New therapeutic strategy for Acute Promyelocytic Leukemia & novel mutations
12 Oct 2016
Novel therapeutic strategy for Acute Promyelocytic Leukemia (APL) and novel mutations in APL patients
By Dr Vikram Mathews, Senior Fellow
Acute promyelocytic leukemia (APL), a type of cancer of the white blood cells, is characterized by mutations in a novel PML-RARA oncogene (1-4). Arsenic trioxide (ATO) has proven efficacy as first line therapy in the treatment of acute promyelocytic leukemia (APL) (5). The relative specificity of ATO in the treatment of APL results from the ability of ATO to bind directly to the PML (promyelocytic leukemia) and chimeric PML-RARA (promyelocytic leukemia–retinoic acid receptor-α) protein which in turn leads to its degradation inside the cell. The existing literature suggests that this degradation is mediated by intra-cellular organelles called proteasomes (6, 7). Based on the current understanding of the mechanism of action of ATO in APL, proteasomal inhibition would be antagonistic to the action of ATO.
We had previously reported, in an in vitro model, that there was evidence of significant microenvironment mediated de novo drug resistance (EM-DR) to ATO (8). This suggests that malignant cells residing in specific micro-environments as in the bone marrow were likely to be more resistant to treatment than when they were removed from this environment. We had also demonstrated that co-culturing malignant cells with stromal cells in vitro (mimicking the effect of the micro-environment in vitro) resulted in upregulation of the NF-κB pathway in malignant cells and inhibiting this pathway could overcome the micro-environment mediated drug resistance to ATO. Further, we had recently reported that by using a proteasome inhibitor (bortezomib)(9)we could overcome such drug resistance and that this drug synergizes with ATO in the treatment of APL.
On a separate note there has been a recent concern of ATO resistance in patients treated with upfront ATO (10). The focus of ATO resistance has centered on mutations in PML-RARA gene (10-12), specifically missense or point mutations in the B2 domain of the PML gene that results in the inability of ATO to directly bind to the PML and PML-RARA oncoprotein leading to resistance (12). While additional mutations have been noted in up to a third of relapsed APL patients in the PML-RARA gene it is not clear whether such mutations are associated with secondary ATO resistance as described for those in the PML B2 domain (10). However, the published data suggests that patients with such mutations have an unfavorable clinical outcome (10-12). There is a need to address novel strategies and therapeutic modalities to treat relapsed APL, especially those that have received ATO as upfront therapy.
Our in vitro observation of a possible synergistic cytotoxic effect of combining bortezomib, a proteasome inhibitor, and ATO on malignant promyelocyte in a stromal co-culture system was contradictory to existing dogma on the mechanism of action of ATO. The mechanism of such a synergy has not been previously evaluated and the theoretical antagonism of combining these two agents on PML and PML-RARA degradation, which is central to clearance of the leukemia initiating compartment in APL and achieving cure (12, 13)has not been previously addressed. In this study we evaluated the mechanism of bortezomib induced cytotoxicity against malignant promyelocytes, its potential mechanism of synergy with ATO and the fate of PML-RARA when this combination was used.
We demonstrate an alternative mechanism of PML-RARA degradation following treatment with ATO and bortezomib and also demonstrate synergy between these two agents. Through a series of in-vitro experiments, animal models and preliminary clinical trial data we have validated the beneficial effect of this combination in the management of APL. Most interestingly this combination is highly effective against ATO resistant APL cell lines and in patients with relapsed APL (14).
The overall management of APL has evolved into a de-escalation strategy from intensive myelo-toxic combination chemotherapy to non myelo-toxic regimens (15). This evolution has been achieved by remarkable progress in the understanding of the biology of the disease and the mechanism of action of agents, such as ATO and ATRA, used to treat this condition. Data in this research publication along the same lines, demonstrates the potential of the use of another non-myelotoxic agent (proteasome inhibitor) in combination with ATO being effective even in ATO resistant and relapsed APL. The potential is that this group of drugs could replace potent myelo-toxic anthracylines even in high risk and relapsed APL where they continue to be used in standard of care.
These observations bring to the forefront novel biology, illustrate new therapeutic targets and have significant translational potential which could have a bearing on other leukemia’s and cancers beyond APL.
In another study, in collaboration with Dr. Phillip Koeffler’s laboratory at National University of Singapore, we did an international multi-center co-operative study to comprehensively analyze the spectrum of mutations in newly diagnosed and relapsed APL. As previously reported we also noted the frequent mutations in the PML and PML-RARA genes in relapsed APL patients in comparison to newly diagnosed patients. Additionally, we demonstrated an increased frequency of previously unidentified mutations in the ARID1B and ARID1A genes in relapsed patients and the potential mechanism by which they mediate drug resistance in relapsed patients(16).
These two publications together address the pathological mechanisms of drug resistance and relapse in acute promyelocytic leukemia along with a novel combination of approved drugs to overcome such resistance and improve clinical outcomes in these patients.
Rationale and efficacy of proteasome inhibitor combined with arsenic trioxide in the treatment of acute promyelocytic leukemia. Ganesan S1, Alex AA1, Chendamarai E, Balasundaram N, Palani HK, David S, Kulkarni U, Aiyaz M, Mugasimangalam R, Korula A, Abraham A,Srivastava A, Padua RA, Chomienne C, George B, Balasubramanian P, Mathews V.Leukemia. 2016 Sep 2. doi: 10.1038/leu.2016.227
Comprehensive mutational analysis of primary and relapse acute promyelocytic leukemia.
Madan V, Shyamsunder P, Han L, Mayakonda A, Nagata Y, Sundaresan J, Kanojia D, Yoshida K, Ganesan S, Hattori N, Fulton N, Tan KT,Alpermann T, Kuo MC, Rostami S, Matthews J, Sanada M, Liu LZ, Shiraishi Y, Miyano S, Chendamarai E, Hou HA, Malnassy G, Ma T, Garg M, Ding LW, Sun QY, Chien W, Ikezoe T, Lill M, Biondi A, Larson RA, Powell BL, Lübbert M, Chng WJ, Tien HF, Heuser M,Ganser A, Koren-Michowitz M, Kornblau SM, Kantarjian HM, Nowak D, Hofmann WK, Yang H, Stock W, Ghavamzadeh A, Alimoghaddam K, Haferlach T, Ogawa S, Shih LY, Mathews V, Koeffler HP. Leukemia. 2016 Aug;30(8):1672-81. doi: 10.1038/leu.2016.69. Epub 2016 Apr 11.
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14. Ganesan S, Alex AA, Chendamarai E, Balasundaram N, Palani HK, David S, et al. Rationale and efficacy of proteasome inhibitor combined with arsenic trioxide in the treatment of acute promyelocytic leukemia. Leukemia. 2016.
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16. Madan V, Shyamsunder P, Han L, Mayakonda A, Nagata Y, Sundaresan J, et al. Comprehensive mutational analysis of primary and relapse acute promyelocytic leukemia. Leukemia. 2016;30(8):1672-81.
Image Credit Wellcome Photo Library, Wellcome Images. Photomicrograph of peripheral blood in acute promyelocytic leukaemia. Shows abnormal immature myeloblastic and promyelocytic white cells.