Effect of aspirin on photodynamic therapy utilizing chloroaluminum sulfonated phthalocyanine (CASP)

The efficacy of photodynamic therapy (PDT) is mediated through a direct vascular effect. Interference with platelet function and resulting vascular stasis have been recently demonstrated utilizing the photosensitizer dihematoporphyrin ether (DHE). We examined the effect of aspirin, a known inhibitor of both cyclooxygenase and platelet activity, on PDT using chloroaluminum sulfonated phthalocyanine (CASP). Thirty-six rats implanted with a window chamber were given either 0.1 mg/kg (low dose) or 10 mg/kg (high dose) aspirin immediately before, immediately after, or 6 hours after the completion of CASP-PDT. Aspirin in either dosage did not appear to have any effect on the window vasculature when given immediately after light exposure. A moderate inhibition of vascular response was seen in animals treated with aspirin pre-PDT, whereas high-dose aspirin completely abrogated the CASP-PDT vascular response when given 6 hours post-PDT. These data indicate that aspirin can effect CASP-PDT in both time-dependent and dose-dependent fashions.