title: Transport mechanisms of hepatic uptake and bile excretion in clinical hepatobiliary scintigraphy with 99mTc-N-pyridoxyl-5-methyltryptophan.
abstract: BACKGROUND
  In clinical hepatobiliary scintigraphy, (99m)Tc-N-pyridoxyl-5-methyltryptophan ((99m)Tc-PMT) is an effective radiotracer among the (99m)Tc-pyridoxylaminates. However, the mechanisms of human hepatic uptake and bile excretion transport of (99m)Tc-PMT have not been determined. We thus investigated the transport mechanisms of human hepatic uptake and bile excretion in hepatobiliary scintigraphy with (99m)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 of (99m)Tc-PMT incubation. Metabolic analysis of (99m)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 of (99m)Tc-PMT.
  
  
  RESULTS
  Uptake of (99m)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 of (99m)Tc-PMT was also identified by comparison between normal and EHBR rats with and without verapamil on in-vivo imaging.
  
  
  CONCLUSIONS
  Human hepatic uptake of (99m)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.
authors.affiliation: School of Health Sciences, College of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japan. Electronic address: kobayasi@mhs.mp.kanazawa-u.ac.jp.
authors.forename: Masato
authors.initials: M
authors.lastname: Kobayashi
authors.affiliation: School of Pharmaceutical Sciences, College of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japan.
authors.forename: Takeo
authors.initials: T
authors.lastname: Nakanishi
authors.affiliation: School of Health Sciences, College of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japan.
authors.forename: Kodai
authors.initials: K
authors.lastname: Nishi
authors.affiliation: Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, Okayama, Japan; Graduate School of Pharmaceutical Sciences, Chiba University, Chiba, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Higaki
authors.affiliation: School of Health Sciences, College of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Okudaira
authors.affiliation: School of Health Sciences, College of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japan.
authors.forename: Masahiro
authors.initials: M
authors.lastname: Ono
authors.affiliation: School of Health Sciences, College of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japan.
authors.forename: Takafumi
authors.initials: T
authors.lastname: Tsujiuchi
authors.affiliation: School of Health Sciences, College of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japan.
authors.forename: Asuka
authors.initials: A
authors.lastname: Mizutani
authors.affiliation: Department of Radiology, Faculty of Medicine University of Miyazaki, Miyazaki, Japan.
authors.forename: Ryuichi
authors.initials: R
authors.lastname: Nishii
authors.affiliation: School of Pharmaceutical Sciences, College of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japan.
authors.forename: Ikumi
authors.initials: I
authors.lastname: Tamai
authors.affiliation: Graduate School of Pharmaceutical Sciences, Chiba University, Chiba, Japan.
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Arano
authors.affiliation: School of Health Sciences, College of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japan.
authors.forename: Keiichi
authors.initials: K
authors.lastname: Kawai
mesh_terms: D000818:Animals; D001646:Bile / Q000000981:diagnostic imaging* / Q000378:metabolism*; D001692:Biological Transport; D057809:HEK293 Cells; D006367:HeLa Cells; D006801:Humans; D008099:Liver / Q000000981:diagnostic imaging* / Q000378:metabolism*; D008297:Male; D029362:Organic Anion Transporters, Sodium-Independent / Q000378:metabolism; D015609:Organotechnetium Compounds; D011730:Pyridoxal / Q000031:analogs & derivatives*; D011877:Radionuclide Imaging; D051381:Rats; D014364:Tryptophan / Q000031:analogs & derivatives*
chemical_list: D029362:Organic Anion Transporters, Sodium-Independent; D015609:Organotechnetium Compounds; C038081:technetium Tc 99m pyridoxyl-5-methyltryptophan; D011730:Pyridoxal; D014364:Tryptophan
keywords: (99m)Tc-PMT; (99m)Tc-pyridoxylaminates; ABC transporter; Hepatobiliary scintigraphy; SLC transporter; Transport mechanism