Human serum albumin as key mediator of the differential accumulation of hypericin in normal urothelial cell spheroids versus urothelial cell carcinoma spheroids.

Hypericin is a bright red fluorescent compound that can be used in urological medicine as a photodiagnostic to detect non-muscle-invasive bladder cancer lesions. To this end a bladder instillation fluid is prepared in which the water-insoluble hypericin is solubilized by the presence of human serum albumin (HSA) to which the compound binds. In the present study, we explored the possibility that besides acting as a passive hypericin carrier, HSA also actively contributes to the selective localization of the compound. By using multicellular spheroids prepared from normal human urothelial (NHU) cells and from different urothelial carcinoma cell (UCC) lines (T24, RT-112 and RT-4), we simulated three-dimensionally the normal urothelium and urothelial cell carcinomas present in the bladder of patients. The distribution of hypericin in these spheroids was investigated in the presence or absence of HSA. Our data show that when hypericin is solubilized by HSA, an excellent differentiation in distribution of hypericin in normal urothelial spheroids and malignant spheroids is observed, clearly suggesting a key role for albumin in the specific localization of hypericin in non-muscle-invasive bladder tumours. Furthermore, PDT results show that both the hypericin-PDT effect on tumour spheroids and the selective character of the treatment can significantly be increased by the presence of HSA. Interestingly, we also observed that the presence of HSA did not convey tumouritropic characteristics to other photosensitizers like pheophorbide a and mTHPP, implying that both the particular characteristics of the photosensitizer and HSA contribute to the final selective accumulation of the compound in tumoural tissue.