The use of a cocktail of single chain Fv antibody fragments to improve the in vitro and in vivo targeting of melanoma

Abstract

Radioscintigraphy using single chain antibody fragments (scFvs) offers a potential means of early detection of melanoma metastases. However, previous studies have shown suboptimal levels of tumour localization and non-specific background accumulation which may be due to antigen heterogeneity. We aimed to improve tumour localization by using a cocktail of different scFvs targeting different epitopes on melanoma cells. We have previously developed three scFvs against distinct and highly tumour-specific melanoma cell-surface antigens by chain shuffling and antibody phage selection on melanoma cells. Three scFvs, RAFT3, B3 and B4 were labeled with 125Iodine and tested both individually and as a cocktail in a nude mouse xenograft model for human melanoma. Results demonstrated improved tumour localization in vivo when compared to the individual scFvs. Tumour uptake of the cocktail at 1 hour was 24.220% ID/g (injected dose/gram) compared with 2.854%, 2.263% and 1.355% for B4, RAFT3 and B3, respectively, when injected individually. In addition, the cocktail exhibited significantly superior tumour to normal tissue ratios for muscle and spleen (p<0.05). A combination or 'cocktail' of scFv clones may have an advantage over individual scFvs for melanoma targeting in patients because of heterogeneity in the expression of different epitopes of antigens on melanoma cells.