Vascular endothelial growth factor receptor tyrosine kinase inhibitors versus bevacizumab in metastatic colorectal cancer: A systematic review and meta-analysis

Bevacizumab has demonstrated a survival benefit in patients with metastatic colorectal cancer (mCRC) when combined with chemotherapy. Several randomized clinical trials comparing the efficacy and toxicity of vascular endothelial growth factor receptor (VEGFR) tyrosine kinase inhibitors (TKIs) against bevacizumab have been reported. The present meta-analysis was conducted to identify the potentially significant benefit of the combined treatment regimens in patients with mCRC. PubMed, Embase and Cochrane Library databases were searched for the randomized controlled trials published on or before September 2014, which compared the efficacy and toxicity of VEGFR TKIs with bevacizumab in combination with chemotherapy in patients with mCRC. The primary endpoints included progression-free survival (PFS), overall survival (OS) and overall response rate (ORR), and secondary endpoints were the toxicity profiles. Relative risks (RRs) with 95% confidence intervals (CIs) for response rate and adverse events (AEs) were calculated, as well as hazard ratios (HRs) for PFS and OS. The final analysis included 4 studies comprising a total of 1,929 intent-to-treat patients with mCRC, which compared VEGFR TKIs (cediranib and axitinib) plus chemotherapy with bevacizumab plus chemotherapy. Results demonstrated that VEGFR TKIs plus chemotherapy significantly resulted in a modest but significantly shorter PFS [hazard ratio (HR), 1.12; 95% CI, 1.00-1.25; P=0.05] compared with that of bevacizumab plus chemotherapy but not in OS (HR, 1.10; 95% CI, 0.88-1.17; P=0.87) and ORR (RR, 0.95; 95% CI, 0.85-1.05; P=0.30). VEGFR TKIs treatment showed a less favorable AE profile compared with bevacizumab, with higher rates of grade-III/IV diarrhea, fatigue, hypertension, neutropenia and thrombocytopenia, whereas a higher incidence of peripheral neuropathy associated with the bevacizumab group was observed. In conclusion, the addition of VEGFR TKIs to chemotherapy resulted in a modest but significantly shorter PFS but not in OS and ORR compared with bevacizumab. The VEGFR TKIs group showed a less favorable AE profile with higher rates of diarrhea, fatigue, hypertension, neutropenia and thrombocytopenia, whereas a higher incidence of peripheral neuropathy associated with the bevacizumab was observed.

Cediranib plus FOLFOX/CAPOX versus placebo plus FOLFOX/CAPOX in patients with previously untreated metastatic colorectal cancer: a randomized, double-blind, phase III study (HORIZON II)

PURPOSE

Cediranib is a highly potent inhibitor of vascular endothelial growth factor (VEGF) signaling with activity against all three VEGF receptors. HORIZON II [Cediranib (AZD2171, RECENTIN) in Addition to Chemotherapy Versus Placebo Plus Chemotherapy in Patients With Untreated Metastatic Colorectal Cancer] assessed infusional fluorouracil, leucovorin, and oxaliplatin/capecitabine and oxaliplatin (FOLFOX/CAPOX) with or without cediranib in patients with previously untreated metastatic colorectal cancer (mCRC).

PATIENTS AND METHODS

Eligible patients were initially randomly assigned 1:1:1 to receive cediranib (20 or 30 mg per day) or placebo plus FOLFOX/CAPOX. In an early analysis of this and two other cediranib studies (HORIZON I [Cediranib Plus FOLFOX6 Versus Bevacizumab Plus FOLFOX6 in Patients With Previously Treated Metastatic Colorectal Cancer] and HORIZON III [Cediranib Plus FOLFOX6 Versus Bevacizumab Plus FOLFOX6 in Patients With Untreated Metastatic Colorectal Cancer]), the 20-mg dose met the predefined criteria for continuation. Subsequent patients were randomly assigned 2:1 to the cediranib 20 mg or placebo arms. Progression-free survival (PFS) and overall survival (OS) were coprimary end points.

