HP and cancer: Most of them are coloured dyes, such as derivatives of acridines29,30–37, flavins phenothiazines38–49, quinolones50–60, cyanines49,61–69, and biological compounds of hematoporphyrin (Hp) and its precursors, like porphyrin (Pf)70, 5-amino levulinic acid (ALA)71–73, etc. The ideal photosensitizer should: (i) accumulate specifically in cancer cells, sparing normal cells; (ii) kill only cancer cells after light beam irradiation and (iii) be not toxic for the human body, even after irradiation.