Theranostics 2017; 7(11):2806-2821. doi:10.7150/thno.19081

Research Paper

Non-invasive in vivo imaging of tumour-associated cathepsin B by a highly selective inhibitory DARPin

Lovro Kramer1, 2, Miha Renko1, Janja Završnik1, 2, Dušan Turk1, 3, Markus A. Seeger4, Olga Vasiljeva1, Markus G. Grütter5, Vito Turk1, Boris Turk1, 3✉

1. Jozef Stefan Institute, Department of Biochemistry and Molecular and Structural Biology, Ljubljana, Slovenia;
2. International Postgraduate School Jozef Stefan, Ljubljana, Slovenia;
3. Center of Excellence CIPKEBIP, Ljubljana, Slovenia;
4. Institute of Medical Microbiology, University of Zürich, Zürich, Switzerland;
5. Department of Biochemistry, University of Zürich, Zürich, Switzerland.

Abstract

Cysteine cathepsins often contribute to cancer progression due to their overexpression in the tumour microenvironment and therefore present attractive targets for non-invasive diagnostic imaging. However, the development of highly selective and versatile small molecule probes for cathepsins has been challenging. Here, we targeted tumour-associated cathepsin B using designed ankyrin repeat proteins (DARPins). The selective DARPin 8h6 inhibited cathepsin B with picomolar affinity (Ki = 35 pM) by binding to a site with low structural conservation in cathepsins, as revealed by the X-ray structure of the complex. DARPin 8h6 blocked cathepsin B activity in tumours ex vivo and was successfully applied in in vivo optical imaging in two mouse breast cancer models, in which cathepsin B was bound to the cell membrane or secreted to the extracellular milieu by tumour and stromal cells. Our approach validates cathepsin B as a promising diagnostic and theranostic target in cancer and other inflammation-associated diseases.

Keywords: protease, cathepsin B, designed ankyrin repeat protein (DARPin), non-invasive diagnostic imaging, tumour microenvironment.

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How to cite this article:
Kramer L, Renko M, Završnik J, Turk D, Seeger MA, Vasiljeva O, Grütter MG, Turk V, Turk B. Non-invasive in vivo imaging of tumour-associated cathepsin B by a highly selective inhibitory DARPin. Theranostics 2017; 7(11):2806-2821. doi:10.7150/thno.19081. Available from http://www.thno.org/v07p2806.htm