Antiangiogenic Cancer Therapy : Monitoring with Molecular US and a Clinically Translatable Contrast Purpose : Methods : Results :

Purpose: Materials and Methods: To develop and test human kinase insert domain receptor (KDR)-targeted microbubbles (MBs) (MB KDR ) for imaging KDR at the molecular level and for monitoring antiangiogenic therapy in a human colon cancer xenograft tumor model in mice. Animal studies were approved by the Institutional Adminis- trative Panel on Laboratory Animal Care. A heterodimeric peptide that binds to human KDR with low nanomolar affi nity ( K D = 0.5 nmol/L) was coupled onto the surface of perfl uorobutane-containing lipid-shelled MBs (MB KDR ). Binding specifi city of MB KDR to human KDR and cross- reactivity with murine vascular endothelial growth factor (VEGF) receptor 2 (VEGFR2) were tested in cell culture under fl ow shear stress conditions (at 100 sec 2 1 ). In vivo binding specifi city of MB KDR to VEGFR2 was tested in human LS174T colon cancer xenografts in mice with a 40-MHz ul- trasonographic (US) transducer. Targeted contrast material– enhanced US imaging signal by using MB KDR was longi- tudinally measured during 6 days in tumors with ( n = 6) and without ( n = 6) antiangiogenic treatment (anti-VEGF antibody). Ex vivo VEGFR2 staining and microvessel den- sity analysis were performed. Signifi cant differences were evaluated ( t , Mann-Whitney, or Wilcoxon test). Results: Cell culture experiments showed four times greater bind- ing specifi city of MB KDR to human KDR and cross-reactivity to murine VEGFR2 ( P = .01). In vivo imaging signal was more than three times higher ( P = .01) with MB KDR compared with control MBs and decreased signifi cantly (approximately fourfold lower, P = .03) following in vivo receptor block- ing with anti-VEGFR2 antibody. One day after initiation of antiangiogenic therapy, imaging signal was signifi cantly decreased (approximately 46% lower, P = .02) in treated versus untreated tumors; it remained signifi cantly lower (range, 46%–84% decreased; P = .038) during the follow- ing 5 days. Microvessel density was signifi cantly reduced ( P = .04) in treated (mean, 7.3 microvessels per square millimeter 6 4.7 [standard deviation]) versus untreated tumors (mean, 22.0 microvessels per square millimeter 6 9.4); VEGFR2 expression was signifi cantly decreased ( . 50% lower, P = .03) in treated tumors. Conclusion: Human MB KDR allow in vivo imaging and longitudinal monitoring of VEGFR2 expression in human colon cancer xenografts.