Transport mechanism and affinity of [99mTc]Tc-mercaptoacetyltriglycine ([99mTc]MAG3) on the apical membrane of renal proximal tubule cells.

Technetium-99m-labeled mercaptoacetyltriglycine ([Tc]MAG3) is widely used for evaluation of transplanted kidneys, diagnosis of tubular necrosis, and scintigraphic studies of tubular function. [Tc]MAG3 is a substrate for organic anion transporter (OAT)1 and OAT3 on the basolateral membrane side for renal secretion. We investigated the transport mechanism and affinity of [Tc]MAG3 on the apical membrane of renal proximal tubule cells for renal secretion. Adenosine triphosphate-binding cassette (ABC) transporters for renal secretion of [Tc]MAG3 were examined using ABC transporter vesicles expressing multiple drug resistance 1 (MDR1), breast cancer resistance protein (BCRP), multidrug resistance-associated protein (MRP)2, and MRP4. MK-571, a MRP inhibitor, was applied to measure the K and V of MRP2 and MRP4 in a vesicle transport assay. Single photon emission computed tomography (SPECT) was performed in normal rats and MRP2-deficient Eisai hyperbilirubinuria rats (EHBR) using [Tc]MAG3 with and without MK-571. [Tc]MAG3 uptake in adenosine triphosphate was significantly higher than that in adenosine monophosphate in vesicles that highly expressed MRP2 and MRP4. The affinity of [Tc]MAG3 for MRP4 was higher than that for MRP2. Renal secretion via MRP2 and MRP4 was identified by comparing normal and EHBR rats with and without MK-571 on SPECT. [Tc]MAG3 is transported via MRP2 and MRP4 on the apical membrane of renal proximal tubule cells. The affinity of MRP4 is higher than that of MRP2. SIGNIFICANCE STATEMENT: [Tc]MAG3, widely used for evaluation of transplanted kidneys, diagnosis of tubular necrosis, and scintigraphic studies of tubular function, is transported via MRP2 and MRP4 on the apical membrane of renal proximal tubule cells. The affinity of MRP4 is higher than that of MRP2.