The protein hormone leptin acts to regulate body fat and energy expenditure. Resistance to this hormone is implicated in human obesity and its pathophysiological consequences. In order to gain insight into the mechanism of leptin resistance, an (18)F-labeled derivative was developed to study the biodistribution of the hormone using positron emission tomography (PET). A two-step, site specific ligation approach was developed for this purpose, in which an aminooxy-reactive group was incorporated at the C-terminus of leptin using expressed protein ligation (EPL), which was subsequently derivatized with [ (18)F]fluorobenzaldehyde using an aniline-accelerated radiochemical oximation reaction. The modified hormone was shown to be biologically active in vitro and in vivo, and it was applied to PET imaging in ob/ ob mice. These protocols will allow for the routine production of site-specifically (18)F radiolabeled leptin, as well as other proteins, for use in PET imaging in systems from mouse to man.

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Cocaine addiction is a major problem for which there is no approved pharmacotherapy. We have developed a vaccine to cocaine (dAd5GNE), based on the cocaine analog GNE linked to the capsid proteins of a serotype 5 adenovirus, designed to evoke anti-cocaine antibodies that sequester cocaine in the blood, preventing access to the CNS. To assess the efficacy of dAd5GNE in a large animal model, positron emission tomography (PET) and the radiotracer [11 C]PE2I were used to measure cocaine occupancy of the dopamine transporter (DAT) in nonhuman primates. Repeat administration of dAd5GNE induced high anti-cocaine titers. Before vaccination, cocaine displaced PE2I from DAT in the caudate and putamen, resulting in 62±4% cocaine occupancy. In contrast, dAd5GNE-vaccinated animals showed reduced cocaine occupancy such that when anti-cocaine titers were 44 Â 10 5 , the cocaine occupancy was reduced to levels of o20%, significantly below the 47% threshold required to evoke the subjective 'high' reported in humans.

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Summary Here we report the first PET imaging studies of leptin's systemic biodistribution in vivo. PET imaging using 18F and 68Ga labeled leptin revealed that in mouse the hormone was rapidly taken up by megalin (gp330/LRP2), a multiligand endocytic receptor localized in renal tubules. In addition, in rhesus monkeys 15% of labeled leptin localized to red bone marrow which was consistent with hormone uptake in rodent tissues. These data confirm a megalin-dependent mechanism for renal uptake in vivo. The significant binding to immune cells and blood cell precursors in bone marrow is also consistent with prior evidence showing that leptin modulates immune function. These experiments set the stage for similar studies in humans to assess the extent to which alterations of leptin's biodistribution might contribute to obesity and also provide a novel and general chemical strategy for 18F labeling of proteins for PET imaging of other polypeptide hormones.

A reduction in PK11195 uptake was observed in both enhancing and chronic lesions after the start of natalizumab. PK11195 PET can be used as tool to assess the longitudinal change in MS lesions.

The neurobiology of anorexia nervosa remains incompletely understood. Here we utilized PET imaging with the radiotracer [11C]raclopride to measure striatal dopamine type 2 (D2) receptor availability in patients with anorexia nervosa. 25 women with anorexia nervosa who were receiving treatment in an inpatient program participated, as well as 25 control subjects. Patients were scanned up to two times with the PET tracer [11C]raclopride: once while underweight, and once upon weight restoration. Control subjects underwent one PET scan. In the primary analyses, there were no significant differences between underweight patients (n=21) and control subjects (n=25) in striatal D2 receptor binding potential. Analysis of subregions (sensorimotor striatum, associative striatum, limbic striatum) did not reveal differences between groups. In patients completing both scans (n=15), there were no detectable changes in striatal D2 receptor binding potential after weight restoration. In this sample, there were no differences in striatal D2 receptor binding potential between patients with anorexia nervosa and control subjects. Weight restoration was not associated with a change in striatal D2 receptor binding. These findings suggest that disturbances in reward processing in this disorder are not attributable to abnormal D2 receptor characteristics, and that other reward-related neural targets may be of greater relevance.

Determination of the immunoreactive fraction (IF) of radiolabeled monoclonal antibodies (MAb) is essential to the understanding of the effects of radiolabeling and subsequent target-specific tumor localization. There has been generally no accepted method of determining the IF of MAbs. The conventional method is based on a radioimmunoassay technique in which the fraction of radiolabeled MAb bound to antigen under conditions of "antigen excess" is determined. Lindmo et al. introduced a modified method in which the IF is determined by extrapolation to conditions representing "infinite antigen excess." Although the Lindmo method, in principle, is insensitive to experimental parameters, it does not always provide a reliable estimate of IF. We, therefore, evaluated an alternate method in which percent cell bound fraction is measured under conditions of fixed antigen concentration and various dilutions of radiolabeled MAb. We developed a mathematical equation to estimate immunoreactivity. J591 MAb specific for prostate-specific membrane antigen was radiolabeled with (111)In, (90)Y and (177)Lu to specific activities of 1-20 mCi/mg. We compared the effect of several experimental conditions on the determination of IF using all three different methods. The Lindmo method requires careful optimization of experimental conditions for each radiolabeled MAb. The alternate method, based on a fixed antigen concentration, appears to be practical and may provide a more reliable measure of immunoreactivity.