Antisense HIF-1alpha prevents acquired tumor resistance to angiostatin gene therapy

Angiostatin is a naturally occurring inhibitor of angiogenesis that is being developed as a drug to fight cancer. In this study we reveal that EL-4 tumors established in mice rapidly develop resistance to angiostatin gene therapy by upregulating hypoxia-inducible pathways. Angiostatin initially delayed tumor growth for 6 days by reducing blood vessel density. However, tumors quickly responded by upregulating the production of hypoxia-inducible factor-1alpha (HIF-1alpha) and its effector vascular endothelial growth factor (VEGF) in response to increasing tumor hypoxia, leading to restored angiogenesis and rapid tumor growth. Theoretically, blockade of HIF-1 should prevent resistance to anti-angiogenic therapy by preventing a tumor from responding to induced hypoxia. Antisense HIF-1alpha inhibited the expression of HIF-1alpha and of the HIF-1 effectors VEGF, glucose transporter-1 and lactate dehydrogenase. As a monotherapy, it was effective in eradicating small 0.1 cm diameter tumors, but only delayed the growth of large 0.4 cm diameter tumors. In contrast, timed injection of a combination of angiostatin and antisense HIF-1alpha plasmids completely eradicated large EL-4 tumors within 2 weeks, and prevented upregulation of hypoxia-inducible pathways induced by angiostatin. The data indicate that blocking hypoxia-inducible pathways by antisense HIF-1alpha can circumvent hypoxia-induced drug resistance and thereby augment the efficacy of anti-angiogenic therapies.