Photodynamic therapy (PDT) is a noninvasive therapy based on the photodynamic effect. In this study, we sought to determine intracellular uptake and in vivo photodynamic therapy potential of Zn phthalocyanine-loaded mesoporous silica nanoparticles (MSNPS) against pancreatic cancer cells. MSNPS were labeled with I-131; the radiolabeling efficiency was found to 95.5 +/- 1.2% in pH 9 and 60 min reaction time. Besides, the highest intracellular uptake yields of I-131-MSNPS nanoparticles in MIA PaCa-2, AsPC-1, and PANC-1 cells were determined as 43.9 +/- 3.8%, 41.8 +/- 0.2%, and 37.9 +/- 1.3%, respectively, at 24 h incubation time. In vivo PDT studies were performed with subcutaneous xenograft cancer model nude mice with AsPC-1 pancreatic cancer cells. For photodynamic therapy, 685 nm red laser light 100 J/cm(2) light dose using and 5-20 mu M ZnPc containing MSNPS concentrations were applied. Histopathological studies revealed that the ratio of necrosis in tumor tissue was higher in the treatment group than the control groups.