Transport of the new chemotherapeutic agent β- d -glucosylisophosphoramide mustard (D-19575) into tumor cells is mediated by the Na + - d -glucose cotransporter SAAT1

For β-d -glucosylisophosphoramide mustard (β-d -Glc-IPM), a new alkylating drug in which isophosphoramide mustard is stabilized, a higher selectivity and lower myelotoxicity was observed than for the currently used cytostatic ifosfamide. Because β-d -Glc-IPM is hydrophilic and does not diffuse passively through the lipid bilayer, we investigated whether a transporter may be involved in the cellular uptake. A variety of cloned Na+ -sugar cotransporters were expressed inXenopus oocytes, and uptake measurements were performed. By tracer uptake and electrical measurements it was found that β-d -Glc-IPM was transported by the low-affinity Na+ -d -glucose cotransporter SAAT1, which had been cloned from pig and is also expressed in humans. At membrane potentials between −50 and −150 mV, a 10-fold higher substrate affinity (K m ≈ 0.25 mM) and a 10-fold lowerV max value were estimated for β-d -Glc-IPM transport than for the transport ofd -glucose or methyl-α-d -glucopyranoside (AMG). Transport of β-d -Glc-IPM and glucose by SAAT1 is apparently performed by the same mechanism because similar sodium dependence, dependence on membrane potential, electrogenicity, and phlorizin inhibition were determined for β-d -Glc-IPM,d -glucose, and AMG. Transcription of human SAAT1 was demonstrated in various human carcinomas and tumor cell lines. In one of these, the human carcinoma cell line T84, phlorizin inhibitable uptake of β-d -Glc-IPM was demonstrated with substrate saturation and an apparentK m of 0.4 mM. The data suggest that the Na+ -d -glucose cotransporter SAAT1 transports β-d -Glc-IPM into human tumor cells and may accumulate the drug in the cells. They provide an example for drug targeting by employing a plasma membrane transporter.