Radiolabeled Bis-MPP as 5-HT1A receptor imaging radiotracer with enhanced affinity: Design, synthesis and biological evaluation

Objectives Mostly CNS based diseases are categorized on the basis of their receptors like dopaminergic, serotonergic, benzodiazepin and cholinergic receptors. It is crucial to develop receptor specific non-invasive neuroimaging agents based on 5HT1A receptor. Our objective was to design and develop a specific bioconjugate to label efficiently with improved binding affinity for 5HT1A receptor. Here we report synthesis, characterization, radiolabeling (using SiF chemistry) and biological evaluation to enhance the concentration of ligand at site to improve binding affinity and uptake to 5HT1A receptors for early detection of CNS disorders.

Methods To enhance 5-HT1A receptors binding affinity, bis-MPP, based on 1-(2-methoxyphenyl)piperazine and di-tert-butylsilyl group, have been designed, synthesized and fully characterized by NMR and MS. Radiolabeling of the obtained bis-MPP conjugate was performed under acidic condition in presence of nucleophilic 18F.Competition binding experiments were performed using [3H]-8-OH-DPAT as radioligand on 5HT1A transfected Neuro 2a cells. IC50 values were determined in hippocampal primary cultures which is a rich source of 5HT1A receptors.

Results Bis-MPP has much better targeting capability as evidenced by its higher binding affinity and brain uptake as compared to its monomeric analog of MPP. Bis-derivative of ligand enhances local MPP concentration contributing to high binding affinity of bis-MPP. Successfully, Bis derivative of MPP using silicon based building block was designed and developed, radiolabeling with 18F was performed in one step and purified by HPLC (EOS 28-42 %) in specific activity of 1564 GBq/mmol. Binding affinity, IC50 of bis-MPP on hippocampal primary cultures was 4.8 nmol/L and found comparable to the reported derivatives.

Conclusions In vitro binding of bis-MPP in the tranfected neuro 2a cells expressing 5HT1A with Kd 0.61nM demonstrate the applicability with enhanced affinity in vivo