The ultimate goal of this project is to develop Positron Emission Tomography (PET) imaging agents to measure the level of vesicular acetylcholine transporter (VAChT) expression in the human brain. VAChT is a unique and novel marker for cholinergic neuron function. This protein is a key biomarker for measuring the loss of cholinergic neurons. One the most common physiological characteristics of neurodegenerative diseases such as Alzheimer s disease, Down s syndrome, and Parkinson s disease is the cholinergic deficit in the central cholinergic system. Changes in the level of VAChT expression also occur during antipsychotic therapy and treatment of drug addiction. To date, no clinically suitable radiotracer for imaging VAChT with PET has been identified. Therefore, a radiotracer that possesses a high affinity and high selectivity for VAChT versus sigma receptors and good pharmacokinetic characteristics would be of tremendous use to the PET community. Although (+)-[18F]FBT was useful for imaging the VAChT in anesthetized rhesus monkeys, the high affinity of (+)-FBT for sigma receptors and the slow rate of washout from brain in human subjects indicated that it is not clinically useful for imaging the VAChT. In order to find an acceptable VAChT PET tracer, we incorporated a ketone group between C region and B region in the structure of vesamicol (see Figure 1). At the same time, we exploited the increased VAChT binding affinity of the benzovesamicol analogs than the vesamicol analogs. We will further explore the C region by introducing different substituted group such as F, Br, I and others in order to identify a position to be labeled with PET isotopes such as carbon-11 or fluorine-18. Additionally we will apply our ketone strategy to develop new FBT analogs with reduced sigma affinity. Several of our new compounds have both high VAChT affinity and high selectivity. One of them, (-)-4-fluorobenzoyl benzovesamicol ((-)-FBBV) is a promising candidate: Ki-VAChT = 4.1 1 0.6 nM, Ki-s1 = 660 1 120 nM, Ki-s2 = 320 1 35 nM. (-)-[18F]FBBV was initially evaluated in vivo in rats and monkeys. The initial results are very encouraging, but its relatively high lipophilicity could be a challenge for generating a high signal : noise ratio in PET imaging studies. The goal of this project is to conduct a structure-activity relationship study aimed at optimizing lipophilicity versus VAChT binding affinity (Ki value) and selectivity. To achieve the goal, molecular modeling and QSAR studies will be used for designing the new compounds. The potential compounds will be radiosynthesized with F-18 or C-11 and evaluated in rats to confirm the specific binding to VAChT in vivo.
The Specific aims of this grant application are: 1. Synthesize and measure the affinities of the target compounds to identify ligands having a high affinity for the VAChT (Ki <10 nM) and high selectivity for VAChT. Racemic mixtures meeting this criterion will be resolved into optical isomers and the individual affinities will be determined. 2. Radiosynthesize of Carbon-11 or Fluorine-18 labeled radiotracers meeting Specific Aim 1 and 3. Measure the lipophilicity of the radiotracers to determine if their log P value is in the range ~ 1.0 3.0. 4. Perform biodistribution and brain uptake studies in rats to select the potential radiotracers for further in vivo evaluation.
Neurodegenerative diseases are the most prevalent diseases of aged population in USA. Identifying a novel PET tracer of imaging VAChT that could be used on clinic to measure the loss or recovery of cholinergic neuron will be tremendous useful on clinic assessing the severity of cholinergic deficient diseases or responding to treatments targeting on the cholinergic dysfunction patients.
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