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The cathepsin K (CatK) enzyme is abundantly expressed in osteoclasts, and CatK inhibitors have been developed for the treatment of osteoporosis. In our effort to support discovery and clinical evaluations of a CatK inhibitor, we sought to discover a radioligand to determine target engagement of the enzyme by therapeutic candidates using positron emission tomography (PET). L-235, a potent and selective CatK inhibitor, was labeled with carbon-11. PET imaging studies recording baseline distribution of [11 C]L-235, and chase and blocking studies using the selective CatK inhibitor MK-0674 were performed in juvenile and adult nonhuman primates (NHP) and ovariectomized rabbits. Retention of the PET tracer in regions expected to be osteoclast-rich compared with osteoclast-poor regions was examined. Increased retention of the radioligand was observed in osteoclast-rich regions of juvenile rabbits and NHP but not in the adult monkey or adult ovariectomized rabbit. Target engagement of CatK was observed in blocking studies with MK-0674, and the radioligand retention was shown to be sensitive to the level of MK-0674 exposure. [11 C]L-235 can assess target engagement of CatK in bone only in juvenile animals. [11 C]L-235 may be a useful tool for guiding the discovery of CatK inhibitors.

The fluorine-18-labeled positron emission tomography (PET) radiotracer [(18) F]MK-9470 is a selective, high affinity inverse agonist that has been used to image the cannabinoid receptor type 1 in human brain in healthy and disease states. This report describes a simplified, one-step [(18) F]radiofluorination approach using a GE TRACERlab FXFN module for the routine production of this tracer. The one-step synthesis, by [(18) F]fluoride displacement of a primary tosylate precursor, gives a six-fold increase in yield over the previous two-step method employing O-alkylation of a phenol precursor with 1,2-[(18) F]fluorobromoethane. The average radiochemical yield of [(18) F]MK-9470 using the one-step method was 30.3 ± 11.7% (n = 12), with specific activity in excess of 6 Ci/µmol and radiochemical purity of 97.2 ± 1.5% (n = 12), in less than 60 min. This simplified, high yielding, automated process was validated for routine GMP production of [(18) F]MK-9470 for clinical studies.

We report herein the discovery of a fatty acid amide hydrolase (FAAH) positron emission tomography (PET) tracer. Starting from a pyrazole lead, medicinal chemistry efforts directed toward reducing lipophilicity led to the synthesis of a series of imidazole analogues. Compound 6 was chosen for further profiling due to its appropriate physical chemical properties and excellent FAAH inhibition potency across species. [(11)C]-6 (MK-3168) exhibited good brain uptake and FAAH-specific signal in rhesus monkeys and is a suitable PET tracer for imaging FAAH in the brain.

A novel tritium or flourine-18-labelled benzamidine-containing NR2B-selective NMDA receptor ligand has been synthesized. This compound was designed to contain the fluoromethoxy group to allow for the synthesis of a high specific activity, fluorine-18-labelled PET tracer for imaging studies of the NR2B receptor. In addition to the fluorine-18-labelled compound, this compound was also tritium labelled. The tritiated ligand (11 Ci/mmol) was synthesized by a gas tritiation reaction of an aryl bromide precursor. The fluorine-18 ligand (2916 Ci/mmol), which was deuterated in the fluoromethoxy group to aid in metabolic stability, was synthesized by alkylating a phenolic precursor with [18F]fluoromethylbromide-d2. Copyright © 2004 John Wiley & Sons, Ltd.

