Biodistribution and radiation dosimetry of the dopamine transporter ligand.

UNLABELLED: 18F-labeled 2 beta-carbomethoxy-3beta-(4-chlorophenyl)-8-(-2-fluoroethyl)nortropane ([18F]FECNT) is a recently developed dopamine transporter ligand with potential applications in patients with Parkinson's disease and cocaine addiction. METHODS: Estimates of the effective dose equivalent and doses for specific organs were made using biodistribution data from 16 Sprague-Dawley rats and nine rhesus monkeys. PET images from two rhesus monkeys were used to calculate the residence time for the basal ganglia. The computer program MIRDOSE3 was used to calculate the dosimetry according to the methodology recommended by MIRD. RESULTS: The basal ganglia were the targeted tissues receiving the highest dose, 0.11 mGy/MBq (0.39 rad/mCi). The effective dose equivalent was 0.018 mSv/MBq (0.065 rem/mCi), and the effective dose was 0.016 mSv/MBq (0.058 rem/mCi). CONCLUSION: Our data show that a 185-MBq (5-mCi) injection of [18F]FECNT leads to an estimated effective dose of 3 mSv (0.3 rem) and an estimated dose to the target organ or tissue of 19.4 mGy (1.93 rad).

18F-labeled FECNT: a selective radioligand for PET imaging of brain dopamine transporters.

Fluorine-18 labeled 2beta-carbomethoxy-3beta-(4-chlorophenyl)-8-(2-fluoroethyl)nort ropane (FECNT) was synthesized in the development of a dopamine transporter (DAT) imaging ligand for positron emission tomography (PET). The methods of radiolabeling and ligand synthesis of FECNT, and the results of the in vitro characterization and in vivo tissue distribution in rats and in vivo PET imaging in rhesus monkeys of [18F]FECNT are described. Fluorine-18 was introduced into 2beta-carbomethoxy-3beta-(4-chlorophenyl)-8-(2-fluoroethyl)nort ropane (4) by preparation of 1-[18F]fluoro-2-tosyloxyethane (2) followed by alkylation of 2beta-carbomethoxy-3beta-(4-chlorophenyl)nortropane (3) in 21% radiochemical yield (decay corrected to end of bombardment [EOB]). Competition binding in cells stably expressing the transfected human DAT serotonin transporter (SERT) and norepinephrine transporter (NET) labeled by [3H]WIN 35428, [3H]citalopram, and [3H]nisoxetine, respectively, indicated the following order of DAT affinity: GBR 12909 > CIT >> 2beta-carbomethoxy-3beta-(4-chlorophenyl)-8-(3-fluoropropyl) nortropane (FPCT) > FECNT. The affinity of FECNT for SERT and NET was 25- and 156-fold lower, respectively, than for DAT. Blocking studies were performed in rats with a series of transporter-specific agents and demonstrated that the brain uptake of [18F]FECNT was selective and specific for DAT-rich regions. PET brain imaging studies in monkeys demonstrated high [18F]FECNT uptake in the caudate and putamen that resulted in caudate-to-cerebellum and putamen-to-cerebellum ratios of 10.5 at 60 min. [18F]FECNT uptake in the caudate/putamen peaked in less than 75 min and exhibited higher caudate- and putamen-to-cerebellum ratios at transient equilibrium than reported for 11C-WIN 35,428, [11C]CIT/RTI-55, or [18F]beta-CIT-FP. Analysis of monkey arterial plasma samples using high performance liquid chromatography determined that there was no detectable formation of lipophilic radiolabeled metabolites capable of entering the brain. In equilibrium displacement experiments with CIT in rhesus monkeys, radioactivity in the putamen was displaced with an average half-time of 10.2 min. These results indicate that [18F]FECNT is a radioligand that is superior to 11C-WIN 35,428, [11C]CIT/RTI-55, [18F]beta-CIT-FP, and [18F]FPCT for mapping brain DAT in humans using PET.

Comparison of technetium-99m-ll-EC isomers in rats and humans.

