Sensitivity of myeloid leukemia cells to calcium influx blockade: Application to bone marrow purging

Objective. The aim of this study was to assess the potential of store-operated Ca²⁺ channel (SOC) antagonists as purging agents for leukemia cells. Materials and Methods. Clonogenic, limiting dilution, and nuclear condensation assays were used to evaluate SOC antagonist efficacy. SOC activity and endoplasmic reticulum Ca²⁺ content were measured by flow cytometry. Murine bone marrow transplantation was used to determine purging efficacy and effects on hemopoietic reconstitution. Results. Econazole (Ec) and ketotifen (Ke) were variably effective against human and murine leukemia cell lines after 24 hours of incubation. However, a 2-hour serum and bovine serum albumin-free treatment protocol with Ec was found to maximize differential sensitivity between leukemic cells and normal hemopoietic progenitors. Primary acute myelogenous leukemia blast cell viability was reduced 4.2 to 5.1 logs by 2-hour Ec treatment as measured by limiting dilution. An inverse relationship between endoplasmic reticulum Ca²⁺ content and Ke sensitivity in leukemia and untransformed cells was observed. Nuclear condensation, an index of apoptosis, which occurred after 24-hour treatments with either Ec or Ke, was not observed after 2-hour serum- and bovine serum albumin-free Ec exposures; however, condensed nuclei were observed after an additional 10-hour incubation in growth medium without drug. Using bone marrow deliberately contaminated with 1% P815 cells, we showed that highly effective in vitro purging can be accomplished using Ec with no adverse effects on bone marrow reconstitution in mice. Conclusion. These studies suggest that SOC antagonists have potential as purging agents for residual leukemia cells present in bone marrow in the context of high-dose chemotherapy and autologous transplantation for leukemia.