Preparation of Pb-labeled monoclonal antibody using a novel Ra-based generator solution.

INTRODUCTION

Alpha-emitting radionuclides have gained considerable attention as payloads for cancer targeting molecules due to their high cytotoxicity. One attractive radionuclide for this purpose is Pb, which by itself is a β-emitter, but acts as an in vivo generator for its short-lived α-emitting daughters. The standard method of preparing Pb-labeled antibodies requires handling and evaporation of strong acids containing high radioactivity levels by the end user. An operationally easier and more rapid process could be useful since the 10.6h half-life of Pb puts time constraints on the preparation protocol. In this study, an in situ procedure for antibody labeling with Pb, using a solution of the generator nuclide Ra, is proposed as an alternative protocol for preparing Pb-radioimmunoconjugates.

METHODS

Radium-224, the generator radionuclide of Pb, was extracted from its parent nuclide, Th. Lead-212-labeling of the TCMC-chelator conjugated monoclonal antibody trastuzumab was carried out in a solution containing Ra in equilibrium with progeny. Subsequently, the efficiency of separating the Pb-radioimmunoconjugate from Ra and other unconjugated daughter nuclides in the solution using either centrifugal separation or a PD-10 desalting size exclusion column was evaluated and compared.

RESULTS

Radiolabeling with Pb in Ra-solutions was more than 90% efficient after only 30min reaction time at TCMC-trastuzumab concentrations from 0.15mg/mL and higher. Separation of Pb-labeled trastuzumab from Ra using a PD-10 column was clearly superior to centrifugal separation. This method allowed recovery of approximately 75% of the Pb-antibody-conjugate in the eluate, and the remaining amount of Ra was only 0.9±0.8% (n=7).

CONCLUSIONS

The current work demonstrates a novel method of producing Pb-based radioimmunoconjugates from a Ra-solution, which may be simpler and less time-consuming for the end user compared with the method established for use in clinical trials of Pb-TCMC-trastuzumab.