Membrane Affinity and Metabolism of A/4-Palmitoyl-1-/?-D-Arabinofuranosylcytosine into Cultured KB Cells1

A/4-Palmitoyl-1 -/8-D-arabinofuranosylcytosine (/V4-palmitoyl-ara-C), a lipophilic derivative of 1-yß-D-arabinofuranosylcyto-sine, possessed an affinity for KB cell plasma membrane. Approximately 15 to 25% of the drug incorporated into KB cells was retained in plasma membrane when the cells were treated with the drug for 1 to 32 hr at 10 ¡IM,the concentration required for 50% inhibition of cell growth. Less than 5.3% of the drug was found in the plasma membrane when the cells were treated with 1-ß-D-arabinofuranosylcytosine. W-Palmi-toyl-ara-C in the membrane fraction cosedimented with plasma membrane in a sucrose density gradient at 4°, indicating a close association between the drug and the membrane. The affinity of W-palmitoyl-ara-C for plasma membrane probably contributes to the efficient uptake rate and the strong cytotoxic effect of /V'-palmitoyl-ara-C reported previously. The metabolites of /\/4-palmitoyl-ara-C in KB cells, treated with the drug for 32 hr at the concentration required for 50% inhibition of cell growth (10 ¡IM),were analyzed by diethyl-aminoethyl Sepharose CL-6B column chromatography thin-layer and paper chromatography. This analysis showed that over 98% of the drug present in the KB cell was A/4-palmitoyl-ara-C. The active metabolites, 1-/S-D-arabinofurano- sylcytosine, 1-/?-D-arabinofuranosylcytosine 5'-monophos-phate, A/4-palmitoyl-1-/?-D-arabinofuranosylcytosine 5'-mono-phosphate, and 1-yß-D-arabinofuranosylcytosine 5'-triphos-phate, were found in amounts of 0.41, 0.37, 0.17, and 0.05%, respectively, of the total drug found in the cells. Also found were the inactive metabolites 1-ß-D-arabinofuranosyluracil and 1-y3-D-arabinofuranosylcytosine diphosphate choline in amounts of 0.61 and 0.29%, respectively.