Purpose To develop and characterize a radiolabelled Substance-P antagonist useful for quantitation of neurokinin-1 receptors in the brain via PET imaging. Procedure [18F]SPA-RQ (Substance-P antagonist – receptor quantifier) was synthesized in good yield and high specific activity by alkylation of a BOC protected phenolate anion using [18F]bromofluoromethane. Removal of the BOC protecting group with trifluoroacetic acid gave [18F]SPA-RQ. Results SPA-RQ has high affinity for human, rhesus monkey and guinea pig NK1 receptors (h-IC50 = 67 pM) and has a log P value of 1.8. Biodistribution studies in guinea pig showed that this tracer penetrates the blood-brain barrier and selectively labels NK1 receptors in the striatum and cortex. Conclusion [18F]SPA-RQ is a potent, high affinity Substance-P antagonist that can be conveniently labeled with high specific activity using [18F]fluoromethylbromide. This tracer is a useful tool for noninvasive imaging of central NK1 receptors.

Radiolabelled neurokinin-1 (NK1) receptor antagonists 1b and 2b were required for in vitro/in vivo characterization to support the development of 1a and 2a as fluorine-18 labelled PET ligands. These tritium labelled compounds were synthesized from aryl iodide precursors giving the final tritiated tracers with specific activities of 28 (1b) and 14 (2b) Ci/mmol. Copyright © 2004 John Wiley & Sons, Ltd.

The αvβ3 integrin receptor ligand [125I]L-775,219, 1c, was synthesized for use in an in vitro receptor binding assay with high specific activity (>1500 Ci/mmol) and a radiochemical yield of 15–20%. Conversion of iodide 1b to the corresponding aryl trimethylstannane 2, followed by radioiodination using Na125I/iodobead gave 1c. Copyright © 2001 John Wiley & Sons, Ltd.

The synthesis and preliminary characterization of three radioiodinated α1-adrenergic receptor ligands [125I]5a–c are described. These tracers are analogs of L-760,478, 1b, itself an analog of prazosin, 1a. The highest affinity tracer, [125I]5a, has six fold higher affinity for the α1 receptor subtypes than prazosin. These ligands could be useful for autoradiographic studies of the α1 receptor subtypes. Copyright © 2001 John Wiley & Sons, Ltd.

[ 11 ]L-753,037, [(+)-(5S,6R,7R)-2-butyl-7-[2-((2S)-2-carboxypropyl)-4-[ 11 C]-methoxyphenyl]-5-(3,4-methylenedioxyphenyl)-cyclopenteno[1,2-b]pyridine-6-carboxylate], a potent mixed antagonist of endothelin receptor subtypes ET A and ET B , was synthesized by [ 11 C]-methylation of a phenolic precursor. The time for synthesis, purification, and formulation was 17 minutes with an average specific radioactivity of 2535 mCi/μmol (EOS) and average decay corrected radiochemical yield of 3% based on [C]-methyl iodide.

The synthesis of the fibrinogen receptor antagonist [ 3 H]L-767,679 and its ethyl ester prodrug [ 3 H]L-767,685 is described. Bromination of an appropriate benzolaclam followed by catalytic tritiation with tritium gas gave a labelled benzolactam that was converted to [ 3 H]L-767,685 via a coupling/deprotection sequence. Hydrolysis of [ 3 H]L-767,685 then gave the acid [ 3 H]L-767,679. These two compounds were obtained with a specific activity of 10-16 Ci/mmol.

The synthesis of [3H]L-756,568, an orally active fibrinogen receptor antagonist, is described. Two synthetic pathways were developed using either bromoindoles 2a/2b or bromoaryl sufonamide 11 as the precursor. Use of the bromoaryl sulfonamide precursor led to [3H]L-756,568 with higher radiochemical purity, higher radiochemical yield, and slightly higher specific activity. Copyright © 1999 John Wiley & Sons, Ltd.

The synthesis of three tritiated reverse transcriptase inhibitors is described herein. These compounds contained the benzoxazole ring linked to a 2-pyridinone ring by an aminomethylene group. The tritiated compounds were synthesized by hydrogenolysis of bromobenzoxazole precursors. The final tritiated compounds had specific activities ranging from 3-10 Ci/mmol.

