William B. Mathews

In this practitioner protocol, the radiochemical synthesis of [¹¹C]CPPC is described in detail, and a quality control summary of three validation productions is presented. The results indicate that the radiotracer product can be produced in good radiochemical yield (> 60 mCi (2.22 GBq) at end‐of‐synthesis (EOS)), at high specific activity (molar activity > 11,435 mCi/μmole (423 GBq/μmole) at EOS) and high chemical and radiochemical purity. The entire production conforms to current Good Manufacturing Practice (cGMP) requirements. The final product is formulated as a sterile, pyrogen‐free solution suitable for human injection.

The pathophysiology of post-treatment Lyme disease syndrome (PTLDS) may be linked to overactive immunity including aberrant activity of the brain’s resident immune cells, microglia. Here we used [11C]DPA-713 and positron emission tomography to quantify the 18 kDa translocator protein, a marker of activated microglia or reactive astrocytes, in the brains of patients with post-treatment Lyme disease symptoms of any duration compared to healthy controls. Genotyping for the TSPO rs6971 polymorphism was completed, and individuals with the rare, low affinity binding genotype were excluded. Data from eight brain regions demonstrated higher [11C]DPA-713 binding in 12 patients relative to 19 controls. [11C]DPA-713 PET is a promising tool to study cerebral glial activation in PTLDS and its link to cognitive symptoms.

Tuberculous meningitis (TBM) is a devastating form of tuberculosis (TB), and key TB antimicrobials, including rifampin, have restricted brain penetration. A lack of reliable data on intralesional drug biodistribution in infected tissues has limited pharmacokinetic (PK) modeling efforts to optimize TBM treatments. Current methods to measure intralesional drug distribution rely on tissue resection, which is difficult in humans and generally limited to a single time point even in animals. In this study, we developed a multidrug treatment model in rabbits with experimentally induced TBM and performed serial noninvasive dynamic ¹¹C-rifampin positron emission tomography (PET) over 6 weeks. Area under the curve brain/plasma ratios were calculated using PET and correlated with postmortem mass spectrometry. We demonstrate that rifampin penetration into infected brain lesions is limited, spatially heterogeneous, and decreases rapidly as early as 2 weeks into treatment. Moreover, rifampin concentrations in the cerebrospinal fluid did not correlate well with those in the brain lesions. First-in-human ¹¹C-rifampin PET performed in a patient with TBM confirmed these findings. PK modeling predicted that rifampin doses (≥30 mg/kg) were required to achieve adequate intralesional concentrations in young children with TBM. These data demonstrate the proof of concept of PET as a clinically translatable tool to noninvasively measure intralesional antimicrobial distribution in infected tissues.

DREADD not the designer compound: Designer receptors exclusively activated by designer drugs (DREADDs) constitute a powerful chemogenetic strategy that can modulate nerve cell activity in freely moving animal preparations. Gomez et al. used radioligand receptor occupancy measurements and in vivo positron emission tomography to show that DREADDs expressed in the brain are not activated by the designer compound CNO (clozapine N -oxide). Instead, they are activated by the CNO metabolite clozapine, a drug with multiple endogenous targets. This may have important implications for the interpretation of results obtained with this popular technology. Science , this issue p. 503

Fenobam is a negative allosteric modulator of the metabotropic glutamate receptor subtype 5 (mGluR5) with inverse agonist activity and is expected to contribute to the treatment of neuropsychiatric disorders involving dysfunction of mGluR5 including Fragile X syndrome. This study examined whether [¹¹C]ABP688, an antagonist PET radioligand, competes with fenobam for the same binding site in the nonhuman primate brain and would allow examination of occupancy‐plasma concentration relationships in the evaluation of the drug for target disorders in the human brain. Four paired PET studies with [¹¹C]ABP688 were performed in baboons at a baseline condition and after intravenous treatment with fenobam at different dose levels (0.3–1.33 mg/kg). Total distribution volume (V_T) and binding potential (BP_ND) using the cerebellum as a reference region were obtained by the plasma reference graphical method. Then it was examined whether occupancy follows a dose‐dependent, saturating pattern that was predicted by a modified first‐order Hill equation in individual regions. Baseline regional V_T and BP_ND values agreed with previously published data. Occupancy showed dose‐dependent and saturating patterns in individual regions, reaching >90% occupancy at 1.33 mg/kg dose of fenobam in the majority of regions. To our knowledge, this is the first use of PET to characterize the mGluR5 therapeutic drug fenobam. This study demonstrates a proof of principle for determining the in vivo occupancy of fenobam in primates. The results indicate that [¹¹C]ABP688 and PET may be useful for examination of occupancy of mGluR5 by fenobam, which should prove to be useful for designing future studies and treatment of human disease states. Synapse 68:565–573, 2014.

Over the last years, ligands for angiotensin II subtype 1 receptor (AT1R) have been developed as an alternative to the use of angiotensin-converting enzyme (ACE) inhibitors for controlling high blood pressure. Radiolabeled versions of these ligands have proven vital to the development of more potent and specific drugs for the treatment of hypertension. Imaging studies using radiolabeled AT1R ligands have also elucidated the role these receptors play in angiogenesis as well as in various disease states beyond hypertension.