RESULTS

In all, 860 patients received cediranib 20 mg (n = 502) or placebo (n = 358). The addition of cediranib to FOLFOX/CAPOX resulted in PFS prolongation (hazard ratio [HR], 0.84; 95% CI, 0.73 to 0.98; P = .0121; median PFS, 8.6 months for cediranib v 8.3 months for placebo) but had no impact on OS (HR, 0.94; 95% CI, 0.79 to 1.12; P = .5707; median OS, 19.7 months for cediranib v 18.9 months for placebo). There were no significant differences in the secondary end points of objective response rate, duration of response, or liver resection rate. Median chemotherapy dose-intensity was decreased by approximately 10% in patients treated with cediranib. Adverse events (AEs) associated with cediranib were manageable. CONCLUSION Addition of cediranib 20 mg to FOLFOX/CAPOX resulted in a modest PFS prolongation, but no significant difference in OS. The cediranib AE profile was consistent with those from previous studies. Because of the lack of improvement in OS, cediranib plus an oxaliplatin-based regimen cannot be recommended as a treatment for patients with mCRC.

Cediranib with mFOLFOX6 versus bevacizumab with mFOLFOX6 as first-line treatment for patients with advanced colorectal cancer: a double-blind, randomized phase III study (HORIZON III)

PURPOSE

To compare the efficacy of cediranib (a vascular endothelial growth factor receptor tyrosine kinase inhibitor [VEGFR TKI]) with that of bevacizumab (anti-VEGF-A monoclonal antibody) in combination with chemotherapy as first-line treatment for advanced metastatic colorectal cancer (mCRC).

PATIENTS AND METHODS

HORIZON III [Cediranib Plus FOLFOX6 Versus Bevacizumab Plus FOLFOX6 in Patients With Untreated Metastatic Colorectal Cancer] had an adaptive phase II/III design. Patients randomly assigned 1:1:1 received mFOLFOX6 [oxaliplatin 85 mg/m(2) and leucovorin 400 mg/m(2) intravenously followed by fluorouracil 400 mg/m(2) intravenously on day 1 and then continuous infusion of 2,400 mg/m(2) over the next 46 hours every 2 weeks] with cediranib (20 or 30 mg per day) or bevacizumab (5 mg/kg every 14 days). An independent end-of-phase II analysis concluded that mFOLFOX6/cediranib 20 mg met predefined criteria for continuation; subsequent patients received mFOLFOX6/cediranib 20 mg or mFOLFOX6/bevacizumab (randomly assigned 1:1). The primary objective was to compare progression-free survival (PFS).

RESULTS

In all, 1,422 patients received mFOLFOX6/cediranib 20 mg (n = 709) or mFOLFOX6/bevacizumab (n = 713). Primary analysis revealed no significant difference between arms for PFS (hazard ratio [HR], 1.10; 95% CI, 0.97 to 1.25; P = .119), overall survival (OS; HR, 0.95; 95% CI, 0.82 to 1.10; P = .541), or overall response rate (46.3% v 47.3%). Median PFS and OS were 9.9 and 22.8 months for mFOLFOX6/cediranib and 10.3 and 21.3 months for mFOLFOX6/bevacizumab. The PFS upper 95% CI was outside the predefined noninferiority limit (HR < 1.2). Common adverse events with more than 5% incidence in the cediranib arm included diarrhea, neutropenia, and hypertension. Cediranib-treated patients completed fewer chemotherapy cycles than bevacizumab-treated patients (median 10 v 12 cycles). Patient-reported outcomes (PROs) were significantly less favorable in cediranib-treated versus bevacizumab-treated patients (P < .001).

CONCLUSION

Cediranib activity, in terms of PFS and OS, was comparable to that of bevacizumab when added to mFOLFOX6; however, the predefined boundary for PFS noninferiority was not met. The cediranib safety profile was consistent with previous studies but led to less favorable PROs compared with bevacizumab. Investigation of oral TKIs in CRC continues.

