Evaluation of a photodynamic therapy agent using a canine prostate cancer model

Abstract

Background: Male dogs can develop spontaneous prostate cancer, which is similar physiologically to human disease. Recently, Tweedle and coworkers have developed an orthotopic canine prostate model allowing implanted tumors and therapeutic agents to be tested in a more translational large animal model. We used the canine model to evaluate prostate-specific membrane antigen (PSMA)-targeted gold nanoparticles as a theranostic approach for fluorescence (FL) imaging and photodynamic therapy (PDT) of early stage prostate cancer.

Methods: Dogs (four in total) were immunosuppressed with a cyclosporine-based immunosuppressant regimen and their prostate glands were injected with Ace-1-hPSMA cells using transabdominal ultrasound (US) guidance. Intraprostatic tumors grew in 4-5 weeks and were monitored by ultrasound (US). When tumors reached an appropriate size, dogs were injected intravenously (iv) with PSMA-targeted nano agents (AuNPs-Pc158) and underwent surgery 24 h later to expose the prostate tumors for FL imaging and PDT. Ex vivo FL imaging and histopathological studies were performed to confirm PDT efficacy.

Results: All dogs had tumor growth in the prostate gland as revealed by US. Twenty-four hours after injection of PSMA-targeted nano agents (AuNPs-Pc158), the tumors were imaged using a Curadel FL imaging device. While normal prostate tissue had minimal fluorescent signal, the prostate tumors had significantly increased FL. PDT was activated by irradiating specific fluorescent tumor areas with laser light (672 nm). PDT bleached the FL signal, while fluorescent signals from the other unexposed tumor tissues were unaffected. Histological analysis of tumors and adjacent prostate revealed that PDT damaged the irradiated areas to a depth of 1-2 mms with the presence of necrosis, hemorrhage, secondary inflammation, and occasional focal thrombosis. The nonirradiated areas showed no visible damages by PDT.

Conclusion: We have successfully established a PSMA-expressing canine orthotopic prostate tumor model and used the model to evaluate the PSMA-targeted nano agents (AuNPs-Pc158) in the application of FL imaging and PDT. It was demonstrated that the nano agents allowed visualization of the cancer cells and enabled their destruction when they were irradiated with a specific wavelength of light.

Keywords: PDT; PSMA; canine prostate cancer; fluorescence imaging; nanoparticles.

Figure 1.

Binding of cells to ³H-ZJ24. Cells (1×10⁶) were incubated with various concentrations of ³H-ZJ24 at 37°C. After 1 hour of incubation, cells were washed with cold Tris buffer, and radioactivity of the cell pellet was measured. Data represent mean±SD of triplicates.

Figure 2.

Representative fluorescence images of prostate and Ace-1-hPSMA prostate tumor (Dog #3). A. Surgical site showing the ventral prostate and tumor. B. Prostate gland and tumor in vivo under white light (left), in vivo fluorescence (FL) image of the prostate gland and tumor (middle) and merged FL image with color image showing the highlight of tumor by nano agents (right).

Figure 3.

Representative fluorescence images of PDT treatment for prostate tumor (Dog #3). A. In vivo FL image of prostate tumor before PDT and after PDT treatment, with most of the tumor and majority of the prostate gland covered. B. Ex vivo FL image of whole prostate after PDT (all cover removed; White circles indicates non-irradiated tumor area, green circles indicates irradiated area and blue circles indicates normal prostate gland.)

Figure 4.

Representative H&E images of prostate and prostate tumor from dog #3. A. Prostate tumor after PDT treatment. The black dye is a surface dye used to localize the region of PDT laser treatment. The tumor near the surface had necrosis, hemorrhage, and secondary inflammation. The deeper tumor was viable. B. Normal adjacent prostate with benign prostatic hyperplasia and PDT therapy. There was no evidence of necrosis as a result of the PDT therapy.