STRATEGIES FOR SELECTIVE CANCER PHOTOCHEMOTHERAPY: ANTIBODY‐TARGETED and SELECTIVE CARCINOMA CELL PHOTOLYSIS*

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Abstract
A principle objective in chemotherapy is the development of modalities capable of selectively destroying malignant cells while sparing normal tissues. One new approach to selective photochemotherapy, antibody-targeted photolysis (ATPL) uses photosensitizers (PS) coupled to monoclonal antibodies (MAbs) which bind to cell surface antigens on malignant cells. Selective destruction of human T leukemia cells (HBP-ALL) was accomplished by coupling the efficient PS chlorin e6 to an anti-T cell MAb using dextran carriers. Conjugates with chlorin: MAb ratios of 30: 1 retained > 85% MAb binding activity, and had a quantum yield for singlet oxygen production of 0.7 .+-. 0.1, the same as that of free chlorin e6. Cell killing was dependent on the doses of both MAb-Ps and 630-670 nm light, and occurred only in target cell populations which bound the MAb. On the order of 1010 singlet oxygen molecules were necessary to kill a cell. A second approach to specific photochemotherapy, selective, carcinoma cell photolysis (SCCP), relies on preferential accumulation of certain cationic PS by carcinoma cell mitochondria. We have evaluated several classes of cationic dyes, and in the case of N,N''-bis-(2-ethyl-1,3-dioxolane)-kryptocyanine (EDKC) and some of its analogs, have demonstrated highly selective killing of human squamous cell, bladder and colon carcinoma cells in vitro. In isolated mitochondria, EDKC uptake and fluorescence depended on membrane potential, and the dye specifically photosensitized damage to Complex I in the electron transport chain. N,N''-bis-(2-ethyl-1,3-dioxolane)-kryptocyanine and some of its analogs accumulated within subcutaneous xenografts of human tumors in nude mice with tumor: skin ratios > 8. Photoirradiation caused significant inhibition of tumor growth, without cutaneous phototoxicity.