Synthesis and biological evaluation of two novel DAT-binding technetium complexes containing a piperidine based analogue of cocaine

Two new technetium complexes containing a piperidine template have been synthesized and evaluated as possible leads for the development of dopamine transporter (DAT) imaging agents. Binding data for the corresponding rhenium complexes containing either a monoaminomonoamide (MAMA') or a diaminodithiol (DADT) chelating unit exhibited significant affinity for the DAT. Initial biodistribution studies in rats revealed only a low brain uptake.

Altropane, a SPECT or PET imaging probe for dopamine neurons: I. Dopamine transporter binding in primate brain

Increasing evidence suggests that the dopamine transporter is an important marker for physiological and pathological changes in dopamine neurons. Potent dopamine transport inhibitors of the phenyltropane series (e.g., WIN 35,428 or CFT) are particularly suitable for PET (positron emission tomography) or SPECT (single photon emission computed tomography) imaging of the dopamine transporter in living brain. We investigated whether altropane, an N-iodoallyl analog of WIN 35,428 (IACFT:E-N-iodoallyl-2 -carbomethoxy-3beta-(4-fluorophenyl)tropane), displayed in vitro properties suitable for evaluation as a SPECT imaging agent. In brain striatum of cynomolgus monkey (Macaca fascicularis), the unlabeled E-isomer (IC50: 6.62 +/- 0.78 nM) was more potent than the Z-isomer (IC50: 52.6 +/- 0.3 nM) and displayed a relatively high dopamine:serotonin transporter selectivity (28-fold). In radiolabeled form, [125I]altropane bound to sites in the striatum with a single high affinity (KD: 5.33 +/- 0.55 nM) and with a site density (BMAX: 301 pmol/g original wet tissue weight) that was within the density range reported previously for the dopamine transporter in striatum. Drugs inhibited [125I]altropane binding with a rank order of potency that corresponded closely to their potencies for inhibiting [3H]WIN 35,428 binding (r2: 0.99; P < 0.0001) to the blocking dopamine transport. The favorable binding properties of altropane, together with its rapid entry into primate brain and highly localized distribution in dopamine-rich brain regions, suggest it is a suitable iodinated probe for monitoring the dopamine transporter in vitro and in vivo by SPECT or PET imaging.

Altropane, a SPECT or PET imaging probe for dopamine neurons: III. Human dopamine transporter in postmortem normal and Parkinson's diseased brain

Increasing evidence suggests that the dopamine transporter is situated almost exclusively on dopamine neurons. Accordingly, it is an valuable marker for Parkinson's disease and other pathological states of dopamine neurons. We previously demonstrated that the potent dopamine transport inhibitor [125I]altropane (IACFT:E-N-iodoallyl-2beta-carbomethoxy-3beta-(4-fluor ophenyl)tropane) is a high affinity selective probe for the dopamine transporter in monkey brain and an effective SPECT imaging agent in nonhuman primate brain. We now report the binding properties of [125I]altropane in postmortem tissue of normal human brain and compare the findings to Parkinson's diseased brain. In homogenates of human brain putamen, [125I]altropane bound with high affinity (KD: 4.96 +/- 0.38 nM, n = 4) and site density (BMAX: 212 +/- 41.1 pmol/g original wet tissue weight) well within the density range reported previously for the dopamine transporter in this brain region. Drugs inhibited [125I]altropane binding with a rank order of potency that corresponded closely to their rank order for blocking dopamine transport (r 0.98, P < 0.001). In postmortem Parkinson's diseased brain, bound [125I]altropane (1 nM) was markedly reduced (89%, 99% in putamen, depending on measures of nonspecific binding) compared with normal aged-matched controls (normal putamen: 49.2 +/- 8.1 pmol/g; Parkinson's diseased putamen: 0.48 +/- 0.33 pmol/g; n = 4). In vitro autoradiography, conducted in tissue sections at a single plane of the basal ganglia, revealed high levels of [125I]altropane binding the caudate nucleus and putamen, but lower levels (73% of the caudate-putamen) in the nucleus accumbens (n = 7). In Parkinson's diseased brains (n = 4), [125I]altropane binding was 13% of the levels detected in normal putamen, 17% of normal values in the caudate nucleus, and 25% of normal levels in nucleus accumbens. The association of [125I]altropane to the dopamine transporter in human postmortem tissue, the marked reduction of [125I]altropane binding in Parkinson's diseased brains, its rapid entry into brain and highly localized distribution in dopamine-rich brain regions, support its use as a probe for monitoring the dopamine transporter in vitro and in vivo by SPECT imaging.

