Preparation and testing of bevacizumab radioimmunoconjugates with bismuth-213 and bismuth-205/bismuth-206

Bevacizumab, a humanized anti-VEGF monoclonal antibody has shown promise in various clinical trials. We report the development and testing of Bi-213 (an alpha-emitting radionuclide) labeled bevacizumab for in vitro and in vivo studies using two different chelators viz cDTPA and CHX-A. The developed labeling method showed high labeling yields of 93.6% and 89.7% for cDTPA and CHX-A respectively and the results were reproducible. The in vitro and in vivo stability tests were carried out using Bi-213 and long half-life Bismuth isotope (Bi-205/ Bi-206) for pharmacokinetics. The in vitro results showed remarkable stability of the radiolabeled bevacizumab regardless of the chelator. The in vivo pharmacokinetics studies however, showed that the uptake and retention of cDTPA-bevacizumab was significantly higher in kidneys (p-value 0.02) and lower in liver and spleen (p-value < 0.0001 and 0.0009 respectively). The values for the two conjugates compared well in blood but the longer term data for CHX-A conjugate showed slow clearance resulting in a significantly longer blood half-life of the product (211 hours compared to 4 hours for cDTPA-bevacizumab). Preliminary in vivo results showed increased efficacy of the combination therapy compared to bevacizumab only. The tumour area (mm(2)) decreased from 24.8 +/- 3.6 and 12.8 +/- 1.7 for 1 and 3 mg/ kg cold bevacizumab only to 6.5 +/- 0.7 and 7.5 +/- 4.8 when single dose of 333 MBq/kg of Bi-213-bevacizumab was administered as combination therapy. In conclusion it can be said that stable radiolabeled bevacizumab conjugates can be prepared with Bi-213 with either chelators used. The shorter blood half-life with cDTPA-bevacizumab may not be a major concern with Bi-213 as its own half-life is 46 minutes only. The combination therapy proved superior to bevacizumab alone therapy, a phenomenon that can be particularly useful in cancers where bevacizumab alone has shown limited success like prostate cancer.