Internalisation enhances photo-induced cytotoxicity of monoclonal antibody-phthalocyanine conjugates.

Immunophototherapy of cancer combines the specificity of a monoclonal antibody (MAb) to an overexpressed tumor marker with the phototoxic properties of the conjugated dye. To analyze the potential role of internalisation of the dye on photo-induced cytotoxicity, we compared two target antigens, carcinoembryonic antigen (CEA) that does not internalise and ErbB2 that does. Human ovarian carcinoma SKOv3 cells that express a high level of ErbB2 were transfected with the CEA cDNA. Using FACS analysis, the resulting cell line, SKOv3-CEA-1B9, demonstrated comparable levels of expression of the two target antigens. Aluminium tetrasulfophthalocyanine (AlPcS(4)) was covalently coupled to anti-CEA MAb 35A7, anti-ErbB2 MAb FSP77 and a non-specific MAb PX, via a five-carbon sulfonamide spacer chain (A(1)) at molar ratios ranging from 6 to 9 moles of AlPcS(4) per mole of MAb. The 35A7-(AlPcS(4)A(1))(8) conjugate induced 68% growth inhibition of the SKOv3-CEA-1B9 cell line after a 20 h incubation at 2.50 microg/ml (based on AlPcS(4)A(1) content) following light exposure. However, the FSP77-(AlPcS(4)A(1))(6) conjugate gave a 51% growth inhibition for an AlPcS(4)A(1) concentration as low as 0.04 microg/ml after the same incubation time and exposure to the same light dose. At a 1.25 microg/ml AlPcS(4)A(1) concentration, the FSP77-(AlPcS(4)A(1))(6) conjugate gave a 67% growth inhibition after an incubation time as short as 1 h, reaching a 96% inhibition after an 8 h incubation time. Using an unique cell line that expresses two different target antigens, we demonstrated a clear advantage of an internalising over a non-internalising MAb-dye conjugate in terms of phototoxic efficacy. In vivo evaluation of the photodynamic properties of the conjugates is in progress.