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Receptor-Mediated DNA-Targeted Photoimmunotherapy

¹ Molecular Radiation Laboratory, Research Division, Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia and ² Department of Pathology and ³ School of Chemistry, The University of Melbourne, Parkville, Victoria, Australia

Requests for reprints: Roger F. Martin, Peter MacCallum Cancer Centre, Research Level 2, Locked Bag No. 1, A'Beckett Street, East Melbourne, Victoria, Australia. E-mail: roger.martin{at}petermac.org.

We show the efficacy of a therapeutic strategy that combines the potency of a DNA-binding photosensitizer, UV_ASens, with the tumor-targeting potential of receptor-mediated endocytosis. The photosensitizer is an iodinated bibenzimidazole, which, when bound in the minor groove of DNA and excited by UV_A irradiation, induces cytotoxic lesions attributed to a radical species resulting from photodehalogenation. Although reminiscent of photochemotherapy using psoralens and UV_A irradiation, an established treatment modality in dermatology particularly for the treatment of psoriasis and cutaneous T-cell lymphoma, a critical difference is the extreme photopotency of the iodinated bibenzimidazole, 1,000-fold that of psoralens. This feature prompted consideration of combination with the specificity of receptor-mediated targeting. Using two in vitro model systems, we show the UV_A cytotoxicity of iodo ligand/protein conjugates, implying binding of the conjugate to cell receptors, internalization, and degradation of the conjugate-receptor complex, with release and translocation of the ligand to nuclear DNA. For ligand-transferrin conjugates, phototoxicity was inhibited by coincubation with excess native transferrin. Receptor-mediated UV_A-induced cytotoxicity was also shown with the iodo ligand conjugate of an anti-human epidermal growth factor receptor monoclonal antibody, exemplifying the potential application of the strategy to other cancer-specific targets to thus improve the specificity of phototherapy of superficial lesions and for extracorporeal treatments. (Cancer Res 2006; 66(21): 10548-52)