Anemia: A Potential Source of Bias in Clinical Trials of Angiogenesis Inhibitors: A Hypothesis

Although anemia may cause angiogenesis and neovascularization, especially in ocular situations, neither published nonran-domized clinical trials nor registered clinical trials have reported the anemia status as inclusion or exclusion criteria in their design. Increases in the circulating levels of erythropoietin and vascular endothelial growth factor are proportional to the levels of tissue hypoxia, which are influenced by hematocrit. Erythropoietin is a potent retinal angiogenic factor that is independent of endothelial growth factor and is capable of stimulating ischemia-induced retinal angiogenesis. We suggest that clinical trials investigating anti-vascular endothelial growth factor treatment for retinal neovascularization should measure appropriate variables such as serum erythropoietin, vascular endothelial growth factor, hemoglobin, and hematocrit to yield preliminary data for future trials of angiogenesis inhibitors. Ignoring the anemia status, serum erythropoietin, and/or vascular endothelial growth factor levels could create clinical uncertainty and subtle statistical bias in both systematic reviews and nonrandomized clinical trials that aim to evaluate the efficiency of angiogenesis inhibitors in several medical situations, including but not limited to ocular alterations, rheumatoid arthritis, and many types of cancer, just to mention a few. Implications of this type of bias could be involved in other disease situations in which angiogenesis inhibitors are used for medication, such as different carcinomas as well as metastases. In this hypothesis paper, we suggest that clinical trials of angiogenesis inhibitors should measure appropriate variables such as serum erythropoietin, hemoglobin, and hematocrit and match their participants by anemia and its severity to avoid a game-changing bias in their data analysis.