Photodynamic therapy (PDT) generates free radicals through the absorption o
f light by photosensitizers. PDT shows promise in the treatment of intimal
hyperplasia, which contributes to restenosis, by completely eradicating cel
ls in the vessel wall. This study investigates the mechanisms of PDT-induce
d cell death. PDT, using the photosensitizer chloroaluminum-sulfonated phth
alocyanine (1 mg/kg) and laser light (lambda = 675 nm) 100 J/cm(2) was admi
nistered to rat carotid arteries after balloon injury-induced intimal hyper
plasia. Apoptosis was determined by cell morphology with light microscopy a
nd transmission electron microscopy, DNA cleavage by terminal dUTP nick-end
labeling staining, and nucleosomal fragmentation (ladder pattern) by DNA a
garose gel electrophoresis. Four hours after PDT, apoptosis was observed in
vascular cells, as evidenced by terminal dUTP nick-end labeling staining a
nd transmission electron microscopy. Within 24 hours no cells were present
in the neointima and media. Immunofluorescence using an cu-smooth muscle ce
ll actin antibody confirmed the disappearance of all neointimal and medial
cells within 24 hours. No inflammatory cell infiltrate was observed during
this time frame. Apoptosis was sharply confined to the PDT treatment held.
These data demonstrate that vascular PDT induces apoptosis as a mechanism o
f rapid, complete, and precise cell eradication in the artery wall. These f
indings and the lack of inflammatory reaction provide the basis for underst
anding and developing PDT for a successful clinical application in the trea
tment of hyperplastic conditions such as restenosis.