Tumor Imaging Using Radiolabeled Matrix Metalloproteinase-Activated Anthrax Proteins

J Nucl Med. 2019 Oct;60(10):1474-1482. doi: 10.2967/jnumed.119.226423. Epub 2019 Apr 6.


Increased activity of matrix metalloproteinases (MMPs) is associated with worse prognosis in different cancer types. The wild-type protective antigen (PA-WT) of the binary anthrax lethal toxin was modified to form a pore in cell membranes only when cleaved by MMPs (to form PA-L1). Anthrax lethal factor (LF) is then able to translocate through these pores. Here, we used a 111In-radiolabeled form of LF with the PA/LF system for noninvasive in vivo imaging of MMP activity in tumor tissue by 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-radiolabeled PA-L1, 111In-PA-WTK563C, or 111In-LFE687A (a catalytically inactive LF mutant) in tumor 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-LFE687A was demonstrated and was corroborated using confocal microscopy with fluorescently labeled LFE687A Uptake was blocked by the broad-spectrum MMP inhibitor GM6001. In vivo imaging showed selective accumulation of 111In-PA-L1 in MDA-MB-231 tumor xenografts (5.7 ± 0.9 percentage injected dose [%ID]/g) at 3 h after intravenous administration. 111In-LFE687A was selectively delivered to MMP-positive MDA-MB-231 tumor 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 noncleavable PA variant control, PA-U7 (2.74 ± 0.24 %ID/g). Conclusion: Taken together, our results indicate that radiolabeled forms of mutated anthrax lethal toxin hold promise for noninvasive imaging of MMP activity in tumor tissue.

Keywords: MMP; SPECT; anthrax lethal toxin; cancer; pretargeting.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Antigens, Bacterial / chemistry*
  • Antigens, Bacterial / genetics*
  • Bacterial Toxins / chemistry*
  • Bacterial Toxins / genetics*
  • Biological Transport
  • Cell Line, Tumor
  • Humans
  • Indium Radioisotopes / chemistry*
  • Kinetics
  • MCF-7 Cells
  • Matrix Metalloproteinase 14 / chemistry
  • Matrix Metalloproteinase 2 / chemistry
  • Matrix Metalloproteinases / metabolism
  • Mice
  • Mutation
  • Neoplasm Transplantation
  • Neoplasms / diagnostic imaging*
  • Tomography, Emission-Computed, Single-Photon*


  • Antigens, Bacterial
  • Bacterial Toxins
  • Indium Radioisotopes
  • anthrax toxin
  • Matrix Metalloproteinases
  • MMP2 protein, human
  • Matrix Metalloproteinase 2
  • MMP14 protein, human
  • Matrix Metalloproteinase 14