Angiogenesis is the process by which new blood vessels sprout from an existing vascular (blood vessel) bed. Although new blood vessels grow rapidly in the developing embryo, in normal adults, blood vessel growth rarely occurs. There are “switches” in the body that normally turn blood vessel growth on and off. The process is tightly controlled by an array of stimulatory and inhibitory factors, with intense bursts of new blood vessel formation largely restricted to the menstrual cycle, pregnancy, and wound healing.
Interestingly, there is a large body of evidence suggesting that abnormal angiogenesis contributes to the development of a variety of diseases, including diabetic retinopathy (changes in the back of the eye), psoriasis, rheumatoid arthritis, and cancer.
Hence, therapies designed to inhibit the growth of new blood vessels may be useful treatments in cancer and other diseases. In cancer, blood vessels probably “feed” the tumor, allowing it to grow beyond an initially small size. Now that we are beginning to understand how small tumors recruit new blood vessels, we can use this information to develop therapies to stop the formation of these new blood vessels and “starve” the tumor (antiangiogenesis). Bevacizumab (Avastin) was the first dedicated antiangiogenic agent to be approved for human use. Dozens of other antiangiogenic agents are in development.
Antiangiogenesis and Cancer
It is now generally accepted that tumor growth and spread to distant organs (metastasis) require growth of new blood vessels. Prior to the recruitment of new vessels (termed the angiogenic switch), tumor size is limited to about the size of a pinhead. New capillaries are required for tumor growth because they provide a mechanism for waste and nutrient exchange, as well as a route for metastasis. Some studies have shown that the angiogenic switch is activated during early (precancerous) stages of tumor progression in many cancers, even before invasion has occurred. Consequently, researchers think that antiangiogenic agents may someday be used to prevent, as well as treat, invasive cancer. The available data suggest that this type of therapy may be more effective when combined with chemotherapy, radiation, or other biologic agents than when given as a single agent. In addition, antiangiogenic agents may potentially prove most effective in the treatment of early cancer, or in preventing the growth of new cancers after cancer surgery. In patients with advanced disease, these agents may be more likely to stabilize disease than to cause a major shrinkage.
Regulation of Angiogenesis
The process of angiogenesis involves multiple steps, such as breakdown of the tissue surrounding the blood vessel by proteins called proteases, production and proliferation of new blood vessel lining (endothelial cells [ECs]), and migration of ECs to generate a new capillary sprout that eventually matures into a tube that forms the new tiny blood vessel. The regulation of angiogenesis appears to be accomplished by many regulators working together in our bodies. During the past two decades, dozens of stimulatory and inhibitory counterparts (see the table below) have been discovered. The balance of regulators at any one time in a given tissue probably determines whether or not angiogenesis occurs.
|IL-8||TIMP-1 and TIMP-2|
|Thymidine phos-phorylase (PD-ECGF)||Angiostatic steroids|
|HGF||16 kD prolactin|
Why Should It Work? There are several reasons why we think that inhibiting blood vessel growth may be beneficial in patients with cancer. New capillaries appear to be required for tumor growth and metastasis. Furthermore, antiangiogenic agents may preferentially target vessels in the tumor, since normal adult blood vessels are essentially dormant. Additional differences probably exist between normal endothelial cells and those associated with a tumor. These alterations can theoretically be exploited to develop cancer-specific antiangiogenic agents, thus potentially avoiding the toxicity associated with traditional cancer therapies like chemotherapy and radiation. Finally, drugs in this class might be broadly useful across a variety of tumor types, since the blood vessels are targeted rather than the cancer cells themselves (which are prone to mutations and perhaps more likely to develop resistance to anticancer drugs).
Treatment Strategies A number of approaches to antiangiogenesis are being explored, all of which stem from advances in our understanding of the process by which angiogenesis is regulated:
1. Block activity of factors that normally stimulate angiogenesis.
2. Increase activity of inhibitory factors by administration of extra factor made outside the body or by gene therapy to enhance production of the factor within the patient.
3. Interfere with the normal relationship between endothelial cells and the surrounding tissue.
4. Inhibit an integral step of angiogenesis (e.g., by specifically blocking endothelial cell migration, division, or tubule formation so new blood vessels can’t form).
