AIM: To investigate the anti-tumor effects of combined cytotoxic drug (gemcitabine) and photodynamic therapy (PDT) on human pancreatic cancer xenograft in nude mice.

METHODS: Human pancreatic cancer cell line SW1990 was used in the investigation of the in vivo effect of combined gemcitabine and PDT on human pancreatic cancer xenograft in mice. Sixty mice were randomly allocated into a control group (without treatment), photosensitizer treatment group (2 mg/kg photosan, without illumination), chemotherapy group (50 mg/kg gemcitabine i.p.), PDT group (2 mg/kg photosan + laser irradiation) and combined treatment group (photosan + chemotherapy), with 12 mice in each group. Tumor size was measured twice every week. Anti-tumor activity in different groups was evaluated by tumor growth inhibition (TGI).

RESULTS: No significant anti-tumor effect was observed either in photosensitizer treatment group or in chemotherapy group. PDT led to necrosis in cancer lesions and significantly reduced tumor volume compared with photosensitizer on day 6 and at the following time points after initialization of therapy (0.24 ± 0.15-0.49 ± 0.08 vs 0.43 ± 0.18-1.25 ± 0.09, P < 0.05). PDT significantly reduced tumor volume in combined treatment group compared with photosensitizer treatment group (0.12 ± 0.07-0.28 ± 0.12 vs 0.39 ± 0.15-1.20 ± 0.11, P < 0.05), small dose chemotherapy group (0.12 ± 0.07-0.28 ± 0.12 vs 0.32 ± 0.14-1.16 ± 0.08, P < 0.05) and control group (0.12 ± 0.07-0.28 ± 0.12 vs 0.43 ± 0.18-1.25 ± 0.09, P < 0.05). TGI was higher in the combined treatment group (82.42%) than in the PDT group (58.18%).

CONCLUSION: PDT has a significant anti-tumor effect, which is maintained for a short time and can be significantly enhanced by small doses of gemcitabine.

Pancreatic cancer remains a lethal disease. Most pancreatic cancer patients are at advanced stage when apparent symptoms occur. Pancreatic cancer patients undergoing resection at the time of initial diagnosis account for less than 20%[1], and the 5-year survival rates after complete and partial pancreatic resection are 15%-25%[23] and 8%-14%[45], respectively. Treatment modalities for inoperable patients are largely limited to radiotherapy, chemotherapy, or their combination. Pancreas is a retroperitoneal organ, with stomach, intestine, and spinal cord around it. Since the sensitivity of pancreatic cancer to radiotherapy is poor, and the tolerant dosage of pancreas tissue is low, radiotherapy does not lead to a convincing beneficial effect on survival. Chemotherapy is the main therapeutic method for advanced pancreatic cancer. 5-fluorouacil is probably the most useful single agent for symptomatic relief[6]. Gemcitabine may also have a value for palliation[7–9]. Overall, the prognosis of pancreatic cancer patients is poor with a 1-year survival rate of about 10%[10]. Therefore, new treatment modalities are urgently needed.

Photodynamic therapy (PDT) produces localized tissue necrosis with light (most conveniently from a laser), after administration of a photosensitizing agent in the presence of oxygen[10–12], based on the use of photosensitizing compounds that localize quite selectively in neoplastic/hyperplastic tissues and become cytotoxic when exposed to light[12–14]. In view of the antitumor effect of single treatment, PDT is local. PDT in combination with surgery[15], radiotherapy[16] or chemotherapy[17], has become a subject of research. New photosensitizers with improved spectroscopic, photochemical and tissue-localizing properties and improved laser instrumentation have stimulated attempts to establish clinical protocols for incorporation of PDT into multi-treatment modalities[18–24].

Most studies on PDT to date have been on lesions of the skin or in the wall of hollow organs, but recent interest is more in its potential for treating lesions of solid organs such as the pancreas[101120]. We have undertaken experiments on treating pancreatic cancer of mice with different doses of gemcitabine[25]. The results indicate that the growth of transplanted tumors can be inhibited by gemcitabine at 100 mg/kg, which shows severe side effects such as diarrhea, dehydration and loss of weight[25]. When gemcitabine at 100 mg/kg is used, the growth of transplanted tumors could not be controlled[25]. On the other hand, combined angiogenetic inhibitors can decrease side effects of gemcitabine, meanwhile the growth and metastasis of transplanted tumors are effectively inhibited[25].

In this study, gemcitabine was used as a cytotoxic drug. The cytotoxic and antitumor effects of combined gemcitabine and PDT were evaluated. Human pancreatic cancer cell line SW1990 was used in experiments to assess the effect of gemcitabine or PDT, or their combination, on pancreatic cancer in accordance with institutional guidelines.