Thyroid cancer: pathogenesis and targeted therapy

Therapeutic options for advanced, unresectable radioiodine-resistant thyroid cancers have historically been limited. Recent progress in understanding the pathogenesis of the various subtypes of thyroid cancer has led to increased interest in the development of targeted therapies, with potential strategies including angiogenesis inhibition, inhibition of aberrant intracellular signaling in the MAPK and PI3K/AKT/mTOR pathways, radioimmunotherapy, and redifferentiation agents. On the basis of a recent positive phase III clinical trial, the RET, vascular endothelial growth factor receptor (VEGFR), and epidermal growth factor receptor (EGFR) inhibitor vandetanib has received FDA approval as of April 2011 for use in the treatment of advanced medullary thyroid cancer. Several other recent phase II clinical trials in advanced thyroid cancer have demonstrated significant activity, and multiple other promising therapeutic strategies are in earlier phases of clinical development. The recent progress in targeted therapy is already revolutionizing management paradigms for advanced thyroid cancer, and will likely continue to dramatically expand treatment options in the coming years.

Distinct patterns of cytokine and angiogenic factor modulation and markers of benefit for vandetanib and/or chemotherapy in patients with non-small-cell lung cancer

PURPOSE

There is an unmet need for biomarkers for identifying patients likely to benefit from anticancer treatments, selecting dose, and understanding mechanisms of resistance. Plasma vascular endothelial growth factor (VEGF) and soluble VEGF receptor 2 (sVEGFR-2) are known to be modulated by VEGF pathway inhibitors. It is unknown whether chemotherapy or VEGFR inhibitor/chemotherapy combinations induce changes in these or other cytokines and angiogenic factors (CAFs) and whether such changes could be markers of benefit.

METHODS

Thirty-five plasma CAFs were analyzed using multiplexed bead arrays and enzyme-linked immunosorbent assays from 123 patients with non-small-cell lung cancer in a randomized phase II study who received vandetanib, a VEGFR and epidermal growth factor receptor inhibitor, monotherapy carboplatin and paclitaxel (CP), or the combination (VCP). Changes in CAFs at days 8, 22, and 43 from baseline were correlated with progression risk.

RESULTS

VEGF increased and sVEGFR-2 decreased by day 43 in the vandetanib arm, whereas a distinct pattern was observed in the CP and VCP arms, with significant decreases in interleukin (IL) -12, IL-1 receptor antagonist, and matrix metalloproteinase 9 (MMP-9) and increased macrophage chemoattractant protein 1. In each treatment arm, changes in different markers were associated with progression risk. For example, increases in IL-8 with VCP, MMP-9 with CP, and VEGF with vandetanib monotherapy were associated with increased progression risk, and increase in intercellular adhesion molecule 1 with vandetanib was associated with decreased risk.

CONCLUSION

Vandetanib and chemotherapy treatment led to distinct patterns of CAF changes; the combination resembled chemotherapy alone. Changes in specific CAFs correlated with clinical outcome, but markers differed for each treatment arm. CAF profiling may provide insights into the biologic effects of treatment and identify drug-specific markers of activity and clinical benefit.

AMG 706, an oral, multikinase inhibitor that selectively targets vascular endothelial growth factor, platelet-derived growth factor, and kit receptors, potently inhibits angiogenesis and induces regression in tumor xenografts

The growth of solid tumors is dependent on the continued stimulation of endothelial cell proliferation and migration resulting in angiogenesis. The angiogenic process is controlled by a variety of factors of which the vascular endothelial growth factor (VEGF) pathway and its receptors play a pivotal role. Small-molecule inhibitors of VEGF receptors (VEGFR) have been shown to inhibit angiogenesis and tumor growth in preclinical models and in clinical trials. A novel nicotinamide, AMG 706, was identified as a potent, orally bioavailable inhibitor of the VEGFR1/Flt1, VEGFR2/kinase domain receptor/Flk-1, VEGFR3/Flt4, platelet-derived growth factor receptor, and Kit receptors in preclinical models. AMG 706 inhibited human endothelial cell proliferation induced by VEGF, but not by basic fibroblast growth factor in vitro, as well as vascular permeability induced by VEGF in mice. Oral administration of AMG 706 potently inhibited VEGF-induced angiogenesis in the rat corneal model and induced regression of established A431 xenografts. AMG 706 was well tolerated and had no significant effects on body weight or on the general health of the animals. Histologic analysis of tumor xenografts from AMG 706-treated animals revealed an increase in endothelial apoptosis and a reduction in blood vessel area that preceded an increase in tumor cell apoptosis. In summary, AMG 706 is an orally bioavailable, well-tolerated multikinase inhibitor that is presently under clinical investigation for the treatment of human malignancies.

