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 in Xenopus 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 (Km ≈ 0.25 mM) and a 10-fold lower Vmax value were estimated for β-d-Glc-IPM transport than for the transport of d-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 apparent Km 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
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