UNLABELLED: Technetium-99m-L,L-ethylenedicysteine (99mTc-LL-EC) is a new renal imaging agent with pharmacokinetic properties reported to be slightly superior to those of 99mTc-mercaptoacetyltriglycine (99mTc-MAG3); however, to better define the potential of the enantiomer 99mTc-DD-EC and the diastereomer 99mTc-DL-EC as renal imaging agents, we compared the three EC stereoisomers with 131I-orthoiodohippurate (OIH) in a series of rats and humans. METHODS: Each 99mTc-EC stereoisomer was coinjected with OIH in six Sprague-Dawley rats for measurements of clearance and extraction fraction. Each stereoisomer was also coinjected with OIH in three human volunteers followed by sequential imaging, plasma clearance measurements and timed urine collections. RESULTS: Technetium-99m-DD-EC had the highest clearance and extraction efficiency in rats (p < or = 0.02). In humans, image quality was good with all three agents. The clearance ratio (EC/OIH) was 82% +/- 8% for 99mTc-DD-EC compared to 70% +/- 3% and 40% +/- 5% for 99mTc-LL-EC and 99mTc-DL-EC, respectively. Technetium-99m-DD and 99mTc-LL-EC were excreted more rapidly than 99mTc-DL-EC. CONCLUSION: Technetium-99m-DD-EC has excellent imaging properties and the data suggest that its clearance may approach that of OIH more closely than any other 99mTc renal agent. A potential limitation is the fact that both 99mTc-DD and LL-EC exist in dianionic (80%) and monoanionic (20%) forms at physiological pH and it is unlikely that these two forms have the same clearance or protein binding affinity.

Fluorine-18-FPCT: a PET radiotracer for imaging dopamine transporters.

UNLABELLED: Fluorine-18-labeled 2 beta-carbomethoxy-3 beta-(4-chlorophenyl)-8-[-3-fluoropropyl) nortropane (FPCT) has been synthesized as a new dopamine transporter imaging agent. METHODS: Fluorine-18 was introduced into 2 beta-carbomethoxy-3 beta-(4-chlorophenyl)-8-[-3-fluoropropyl) nortropane by preparation of 1-[18F]fluoro-3-iodopropane followed by alkylation of 2 beta-carbomethoxy-3 beta-(4-chlorophenyl)nortropane. RESULTS: Tissue distribution studies in rats with [18F]FPCT showed high striatal uptake (0.70% dose/g at 60 min; 0.38% dose/g at 120 min) and good striatal-to-cerebellum ratios (5.5 at 60 min; 6.2 at 120 min). Imaging studies in rhesus monkeys (n = 2) with [18F]FPCT showed high uptake and retention in the putamen (P) (P = 0.03%-0.12% dose/g; at 115 min) and good putamen-to-cerebellum ratios of 3.40-3.43 at 115 min. Plasma metabolites were analyzed in rhesus monkeys (n = 2) by ether extraction and HPLC. The radioactivity in the ether-extractable fraction displayed a single peak that corresponded on HPLC to unmetabolized authentic FPCT. CONCLUSION: These results suggest that [18F]FPCT is an excellent candidate for PET imaging of dopamine transporters.

Evaluation of technetium-99m-triamide-mercaptide complexes designed to identify properties favoring renal tubular transport.

UNLABELLED: To aid in the design of an improved 99mTc-labeled renal agent, several new [99mTcO(MAG3)]2- analogs were synthesized to determine the effects of varying the position and chemical form of the terminal charged group on renal clearance. METHODS: Clearance, extraction efficiency and plasma protein binding were measured in six Sprague-Dawley rats per complex for ortho, meta and para isomers of [99mTcO(MAG2-ABA)]2-, with MAG2- = mercaptoacetylglycylglycyl- and ABA = aminobenzoate; [99mTcO(MAG2-pASA)]2-, with pASA = p-aminosalicylate; [99mTcO(MAG2-AMS]2-, with AMS = aminomethylsulfonate; and [99mTcO(MAG2-AMP]3-, with AMP = aminomethylphosphonate. For agents with relatively poor clearances, hepatobiliary excretion was evaluated by using a camera-based method. RESULTS: The clearances of the ortho, meta and para isomers of [99mTcO(MAG2-ABA)]2- were 17%, 20% and 59% of those of OIH, respectively. The clearances of [99mTcO(MAG2-pASA)]2-, [99mTcO(MAG2-AMS)]2- and [99mTcO(MAG2-AMP)]3- were 32%, 46% and 39% those of OIH, respectively. CONCLUSION: Optimal tubular transport appears to require a terminal anionic group; a planar carboxylate is preferred over nonplanar -SO3- or -PO3(2-) substituents, suggesting that the smaller size and/or planar shape of the carboxylate group are probably more important than the total charge or charge distribution. Optimal transport also appears to depend on the oxo-carboxylate conformation (syn or anti) and the oxo-carboxylate distance, although these relationships can be modulated by steric interactions. These structure-distribution relationships are important factors to consider in the future design of renal radiopharmaceuticals.

Treatment of tyramine-induced brain edema with anion transport inhibitor L-644,711.