5. Employ endothelial-specific toxins. Agents in this class would be designed to bind endothelial cells in order to deliver a toxin such as chemotherapy or radiation.
Examples of each of these classes of inhibitors are already being tested. Many others are in various stages of development.
Clinical Experience Dozens of antiangiogenic agents have already entered the clinic for testing in patients with cancer. Combinations with chemotherapy, radiation, and/or other biologically based therapies are also being explored.
|Selected Antiangiogenic Agents in Clinical Trials|
|AGENT||STAGE OF TESTING||SPONSOR|
|Anti-VEGF antibody (bevacizumab)||Phase IV (approved in lung and colon cancer)||Genentech|
|Lenalidomide (thalidomide analog)||Phase IV||NCI, Celgene (commercially available)|
|Thalidomide (immunomodulatory, anti-inflammatory and anti-angiogenic properties)||Phase III||Celgene (commercially available)|
|SU11248 (sunitinib) (receptor TKI)||Phase III (approved for GIST and RCC)||Pfizer|
|Sorafenib (Bay 43-9006) (receptor TKI)||Phase III (approved for RCC)||Bayer|
|Celecoxib (Celebrex) (Cox-2 inhibitor)||Phase III||Pfizer (commercially available)|
|AZD2171 (receptor TKI)||Phase II/III||Astra Zeneca|
|AVE0005 (VEGF Trap)||Phase II/III||Regeneron Pharmaceuticals/Sanofi-Aventis|
|GW786034 (pazopanib) (receptor TKI)||Phase III||Glaxo Smithkline|
|ZD6474 (receptor TKI)||Phase III||AstraZeneca|
|PTK787/ZK222584 (receptor TKI)||Phase III||Novartis|
|AMG706 (receptor TKI)||Phase I/II||Amgen|
|ABT-510 (thrombospondin analog)||Phase I/II||NCI|
|ATN-224 (2nd generation||Phase I/II||Attenuon tetrathiomolybdate)|
|Combretastatin (tumor vascular-targeting agent)||Phase II||NCI|
|PI-88 (heparinase inhibitor)||Phase I/II||Progen|
|PXD-101 (histone deacetylase inhibitor)||Phase II||CuraGen/TopoTarget|
|AG-013736 (receptor TKI)||Phase II||Pfizer|
|Panzem (2-ME) (estradiol metabolite with||Phase II||EntreMed antiangiogenic activity)|
|SU-014813 (receptor TKI)||Phase II||Pfizer|
|FR901228 (histone deacetylase inhibitor)||Phase II||NCI|
|BMS-582664 (receptor TKI)||Phase II||Bristol-Myers Squibb Company|
|M200 (volociximab) (anti-alpha5 beta 1 integrin antibody)||Phase II||Protein Design Labs|
|EMD121974 (cilengitide) (Cyclic peptide, inhibits alpha(v)beta(3) and alpha(v)-beta(5) integrins)||PPhase I/II||NCI|
|AMG 386 (Fc-peptide fusion protein, angiopoietin inhibitor)||Phase I||Amgen|
|NPI-2358 (vascular disrupting agent)||Phase I||Nereus Pharmaceuticals, Incorporated|
|Adapted from the National Cancer Institute database, www.cancer.gov/clinicaltrials (updated March 2007)|
Many of the antiangiogenic agents under evaluation are directed against vascular endothelial growth factor (VEGF), which has emerged as a central therapeutic target in cancer. VEGF is a potent, positive regulator of angiogenesis that signals through receptors on the surface of endothelial cells. VEGF, as well as its receptors, are often overexpressed in cancers; levels of expression often inversely correlate with patient outcome. In addition to direct effects on endothelial cells, experimental evidence suggests that VEGF inhibitors may help to “normalize” the aberrant tumor blood supply, thus potentially improving delivery of chemotherapy and other therapeutic agents.
To date, the most impressive results have been associated with the use of a humanized monoclonal antibody designed to bind and inhibit VEGF function called bevacizumab (Avastin). Bevacizumab was approved by the United States Food and Drug Administration (FDA) in 2004 for use in in patients with previously untreated metastatic colorectal cancer (in combination with chemotherapy). Recently, it also gained approval for use in the first-line treatment of advanced lung cancer. Numerous other trials with this agent are ongoing.
