanism underlying differential killing of myeloblasts between patients. In this study, we tested the hypothesis that the differential mitochondrial tendency to apoptosis, or priming, measured by BH3 profiling explains the differential cellular and clinical response to cytotoxic chemotherapy. Our results support this hypothesis, as reflected in both initial response () and longer term freedom from relapse (). This lends additional support to the concept that mitochondrial priming is an important determinant of clinical chemosensitivity that we supported in prior work in other cancers including multiple myeloma, acute lymphoblastic leukemia, and ovarian cancer (Ni Chonghaile et al., 2011
Another important observation is that patients who relapse following an initial complete remission to induction chemotherapy are unlikely to achieve long-term remissions from subsequent chemotherapy, no matter what combination of agents is used. It is known that relapsed AML tends to be broadly more chemoresistant than that of the initial presentation, but the mechanism underlying this chemoresistance has been unclear. Our results suggest that selection for reduced mitochondrial priming in relapsed AML may well be an important determinant of this chemoresistant phenotype ().
Success of induction chemotherapy depends on a therapeutic index. That is, there must be a feature of myeloblasts that renders them selectively more chemosensitive than critical normal tissues. Perhaps surprisingly, given its centrality to the treatment of AML, the biologic basis of this feature remains poorly understood. There is no obvious AML-specific target exploited by standard induction chemotherapy, as it acts primarily to damage DNA, a target present in normal as well as malignant cells. We found that mitochondrial priming was a key determinant of the therapeutic index between myeloblasts and normal cells () and therefore one answer to the question, “Why does chemotherapy work?” We could not identify a BCL-2 family protein whose level replicated the performance of BH3 profiling, supporting the concept that priming is likely the result of the simultaneous contribution of many proteins, perhaps even including some outside the BCL-2 family.
Two important points go beyond the elucidation of biological mechanisms of clinical behavior of AML and into potential clinical application. The first is the identification of a therapeutic index and a potential predictive biomarker for BCL-2 inhibition. We have made the observation not only of myeloblast sensitivity, but also relative HSC insensitivity to BCL-2 inhibition. Others have previously made a similar observation (Konopleva et al., 2006
). However, we demonstrated, based on our mitochondrial BH3 profiling studies, that this is an on-target effect, based in the mitochondrion, and, further, that BH3 profiling is a potential predictive biomarker. Significantly, we found that BCL-2 dependence is observed even in cases where there was a poor response to conventional induction chemotherapy, indicating a potential strategy to rescue this difficult to treat population. Notably, identifying this therapeutic index and validating a predictive biomarker is of more than purely scientific interest, since clinical BCL-2 inhibition is now a practicable clinical approach. Currently, Abbott Laboratories has two drugs in clinical trials that directly target BCL-2, ABT-263 and ABT-199 (Roberts et al., 2012
; Tse et al., 2008
; Wilson et al., 2010
). Both are orally available counterparts of ABT-737. An on-target toxicity, thrombocytopenia, due to high affinity binding to BCL-XL on which platelets depend, may limit testing of ABT-263 in AML, since patients very commonly present with existing thrombocytopenia. However, ABT-199 has greater selectivity for BCL-2, and lower affinity for BCL-XL, so the chances are better for achieving in vivo BCL-2 antagonism without exacerbating thrombocytopenia.
The second important advance is our identification of BH3 profiling as a potential predictive biomarker in AML, not only for BCL-2 inhibition but also for conventional chemotherapy. The understandable enthusiasm for BCL-2-targeted therapy notwithstanding, we expect that conventional chemotherapy and allogeneic bone marrow and stem cell transplantation, with their demonstrated curative potential, will remain a mainstay of AML therapy for many years to come. Therefore, it is worth considering how our findings could be used to better direct use of these modalities. There are two important dilemmas often encountered in the treatment of AML patients. In those under 60, what is the best post-remission strategy? In other words, who should receive an Allo-SCT in first complete remission? Allo-SCT has the potential to cure patients who are at high risk of relapse, but it is bears a higher treatment related mortality, and can be accompanied by years of chronic graft versus host disease. Therefore, the optimal strategy is to identify those that are most likely to relapse following complete remission, and selectively direct them to Allo-SCT. Currently, predictive tools such as those used by the ELN employ a combination of genetic and cytogenetic markers to perform such prediction. While useful, these still appear to be imprecise tools, and we have found that BH3 profiling can actually improve the prognostic capabilities of the conventional prognostic approach () and by itself identify a subpopulation that apparently requires ALLO-SCT for cure ().
Another decision-making dilemma faced by clinicians is whether to administer high-dose induction chemotherapy to newly diagnosed patients over 60 years of age. In older patients, treatment related mortality is higher and complete remission rates are lower. Clinical benefit appears to be restricted to those who attain complete remission. Therefore, it would be useful to be able to predict which patients are most likely to achieve a complete remission, and direct them to standard induction chemotherapy, sparing those patients unlikely to achieve remission the significant side effects. BH3 profiling apparently can identify those patients most likely to achieve a complete remission following induction chemotherapy (). We will be testing the predictive utility of BH3 profiling in these two clinical settings in followup prospective clinical trials.
Our studies here do not directly demonstrate what upstream factors determine the relative priming of different cells. It is likely that activation of different oncogenes contributes, as this by itself can affect sensitivity to apoptosis. It is also likely that differential activation of any of a number of tyrosine kinase driven pathways could affect priming, since it is clear that killing via inhibition of these pathways proceeds by perturbation of BCL-2 family proteins and utilization of the mitochondrial pathway of apoptosis. One strategy for improving response to chemotherapy in poorly primed AML might be to selectively increase priming in AML cells, perhaps with an agent that is highly selective, but less potent that conventional chemotherapy. Once the AML is primed into a range consistent with good clinical response, then chemotherapy might be added. We tested such a strategy in vitro, and showed it to work using BCL-2 inhibition to prime AML cells and render them more sensitive to chemotherapy (). We propose that such an approach merits testing in clinical trials, and that guidance by BH3 profiling might assist with identifying useful priming agents.
There is considerable and appropriate interest in better personalization of therapy in cancer patients, including in those with AML. The vast majority of these personalization strategies are based on genetics, and are directed toward targeted therapies. Even in these therapies, the gulf between genotype and phenotype can be difficult to bridge. Here we demonstrate that BH3 profiling, an assay of mitochondrial apoptotic function, can provide information that can potentially be exploited for personalization of AML therapy in the application of BCL-2 antagonists, allogeneic bone marrow transplant, and, likely to be a therapeutic remain a mainstay for many years, conventional chemotherapy.