PHOTOTOXICITY OF LUMIDOXYCYCLINE

Doxycycline (DOTC) is a photosensitizing drug whose mechanism of photo toxicity is complicated by the large variety of stable photoproducts f ormed. To assess the role of a DOTC photoproduct, lumidoxycycline (LuD OTC), in the photosensitization mechanism of DOTC, MGH-U1 human bladde r carcinoma cells were treated in vitro with either DOTC or LuDOTC, an d irradiated with the 351-nm emission of an argon-ion laser. Both DOTC and LuDOTC were phototoxic and caused radiant-exposure-dependent inhi bition of cellular incorporation of tritiated thymidine. On an absorbe d-photon basis, DOTC was about five times as phototoxic as LuDOTC. Cel lular uptake of DOTC was about five times as great as that of LuDOTC. Epifluorescence microscopy showed localization of LuDOTC predominantly within cellular membranes, particularly of mitochondria, as well as a low level of LuDOTC fluorescence diffusely within the cytoplasm. Epif luorescence microscopy of cells labeled with the mitochondrial probe, rhodamine 123, showed mitochondrial fragmentation and altered mitochon drial membrane integrity after LuDOTC photosensitization; these effect s depended on radiant exposure and were partially reversible by 24 h a fter irradiation. For both DOTC and LuDOTC phototoxicity was increased by irradiation in the presence of deuterium oxide and decreased in th e presence of sodium azide, effects consistent with an important mecha nistic role for singlet oxygen, O2(1DELTAg), in the injury. In solutio n, LuDOTC and DOTC had similar quantum yields for generation of O2(1DE LTAg) as measured by time-resolved spectroscopy and by O2(1DELTAg) tra pping. LuDOTC was photostable in solution, but DOTC underwent signific ant photodegradation. These data demonstrate that DOTC photoproducts s uch as LuDOTC have significant photobiologic activity and may play an important role in the phototoxicity mechanism of DOTC.