Transport mechanisms of hepatic uptake and bile excretion in clinical hepatobiliary scintigraphy with99m Tc- N -pyridoxyl-5-methyltryptophan

Abstract Introduction In clinical hepatobiliary scintigraphy,99m Tc- N -pyridoxyl-5-methyltryptophan (99m Tc-PMT) is an effective radiotracer among the99m Tc-pyridoxylaminates. However, the mechanisms of human hepatic uptake and bile excretion transport of99m Tc-PMT have not been determined. We thus investigated the transport mechanisms of human hepatic uptake and bile excretion in hepatobiliary scintigraphy with99m Tc-PMT. Methods Four solute carrier (SLC) transporters involved in hepatic uptake were evaluated using human embryonic kidney (HEK) and HeLa cells with high expression of SLC transporters (organic anion transporting polypeptide (OATP)1B1, OATP1B3, OATP2B1, organic anion transporters (OAT)2 and organic cation transporters (OCT)1) after 5 min of99m Tc-PMT incubation. Metabolic analysis of99m Tc-PMT was performed using pooled human liver S9. Adenosine triphosphate (ATP)-binding cassette (ABC) transporters for bile excretion were examined using hepatic ABC transporter vesicles human expressing multiple drug resistance 1 (MDR1), multidrug resistance-associated protein 2 (MRP2), breast cancer resistance protein or bile salt export pump.99m Tc-PMT was incubated for 1, 3 and 5 min with ATP or adenosine monophosphate and these vesicles. SPECT scans were performed in normal and Eisai hyperbilirubinemic (EHBR) model rats, deficient in Mrp2 transporters, without and with verapamil (rat Mdr1 and human MDR1 inhibitor) after intravenous injection of99m Tc-PMT. Results Uptake of99m Tc-PMT in HEK293/OATP1B1 and HeLa/OATP1B3 was significantly higher than that in HEK293- and HeLa-mock cells.99m Tc-PMT was not metabolized in the human liver S9. In vesicles with high expression of ABC transporters, uptake of MDR1 or MRP2 was significantly higher at all incubation times. Bile excretion of99m Tc-PMT was also identified by comparison between normal and EHBR rats with and without verapamil on in-vivo imaging. Conclusions Human hepatic uptake of99m Tc-PMT was transferred by OATP1B1 and OATP1B3, and excretion into bile canaliculi via MDR1 and MRP2.99m Tc-PMT hepatobiliary scintigraphy may be a useful ligand as a noninvasive method of visualizing and quantifying hepatobiliary transporter functionality, which could predict drug pharmacokinetics.