Spectrophotometric determination of ligand–protein ratio in B2M3–DTPA monoclonal conjugates based on yttrium–methylthymol blue complex

Monoclonal conjugates using radionuclides are widely used in the diagnosis or therapy of cancer. A number of ligand molecules per protein molecule (L/P ratio) are a crucial factor affecting the monoclonal conjugate efficacy and safety. For proper and accurate dosing of monoclonal conjugates, it is necessary to establish the L/P ratio before they are applied. The aim of this work was the development of a simple, efficient, non-radioactive, and low-cost spectrophotometric method for L/P ratio measurement. The method is based on transchelation reaction between the monoclonal conjugate and yttrium(III)–methylthymol blue complex followed at wavelength of 590 nm. The method operates in the L/P ratio range of 0.32–8.00 and the results are satisfactorily comparable to the conventional mass spectrometric method. Graphical abstract