Releasing pressure in tumors: what do we know so far and where do we go from here? A review

Tumor interstitial pressure is a fundamental feature of cancer biology. Elevation in tumor pressure affects the efficacy of cancer treatment. It causes heterogenous intratumoral distribution of drugs and macromolecules. It also causes the development of hypoxia within tumor bulk, leading to reduced efficacy of therapeutic drugs and radiotherapy. Tumor pressure has been associated with increased metastatic potential and poor prognosis in some tumors. The formation of increased pressure in solid tumors is multifactorial. Factors known to affect tumor pressure include hyperpermeable tortuous tumor vasculatures, the lack of functional intratumoral lymphatic vessels, abnormal tumor microenvironment, and the solid stress exerted by proliferating tumor cells. Reducing this pressure is known to enhance the uptake and homogenous distribution of many therapies. Pharmacologic and biologic agents have been shown to reduce tumor pressure. These include antiangiogenic therapy, vasodilatory agents, antilymphogenic therapy, and proteolytic enzymes. Physical manipulation has been shown to cause reduction in tumor pressure. These include irradiation, hyperbaric oxygen therapy, hyper- or hypothermic therapy, and photodynamic therapy. This review explores the methods to reduce tumor pressure that may open up new avenues in cancer treatment.