Tyramine induces coma in phenelzine-treated dogs. Development of coma in these animals is associated with brain edema, abnormal brain scans of Tc-99m-diethylene-triamine-penta-acetic acid (Tc-99m-DTPA), and elevated levels of CSF catecholamines. We found that the intravenous administration of 6-7 mg/kg of a single dose of L-644,711 given fifteen minutes after the oral administration of tyramine to phenelzine-pretreated animals followed by an infusion of normal saline containing 6-7 mg/kg of the drug given over a period of 2 hr caused reversal of brain injury. This was accompanied by full recovery within a period of 24 hr of all the animals tested. A follow-up study revealed that 24 hr after treatment with L-644,711 CSF levels of catecholamines and brain images of Tc-99m-DTPA were indistinguishable from normal controls. Animals that received no drug died from unresolved coma within 4 to 24 hr. Animals that had recovered due to therapy with L-644,711 were given 10-14 days rest followed by a repetition of the phenelzine and tyramine treatment but denied L-644,711 therapy. These animals also died of unresolved coma within 24 hr. This preliminary study suggest that the use of L-644,711 may constitute an important advance in treatment of brain edema of a wide range of neurological disorders.

Tyramine neurotoxicity and first-pass brain technetium-99m-diethylenetriamine penta-acetic acid.

Tyramine induces coma in phenelzine-treated dogs with liver disease. The objective of the present investigation was to examine the influence of tyramine in these monoamine oxidase-inhibited dogs on the kinetics of Tc-99m-diethylenetriamine penta-acetic acid (Tc-99m-DTPA) during its first passage through the brain by nuclear imaging techniques. The study began with anesthetized mongrel dogs (n = 10) in a supine position over the camera detector. Data acquisition was started simultaneously after the rapid intracarotid injection of Tc-99m-DTPA (5 mCi) and 60 0.5-sec images of the brain were taken. Tyramine induced increased uptake with a concomitant impairment in the elimination of Tc-99m-DTPA from the brain of these phenelzine-treated animals with hepatic injury (n = 5) as compared to pretreated animals serving as a control group or phenelzine-treated animals without liver disease. This was accompanied by an appreciable reduction in hemispheric cerebral blood flow (50.5 +/- 19.3 vs. 110 +/- 16 ml/100 g/min), respectively. Increased cerebrovascular permeability of Tc-99m-DTPA and decreased cerebral blood flow occurred concomitantly with increased cerebrospinal fluid pressure and elevation in cerebrospinal fluid catecholamines of monoamine oxidase-inhibited animals with hepatic injury.

Quantitative evaluation of brain edema by radionuclide imaging technique.

The objective of the present investigation was to quantify metabolic cerebral edema in dogs with impaired monoamine oxidase (MAO) function and liver disease using dynamic imaging of the brain with 99mTc-diethylenetriamine pentacetic acid (99mTc DTPA). Data acquisition was started following a rapid intracarotid injection of 99mTc DTPA (5 mCi), and sixty 0.5-s images of the left or right hemisphere were taken. First-pass time-activity curves were obtained by selecting regions of interest for the appropriate brain hemisphere. The results of this study demonstrated that in phenelzine-treated animals there was a 6 to 38% reduction in brain washout slopes of 99mTc DTPA (24.0 +/- 11.5% reduction). These washout slopes were further reduced (range 24.0-86.0; 50.6 +/- 18.5% reduction) following the oral administration of tyramine (1 mg/kg). A significant correlation was noted between changes in washout slopes and the development of coma in these animals.

Technetium 99m pertechnetate measurement of renal blood flow distribution.

The accuracy of measuring anatomical blood flow using the rising slopes of first-pass time-activity curves were studied in six isolated dog kidneys. In each case, the canine kidney was placed on an open-circuit in vitro perfusion apparatus, which included an in-line flow meter and an in-line variable resistance for adjusting the flow rate. Time-activity curves from images acquired at 2-second intervals were generated for each kidney following injection of 4 mCi of 99mTc pertechnetate into the perfusate solution for each flow rate condition studied. The flow rate was calculated from the average slope of the ascending segment, initial rise to peak, of the time-activity curves. Excellent agreement was found between the radioactive tracer and established flow meter measurements (r = 0.995). With the availability of mobile gamma camera/computer systems, this technique could easily be utilized in evaluating perfusion function of transplant kidneys prior to surgery. Also, this technique can be useful in monitoring the renal anatomical distribution of blood flow in postsurgical patients.

Preparation and biodistribution of 99mTc-labeled tyramine iminodiacetic acid.

