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INTRODUCTION: Our previous work demonstrated that the (99m)Tc renal tracer, (99m)Tc(CO)(3)(FEDA) ((99m)Tc-1), has a rapid clearance comparable in rats to that of (131)I-OIH, the radioactive gold standard for the measurement of effective renal plasma flow. The uncharged fluoroethyl pendant group of (99m)Tc-1 provides a route to the synthesis of a structurally analogous rhenium-tricarbonyl (18)F renal imaging agent, Re(CO)(3)([(18)F]FEDA) ((18)F-1). Our goal was to develop an efficient one-step method for the preparation of (18)F-1 and to compare its pharmacokinetic properties with those of (131)I-OIH in rats. METHODS: (18)F-1 was prepared by the nucleophilic (18)F-fluorination of its tosyl precursor. The labeled compound was isolated by HPLC and subsequently evaluated in Sprague-Dawley rats using (131)I-OIH as an internal control and by dynamic PET/CT imaging. Plasma protein binding (PPB) and erythrocyte uptake (RCB) were determined and the urine was analyzed for metabolites. RESULTS: (18)F-1 was efficiently prepared as a single species with high radiochemical purity (>99%) and it displayed high radiochemical stability in vitro and in vivo. PPB was 87% and RCB was 21%. Biodistribution studies confirmed rapid renal extraction and high specificity for renal excretion, comparable to that of (131)I-OIH, with minimal hepatic/gastrointestinal elimination. The activity in the urine, as a percentage of (131)I-OIH, was 92% and 95% at 10 and 60 min, respectively. All other organs (heart, spleen, lungs) showed a negligible tracer uptake (less than 0.4% ID). Dynamic microPET/CT imaging demonstrated rapid transit of (18)F-1 through the kidneys and into the bladder; there was no demonstrable activity in bone verifying the absence of free [(18)F]fluoride. CONCLUSIONS: (18)F-1 exhibited a high specificity for the kidney, rapid renal excretion comparable to that of (131)I-OIH and high in vivo radiochemical stability. Not only is (18)F-1 a promising PET renal tracer, but it provides a route to the development of a pair of analogous (18)F/(99m)Tc renal imaging agents with almost identical structures and comparable pharmacokinetic properties. These promising in vivo results warrant subsequent evaluation in humans.

Introduction 99m Tc(CO) 3 -nitrilotriacetic acid, 99m Tc(CO) 3 (NTA), is a new renal tubular agent with pharmacokinetic properties comparable to those of 131 I-OIH but the clearance of 99m Tc(CO) 3 (NTA) and 131 I-OIH is still less than the clearance of PAH, the gold standard for the measurement of effective renal plasma flow. At physiological pH, dianionic 99m Tc(CO) 3 (NTA) has a mononegative inner metal-coordination sphere and a mononegative uncoordinated carboxyl group. To evaluate alternate synthetic approaches, we assessed the importance of an uncoordinated carboxyl group, long considered essential for tubular transport, by evaluating the pharmacokinetics of three analogs with the 99m Tc(CO) 3 (NTA) metal-coordination sphere but with uncharged pendant groups. Methods 99m Tc(CO) 3 complexes with N -(2-acetamido)iminodiacetic acid (ADA), N -(2-hydroxyethyl)iminodiacetic acid (HDA) and N -(fluoroethyl)iminodiacetic acid (FEDA) were prepared using a tricarbonyl kit and isolated by HPLC. The pharmacokinetics were evaluated in Sprague–Dawley rats, with 131 I-OIH as an internal control; urine was analyzed for metabolites. Plasma protein binding and erythrocyte uptake were determined from the 10min blood samples. Re(CO) 3 (FEDA), the analog of 99m Tc(CO) 3 (FEDA), was prepared and characterized. Results 99m Tc(CO) 3 (ADA), 99m Tc(CO) 3 (HDA) and 99m Tc(CO) 3 (FEDA) were efficiently prepared as a single species with high radiochemical purities (>99%). These new monoanionic 99m Tc(CO) 3 tracers with uncharged dangling groups all showed rapid blood clearance and high specificity for renal excretion. Activity in the urine, as a percent of 131 I-OIH at 10 and 60min, was 96% and 99% for ADA, 96% and 100% for HDA, and 100% and 99% for FEDA, respectively. Each new tracer was excreted unchanged in the urine. The Re(CO) 3 (FEDA) structure adds compelling evidence that such 99m Tc(CO) 3 (NTA) analogs have metal-coordination spheres identical to that of 99m Tc(CO) 3 (NTA). Conclusions New tracers lacking the negatively charged pendant carboxyl group previously thought to be essential for rapid renal extraction, 99m Tc(CO) 3 (ADA), 99m Tc(CO) 3 (HDA) and 99m Tc(CO) 3 (FEDA), exhibit pharmacokinetics in rats comparable to those of 99m Tc(CO) 3 (NTA) and 131 I-OIH. Furthermore, these encouraging results in rats warrant evaluation of this new tracer type in humans.

