Synthesis and biodistribution studies of iodine-131 D-amino acid YYK peptide as a potential therapeutic agent for labeling an anti-CD20 antibody

Abstract

A major drawback of conventionally radioiodinated monoclonal antibodies for radioimmunotherapy is in vivo dehalogenation of iodine as a result of deiodinase recognition. To solve this problem we have synthesized a YYK tri-peptide consisting of non-metabolizable D-amino acids modified with the N-succinimidyl (N-Succ) function. The chemical purity of the synthesized peptide as assessed by analytical high performance liquid chromatography was 95%. Labeling of the Fmoc-D-Tyr(tBu)-D-Tyr(tBu)-D-Lys(Boc)-N-Succ was performed using the chloramine-T method and the conventional extraction, resulting in a radiochemical yield of 50‐71% and a radiochemical purity of 495%. Radioiodination of the peptide was followed by conjugation to anti-CD20 antibody with 65‐75% labeling efficiency and 90% radiochemical purity. The effect of radioiodinated peptide on the biological behavior of the conjugate was evaluated through biodistribution studies in normal Lewis rats. Thyroid and stomach levels from Rituximab labeled with [ 131 I]-YYK-peptide were two- to fourfold less than those with directly labeled [ 131 I]-Rituximab, suggesting low recognition of its D-iodotyrosine residue by endogenous deiodinases. The favorable in vitro/in vivo stability and biodistribution profiles suggest that this radioiodinelabeled YYK peptide is a good candidate for further exploration of its potential clinical application.