1.1 Dopamine transporter radiotracers

1 Ligand Development

1.1 Dopamine transporter radiotracers

A radiotracer for the brain dopamine transporter (DAT) was required to study a rodent model of Parkinson’s disease (PD). [¹⁸F]FECNT (2) appeared attractive because of its high binding affinity, selectivity and favorable time-course in attaining specificbinding equilibrium.^1,2 However, our Branch observed³ a timedependent increase in the distribution volume (V) due to accumulation of radiometabolite in rat brain. A dual kinetic model^4,5 was shown to be suitable to deal with this situation.⁶ By application of radio-HPLC in conjunction with LC-MS-MS, we established⁷ that an inactive [¹⁸F]fluoroalkyl metabolite of [¹⁸F]FECNT, arising from Phase 1 dealkylation, originates in the periphery and confounds the measurement of DAT with a reference tissue model. Consistent with the rodent data, the apparent V in cerebellum of both human and nonhuman primates continuously increased after injection, again probably due to brain entry of the radiometabolite. Thus, reference tissue modeling of [¹⁸F]FECNT is prone to much greater error than a more practically demanding analysis requiring a measured arterial input function. For this reason, we chose to consider other reportedly promising DAT radiotracers.

[¹¹C]PE2I⁸ (3) has been used to study DAT in non-human primates and human subjects, and gives high target to non-target radioactivity ratios. Baseline PET experiments in rats revealed the expected high ratio (~ 6) of radioactivity in striatum compared to that in cerebellum. However, our initial analysis indicated heterogeneity in the radioactive species being measured in brain.⁹ Two less polar radioactive metabolites were detected, one formed by hydroxylation of the aryl methyl group (metabolite A) and the other by oxidation of this metabolite to the carboxylic acid (metabolite B), as shown by our combined application of radio-LC and LC-MS-MS to rodent brain extracts. Metabolite A accumulated selectively in striatum, so indicating it is bioactive, probably with high affinity to DAT, whereas metabolite B is bio-inactive.¹⁰ The metabolic fate of [¹¹C]PE2I appears similar in primates. Hence, [¹¹C]PE2I was concluded to be unsuitable for our envisaged rodent application, again due to radioactive metabolites, active and inactive, accumulating in brain.

Further radiotracers (e.g. [¹¹C]CFT¹¹, 4; [¹¹C]altropane¹², 5; [¹¹C]JM10, 21) were also tested to see if they produced radioactive metabolites in rodent brain. Only [¹¹C]CFT proved devoid of this problem; > 99% of the radioactivity in brain was parent radiotracer at 30 min after administration to mice. Hence, on the basis of this encouraging data, [¹¹C]CFT was introduced for PET studies in rodent models of PD.