Radioiodine D Amino Acids Labeling of Rituximab, A New Method for Enhancing the Radiopharmaceutical Targetingand Biostability

Radioimmunotherapy (RIT) is a very promising new therapy for the treatment of recurrent B-Cell non-Hodgkin’s lymphoma (NHL). Iodine-131 is the most frequently used nuclide in clinical RIT, but its usefulness has been limited by dehalogenation of monoclonal antibodies labeled via conventional methods. To circumvent this problem, we have synthesized a tri-peptide consisting of non-metabolizable D amino acids attached to N-Hydroxysuccinimide (NHS). 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. Radioiodination of tri-peptide was followed by conjugation to anti-CD20 antibody. In vitro stability of labeled antibody in serum and phosphate buffered saline (PBS) was measured for 48hr by (thin layer chromatography) TLC. Raji cell line was used to test cell binding of the labeled anti-CD20. The chemical purity of synthesized peptide as assessed by analytical (high performance liquid chromatography) HPLC was 95%. Labeling of tri-peptide resulted in a radiochemical yield of 50-71% with radiochemical purity of > 95%. At Rituximab concentration of 10mg/ml, coupling efficiencies of 65-80% was obtained with radiochemical purity of 95% and Specific activity (SA) of 185MBq/mg (5mCi/mg). This study showed that labeling monoclonal antibodies with radioiodine by non-metabolizable D amino acids will improve bio-stability of the product.