The primary goal of this project has been to develop radiotracers for the noninvasive assessment of cardiac sympathetic innervation. To this end, our laboratory has developed several successful radiotracers for imaging cardiac sympathetic neurons, including radioiodinated meta iodobenzylguanidine (MIBG), 11C]meta-hydroxyephedrine (HED) and [11C]epinephrine (EPI). All of these tracers are avidly taken up into cardiac sympathetic nerves by the neuronal norepinephrine transporter (NET) and stored in vesicles by the vesicular monoamine transporter (VMAT2). However, while the very rapid neuronal uptake of these agents provides high quality images, it also makes tracer kinetic analyses problematic. The major goal of this proposal is to develop kinetically superior, more information-rich tracers that possess optimal kinetic properties for quantitative analyses. Such tracers would provide more sensitive and accurate measures of nerve density than are currently possible. This would allow detection of early denervation in patients with diseases such as diabetic autonomic neuropathy and heart failure, before nerve losses become severe. Early detection of nerve losses may be critically important in providing effective therapies to halt or reverse cardiac denervation. We hypothesize that a radiolabeled NET substrate must possess 2 kinetic properties to be 'ideal' for tracer kinetic analyses: (1) a slow neuronal uptake rate, and (2) a very long neuronal retention time, through rapid and efficient vesicular storage. We further hypothesize that a tracer with these properties can be found among the many guanidines known to exert potent pharmacological effects on sympathetic neurons. A radiosynthetic method for incorporating carbon-11 into guanidines will be used to synthesize and evaluate 2 series of [11C]guanidines as sympathetic nerve tracers with improved kinetics. Radiolabeling with a N-[11C]guanyl moiety instead of a N-[11C]methyl group offers several advantages, including higher polarity, resistance to metabolism, and stability in blood. Series I is comprised of [11C]phenethylguanidines. Pilot studies show that several [11C]phenethylguanidines possess the desired slow neuronal uptake rate and long neuronal retention time. In an effort to develop an [18F] guanidine with ideal kinetics, Series II consists of ring-fluorinated phenethylguanidines. These will be evaluated with HC-labeling before 18F-labeling the best agents. Bioevaluation of [11C]guanidines will start with kinetic studies in the isolated rat heart and cellular studies of NET and VMAT2 transport kinetics. The biodistribution of promising compounds will be measured in rats. Finally, microPET imaging in monkeys will be performed with the best tracers. This systematic study of [11C]guanidines should result in the development of a tracer with optimal kinetics for quantifying cardiac sympathetic nerve density with PET.
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