In Vitro Studies of Bortezomib with Daunorubicin and Cytarabine:Sequence of Administration Affects Leukemia Cell Chemosensitivity.

Induction chemotherapy with cytarabine and an anthracycline has been a standard treatment of newly diagnosed AML for the past 20 years. While the complete remission rate is high, only 20–40% of patients have prolonged leukemia-free survival and thus novel treatments are required. Bortezomib represents a first-in class proteasome inhibitor with activity in a variety of hematologic malignancies. Recent evidence has demonstrated synergistic cytotoxicity between the proteasome inhibitor, MG-132, and the anthracycline, idarubicin, in primitive leukemia cells.(Guzman et al. Proc Natl Acad Sci U S A 99 2002 16220-5) In order to determine if adding bortezomib to the conventional antileukemic therapies daunorubicin and cytarabine enhances cellular cytotoxicity, we conducted in vitro studies on the acute myeloid leukemia cell line, KG-1. Proliferation assays demonstrated that single-agent daunorubicin or single-agent cytarabine impaired proliferation to a greater extent than when either agent was simultaneously combined with bortezomib. Reasoning that treatment with bortezomib resulted in cell cycle arrest and protection from cycle-dependent agents, we treated cells with bortezomib and observed an accumulation of cells in the G2/M phase of the cell cycle, supporting our hypothesis. We then sought to determine whether sequential administration of bortezomib and chemotherapy affected leukemia cell proliferation. Cells were treated with bortezomib either 24 hours before or after idarubicin or cytarabine. Surprisingly, pretreatment of cells with bortezomib followed by chemotherapy resulted in increased proliferation relative to chemotherapy alone while initial treatment of cells with chemotherapy followed by bortezomib resulted in enhanced cytotoxicity. In conclusion, these data indicate that in in vitro leukemia cell cultures the timing of administration of bortezomib relative to conventional anti-leukemic agents critically affects cytotoxicity.