1. Resistance to Targeted Therapy in Chronic Myelogenous Leukemia Andreas Hochhaus, Philipp Erben, Thomas Ernst, and Martin C. Mueller Seminars in Hematology 44(suppl 1):S15-S24,2007 Prof. 윤휘중 / R2 이윤정

2. INTRODUCTION Chronic myelogenous leukemia (CML) : 1.5 cases per 100,000 people, 15% of all patients with leukemia Imatinib (Bcr-Abl selective tyrosine kinase inhibitor, imatinib mesylate (Glivec)) therapy for CML : annual mortality rate  15% to 20% down to 2%, based on the current data bcr-abl fusion gene( pluripotential hematopoietic stem cells) : expansion of a hematopoietic clone  development of CML

3. Molecular Biology of CML Semin Hematol 44(suppl 1):S4-S14,2007

4. Defining Resistance to Imatinib in CML Initially refractory to imatinib treatment, lose imatinib sensitivity over time, experience relapse in chronic-phase ( CP) CML, advanced phase of CML

5. Defining Resistance to Imatinib in CML Hematologic resistance : lack or loss of normalization of peripheral blood counts, the differential leukocyte count, and spleen size, in advanced phases of CML, failure to return to CP Cytogenetic resistance : lack or loss of major cytogenetic response (MCR<= 35% Ph-positive [Ph] metaphases) or complete cytogenetic response (CCR; 0% Ph metaphases) Molecular resistance : lack or loss of CMR (absence of detectable Bcr-Abl transcripts by reverse-transcriptase polymerase chain reaction (RT-PCR)). Molecular relapse : complete cytogenetic responders, increase in Bcr-Abl transcript levels by five- to 10-fold.

6. Resistance to Imatinib Monotherapy Chronic Phase CML Early phase II studies : imatinib involved CP CML patients in whom interferon- (IFN-) therapy had failed 454 patients, 5% did not achieve a CHR 18 months median follow-up (primary resistance)  40% had less than an MCR 24 months follow-up  secondary resistance or relapse was approximately 13%. The International Randomized Study of Interferon and STI571 (IRIS) :imatinib efficacy in 553 patients newly diagnosed with CP CML and treated with imatinib without prior therapy 18 months median follow-up (primary resistance)  approximately 5% 24 months follow-up  estimated rate of relapse or progression was 10%

7. Pathogenesis of Resistance 1.Bcr-Abl Mutations Frequency of Bcr-Abl mutations in resistant patients : 42%- 90% The T315I mutation and some mutations : ATP phosphate-binding loop (the “Ploop”) was greater level of resistance Detection of a kinase domain mutation was important 2. Activation of Alternative Pathways PI3K/Akt activation was found to be a critical mediator of cell survival in vitro during the early phase of imatinib resistance development Inhibiting imatinib-induced Akt activation in vitro using mTOR inhibitors and Akt-specific siRNA, imatinib resistance can be effectively antagonized

8. Pathogenesis of Resistance 3. 3. Clonal Cytogenetic Evolution Clonal evolution has been demonstrated to occur during imatinib therapy and it has also been associated with disease progression : del9q, p53 mutations and loss of p53 4. Chromosomal Aberrations in Ph-Negative Cells The underlying mechanism is not completely understood 5. Pharmacologic Interaction, Drug Transport Overexpression of P-glycoprotein (Pgp), is encoded by the multidrug resistance (MDR1) gene Differential expression of influx, facilitated by transporters such as by the human organic cation transporter (hOCT1), and efflux, facilitated by MDR1 Acid 1 glycoprotein (AGP) binds to imatinib with high affinity was inhibit its activity

9. Strategies to Prevent Resistance Optimal Dosing at Diagnosis Full therapeutic doses as early as possible 400 mg/d for CP CML and 600 mg/d for advanced disease can aid in diminishing the risk of relapse. Clinical evidence Phase I study : dose-escalating trial in patients with CP CML  dose-response relationship Dose escalation may be the optimal approach for avoiding imatinib resistance in patients with either newly diagnosed CML or no prior therapy subsequent to IFN failure.

10. Strategies to Treat Resistant Patients 1. Dose Escalation Clinical Evidence. 54 patients with CP CML resistant or refractory to imatinib were given doses of 300 or 400 mg imatinib, escalated from once to twice daily. Improve response in patients with accelerated disease, who had higher response rates when treated with 600 mg of imatinib compared with 400 mg. Higher imatinib dosages is a higher rate of adverse effects  myelosuppression and fluid retention Imatinib phase II trials demonstrated efficacy and feasibility of administering higher-than-standard doses of imatinib in advanced phases of CML

11. Strategies to Treat Resistant Patients 1. Dose Escalation Rationale for Use of Higher-than-standard Dose of Imatinib to Overcome Resistance. Bcr-Abl kinase domain point mutations Three main regions of the Bcr-Abl kinase domain: (1) the P- loop, (2) the catalytic domain and intervening sequences containing amino acids that contact imatinib (3) the activation loop Imatinib resistance retain dependency on Bcr-Abl kinase activity  respond to increasing concentrations of imatinib. Multidrug-resistance mechanisms  overexpression of Pgp, increasing imatinib concentrations can assist in overcoming resistance

12. Strategies to Treat Resistant Patients 2. Interruption or Cessation of Imatinib Therapy Clinical Evidence. Reduce a clone of cells bearing a Bcr-Abl Y253H (P-loop) mutation Rationale. High frequency of mutations that impair the binding of Bcr-Abl to imatinib, such as E255K or Y253F, and the poor prognosis portended by P-loop mutations Cessation or interruption of imatinib of resistant patients. Nonmutant Bcr-Abl leukemic clones to suppress the mutant clone by removing its competitive advantage.

13. Strategies to Treat Resistant Patients 3. Upfront Combination Therapy Clinical Evidence combination of imatinib plus IFN, Cytarabine Dasatinib (Novel Bcr-Abl inhibitors) Imatinib plus lonafarnib (Farnesyltransferase inhibitors) The serine/threonine protein kinase mTOR is a downstream component of the PI3K/Akt pathway  controlling cell growth and proliferation. mTOR inhibitor (rapamycin) with imatinib markedly enhanced this growth-inhibitory effect.

14. Strategies to Treat Resistant Patients 4. Novel Bcr-Abl Inhibitors Dasatinib Thiazolecarboxamide  bind to the Abl kinase domain in both the active (open) and inactive (closed) conformations, inhibits Src family kinases Nilotinib Aminopyrimidine  binds the Abl kinase domain in the inactive conformation Phase I and II studies : dasatinib and nilotinib show encouraging hematologic and cytogenetic response rates with good tolerability

15. Strategies to Treat Resistant Patients 5.Treatment of Minimal Residual Disease Persistence of leukemic cells following imatinib therapy raises question Determine the duration of treatment with imatinib necessary to sustain molecular response Combination therapy

16. CONCLUSION Molecular-targeted therapy with imatinib has improved treatment of CML Resistance and relapse plays a role in the complex interaction between imatinib and CML progression Combination therapies with cytotoxic agents or agents that target multiple sites within the Bcr-Abl signal transduction pathway