Photodynamic therapy of cottontail rabbit papillomavirus-induced papillomas in a severe combined immunodeficient mouse xenograft system

Abstract

Objectives/hypothesis: To evaluate the efficacy of photodynamic therapy (PDT) with the phthalocyanine photosensitizer Pc 4 for treating an animal model of recurrent respiratory papillomatosis (RRP).

Methods: Rabbit skin was grafted onto the dorsum of severe combined immunodeficient mice, two xenografts per animal. After the graft healed, it was inoculated with cottontail rabbit papillomavirus (CRPV). When papillomas developed, Pc 4 (0.6 or 1.0 mg/kg) was administered systemically, and 48 hours later, one papilloma of the two on each animal was exposed to 675-nm photoactivating light at either 100 or 150 J/cm(2). In addition to the contralateral tumors, which received Pc 4 but no light, other controls included animals receiving light only or neither agent. Response was assessed by measuring papilloma size with a caliper. Some papillomas and residual skin were harvested for histological assessment.

Results: For the lower-dose PDT regimens, papilloma growth rates were not significantly different from the controls. In contrast, 13 of 15 papillomas receiving the higher Pc 4 dose (1.0 mg/kg) and the higher light fluence (150 J/cm(2)) regressed completely and did not regrow within the observation period of up to 79 days. The response of these papillomas was significantly different from the controls (P < .001). Histological analysis confirmed the absence of residual tumor following complete response and replacement with near-normal epithelium.

Conclusions: Pc 4-PDT is highly effective in treating virally induced (CRPV) papillomas in a murine model of RRP, and thus warrants further study as a treatment for HPV-induced papillomas.

Fig. 1

Experimental design. SCID = severe combined immunodeficient; CRPV = cottontail rabbit papillomavirus.

Fig. 2

Rabbit xenografts in severe combined immunodeficient mice. Appearance of successful xenografts at approximately 2 weeks (A) and 10 weeks (B) postimplantation.

Fig. 3

Growth of papillomas and effect of treatments. (A) Growth of papillomas in treatment groups given both Pc 4 and light, and in controls with no Pc 4, or with Pc 4 or laser light only. For each treatment group, the mean volumes ± one standard error are plotted against post-treatment time. (B) Growth of individual papillomas in treatment group PC1.0L150. The size of each papilloma is expressed as a percentage of its size at time 0. To better show the data, the vertical axis is cut off at 300%. One tumor (line with arrow, asterisk) increased rapidly above 300% after week 4. *Values beyond week 4 are not shown because the animal that harbored this tumor was euthanized according to protocol at week 7 due to growth of the treated and contralateral tumors above 15% of the body weight of the animal. **This tumor regressed to near 0 but eventually regrew. All others regressed to 0 volume and did not regrow within the 10-week post-treatment observation period.

Fig. 4

Regression of photodynamic therapy-treated papilloma. The white arrow shows normal epithelium on gross examination after papilloma regression; the black arrow points to a dense keratin horn of a nonirradiated papilloma (Pc 4 only).

Fig. 5

Representative pretreatment photomicrographof virus-induced papilloma on xenografted severe combined immunodeficient mouse. (A) Low-power hematoxylin and eosin (H&E) stain of papilloma with an overlying dense keratin cap. (B) High-power H&E stain showing typical histological features of robust papilloma growth with diffuse hyperkeratosis and parakeratosis.

Fig. 6

Representative post-treatment photomicrograph of virus-induced papilloma on xenografted severe combined immunodeficient mouse. (A) Low-power hematoxylin and eosin (H&E) stain of thin but normal-appearing epithelial layer following regression of the tumor. Mild stromal changes consistent with skin grafting and photodynamic therapy are observed. (B) High-power H&E stain of normal keratinizing squamous epithelium with focal cellular changes consistent with early viral infection (arrow).