Lack of comparability between binding of monoclonal antibodies to melanoma cells in vitro and localization in vivo.

The affinity and specificity of monoclonal antibodies (MAbs) to tumor-associated antigens are often determined by in vitro assays. The specific binding of two anti-human melanoma antibodies (96.5 and ZME-018) and a control antibody (ZCE-025) to three human melanoma cell lines (DX3, A375-M, and Hs294t) was examined under in vitro conditions and compared to in vivo localization of 111In-labeled antibodies to the same cells growing as solid tumors in the subcutis of nude mice. The in vitro binding of the specific MAbs to the tumor cells did not predict in vivo localization. Under in vitro conditions, MAb ZME-018 bound to all three cell lines at levels exceeding that of 96.5, yet ZME-018 did not show superior localization to subcutaneous tumors. MAb 96.5 bound to cultured DX3 cells at levels exceeding those observed with A375-M cells. Yet, 96.5 localized better to A375-M xenografts in nude mice than to DX3 or Hs294t xenografts. Antigen expression differed between in vitro and in vivo growing cells, as evidenced by alteration in binding of 96.5 to tumor cells dissociated from solid subcutaneous tumors. Collectively, the data suggest that in vitro parameters do not predict the clinically relevant localization of MAbs to tumors.