Molecular targeted therapy in acute myeloid leukemia

Keywords:

• Acute myeloid leukemia
• FLT3 protein
• RAS
• JAK2

The standard of care treatment for acute myeloid leukemia (AML) has remained relatively unchanged over the past four decades. Therapy typically consists of intensive induction therapy, most commonly with a combination of an anthracycline and cytarabine, followed by post-remission consolidation, with cytarabine-based chemotherapy or stem cell transplantation (SCT).1 However, only approximately 40–50% of younger patients (<60 years) and 10–20% of older patients (>60 years) will the cured.2 Recent advances in molecular technology and a better understanding of the biology of AML have led to the identification of novel molecular markers in AML.3 These markers may play a role in defining prognosis, developing targeted therapies and detecting minimal residual disease in patients with AML. A number of molecular markers have been identified, including FMS like tyrosine kinase 3 (FLT3) mutations, RAS/RAF/MEK/ERK pathway (RAS) mutations and mammalian Janus kinase (JAK2) mutations.

FLT3

FLT3 plays a crucial role in normal hematopoiesis and cellular growth in primitive hematopoietic stem and progenitor cells.4 Signaling via receptor tyrosine kinases (RTKs) is frequently deregulated in hematological malignancies. FLT3 is a transmembrane tyrosine kinase that belongs to the class 3 family of RTKs. FLT3 is activated following binding of FLT3 ligand, leading to activation of downstream signaling pathways including Stat5, Ras, and PI3 kinase. FLT3 stimulates survival and proliferation of leukemic blasts. FLT3 is expressed on the leukemic cells of 70–100% of patients with AML. Importantly, activating mutations in FLT3 are observed in approximately 30% of adult AML patients. The two leading types of mutations found in AML include internal tandem duplications (ITDs) in the juxtamembrane domain (17–34%) and mutations in the activation loop (approximately 7%).5 Patients with mutations in FLT3 have a worse prognosis when treated with conventional chemotherapy, compared to patients with wild-type FLT.6 Initial studies using small molecule FLT3 inhibitors have offered encouragement that a more selective therapy may improve the outcome of patients. In addition, when FLT3 inhibitors are added to the conventional arsenal of AML treatments patients with AML expressing mutated FLT3 may experience significant clinical benefit.

Quizartinib (AC220)

Quizartinib (AC220) is a novel second-generation class 3 RTK inhibitor with potent FLT3 activity in vitro and in vivo.7 In addition to FLT3, quizartinib inhibits c-KIT, platelet derived growth factor, and RET. AC220 has been explored in a phase 1 trial in 76-relapsed/refractory or untreated, elderly AML patients.8 Quizartinib had significant clinical activity, inducing a reduction in blast counts, including full complete remission (CR) in some patients. Ten of 76 (13%) patients had a CR (2 CR, 6 CR with incomplete blood count recovery, and two CR with incomplete platelet recovery) and 13 (17%) had a partial remission (PR). The median duration of response was 14 weeks, with duration of responses up to 67+ weeks. Higher overall response rates and CR rates were observed in FLT3-ITD patients (56 and 28%, respectively) compared with those lacking the mutation (20 and 7%, respectively). The most commonly reported adverse events that were possibly drug-related were reversible prolongation of QTc; others were mainly grade 2 and included peripheral edema, dysgeusia, and nausea. Phase 2/3 studies combining quizartinib with standard chemotherapy agents are currently ongoing.

Sorafenib (BAY 43-9006)

Sorafenib (BAY 43-9006) is a multikinase inhibitor that targets B-RAF, platelet derived growth factor, and FLT.9 Sorafenib was shown to be safe and clinically effective in two phase 1 trials that examined the effects of both dose and schedule of sorafenib in relapsed or refractory AML patients. The most common toxicities were fatigue (16%) and hypokalemia (13%). A randomized phase 1 trial examining two schedules of sorafenib, either continuous or intermittent, was conducted in patients with relapsed or refractory AML (n = 38) or untreated myelodysplastic syndrome (MDS, n = 4).10 One CR was noted in an FLT3-ITD-positive AML patient. Sorafenib administered either before or after allogeneic SCT (allo-SCT) in AML patients has also been explored. Sorafenib-induced remission allowed for allo-SCT in two of the three refractory AML patients. Two of the four patients treated after allo-SCT survived 216 and 221 days, respectively, while the other two remained in ongoing complete molecular remission.11 Sorafenib in combination with idarubicin and cytarabine is currently under investigation in a phase 1/2 trial in newly diagnosed patients <65 years of age.12 Of 51 evaluable patients, 38 (75%) achieved a CR, including 12 (92%) of 13 FLT3-ITD, 2 (100%) of 2 patients with FLT3-TKD, and 24 (66%) of 36 FLT3-wild-type patients. The difference in CR rate between the FLT3-mutated and FLT3-wild-type patients was statistically significant (P = 0·033). Sorafenib is currently being used in combination with azacytidine, plerixafor/granulocyte colony-stimulating factor, and other agents in ongoing phase 2 studies.

