Site-specifically 89Zr-labeled monoclonal antibodies for ImmunoPET
Received 10 October 2009; received in revised form 25 November 2009; accepted 27 November 2009. published online 11 February 2010.
Abstract
Three thiol reactive reagents were developed for the chemoselective conjugation of desferrioxamine (Df) to a monoclonal antibody via engineered cysteine residues (thio-trastuzumab). The in vitro stability and in vivo imaging properties of site-specifically radiolabeled 89Zr-Df-thio-trastuzumab conjugates were investigated.
Methods
The amino group of desferrioxamine B was acylated by bromoacetyl bromide, N-hydroxysuccinimidyl iodoacetate, or N-hydroxysuccinimidyl 4-[N-maleimidomethyl]cyclohexane-1-carboxylate to obtain thiol reactive reagents bromoacetyl-desferrioxamine (Df-Bac), iodoacetyl-desferrioxamine (Df-Iac) and maleimidocyclohexyl-desferrioxamine (Df-Chx-Mal), respectively. Df-Bac and Df-Iac alkylated the free thiol groups of thio-trastuzumab by nucleophilic substitution forming Df-Ac-thio-trastuzumab, while the maleimide reagent Df-Chx-Mal reacted via Michael addition to provide Df-Chx-Mal-thio-trastuzumab. The conjugates were radiolabeled with 89Zr and evaluated for serum stability, and their positron emission tomography (PET) imaging properties were investigated in a BT474M1 (HER2-positive) breast tumor mouse model.
Results
The chemoselective reagents were obtained in 14% (Df-Bac), 53% (Df-Iac) and 45% (Df-Chx-Mal) yields. Site-specific conjugation of Df-Chx-Mal to thio-trastuzumab was complete within 1 h at pH 7.5, while Df-Iac and Df-Bac respectively required 2 and 5 h at pH 9. Each Df modified thio-trastuzumab was chelated with 89Zr in yields exceeding 75%. 89Zr-Df-Ac-thio-trastuzumab and 89Zr-Df-Chx-Mal-thio-trastuzumab were stable in mouse serum and exhibited comparable PET imaging capabilities in a BT474M1 (HER2-positive) breast cancer model reaching 20–25 %ID/g of tumor uptake and a tumor to blood ratio of 6.1–7.1.
Conclusions
The new reagents demonstrated good reactivity with engineered thiol groups of trastuzumab and very good chelation properties with 89Zr. The site-specifically 89Zr-labeled thio-antibodies were stable in serum and showed PET imaging properties comparable to lysine conjugates.
Genentech Research and Early Development, Genentech Inc., South San Francisco, CA 94080, USA
Corresponding author. Department of Biomedical Imaging, Genentech, Inc., South San Francisco, CA 94080, USA. Tel.: +1 650 225 8821; fax: +1 650 467 7991.
ABSTRACT