[18F]MK-9470, a positron emission tomography (PET) tracer for in vivo human PET brain imaging of the cannabinoid-1 receptor

Abstract

[(18)F]MK-9470 is a selective, high-affinity, inverse agonist (human IC(50), 0.7 nM) for the cannabinoid CB1 receptor (CB1R) that has been developed for use in human brain imaging. Autoradiographic studies in rhesus monkey brain showed that [(18)F]MK-9470 binding is aligned with the reported distribution of CB1 receptors with high specific binding in the cerebral cortex, cerebellum, caudate/putamen, globus pallidus, substantia nigra, and hippocampus. Positron emission tomography (PET) imaging studies in rhesus monkeys showed high brain uptake and a distribution pattern generally consistent with that seen in the autoradiographic studies. Uptake was blocked by pretreatment with a potent CB1 inverse agonist, MK-0364. The ratio of total to nonspecific binding in putamen was 4-5:1, indicative of a strong specific signal that was confirmed to be reversible via displacement studies with MK-0364. Baseline PET imaging studies in human research subject demonstrated behavior of [(18)F]MK-9470 very similar to that seen in monkeys, with very good test-retest variability (7%). Proof of concept studies in healthy young male human subjects showed that MK-0364, given orally, produced a dose-related reduction in [(18)F]MK-9470 binding reflecting CB1R receptor occupancy by the drug. Thus, [(18)F]MK-9470 has the potential to be a valuable, noninvasive research tool for the in vivo study of CB1R biology and pharmacology in a variety of neuropsychiatric disorders in humans. In addition, it allows demonstration of target engagement and noninvasive dose-occupancy studies to aid in dose selection for clinical trials of CB1R inverse agonists.

Fig. 1.

Chemical structure of CB1R-selective PET tracer, [¹⁸F]MK-9470; MK-0364, a structurally analogous CB1R inverse agonist used in the occupancy drug studies in this paper; and AM251, a structurally unique CB1R inverse agonist used as the blocking compound in the autoradiographic studies.

Fig. 2.

Autoradiographic distribution of bound [¹⁸F]MK-9470 in the rhesus brain. Coronal sections (20 μm) of rhesus brain were incubated with [¹⁸F]MK-9470. Specific binding was assessed by incubating replicate sections with (lower two rows) and without (upper two rows) 1 × 10⁻⁵ M AM251, a structurally unique CB1R inverse agonist. The sections were exposed to phosphorimaging plates for 20 min, scanned, and analyzed with MCID software. Amg, amygdala; Cb, cerebellum; cc, corpus callosum; Cd, caudate; cgmb, central gray substance midbrain; Gpe, external globus pallidus; Gpi, internal globus pallidus; Hi, hippocampus; lv, lateral ventricle; MnR, median raphe nucleus; PH, posterior hypothalamus; pphg, posterior hippocampal gyrus; Pu, putamen; pvga, periventricular gray area; Rt, reticular nucleus; SN, substantia nigra; Th, thalamic nuclei; VTA, ventral tegmental area; 3V, third ventricle.

Fig. 3.

In vivo PET images of [¹⁸F]MK-9470 in rhesus monkey brain. (A) Baseline images (section through the basal ganglia and cerebellum, 60–120 min after tracer injection) demonstrate high tracer uptake in the brain with a regional distribution consistent with the known in vitro CB1R distribution. (B) [¹⁸F]MK-9470 PET images acquired after MK-0364 dosing via bolus plus constant infusion to steady-state plasma levels sufficient to provide a high level of blockade of CB1R. Color scale indicates SUV units. (C) TACs with open symbols showing tracer binding in baseline conditions (0–120 min). At 120 min into the baseline scan, a chase MK-0364 dose was administered via bolus plus constant infusion, resulting in rapid displacement of the tracer from the CB1R, confirming reversible binding. ○, putamen; □, occipital cortex; ◇, cerebellum; ▿, thalamus; and ×, white matter. Data are corrected for the physical decay of ¹⁸F.

Fig. 4.

Relationship between MK-0364 plasma concentration and in vivo CB1R occupancy measured by using [¹⁸F]MK-9470 in rhesus monkey brain. PET studies were performed after a single dose (▿) and multiple doses (○) of MK-0364. When the data were fitted by using a Hill equation (dashed line, n_H = 1 = slope of the Hill plot), the projected plasma concentration producing 50% receptor occupancy (Occ₅₀) was estimated to be 34 nM.

Fig. 5.

Uptake of [¹⁸F]MK-9470 in human brain. (A) Baseline images (120–180 min after tracer injection, day 1 of subject 1) show the regional distribution of tracer binding was consistent with labeling CB1R and with rhesus monkey images. Color scale corresponds to SUV units. (B) Tissue TACs (symbols) and [¹⁸F]MK-9470 plasma curve (solid line). ○, putamen; □, occipital cortex; ◇, cerebellum; ▿, thalamus; and ×, white matter. Data are corrected for the physical decay of ¹⁸F. (C) Average [¹⁸F]MK-9470 fraction in arterial plasma (n = 8), as measured in the four subjects participating in the test–retest study. Error bars represent SD.

Fig. 6.

PET images of CB1R were acquired in normal healthy male subjects before and after treatment with MK-3640 or placebo. (A) Representative transverse parametric SUV images at the level of the basal ganglia for two human subjects, acquired before (Left) and 24 h after (Right) treatment with either placebo (Upper) or a 7.5 mg dose of MK-0364 (Lower). (B) Graphs show the corresponding TACs: ○, putamen; □, occipital cortex; ◇, cerebellum; and ▿, thalamus. The open symbols denote baseline values, and the closed symbols denote the scan after treatment.

Fig. 7.

Relationship between MK-0364 dose and CB1R occupancy using [¹⁸F]MK-9470 in human brain. Baseline PET scans were performed before drug administration. Subjects received a daily dose of MK-0364 or placebo for 14 days and were scanned ≈24 h after the last dose. Error bars represent SD.