Tissue blood flow estimation with copper(II)-pyruvaldehyde bis (N-4-methylthiosemicarbazone) and PET

Copper (II)-pyruvaldehyde bis (N-4-methylthiosemicarbazone) (Cu-PTSM) labelled with 62Cu or 64Cu is currently under investigation as a radiotracer for imaging the distribution of blood flow with positron emission tomography (PET). The application of a simple trapped tracer model in conjunction with tissue uptake and continuous arterial sampling to estimate blood flow has been compared with the 57Co-microsphere method in the rat. After intraventricular injection the cumulative arterial function for 64Cu increased progressively due to the presence of circulating non lipophilic complexes. The cumulative function for lipophilic 64Cu-PTSM extracted in n-octanol plateaued at levels corresponding to those reached by 57Co-microspheres. No consistent disagreement was found between cardiac output and blood flow estimated by 64Cu-PTSM and 57Co-microspheres in low to moderate flow tissues: muscle (0.08, 0.07 mL/min/g; 64Cu mean, 57Co mean), brain (0.52, 0.43 mL/min/g) and kidney (2.29, 2.45 mL/min/g). However, 64Cu-PTSM underestimated blood flow measured by 57Co-microspheres in myocardium (4.09, 6.55 mL/min/g). A simple tissue trapping model may therefore be suitable for the derivation of blood flow estimates in low to moderate flow tissues using 62,64Cu-PTSM, PET imaging and continuous arterial sampling with n-octanol extraction.

Evaluation of copper(II)-pyruvaldehyde bis (N-4-methylthiosemicarbazone) for tissue blood flow measurement using a trapped tracer model

Copper(II)-pyruvaldehyde bis (N-4-methylthiosemicarbazone) (Cu-PTSM) labelled with 62,64Cu is a promising radiotracer for the study of blood flow using positron emission tomography (PET). We have investigated the application of a simple trapped tracer model to measurements of tissue 64Cu-PTSM uptake combined with continuous arterial sampling. A dual-tracer method was used to compare blood flow estimated by 64Cu-PTSM with values derived from measurements using cobalt-57 microspheres in the rat. Prolonged retention of 64Cu-PTSM following intravenous administration was initially confirmed in both normal tissues and tumours. After intraventricular 64Cu-PTSM infusion, cumulative arterial 64Cu activity increased progressively, and after extraction in n-octanol was found to plateau to levels corresponding with those reached following administration of 57Co microspheres. Rapid and species-dependent rates of 64Cu-PTSM decomposition to non-extractable 64Cu complexes were found in rat and human blood in vitro (70% +/- 6% and 43 +/- 5% respectively at 16 min), demonstrating the need for immediate processing of arterial samples. Close agreement was found between blood flow estimated by 64Cu-PTSM and 57Co microsphere methods in tissues of low to moderate flow: muscle (0.01, 0.08, 0.07 ml/min per gram; mean difference, mean 64Cu, mean 57Co), brain (0.09, 0.52, 0.43 ml/min per gram) and kidney (-0.16, 2.29, 2.45 ml/min per gram). Estimates of cardiac output also compared favourably between the two methods (5.7, 59.8, 54.1 ml/min). We conclude that a simple tissue trapping model may be suitable for the derivation of blood flow estimates using 62,64Cu-PTSM, PET imaging and continuous arterial blood sampling.

Pharmacokinetics of [18F]fleroxacin in healthy human subjects studied by using positron emission tomography