The synthesis of [ 3 H]L-734,217, 1b, an orally active fibrinogen receptor antagonist, is described. The conversion of 3-amino crotonate 6 to [ 3 H]L-734,217 was carried out via a two step sequence of catalytic tritiation followed by basic hydrolysis. Deuterium model reactions showed that the reduction of 6 with PtO2 occurred via hydrogen transfer from the solvent (ethanol or acetic acid) leading to poor isotope incorporation. When this reduction was conducted in methanol with 10% Pd/C, good isotope incorporation resulted. Ultimately, [ 3 H]L-734,217 was formed with a specific activity of 42 Ci/mmol.

The cyclophosphamide metabolite phosphoramide mustard (PM) was synthesized with isotopic enrichment at each nitrogen and oxygen site. Sequential reaction of N,N-bis(2-chloroethyl)phosphoramidic dichloride [Cl2P(O)N(CH2CH2Cl)2] with benzyl alcohol and ammonia gave N,N-bis(2-chloroethyl)phosphorodiamidic acid phenylmethyl ester [BzO(H2N)P(O)N(CH2CH2Cl)2]. Catalytic hydrogenation of this benzyl ester followed by the addition of cyclohexylamine (CHA) provided PM as the CHA salt. Incorporation of 15NH3 into this general scheme gave PM with a 15NH2 moiety. Glycine-15N was converted to bis(2-chloro-ethyl)amine-15N hydrochloride which, in turn, provided for N,N-bis(2-chloroethyl)phosphoramidic-15N dichloride. Use of this compound in the general synthetic pathway yielded PM°CHA with 15N in the mustard moiety. 17O-Enriched PM was generated through the use of benzyl alcohol-17O. To obtain the alcohol, labelled benzaldehyde was made by exchange with 17OH2 and was then reduced with sodium borohydride.

The automation of routinely produced radiotracers is an important process for any PET center. Automation allows frequent, highly reproducible syntheses with large amounts of radioactivity while minimizing exposure to the radiochemist in comparison with manual radiotracer synthesis. We found the Gilson SK233 Work Station, coupled with Gilson's 735 Sampler Software and a Gilson SPE (solid phase extraction) module, to be a versatile, user-friendly, and cost-effective solution to our need for automation in the development and routine synthesis of radiotracers.

Amalgamation of the structure−activity relationship of two series of GlyT1 inhibitors developed at Merck led to the discovery of a clinical candidate, compound 16 (DCCCyB), which demonstrated excellent in vivo occupancy of GlyT1 transporters in rhesus monkey as determined by displacement of a PET tracer ligand.

The human immunodeficiency virus integrase inhibitor, [18F]MK-0518, was prepared via a three-step, one-pot radiosynthesis. [18F]4-Fluorobenzylamine was produced from the fluorination of 4-cyano-N,N,N-trimethylammonium triflate with [18F]fluoride and reduction with borane methylsulfide complex in 50–68% radiochemical yield. The final step, the coupling of [18F]4-fluorobenzylamine with an ester coupling partner, achieved an overall uncorrected radiochemical yield after HPLC purification of ∼2%, based on the starting [18F]fluoride. In a typical run, the total synthesis time was about 90 min and gave 0.37–1.74 GBq (10–47 mCi) of [18F]MK-0518. The radiochemical purity of [18F]MK-0518 was>98% and the specific activity was 243–1275 Ci/mmol (EOS, n=4). A convenient three-step, one-pot radiosynthesis of [18F]MK-0518 via [18F]4-fluorobenzylamine has been developed, giving sufficient quantities of [18F]MK-0518 for animal positron emission tomography studies. Copyright © 2010 John Wiley & Sons, Ltd.