The endothelin subtype-A (ETA) receptor is a member of a family of G-protein–coupled receptors that plays a central role in vasoconstriction, cell proliferation, and hormone production. The aim of this study was to synthesize and evaluate in vivo ¹¹C- and ¹⁸F-labeled analogs of the potent and selective ETA antagonist N-[[2′-[[(4,5-dimethyl-3-isoxazolyl)amino]sulfonyl]-4-(2-oxazolyl)[1,1′-biphenyl]-2-yl]methyl]-N,3,3-trimethylbutanamide (BMS-207940). Methods: Protected precursors and authentic nonradioactive standards were synthesized by reductive amination and subsequent alkylation of protected aldehyde 1. Desmethyl precursor 2 was reacted with ¹¹C-methyl iodide followed by deprotection and high-performance liquid chromatography purification to produce ¹¹C-(N-[[2′-[[(4,5-dimethyl-3-isoxazolyl)amino]sulfonyl]-4-(2-oxazolyl)[1,1′-biphenyl]-2-yl]methyl]-N,1-methylindolecarboxamide) (¹¹C-BMS-5p) 3. Nitro precursor 4 was reacted with ¹⁸F-fluoride and purified by high-performance liquid chromatography to produce ¹⁸F-(N-[[2′-[[(4,5-dimethyl-3-isoxazolyl)amino]sulfonyl]-4-(2-oxazolyl)[1,1′-biphenyl]-2-yl]methyl]-N,4-fluorobenzamide) (¹⁸F-FBzBMS) 5. Biodistribution was determined by injecting male CD-1 mice via the tail vein with either ¹¹C-BMS-5p 3 or ¹⁸F-FBzBMS 5. Specific binding was determined by pretreatment with 1 mg of BMS-207940 per kilogram. PET scanning of a baboon using either ¹¹C-BMS-5p 3 or ¹⁸F-FBzBMS 5 was performed at baseline and 40 min after intravenous administration of 1 mg of BMS-207940 per kilogram. Results: ¹¹C-BMS-5p 3 was synthesized with 1.5% radiochemical yield in 36 min, with an average specific activity of 1,051 GBq/μmol (28,400 mCi/μmol; n = 5) at the end of synthesis. ¹⁸F-FBzBMS 5 was synthesized with 0.54% radiochemical yield in 130 min, with an average specific activity of 12.9 GBq/μmol (349 mCi/μmol, n = 7) at the end of synthesis. In mice, the highest uptake of both radioligands was in the liver, lungs, and heart. Radioactivity in the liver washed out over time, and uptake in the lungs and heart remained relatively stable. Mice pretreated with 1 mg of BMS-207940 per kilogram showed greater than 64% specific binding in the lungs and kidneys at 60 min. Specific binding in the heart was determined to be 63% for ¹¹C-BMS-5p 3 and 81% for ¹⁸F-FBzBMS 5. PET studies in a baboon showed high uptake of both radioligands in the myocardium. Between 35 and 85 min, the heart-to-blood ratio was 4.7 to 1. Pretreatment with a 1 mg/kg dose of BMS-207940 showed 85% specific binding in the myocardium at 85 min after injection. Conclusion: Both ¹¹C-BMS-5p 3 and ¹⁸F-FBzBMS 5 bind selectively to the ETA receptor in vivo. Further development of these radioligands for imaging the ETA receptor in humans is warranted.

5‐(2‐Phenylethynyl)pyridine‐3‐[¹¹C]carbonitrile ([¹¹C]LY2232645), a metabotropic glutamate 5 receptor (mGluR5) antagonist, was synthesized by a no‐carrier‐added nucleophilic halogen displacement with [¹¹C]copper(I) cyanide. The average radiochemical yield was 2.5%, and the average specific activity was 1365 mCi/µmol at end‐of‐synthesis. The total time of synthesis, purification, and formulation was 26 min. Copyright © 2006 John Wiley & Sons, Ltd.

Imaging purine receptors and adenylate biodistribution in vivo may be of clinical importance not only for the investigation of normal adenylate metabolism but also in pathological conditions where adenylate uptake and/or release from certain tissues and organs may be altered, such as some types of cancer. In order to develop a tracer for positron emission tomography (PET) that would not be subject to loss of its radioisotope, adenosine 5′-monophosphate (AMP) was intrinsically labeled at the C-8 position with carbon-11. [¹¹C]AMP was synthesized by reacting 5-amino-1-β-d-ribofuranosylimidazole-4-carboxamidine-5′-phosphate with [¹¹C]formaldehyde. The metabolism of [¹¹C]AMP in human blood was determined in vitro both in the presence and absence of dipyridamole. The ex vivo biodistribution of [¹¹C]AMP and its in vivo dosimetry were determined in normal mice. The effect of dipyridamole on the distribution of [¹¹C]AMP in mice was also determined. [¹¹C]AMP was reliably synthesized in 34 minutes (n = 7) with an average radiochemical yield of 2.4% and an average specific activity of 90.10 GBq/μmol (2435 mCi/μmol) at end of synthesis. In normal mice, the highest uptake of [¹¹C]AMP was in the lungs, blood, and heart. The ex vivo mouse experiments showed that the uptake of ¹¹C radiotracer in the lungs at 60 minutes postinjection was significantly lower for dipyridamole-treated animals than controls. Dosimetry showed that the critical organs for radiation dose burden are kidneys and bladder. Treatment with dipyridamole blocked the red blood cell uptake of extracellular adenosine and therefore its subsequent intracellular conversion to ATP. The biodistribution studies indicate that the tracer has substantial accumulation in the kidneys, lungs, heart, and blood. [¹¹C]AMP is promising as a PET-imaging agent to trace adenylate biology in vivo.