Midostaurin

A phase 2b study of midostaurin in AML and MDS patients with either wild-type (n = 60) or mutant (n = 35) FLT3 has been reported.13 Among 92 patients evaluable for response, 71% of FLT3-mutant patients had a ⩾50% reduction in peripheral blood or bone marrow blasts compared to 42% of FLT3-wild-type patients. One partial response was observed in an FLT3-mutant patient on the 100 mg dose regimen. Grade 1/2 nausea and vomiting were the most common adverse events, respectively, observed in 60 and 48% of patients.

Other FLT3 inhibitors include lestaurtinib, sunitinib, and KW-2449.

RAS

The RAS is critical for proliferation of many human cancers. Normally, this pathway is activated by the binding of extracellular growth factors to membrane receptors which leads to activation of the small guanosine triphosphate-binding protein RAS. As a result, RAS adopts an activated conformation, which stimulates downstream signaling. One of the critical downstream proteins is RAF, which phosphorylates MEK. Activated MEK then phosphorylates residues on the MAP kinases: ERK1 and ERK2. Phosphorylated ERK dimerizes and translocates to the nucleus, where it is involved in cellular proliferation, nuclear transport, DNA repair, nucleosome assembly, mRNA processing, and translation.14 Mutated oncogenic forms of RAS are found in approximately 15% of all cancer including approximately 30% of AML and acute lymphocytic leukemia.15 Because of its central role in the ERK pathway, MEK is thought to be an important therapeutic target in AML.

Final and preliminary clinical data are available for three MEK inhibitors in solid tumor phase 1/2 trials (CI-1040, PD-0325901, and AZD6244). To date, these agents have been dosed in a total of approximately 500 patients. In total, there have been 12 PRs and one CR. The most common toxicities in the completed trials include rash, diarrhea, nausea, vomiting, peripheral edema, and fatigue. More concerning are reports of optical disturbances, including transient blurred vision, RVO, or optic neuropathy. A decrease in cardiac ejection fraction and transaminitis have also been reported.16

GSK1120212 is an oral selective inhibitor of MEK1/MEK1 activation and kinase activity.¹⁷17,18 To date, GSK1120212 has been administered, in approximately 300 subjects with a variety of refractory cancers. Studies aimed at identifying recommended doses and regimens of GSK1120212 in AML and other leukemias are ongoing.

MSC1936369B is a potent, selective MEK inhibitor, demonstrating robust antitumor activity in cell proliferation and clonogenic assays in vitro. The preclinical in vivo and in vitro antileukemia and antimyeloma activity of MSC1936369B has been demonstrated. Phase 1/2 trials using MSC1936369B in relapsed/refractory AML are currently accruing.

JAKs

The mammalian JAKs protein family consists of four cytoplasmic tyrosine kinases (JAK1, JAK2, JAK3, and TYK2) that play a role in hematopoiesis. Upon activation by an associated receptor, JAKs phosphorylate cytoplasmic signal transducer and activator of transcriptions (STATs), resulting in altered expression of target genes. Aberrant activation of JAKs has been associated with increased malignant cell proliferation and survival in patients with Philadelphia chromosome negative myeloproliferative disease.19 Although JAK2 mutations are extremely rare in AML, STAT3 and/or STAT5 are activated in a majority of AML samples.20 This may occur via mutations in costimulatory molecules. For example, it has been shown that mutations in FLT3 domain can constitutively activate this receptor kinase without ligand binding, resulting in the activation of downstream prosurvival signals including JAK/STAT5. Thus, there may be a role for JAK/STAT5 inhibitors in AML.

Ruxolitinib (INCB018424) is a JAK2 inhibitor that has been extensively studied in patients with myeloproliferative neoplasms. In a phase 2 study of ruxolitinib in patients with relapsed/refractory leukemias, 18 patients with relapsed and refractory leukemias [nine de novo AML, three secondary AML (sAML), two acute lymphocytic leukemia, one MDS, two chronic myelomonocytic leukemia (CMML), and one chronic myeloid leukemia] were treated with ruxolitinib. Five patients (one with AML, two with sAML, and three with CMML) had the JAK2 V617F mutation. Three patients (including two with sAML and one with CMML, all with JAK2 mutation) had significant declines in their bone marrow blasts (to <5%) and clinical improvement.21

Other drugs acting on the JAK2 pathway include AZ 960 (potent and selective ATP competitive inhibitor of the JAK2 kinase), Ki11502 (a novel multitargeted RTK inhibitor), CYT387, SAR302503 (formerly TG101348, a novel JAK2-FLT3 inhibitor), and SB1518 (also JAK2-FLT3 inhibitor). These are currently in clinical trials.

Conclusion

Growing evidence suggests that molecular targeted therapy have clear clinical value for patients with AML. Our challenge is to further understand the molecular biology of AML and to find appropriate treatment niches for these targeted therapies, either as single agents or in combination regimens. This may ultimately improve the outcomes for patients with AML.