Role of anti-angiogenesis agents in treating NSCLC: focus on bevacizumab and VEGFR tyrosine kinase inhibitors

OPINION STATEMENT

Successful inhibition of angiogenesis with the anti-vascular endothelial growth factor (VEGF) antibody bevacizumab has improved the efficacy seen with standard cytotoxic therapy in NSCLC. The addition of bevacizumab to first-line chemotherapy improved response rate and progression free survival and added 2 months to median overall survival for those patients with advanced stage NSCLC on the treatment arm of E4599. Bevacizumab is now a standard agent to add to frontline carboplatin and paclitaxel for patients with newly diagnosed NSCLC who meet the eligibility criteria from the landmark E4599 trial. Unfortunately about half of all patients are not eligible either because they have squamous histology, brain metastases, or are on anti-coagulation. Ongoing trials are further exploring the safety of bevacizumab in these patient populations, as well as in combination with other cytotoxic regimens. Exploration of other applications of bevacizumab in the second-line and adjuvant setting are ongoing as well. The largest class of drugs that block angiogenesis are the multi-targeted tyrosine kinase inhibitors (TKIs) that target the VEGF receptor (VEGFR). These drugs are still in development, and though two are now on the market for treating other malignancies, their role in NSCLC is under investigation. These agents have the advantages of hitting multiple targets, convenient oral administration, and potential for lower cost. Their lack of target specificity leads to unexpected toxicity, but also promising efficacy. For example, the overall objective response rate of 9.5% with single agent sunitinib compares similarly to that of pemetrexed or docetaxel in previously treated NSCLC patients, but toxicity, notably fatigue, lead to discontinuation in 38% of patients. Hypertension, hemorrhage and cavitation are common toxicities amongst this class of agents. Rash, fatigue, myalgia, and hand-foot syndrome are more specifically seen with TKIs. These compounds may also be synergistic or additive with traditional cytotoxic chemotherapy drugs and other novel compounds. In early trials sorafenib as a single agent has shown no clinical response in previously treated NSCLC patients, whereas clinical benefit in combination with erlotinib or chemotherapy has been seen in early studies. Vandetanib has demonstrated objective responses as a single agent and in combination with chemotherapy in previously treated NSCLC patients. A phase I trial of AZD2171 with carboplatin and paclitaxel in newly diagnosed advanced stage NSCLC also demonstrated promising results with 6 of 15 patients achieving partial responses. NSCLC specific trials are also underway, or in development for pazopanib, axitinib, AMG 706, XL647, enzastaurin, and other TKIs. Other anti-angiogenesis agents with different mechanisms of action include thalidomide and its derivatives, monoclonal antibodies to the VEGFRs, and VEGF Trap, a chimeric molecule which combines extracellular portions of VEGFR1 and VEGFR2 with the Fc portion of immunoglobulin G1 to form a molecule that binds and "traps" VEGF. Despite modest improvements, prognosis continues to be poor for patients with advanced NSCLC. Bevacizumab is a first step into the world of angiogenesis inhibitors for NSCLC and though it only offers a modest survival benefit in a limited patient population, it paves the way for the development of the next generation of anti-angiogenesis inhibitors. We can hope that further improvements in survival will follow.

