Tumorigenesis and Neoplastic Progression Tumor Recovery by Angiogenic Switch from Sprouting to Intussusceptive Angiogenesis after Treatment with PTK 787 / ZK 222584 or Ionizing Radiation

Inhibitors of angiogenesis and radiation induce compensatory changes in the tumor vasculature both during and after treatment cessation. To assess the responses to irradiation and vascular endothelial growth factor-receptor tyrosine kinase inhibition (by the vascular endothelial growth factor tyrosine kinase inhibitor PTK787/ZK222854), mammary carcinoma allografts were investigated by vascular casting; electron, light, and confocal microscopy; and immunoblotting. Irradiation and anti-angiogenic therapy had similar effects on the tumor vasculature. Both treatments reduced tumor vascularization, particularly in the tumor medulla. After cessation of therapy, the tumor vasculature expanded predominantly by intussusception with a plexus composed of enlarged sinusoidal-like vessels containing multiple transluminal tissue pillars. Tumor revascularization originated from preserved alpha-smooth muscle actin-positive vessels in the tumor cortex. Quantification revealed that recovery was characterized by an angiogenic switch from sprouting to intussusception. Up-regulated alpha-smooth muscle actin-expression during recovery reflected the recruitment of alpha-smooth muscle actin-positive cells for intussusception as part of the angio-adaptive mechanism. Tumor recovery was associated with a dramatic decrease (by 30% to 40%) in the intratumoral microvascular density, probably as a result of intussusceptive pruning and, surprisingly, with only a minimal reduction of the total microvascular (exchange) area. Therefore, the vascular supply to the tumor was not severely compromised, as demonstrated by hypoxia-inducible factor-1alpha expression. Both irradiation and anti-angiogenic therapy cause a switch from sprouting to intussusceptive angiogenesis, representing an escape mechanism and accounting for the development of resistance, as well as rapid recovery, after cessation of therapy. Tumorigenesis and Neoplastic Progression Tumor Recovery by Angiogenic Switch from Sprouting to Intussusceptive Angiogenesis after Treatment with PTK787/ZK222584 or Ionizing Radiation Ruslan Hlushchuk,* Oliver Riesterer, Oliver Baum,* Jeanette Wood, Guenther Gruber, Martin Pruschy, and Valentin Djonov* From the Institute of Anatomy,* University of Bern, Bern, Switzerland; Department of Radiation Oncology, University Hospital, Zurich, Switzerland; Novartis Pharma AG, Basel, Switzerland; Department of Radiation Oncology, University Hospital, Bern, Switzerland; Institute of Anatomy, University of Fribourg, Fribourg, Switzerland Inhibitors of angiogenesis and radiation induce compensatory changes in the tumor vasculature both during and after treatment cessation. To assess the responses to irradiation and vascular endothelial growth factor-receptor tyrosine kinase inhibition (by the vascular endothelial growth factor tyrosine kinase inhibitor PTK787/ZK222854), mammary carcinoma allografts were investigated by vascular casting; electron, light, and confocal microscopy; and immunoblotting. Irradiation and anti-angiogenic therapy had similar effects on the tumor vasculature. Both treatments reduced tumor vascularization, particularly in the tumor medulla. After cessation of therapy, the tumor vasculature expanded predominantly by intussusception with a plexus composed of enlarged sinusoidal-like vessels containing multiple transluminal tissue pillars. Tumor revascularization originated from preserved -smooth muscle actin-positive vessels in the tumor cortex. Quantification revealed that recovery was characterized by an angiogenic switch from sprouting to intussusception. Up-regulated -smooth muscle actin-expression during recovery reflected the recruitment of -smooth muscle actinpositive cells for intussusception as part of the angioadaptive mechanism. Tumor recovery was associated with a dramatic decrease (by 30% to 40%) in the intratumoral microvascular density, probably as a result of intussusceptive pruning and, surprisingly, with only a minimal reduction of the total microvascular (exchange) area. Therefore, the vascular supply to the tumor was not severely compromised, as demonstrated by hypoxia-inducible factor-1 expression. Both irradiation and anti-angiogenic therapy cause a switch from sprouting to intussusceptive angiogenesis, representing an escape mechanism and accounting for the development of resistance, as well as rapid recovery, after cessation of therapy. (Am J Pathol 2008, 173:1173–1185; DOI: 10.2353/ajpath.2008.071131) Tumor relapse and the development of drug resistance is a major problem in the management of solid tumors. It may be inherent to the tumor-cell compartment, to the aberrant and inefficient vasculature, or to the surrounding stroma. In the first case, a refractory subpopulation of clonogenic cancer cells survives and proliferates. In the second case, the potency of therapy is hampered due to limited tumor perfusion. The tumor vasculature can be damaged by chemotherapeutics, but interestingly, in particular in response to inhibitors of angiogenesis, the tumor vasculature may undergo morphological changes that normalize the tumor vasculature. This tumor vascular normalization concept claims that treatment with low doses of angiogenesis inhibitors preferentially targets immature vessels and thereby creates a normalization of the blood vessel with improved functionality and results in better perfusion of the remaining tumor mass. This could diminish tumor hypoxia by improved delivery of oxygen and also improve the delivery of chemotherapeutics. The administration of anti-angiogenic agents is now generally recognized as a promising therapeutic approach in Supported by the grant No3100A0-116243 from the Swiss National Sci-