INTRACELLULAR CONCENTRATIONS OF MITOXANTRONE IN LEUKEMIC-CELLS INVITRO VS INVIVO

The aim of this study was to determine the intracellular pharmacokinet ics of mitoxantrone in vivo and to use these results to establish how leukemic cells should be incubated to perform clinically relevant in v itro studies of this drug. Blood samples were obtained from 11 patient s with acute nonlymphoblastic leukemia at certain intervals up to 20 h after the infusion of mitoxantrone 12 mg/m2. Plasma and leukemic cell s were separated and the drug concentrations were determined with HPLC . Before treatment, leukemic cells from 12 patients were incubated wit h 0.02, 0.05, 0.1, 0.2 and 1.0 muM mitoxantrone for 1-4 h and thereaft er cultured in suspension culture for 20 h; during this time cell samp les were taken at certain intervals for drug determination. In cells i ncubated with 0.05 and 0.2 muM mitoxantrone the cytotoxic effect was m easured with the DiSC assay after cultivation for 4-5 days. In vivo, t he intracellular levels exceeded the plasma concentrations already at the end of infusion and after 2 h the intracellular concentrations wor e 200-300 times higher than in plasma. In vitro, the intracellular ste ady state level of mitoxantrone was reached after 1-2 h and there was a pronounced intracellular retention even after 20 h culture in drug-f ree medium. Incubation with 0.05 muM during 1 h gave intracellular con centrations of mitoxantrone similar to those achieved in vivo. This in cubation concentration gave a mean cytotoxic effect of 53% living cell s measured with the DiSC assay, which gives good possibilities to disc riminate between mitoxantrone-sensitive and unsensitive cells. We beli eve that exposing leukemic cells in vitro for in vivo mimicking mitoxa ntrone concentrations could increase the clinical relevance of predict ive assays.