New indications for established drugs: Combined tumor-stroma-targeted cancer therapy with PPAR gamma agonists, COX-2 inhibitors, mTOR antagonists and metronomic chemotherapy

In search of new strategies for the treatment of cancer, the interaction between tumor and stroma attracts more and more attention. Disruption of stroma functions, e.g. angiogenesis, has evolved into a promising target for cancer therapies. Since stromal cells are genetically stable, stroma-targeted therapies seem to be less susceptible to the development of drug resistance. Several well-established drugs, which had initially been developed for other indications, also exhibit antitumor activity. Among those, PPAR gamma agonists, COX-2 inhibitors, and mTOR antagonists are the most remarkable examples. Current research data and clinical experience suggest that these drugs target stroma functions in cancer, in particular angiogenesis, but immunological mechanisms and direct antitumor effects seem to participate as well. In addition to these drugs, frequent administration of low-dose chemotherapeutics, referred to as metronomic chemotherapy, reveals profound anti-angiogenic effects. In the meantime, a multitude of preclinical and clinical studies have been undertaken, which demonstrate the efficacy of these drugs in cancer therapy. Combinatorial use of these agents has been suggested to be superior in terms of antitumor efficacy and prevention of drug resistance. The toxicity of these therapies is surprisingly low compared with conventional high-dose chemotherapy regimens. Patients with advanced disease, often heavily pretreated and presenting multiple drug resistance, could particularly profit from such tumor-stroma-targeted therapies. However, larger randomized clinical trials are required for further evaluation and optimization of this concept.