Radiolabeling and Preclinical Evaluation of a New S-Alkylated Cysteine Derivative Conjugated to C-Substituted Macrocycle for Positron Emission Tomography

A new S-alkylated cysteine-derivatized tumor targeting agent, 2,2'-(12-(2-((2-acetamido-2-carboxyethyl)thio)acetamido)-11,13-dioxo-1,4,7,10-tetraazacyclotridecane-4,7-diyl)diacetic acid was developed for positron emission tomography (PET) imaging. N -Acetyl cysteine (NAC) was conjugated to ATRIDAT as a specific targeting agent toward L-type and ASC amino acid transporter systems in the oncogenic cells. NAC was attached via S-alkylation to prevent its incorporation at undesired recognition sites affecting the signal-to-noise ratio. NAC-ATRIDAT was subjected to gallium-68 complexation with >75% radiolabeling yield. The radiocomplex was purified through the tc18 cartridge to obtain 99.89% radiochemical yield. IC-50 of the NAC-ATRIDAT conjugate was 0.8 mM in A549 cells as evaluated through 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazonium bromide assay. Binding affinity experiments on A549 cells showed noteworthy binding with K D in the nanomolar range. A time course study showed a K m value of 0.19 μM and V max  value of 0.49 pmol/μg protein/min showing reasonable tumor kinetics. Efflux studies showed that the synthesized radioligand is transported majorly by LAT followed by the ASC system. Clearance was found to be renal with 7.67 ± 1.48% ID/g uptake at 30 min which substantially declined to 0.52 ± 0.% ID/g at 4 h. A significant uptake of 10.06 ± 1.056% ID/g was observed at the tumor site in mice at 1 h. μPET images revealed a high contrast with a tumor-to-kidney ratio of 4.8 and a tumor-to-liver ratio of 35.85 at 1 h after injection. These preclinical in vitro and in vivo evaluation supports its potential on the way of becoming a successful 68 Ga-radiolabeled amino acid-based PET imaging agent.