1. Biodistribution of 212Pb Conjugated Trastuzumab in Mice
N.R. Schneider1, S.E. Glover1, H.B. Spitz1, M. Lobaugh1, P. Sandwall1, M. Murry1, Zongqing Tan2, Pingping Chen2, Lingling Cui2, and Zhongyun Dong2
University of Cincinnati, College of Engineering & Applied Science, Nuclear & Radiological Engineering
1College of Engineering and Applied Sciences, 598 Rhodes Hall, Cincinnati, OH 45221
2Divison of Hematology-Oncology, UC Cancer Institute, Cincinnati, OH 45267
Introduction
212Pb-trastuzumab is an alpha-emitting radioimmunoconjugate that can deliver a short-range, high LET dose to HER-2 expressing targeted tissue. The decay products of 212Pb include 212Bi and 212Po which emit alpha particles with energy from 6.09 MeV to 8.78 MeV (Fig.1). The radiation dose from 212Pb and its short-lived decay products is delivered within 4 days of administering the radiolabeled antibody. Uptake and biodistribution of 212Pb-trastuzumab in mice was determined by measuring 212Pb in organs obtained from groups of serially-sacrificed mice bearing orthotopic PC-3MM2 human prostate carcinoma.
Methods
Conjugation
A generator containing 3 mCi 224Ra was used to elute 212Pb in 4.5 mL of 2 M HCl. The solution was converted to nitrate form, chelated with sodium ascorbate and NH4OAc, and incubated with trastuzumab for 1 hour. The reaction was quenched with EDTA then purified, resulting in yields as high as 72% in 8.35 mL.
Tissue Samples
Each of 28 mice were injected (IV) with ≈ 20 μCi of 212Pb-trastuzumab. Tissue samples were collected at 30min, 1hr, 4hr, 8hr, 12hr, 24hr, 36hr, 48hr, 60hr, 72hr post injection. Organs of interest included blood, brain, heart, liver, lung, kidney, spleen, bladder, intestine, and tumor.
Measurements
High-resolution gamma spectrometry was used to measure 212Pb-trastuzumab at 239 keV in each tissue sample. The average percent injected dose of the antibody per gram (%ID/g) was determined for each tissue at the time of sacrifice from the activity of 212Pb corrected for radiological decay and normalized based on total organ mass. Blood is reported as %ID/mL.
Results
Although PC-3MM2 cells express limited HER2 receptors, ≈ 8 % ID of 212Pb-trastuzumab was observed in the tumor at 72-hr post I.V. injection. Liver, spleen, and kidney exhibit the greatest concentrations. Uptake of 212Pb-trastuzumab from blood at 72-hr post IV injection was 0.1% in the bladder, 1.5% in the tumor, 2.5% in the spleen, 5.4% in the kidney, 6.0% in the intestine, 12.0% in the liver, and 72% in other tissue.
Discussion
This study demonstrates an increased uptake rate of 212Pb-trastuzumab in PC-3MM2 human prostate tumor over a 72 hr period of observation.
No clinical evidence of systemic toxicity3.
Uptake of 212Pb-trastuzumab in other organs should be considered in evaluating the risk of lethal radiation toxicity.
Future Work
Microdosimetry of radiolabeled 212Pb-trastuzumab to determine alpha and beta particle energy distribution in normal organ tissues.
Determine optimal dosing regimens for trastuzumab as a therapeutic agent.
Acknowledgements
This research was supported, in part, by Areva Med LLC, who provided the radium generator.
References
[1] Milenic D.E., Garestani K. et al. “Potentiation of High-LET Radiation by Gemcitabine: Targeting HER2 with Trastuzumab to Treat Disseminated Peritoneal Disease”. Clin Cancer Res 2007; 13,
[2] McDevitt M.R. et al. “An α-particle Emitter Antibody ([213Bi]J591) for Radioimmunotherapy of Prostate Cancer”. Cancer Res 2000; 60,
[3] Tan Z., Chen P. et al. “Significant Systemic Therapeutic effects of high-LET immunoradiation by 212-Pb-Trastuzumab against prostatic tumors of androgen-independent human prostate cancer in mice”. International Jour of Oncology 2012.
Figure 3 a-b: Biodistribution in organs up to 72 hr
Figure 1: 212Pb decay chain
Figure 4: Percent uptake of of 212Pb-trastuzumab from blood up to 72-hr post IV injection
Figure Pb elution from 224Ra generator
Presented at the 9th International Conference on the Methods and Applications of Radioanalytical Chemistry (MARC IX), March 2012, Kona, HI.
Figure 2 a – f: Uptake of 212Pb-trastuzumab in organs of mouse