PHOTODYNAMIC THERAPY OF NATURALLY-OCCURRING TUMORS IN ANIMALS USING ANOVEL BENZOPHENOTHIAZINE PHOTOSENSITIZER

5-Ethylamino-9-diethylaminobenzo[a]phenothiazinium chloride (EtNBS) is a novel photodynamic therapy (PDT) photosensitizer with efficacy agai nst experimental murine tumors. In this preliminary study, dogs and ca ts with naturally occurring tumors were treated with EtNBS-PDT to dete rmine safety and efficacy. Fifteen treatments were performed on 13 ani mals (9 treatments in 8 cats and 6 treatments in 5 dogs), generally us ing 400 J of 652 nm light. Two feline sublingual squamous cell carcino mas (SCCs) responded briefly (minor response), Sis feline facial SCCs were treated, resulting in two partial responses and four long-term co mplete responses (CR), Two canine intraoral SCCs were treated; one res ponded minimally for 2 weeks (minor response), and one achieved long-t erm CR, One canine cutaneous mast cell tumor achieved CR, and one cani ne ocular mast cell. tumor responded briefly, One canine ocular melano ma did not respond to treatment. Systemic reactions included nausea as sociated with photosensitizer injection in two cats and two dogs, elev ated body temperatures during treatment in two dogs, elevated body tem perature 2 days after PDT in one cat, and inappetance for 2 weeks in o ne cat. A peripheral neuropathy of undetermined cause occurred in one cat 2 weeks after PDT and resolved without treatment, Local reaction w as well tolerated in 13 of 15 treatments. All animals were exposed to normal daylight after less than 5 days (mean, 3.5 days) without residu al photosensitization, EtNBS-PDT is safe for dogs and cats and has act ivity against selected naturally occurring tumors, with an overall obj ective response rate (partial response + CR) of 61.5%.