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INTRODUCTION: Trastuzumab, a humanized antibody directed against the Her2 receptor, induces the expression of p27(kip1), an intranuclear cyclin-dependent kinase inhibitor in some breast cancer cells. The aim of this study was to develop a radioimmunoconjugate (RIC) to monitor trastuzumab-induced p27(kip1) protein up-regulation in vivo. MATERIALS AND METHODS: Anti-p27(kip1) IgG was purified, and conjugated to diethylenetriaminopentaacetate, to allow radiolabeling with (111)In for in vivo detection. Then tat peptide (GRKKRRQRRRPPQGYG), containing a nuclear localization sequence (underlined), was conjugated to the Fc-domain of IgG, using NaIO(4) oxidation of carbohydrates and the resulting Schiff base stabilized with NaCNBH(3). The conjugate was radiolabeled with (111)In, yielding [(111)In]-anti-p27(kip1)-tat. (111)In labeling efficiency, purity and p27(kip1) binding were measured. Trastuzumab-induced p27(kip1) up-regulation was assessed in a panel of breast cancer cell lines by Western blot analysis. Uptake and retention of [(111)In]-anti-p27(kip1)-tat were measured in MDA-MB-361 and SKBr3 cells after exposure to trastuzumab. Uptake of [(111)In]-anti-p27(kip1)-tat was determined at 72 h postintravenous injection in MDA-MB-361 xenografts in athymic mice treated with trastuzumab or saline. RESULTS: [(111)In]-anti-p27(kip1)-tat was synthesized to 97% purity. The RIC was able to bind to p27(kip1) protein and internalized in the cells and was transported to the nuclei of MDA-MB-361 cells. The level of p27(kip1) protein in MDA-MB-361 cells was increased after exposure to clinically relevant doses of trastuzumab for 3 days. Trastuzumab-mediated induction of p27(kip1) was not associated with increased cellular uptake or nuclear localization of [(111)In]-anti-p27(kip1)-tat (6.53+/-0.61% vs. 6.98+/-1.36% internalized into trastuzumab-treated vs. control cells, respectively). However, retention of [(111)In]-anti-p27(kip1)-tat at 72 h was increased approximately twofold (13.5+/-1.3% vs. 6.6+/-0.6% of internalized [(111)In]-anti-p27(kip1)-tat was retained in trastuzumab-treated vs. control cells, respectively; P=.016). Immunohistochemistry showed up-regulation of p27(kip1) in trastuzumab-treated xenografts. Tumour uptake of [(111)In]-anti-p27(kip1)-tat was significantly higher in trastuzumab-treated compared to control animals (6.5+/-0.9 vs. 4.8+/-0.1 %ID/g at 72 h postinjection, respectively; P=.0065). CONCLUSION: [(111)In]-Anti-p27(kip1)-tat may be useful for monitoring changes in the expression of the intranuclear protein, p27(kip1). Up-regulation of p27(kip1) resulted in increased retention of [(111)In]-anti-p27(kip1)-tat in cells treated with trastuzumab. Modest, but statistically significantly higher, retention was also observed in tumours in mice treated with trastuzumab. Since responsiveness to trastuzumab correlated to up-regulation of p27(kip1), it may be possible to use [(111)In]-anti-p27(kip1)-tat to guide treatment with Herceptin and other drugs which alter p27(kip1) expression.

Original publication




Journal article


Nucl Med Biol

Publication Date





811 - 819


Amino Acid Sequence, Animals, Antibodies, Monoclonal, Antibodies, Monoclonal, Humanized, Breast Neoplasms, Cell Line, Tumor, Cell Transformation, Neoplastic, Cyclin-Dependent Kinase Inhibitor p27, Gene Expression Regulation, Neoplastic, Humans, Immunoconjugates, Intranuclear Space, Isotope Labeling, Mice, Molecular Sequence Data, Pentetic Acid, Peptides, Protein Transport, Time Factors, Tissue Distribution, Trastuzumab, Treatment Outcome, Up-Regulation