Doxycycline (
DOTC) is a photosensitizing
drug whose mechanism of
phototoxicity is complicated by the large variety of stable photoproducts formed. To assess the role of a
DOTC photoproduct,
lumidoxycycline (
LuDOTC), in the
photosensitization mechanism of
DOTC, MGH-U1 human bladder
carcinoma cells were treated in vitro with either
DOTC or
LuDOTC, and irradiated with the 351-nm emission of an
argon-ion laser. Both
DOTC and
LuDOTC were phototoxic and caused radiant-exposure-dependent inhibition of cellular incorporation of tritiated
thymidine. On an absorbed-photon basis,
DOTC was about five times as phototoxic as
LuDOTC. Cellular 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. Epifluorescence microscopy of cells labeled with the mitochondrial probe,
rhodamine 123, showed mitochondrial fragmentation and altered mitochondrial membrane integrity after
LuDOTC photosensitization; these effects depended on radiant exposure and were partially reversible by 24 h after irradiation. For both
DOTC and
LuDOTC,
phototoxicity was increased by irradiation in the presence of
deuterium oxide and decreased in the presence of
sodium azide, effects consistent with an important mechanistic role for
singlet oxygen, O2(1 delta g), in the injury. In
solution,
LuDOTC and
DOTC had similar quantum yields for generation of O2(1 delta g) as measured by time-resolved spectroscopy and by O2(1 delta g) trapping.
LuDOTC was photostable in
solution, but
DOTC underwent significant photodegradation. These data demonstrate that
DOTC photo-products such as
LuDOTC have significant photobiologic activity and may play an important role in the
phototoxicity mechanism of
DOTC.