Comparison of in vitro assays for detecting subpopulations of P388 leukemic cells resistant to AraC.

The kinetics of cytosine arabinoside (AraC) were studied in vitro using P388 murine leukemia lines sensitive (P388-S) and resistant (P388-R) to the drug. Using P388-S cells, the 3H-thymidine index (L.I.) was paralleled by the 3H-AraC index, i.e. any cell in the S-phase of the cycle also incorporated the 3H-AraC into DNA. With the resistant line, the 3H-AraC index was zero in spite of high L.I. This discrepancy between the L.I. and 3H-AraC index could be used to predict the percentage of resistant cells when known mixtures of sensitive and resistant populations were used. The reason for lack of incorporation into DNA by the resistant cells was shown to be the presence of very low levels of the enzyme deoxycytidine kinase in P388-R cells resulting in an inability to phosphorylate AraC to its active form AraCTP. Immediate inhibition of DNA synthesis caused by AraC was directly proportional to the number of sensitive cells present in the population, but was not as accurate a predictor of sensitivity as the 3H-AraC index. Cloning in methylcellulose was found to be the least sensitive predictor of the percentage of sensitive and resistant cells, most likely related to the difference in cloning efficiency of the sensitive and resistant lines.