A real-time in vitro assay as a potential predictor of in vivo tumor imaging properties

INTRODUCTION: Selective tumor targeting strategies based on cell surface molecules enable new personalized diagnosis and treatments, potentially lowering adverse effects and increasing efficacy. Radio-immunotargeting generally relies on a molecule binding to a cancer-specific target. It is therefore important to understand the properties of molecular interactions in their working environment and how to translate these properties measured in vitro into the in vivo molecular imaging situation. METHODS: Time resolved interaction analysis in vitro was compared with a corresponding in vivo xenograft mouse model. The antibody fragment AbD15179 was labeled with (125)I or (111)In, and analyzed on cell lines with differing CD44v6 expression in vitro, and in a dual tumor xenograft model derived from the same cell lines. In vitro LigandTracer measurements were analyzed with TraceDrawer and Interaction Map. Conjugate sensitivity, kinetics, and signal-to-background ratios were assessed for both tumor cells in vitro and xenograft tumors in vivo. RESULTS: In vitro results revealed a general biphasic appearance of a high- and a low-affinity interaction event. The (111)In-labeled fragment displayed the largest proportion of the high-affinity interaction with increased sensitivity and retention compared to (125)I-Fab. In vivo results were in agreement with in vitro data, with increased retention, higher sensitivity and better contrast for the (111)In-labeled fragment compared to (125)I. CONCLUSIONS: Time resolved binding characteristics measured in vitro largely matched the in vivo performance for the conjugates, which is promising for future studies. In vitro time-resolved LigandTracer assays are efficient, rapid, and in this study shown to be able to predict in vivo outcomes. ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE: Further studies are needed to confirm these findings, but the method is promising considering the ethical need to reduce the use of laboratory animals, as well as reducing costs for the development of tumor targeting compounds in the future.