Positron emission tomography (PET) with [18F]fleroxacin was used to study the pharmacokinetics of fleroxacin, a new broad-spectrum fluoroquinolone, in 12 healthy volunteers (9 men and 3 women). The subjects were infused with a standard therapeutic dose of fleroxacin (400 mg) supplemented with approximately 20 mCi of [18F]fleroxacin. Serial PET images were made and blood samples were collected for 8 h, starting at the initiation of the infusion. The subjects were then treated with unlabeled drug for 3 days (400 mg/day). On the fifth day, infusion of radiolabeled drug, PET imaging, and blood collection were repeated. In most organs, there was rapid accumulation of radiolabeled drug, with stable levels achieved within 1 h after completion of the infusion. Especially high peak concentrations (in micrograms per gram) were achieved in the kidney (> 34), liver (> 25), lung (> 20), myocardium (> 19), and spleen (> 18). Peak concentrations of drug more than two times the MIC for 90% of Enterobacteriaceae strains tested (> 10-fold for most organisms) were achieved in all tissues except the brain and remained above this level for more than 6 to 8 h. The plateau concentrations in tissues (2 to 8 h, in micrograms per gram +/- standard error of the mean) of drug were as follows: brain, 0.83 +/- 0.032; myocardium, 4.53 +/- 0.24; lung, 5.80 +/- 0.48; liver, 7.31 +/- 0.33; spleen, 6.00 +/- 0.47; bowel, 3.53 +/- 0.74; kidney, 8.85 +/- 0.64; bone, 2.87 +/- 0.29; muscle, 4.60 +/- 0.33; prostate, 4.65 +/- 0.48; uterus, 3.87 +/- 0.39; breast, 2.68 +/- 0.11; and blood, 2.35 +/- 0.09. Concentrations of fleroxacin in tissue were similar in males and females, before and after pretreatment with unlabeled drug.

Pharmacokinetics of fleroxacin as studied by positron emission tomography and [18F]fleroxacin

A new method of tracing the disposition of fleroxacin was tested in infected and noninfected animals in an effort to develop a technique that might be applicable in humans. [18F]fleroxacin was synthesized and shown to be identical physically, chemically, and in its antimicrobial activity to the commercially produced product. Tracer amounts of [18F]fleroxacin were coinjected with a pharmacologic dose of unlabeled drug (10 mg/kg) into normal mice, rats with focal thigh infection due to Escherichia coli, and normal and infected rabbits. The rats and mice were killed at fixed time intervals after injection, and the concentration of drug was determined by radioactive counting in a well-type counter; the rabbits were studied both by this method and by positron emission tomographic (PET) imaging. These studies validated the reliability of the new approach and suggested that it could be applied safely to humans. In all three animal species studied, delivery of [18F]fleroxacin to most tissues was rapid, with the notable exception of the brain. Accumulation of drug in infected thigh muscle was similar to that in normal muscle. The concentrations of drug reached in various tissues suggest that fleroxacin will be particularly useful in the treatment of gastrointestinal, urinary tract, hepatobiliary, and skeletal infections and that it shows promise for the treatment of lung and soft tissue infection. The minimal concentrations of drug delivered to the brain should decrease the occurrence of central nervous system toxicity with this particular fluoroquinolone.

Synthesis and biodistribution of 18F-labeled fleroxacin

[18F]Fleroxacin (6,8-difluoro-1,4-dihydro-1-(2-[18F]fluoroethyl)-4- oxo-7-(4-methyl-1-piperazinyl)-3-quinolinecarboxylic acid) was synthesized from its methylsulfonyl ester precursor. 6,7,8-Trifluoro-4-hydroxyquinoline-3-carboxylic acid ethyl ester (Ro 19-7423) was alkylated with 2-bromoethanol to produce 6,7,8-trifluoro-1,4-dihydro-1-(2-hydroxyethyl)-4-oxo-3-quinolinecarboxyl ic acid ethyl ester in 76% yield which was then condensed with 1-methyl-piperazine to produce 6,8-difluoro-1,4-dihydro-1-(2-hydroxyethyl)-7-(4-methyl-1-piperazinyl)4- oxo-3- quinolinecarboxylic acid ethyl ester in 67% yield. This product was reacted with methanesulfonyl chloride to produce the mesylate precursor of fleroxacin in 66% yield. Nucleophilic substitution of the mesylate with 18F- in the presence of Kryptofix 2.2.2 followed by basic hydrolysis produced [18F]fleroxacin with a radiochemical yield of 5-8% [EOS] within 90 min. The pattern of biodistribution of [18F]fleroxacin was similar to the 14C-labeled drug.

Circulating levels of epithelial membrane antigen in patients with breast or colorectal-cancer

Epithelial membrane antigen (EMA) is expressed by adenocarcinomas of the breast, ovary and colon and has been suggested as a circulating tumour marker. Serum EMA levels were measured in 126 patients, 31 with colorectal cancer, 52 with breast cancer and 43 age matched controls using a competitive binding radioimmunoassay and the rat anti-EMA monoclonal antibody (MAb) ICR2. The EMA levels in the control group varied widely from 90-3240ng/ml with a median value of 570ng/ml. This was not significantly different from the levels in patients with colorectal cancer (60-8530, median 580ng/ml) or those with breast cancer (210-13300, median 655ng/ml). However, the highest EMA levels (>5000ng/ml) were found in patients with cancer. The wide range of EMA levels in the control group prohibit its use for screening.

