Early results in the irrational design of new bifunctional chelators

BACKGROUND The development of a simple route for the synthesis of the N‐hydroxysuccinimide (NHS) ester of S‐acetyl‐protected mercaptoacetyltriglycine (MAG3) has opened the possibility of preparing novel bifunctional N3S chelators for technetium‐99m (99mTc) and other radionuclides. In particular, the synthesis may be applied to a vast number of tripeptides in place of triglycine, to provide a "library" of bifunctional N3S chelators, each with unique properties related to the particular amino acid residues within each tripeptide. METHODS The authors have synthesized by this simple route the NHS esters of four N3S chelators by reacting NHS‐S‐acetylthioglycolic acid with ala‐gly‐gly, phe‐gly‐gly, pro‐gly‐gly, and ser‐ser‐ser, in addition to gly‐gly‐gly. Each bifunctional chelator was conjugated to biocytin as a model primary amine and radiolabeled with 99mTc. The properties of the four chelators were compared with MAG3 with respect to the stability of the label in saline and serum, the extent of serum protein binding, and the instability to cysteine challenge. RESULTS A range of values was observed. Labeled mercaptoacetyltriserine showed stability towards transchelation to cysteine similar to that of MAG3 as well as lower serum protein binding; labeled mercaptoacetylalanyldiglycine showed slightly higher serum protein binding than labeled MAG3 but greater stability to cysteine challenge. CONCLUSIONS The authors concluded that this simple synthesis and evaluation scheme may be used to prepare and screen a large library of bifunctional chelators for those with useful properties. Cancer 1997; 80:2347‐53. © 1997 American Cancer Society. The properties of tripeptide chelators labeled with technetium‐99m were found to be influenced by the nature of the peptides selected. A simple synthesis and assay scheme was developed, which may ultimately be used in the irrational design of superior chelators for radiopharmaceutical use.