The synthesis and biodistribution properties of 99mTc-labeled N-substituted tyramine, [N-(4-hydroxyphenethyl)iminodiacetic acid] are described. Tissue distribution studies in rats were indicative of high hepatic and kidney extraction, accompanied by rapid plasma and urinary clearance and minimal biliary excretion. These findings were substantiated by organ image analysis. The preliminary data indicate that this labeled material may represent a new class of radiopharmaceuticals for the evaluation of hepatic and renal functions.

Similarity between tyramine-induced neurotoxicity and the coma of Reye's syndrome.

The objective of the present investigation was to determine whether or not tyramine induces coma in experimental animals with impaired mitochondrial monoamine oxidase function, and whether the coma in these animals was a function of increased cerebrospinal fluid (CSF) pressure. Ten mongrel dogs were treated (orally) daily with the monoamine oxidase-inhibiting drug, phenelzine (4.5 mg/kg), over a period of 1 month. The present studies indicated that in phenelzine-treated animals with liver disease and behavioral side effects (n = 4), the i.v. administration of tyramine (1 mg/kg) caused substantial elevation in CSF pressure that exceeded 30 mm Hg (initial pressure 12.5 +/- 2.1). This was followed by substantial accumulation of tyramine, dopamine and norepinephrine concentrations in CSF of these animals. The animals became comatose soon afterward. The administration of tyramine to pretreated (n = 10) or phenelzine-treated animals without liver disease (n = 6) caused only the expected transient increase in blood pressure but with no significant effect on CSF pressure of these animals. These animals recovered fully from the experiment without any ill effect. These studies suggest that tyramine may have obvious implications in the development of intracranial hypertension in Reye's syndrome.

Hepatorenal failure induced by tetracycline in dogs with portacaval shunt.

The aim of the present investigation was to determine whether tetracycline accelerated the hepatic toxicity of portacaval anastomosis and whether this could be reflected by changes in pharmacokinetics of the drug. Side-to-side portacaval shunts were constructed and converted to end-to-side by ligating the hepatic side of the portal vein in dogs. The results of this study showed that the i.v. infusion of tetracycline (50 mg/kg) to shunted animals caused rapid deterioration in hepatic and renal functions followed by the eventual transition of these animals from stage I to stage II liver disease. This was reflected by a 3- to 6-fold increase in the serum level of bilirubin, alkaline phosphatase, serum glutamic-oxalacetic transaminase, blood urea nitrogen and creatinine as compared with the levels of those produced in serum of dogs before and after the construction of the shunt before the administration of tetracycline. Kinetic analysis revealed a significant prolongation in the elimination half-life (38.2 +/- 4.2 hr) of the shunted dogs as compared with controls (14.5 +/- 1.5 hr) after the i.v. administration of tetracycline. This was accompanied by an appreciable reduction of elimination rate constant. In contrast to shunted animals, control animals exhibited no behavioral side effects after the administration of tetracycline.

Evidence for central hypertyraminemia in hepatic encephalopathy.

In mongrel dogs, the effect of end-to-side portacaval shunt on plasma, cerebrospinal fluid (CSF) and brain tyramine, tyrosine, dopamine, norepinephrine, and epinephrine were studied. It was found that the level of tyramine in plasma, CSF, and selected brain regions increased steadily after the construction of the shunts. These elevations became more pronounced when the dogs manifested symptoms of hepatic encephalopathy. In postshunted dogs with stage II and III hepatic encephalopathy, tyramine concentration in corpus striatum (1,312 +/- 371), hypothalamus (400 +/- 67.0), and midbrain (660 +/- 78.7 ng/g) was significantly (P less than 0.05) higher than the level in dogs with stage 0 and I hepatic encephalopathy and sham-operated dogs serving as controls (corpus striatum, 831 +/- 140; hypothalamus, 167 +/- 40.0; and midbrain, 132 +/- 37.4 ng/g). This was followed by a concomitant depletion of dopamine and norepinephrine in these brain regions (postshunt: dopamine 104 +/- 20.0, 3,697 +/- 977, and 105 +/- 14.1; norepinephrine 521 +/- 71.6, 81.6 +/- 13.7, and 218 +/- 31.7 ng/g; vs. sham group: dopamine 532 +/- 83.1, 8,210 +/- 1,126, and 192 +/- 35.0; norepinephrine 1,338 +/- 425, 124 +/- 21.3, and 449 +/- 89.7 ng/g) of encephalopathic dogs with portacaval shunt. Furthermore, tyramine, tyrosine, dopamine, and norepinephrine levels in plasma and CSF increased markedly as clinical features in the dogs' behavior characteristic of hepatic encephalopathy occurred, including hypersalivation, ataxia, flapping tremor, somnolence, and coma. Cerebral hypertyraminemia and a defect in sympathetic neurotransmission may contribute to the development of hepatic encephalopathy of liver disease.