In addition to antibody-based strategies, significant efforts have been placed on generating small molecule inhibitors of VEGF receptor signaling. While many different inhibitors are in development (which vary in terms of specificity, potency, and oral bioavailability), two agents in this class (sorafenib [Nexavar] and sunitinib) have already been approved for use in human cancer. Many other VEGF inhibitors, as well as numerous other antiangiogenic strategies, are being explored.
Bevacizumab (Avastin) Bevacizumab is a recombinant humanized monoclonal antibody directed against VEGF. The approval in colorectal cancer stemmed from a Phase III clinical trial showing that patients with previously untreated metastatic colorectal cancer (mCRC) who were randomized to receive 5-fluorouracil + leucovorin (LV) + irinotecan (IFL) chemotherapy plus bevacizumab lived nearly five months longer than patients treated with chemotherapy alone. Use of bevacizumab was also associated with statistically significant increases in radiographic response rate and time to tumor progression (the time it takes the tumor to grow or new tumors to form). Bevacizumab was given intravenously every two weeks and was generally well tolerated, although treatment-associated high blood pressure was seen. In addition, small but clinically significant increases in gastrointestinal perforations and arterial thromboembolic events (like strokes and heart attacks) were observed.
Subsequent studies showed that bevacizumab improved the activity of oxaliplatin-based chemotherapy in the second-line setting, and of 5-FU/LV chemotherapy in the first-line treatment of patients with mCRC. Taken together, these data led to the approval of bevacizumab in combination with 5-FU–based chemotherapy in the first- and second-line treatment of patients with mCRC. Ongoing studies are aimed at exploring the incremental benefit of bevacizumab in the adjuvant setting (i.e., after potentially curative removal of the primary tumor), and in combination with other treatment regimens (including ones that incorporate oral 5-FU drugs and other biologically based agents).
The ECOG 4599 trial was designed to evaluate the use of bevacizumab in combination with chemotherapy (carboplatin/paclitaxel) in patients with previously untreated metastatic nonsmall cell lung cancer. The addition of bevacizumab improved progression-free and overall survival in these patients (by around two months), leading to the FDA approval of bevacizumab for this indication. The relative efficacy of this combination compared to numerous other potential first-line treatment regimens for this patient population is unknown, and is the subject of ongoing clinical trials.
The preliminary results of a large, randomized clinical trial for patients with previously untreated metastatic or locally recurrent breast cancer have also been reported. The study showed that patients who received bevacizumab in combination with standard chemotherapy with paclitaxel had a longer time period before their disease worsened compared to patients who received chemotherapy alone. The study results suggest that bevacizumab has activity in breast cancer, however, the drug has not yet been approved for use in this disease. Interestingly, a previous study in patients assigned to a different chemotherapy (capecitabine [Xeloda]), with or without bevacizumab, failed to show that bevacizumab improved the outcome. The fact that the study was designed for women who had received prior chemotherapy for advanced disease may be significant and suggests that the benefit of bevacizumab in breast cancer may depend on the extent of prior therapy and/or the agent(s) with which bevacizumab is combined.
The approval of bevacizumab for use in humans validated VEGF as a target for therapy in cancer and fueled interest in antiangiogenesis as a therapeutic strategy. However, the potential benefits of bevacizumab must be carefully weighed against the potential risks. The cumulative data with this drug suggest that it is associated with high blood pressure, an increased risk of gastrointestinal perforations (overall incidence of about 1 percent), and spilling of protein in the urine.
Treatment with bevacizumab increases the risk of stroke and heart attack (especially in patients older than sixty-five and with a prior history of similar events). Use of bevacizumab has also been associated with two types of bleeding: (1) minor bleeding, usually in the form of nosebleeds and (2) massive, sometimes fatal, bleeding. While gastrointestinal bleeding has been observed, the association with potentially fatal bleeding from the lungs is perhaps most concerning. Patients with squamous cell lung cancers and central, cavitary lung cancers seem to be particularly at risk; the safety of bevacizumab in this population has not been established.