Ligands that coordinate via dianionic phosphonate groups have not been widely utilized in radiopharmaceuticals. N-(phosphonomethyl)iminodiacetic acid (1, PMIDA) and N-(phosphonomethyl)glycine (2, PMG) were investigated as new chelators for the (99m)Tc/Re-tricarbonyl “core” (fac-M(CO)(3), M = (99m)Tc, Re) present in a major class of radiopharmaceuticals. fac-M(CO)(3)(PMIDA) and fac-M(CO)(3)(PMG) complexes were studied by HPLC and (1)H/(13)C/(31)P NMR methods for M = Re (Re-1 and Re-2) and by HPLC for M = (99m)Tc ((99m)Tc-1 and (99m)Tc-2). Re-1 and (99m)Tc-1 complexes exhibit a similar pH-dependent equilibrium between geometric linkage isomers (M-1a and M-1b). However, only one isomer exists for M-2 under all conditions. Structural characterization by X-ray crystallography reveals the presence of a bond between a phosphonate oxygen and the Re(I) center in fac-Re(CO)(3)(PMG) (Re-2). Detailed comparisons of NMR data for Re-2 conclusively demonstrate that the phosphonate group is coordinated in Re-1b (isomer favored at high pH) but not in Re-1a, which has a dangling N-(phosphonomethyl) group. To our knowledge, Re-1b and Re-2 and their (99m)Tc analogs are the first well-documented examples of complexes with these metal-tricarbonyl cores having a dianionic phosphonate group directly coordinated in a fac-M(CO)(3)-O-P grouping. Pharmacokinetic studies using Sprague-Dawley rats reveal that (99m)Tc-2 is a robust tracer. Hence, phosphonate groups should be considered in designing (99m)Tc and (186/188)Re radiopharmaceuticals, including agents with bioactive moieties attached to dangling carboxylate or phosphonate groups.

We previously identified two new agents based on the [99mTcVO]3+ core with renal clearances in human volunteers 30% higher than that of the widely used clinical tracer 99mTc-MAG3 (MAG35– = penta-anion of mercaptoacetyltriglycine). However, renal agents with even higher clearances are needed. More recently, we changed our focus from the [99mTcVO]3+ core to the discovery of superior tracers based on the fac-[99mTcI(CO)3]+ core. Compared to 99mTc-MAG3, fac-[99mTcI(CO)3(NTA)]2– (NTA3– = trianion of nitrilotriacetic acid) holds great promise by virtue of its efficient renal clearance via tubular secretion and the absence of hepatobiliary elimination, even in patients with severely reduced renal function. We report here NMR, molecular (X-ray) structure, and solution data on fac-[ReI(CO)3(NTA)]2– with a −CH2CO2– dangling monoanionic chain and on two fac-[ReI(CO)3(L)]− analogues with either a −CH2CONH2 or a −CH2CH2OH dangling neutral chain. In these three fac-[ReI(CO)3(L)]n− complexes, the fac-[ReI(CO)3(N(CH2CO2)...

To develop the first radiotracer targeting GPR91, a cell membrane-bound receptor that modulates the cellular response to hyperglycemia and hypoxia, we designed and prepared a small series of compounds based on a published series of 1,8-naphthyridines with high affinity to GPR91. Our approach provides a mechanism to incorporate radioactive atoms ((99m)Tc and (18)F) into the GPR91 pharmacophore as the final synthetic step. Pharmacological assays confirmed lead compounds for (99m)Tc and (18)F GPR91 radiotracers within the series.

