A Cystine Knot Peptide Targeting Integrin αvβ6 for Photoacoustic and Fluorescence Imaging of Tumors in Living Subjects

J Nucl Med. 2016 Oct;57(10):1629-1634. doi: 10.2967/jnumed.115.169383. Epub 2016 May 26.


Photoacoustic imaging is a nonionizing biomedical imaging modality with higher resolution and imaging depth than fluorescence imaging, which has greater sensitivity. The combination of the 2 imaging modalities could improve the detection of cancer. Integrin αvβ6 is a cell surface marker overexpressed in many different cancers. Here, we report the development and evaluation of a dye-labeled cystine knot peptide, which selectively recognizes integrin αvβ6 with high affinity, for photoacoustic and fluorescence imaging. The new dual-modality probe may find clinical application in cancer diagnosis and intraoperative imaging of integrin αvβ6-positive tumors.

Methods: An engineered cystine knot peptide, R01, that recognizes integrin αvβ6 was labeled with Atto 740 (A740) and evaluated for its specific cell uptake and its sensitivity threshold. A740-R01 was injected via the tail vein into nude mice xenografted with A431 (integrin αvβ6-positive) or 293T (integrin αvβ6-negative) tumors. Photoacoustic and fluorescence scans of tumors were acquired before and at 0.5, 1, 2, and 4 h after injection of A740-R01. Dynamic photoacoustic scans of various normal organs were also acquired. Ex vivo fluorescence imaging of tissues was performed 1 h after injection.

Results: The A740-R01 demonstrated integrin αvβ6-dependent binding to A431 cells in culture. Sensitivity studies indicated that the probe may potentially detect lesions as small as 1 or 6 mm3 by fluorescence or photoacoustic imaging, respectively. The photoacoustic and fluorescence signals of A431 xenografts at 1 h after injection were 1.87 ± 0.25 arbitrary units (AU) and 8.27 ± 0.87 AU, respectively. Target specificity was confirmed by low tumor uptake in 293T tumors at 1 h after injection (1.07 ± 0.15 AU and 1.10 ± 0.14 AU for photoacoustic and fluorescence signals, respectively). A740-R01 exhibited hepatobiliary clearance marked by high uptake in the liver, spleen, and intestine but low uptake in the kidneys.

Conclusion: A740-R01 specifically targeted integrin αvβ6 with low nanomolar affinity. A740-R01 was able to detect integrin αvβ6 both in vitro and in vivo by photoacoustic and fluorescence imaging. A740-R01 is able to detect αvβ6-positive tumors in living subjects and may have clinical application in cancer diagnosis and real-time image-guided surgery.

Keywords: cystine knot; integrin αvβ6; photoacoustic fluorescence imaging.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Antigens, Neoplasm / metabolism*
  • Cell Line, Tumor
  • Cystine*
  • Female
  • Humans
  • Integrins / metabolism*
  • Mice
  • Mice, Nude
  • Models, Molecular
  • Optical Imaging / methods*
  • Peptides / chemistry*
  • Peptides / metabolism*
  • Photoacoustic Techniques / methods*
  • Protein Conformation
  • Protein Transport
  • Tissue Distribution


  • Antigens, Neoplasm
  • Integrins
  • Peptides
  • integrin alphavbeta6
  • Cystine