	The use of tyrosine kinase inhibitors (TKIs) to inhibit BCR-ABL protein in chronic myeloid leukemia (CML) is one of the major milestones in modern oncology. After the FDA approval of imatinib (a first-generation TKI), dasatinib, nilotinib, and later bosutinib, recurrence of disease due to multiple mechanisms of resistance became the new challenge in CML treatment. The 

 mutation is of special interest as it continues to be an obstacle to the use of TKIs. In this review, we cover the clinical efficacy data of the approved agents for this indication, ponatinib and omacetaxine, in addition to discussing the role of stem cell transplantation and other novel agents in the treatment of CML.

Abelson tyrosine kinase (ABL1) is a nonreceptor tyrosine kinase involved in cell growth and proliferation. Chronic myeloid leukemia (CML) arises from the fusion of the ABL1 gene (chromosome 9q34) with the breakpoint cluster region (BCR) gene (chromosome 22q11.2), generating the Philadelphia chromosome expressing BCR-ABL1. BCR-ABL1 is a constitutively activated tyrosine kinase that activates many signaling pathways, thus providing a proliferative advantage.

 Imatinib, the first tyrosine kinase inhibitor (TKI), competitively inhibiting the BCR-ABL1 kinase, was approved based on the results of the International Randomized Study of Interferon versus STI571 (IRIS) trial showing that at 18 months, 76.2% of the patients achieved complete cytogenetic response (CCyR) and 96.7% were free from disease progression.

 Longer follow-up revealed significant relapses

; of these patients, 40% to 50% were identified as having point mutations at the ATP-binding site of the ABL1 kinase domain.

Development of second-generation TKIs (dasatinib, nilotinib) helped overcome most mutations except 

 Dasatinib was able to overcome 14 of the initially identified 15 imatinib-resistant mutations.

 Subsequently, dasatinib was found to be less effective against additional mutations such as F317L and T315A due to the important role of phenylalanine 317 and threonine 315 serving as contact residues to imatinib and dasatinib.

 Nilotinib is not affected by the phenylalanine 317 position. Thus, F317L is not considered resistant to nilotinib. However, other mutations, such as E459K and Y253H, were found to be resistant to nilotinib.

, a threonine to isoleucine gatekeeper mutation results in alteration of the structure of the ATP-binding pocket by eliminating a hydrogen-bonding interaction involved in binding first- and second-generation TKIs.

 Several third-generation TKIs (including bosutinib) were designed to target 

, but only a few agents have shown clinical activity.

Ponatinib was designed to accommodate the 

 side chain via a carbon-carbon triple bond and was confirmed to bind to the inactive mode of the kinase domain in the murine ABL1

. Preclinical data suggested that it prevents the autophosphorylation of both native and 

-mutant kinase and inhibits growth and signaling in the cells expressing native or mutant 

. Dose-dependent antitumor activity was shown in a BCR-ABL1

 mouse xenograft model, with significant response at daily doses of 10 mg/kg and 30 mg/kg.

 It was also shown to inhibit compound mutations, in addition to multiple other kinase pathways such as the SRC family kinases, platelet-derived growth factor receptor-, vascular endothelial growth factor receptor pathways, and 

Based on these results, a phase I trial enrolled 48 patients (31 with refractory chronic-phase [CP] CML, six with accelerated-phase [AP] CML, five with blast-phase [BP] CML, and two with Philadelphia-positive acute lymphoblastic leukemia [ALL]), 18 of whom had the 

 Interim results showed that pancreatitis was the dose-limiting toxicity at 60 mg orally daily, and 45 mg became the recommended dosage for future studies. CCyR was noted in 33%, and major molecular response (MMR) in 48% of all patients. CCyR was achieved in 57% of patients with the 

mutation. Significant adverse events (AEs) included nausea (20%) and fatigue (15%). Additional data on 11 patients with the 

 mutation from the same trial showed that 82% achieved major cytogenetic response (MCyR) and 40% achieved MMR.

 The final results of the dose-escalation trial (n = 81) were recently published.

-mutant CP CML, 100% had a complete hematologic response (CHR) and 92% had a MCyR. AEs were similar to previously published results.

