Hypoxia and anemia: effects on tumor biology and treatment resistance

In locally advanced solid tumors, oxygen (O2) delivery is frequently reduced or even abolished. This is due to abnormalities of the tumor microvasculature, adverse diffusion geometries, and tumor-associated and/or therapy-induced anemia. Up to 50-60% of locally advanced solid tumors may exhibit hypoxic and/or anoxic tissue areas that are heterogeneously distributed within the tumor mass. In approximately 30% of pretreatment patients, a decreased O2 transport capacity of the blood as a result of tumor-associated anemia can greatly contribute to the development of tumor hypoxia. While normal tissues can compensate for this O2 deficiency status by a rise in blood flow rate, locally advanced tumors (or at least larger tumor areas) cannot adequately counteract the restriction in O2 supply and thus the development of hypoxia. Hypoxia-induced alteration in gene expression and thus in the proteome (< 1% O2, or < 7 mmHg), and/or genome changes (< 0.1% O2, or < 0.7 mmHg) may promote tumor progression via mechanisms enabling cells to overcome nutritive deprivation, to escape from the hostile metabolic microenvironment and to favor unrestricted growth. Sustained hypoxia may thus lead to cellular changes resulting in a more clinically aggressive phenotype. In addition, hypoxia is known to directly or indirectly confer resistance to X- and gamma-radiation, and some chemotherapies leading to treatment failures. Whereas strong evidence has accumulated that hypoxia plays a pivotal role in tumor progression and acquired treatment resistance, the mechanism(s) by which treatment efficacy and survival may be compromised by anemia (independent of hypoxia) are not fully understood.