In this current study, we confirm the ability of the anti-VEGF monoclonal antibody bevacizumab to inhibit endothelial cell proliferation and disrupt the formation of capillary-like networks in culture. In the H&N and lung cancer xenograft models, treatment with bevacizumab inhibited tumor vascularization and inhibited volume growth of both SCC1 and H226 tumors. However, the growth inhibitory effect of bevacizumab is not complete, suggesting the potential value of combining bevacizumab with other cytotoxic modalities, such as radiation to achieve more potent therapeutic effects.
In this work, we demonstrate that radiation combined with bevacizumab reduced the formation of tumor vasculature and inhibited tumor growth in SCC1 and H226 cancer xenograft models more strongly than either modality alone (Figure
). This is consistent with prior work using the recombinant human monoclonal anti-VEGF 165 antibody in mouse models bearing other human cancers
]. Our findings confirm that neutralizing the VEGF ligand with bevacizumab can augment tumor response to radiation. Works from our laboratory and others have previously demonstrated that radiation response is enhanced by blocking the VEGF signaling pathway using small molecule VEGF receptor tyrosine kinase inhibitors such as ZD6474
] and PTK787/ZK222584
], or by directly targeting tumor blood vessels with vascular targeting agents such as ZD6126
] and combretastatin
The anti-tumor effect of this combination approach is consistent with the two-compartment model described by Folkman
]. According to this model, tumors are comprised of distinct compartments including tumor cells and endothelial cells. By targeting the endothelial cell compartment, bevacizumab not only inhibits the supply of oxygen and nutrients to the tumor, but also interrupts the “paracrine effect” by inhibiting endothelial secretion of growth factors such as IGF1, bFGF, and HB-EGF, which can stimulate tumor proliferation. In parallel, by targeting the tumor compartment, radiation kills cancer cells and thereby shuts down their production of “pro-angiogenic” factors, thus indirectly affecting the endothelial compartment. We have also observed that treatment with radiation can inhibit endothelial cell proliferation and stimulate apoptosis
] and G2/M arrest (nonpublished data), suggesting direct inhibitory effects of radiation on this compartment.
A current question of interest in clinical trial design regards the optimal sequencing of radiation and anti-angiogenic drugs to achieve maximal benefit. A valid concern is whether targeting the tumor vasculature will decrease tumor blood perfusion, resulting in tumor hypoxia, and thereby diminishing the effects of radiation. To investigate the impact of treatment sequencing on tumor response, we designed sequence experiments as described in Figure
. In the SCC-1 model, it appeared that tumor control was best achieved with the regimen of radiation followed by bevacizumab. This result supports the hypothesis that hypoxia induced by bevacizumab may hinder radiation effect. However, we found no clear difference between sequence regimens in the H226 tumors.
Consistent with our observation in the SCC-1 tumors, preclinical studies have shown that delivering ZD6126 prior to radiation to U87 glioblastoma xenografts resulted in acute drop in tumor oxygen tension and attenuation of the killing effects of radiation
]. Further, in KHT sarcoma models, the strongest anti-tumor activity was achieved when ZD6126 was administered one hour following radiation
]. These observations suggest a negative impact of ZD6126-induced hypoxia on radiation effect. However, the concept of normalization of tumor vasculature proposed by Jain et al.
supports a strategy of using anti-angiogenic drugs to improve efficacy of radiation
]. This theory suggests that short term treatment of anti-angiogenic agents may “normalize” the network of abundant but chaotic, leaky and dysfunctional tumor vasculature, thus restore the integrity and function of the blood vessels, leading to a decrease in interstitial fluid pressure and the improvement in tumor oxygenation
]. Therefore, this process of vascular normalization could enhance the tumor killing activity of radiation as well as improve drug delivery into the tumor
]. Although the induction of vascular normalization by anti-angiogenic agents has been supported by preclinical studies
], it remains a challenge to capture the transient “tumor oxygenation window” for the delivery of radiation. We are commencing real-time imaging of tumor hypoxia profiles in animals during treatment to help explore optimal strategies for this combined therapy.
In the clinic, several clinical phase I/II studies have been conducted to investigate the safety and efficacy of radiation and bevacizumab in cancer patients. The first report came from a series of 6 patients with locally advanced rectal carcinoma who were treated in a phase I trial with induction therapy of bevacizumab (5
mg/kg x 1 dose) followed by radiation in combination with bevacizumab and 5-fluorouracil, then surgical resection
]. This pilot study demonstrated that a single dose of bevacizumab induction lead to a significant decrease in interstitial fluid pressure, tumor blood perfusion, and microvascular density on day 12
]. The subsequent phase II trial in the same patient population demonstrated that bevacizumab induction therapy followed by concurrent bevacizumab and chemoradiation appeared safe and active with a 5-year local control and overall survival of 100%
]. The combination of bevacizumab with radiation was also investigated in early clinical studies in other diseases including pancreatic cancer
] and head and neck cancer
], in which bevacizumab was started either prior or concurrently with chemoradiation.