Verteporfin-based photodynamic therapy overcomes gemcitabine insensitivity in a panel of pancreatic cancer cell lines

Abstract

Pancreatic cancer is notoriously difficult to treat and resistant to virtually all therapeutics including gemcitabine, the standard front line agent for palliative chemotherapy. Early clinical studies point to a potential role for photodynamic therapy (PDT) in the management of this deadly disease. Here we examine PDT with verteporfin for treatment of cells that are nonresponsive to gemcitabine and identify intracellular and extracellular factors that govern sensitivity to each modality. Using MTS we assess cytotoxicity of verteporfin-PDT in gemcitabine-treated nonresponsive populations from a panel of five pancreatic cancer cell lines representing a range of tumor histopathology and origin. We conduct Western blots for pro-/anti-apoptotic proteins bax and Bcl-XL to identify factors relevant to PDT and gemcitabine sensitivity. To examine the role of extracellular matrix influences we compare response to each modality in traditional cell culture conditions and cells grown on a laminin-rich basement membrane. All cell lines have gemcitabine nonresponsive populations (17-33%) at doses up to 1 mM while moderate total verteporfin PDT doses (1-6 µM J/cm2) produce nearly complete killing. Our data shows that cells that are nonresponsive to sustained gemcitabine incubation are sensitive to verteporfin PDT indicating that the latter is agnostic to gemcitabine sensitivity. Verteporfin-based PDT decreases Bcl-XL and increases the bax/Bcl-XL ratio toward a pro-apoptotic balance. Insensitivity to gemcitabine is increased in cells that are adherent to basement membrane relative to traditional tissue culture conditions. Collectively these results indicate the ability of verteporfin-based PDT to bypass intracellular and extracellular cues leading to gemcitabine resistance and point to the emerging role of this therapy for treatment of pancreatic cancer.