Decarboxylation to tyramine: an important route of tyrosine metabolism in dogs with experimental hepatic encephalopathy.

Tyrosine metabolism via decarboxylation to tyramine was evaluated in dogs with functional end-to-side portacaval shunt. It was found that the endogenous plasma levels of both tyrosine and tyramine increased steadily after the construction of the shunt. These elevations became more pronounced when the dogs manifested symptoms of hepatic encephalopathy. In encephalopathic dogs, average endogenous plasma tyrosine and tyramine concentrations were 110.1 mumoles per liter and 7.6 ng per ml as compared to 55.4 and 1.2 in control dogs, respectively. The pattern of plasma concentrations of tyrosine and tyramine after an oral dose of L-tyrosine (50 mg per kg) was also investigated in control and shunted dogs. There was a progressive rise in peak levels of tyramine (to about 50-fold increase, at 6 weeks) after the construction of the shunt, as compared to levels obtained in pre- and at 1 and 4 weeks postoperatively (70.6 versus 1.20, 3.9, and 8.11 ng per ml). Similar observations were made with levels of plasma tyrosine. Six weeks after portacaval shunt, mean peak levels of plasma tyrosine, achieved at 5 hr after dose administration, were 450 as compared to 85 mumoles per liter obtained in preshunted dogs. These studies demonstrated a correlation between abnormalities in tyrosine metabolism and postshunt hepatic encephalopathy.

Cyclic GMP-dependent and cyclic AMP-dependent protein kinases, protein kinase modulators and phosphodiesterases in arteries and veins of dogs. Distribution and effects of arteriovenous fistula and arterial occlusion.

Possible involvement of cyclic GMP-dependent and cyclic AMP-dependent protein kinases, protein kinase modulators and cyclic nucleotide phosphodiesterases in functions of vascular tissues were investigated in the dog. All of the above activities, localized in the smooth muscle-rich inner layer of the blood vessels, were found to be higher in the arteries than in the veins. The peripheral arteries were disproportionately richer in cyclic GMP-dependent protein kinase (as indicated by high ratios of cyclic GMP-dependent to cyclic AMP-dependent protein kinase) than were the veins, with the exception of the pulmonary artery, an atypical arterial tissue exposed to low blood pressure. Interestingly, the protein kinase ratio for the aorta, an artery with no significant role in blood pressure regulation, was not higher than that for the vena cava. Creation of femoral arteriovenous fistulae in the dogs led to preferential reductions in the cyclic GMP-dependent enzyme activity both in the proximal and distal arteries, whereas it was elevated in the stressed vein distal to the anastomotic site. The cyclic GMP-dependent enzyme was preferentially reduced in the saphenous artery distal to occlusion. Changes in the cyclic GMP-dependent enzyme activity appeared to precede gross atrophy or hypertrophy of the vessels. It is suggested that the vascular cyclic GMP-dependent protein kinase may be closely related to peripheral resistance and its regulation.

Studies of the fate of tyramine in dogs: the effect of monoamine oxidase inhibition, portafemoral shunt and coronary artery ligation on the kinetics of tyramine.

Radioimmunoassay of tyramine (T) was used to investigate the kinetics of T in plasma of three groups of dogs (control, pretreated with monoamine oxidase inhibitor and those with portafemoral shunt). Furthermore, the influence of coronary artery ligation on the T content of the heart was studied. After i.v. administration of T-HCl (1.7mumol/kg, 0.3 mg/kg), there was a rapid initial decline in T plasma levels with an average T 1/2 of 4.3 minutes. Similar results were obtained in experiments in which the same dose of T-3H was used. There was a 10-fold difference between 3H and T concentrations. Pretreatment with a monoamine oxidase inhibitor resulted in a decrease in T metabolism as reflected by changes in pharmacokinetic parameters (estimated area under the curve AUC, 8166 vs. 1000 ng x min x ml-1, P less than .001; total body clearance, BC, 35.7 vs. 285 ml/min/kg. P less than .005). Similar results were obtained in dogs with portafemoral shunt. Coronary artery ligation resulted in an increase in the level of T in the infarction [1.2. +/- 0.3 (S.E.M.) ng/ml] compared to those of adult volunteers.