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Elemental selenium generated by the photobleaching of selenomerocyanine dyes forms conjugates with serum albumin and serum lipoproteins that are toxic to leukemia and selected solid tumor cells but well tolerated by normal CD34-positive hematopoietic stem and progenitor cells. Serum albumin and lipoproteins act as Trojan horses that deliver the cytotoxic entity (elemental selenium) to tumor cells as part of a physiological process. They exploit the fact that many tumors have an increased demand for albumin and/or low-density lipoprotein. Se(0)-protein conjugates are more toxic than selenium dioxide, sodium selenite, selenomethionine, or selenocystine. They are only minimally affected by drug resistance mechanism, and they potentiate the cytotoxic effect of ionizing radiation and several standard chemotherapeutic agents. The cytotoxic mechanism of Se(0)-protein conjugates is not yet fully understood. Currently available data are consistent with the notion that Se(0)-protein conjugates act as air oxidation catalysts that cause a rapid depletion of intracellular glutathione and induce apoptosis. Drugs modeled after our Se(0)-protein conjugates may prove useful for the local and/or systemic therapy of cancer.

Many Type II photosensitizers are substrates of the singlet oxygen they generate. They are, therefore, converted to photoproducts when exposed to light in the presence of oxygen. While most photoproducts appear to be biologically inert, one of the photoproducts generated by selenomerocyanine photosensitizers is a form of elemental selenium that is surprisingly toxic to tumor cells if allowed to form conjugates with serum albumin or lipoproteins. Albumin and lipoproteins act as delivery vehicles for the cytotoxic entity, exploiting the fact that many tumor cells have an increased capacity to bind and internalize albumin and/or lipoproteins. The cytotoxic mechanism of Se(0)-protein conjugates is not yet fully understood but is obviously quite different from the singlet oxygen-mediated mechanism of merocyanine-mediated photodynamic therapy (PDT). Whereas merocyanine-PDT is directed against the plasma membrane, is more effective at 4 °C than at room temperature, and is inhibited by excess albumin and lipoproteins, selenomerocyanine-derived cytotoxic photoproducts act on intracellular targets, are ineffective at low temperatures, and require albumin or lipoproteins as carrier molecules. The discovery of cytotoxic Se(0)-protein conjugates provides a rare example of photoproducts contributing substantially to the cytotoxic effect of PDT and challenges the widely held notion that elemental selenium is biologically inert.

The efficacy of chloroaluminum phthalocyanine (AlPc) for photodynamic therapy (PDT) has been evaluated in vitro on acute myeloid leukemia (AML) cells, normal peripheral blood leukocytes (PBL) and mobilized peripheral blood stem cells (mPBSC). The selectivity of the treatment has been evaluated by mixing PBL and TF-1, an erythroleukemic cell line. Upon photoradiation, this photosensitizer leads to a significant and preferential photokilling of leukemia cells in comparison to normal cells. The use of stimulated lymphocytes in PBL/TF-1 mixtures instead of resting cells also leads to a preferential killing towards TF-1 although activated PBL are more affected than resting PBL. The analysis of AlPc intracellular emission by flow cytometry shows that the uptake of the dye by leukemia cells is faster. This good efficacy towards AML and the observed lower phototoxicity towards normal cells (PBL, normal progenitors) suggest that this phthalocyanine is a potential bone marrow purging agent.

The photodynamic therapy (PDT) efficiency of five phthalocyanines, chloroaluminum phthalocyanine (AlPc), dichlorosilicon phthalocyanine (SiPc), bis(tri-n-hexylsiloxy)silicon phthalocyanine (PcHEX), bis(triphenylsiloxy)silicon phthalocyanine (PcPHE) and nickel phthalocyanine (NiPc), was assessed on two leukemic cell lines TF-1 and erythroleukemic and B lymphoblastic cell lines, Daudi, respectively. AlPc showed the best photocytotoxicity leading to 0.008 surviving fraction at 2 x 10(-9) M for TF-1 and 4 x 10(-9) M for Daudi. A1 5 x 10(-7) M, SiPc and PcHEX induced a significant photokilling, whereas NiPc and PcPHE were inactive. Laser flash photolysis and photoredox properties of the phthalocyanines were investigated to try to relate these parameters with the biological effects. AlPc showed the longest triplet life-time: 484 microseconds in dimethyl sulfoxide/H2O. This value was increased up to 820 microseconds when AlPc was complexed with human serum albumin used as a membrane model. Such an enhancement was not observed with the silicon phthalocyanines. Upon irradiation, all the phthalocyanines generated singlet oxygen with 0.29-0.37 quantum yield values. The reduction potentials of the excited states obtained from measurement in the ground state and energy of the excited triplets show that AlPc is the best electron acceptor. The in vitro photocytotoxicity observed and the measured parameters are in agreement with a key role of electron transfer in PDT assays involving these phthalocyanines.