The PACE (Ponatinib Philadelphia-positive ALL and CML Evaluation) trial,

 a pivotal phase II study, was conducted in 449 heavily pretreated patients (including 85 with AP CML, 62 with BP CML, and 32 with Philadelphia-positive ALL) who were resistant/intolerant to nilotinib or dasatinib or had the 

 mutation at baseline, and were followed for a median of 11 months (range, 0.1-21 months). In patients with CP CML with the T315I mutation, 70% achieved the primary endpoint of MCyR. For AP CML, BP CML, and Philadelphia-positive ALL, the primary endpoint was major hematologic response (MaHR) and was achieved in 50% and 33% (combined for the latter two), respectively. A post-hoc analysis showed that the presence of the mutation was not a predictor of response, which was instead explained by the younger age of the patients, exposure to less treatment, and shorter duration of leukemia in the patients with the 

. At 45 mg daily, ponatinib most commonly caused thrombocytopenia (36%) and rash (33%). The most common serious AE was pancreatitis (5%). Arterial thrombotic events and hepatotoxicity were also noted and added to the black box warnings for this drug.15 Based on the PACE study, the FDA approved ponatinib as a treatment option for all phases of CML that are resistant or intolerant to TKIs.

Although ponatinib was originally developed as a pan BCRABL inhibitor, recent studies identified new mutations that are resistant to ponatinib in vitro, such as 

 Further, a follow-up study (based on the PACE data) compared the impact of baseline mutations and endof- treatment mutations on response to ponatinib. MCyR was achieved in 49% of patients with no mutations at baseline, 64% of patients with one mutation, and 62% in patients with two or more mutations present at baseline.

 These data suggested that patients without mutations harbored other, yet unknown mechanisms of resistance. In addition, MCyR was achieved in 57% of patients with mutations other than T315I, 74% of patients with 

, and 57% of patients with T315I and other mutations. Multivariate analysis still showed that 

 is not an independent predictor of response as it was noted in the initial PACE study. Among 56 patients who had an end-oftreatment mutation evaluation, 46 had no change, five lost a mutation, and five gained mutations, again suggesting other, not-yet-discovered mechanisms of resistance.

In the 1970s, homoharringtonine (HHT; an omacetaxine precursor) and other esters of alkaloid cephalotaxines were isolated from the bark of the Cephalotaxus fortunei evergreen tree and found to have activity against murine leukemia by decreasing protein synthesis and inducing apoptosis.

 HHT was found to induce apoptosis in vitro in CML cells (including 

-mutant cells), and also in the murine model of CML and 

 It is different from TKIs as it induces apoptosis in both CML and non-CML stem cells in vitro and in murine models, which makes it an effective strategy to eradicate resistant disease.

 Interestingly, before the imatinib era, HHT was considered an effective agent for CML that had failed interferon-alpha therapy.

 Anecdotal reports illustrated HHT activity in patients with T315I-mutant CML refractory to TKIs.

Omacetaxine, a subcutaneous, semisynthetic HHT, was evaluated in 2006 in a phase II trial (CML-202) in imatinibresistant, 

 In this study, omacetaxine was given at 1.25 mg/m2 twice daily for 14 days every 28 days until CHR, followed by maintenance at the same dosage for 7 days every 28 days, for up to 24 months. The preliminary data included 55 patients (32 CP, 14 AP, and 9 BP CML); 53% had failed at least two TKIs and 27% had failed at least three TKIs. CHR rates were 80%, 45%, and 13%, respectively, per disease phase. Cytogenetic response (CyR) rates were 28%, 9%, and 0%. The 2-year progression-free survival (PFS) was 70% for patients with CP CML. The most common grade 3/4 AEs were thrombocytopenia (53%) and neutropenia (28%). The final results from this study were published in 2012 and illustrated that among 62 patients with CP CML, MCyR was 23% and CCyR was 16%, with a median PFS of 7.7 months.

 A median overall survival (OS) was not reached at a median follow-up to 19 months.

Another phase II study (N = 122) was conducted in patients with CP and AP CML.

