Binding of αβ Integrin-Specific Radiotracers Is Modulated by Both Integrin Expression Level and Activation Status.

Purpose: Molecular imaging of αβ integrin has exhibited real potential to guide the appropriate use of anti-angiogenic therapies. However, an incomplete understanding of the factors that influence binding of αβ integrin-specific radiotracers currently limits their use for assessing response to therapy in cancer patients. This study identifies two fundamental factors that modulate uptake of these radiotracers. Procedures Experiments were performed in prostate cancer (PC3) and glioblastoma (U87MG) cells, which differentially express αβ integrin. αβ integrin-specific radiotracers were used to investigate the effect of manipulating αβ integrin expression or activation in cellular binding assays. β integrin and αβ integrin expression were measured by western blotting and flow cytometry, respectively. The effect of select pharmacological inhibitors on αβ integrin activation and expression was also determined.

Results: Radiotracer binding was proportional to αβ integrin expression when it was decreased (β knock-down cells) or increased, either using pharmacological inhibitors of cell signalling or by culturing cells for different times. Studies with both small molecule and arginine-glycine-aspartic acid (RGD)-based radiotracers revealed increased radiotracer binding after activation of αβ integrin with Mn or talin head domain. Moreover, inhibition of fundamental signalling pathways (mitogen-activated protein kinase kinase (MEK), Src and VEGFR2) decreased radiotracer binding, reflecting reduced αβ integrin activity.

Conclusion: Binding of small molecule ligands and radiolabelled RGD peptides is modulated by expression and activation status of αβ integrin. αβ integrin-specific radiotracers can provide otherwise inaccessible information of the effect of signalling pathways on αβ integrin. This has significant implications for assessing response to anti-angiogenic therapies in clinical studies.