PET imaging of brain aromatase in humans and rhesus monkeys by 11C-labeled cetrozole analogs

Abstract

Aromatase is an estrogen synthetic enzyme that plays important roles in brain functions. To quantify aromatase expression in the brain by positron emission tomography (PET), we had previously developed [¹¹C]cetrozole, which showed high specificity and affinity. To develop more efficient PET tracer(s) for aromatase imaging, we synthesized three analogs of cetrozole. We synthesized meta-cetrozole, nitro-cetrozole, and iso-cetrozole, and prepared the corresponding ¹¹C-labeled tracers. The inhibitory activities of these three analogs toward aromatase were evaluated using marmoset placenta, and PET imaging of brain aromatase was performed using the ¹¹C-labeled tracers in monkeys. The most promising analog in the monkey study, iso-cetrozole, was evaluated in the human PET study. The highest to lowest inhibitory activity of the analogs toward aromatase in the microsomal fraction from marmoset placenta was in the following order: iso-cetrozole, nitro-cetrozole, cetrozole, and meta-cetrozole. This order showed good agreement with the order of the binding potential (BP) of each ¹¹C-labeled analog to aromatase in the rhesus monkey brain. A human PET study using [¹¹C]iso-analog showed a similar distribution pattern of binding as that of [¹¹C]cetrozole. The time-activity curves showed that elimination of [¹¹C]iso-cetrozole from brain tissue was faster than that of ¹¹C-cetrozole, indicating more rapid metabolism of [¹¹C]iso-cetrozole. [¹¹C]Cetrozole has preferable metabolic stability for brain aromatase imaging in humans, although [¹¹C]iso-cetrozole might also be useful to measure aromatase level in living human brain because of its high binding potential.

Figure 1

The chemical structures of [¹¹C]cetrozole (A) and its analogs, [¹¹C]meta-cetrozole (B), [¹¹C]nitro-cetrozole (C), and [¹¹C]iso-cetrozole (D). The methyl moiety in [¹¹C]meta-cetrozole showed a different position from that in [¹¹C]cetrozole. [¹¹C]Nitro-cetrozole contained a nitro group instead of the cyano group of [¹¹C]cetrozole. [¹¹C]Iso-cetrozole showed a different nitrogen position in the triazole in comparison with [¹¹C]cetrozole.

Figure 2

The distribution volume ratio images of [¹¹C]cetrozole (A), [¹¹C]meta-cetrozole (B), [¹¹C]nitro-cetrozole (C), and [¹¹C]iso-cetrozole (D) in rhesus monkey brain (coronal section). The slices contain the amygdala indicated by arrowhead. The scale ranges of the color bar are 0.4–1.5 for [¹¹C]cetrozole, [¹¹C]meta-cetrozole, and [¹¹C]nitro-cetrozole, and 0.4–2.0 for [¹¹C]iso-cetrozole.

Figure 3

BP_ND values in the amygdala, hypothalamus, nucleus accumbens, and the white matter of [¹¹C]cetrozole, [¹¹C]meta-cetrozole, [¹¹C]nitro-cetrozole, and [¹¹C]iso-cetrozole (N = 4, mean ± S.E.) in rhesus monkey brain. [¹¹C]Meta-cetrozole showed a lower BP_ND than [¹¹C]cetrozole in the amygdala, hypothalamus, and nucleus accumbens (**P < 0.01). [¹¹C]Iso-cetrozole showed a higher BP_ND than [¹¹C]cetrozole in the amygdala, nucleus accumbens (*P < 0.05), and hypothalamus (**P < 0.01).

Figure 4

Time-activity curves of [¹¹C]-cetrozole (A), [¹¹C]meta-cetrozole (B), [¹¹C]nitro-cetrozole (C), and [¹¹C]iso-cetrozole (D) in rhesus monkey brain (N = 4, mean ± S.E.). Aromatase-rich regions (amygdala, hypothalamus, and nucleus accumbens), nonspecific region (white matter), and reference region for Logan reference tissue model analysis (cerebellum) are shown.

Figure 5

The representative SUV images of [¹¹C]cetrozole (A,B) and [¹¹C]iso-cetrozole (C,D) in the brains of similar individuals (A,C transaxial slices; B,D sagittal slices). Arrow heads indicate the thalamus.

Figure 6

Time-activity curves of [¹¹C]cetrozole (A, N = 21) and [¹¹C]iso-cetrozole (B, N = 6) in the human brain (mean ± S.E.). The aromatase-rich regions (thalamus, amygdala, and hypothalamus) and the reference region for Logan reference tissue model analysis (cerebellum) are shown. The [¹¹C]iso-cetrozole curves demonstrate rapid clearance from the tissues in comparison with cetrozole.

Figure 7

BP_ND values in the thalamus, hypothalamus, amygdala, white matter, temporal cortex, and nucleus accumbens of [¹¹C]cetrozole (N = 21) and [¹¹C]iso-cetrozole (N = 6) in the human brain. Each dot indicates individual value. Mean and 95% confidence intervals are also shown. In the hypothalamus, [¹¹C]iso-cetrozole showed significantly higher BP_ND than [¹¹C]cetrozole. No significant difference was observed between the BP_ND of [¹¹C]iso-cetrozole and [¹¹C]cetrozole in the other regions.

Figure 8

Parent composition of [¹¹C]cetrozole (A, N = 21) and [¹¹C]iso-cetrozole (B, N = 5) in human plasma (mean ± SD). The parent fraction of [¹¹C]cetrozole and [¹¹C]iso-cetrozole remained 80% and 27%, respectively, at 20 min after the injection.