Furthermore, bevacizumab impairs wound healing in animal models and should be discontinued several weeks before elective surgery (it takes weeks to months to clear from the body). It should not be resumed until the surgical incision is fully healed (at least twenty-eight days after major surgery). Its use in patients with brain metastases or significant underlying heart disease has not been studied. Finally, bevacizumab may impair fertility and, if possible, should not be used in pregnant women.
Sunitinib Sunitinib is an oral, small molecule receptor tyrosine kinase inhibitor (TKI) of VEGF receptor signaling (VEGFR-1, -2, -3). In contrast to bevacizumab, sunitinib is a pill and is not specific for VEGF. It also inhibits the activity of the platelet-derived growth factor (PDGF)–alpha and PDGF-beta receptors—both of which are also known to be involved in the regulation of angiogenesis. In addition, it inhibits signaling through the c-Kit, RET, and FLT3 genes, which might also be valid therapeutic targets in cancer.
In 2006, sunitinib received FDA approval for the treatment of renal cell carcinoma (RCC), as well as gastrointestinal stromal tumor (GIST). GIST is a rare form of stomach and intestinal cancer that is exceedingly resistant to chemotherapy. Advances in our understanding of the molecular mechanisms underlying tumor progression led to the discovery that tumor growth is dependent on abnormal c-Kit receptor function. Treatment with the small molecule inhibitor imatinib mesylate (Gleevec) revolutionized the treatment of patients with advanced disease. However, until sunitinib, no treatment was available for patients with imatinib-resistant disease or intolerance to imatinib. Sunitinib was approved based on the results of a study suggesting that it delays time to tumor progression in these patients.
RCC (a form of kidney cancer) is unusually dependent on VEGF. A quarter of patients with RCC have locally advanced disease at diagnosis; one-third of patients have metastatic disease at presentation. The disease is inherently resistant to chemotherapy, and until recently, the mainstay of therapy was cytokine therapy with interleukin-2 (IL-2) and interferon. The results of two small studies exploring second-line treatment with single-agent sunitinib showed a 30 to 40 percent response rate and a median time to disease progression of eight to nine months.
A planned interim analysis of a Phase III study of interferon versus sunitinib (as first-line therapy for patients with metastatic RCC) subsequently showed that patients assigned to sunitinib experienced a higher response rate (34 percent versus 6 percent) and improved time to tumor progression (eleven months versus five months) compared to interferon.
Of note, bevacizumab has also shown activity in RCC but is not approved for this indication. Like bevacizumab, treatment with sunitinib is associated with hypertension. Other common side effects include diarrhea, hand-foot reaction (redness and tenderness of the palms and soles), skin rash, and fatigue.
Sorafenib (Nexavar) Sorafenib, like sunitinib, is a receptor TKI that interferes with VEGF receptor signaling. It is also oral, and not entirely specific. While originally developed as an inhibitor of c-Raf kinase, it also inhibits VEGF receptors (VEGFR-1 and -2), PDGFR-beta, FLT3, and c-Kit. In addition to direct effects on tumor cells, sorafenib inhibits angiogenesis in experimental models. The drug was approved for the treatment of RCC in 2005 based on the results of two studies suggesting that treatment with sorafenib delays disease progression. It has a side-effect profile that is fairly similar to that of sunitinib.
Over the past several years, we have witnessed an explosion in the number of antiangiogenic agents in development. A myriad of approaches are under investigation—the majority involving the inhibition of VEGF, a potent regulator of angiogenesis. Several large Phase III trials have validated VEGF as a target for therapy in cancer. Bevacizumab (Avastin) has received FDA approval for the treatment of patients with advanced lung cancer or metastatic colorectal cancer in combination with chemotherapy. The small molecule receptor tyrosine kinase inhibitors, sorafenib (Nexavar) and sunitinib, have antiangiogenic activity and were also recently approved for use in humans.
Future efforts will be aimed at identifying safer and more specific inhibitors of angiogenesis; improving patient selection (i.e., predicting response to therapy and/or toxicity); and optimizing the best approaches to treatment (stage of disease, duration of therapy, type of disease, dose, schedule, and combinations with other agents/treatment modalities). Hundreds of clinical trials are ongoing, in which the use of antiangiogenic agents in a variety of diseases, and in combination with a range of other agents, are being studied.