By analogy to the recently described single amino acid chelate (SAAC) technology for complexation of the {M(CO)3}+ core (M = Tc, Re), a series of tridentate ligands containing thiolate and thioether groups, as well as amino and pyridyl nitrogen donors, have been prepared: (NC5H4CH2)2NCH2CH2SEt (L1); (NC5H4CH2)2NCH2CH2SH (L2); NC5H4CH2N(CH2CH2SH)2 (L3); (NC5H4CH2)N(CH2CH2SH)(CH2CO2R) [R = H (L4); R = -C2H5 (L5). The {Re(CO)3}+ core complexes of L1-L5 were prepared by the reaction of [Re(CO)3(H2O)3]Br or [NEt4]2[Re(CO)3Br3] with the appropriate ligand in methanol and characterized by infrared spectroscopy, 1H and 13C NMR spectroscopy, mass spectrometry, and in the case of [Re(CO)3(L2)] (Re-2) and [Re(CO)3(L1)Re(CO)3Br2] (Re-1a) by X-ray crystallography. The structure of Re-2 consists of discrete neutral monomers with a fac-Re(CO)3 coordination unit and the remaining coordination sites occupied by the amine, pyridyl, and thiolate donors of L2, leaving a pendant pyridyl arm. In contrast, the structure of Re-1a consists of discrete binuclear units, constructed from a {Re(CO)3(L1)}+ subunit linked to a {Re(CO)3Br2}- group through the sulfur donor of the pendant thioether arm. The series of complexes establishes that thiolate donors are effective ligands for the {M(CO)3}+ core and that a qualitative ordering of the coordination preferences of the core may be proposed: pyridyl nitrogen approximately thiolate > carboxylate > thioether sulfur > thiophene sulfur. The ligands L1 and L2 react cleanly with [99mTc(CO)3(H2O)3]+ in H2O/DMSO to give [99mTc(CO)3(L1)]+ (99m)Tc-1) and [99mTc(CO)3(L2)] (99mTc-2), respectively, in ca. 90% yield after HPLC purification. The Tc analogues 99mTc-1 and 99mTc-2 were subjected to ligand challenges by incubating each in the presence of 1000-fold excesses of both cysteine and histidine. The radiochromatograms showed greater than 95% recovery of the complexes.

Abstract[ 11 C]Loperamide has been proposed for imaging P-glycoprotein (P-gp) function with positron emission tomography (PET), but its metabolism to [N-methyl-11 C]N-desmethyl-loperamide ([ 11 C] dLop; [ 11 C]3) precludes quantification. We considered that [ 11 C]3 might itself be a superior radiotracer for imaging brain P-gp function and therefore aimed to prepare [ 11 C]3 and characterize its efficacy. An amide precursor (2) was synthesized and methylated with [ 11 C]iodomethane to give [ 11 C]3. After administration of [ 11 C]3 to wild type mice, brain radioactivity uptake was very low. In P-gp (mdr-1a (−/−)) knockout mice, brain uptake of radioactivity at 30 min increased about 3.5 fold by PET measures, and over seven-fold by ex vivo measures. In knockout mice, brain radioactivity was predominantly (90%) unchanged radiotracer. In monkey PET experiments, brain radioactivity uptake was also very low, but after P-gp blockade increased more than seven-fold. [ 11 C]3 is an effective new radiotracer for imaging brain P-gp function and, in favor of future successful quantification, appears free of extensive brain-penetrant radiometabolites.

Loperamide, an opiate receptor agonist, does not cross the blood-brain barrier because it is a substrate for the permeabilityglycoprotein (P-gp) efflux pump. We evaluated 11 C-loperamide as a PET radiotracer to measure P-gp function in vivo. Methods: Monkeys were injected with 11 C-loperamide, and PET brain images were acquired for 120 min. The baseline scans were followed by scans acquired after administration of either of 2 P-gp inhibitors, (2R)-anti-5-f3-[4-(10,11-dichloromethanodibenzo-suber-5-yl)piperazin-1-yl]-2-hydroxypropoxygquinoline trihydrochloride (DCPQ) or tariquidar. Both the PET scans and ex vivo measurements were obtained in P-gp knockout and wildtype mice. Results: Pharmacologic inhibition of P-gp in monkeys dose-dependently increased brain activity, with a 3.7-fold effect at the highest DCPQ dose (8 mg/kg intravenously). This increase of brain activity was not caused peripherally, because DCPQ insignificantly changed the plasma concentration and plasma protein binding of radiotracer. Furthermore, the structurally dissimilar inhibitor, tariquidar, also increased brain uptake with potency equal to that of DCPQ. P-gp knockout mice had 3-fold higher brain activity on PET than did wild-type animals. Four radiometabolites were detected in the plasma and brains of ex vivo mice. The most lipophilic radiometabolite was found to be comobile with reference dLop on high-performance liquid chromatography. The brain concentrations of 11 C-loperamide and the putative 11 C-dLop were about 16-fold greater in P-gp knockout mice than in wild-type mice. Conclusion: Both 11 C-loperamide and its putative radiometabolite 11 C-dLop are avid P-gp substrates. 11 C-dLop may be superior to 11 C-loperamide in measuring P-gp function at the blood-brain barrier, because further demethylation of 11 CdLop will generate radiometabolites that have little entry into the brain.