Pharmacokinetics of 18F-labeled fleroxacin in rabbits with Escherichia coli infections, studied with positron emission tomography

18F-labeled fleroxacin was used to measure the pharmacokinetics of fleroxacin in healthy and infected animals by positron emission tomography (PET) and tissue radioactivity measurements. In all experiments, a pharmacological dose of unlabeled drug (10 mg/kg) was coinjected with the tracer. The pharmacokinetics of [18F]fleroxacin was measured in groups of healthy mice (n = six per group) at 10, 30, 60, and 120 min after injection and in groups of rats with Escherichia coli thigh infections (n = six per group) at 60 and 120 min after injection by radioactivity measurements in excised tissues. In healthy rabbits (n = 4) and in rabbits with E. coli thigh infections (n = 4), tissue concentrations of drug were determined by serial PET imaging over 2 h; after the final image was acquired, animals were sacrificed and concentrations measured by PET were compared with the results of tissue radioactivity measurements. In all three species, there was rapid equilibration of [18F]fleroxacin to significant concentrations in most peripheral organs; low concentrations of drug were detected in the brain. Accumulations of radiolabeled drug in infected and healthy thigh muscles were similar. Peak concentrations of drug of more than three times the MIC for 90% of members of the family Enterobacteriaceae (greater than 100-fold for most organisms) were achieved in all tissues except brain and remained above this level for more than 2 h. Especially high peak concentrations were achieved in the kidney (greater than 75 micrograms/g), liver (greater than 50 micrograms/g), blood (greater than 25 micrograms/g), and bone and lung (greater than 10 micrograms/g). Since the MICs for 90% of all Enterobacteriaceae are <2 micrograms/ml, fleroxacin should be particularly useful in treating gram-negative infections affecting these tissues. In contrast, the low concentration of drug delivered to the brain should limit the toxicity of the drug for the central nervous system.

Preoperative imaging of colorectal cancers. Targeting the epithelial membrane antigen with a radiation-labeled monoclonal antibody

The epithelial membrane antigen (EMA) is expressed by the majority of colorectal cancers but has not previously been investigated as a target for radiation-labeled monoclonal antibodies (MoAb) in the imaging of patients with colorectal cancer. A rat IgG2a MoAb that recognizes EMA, ICR2, was labeled with Indium-111 (100 megabecquerel per milligram [MBq/mg]MoAb) using the bicyclic anhydride of the chelating agent diethylene triamine pentacetic acid (ccDTPA) and was administered intravenously to 22 patients known to have or thought to have colorectal cancer. Daily gamma camera imaging was performed for 3 days during the time between the administration of the radiation-labeled antibody and surgical procedure. At operation, the biopsies were done of the tumors and the normal colon, and the uptake of radiation-labeled MoAb was measured in a gamma well-counter. Immunocytochemistry for EMA expression also was done on resected tumors. Independent unblinded and blinded reporting was done on all scans. The sensitivity of 111In-ICR2 for detecting cancers preoperatively was 80% and 60%, respectively, on unblinded and blinded reporting, and the corresponding specificity 20% and 60%. The low unblinded specificity was attributable to a false-positive localization in severely dysplastic benign tumors (n = 2) and inflammatory tissue (n = 2). Liver metastases present in three patients were cold relative to normal liver. Lymph node metastases were localized in 1 of 6 patients preoperatively. The mean absolute uptake of 111In-ICR2 in tumor tissue was 7.75 +/- 3.77 x 10(-3) percent of injected dose per gram, and the ratio to normal colon was 2.10 +/- 0.92:1. On immunohistochemistry, EMA was expressed by 16 of the 17 primary cancers, both dysplastic adenomas, and all nodal metastatic deposits. EMA-negative tumors (1 cancer + 1 colonic lipoma) had negative antibody scans, and patients whose tumor was negative or only focally positive for EMA expression had lower tumor/normal colon ratios of radioactivity (1.30 +/- 0.26 versus 2.45 +/- 0.65, P = 0.005) on gamma well-counting of excised specimens. These results suggest a possible role for 111In-ICR2 in the detection of colorectal cancer and metastases but not its liver deposits.

