H-CRRETAWAC-OH, a lead structure for the development of radiotracer targeting integrin α5 β1?

Abstract

Imaging of angiogenic processes is of great interest in preclinical research as well as in clinical settings. The most commonly addressed target structure for imaging angiogenesis is the integrin αvβ3. Here we describe the synthesis and evaluation of [18F]FProp-Cys∗-Arg-Arg-Glu-Thr-Ala-Trp-Ala-Cys∗-OH, a radiolabelled peptide designed to selectively target the integrin α5β1. Conjugation of 4-nitrophenyl-(RS)-2-[18F]fluoropropionate provided [18F]FProp-Cys∗-Arg-Arg-Glu-Thr-Ala-Trp-Ala-Cys∗-OH in high radiochemical purity (>95%) and a radiochemical yield of approx. 55%. In vitro evaluation showed α5β1 binding affinity in the nanomolar range, whereas affinity to αvβ3 and αIIbβ3 was >50 M. Cell uptake studies using human melanoma M21 (αvβ3-positive and α5β1-negative), human melanoma M21-L (αvβ3-negative and α5β1-negative), and human prostate carcinoma DU145 (αvβ3-negative and α5β1-positive) confirmed receptor-specific binding. The radiotracer was stable in human serum and showed low protein binding. Biodistribution studies showed tumour uptake ranging from 2.5 to 3.5% ID/g between 30 and 120 min post-injection. However, blocking studies and studies using mice bearing α5β1-negative M21 tumours did not confirm receptor-specific uptake of [18F]FProp-Cys∗-Arg-Arg-Glu-Thr-Ala-Trp-Ala-Cys∗-OH, although this radiopeptide revealed high affinity and substantial selectivity to α5β1 in vitro. Further experiments are needed to study the in vivo metabolism of this peptide and to develop improved radiopeptide candidates suitable for PET imaging of α5β1 expression in vivo.