The liposomal formulation of the photosensitizer meta-(tetrahydroxyphenyl) chlorin (m-THPC) was investigated for use in photodynamic therapy (PDT) of feline cutaneous squamous cell carcinoma. Liposomal m-THPC (0.15 mg/kg body weight) was administered intravenously to 18 cats with 20 cutaneous squamous cell carcinomas. All tumors were located on the head (primarily the nasal planum), most were early stage,and none of the cats had evidence of metastatic disease. PDT was performed either 16 hours (n = 9) or 6 hours (n = 11) after injection using a 652-nm diode laser for a treatment time of 200 seconds. Cats were anesthetized during illumination but not for drug injection. Acute local adverse effects were seen in 15% of the cats and included edema and erythema of the tumor and surrounding skin. These effects were mild. No general adverse effects were noted in any cats, and the injection seemed well tolerated. An initial complete response rate of 100% was seen with an overall 1-year control rate of 75%. The overall recurrence rate was 20% with median time to recurrence of 172.25 (+/- 87.1 days). Potential prognostic indicators-including tumor location, presence of adverse effects, and pre-PDT plasma concentrations of vascular endothelial growth factor-were investigated and found not to be significant. Liposomal m-THPC was successful for treatment of superficial cutaneous squamous cell carcinoma in cats.

COMMENTARY: In human medicine, photodynamic therapy (PDT) is more commonly used in Europe than in North America. Several reports on PDT in a variety of species, including snakes and birds, have been published. PDT is an effective treatment, particularly for oral squamous cell carcinomas in dogs and cutaneous squamous cell carcinomas in cats. The noteworthy benefit of this particular agent is the smaller amount of light required, which allows for much shorter treatment time. The major drawback of PDT is the cost of photosensitizing agents and light sources. -- Dudley McCaw, DVM, Diplomate ACVIM (Internal Medicine, Oncology)

Photodynamic therapy of feline cutaneous squamous cell carcinoma using a newly developed liposomal photosensitizer: Preliminary results concerning drug safety and efficacy. Buchholz J,Wergin M,Walt H, et al. J VET INTERN MED 21:770-775, 2007.