In vivo studies of the SERT-selective [18F]FPBM and VMAT2-selective [18F]AV-133 radiotracers in a rat model of Parkinson's disease

Abstract

Introduction: The utility of [(18)F]FPBM [2-(2'-((dimethylamino)methyl)-4'-(3-[(18)F]-fluoropropoxy)phenylthio)benzenamine], a selective serotonin transporter (SERT) tracer, and [(18)F]AV-133 [(+)-2-Hydroxy-3-isobutyl-9-(3-fluoropropoxy)-10-methoxy-1,2,3,4,6,7-hexahydro-11bH-benzo[a]quinolizine], a selective vesicular monoamine transporter 2 (VMAT2) tracer, were tested in the 6-hydroxydopamine (6-OHDA) unilateral lesioned rat model.

Methods: Positron emission tomography (PET) imaging of three 6-OHDA unilateral lesioned male Sprague Dawley rats (Rats 1-3) were performed with [(18)F]FPBM and [(18)F]AV-133 to examine whether changes in SERT and VMAT2 binding, respectively, could be detected in the brain. The brains of the three rats were then removed and examined by in vitro autoradiography with [(18)F]FPBM and the dopamine transporter ligand, [(125)I]IPT [N-(3'-[(125)I]-iodopropen-2'-yl)-2-beta-carbomethoxy-3-beta-(4-chloro phenyl) tropane, for confirmation. Biodistribution of [(18)F]FPBM in a separate group of p-chloroamphetamine (PCA) treated rats were also performed.

Results: PET image analysis showed varying levels of SERT binding reduction (Rat 1=-11%, Rat 2=-4%, Rat 3=-43%; n=2) and a clear and definitive loss of VMAT2 binding (Rat 1=-87%, Rat 2=-72%, and Rat 3=-91%; n=1) in the left striatum when compared to the right (non-lesioned side) striatum. The results from PET imaging were corroborated with quantitative in vitro autoradiography. Rats treated with a selective serotonin toxin (p-chloroamphetamine) showed a significant reduction of [(18)F]FPBM uptake in the cortex and hypothalamus regions of the brain.

Conclusion: The preliminary data suggest that [(18)F]FPBM and [(18)F]AV-133 may be useful for the examination of serotonergic and dopaminergic neuron integrity, respectively, in the living brain.

FIGURE 1

Structure of a potentially new PET radiotracer, [¹⁸F]FPBM (2-(2′-((dimethylamino)methyl)-4′-(3-fluoropropoxy)phenylthio)benzenamine), for the imaging of serotonin transporters in the brain.

FIGURE 2

PET imaging was performed to confirm that the 6-hydroxydopamine unilateral lesion of rats #1-3 were successful (n = 1). 50-57 MBq of [¹⁸F]AV-133, a VMAT2 ligand, was injected through a catheter placed in the tail vein of the rat while under isoflurane anesthesia. In each of the three rats summed PET images (4 hrs) showed [¹⁸F]AV-133 localization to the right striatum but not the left striatum indicating that the lesion was successful. Some non-specific binding can be seen in the nasal cavity.

FIGURE 3

The utility of [¹⁸F]FPBM, a SERT ligand, was evaluated in the 6-hydroxydopamine unilateral lesioned rats (rats #1-3) through PET imaging. [¹⁸F]FPBM (37-57 MBq), was injected through a catheter placed in the tail vein of the rat while under isoflurane anesthesia. (A) Summed PET images (4 hrs) of a normal control rat showed [¹⁸F]FPBM localization to SERT-rich regions such as the thalamus and striatum. (B) In rat #1 there was [¹⁸F]FPBM binding in the right striatum but decreased binding in the left striatum (white arrow). (C) In Rat #2 there appeared to be comparable amounts of [¹⁸F]FPBM binding between the left (white arrow) and right striatum. (D) There was a clear loss of [¹⁸F]FPBM binding in the left striatum (white arrow) of rat #3. Nonspecific binding in the nasal cavity (NC) and Harderian glands (HG) can also be seen in these images.

FIGURE 4

Time activity curves were generated from PET image analysis using the AMIDE software. Points in the curve represent activity in the striatum after subtracting out the activity in the cerebellum at the same time point. Percent [¹⁸F]FPBM binding change in the left striatum was expressed as ((total activity in right striatum − total activity in cerebellum) − (total activity in left striatum − total activity in cerebellum))/(total activity in right striatum − total activity in cerebellum) × 100. (A) The normal control rats had a +0.5% average increase in total activity in the left striatum (n = 2, two separate rats). (B) Rat #1 showed an −11% average decrease (n = 2). (C) Rat #2 displayed a −4% average decrease (n = 2). (D) Rat #3 had a −43% average decrease (n = 2).

FIGURE 5

[¹²⁵I]IPT and [¹⁸F]FPBM binding in the striatum of the three Parkinson’s model rats were examined through in vitro autoradiography. (A) In the control rat, the dopamine transporter ligand, [¹²⁵I]IPT, strongly binds in both the left and right striatum, as expected. [¹⁸F]FPBM also appears to have equal amounts of binding in the left and right striatum. Binding of [¹⁸F]FPBM appears less intense than [¹²⁵I]IPT due to the natural lower level of serotonergic innervation compared to dopaminergic innervation in the striatum. (B) [¹²⁵I]IPT binding in the left striatum is clearly reduced in rat #3. Rat #3 also had visible differences in [¹⁸F]FPBM binding between the left (red arrows) and right striatum. Rat #1 and #2 also showed reduced [¹²⁵I]IPT binding in the left striatum but did not show any differences with [¹⁸F]FPBM (not shown). ¹²⁵I standards were exposed to the film along with the brain sections. DAT: dopamine transporter, SERT: serotonin transporter, St: striatum, OT: olfactory tubercle, GP: globus pallidus, Hp: hippocampus, Th: thalamus, Am: amygdala, Cb: cerebellum, LC: locus coeruleus (Please note: 1nCi = 0.037 Bq).

FIGURE 6

Results from PET imaging and in vitro autoradiography show a similar trend. (A) The results from both PET imaging of VMAT2 with [¹⁸F]AV-133 and in vitro autoradiography of dopamine transporters with [¹²⁵I]IPT suggest that rat #3 had the greatest loss of dopaminergic neurons followed by rat #1 then rat #2. (B) For in vitro autoradiography the percent [¹⁸F]FPBM binding reduction was calculated by averaging brain sections containing the rostral striatum (n = 2 for control, n = 3 for rats #1-3). There was variation in values between the brain sections, but the averaged results did rank the rats in the same order of SERT binding loss as the results from PET imaging with [¹⁸F]FPBM: rat #3 > rat #1 > rat #2. *Separate control rats were used for PET imaging and in vitro autoradiography.