title: Characterization of a 67Ga/68Ga radiopharmaceutical for SPECT and PET of MDR1 P-glycoprotein transport activity in vivo: validation in multidrug-resistant tumors and at the blood-brain barrier.
abstract: Overexpression of multidrug resistance (MDR1) P-glycoprotein (Pgp) remains an important barrier to successful chemotherapy in cancer patients and impacts the pharmacokinetics of many important drugs, thus evoking a need to noninvasively interrogate Pgp transport activity in vivo.
  
  
  METHODS
  Cell tracer transport experiments as well as mouse biodistribution and microPET imaging studies were performed to characterize a nonmetabolized gallium(III) complex, gallium(III)-(bis(3-ethoxy-2-hydroxy-benzylidene)-N,N'-bis(2,2-dimethyl-3-amino-propyl)ethylenediamine) (Ga-[3-ethoxy-ENBDMPI])(+), as a candidate SPECT ((67)Ga) and generator-produced PET ((68)Ga) radiopharmaceutical recognized by MDR1 Pgp.
  
  
  RESULTS
  The (67)Ga-complex showed high membrane potential-dependent accumulation in drug-sensitive KB3-1 cells and modulator-reversible low accumulation in MDR KB8-5 cells. In KB8-5 cells, the median effective concentrations (EC(50)) of MDR modulators LY335979, PSC 833, and cyclosporin A were 69 nmol/L, 1 micromol/L, and 3 micromol/L, respectively. Using a variety of cells stably expressing MDR1 Pgp, multidrug resistance-associated proteins (MRP1-MRP6), or the breast cancer resistance protein (BCRP/MXR), the (67)Ga-complex was shown to be readily transported by MDR1 Pgp and, to a much lesser extent, by MRP1, but not MRP2-MRP6 or BCRP/MXR. In a nude mouse xenograft tumor model, the (67)Ga-complex produced a readily detected 3-fold difference between Pgp-expressing tumors and drug-sensitive tumors in the opposite flank. In mdr1a/1b(-/-) gene-deleted mice, the (67)Ga-complex showed 17-fold greater brain uptake and retention compared with wild-type mice with no net difference in blood pharmacokinetics, consistent with transport in vivo by Pgp expressed at the capillary blood-brain barrier. This could be readily observed with microPET using the (68)Ga-complex. Incidentally, wild-type mice showed heart-to-blood ratios of >100 by 1 h after injection and heart-to-liver ratios of 2.2 by 120 min.
  
  
  CONCLUSIONS
  Molecular imaging of the functional transport activity of MDR1 Pgp with ((67/68)Ga-[3-ethoxy-ENBDMPI])(+) may enable noninvasive SPECT/PET monitoring of the blood-brain barrier, chemotherapeutic regimens, and MDR1 gene therapy protocols in vivo. These Pgp-directed properties of the radiopharmaceutical may also translate favorably to myocardial perfusion imaging.
authors.affiliation: Molecular Imaging Center, Mallinckrodt Institute of Radiology, St. Louis, Missouri, USA.
authors.forename: Vijay
authors.initials: V
authors.lastname: Sharma
authors.forename: Julie L
authors.initials: JL
authors.lastname: Prior
authors.forename: Martin G
authors.initials: MG
authors.lastname: Belinsky
authors.forename: Gary D
authors.initials: GD
authors.lastname: Kruh
authors.forename: David
authors.initials: D
authors.lastname: Piwnica-Worms
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism*; D000818:Animals; D001812:Blood-Brain Barrier / Q000000981:diagnostic imaging* / Q000378:metabolism*; D002294:Carcinoma, Squamous Cell / Q000000981:diagnostic imaging* / Q000378:metabolism*; D045744:Cell Line, Tumor; D005710:Gallium Radioisotopes / Q000493:pharmacokinetics*; D006801:Humans; D008297:Male; D008657:Metabolic Clearance Rate; D051379:Mice; D018345:Mice, Knockout; D008819:Mice, Nude; D009928:Organ Specificity; D049268:Positron-Emission Tomography / Q000379:methods; D021381:Protein Transport; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D014018:Tissue Distribution; D015899:Tomography, Emission-Computed, Single-Photon / Q000379:methods
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D005710:Gallium Radioisotopes; D019275:Radiopharmaceuticals