Two histamine H3 receptor (H3R) inverse agonist PET tracers have been synthesized and characterized in preclinical studies. Each tracer has high affinity for the histamine H3 receptor, has suitable lipophilicity, and neither is a substrate for the P-glycoprotein efflux pump. A common phenolic precursor was used to synthesize each tracer with high specific activity and radiochemical purity by an alkylation reaction using either [11C]MeI or [18F]FCD2Br. Autoradiographic studies in rhesus monkey and human brain slices showed that each tracer had a widespread distribution with high binding densities in frontal cortex, globus pallidus and striatum, and lower uptake in cerebellum. The specificity of this expression pattern was demonstrated by the blockade of the autoradiographic signal by either the H3R agonist R-α-methylhistamine or a histamine H3R inverse agonist. In vivo PET imaging studies in rhesus monkey showed rapid uptake of each tracer into the brain with the same distribution seen in the autoradiographic studies. Each tracer could be blocked by pretreatment with a histamine H3R inverse agonist giving a good specific signal. Comparison of the in vitro metabolism of each compound showed slower metabolism in human liver microsomes than in rhesus monkey liver microsomes, with each compound having a similar clearance rate in humans. The in vivo metabolism of 1b in rhesus monkey showed that at 60 min, ∼35% of the circulating counts were due to the parent. These tracers are very promising candidates as clinical PET tracers to both study the histamine H3R system and measure receptor occupancy of H3R therapeutic compounds. Synapse 63:1122–1132, 2009. © 2009 Wiley-Liss, Inc.

Three metabotropic glutamate receptor subtype 5 (mGluR5) PET tracers have been labeled with either carbon-11 or fluorine-18 and their in vitro and in vivo behavior in rhesus monkey has been characterized. Each of these tracers share the common features of high affinity for mGluR5 (0.08-0.23 nM vs. rat mGluR5) and moderate lipophilicity (log P 2.8-3.4). Compound 1b was synthesized using a Suzuki or Stille coupling reaction with [11C]MeI. Compounds 2b and 3b were synthesized by a SNAr reaction using a 3-chlorobenzonitrile precursor. Autoradiographic studies in rhesus monkey brain slices using 2b and 3b showed specific binding in cortex, caudate, putamen, amygdala, hippocampus, most thalamic nuclei, and lower binding in the cerebellum. PET imaging studies in monkey showed that all three tracers readily enter the brain and provide an mGluR5-specific signal in all gray matter regions, including the cerebellum. The specific signal observed in the cerebellum was confirmed by the autoradiographic studies and saturation binding experiments that showed tracer binding in the cerebellum of rhesus monkeys. In vitro metabolism studies using the unlabeled compounds showed that 1a, 2a, and 3a are metabolized slower by human liver microsomes than by monkey liver microsomes. In vivo metabolism studies showed 3b to be long-lived in rhesus plasma with only one other more polar metabolite observed.

Diketo acids such as L-731,988 are potent inhibitors of HIV-1 integrase that inhibit integration and viral replication in cells. These compounds exhibit the unique ability to inhibit the strand transfer activity of integrase in the absence of an effect on 3′ end processing. To understand the reasons for this distinct inhibitory profile, we developed a scintillation proximity assay that permits analysis of radiolabeled inhibitor binding and integrase function. High-affinity binding of L-731,988 is shown to require the assembly of a specific complex on the HIV-1 long terminal repeat. The interaction of L-731,988 with the complex and the efficacy of L-731,988 in strand transfer can be abrogated by the interaction with target substrates, suggesting competition between the inhibitor and the target DNA. The L-731,988 binding site and that of the target substrate are thus distinct from that of the donor substrate and are defined by a conformation of integrase that is only adopted after assembly with the viral end. These results elucidate the basis for diketo acid inhibition of strand transfer and have implications for integrase-directed HIV-1 drug discovery efforts.

Improved communication and cooperation between research-driven drug companies and academic positron emission tomography (PET) centers, coupled with improvements in PET camera resolution, the availability of small animal PET cameras and a growing list of neuroreceptor-specific PET tracers, have all contributed to a substantial increase in the use and value of PET as a tool in central nervous system drug discovery and development.