Combination of vascular endothelial growth factor receptor/platelet-derived growth factor receptor inhibition markedly improves radiation tumor therapy.
OBJECTIVE
Investigations on the combination of radiotherapy with vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF) antiangiogenic agents, which has the potential to improve the clinical outcome in cancer patients.


METHODS
Here, we analyze the combined VEGF (SU5416) and PDGF (SU6668) receptor tyrosine kinase inhibition with irradiation in human endothelium (HUVEC), prostate cancer (PC3), and glioblastoma (U87) in vitro and in vivo.


RESULTS
Combined inhibition of VEGF and PDGF signaling resulted in enhanced apoptosis, reduced cell proliferation, and clonogenic survival as well as reduced endothelial cell migration and tube formation compared with single pathway inhibition. These effects were further enhanced by additional irradiation. Likewise, in PC3 and U87 tumors growing s.c. on BALB/c nu/nu mice, dual inhibition of VEGF and PDGF signaling significantly increased tumor growth delay versus each monotherapy. Interestingly, radiation at approximately 20% of the dose necessary to induce local tumor control exerts similar tumor growth-inhibitory effects as the antiangiogenic drugs given at their maximum effective dose. Addition of radiotherapy to both mono- as well as dual-antiangiogenic treatment markedly increased tumor growth delay. With respect to tumor angiogenesis, radiation further decreased microvessel density (CD31 count) and tumor cell proliferation (Ki-67 index) in all drug-treated groups. Of note, the slowly growing PC3 tumor responded better to the antiangiogenic drug treatments than the faster-growing U87 tumor. In addition to the beneficial effect of abrogating VEGF survival signaling when combined with radiation, we identified here a novel mechanism for the tumor escape from radiation damage. We found that radiation induced up-regulation of all four isoforms of PDGF (A-D) in endothelial cells supporting adjacent smooth muscle cells resulting in a prosurvival effect of radiation. The addition of SU6668 attenuated this undesirable paracrine radiation effect, which may rationalize the combined application of radiation with PDGF signaling inhibition to increase antitumor effects.


CONCLUSIONS
A relative low radiation dose markedly enhances local antitumor effects of combined VEGF and PDGF signaling inhibition, suggesting a promising combination regimen for local tumor treatment with radiotherapy remaining an essential element.
Timke
Carmen
C
Department of Radiation Oncology, German Cancer Research Center, Heidelberg, Germany.
Zieher
Heike
H
Roth
Alexandra
A
Hauser
Kai
K
Lipson
Kenneth E
KE
Weber
Klaus J
KJ
Debus
Jürgen
J
Abdollahi
Amir
A
Huber
Peter E
PE
D020533:Angiogenesis Inhibitors / Q000008:administration & dosage*; D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D003131:Combined Modality Therapy; D042783:Endothelial Cells / Q000187:drug effects; D005434:Flow Cytometry; D006801:Humans; D007150:Immunohistochemistry; D007211:Indoles; D051379:Mice; D008819:Mice, Nude; D009369:Neoplasms / Q000098:blood supply / Q000188:drug therapy* / Q000532:radiotherapy*; D009389:Neovascularization, Pathologic / Q000188:drug therapy; D000078183:Oxindoles; D011422:Propionates; D047428:Protein Kinase Inhibitors / Q000008:administration & dosage; D011758:Pyrroles; D011838:Radiation-Sensitizing Agents / Q000008:administration & dosage*; D011878:Radiotherapy; D017479:Receptors, Platelet-Derived Growth Factor / Q000037:antagonists & inhibitors*; D040262:Receptors, Vascular Endothelial Growth Factor / Q000037:antagonists & inhibitors*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D023041:Xenograft Model Antitumor Assays
D020533:Angiogenesis Inhibitors; D007211:Indoles; D000078183:Oxindoles; D011422:Propionates; D047428:Protein Kinase Inhibitors; D011758:Pyrroles; D011838:Radiation-Sensitizing Agents; C116890:Semaxinib; C412603:orantinib; D017479:Receptors, Platelet-Derived Growth Factor; D040262:Receptors, Vascular Endothelial Growth Factor