 In patients with CP CML who had failed two or more TKIs, MCyR was achieved in 27%, with a median survival of 30 months. Pooled safety analysis of 207 patients in the two phase II studies showed that the most common AEs were thrombocytopenia, anemia, diarrhea, neutropenia, and nausea.29 Febrile neutropenia occurred in 11% of patients. Based on these data, omacetaxine was approved by the FDA in 2012 for CP and AP CML intolerant to other therapy or following failed therapy with two or more TKIs. The 

 summarizes the clinical efficacy data of omacetaxine and ponatinib in 

	Hematopoietic Stem Cell Transplantation (HSCT)

HSCT is another treatment option for patients with the 

 mutation, although evidence in this area is scarce. A retrospective study identified eight patients with 

-mutated CML who underwent nine unrelated allogeneic HSCTs; two patients had CP CML, three had AP CML, and three were in second CP CML.

 Two of the patients underwent myeloablative conditioning, four had reduced-intensity conditioning, and one had a myeloablative preparatory regimen in the first HSCT, followed by a reduced-intensity regimen in the second. CCyR was achieved in four patients, and complete molecular remission (CMR) was achieved in four patients as the best outcome. The best outcomes were noted in two patients who underwent HSCT in CP CML and achieved CMR; they remained alive without recurrence for 14 and 42 months. At a median follow-up of 13 months, five patients were still alive, three of them in CMR, one in CCyR, and one in CHR.

		Table. Summary of Clinical Efficacy Data of Ponatinib and Omacetaxine in 

					Thrombocytopenia (20%) Neutropenia (10%)

					Thrombocytopenia (76%) Neutropenia (44%)

		AEs indicates adverse events; AP, accelerated phase; BP, blast phase; CHR, complete hematologic response; CCyR, complete cytogenetic response; CP, chronic phase; MaHR, major hematologic response; MCyR, major cytogenetic response; MMR, major molecular response; n, number; Ph+ ALL, Philadelphia chromosome-positive acute lymphoblastic leukemia.

A recent larger study of patients with T315I-mutated CML (N = 64) concluded that allogeneic HSCT resulted in a median OS of 10.3 months in CP CML and 7.4 months in BP CML.

 Unrelated donor stem cells correlated with poor outcome. The available data suggest that HSCT is a viable option for patients with the T315I mutation, but it has not been proven to be superior or inferior to omacetaxine or ponatinib.

DCC-2036 (rebastinib) is a switch pocket inhibitor that prevents ABL1 kinase activation when blocking essential conformation change in the switch pocket. A phase I study in 30 patients with CML (including 

 mutations) suggested the median tolerated dosage to be 150 mg orally twice a day, and resulted in two out of 19 CyRs in patients with CP CML.

 Aurora kinase inhibitors, including MK-0457 (tozasertib), may also block ABL1 kinase at the gatekeeper site. As noted in a phase I/II study conducted in 18 patients with 

-mutated CML, eight hematologic responses were achieved.

 The combination of MK-0457 and vorinostat (a histone deacetylase inhibitor) suggested synergistic effect in imatinibresistant CML cells with the 

 mutation, which could be the beginning of an era of targeted therapy combinations in CML clinical trials.

The success story of CML and TKIs continues to mature with more available options that could tackle treatment challenges. Although more advances have been recently made in 

-mutant CP, AP, and BP CML, this is still an area that could use more effective agents. The current direction of research is focused on novel experimental agents, combinations of known TKIs, and targeting CML stem cells in hopes of finding the cure for this disease.

Dr. Al Ustwani has no conflicts of interest to report.

Dr. Wetzler has been a consultant/advisory board member for TEVA Pharmaceutical Industries, Ltd. and Ariad Pharmaceuticals.

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			O’Hare T, Shakespeare WC, Zhu X, et al. AP24534, a pan-BCR-ABL inhibitor for chronic myeloid leukemia, potently inhibits the T315I mutant and overcomes mutation-based resistance. 

			Karaman MW, Herrgard S, Treiber DK, et al. A quantitative analysis of kinase inhibitor selectivity.

			Talpaz M, Deininger M W, Shah NP, et al. Phase I trial of AP24534 in patients with refractory chronic myeloid leukemia (CML) and hematologic malignancies. 

			Cortes JE, Talpaz M, Bixby D, et al. Phase 1 trial of oral ponatinib (AP24534) in patients with refractory chronic myelogenous leukemia (CML) and other hematologic malignancies: emerging safety and clinical response findings. 

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Articles

Abelson tyrosine kinase (ABL1) is a nonreceptor tyrosine kinase involved in cell growth and proliferation. 

Omar Al Ustwani, MD

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Novel Therapies for T315I-Mutant Chronic Myeloid Leukemia