Combining radiotherapy with AZD2171, a potent inhibitor of vascular endothelial growth factor signaling: pathophysiologic effects and therapeutic benefit

AZD2171 is a highly potent, orally active inhibitor of vascular endothelial growth factor receptor signaling. The potential for AZD2171 to enhance the antitumor effects of radiotherapy was investigated in lung (Calu-6) and colon (LoVo) human tumor xenograft models. Combined treatment resulted in a significantly enhanced growth delay compared with either modality alone. The enhancement was independent of whether chronic once daily AZD2171 treatment was given 2 h prior to each radiation fraction (2 Gy daily for 3 or 5 consecutive days), and daily thereafter, or commenced immediately following the course of radiotherapy. Histologic assessments revealed that 5 days of radiation (2 Gy) or AZD2171 (3 or 6 mg/kg/d) reduced vessel density and perfusion. Concomitant AZD2171 and radiation enhanced this effect and produced a significant increase in tumor hypoxia. Concomitant AZD2171 (6 mg/kg/d) was also found to reduce tumor growth significantly during the course of radiotherapy (5 x 2 Gy). However, the extent and duration of tumor regression observed postradiotherapy was similar to sequentially treated tumors, suggesting that preirradiated tumors were sensitized to AZD2171 treatment. An enhanced antivascular effect of administering AZD2171 postradiotherapy was observed in real-time in Calu-6 tumors grown in dorsal window chambers. Collectively, these data support the clinical development of AZD2171 in combination with radiotherapy.

Vascular endothelial growth factor receptor tyrosine kinase inhibitors vandetanib (ZD6474) and AZD2171 in lung cancer

Vascular endothelial growth factor (VEGF) is a rational target for advanced non-small cell lung cancer (NSCLC), a hypothesis validated by the recent Eastern Cooperative Oncology Group E4599 trial showing that the addition of the VEGF monoclonal antibody bevacizumab to chemotherapy prolongs overall survival. Several new tyrosine kinase inhibitors targeting the VEGF pathway are currently in advanced clinical development for NSCLC and offer several possible advantages compared with monoclonal antibodies, including oral administration, more flexible dosing, a broader spectrum of target inhibition, and different toxicity profiles. Among these agents, vandetanib (ZD6474), an inhibitor of the VEGF receptor (VEGFR)-2 and epidermal growth factor receptor tyrosine kinase, has been the most extensively studied. In a randomized phase II study of patients with platinum-refractory NSCLC, including squamous histology, vandetanib prolonged progression-free survival compared with gefitinib. In another phase II trial, an improvement in progression-free survival was observed for vandetanib in combination with docetaxel compared with docetaxel alone. AZD2171 is an inhibitor of VEGFR-1, VEGFR-2, and VEGFR-3 and other tyrosine kinases that has shown clinical activity in NSCLC in combination with carboplatin and paclitaxel. Several phase III trials are under way testing these agents either as monotherapy or in combination with chemotherapy in patients with lung cancer. Early results with these agents, and others being tested, raise the possibility that there will eventually be multiple VEGF-targeted therapies available in the clinic that can potentially benefit a broader range of patients with advanced-stage NSCLC.

Imaging tumor vascular heterogeneity and angiogenesis using dynamic contrast-enhanced magnetic resonance imaging

This article reviews the application of dynamic contrast-enhanced magnetic resonance imaging in both clinical studies and early-phase trials of angiogenesis inhibitors. Emphasis is placed on how variation in image acquisition and analysis affects the meaning and use of derived variables. We then review the potential for future developments, with particular reference to the application of dynamic contrast-enhanced magnetic resonance imaging to evaluate the heterogeneity of tumor tissues.

Antiangiogenics: the potential role of integrating this novel treatment modality with chemoradiation for solid cancers

Although still in very early stages of clinical development, the combination of antiangiogenics with contemporary chemoradiotherapy regimens has emerged as a feasible and promising approach to many cancers. We review the rationale and the current understanding of antiangiogenics and their therapeutic potential in combination with chemoradiotherapy. Finally, we offer a perspective on future research directions aimed at making this complex therapeutic approach successful in the clinic.

