Background: C-kit (CD117), a tyrosine kinase receptor, is expressed on most myeloid blasts and is thought to be important in the pathogenesis of AML. Activation of the c-kit receptor leads to phosphorylation and activation of downstream signaling proteins which are important for cell survival and proliferation. Previous studies have demonstrated conflicting results regarding the prognostic impact of c-kit expression. This discordance is likely secondary to limitations in the current method used to measure c-kit. The aim of this study was to evaluate c-kit quantitatively using the mean fluorescent index (MFI), and determine whether this index predicted outcome in patients with newly diagnosed AML.

Methods: We conducted a retrospective review of 183 patients with newly diagnosed AML treated with induction chemotherapy at the Cleveland Clinic between January 1998 and December 2005. Flow cytometry was performed on bone marrow or peripheral blood. Blasts were stained with antibodies against CD45 and c-kit (BD Biosciences San Jose, CA). CD45-stained cells without c-kit antibody were used as a negative control. Flow cytometry was performed on FACS Caliber instruments, and data were acquired using Cell Quest software. Using a CD45/orthogonal gate to isolate blasts, the MFI was calculated as the mean channel number (MCN) of the blasts/MCN autofluorescence. Known prognostic factors including: age, WBC count at diagnosis, AML etiology (de novo vs. secondary AML) and cytogenetics (as defined by CALGB criteria) were obtained. Cox proportional hazards analysis was used to identify univariate and multivariate correlates of complete remission (CR), overall survival (OS), relapse after remission, and progression-free survival (PFS). Only variables significant in the univariate setting were included in multivariate analysis. Results were summarized as the hazard ratio (HR) with 95% confidence intervals (CI).

Results: All patients underwent anthracycline-based induction chemotherapy. 96 (52.5%) patients were male with a median age of 57 years (17–79). Cytogenetics were favorable in 24 patients (13.1%), intermediate in 98 (53.6%), poor in 47 (25.7%) and unknown in 14 (7.6%). AML was secondary in 52 patients (28.4%) and de novo in 131 patients (71.6%). Median WBC count at diagnosis was 10.3 k/uL [4–259 k/uL]. 139 patients (76%) achieved a CR following induction chemotherapy, and 66 patients (36.1%) relapsed after achieving CR. Median c-kit MFI was 13.1 [0–251.1]. On univariate analysis, c-kit MFI(per 50 unit increase) was associated with an increased risk of AML relapse (HR=1.33 [CI=1.01–1.74 p=.041] ), and a trend towards inferior PFS and OS, HR=1.24 (CI=.97–1.57 p=.08) and HR=1.29 (CI=.99–1.67 p=.06), respectively. On multivariate analysis, c-kit MFI correlated with worse OS (HR=1.37 [CI=1.04–1.82 p=.026]) and an increased risk of relapse (HR=1.32 [CI=1.01–1.73 p=.043]). A trend towards decreased PFS was also seen, but did not reach statistical significance (HR=1.21 [CI=.96–1.53 p=.11]).

Conclusion: C-kit MFI, independently of known prognostic factors, predicts OS and risk of relapse in patients with newly diagnosed AML. Whether inhibiting c-kit in patients with AML will alter prognosis is the basis of ongoing clinical trials.

Disclosure: No relevant conflicts of interest to declare.

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