Transport mechanisms of hepatic uptake and bile excretion in clinical hepatobiliary scintigraphy with ^99mTc-N-pyridoxyl-5-methyltryptophan

Introduction: In clinical hepatobiliary scintigraphy, ^99mTc-N-pyridoxyl-5-methyltryptophan (^99mTc-PMT) is an effective radiotracer among the ^99mTc-pyridoxylaminates. However, the mechanisms of human hepatic uptake and bile excretion transport of ^99mTc-PMT have not been determined. We thus investigated the transport mechanisms of human hepatic uptake and bile excretion in hepatobiliary scintigraphy with ^99mTc-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 5min of ^99mTc-PMT incubation. Metabolic analysis of ^99mTc-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. ^99mTc-PMT was incubated for 1, 3 and 5min 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 of ^99mTc-PMT. Results: Uptake of ^99mTc-PMT in HEK293/OATP1B1 and HeLa/OATP1B3 was significantly higher than that in HEK293- and HeLa-mock cells. ^99mTc-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 of ^99mTc-PMT was also identified by comparison between normal and EHBR rats with and without verapamil on in-vivo imaging. Conclusions: Human hepatic uptake of ^99mTc-PMT was transferred by OATP1B1 and OATP1B3, and excretion into bile canaliculi via MDR1 and MRP2. ^99mTc-PMT hepatobiliary scintigraphy may be a useful ligand as a noninvasive method of visualizing and quantifying hepatobiliary transporter functionality, which could predict drug pharmacokinetics.