Preclinical studies in rats showed that two of (99m)Tc(CO)3(ASMA) isomers (rac- and L-ASMA) had pharmacokinetic properties equivalent to that of (131)I-OIH, the radiopharmaceutical standard for the measurement of effective renal plasma flow. The aim of this study was to evaluate the pharmacokinetics of (99m)Tc(CO)3(ASMA) isomers in healthy human subjects.Three ASMA ligands (rac-, L- and D-ASMA) were labeled with (99m)Tc(CO)3 using an IsoLink kit (Covidien), and each formed (99m)Tc(CO)3(ASMA) tracer was co-injected with (131)I-OIH into healthy human subjects followed by sequential imaging, plasma clearance measurements and timed urine collection. Plasma protein binding, red cell uptake and percent injected dose in the urine were determined. Urine from each group of volunteers was analyzed for metabolites by HPLC.Image quality was excellent with all three agents. Each (99m)Tc(CO)3(ASMA) preparation was excreted unchanged in the urine. The plasma clearance ratio ((99m)Tc(CO)3(ASMA)/(131)I-OIH) was 81 ± 3 % for D-ASMA compared to only 20 ± 4 % for L-ASMA and 37 ± 7 % for rac-ASMA; the 81 % clearance ratio for D-ASMA isomer is still ∼ 30 % higher than the (99m)Tc-MAG3/(131)I-OIH clearance ratio (∼50-60 %). Red cell uptake was similar for all three tracers (6-9 %), and all tracers had a relatively rapid renal excretion; at 3 h, the (99m)Tc(CO)3(ASMA)/(131)I-OIH urine ratio was 100 ± 3 % for D-ASMA, 80 ± 2 % for L-ASMA and 88 ± 1 % for rac-ASMA.The renal excretion characteristics of (99m)Tc(CO)3(D-ASMA) in humans are superior to those of the other two (99m)Tc(CO)3(ASMA) isomers studied, but are still inferior to (131)I-OIH, even though there was no difference in the clearance of two of (99m)Tc(CO)3(ASMA) isomers and (131)I-OIH in rats. The work described here demonstrates the sensitivity in in vivo biological behavior of (99m)Tc(CO)3(ASMA) isomers to their subtle structural differences.

New tracers lacking the negatively charged pendant carboxyl group previously thought to be essential for rapid renal extraction

A series of piperazin-1-yl substituted unfused heterobiaryls was synthesized as ligands of the 5-HT7 receptors. The goal of this project was to elucidate the structural features that affect the 5-HT7 binding affinity of this class of compounds represented by the model ligand 4-(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (2). The SAR studies included systematical structural changes of the pyrimidine core moiety in 2 to quinazoline, pyridine and benzene, changes of the 3-furyl group to other heteroaryl substituents, the presence of various analogs of the 4-methylpiperazin-1-yl group, as well as additional substitutions at positions 5 and 6 of the pyrimidine. Substitution of position 6 of the pyrimidine in the model ligand with an alkyl group results in a substantial increase of the binding affinity (note a change in position numbers due to the nomenclature rules). It was also demonstrated that 4-(3-furyl) moiety is crucial for the 5-HT7 binding affinity of the substituted pyrimidines, although, the pyrimidine core can be replaced with a pyridine ring without a dramatic loss of the binding affinity. The selected ethylpyrimidine (12) and butylpyrimidine (13) analogs of high 5-HT7 binding affinity showed antagonistic properties in cAMP functional test and varied selectivity profile—compound 12 can be regarded as a dual 5-HT7/5-HT2AR ligand, and 13 as a multi-receptor (5-HT7, 5-HT2A, 5-HT6 and D2) agent.

