Tumor imaging using radiolabelled matrix metalloproteinase-activated anthrax proteins

Journal article

Purpose: Increased activity of matrix metalloproteinases (MMPs) is associated with worse prognosis in different cancer types. The protective antigen (PA-WT) of the binary anthrax lethal toxin was modified to form a pore in cell membranes only when cleaved by MMPs (PA-L1). Anthrax lethal factor (LF) is then able to translocate through these pores. Here, we used an ¹¹¹In-radiolabelled form of LF with the PA/LF system for non-invasive in vivo imaging of MMP activity in tumour tissue by single photon emission computed tomography (SPECT).

Methods: MMP-mediated activation of PA-L1 was correlated to anthrax receptor expression and MMP activity in a panel of cancer cells (HT1080, MDA-MB-231, B8484 and MCF7). Uptake of ¹¹¹In-radiolabelled PA-L1, ¹¹¹In-PA-WT^K563C or ¹¹¹In-LF^E687A (a catalytically inactive LF mutant) in tumour and normal tissues was measured using SPECT/CT imaging in vivo.

Results: Activation of PA-L1 in vitro correlated with anthrax receptor expression and MMP activity (HT1080>MDA-MB-231>B8484>MCF7). PA-L1-mediated delivery of ¹¹¹In-LF^E687A was demonstrated, and corroborated using confocal microscopy with fluorescently labelled LF^E687A. Uptake was blocked by the broad-spectrum MMP inhibitor GM6001. In vivo imaging showed selective accumulation of ¹¹¹In-PA-L1 in MDA-MB-231 tumour xenografts (5.7±0.9%ID/g) at 3 h post intravenous administration. ¹¹¹In-LFE687A was selectively delivered to MMP-positive MDA-MB-231 tumour tissue by MMP-activatable PA-L1 (5.98±0.62%ID/g), but not by furin cleavable PA-WT (1.05±0.21%ID/g), or a non-cleavable PA variant control, PA-U7 (2.74 ± 0.24%ID/g).

Conclusion: Taken together, our results indicate that radiolabelled forms of mutated anthrax lethal toxin hold promise for non-invasive imaging of MMP activity in tumour tissue.

Authors: Xavier, M; Liu, S; Bugge, T; Baguna-Torres, J; Mosley, M

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Society of Nuclear Medicine and Molecular Imaging
• Journal: Journal of Nuclear Medicine
• Volume: 60
• Issue: 10
• Pages: 1474–1482
• Publication date: 2019-04-06
• Acceptance date: 2019-03-13
• DOI: 10.2967/jnumed.119.226423
• EISSN: 2159-662X
• ISSN: 0161-5505
• Keywords: animal imaging; molecular imaging; cancer; SPECT; MMP; pretargeting; anthrax lethal toxin; oncology: breast; radiopharmaceuticals