Vascular shut down effect on the microcirculation in photodynamic therapy using zinc coproporphyrin

The effect of photodynamic therapy (PDT) using Zn coproporphyrin III (Zn CP-III) on the microcirculation was examined. Animals were given Zn CP-III doses of 3 mg/kg or 5 mg/kg. Microvessels in the mesentery were exposed to 450 J/cm² laser light of Nd:YAG at 532 nm. Significant reductions in blood flow and complete blood flow stasis (vascular shut down: VSD) were observed during light treatment. VSD was a result of platelet aggregation and the mechanical obstruction of flow. In venules, VSD was more prominent than in arterioles, resulting in stasis after 300-350 J/cm² light treatment. VSD was dependent on Zn CP-III dose and initial vessel diameter. Animals given only either Zn CP-III or light treatment showed no apparent change in vessels. The authors conclude that the photoactivation of Zn CP-III results in significant thrombosis of normal vessels and it may contribute to the mechanism by which PDT results in tumor necrosis