Antiproliferative Activity of ELACYT™ (CP-4055) in Combination with Cloretazine (VNP40101M), Idarubicin or Gemcitabine in HL-60 Human Myeloid Leukemia Cells.

CP-4055 (ara-C-5′-elaidic acid ester; ELACYT™) is the C18:1^Δ9,trans unsaturated fatty acid ester of cytarabine, a deoxycytidine analog used clinically in the treatment of acute myeloid and lymphoblastic leukemias and lymphomas. Unlike cytarabine, CP-4055 also has shown activity in solid tumors, likely due to increased cellular uptake via a nucleoside carrier-independent mechanism. The objective of the current study was to evaluate drug partners for inclusion in combination chemotherapy regimens with CP-4055 in acute myeloid leukemia. The antiproliferative activity of CP-4055 was compared to that of cloretazine (VNP40101M, alkylating agent), idarubicin (topoisomerase II inhibitor), and gemcitabine (a DNA antimetabolite) in HL-60, an established in vitro model for human acute promyelocytic leukemia, following a 48 h continuous exposure (~2 cell doublings). Growth inhibition was assessed by an ATP microplate assay (ATPLite, Perkin-Elmer). Drug interaction was determined by the combination index method of Chou and Talalay and by the 3-dimensional method of Kanzawa. The results revealed a 5-log range of potency, with IC50’s of 1 nM (gemcitabine), 12 nM (Idarubicin), 13 μM (CP-4055), and 132 μM (cloretazine). Drug synergy was observed for the combination of CP-4055 with gemcitabine (CP:GEM = 1: 0.00025); CI = 0.42 ± 0.01), while combinations of CP-4055 with cloretazine (1:10) and with idarubicin (1:0.0025) were additive (CI = 0.9 ± 0.1). The dose reduction index for the CP-4055-gemcitabine combination indicated that for an IC50 level of activity, the dose of CP-4055 could be reduced 20-fold in combination to produce the same effect as the agent used alone. Furthermore, dose reduction increased as the level of activity increased. This interaction was independent of drug sequence, while interactions of CP-4055 with cloretazine and idarubicin became antagonistic when CP-4055 was administered 24 h prior to addition of the second drug. Three-dimensional analysis confirmed these interactions, but in addition revealed that synergy of CP-4055 + gemcitabine was largely independent of drug ratio, with optimal interaction occurring at CP-4055 concentrations of 0.6 – 40 μM and gemcitabine concentrations of 0.2 – 2 nM. Since drug ratio is difficult to control in vivo, ratio independence is a significant advantage for this combination. Finally, tertiary combinations with CP-4055 were tested. CP-4055 combined with cloretazine and idarubicin (1:10:0.001) produced an additive response, while combinations of CP-4055 with gemicitabine and either idarubicin (1:0.0001:0.001) or cloretazine (1:0.0001:10) did not increase synergy over that of the binary combination of CP-4055 and gemcitabine.

Conclusion: CP-4055 is active against human myeloid leukemia cells in vitro. Growth inhibition can be enhanced by combination with gemcitabine and the synergistic interaction is independent of both drug sequence and drug ratio. Combinations with cloretazine and with idarubicin produce additive activity across a broad range of drug ratios but are optimal only with simultaneous exposure. Of interest is that synergy was observed with agents from the same DNA antimetabolite drug class and that both are analogs of deoxycytidine. The underlying mechanism for this interaction is currently unknown.