The achievement of durable complete cytogenetic remission in late chronic and accelerated phase patients with CML treated with Imatinib mesylate predicts for prolonged response at 6 years

Potential mechanisms of disease progression and management of advanced-phase chronic myeloid leukemia

Despite vast improvements in the treatment of Philadelphia chromosome-positive chronic myeloid leukemia (CML) in chronic phase (CP), advanced stages of CML, accelerated phase or blast crisis, remain notoriously difficult to treat. Treatments that are highly effective against CML-CP produce disappointing results against advanced disease. Therefore, a primary goal of therapy should be to maintain patients in CP for as long as possible, by (1) striving for deep, early molecular response to treatment; (2) using tyrosine kinase inhibitors that lower risk of disease progression; and (3) more closely observing patients who demonstrate cytogenetic risk factors at diagnosis or during treatment.

The expanding options for front-line treatment in patients with newly diagnosed CML

The past decade has seen remarkable advances in the treatment of chronic myeloid leukemia (CML). The discovery of the underlying cause of CML, a chromosomal translocation resulting in the expression of an aberrant tyrosine kinase, has enabled the rational development of targeted therapy with tyrosine kinase inhibitors (TKIs). The first available TKI, imatinib, dramatically improved survival rates and demonstrated the potential for long-term treatment. A number of additional strategies have been tested to further maximize outcomes in patients with newly diagnosed CML, including newer TKIs, imatinib dose escalation, and combination therapy. The advanced, more potent TKIs, nilotinib and dasatinib, have proven effective for newly diagnosed patients and for those who experience inadequate response or intolerance to imatinib. Randomized phase 3 studies have shown that nilotinib and dasatinib are more efficacious than imatinib in achieving primary study endpoints. Nilotinib was superior to imatinib in the rate of major molecular response at 12 months; dasatinib was superior to imatinib in the rate of complete cytogenetic response by 12 months. These phase 3 studies are ongoing to further define longer-term efficacy and safety. Research on additional contributing signaling pathways in CML, T315I mutations, and other causes of treatment resistance has identified additional potential treatments that are now in early stages of clinical development, with encouraging preliminary results. With continued advances, it is conceivable that the ultimate goal - a cure for CML - is in our sights.

Suboptimal responses in chronic myeloid leukemia: implications and management strategies

The high response rates and increased survival associated with imatinib therapy prompted a paradigm shift in the management of chronic myeloid leukemia. However, 25% to 30% of imatinib-treated patients develop drug resistance or intolerance, increasing the risk of disease progression and poor prognosis. In 2006, the European LeukemiaNet proposed criteria to identify patients with a suboptimal response to, or failure associated with, imatinib; these recommendations were updated in 2009. Suboptimal responders represent a unique treatment challenge. Although they may respond to continued imatinib therapy, their long-term outcomes may not be as favorable as those for optimally responding patients. Validation studies demonstrated that suboptimal responders are a heterogeneous group, and that the prognostic implications of suboptimal response vary by time point. There are few data derived from clinical trials to guide therapeutic decisions for these patients. Clinical trials are currently underway to assess the efficacy of newer tyrosine kinase inhibitors in this setting. Identification of suboptimal responders or patients failing treatment using hematologic, cytogenetic, and molecular techniques allows physicians to alter therapy earlier in the treatment course to improve long-term outcomes.

Comprehensive evaluation of time-to-response parameter as a predictor of treatment failure following imatinib therapy in chronic phase chronic myeloid leukemia: which parameter at which time-point does matter?

Early recognition of high-risk patient is important to improve long-term outcomes following imatinib therapy for chronic myeloid leukemia (CML). Some controversy surrounds the question, which of short-term response parameters at which time-point, including complete cytogenetic response (CCyR) or major molecular response (MMR) at 6 or 12 months, is the best predictor for treatment outcomes. In this comprehensive analysis, we adopted landmark analysis method, time-dependent Cox's proportional hazard model, and receiver-operating characteristics (ROC) method to analyze time-to-response parameter as predictor of long-term outcomes in 187 chronic phase (CP) CML patients. Regardless of the methods of analysis, earlier achievement of short-term response such as CCyR or MMR could predict the higher probability of achieving better interim outcome (such as treatment failure or loss of response [LOR]). Similar to the findings from other studies, our ROC analysis provided cutoff time points for MMR (18-36 months) and CCyR (6-12 months) that were the best predictors for LOR or treatment failure, which can be an indirect evidence supporting the ELN recommendation. The patient who achieves short-term response rapidly will have a lower risk of losing response or failing after imatinib therapy in CML patients.

Strategic treatment interruptions during imatinib treatment of chronic myelogenous leukemia

Although imatinib is an effective treatment for chronic myelogenous leukemia (CML), and nearly all patients treated with imatinib attain some form of remission, imatinib does not completely eliminate leukemia. Moreover, if the imatinib treatment is stopped, most patients eventually relapse (Cortes et al. in Clin. Cancer Res. 11:3425-3432, 2005). In Kim et al. (PLoS Comput. Biol. 4(6):e1000095, 2008), the authors presented a mathematical model for the dynamics of CML under imatinib treatment that incorporates the anti-leukemia immune response. We use the mathematical model in Kim et al. (PLoS Comput. Biol. 4(6):e1000095, 2008) to study and numerically simulate strategic treatment interruptions as a potential therapeutic strategy for CML patients. We present the results of numerous simulated treatment programs in which imatinib treatment is temporarily stopped to stimulate and leverage the anti-leukemia immune response to combat CML. The simulations presented in this paper imply that treatment programs that involve strategic treatment interruptions may prevent leukemia from relapsing and may prevent remission for significantly longer than continuous imatinib treatment. Moreover, in many cases, strategic treatment interruptions may completely eliminate leukemic cells from the body. Thus, strategic treatment interruptions may be a feasible clinical approach to enhancing the effects of imatinib treatment for CML. We study the effects of both the timing and the duration of the treatment interruption on the results of the treatment. We also present a sensitivity analysis of the results to the parameters in the mathematical model.

Dynamics and potential impact of the immune response to chronic myelogenous leukemia

Recent mathematical models have been developed to study the dynamics of chronic myelogenous leukemia (CML) under imatinib treatment. None of these models incorporates the anti-leukemia immune response. Recent experimental data show that imatinib treatment may promote the development of anti-leukemia immune responses as patients enter remission. Using these experimental data we develop a mathematical model to gain insights into the dynamics and potential impact of the resulting anti-leukemia immune response on CML. We model the immune response using a system of delay differential equations, where the delay term accounts for the duration of cell division. The mathematical model suggests that anti-leukemia T cell responses may play a critical role in maintaining CML patients in remission under imatinib therapy. Furthermore, it proposes a novel concept of an "optimal load zone" for leukemic cells in which the anti-leukemia immune response is most effective. Imatinib therapy may drive leukemic cell populations to enter and fall below this optimal load zone too rapidly to sustain the anti-leukemia T cell response. As a potential therapeutic strategy, the model shows that vaccination approaches in combination with imatinib therapy may optimally sustain the anti-leukemia T cell response to potentially eradicate residual leukemic cells for a durable cure of CML. The approach presented in this paper accounts for the role of the anti-leukemia specific immune response in the dynamics of CML. By combining experimental data and mathematical models, we demonstrate that persistence of anti-leukemia T cells even at low levels seems to prevent the leukemia from relapsing (for at least 50 months). As a consequence, we hypothesize that anti-leukemia T cell responses may help maintain remission under imatinib therapy. The mathematical model together with the new experimental data imply that there may be a feasible, low-risk, clinical approach to enhancing the effects of imatinib treatment.