Angiogenesis Inhibitor Therapies: Focus on Kidney Toxicity and Hypertension

Angiogenesis inhibitors that target the epidermal growth factor (EGF) receptor (EGFR) and vascular endothelial growth factor (VEGF) constitute an important addition to the therapeutic armamentarium for the treatment of patients with metastatic disease. However, because the same growth factors are expressed in the kidneys, these treatment molecules have renal side effects. EGFR is expressed mainly in tubules (mainly distal and collecting segments) and mesangial and parietal epithelial cells. EGF is involved in maintaining tubular integrity and is a potent mitogen for cultured mesangial cells. Few cases of acute renal failure have been reported related to EGFR inhibitors. VEGF and VEGF receptors are still highly expressed in the kidney. VEGF is expressed in podocytes in the glomerulus, and VEGF receptors are present on endothelial, mesangial, and peritubular capillary cells. Signaling between endothelial cells and podocytes is essential for the proper development and maintenance of the filtration function of the kidney glomerulus. The most common renal class effects of VEGF antagonists are both manageable; hypertension and proteinuria commonly regressive on drug withdrawal. There was a dose-dependent increase in risk of proteinuria and hypertension in patients with cancer who received targeted therapies. Furthermore, few patients with glomerulonephritis or thrombotic microangiopathy secondary to treatment were reported. Hypertension is believed to be nitric oxide dependent, whereas proteinuria seems to be related to downregulation of podocyte tight junction protein. This article reviews data relating to hypertension and proteinuria associated with the use of these drugs.

Angiogenesis in brain tumours

Despite aggressive surgery, radiotherapy and chemotherapy, malignant gliomas remain uniformly fatal. To progress, these tumours stimulate the formation of new blood vessels through processes driven primarily by vascular endothelial growth factor (VEGF). However, the resulting vessels are structurally and functionally abnormal, and contribute to a hostile microenvironment (low oxygen tension and high interstitial fluid pressure) that selects for a more malignant phenotype with increased morbidity and mortality. Emerging preclinical and clinical data indicate that anti-VEGF therapies are potentially effective in glioblastoma--the most frequent primary brain tumour--and can transiently normalize tumour vessels. This creates a window of opportunity for optimally combining chemotherapeutics and radiation.

Blood-based biomarkers of SU11248 activity and clinical outcome in patients with metastatic imatinib-resistant gastrointestinal stromal tumor

PURPOSE

There is an unmet need for noninvasive markers to measure the biological effects of targeted agents, particularly those inhibiting the vascular endothelial growth factor (VEGF) receptor (VEGFR) pathway, and identify patients most likely to benefit from treatment. In this study, we investigated potential blood-based biomarkers for SU11248 (sunitinib malate), a multitargeted tyrosine kinase inhibitor, in patients with metastatic imatinib-refractory gastrointestinal stromal tumors.

EXPERIMENTAL DESIGN

Patients (n=73) enrolled in a phase I/II trial received SU11248 daily for 14 or 28 days followed by 14 days without treatment per cycle. Clinical benefit was defined as progression-free survival of >6 months. We assessed plasma markers, including VEGF and soluble VEGFR-2 (sVEGFR-2), and two cellular populations bearing VEGF receptors: monocytes and, in a subset of patients, mature circulating endothelial cells (CEC).

RESULTS

Compared to patients with progressive disease, patients with clinical benefit had significantly greater increases in CECs (0.52 versus -0.01 CEC/microL/d, P=0.03) and smaller decreases in monocyte levels (47% versus 60%, P=0.007) during cycle 1. VEGF increased by 2.2-fold and sVEGFR-2 decreased 25% during the first 2 weeks of treatment. Neither plasma marker correlated with clinical outcome although a modest inverse correlation was observed between sVEGFR-2 changes and plasma drug levels. Monocytes, VEGF, and sVEGFR-2 all rebounded towards baseline off treatment.

