DEVELOPMENT OF NEW PET AND SPECT RADIOTRACERS AND NEW APPROACHES TO PET DATA

National Institute On Drug Abuse

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Abstract

The features of novel radiotracers, specifically the 5-iodo, 2- fluoro, 6-fluoro and 5-iodo-6-fluoro analogs of A-85380, developed to image the nicotinic acetylcholine receptors (nAChRs) with positron emission tomography (PET) and single photon emission computed tomography (SPECT) have been further investigated in terms subtype selectivity and toxicity. Detailed kinetic and saturation analyses confirmed that as with 5-[I-125] iodo-A85380, 2-fluoro-A-85380 labels with very high affinity a homogeneous population of binding sites, presumably the alpha4-beta2 subtype of nAChRs in rodent and human brain. Results of competition assays for four different types of receptors demonstrated that the affinities of 5-iodo- and 2-fluoro-A-85380 for alpha4beta2 receptors exceed their affinities for the mammalian alpha7, muscular, and alpha3beta4 subtypes by three to five orders of magnitude. Further, 5-[I-125] iodo-A85380 shows no binding in any brain region in "knockout" mice lacking the beta2 subunit of nAChRs. These findings and in vivo studies in non-human primates demonstrate that [F-18]2-fluoro-A85380 is superior to other radioligands available for selectively imaging alpha4beta2 nAChRs with PET. Toxicology studies demonstrated that at relatively high doses these ligands have convulsant properties and affect the cardiovascular system. Convulsions in mice were manifested at doses approximately 1000 to 10,000 times larger than doses needed to image nAChRs, demonstrating that the A-85380-related compounds have a wider margin of safety than new imaging ligands that are derivatives of the toxin epibatidine. In preliminary cardiovascular studies in unanesthetized rats, 5-iodo-A-85380, 2-fluoro-A-85380 and nicotine produced equivalent, modest transient increases in blood pressure, but only nicotine produced marked decreases in heart rate, suggesting that derivatives of A-85380 are less cardiotoxic than nicotine. [F-18]-2-Fluoro-A-85380 crosses the blood-brain barrier and distributes in the brains of mice and Rhesus monkeys in proportion to known densities of alpha4beta2 nAChRs. Estimating the ratio of specific to non-specific binding in monkey using the cerebellum, which contains few alpha4beta2 nAChRs, showed a high level of specific binding in the thalamus at 4 h after radiotracer administration. The specificity of binding was confirmed by blocking and displacement studies with cytisine indicating that 2-[F-18]-F-A-85380 is an excellent candidate for PET imaging of central nAChRs in human subjects. Another goal has been to improve PET-FDG methodology by using venous instead of arterial blood sampling to calculate CMRglc. There was a good agreement between the time curves of FDG and glucose concentration obtained from arterial and venous blood samples and values of CMRglc calculated using arterial and venous blood in anesthetized non human primates (NHPs). Substitution of arterial blood sampling by venous sampling will simplify the experimental procedure as applied to both humans and NHPs without a significant loss of accuracy and expands the ability to conduct multiple PET-FDG studies on the same subject.