The effect of circulating antigen and radiolabel stability on the biodistribution of an indium labelled antibody

This study has investigated two of the main problems with radiolabelled antibody imaging, the formation of circulating immune complexes (I.C.) and the non specific binding of radiolabel to the antibody molecule. Patients undergoing immunoscintigraphy with 111In labelled monoclonal antibody ICR2 were divided into three groups who received either the radiolabelled antibody alone (control, n = 12), the radiolabelled antibody which was incubated with the chelating agent diethylene triamine pentacetic acid (DTPA) prior to size exclusion chromatography (n = 6) or whose injectate was treated with DTPA and cold MAb administered intravenously prior to radiolabelled MAb administration (n = 6). Radiolabelled antibody uptake in abdominal organs was measured by region of interest analysis using a gamma camera with online computer and that in tumour and normal tissues by gamma well counting of biopsies. Circulating antigen and immune complex was measured by high pressure liquid chromatography (HPLC). The sensitivity of tumour imaging and the tumour uptake of radiolabelled antibody was not significantly different between the groups. Patients with high circulating antigen levels developed high levels of circulating immune complex but also had high tumour uptakes of radiolabelled antibody. Administration of cold MAb increased the splenic, but did not effect the tumour uptake of radiolabelled antibody and only minimally reduced levels of circulating immune complex. Chelate administration reduced the urinary excretion of radioactivity but increased the liver uptake of radioactivity. These results have shown that successful antibody imaging can be carried out despite high levels of circulating antigen, that large doses of unlabelled antibody are required to prevent immune complex formation and that removal of non specifically bound 111In does not reduce the liver uptake of radioactivity.

The biodistribution and pharmacokinetics of meta-iodobenzylguanidine in childhood neuroblastoma

MIBG is generating considerable interest for the treatment of neuroblastoma. This study has investigated the biological variation in handling of the compound in children with neuroblastoma. The biodistribution of the compound has been characterised in children undergoing tracer administrations of 123I and 131I-mIBG. Estimates of hepatic and whole body radiation dose delivery have been made. The results indicate substantial interpatient variation in hepatic dose delivery. This organ may be critical in some patients undergoing targeted radiotherapy with mIBG.

Quantitative variation in la antigen expression plays a central role in immune regulation

The analysis of la antigen function has focused primarily on allelic variants of Ia molecules. In this review Charles Janeway and his colleagues discuss evidence that quantitative rather than qualitative variation in Ia antigen expression had a major role in immunoregulation and immunologically mediated disease.

Monoclonal antibodies specific for Ia glycoproteins raised by immunization with activated T cells: possible role of T cellbound Ia antigens as targets of immunoregulatory T cells

Two monoclonal antibodies, Y-3P and Y-8P, specific for conventional mouse Ia glycoprotein antigens are described. Both were raised by repeated immunization of primed mice with activated T cells over a period of 1 yr, and both detect a new and broad public Ia specificity. Both of the antibodies react with I-A subregion-controlled A alpha:A beta complexes of all mouse strains apart from the responder strain (H-2d), as well as the equivalent of A alpha:A beta complexes in rats carrying seven independent haplotypes. These antibodies have great utility as almost universal Ia reagents. On the basis of these results, we propose that Ia antigens presented to the immune system bound to activated T cells are immunogenic, and may induce the production of Ia antibodies of novel and broad specificity. In addition, we propose that such bound Ia glycoproteins could be a target for immunoregulatory T cells, and could account for the specificity of suppression of graft-vs-host reactions and Ia-restricted helper T cells observed by others in F1 animals injected with parental T cells.

Monoclonal antibodies specific for Ia glycoproteins raised by immunization with activated T cells: possible role of T cellbound Ia antigens as targets of immunoregulatory T cells

Two monoclonal antibodies, Y-3P and Y-8P, specific for conventional mouse Ia glycoprotein antigens are described. Both were raised by repeated immunization of primed mice with activated T cells over a period of 1 yr, and both detect a new and broad public Ia specificity. Both of the antibodies react with I-A subregion-controlled A alpha:A beta complexes of all mouse strains apart from the responder strain (H-2d), as well as the equivalent of A alpha:A beta complexes in rats carrying seven independent haplotypes. These antibodies have great utility as almost universal Ia reagents. On the basis of these results, we propose that Ia antigens presented to the immune system bound to activated T cells are immunogenic, and may induce the production of Ia antibodies of novel and broad specificity. In addition, we propose that such bound Ia glycoproteins could be a target for immunoregulatory T cells, and could account for the specificity of suppression of graft-vs-host reactions and Ia-restricted helper T cells observed by others in F1 animals injected with parental T cells.