Potential new photosensitizers for photodynamic therapy

In continuation of the effort to search for an ideal photosensitizer, two groups of potential new photosensitizers were synthesized and investigated for their photodynamic actions against tumors in mice. These were derivatives of methyl pheophorbide-a and of silicon naphthalocyanine. Of the former group, the 2(1-O-hexyl)ethyl-desvinyl-methyl pheophorbide-a, or HEDP, was the most active sensitizer. HEDP could be readily produced in large quantities and showed an optimum photodynamic action at 665 nm where it absorbs strongly. Also HEDP was cleared from the mouse skin within 4 days after administration, thus possibly alleviating the long-term phototoxic side-effects observed in Photofrin-based therapy. Of the second group of photosensitizers, the bis(dimethyl hydroxypropylsiloxy)silicon naphthalocyanine (HPSiNc), and the corresponding acetoxy derivative (APSiNc) were of particular interest. At a drug-light dose of 1.0 mg/kg-135 J/cm² (delivered by a laser at 772 nm), they showed antitumor activities comparable to that of Photofrin. Further studies on these photosensitizers are warranted.