Biotinidase Resistant ⁶⁸Gallium-Radioligand Based on Biotin/Avidin Interaction for Pretargeting: Synthesis and Preclinical Evaluation

A new macrocyclic system 2,2′-(12-amino-11,13-dioxo-1,4,7,10-tetraazacyclotridecane-4,7-diyl)­diacetic acid (ATRIDAT) was designed for coordinating metals in +2 and +3 oxidation states particularly ⁶⁸Ga­(III), for PET imaging. ATRIDAT was conjugated to d-biotin for pretargeting via biotin–avidin interaction. This model provides high tumor targeting efficiency and stability to biotinidase activity leading to modest signal amplification at the tumor site. Cyclization of triethylenetetramine with protected diethylamino malonate resulted in the formation of 13 membered diamide ring. d-Biotin was then anchored on the pendant amine rendering α-methyne carbon to the biotinamide bond which blocks the biotinidase enzyme activity. Biotinidase stability assay showed remarkable stability toward the action of biotinidase with ∼95% remaining intact after treatment following 4 h. Binding affinity experiments such as HABA assay, competitive displacement studies with d-biotin and CD showed high binding affinity of the molecule with avidin in nanomolar range. Biotin conjugate was successfully radiolabeled with ⁶⁸Ga­(III) with radiolabeling efficiency of ∼70% and then purified to get 99.9% radiochemical yield. IC₅₀ of the compound was found to be 2.36 mM in HEK cell line and 0.82 mM in A549 as assessed in MTT assay. In biodistribution studies, the major route of excretion was found to be renal. Significant uptake of 4.15 ± 0.35% was observed in tumor in the avidin pretreated mouse at 1 h. μPET images also showed a high tumor to muscle ratio of 26.8 and tumor to kidney ratio of 1.74 at 1 h post-injection after avidin treatment