PDT may be defined as a treatment based on an oxygen-dependent reaction between a photosensitizing dye and light, that is, the light combination of two absolutely non-toxic elements, drug and light, in the presence of oxygen, can result in selective destruction of tissue[1126]. The technique consists of administration of a tumor-localizing photosensitizing agent, which most often requires metabolic synthesis followed by activation of the agent by light with a specified wavelength. Photosensitizing agents used in PDT are macromolecular materials, which contribute to preferential location in neoplastic tissues and delay clearance of neoplastic tissues[27]. Therefore, PDT aims at a sequence of photochemical and photobiological processes that cause irreversible damage to tumor tissues and little damage to connective tissues, and maintain the mechanical integrity of organs[1224]. It was reported that PDT has a selective effect on malignant pancreas but no significant effect on normal pancreas, and could well match other treatment modalities, except for radical surgery[28]. In a pilot clinical trail, Bown et al[10] used PDT in the palliative treatment of 16 patients with cancers in the pancreatic head that could not be treated with surgery because of the advanced nature of the disease or the general condition of the patients, and they concluded that PDT may be of value for treating localized cancers in patients who are poor candidates for definitive surgery or in whom the location of tumor makes pancreatic resection inappropriate. Abulafi et al[29] and Tseng et al[30] reported that patients with pancreatic and ampullary carcinoma who are not suitable for surgery should be treated with PDT, since PDT is both feasible and safe for small tumors.

PDT, on the other hand, has some disadvantages and limitations. Little information is currently available concerning the uptake of photosensitizer by pancreas or pancreatic cancer. Local spread of photodynamic agents to vital organs is common, and may cause perforation of the duodenum and jejunum, leading to death after treatment with PDT[1131]. In addition, large tumor mass limits PDT to penetrate into the effective depth of tissue and needs multiple interstitial optical fibers to increase its volume[32]. Therefore, the ability of chemotherapeutic agents to enhance the effects of PDT in cell culture and transplantable mouse tumors has been studied by several groups[20–22]. Kirveliene et al[20] used murine hepatoma MH-22A to investigate in vitro and in vivo cytotoxic and anti-tumor effects of doxorubicin (Dox), a conventional anticancer drug, and PDT, showing that Dox potentiates therapeutic efficacy of PDT and vice versa, and that the degree of potentiation is influenced by Dox. Peterson et al[21] and Snyder et al[22] reported that combined treatment with PDT and Dox is more effective than treatment with either PDT or Dox alone. In vitro studies have revealed a significant effect of fluoropyrimidines[18] and mitomycin C[19] on the viability of mTHPC-photosensitized cells. Some studies focused on the effects of PDT on pancreas cancer[1011]. As we know, no study about the effect of therapy with photodynamic- cytotoxic agents on pancreatic cancer has been reported.

Gemcitabine is an active nucleoside analogue against a wide variety of cancers, including non-small cell lung cancer and pancreatic cancer[7–9]. Gemcitabine, acting as an antimetabolite, can inhibit ribonucleotide reductase and DNA synthesis, and induce apoptosis[33]. Gemcitabine today remains a first-line drug for patients with advanced pancreatic cancer[7–9]. However, it has a narrow therapeutic index due to rapid enzyme deamination by deoxytidine deaminase into its corresponding inactive uracil derivative, and can also induce drug resistance[3435]. Therefore, a high dose of gemcitabine is needed to achieve the desired therapeutic response with different adverse effects[36]. To overcome such drawbacks, based on the relation between quantity and effect of gemcitabine[25], we used photosan and gemcitabine as representatives of photosensitizing and cytotoxic agents to investigate the cytotoxic and antitumor effects of gemcitabine and PDT on pancreatic cancer in vivo. The results indicate that small dose gemcitabine or photosensitizer can not inhibit the growth of pancreatic cancer. PDT had a significant anti-tumor effect, but lasted a short time. Photodynamic-cytotoxic therapy (small dose gemcitabine) could significantly inhibit the growth of pancreatic cancer, and showed a relative long-duration anti-tumor effect compared with PDT. The inhibition rate of photodynamic-cytotoxic agents and PDT for tumors was 82.42% and 58.18%, respectively. The four treatment modalities did not induce any signs of toxicity such as diarrhea and vomiting. No treatment-related death occurred. Animals in the PDT group and combined treatment group had no skin photosensitivity to light.

In conclusion, low dose gemcitabine increases the anticancer effect of PDT with no obvious adverse effects. Combined PDT and gemcitabine can be used in treatment of pancreatic cancer in patients who are poor candidates for surgery.