Dah-Ren Hwang

In vivo delineation of myocardial hypoxia during coronary occlusion using fluorine-18 fluoromisonidazole and positron emission tomography: A potential approach for identification of jeopardized myocardium☆

Previous studies have demonstrated that the positron-emitting fluorine-18 (18F)-labeled fluoromisonidazole is a specific tracer of myocardial hypoxia. Its fractional extraction is enhanced in ischemic or hypoxic myocardium but returns to baseline levels on reperfusion and recovery of normal function. Thus, this agent might be useful in delineating acutely hypoxic but potentially salvageable myocardium. Accordingly, to delineate the relation between the myocardial extraction of 18F-fluoromisonidazole after intravenous administration and the time of antecedent ischemia in vivo, uptake of tracer was measured with positron emission tomography and direct postmortem tissue analysis in 14 dogs in which tracer was administered within 3 h of coronary occlusion (a time associated with marked potential for salvage on reperfusion); in 4 dogs after 6 h of coronary occlusion (a time associated with minimal salvage of myocardium on reperfusion); and in 8 dogs after greater than 24 h of coronary occlusion (to delineate uptake in tissue that is irreversibly damaged). The residual fraction (that is, the amount of tracer extracted and retained in a region) in ischemic myocardium in the dogs in which 18F-fluoromisonidazole was administered within 3 h after occlusion averaged (+/- standard deviation) 23 +/- 18%, which was higher than the residual fraction in myocardium subjected to ischemia for either 6 or greater than 24 h before tracer administration (12 +/- 7% and 5 +/- 2%, respectively, p less than 0.01 for both). Retention of tracer in remote normal myocardium averaged 2 +/- 1%.(ABSTRACT TRUNCATED AT 250 WORDS)

No-carrier-added synthesis of 3-[18F]fluoro-1-(2-nitro-1-imidazolyl)-2-propanol. A potential PET agent for detecting hypoxic but viable tissues

Four different approaches towards the synthesis of [18F]FMISO have been studied. The first approach was based on the reaction of epoxide 4 and [18F]fluoride. Both specific activity and radiochemical yield (less than 1%) for [18F]FMISO were low. Two new approaches, starting with compounds 8 and 9, have failed to give [18F]FMISO. The fourth approach, based on the reaction of [18F]epifluorohydrin 10, prepared from Tosylate 13 and [18F]KF/Kryptofix 222, has provided a reliable, no-carrier added synthesis of [18F]FMISO. The product was obtained in a radiochemical yield of 7-12% at end-of-synthesis (based on [18F]fluoride) with a specific activity of greater than 400 Ci/mmol and a synthesis time of 1.5 h. Preliminary PET studies suggest that [18F]FMISO may be a promising tracer for delineation of ischemic but viable myocardium.

Positron labeled muscarinic acetylcholine receptor antagonist: 2- and 4-[18F]fluorodexetimide. Syntheses and biodistribution

Two 18F-labeled analogues of dexetimides, 2-[18F]fluorodexetimide (2-FDEX) and 4-[18F]fluorodexetimide (4-FDEX), were prepared and evaluated in vivo as possible agents for the study of the muscarinic acetylcholine receptor (mAChR) with PET. Two synthetic approaches, a 2-step reductive alkylation procedure and a 4-step alkylation approach, were investigated. The alkylation approach with higher overall radiochemical yields was used to prepare 2- and 4-FDEX for biodistribution studies. The overall synthesis time for both compounds was 2.5 h and the overall radiochemical yield at end-of-synthesis was 12%. The specific activity was found to be greater than 600 mCi/mumol. Biodistribution studies of 2-FDEX in rats produced striatum-to-cerebellum and cortex-to-cerebellum ratios of 8.6 +/- 1.1 and 8.4 +/- 1.0 at 1 h after injection, and 12.1 +/- 2.1 and 10.7 +/- 2.2 at 3 h, respectively. Substantial radioactivity detected in bone indicated the in vivo defluorination of 2-FDEX. The striatum-to-cerebellum ratio for 4-FDEX was slightly lower at 1 h (5.9 +/- 0.9) but equally high at 3 h (12.3 +/- 2.0) when compared to 2-FDEX, and there was little bone uptake. The uptake of both 2-FDEX and 4-FDEX into mAChR rich brain regions (e.g. striatum, cortex) was blocked by a dose of dexetimide (5 mg/kg). Our results suggest 4-FDEX is a potential PET agent for study mAChR in vivo.

A new procedure for labeling alkylbenzenes with [18F]fluoride.

