Adenosine 2A Receptor Occupancy by Tozadenant and Preladenant in Rhesus Monkeys

Abstract

Motor symptoms in Parkinson disease (PD) are caused by a loss of dopamine input from the substantia nigra to the striatum. Blockade of adenosine 2A (A 2A ) receptors facilitates dopamine D 2 receptor function. In phase 2 clinical trials, A 2A antagonists (istradefylline, preladenant, and tozadenant) improved motor function in PD. We developed a new A 2A PET radiotracer, 18 F-MNI-444, and used it to investigate the relationship between plasma levels and A 2A occupancy by preladenant and tozadenant in nonhuman primates (NHP). Methods: A series of 20 PET experiments was conducted in 5 adult rhesus macaques. PET data were analyzed with both plasma-input (Logan graphical analysis) and reference-region-based (simplified reference tissue model and noninvasive Logan graphical analysis) methods. Whole-body PET images were acquired for radiation dosimetry estimates. Human pharmacokinetic parameters for tozadenant and preladenant were used to predict A 2A occupancy in humans, based on median effective concentration (EC 50 ) values estimated from the NHP PET measurements. Results: 18 F-MNI-444 regional uptake was consistent with A 2A receptor distribution in the brain. Selectivity was demonstrated by dose-dependent blocking by tozadenant and preladenant. The specific-to-nonspecific ratio was superior to that of other A 2A PET radiotracers. Pharmacokinetic modeling predicted that tozadenant and preladenant may have different profiles of A 2A receptor occupancy in humans. Conclusion: 18 F-MNI-444 appears to be a better PET radiotracer for A 2A imaging than currently available radiotracers. Assuming that EC 50 in humans is similar to that in NHP, it appears that tozadenant will provide a more sustained A 2A receptor occupancy than preladenant in humans at clinically tested doses. Par kinson disease (PD) has a prevalence of 1.6% in individuals over the age of 65 y (1) and a lifetime risk of 6.7% from age 45 to 100 y (2). Motor symptoms, which include tremor, bradykinesia, and rigidity, emerge when there is a loss of more than 50% of dopamine neurons in the substantia nigra (SN) (3,4). Loss of these neurons reduces dopamine input to the striatum, where dopamine binds to both D 1 and D 2 receptors. Most striatal D 1 receptors are localized in the so-called direct pathway, whereas most striatal D 2 receptors are localized in medium spiny neurons that project to the globus pallidus pars externa (indirect pathway). Adenosine signals via 4 different G-protein-coupled receptors: A 1 , A 2A , A 2B , and A 3 (5). A 2A receptors are predominantly expressed in striatum, with lower levels present in cortex and thalamus and even lower in cerebellum (5-9). A 2A receptors may play a role in inflammation (10) and could therefore be important in a variety of neurologic diseases, including multiple sclerosis, in which A 2A receptor density is increased (11). In PD, A 2A receptors may be important because they form heterodimers with D 2 receptors in the striatum (5,12), and agonists of A 2A (e.g., adenosine) reduce the affinity of D 2 for dopamin...

Characterization in Humans of 18F-MNI-444, a PET Radiotracer for Brain Adenosine 2A Receptors

“…18 F-MNI-444 distribution in the brain was consistent with known A 2A densities from autoradiography reports (26,27) and with our prior observations in the nonhuman primate brain (21). The distribution volumes estimated with the Logan graphical analysis showed a good correlation with the 1TCM and 2TCM estimates, and given the fitting issues of 1TCM in lowuptake regions and the convergence issues of 2TCM in high-uptake regions, the graphical analysis was taken as the preferred method The target-to-nontarget ratios and BP ND s measured in this study were expressively higher than those reported by other researchers using 11 C-SCH442416 and [7-methyl-11 C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthine ( 11 C-TMSX), the other 2 PET radiotracers currently in human use.…”

“…Previously, we have observed that this region was suitable for quantification of 123 I-MNI-420 SPECT images. High doses of caffeine or the selective A 2A antagonist preladenant did not reduce 123 I-MNI-420 cerebellar uptake in monkeys (16), and this was confirmed with 18 F-MNI-444 against preladenant or tozadenant, another selective A 2A antagonist (21).…”

“…In addition, BP ND was also estimated using the noninvasive LGA (NI-LGA) (25) with t* 5 20 min and k 2 9 5 0.15 min 21 , as well as from the SUVr (SUV ratio to the reference region) between 60 and 90 min after radiotracer injection as BP ND 5 SUVr-1. The cerebellum, shown to have low to negligible A 2A density in autoradiography experiments (26,27) and previously used in studies with A 2A radiotracers (13,14,21), was used as the reference region for data quantification. Finally, the test-retest reproducibility for V T and BP ND was estimated as ABS(test 2 retest)/AVERAGE(test 1 retest) (where ABS is absolute value).…”

“…The radiolabeling and preparation of 18 F-MNI-444 were described in detail previously (21). In brief, 18 F-MNI-444 was prepared by reaction of the corresponding tosyl precursor, 2-(4-(4-(2-(5-amino-2-(furan-2-yl)-7H-pyrazolo [4,3- Quality control included visual inspection of appearance, pH, strength, pyrogen content, residual solvent, residual Kryptofix 222, radiochemical purity and identity, and specific activity (21).…”

“…In brief, 18 F-MNI-444 was prepared by reaction of the corresponding tosyl precursor, 2-(4-(4-(2-(5-amino-2-(furan-2-yl)-7H-pyrazolo [4,3- Quality control included visual inspection of appearance, pH, strength, pyrogen content, residual solvent, residual Kryptofix 222, radiochemical purity and identity, and specific activity (21). Identity and radiochemical purity were determined by high-performance liquid chromatography using a Phenomenex Kinetex-XB C18 (4.6 · 100 mm) column, eluted with methanol (solvent A) and 0.8% triethylamine in water (solvent B) with a linear gradient going from 50% A to 90% A in 12 min, maintained at 90% A until 15 min and returned to the initial condition at 15.1 min until the end of run at 20 min.…”