Synchronization of L1210 leukemia with hydroxyurea infusion and the effect of subsequent pulse dose chemotherapy

Studies were performed to synchronize L1210 tumor cells in S phase in an effort to maximize the effect of subsequent pulse dose chemotherapy. Hydroxyurea (HU) was administered by continuous iv infusion 5 days after tumor implantation. Perturbation effects on the S-phase cells were measured by serial tritiated thymidine labeling indices. Effects on cell-cycle progression were measured by DNA content distribution analysis using flow cytometry. Synchronization of tumor cells was achieved with HU infusion (48 mg/kg/hour x 24 hours), resulting in 90% of the cells in S phase. Following infusion, synchronous progression of S-phase cells into G2/M and then G1 was apparent from +0 to +10 hours later. The susceptibility of HU-synchronized cells to subsequent chemotherapy was determined by treating mice with cytosine arabinoside (Ara-C), methotrexate (MTX), or Adriamycin (ADR) pulse doses at various intervals following infusion. Synergy, measured by prolongation of survival times, resulted when Ara-C was administered immediately after the end of the infusion. Survival times then decreased as the fraction of cells in S phase decreased. In contrast, the survival times of mice treated with MTX or ADR pulse doses after HU infusion were additive at best and did not correlate with fluctuations in the S phase compartment. Therefore, prior synchronization of tumor cells in S phase was therapeutically advantageous when coupled with appropriately timed Ara-C pulse doses. There was little advantage in combining HU infusion with subsequent MTX or ADR therapy.