Radioiodinated 2-hydroxy-3-(4-iodophenyl)-1-(4-phenylpiperidinyl)propane: potential radiotracer for mapping central cholinergic innervation in vitro

Synthesis and in vitro evaluation of new benzovesamicol analogues as potential imaging probes for the vesicular acetylcholine transporter

Our goal was to synthesize new stereospecific benzovesamicol analogues, which could potentially be used as SPECT or PET radioligands for the vesicular acetylcholine transporter (VAChT). This paper describes the chemical synthesis, resolution and determination of binding affinity for four enantiomeric pairs of derivatives. Their intrinsic affinities were determined by competition against binding of [3H]vesamicol to human VAChT. Of the eight enantiomers, (E)-(R,R)-5-AOIBV [(R,R)-3], and (R,R)-5-FPOBV [(R,R)-4] displayed the highest binding affinities for VAChT (Kd=0.45 and 0.77 nM, respectively), which indicated that an elongation of the chain from 5-idodo as in the case of 5-iodobenzovesamicol (5-IBVM), to a 5-(E)-3-iodoallyloxy or 5-fluoropropoxy substituent, as in 5-AOIBV and 5-FPOBV, respectively, was very well tolerated at the vesamicol binding site. The enantiomer (R,R)-4-MAIBV [(R,R)-16], which retains the basic structure of (-)-5-IBVM but possess an additional aminomethyl substituent in the 4-position of the piperidine ring, displayed lower binding affinity (Kd=8.8 nM). Nevertheless, the result suggests that substitution at this position may be an interesting alternative to investigate for development of new benzovesamicol analogues. As expected, the corresponding (S,S) enantiomers displayed lower Kd values, they were approximately 10-fold lower in the case of (S,S)-5-FPOBV (Kd=8.4 nM) and (E)-(S,S)-5-AOIBV (Kd=4.3 nM). (R,R)-3, and (R,R)-4 showed the same high affinity for VAChT as (-)-5-IBVM and may be suitable as imaging agents of cholinergic nerve terminals.

The potential of radioiodinated (-)-m-iodovesamicol for diagnosing cholinergic deficit dementia

We investigated changes in the brain distribution of (-)-[(125)I]-m-iodovesamicol [(-)-[(125)I]mIV] in cholinergic denervation rats produced by a unilateral lesion of the nucleus basalis magnocellularis (NBM). Dual-tracer ex vivo autoradiographic analysis using (-)-[(125)I]mIV and [(99m)Tc]HMPAO was conducted to the effect of regional cerebral perfusion on the brain distribution of (-)-[(125)I]mIV in a unilateral NBM-lesioned rat. (-)-[(125)I]mIV binding in the ipsilateral cortex to the lesion significantly reduced by 10.4 %, compared with that in the contralateral cortex, while (-)-[(125)I]mIV binding in the ipsilateral caudate putamen, hippocampus and thalamus did not change. The rate of reduction in the (-)-[(125)I]mIV binding (10.4 %) was significantly higher than that of [(99m)Tc]HMPAO accumulation (4.0%) in the ipsilateral cortex to the lesion (P < 0.01). These results suggested that radioiodinated (-)-mIV may be useful in the study of dementia characterized by degeneration of the cholinergic neurotransmitter system, such as Alzheimer's disease.

Myocardial accumulation of a dopamine D2 receptor-binding radioligand, 2'-iodospiperone

125I-2'-iodospiperone (2'-ISP), which has a high and selective affinity for dopamine D2 receptors, produced a high myocardial accumulation of radioactivity in the early phase after intravenous injection into mice. A human scintigraphic study also showed that the myocardium was clearly visualized soon after intravenous injection of the tracer. Analysis of the myocardial homogenate obtained from mice showed that 125I-2'-ISP was metabolically stable and was taken up the myocardium in its intact form. Administration of spiperone significantly reduced the myocardial uptake of 125I-2'-ISP in mice. Treatment with haloperidol and (+) butaclamol, which have a high affinity for dopamine D2 receptors, also tended to reduce the myocardial uptake of radioactivity, while (-)-butaclamol, which has no affinity for dopamine D2 receptors, caused no change in uptake. These findings suggest that the myocardial accumulation of 2'-ISP occurred in association with dopamine D2 (DA2) receptors.