Role of Cyclooxygenase-2 in Pathogenesis and Prevention of Colorectal Cancer

Chemoprevention of colorectal cancer is a promising science that has particular importance due to the limited success of current treatments for most advanced common malignancies. Many chemopreventive agents have been studied including cyclooxygenase (COX) inhibitors. Two isoforms of the COX enzymes are COX-1 and COX-2. COX-1 is constitutively expressed in normal tissue, serving an important role in tissue homeostasis, whereas COX-2 is an inducible enzyme, which is markedly overexpressed at sites of inflammation and colorectal neoplasms. The preventive efficacy of traditional nonsteroidal anti-inflammatory drugs (NSAIDs), which inhibit COX-1 and/or COX-2, has considerable support from animal and epidemiological studies; however, there are well-documented toxicities associated with NSAID use. These adverse effects are attributed to NSAIDs' inhibition of COX-1. The development of COX-2 specific inhibitors gave hopes of bypassing the associated traditional NSAID toxicities while better targeting tissues sustaining inflammation and neoplasia. The PrsSAP, APC and APPROVe trials demonstrated the efficacy of COX-2 specific inhibitors in preventing the recurrence of sporadic colorectal polyps. However, the trials were terminated early due to discovery of significant cardiovascular toxicity, although the exact extent of this toxicity remains unclear. The exact mechanisms through which NSAIDs exert their cancer preventing effects are currently unknown; inhibition of COX-2 is of great importance, but COX-2 independent pathways exist as well. In addition, the efficacy of NSAID use for cancer prevention can differ significantly between individuals. Personalized medicine in this field is also greatly anticipated. Combination therapy is under extensive research in order to improve efficacy while reducing toxicity profiles. Chemoprevention of colorectal cancer is largely possible, but the ultimate drug and proper patient selection, among other elements of the cancer prevention equation, are still needed.