Development of multidrug resistance (MDR) in cancer cells decreases net doxorubicin uptake as a result of either increased efflux, or decreased intracellular sequestration, or decreased membrane permeability. Kinetic parameters of drug uptake can distinguish among these forms of altered transport. Cellular uptake of fluorescent drugs was monitored by a flow cytometric assay using a rapid-injection system and analyzed with a three-compartment model in which rapid diffusion from extracellular fluid into the cell was followed by uptake into a nonexchangeable pool. In agreement with our recent studies of '*C-doxorubicin distribution (Dordal et al.: J Pharmacol Exp Ther 271:12861290, 1994), sequestration of doxorubicin was decreased 2.7-fold in P-glycoprotein-expressing SU-4R lymphoma cells compared to drug-sensitive SU-4 cells Malignant cells may become resistant to anticancer drugs by the overexpression of proteins that alter net cellular drug accumulation. Clinically significant resistance to anthracyclines, vinca alkaloids, and other natural products has been reported in association with increased expression of P-glycoprotein, the most widely studied of these proteins (9,ll). More recently, intracellular proteins have been described that are found in resistant cells that do not express P-glycoprotein but do exhibit a similar phenotype (4,16). P-glycoprotein is believed to act as a drug efflux pump, because cell lines expressing the protein lose intracellular drug much more quickly than their nonresistant progenitors when transferred to drug free medium.However, our recent studies of multidrug-resistant HL-60 and SU-4 subclones suggested that, in these cell lines, the most significant alteration in doxorubicin transport was decreased sequestration in a nonexchangeable compartment (7). The result was that proportionally more intracellular drug existed in exchangeable compartments, where it could freely diffuse from the cell. Drugresistant cells showed no evidence of active efflux (14.0 2 4.8 vs. 5.0 f 0.9 nl s-') without a change in membrane permeability or evidence of active efflux. In contrast, sequestration of the highly fluorescent dye rhodamine 123 was decreased 20-fold (17.1 f 8.3 vs. 0.9 f 0.8 nl s-'). Resistant

In patients with postpartum endometritis, once-daily gentamicin dosing provides consistently high peak serum levels of gentamicin, requires less nurse tasking time, costs less, and is as effective as the 8-hour dosing regimen.