Targeted contrast-enhanced ultrasound imaging of tumor angiogenesis with contrast microbubbles conjugated to integrin-binding knottin peptides.
Targeted contrast-enhanced ultrasound imaging is increasingly being recognized as a powerful imaging tool for the detection and quantification of tumor angiogenesis at the molecular level. The purpose of this study was to develop and test a new class of targeting ligands for targeted contrast-enhanced ultrasound imaging of tumor angiogenesis with small, conformationally constrained peptides that can be coupled to the surface of ultrasound contrast agents.


METHODS
Directed evolution was used to engineer a small, disulfide-constrained cystine knot (knottin) peptide that bound to alpha(v)beta(3) integrins with a low nanomolar affinity (Knottin(Integrin)). A targeted contrast-enhanced ultrasound imaging contrast agent was created by attaching Knottin(Integrin) to the shell of perfluorocarbon-filled microbubbles (MB-Knottin(Integrin)). A knottin peptide with a scrambled sequence was used to create control microbubbles (MB-Knottin(Scrambled)). The binding of MB-Knottin(Integrin) and MB-Knottin(Scrambled) to alpha(v)beta(3) integrin-positive cells and control cells was assessed in cell culture binding experiments and compared with that of microbubbles coupled to an anti-alpha(v)beta(3) integrin monoclonal antibody (MB(alphavbeta3)) and microbubbles coupled to the peptidomimetic agent c(RGDfK) (MB(cRGD)). The in vivo imaging signals of contrast-enhanced ultrasound with the different types of microbubbles were quantified in 42 mice bearing human ovarian adenocarcinoma xenograft tumors by use of a high-resolution 40-MHz ultrasound system.


RESULTS
MB-Knottin(Integrin) attached significantly more to alpha(v)beta(3) integrin-positive cells (1.76 +/- 0.49 [mean +/- SD] microbubbles per cell) than to control cells (0.07 +/- 0.006). Control MB-Knottin(Scrambled) adhered less to alpha(v)beta(3) integrin-positive cells (0.15 +/- 0.12) than MB-Knottin(Integrin). After blocking of integrins, the attachment of MB-Knottin(Integrin) to alpha(v)beta(3) integrin-positive cells decreased significantly. The in vivo ultrasound imaging signal was significantly higher after the administration of MB-Knottin(Integrin) than after the administration of MB(alphavbeta3) or control MB-Knottin(Scrambled). After in vivo blocking of integrin receptors, the imaging signal after the administration of MB-Knottin(Integrin) decreased significantly (by 64%). The imaging signals after the administration of MB-Knottin(Integrin) were not significantly different in the groups of tumor-bearing mice imaged with MB-Knottin(Integrin) and with MB(cRGD). Ex vivo immunofluorescence confirmed integrin expression on endothelial cells of human ovarian adenocarcinoma xenograft tumors.


CONCLUSIONS
Integrin-binding knottin peptides can be conjugated to the surface of microbubbles and used for in vivo targeted contrast-enhanced ultrasound imaging of tumor angiogenesis. Our results demonstrate that microbubbles conjugated to small peptide-targeting ligands provide imaging signals higher than those provided by a large antibody molecule.
Molecular Imaging Program at Stanford, Department of Radiology, School of Medicine, Stanford University, Stanford, California 94305-5105, USA. willmann@stanford.edu
Jürgen K
JK
Willmann
Richard H
RH
Kimura
Nirupama
N
Deshpande
Amelie M
AM
Lutz
Jennifer R
JR
Cochran
Sanjiv S
SS
Gambhir
D000595:Amino Acid Sequence; D000818:Animals; D000911:Antibodies, Monoclonal / Q000737:chemistry / Q000276:immunology; D040761:Biomimetic Materials / Q000737:chemistry / Q000378:metabolism; D002448:Cell Adhesion; D045744:Cell Line, Tumor; D002471:Cell Transformation, Neoplastic; D003287:Contrast Media / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism; D015195:Drug Design; D005260:Female; D005455:Fluorescent Antibody Technique; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D039302:Integrin alphaVbeta3 / Q000378:metabolism; D016023:Integrins / Q000276:immunology / Q000378:metabolism*; D051379:Mice; D045423:Microbubbles; D008969:Molecular Sequence Data; D009369:Neoplasms / Q000098:blood supply* / Q000000981:diagnostic imaging* / Q000235:genetics / Q000473:pathology; D009389:Neovascularization, Pathologic; D009842:Oligopeptides / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism*; D015203:Reproducibility of Results; D014463:Ultrasonography
D000911:Antibodies, Monoclonal; D003287:Contrast Media; D039302:Integrin alphaVbeta3; D016023:Integrins; D009842:Oligopeptides; C047981:arginyl-glycyl-aspartic acid