Discussion

Molecular imaging agents enable non-invasive identification and staging of tumours. To increase preclinical knowledge and limit laboratory animal use, it is important to evaluate conjugate kinetics in relevant in vitro model systems. We investigated if a real-time kinetic cell assay could estimate in vivo imaging properties for a radiolabelled antibody fragment.
Fab Abd15179 that targets CD44v6, an antigen overexpressed in aggressive head & neck squamous carcinoma (SCC), was conjugated to either 111In (CHX-A”-DTPA) or 125I (Chloramine T). Interactions were measured in vitro with LigandTracer on SCC with high (A431) and intermediate (H314) CD44v6 expression. The same batch of 111In- or 125I-Fab conjugates was injected in twenty mice bearing both A431 and H314 tumour xenografts to evaluate tumour uptake, sensitivity and specificity without inter-subject variability.
AbD15179 bound to cells with one high-affinity and one low-affinity interaction. 111In-Fab dissociated slower from tumour cells than 125I-Fab, and clear differences in binding signal between cell lines were present for 111In-Fab but not for 125I-Fab. In accordance with in vitro results, 111In-Fab dissociated slower from tumours than 125I-Fab and made it possible to distinguish between A431 and H314 xenografts in vivo.
Time-resolved LigandTracer cell assays could adequately predict the in vivo performance for two conjugates on two xenograft models. The method therefore shows initial promise to reduce the use of laboratory animals and costs for future development of tumour targeting compounds.