The impact of aggregation on the biodistribution of hypericin

Hypericin is a potent agent in the photodynamic therapy of cancers and accumulates to a large extent in tumor tissue. To better understand the impact of hypericin aggregates present in the delivery vehicle on the biodistribution of the compound, we compared the in vivo tissue accumulation after administering hypericin suspended as coarse aggregates in phosphate-buffered saline, with the biodistribution found after injection of a solution of hypericin in a mixture of DMSO, polyethylene glycol and water. When administered as coarse aggregates, hypericin showed a pronounced uptake in liver, spleen and lung and a slow body clearance with a complete decline in tumor/normal tissue ratios (far less than 1). In contrast, delivery of hypericin as a solution resulted in dramatically improved tumor to normal tissue ratios and a relatively fast elimination from the body. To elucidate the exact localization of hypericin in both conditions, a fluorescence microscopy study was performed on sections of spleen, liver, lung and tumor tissue. At 24 h after injection, fluorescence in spleen, liver and lung was faint and homogeneous for dissolved hypericin, whereas bright fluorescent spots covering the entire tissue sections were found when coarse aggregates were injected. We found that aggregates get trapped within these tissues, followed by a gradual monomerization. A direct involvement of monocytes and macrophages, however, could not be demonstrated. In conclusion, it is of critical importance that the delivery vehicle prevents extensive aggregation of hypericin before injection and assures an efficient transfer to serum lipoproteins upon injection. These results may also be extended to radiolabeled derivatives and other lipophilic photosensitizers, such as porphyrins, phthalocyanines, naphthalocyanines and chlorines, with similar aggregation properties.