Immune stimulation and tumor cells escape-the two sides of the coin of protoporphyrin IX-based photodynamic tumor therapy

Objective: Photodynamic therapy (PDT) uses the combination of a photosensitizing drug and light to cause selective damage to solid tumors. For early or localized disease, PDT can be a curative therapy with many advantages over available alternatives. For more advanced or metastatic disease curative therapy is usually not possible. However, there is increasing evidence that PDT can also induce systemic anti-tumor immunity, indicating that PDT can become a rational therapeutic option, even if not all tumor cells are primarily eliminated by PDT. One prerequisite to minimize tumor relapse in these settings is to counteract repair mechanisms of PDT-induced damage and to aid PDT-triggered immune stimulation.