A new procedure for labeling alkylbenzenes with no-carrier-added (nca) [18F]fluoride is reported. This will allow the use of [18F]-for-nitro aromatic nucleophilic displacement reaction for labeling aromatic compounds with no activating groups on the benzene ring. The new procedure involves (A) the [18F]-for-nitro displacement reaction on nitrophenones, and (B) the reduction of [18F]fluorophenones with triethylsilane and trifluoroacetic acid to alkylfluorobenzenes. The desired 18F-labeled alkylbenzenes were prepared in a synthesis time of 1 h with a radiochemical yield of 20% at end-of-synthesis. The procedure has been successfully applied to the synthesis of 18F-labeled alkylating agents, such as 4-[18F]fluorophenethyl bromide, 4, and 4-[18F]fluorophenbutyl chloride, 5. Using the reaction of piperidine and 4 as a model, the potential use of phenethylbromide 4 for labeling biologically important amines was examined. Initial results indicated that the desired alkylated piperidine was formed in low yields (less than 5%) due to the conversion of halide 4 to [18F]fluorostyrene (greater than 85%) under basic conditions. The new procedure provides an easy method of labeling alkylbenzenes with fluorine-18.

Imaging prostate derived tumors with PET and N-(3-[18F]fluoropropyl)putrescine.

Because of the high uptake of polyamines by the prostate and by prostate derived tumors, polyamines have been considered as potential imaging agents for metastatic prostate cancer. We now report the successful PET imaging of the Dunning R3327H prostatic carcinoma with N-(3-[18F]fluoropropyl)putrescine (FPP), a positron-labeled putrescine analog. Additionally, the biodistribution of FPP in tumor bearing Copenhagen male rats is analyzed. The tumor uptake of FPP was high, and the tumor-to-muscle ratios at 1, 2, 3 and 4.5 h post-injection were 7.2 +/- 1.0, 5.61 +/- 1.65, 4.62 +/- 0.21 and 3.51 +/- 0.91 respectively. The estimated radiation dose for FPP was calculated from rat biodistribution data. The radiation dose estimates suggest that the critical organ, following the administration of FPP, is the upper large intenstine which receives 0.3 rad/mCi administered.

Characterization of 64Cu-DOTA-Conatumumab: A PET Tracer for In Vivo Imaging of Death Receptor 5

Conatumumab is a fully human monoclonal antibody that binds to and activates human death receptor 5 (DR5; also known as TRAIL receptor 2). The purpose of this study was to characterize 64Cu-labeled conatumumab as a PET tracer for imaging DR5 in tumors. Methods: DOTA-conatumumab was synthesized by incubating conatumumab with 2,2′,2″-(10-(2-(2,5-dioxopyrrolidin-1-yloxy)-2-oxoethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (DOTA-NHS). The absolute numbers of DOTA molecules per conatumumab molecules were determined by matrix-assisted laser desorption ionization mass spectrometry and electrospray ionization quadrupole time-of-flight mass spectrometry. 64Cu-DOTA-conatumumab was prepared by incubating 64CuCl2 (33–222 MBq) with DOTA-conatumumab at 37°C for 1 h. Binding of conatumumab and DOTA-conatumumab to Fc-coupled human DR5 (huTR2-Fc) was tested in a kinetic analysis assay, and the biologic activity of copper-DOTA-conatumumab was measured using a caspase-3/7 luminescent assay. In vivo evaluation of DOTA-conatumumab and copper-DOTA-conatumumab was done in severe combined immunodeficiency mice bearing Colo205 xenografts: tissue uptake was determined with biodistribution studies, and small-animal PET and autoradiography were used to determine the uptake of 64Cu-DOTA conatumumab into tumors and other tissues. Results: DOTA-conatumumab was prepared with an average of 5 DOTA molecules per conatumumab molecule. The in vitro median effective concentration required to induce a 50% effect of DOTA-conatumumab and conatumumab from the assay were 389 and 320 pM, respectively. The median effective dose (±SD) of DOTA-conatumumab and conatumumab via the caspase assay was 135 ± 31 and 128 ± 30 pM, respectively. In female CB17 severe combined immunodeficiency mice bearing Colo205 xenografts, DOTA-conatumumab and conatumumab inhibited tumor growth to the same extent. Small-animal PET studies showed tumor uptake at 24 h after injection of the tracer, with a mean standardized uptake value of 3.16 (n = 2). Tumor uptake was decreased by the coadministration of 400 μg of unlabeled conatumumab (mean standardized uptake value, 1.55; n = 2), suggesting saturable uptake. Tissue uptake determined by biodistribution studies was in agreement with the small-animal PET findings. Conclusion: These results suggest that 64Cu-DOTA-conatumumab is a potential PET tracer for imaging DR5 in tumors and may be useful for measuring on-target occupancy by conatumumab.

No-carrier-added radiobromination via cuprous chloride-assisted nucleophilic aromatic bromodeiodination

Cuprous chloride-assisted aromatic bromodeiodination was investigated as a regioselective no-carrier-added labelling technique. Using 77Br-, CuCl and simple iodinated aromatic substrates in DMSO, radiochemical yields ranged from 60 to 90% after 30 min at 135 degrees C. The methodology described here holds potential for the convenient labelling of radiopharmaceuticals with 75Br, 76Br or 77Br.