Synthesis and preclinical validation of 5HT<sub>1A</sub> receptor-targeted carbon dots functionalized with Gd<sup>3+</sup> and <sup>99m</sup>Tc as a multi-modal SPECT and MRI probe for molecular imaging.

Abstract

We report a novel 5HT1A receptor-targeted multimodal imaging probe developed by functionalizing biocompatible nitrogen doped carbon quantum dots (CDMPP-DTPA). The carbon quantum dots were hydrothermally synthesized using methoxyphenyl piperazine (MPP) for neuro receptor targeting and diethylenetriaminepentaacetic acid (DTPA) for metal chelation. Subsequent loading with Gd3+ and radiolabeling with 99mTc (efficiency: 95.5% ± 0.5) enabled MRI-optical and SPECT-optical imaging, respectively. Subsequent to Gd3+ loading, CDMPP-DTPA continued to exhibit excellent fluorescence characteristics at 360-380 nm. A bright contrast of the 5HT1A rich hippocampal region was observed in T1-weighted MRI scans using Gd-CDMPP-DTPA, with a longitudinal relaxivity (r1) of 9.983 mM-1 s-1 and a transverse relaxivity (r2) of 15.262 mM-1 s-1. In vitro assays confirmed the biocompatibility of the resulting probes, which were evaluated using an MTT cytotoxicity assay with HEK cells, and a haemolysis assay. Histopathological analysis confirmed the nontoxic nature of the probes. In vivo T1-weighted MRI and SPECT scans in mice demonstrated significant brain permeability and indicated early brain uptake. Region of interest (ROI) analysis corroborated the invasive organ biodistribution pattern, and a brain uptake of approximately 3.24% ID g-1 (injected dose per gram of tissue) was evident, along with the hepato-biliary and renal clearance pathways. These findings establish the functionalized carbon quantum dots as a promising multimodal imaging nanoplatform for the noninvasive mapping of 5HT1A receptors, particularly relevant in the diagnosis and longitudinal monitoring of neurological conditions.