The reaction of an aminopolycarboxylate ligand, aspartic-N-monoacetic acid (ASMA), with [Re(CO)3(H2O)3]+ was examined. The tridentate coordination of ASMA to this ReI tricarbonyl precursor yielded fac-Re(CO)3(ASMA) as a mixture of diastereomers. The chemistry is analogous to that of the TcI tricarbonyl complex, which yields fac-99mTc(CO)3(ASMA) under similar conditions. The formation, structure, and isomerization of fac-Re(CO)3(ASMA) products were characterized by HPLC, 1H NMR spectroscopy, and X-ray crystallography. The two major fac-Re(CO)3(ASMA) diastereomeric products each have a linear ONO coordination mode with two adjacent five-membered chelate rings, but they differ in the endo or exo orientation of the uncoordinated acetate group, in agreement with expectations based on previous studies. Conditions have been identified for the expedient isomerization of fac-Re(CO)3(ASMA) to a mixture consisting primarily of one major product. Because different isomeric species typically have different pharmacokinetic characteristics, these conditions may provide for the practical isolation of a single 99mTc(CO)3(ASMA) species, thus allowing the isolation of the isomer that has optimal imaging and pharmacokinetic characteristics. This information will aid in the design of future 99mTc radiopharmaceuticals.

In an ongoing effort to develop a renal tracer with pharmacokinetic properties comparable to p-aminohippurate and superior to those of both 99mTc-mercaptoacetyltriglycine and 131I-o-iodohippurate (131I-OIH), we evaluated a new renal tricarbonyl radiotracer based on the aspartic-N-monoacetic acid (ASMA) ligand, 99mTc(CO)3(ASMA). The ASMA ligand features 2 carboxyl groups and an amine function for the coordination of the {99mTc(CO)3}+ core as well as a dangling carboxylate to facilitate rapid renal clearance. Methods:rac-ASMA and l-ASMA were labeled with a 99mTc-tricarbonyl precursor, and radiochemical purity of the labeled products was determined by high-performance liquid chromatography. Using 131I-OIH as an internal control, we evaluated biodistribution in normal rats with 99mTc(CO)3(ASMA) isomers and in rats with renal pedicle ligation with 99mTc(CO)3(rac-ASMA). Clearance studies were conducted in 4 additional rats. In vitro radiotracer stability was determined in phosphate-buffered saline, pH 7.4, and in challenge studies with cysteine and histidine. 99mTc(CO)3(ASMA) metabolites in urine were analyzed by high-performance liquid chromatography. Results: Both 99mTc(CO)3(ASMA) preparations had greater than 99% radiochemical purity and were stable in phosphate-buffered saline, pH 7.4, for 24 h. Challenge studies on both revealed no significant displacement of the ligand. In normal rats, the percentage injected dose in urine at 10 and 60 min for both preparations averaged, respectively, 103% and 106% that of 131I-OIH. The renal clearances of 99mTc(CO)3(rac-ASMA) and 131I-OIH were comparable (P = 0.48). The tracer was excreted unchanged in the urine, proving its in vivo stability. In pedicle-ligated rats, 99mTc(CO)3(rac-ASMA) had less excretion into the bowel (P

The addition reaction of lithium reagents to the 4 position of 2-chloropyrimidine or 2-chloroquinazoline followed by oxidation of the resultant dihydro intermediate product is a powerful tool for the synthesis of 4-substituted 2-chloropyrimidines or 2-chloroquinazolines. 4-Vinyl derivatives undergo a conjugate nucleophilic addition across the vinyl group. A nucleophilic displacement of chloride in 4substituted 2-chloropyrimidines or 2-chloroquinazolines by treatment with 4-methylpiperazine provides compounds that are antagonists of the serotonin 5-HT2A receptor.

Fused dihydrooxazoles are produced by the reaction of 8-bromoteophylline (1), 6-bromo-2-pyridone (7), or 2-bromobenzimidazole (11) with an N-substituted N-(2,3-epoxypropyl)amine. The product derived from 1 undergoes rearrangement to a fused dihydrooxazine while the fused dihydrooxazoles derived from 7 and 11 are stable. J. Heterocyclic Chem., (2011).

Conjugate addition reaction of various nucleophiles across the vinyl group of 2-chloro-4-vinylpyrimidine, 2-chloro-4-(1-phenylvinyl)pyrimidine and 2-chloro-4-vinyl-quinazoline provides the corresponding 2-chloro-4-(2-substituted ethyl)pyrimidines and 2-chloro-4-(2-substituted ethyl)quinazolines. Treatment of these products, without isolation, with N-methylpiperazine results in nucleophilic displacement of chloride and yields the corresponding 2,4-disubstituted pyrimidines and quinazolines.