Myeloid cell factor-1 (Mcl-1) is an anti-apoptotic member of the Bcl-2 protein family that associates with tumor cell survival and drug resistance in chronic lymphocytic leukemia (CLL). Increased Mcl-1 expression is associated with failure to achieve remission after chemotherapy with fludarabine and chlorambucil in CLL patients. However, the influence of Mcl-1 expression has not been examined in CLL trials using treatment regimens that contain rituximab, which has shown substantially improved outcomes for a large subset of patients and have become a mainstay in the therapy of newly diagnosed CLL. We previously reported that combination chemoimmunotherapy with pentostatin, cyclophosphamide, and rituximab (PCR) has significant clinical activity with low accompanying toxicity in previously untreated CLL patients, and is especially well tolerated in older patients in whom the use of fludarabine may be associated with prohibitive toxicities. As part of this study, we prospectively investigated association of Mcl-1 protein expression with response and progression-free survival. Eligible patients received a regimen consisting of pentostatin (2 mg/m2), cyclophosphamide (600 mg/ m2), and rituximab (375 mg/m2) given intravenously on day 1 of a 21-day cycle for a maximum of 6 cycles. Responses were assessed according to NCI 1996 criteria and included bone marrow evaluation and two-color flow cytometry two months after completion of therapy. Flow cytometry-negative status was defined as less than or equal to 1% CD5+/CD19+ cells. Of the 64 patients evaluated in this trial, clinical responses were seen in 58 (91%), with 26 (41%) complete responses (CR), 14 (22%) nodular partial responses (nPR), and 18 (28%) partial responses (PR). Lysates from peripheral blood cells obtained pre-treatment were analyzed for Mcl-1 expression by immunoblot. Mcl-1 as a continuous variable was not found to be associated with other commonly described risk factors. However, Recursive Partitioning Analysis (RPA) was devised employing Mcl-1 expression as a continuous variable, which revealed an optimal cut point of 0.85. At this cutoff, flow cytometry-negative status was significantly different between the two groups (p=0.01) and median PFS was significantly higher (p=0.02) in patients with Mcl-1 levels of <0.85 (50.8 vs. 18.7 months). No difference by Mcl-1 expression was noted in other pre-treatment or response parameters. Our data describes the first prospective validation of Mcl-1 over-expression adversely influencing both the likelihood of attaining a flow cytometry-negative complete remission and extended progression free survival. Mcl-1 expression was also independent of other risk factors, indicating additional benefit for this measure in assessing patients. Mcl-1 expression may therefore predict poor response to chemoimmunotherapy, and these findings advocate pursuing therapeutic agents targeting this important anti-apoptotic protein.

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