Photodynamic therapy of naturally occurring tumors in animals using a novel benzophenothiazine photosensitizer

5-Ethylamino-9-diethylaminobenzo[a]phenothiazinium chloride (EtNBS) is a novel photodynamic therapy (PDT) photosensitizer with efficacy against experimental murine tumors. In this preliminary study, dogs and cats with naturally occurring tumors were treated with EtNBS-PDT to determine safety and efficacy. Fifteen treatments were performed on 13 animals (9 treatments in 8 cats and 6 treatments in 5 dogs), generally using 400 J of 652 nm light. Two feline sublingual squamous cell carcinomas (SCCs) responded briefly (minor response), Sis feline facial SCCs were treated, resulting in two partial responses and four long-term complete responses (CR), Two canine intraoral SCCs were treated; one responded minimally for 2 weeks (minor response), and one achieved long-term CR, One canine cutaneous mast cell tumor achieved CR, and one canine ocular mast cell. tumor responded briefly, One canine ocular melanoma did not respond to treatment. Systemic reactions included nausea associated with photosensitizer injection in two cats and two dogs, elevated body temperatures during treatment in two dogs, elevated body temperature 2 days after PDT in one cat, and inappetance for 2 weeks in one cat. A peripheral neuropathy of undetermined cause occurred in one cat 2 weeks after PDT and resolved without treatment, Local reaction was well tolerated in 13 of 15 treatments. All animals were exposed to normal daylight after less than 5 days (mean, 3.5 days) without residual photosensitization, EtNBS-PDT is safe for dogs and cats and has activity against selected naturally occurring tumors, with an overall objective response rate (partial response + CR) of 61.5%.