Clonal Dasatinib Large Granular Expansion Is Associated with Suboptimal and Optimal Leukemia Net Response Criteria in Chronic Myelogenous Leukemia

Dasatinib a BCR/ABL and Src family tyrosine kinase inhibitor (TKI) used in the treatment of Chronic Myelogenous Leukemia (CML) has recently been associated with large granular (LG) expansion. Its clinical meaning and pathogenic mechanisms are still to be elucidated. Methodology: Based on the knowledge that both LG subsets (NK and T) have cytotoxic properties implicated in anti-tumoral and immune mediated activity, we conducted a prospective study from December 2009 to August 2011 with CML patients to evaluate the clinical significance of LG lymphocytosis (LGL) and T-LG clonality in dasatinib users and characterize if these conditions are specific for dasatinib when compared to imatinib. Imatinib patients (N=28) were vertically studied as a control group. Blood samples were analyzed by complete blood count, immunophenotyping with four-color flow cytometer and RT-PCR to detect T-receptor clonality. Dasatinib group (N=36) was prospectively followed for the same parameters and also evaluated for clinical response (according to the European LeukemiaNet 2009) and side effects. Results: The mean diagnosis time for the imatinib group was 6.2 years (0.3–17.4) and 5.9 years (1.2–18.5) for the dasatinib group. In both, the mean time since beginning current TKI therapy was over than 12 months (4.6 years on imatinib, 1.7 years on dasatinib). The incidence of LGL was higher in the dasatinib group compared to the imatinib group (86% × 53.5%, p=0.005) (table 1). T-LG clonality was statically associated with response by LeukemiaNet definition (p=0.0246) (table 2). It is worth to emphasize that one patient with oligoclonal LGL had major molecular response with just 50mg of dasatinib (dose reduction because of thrombocytopenia). LGL above 800/mm³ was statically associated with clonality (p=0,0224). Excluding hematologic toxicity, significant toxicity was observed in 30% (N=11) (colitis, pleural effusion, hepatic and pancreatic toxicity) of the dasatinib users (table 3). In these patients LGL was observed in 81.8% and clonality in 63.6%. Development of hemorrhagic colitis happened in the oligoclonal patient with the highest LG peak (NK=4224/mm³ and T-LG lymphocyte =1672/mm³).

Previous articles suggest that T-LG lymphocytes expressed mainly two Src kinase family members, Fyn and Lck and that inhibition of this pathway (a feature of dasatinib) interfered with their development which goes against what we find in current practice. In someway dasatinib is capable of stimulating our innate defense and leads to a selection of a T-LG clone that is associated with a better response. Conclusion: We found an important association between absolute lymphocytosis, LGL and clonality in dasatinib group that was not evident with imatinib. We were able to demonstrate that LGL above 800/mm³ was associated with T-LG clonality and that clonality was statistically associated with response by LeukemiaNet criteria for CML. This suggests that clonal T-LG expansion has a role in improving dasatinib ITK function, a feature not observed with imatinib. Perspective: If dasatinib is able to stimulate our innate anti-tumoral defenses its use should be evaluated in other kind of tumors and the molecular way it occurs should be investigated.

Off Label Use: Dasatinib dose reduction due to toxicity.