Iodine-131 D-amino acid peptide, a novel bi-functional chelating agent for improving therapeutic application of Rituximab

Objectives Rituximab radioimmunotherapy (RIT) is a promising tool for the treatment of recurrent B-Cell non-Hodgkin’s Lymphoma. Iodine-131 is the most frequently used nuclide in clinical RIT, but its usefulness has been limited by dehalogenation. To solve this problem we have synthesized a YYK tri-peptide consisting of non-metabolizable D-amino acids modified with the N-succinimidyl function.

Methods Tri-peptide was synthesized by standard Fmoc solid phase synthesis on tritylchloride resin. Labeling of tri-peptide was performed using the chloramine-T method and the conventional extraction. Radioiodinated peptide was conjugated to Rituximab via N-succinimidyl group. The cytotoxicity assessment of both directly and in-directly labeled I-131 Rituximab was done by MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, a tetrazole) assay on Raji cell.

Results The chemical purity of the peptide as assessed by analytical HPLC was 95%. The radiochemical yield of [131I]-peptide was 50-71% with radiochemical purity of > 95%. Radioiodination of peptide was followed by conjugation to Rituximab with 65-75% labeling efficiency. The MTT assay showed that %SD50s for Rituximab labeled with [131I]-peptide was (0.2, 0.2, 0.4, 4.2 nM) in (24, 48, 72 and 96 h) respectively and %SD50s for directly labeled [131I]-Rituximab was (0.8, 0.9, 4.7, 4.9 nM) in (24, 48, 72 and 96 h) respectively.

Conclusions This favorable cytotoxicity of [131I]-peptide-Rituximab suggest that the use of radioiodine labeled peptide is promissing for improving RIT of Rituximab.

Research Support We thank Drs. Thakur and Zhang, Thomas Jefferson University, for their helpful comments