CONCLUSIONS

Monocytes, VEGF, and sVEGFR-2 were consistently modulated by treatment, suggesting that they may serve as pharmacodynamic markers for SU11248. Changes in CECs and monocytes, but not the plasma markers, differed between the patients with clinical benefit and those with progressive disease. These end points merit further investigation in future trials to determine their utility as markers of SU11248 activity and clinical benefit in gastrointestinal stromal tumors and other tumor types.

VEGF-targeted cancer therapy strategies: current progress, hurdles and future prospects

Despite setbacks, the clinical development of antiangiogenic agents has accelerated remarkably over the past 3-4 years. Consequently, there are currently three direct inhibitors of the VEGF pathway approved for use in cancer therapy. Other agents that block the VEGF pathway are in advanced stages of clinical development and have shown promising results. With these exciting developments come crucial questions regarding the use of these new molecular-targeted agents, alone or in combination with standard cytotoxic or targeted agents. Importantly, the mechanisms of action of anti-VEGF therapy remain unknown. Here, we discuss several potential mechanisms of action such as tumor vascular normalization, bone marrow-derived cell recruitment blockade and cytostatic effects of anti-VEGF therapy. We review the current progress, the major stumbling blocks and the future directions for anti-cancer therapy using anti-VEGF agents, emphasizing clarification of the underlying molecular mechanisms of action and biomarker identification and validation.

A review of the latest clinical compounds to inhibit VEGF in pathological angiogenesis

Angiogenesis plays an important role in the formation of new blood vessels and is crucial for tumour development and progression. Imbalance between pro- and antiangiogenesis factors regulates the biological process of angiogenesis. The best characterised of the proangiogenic factors and the most potent is vascular endothelial growth factor (VEGF). The binding of VEGF to one of its transmembrane tyrosine kinase receptors, which are predominantly found on endothelial cells, results in receptor dimerisation, activation and autophosphorylation of the tyrosine kinase domain. This triggers a cascade of complex downstream signalling pathways. Several strategies targeting the VEGF signalling pathway have been developed. These include neutralising antibodies to VEGF (bevacizumab) or VEGF receptors (VEGFRs) (DC101), soluble VEGFR/VEGFR hybrids (VEGF-Trap), and tyrosine kinase inhibitors of VEGFRs (BAY43-9006, SU11248, ZD6474, AZD2171, PTK/ZK and others). Several of these agents are now being investigated in clinical trials.

Angiogenesis inhibitors in the treatment of lung cancer

Despite improvements in cytotoxic chemotherapy and combined modality therapies for lung cancer, the prognosis for patients remains poor, and the majority of patients die from the disease. Angiogenesis, i.e. the formation of new blood vessels, is important for tumor growth, invasion and metastasis and represents a rational target in the development of more effective treatments. The vascular endothelial growth factor (VEGF) signaling pathway plays a crucial role in the angiogenic process and consequently, inhibitors of this system are currently under development. The most studied anti-angiogenic agents include anti-VEGF monoclonal antibodies and VEGF receptor tyrosine kinase inhibitors. Recent clinical trials have yielded promising results. This article will review angiogenesis inhibitors targeting the VEGF pathway which are currently being developed for the treatment of lung cancer.

Anti-angiogenic therapy in the treatment of advanced renal cell cancer

Metastatic renal cell cancer is associated with a poor prognosis and is resistant to traditional chemotherapy agents. The majority of tumours are associated with inactivation of the von Hippel-Lindau gene and subsequent overexpression of proangiogenic factors, including vascular endothelial growth factor (VEGF). Drugs targeting these pathways have undergone clinical testing in renal cell cancer with encouraging results. This type of therapy is set to revolutionise the treatment of renal cell cancer and this review outlines recent evidence from clinical trials investigating the most promising of these agents.

DCE-MRI biomarkers in the clinical evaluation of antiangiogenic and vascular disrupting agents

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is now frequently used in early clinical trial assessment of antiangiogenic and vascular disrupting compounds. Evidence of drug efficacy and dose-dependent response has been demonstrated with some angiogenesis inhibitors. This review highlights the critical issues that influence T₁-weighted DCE-MRI data acquisition and analysis, identifies important areas for future development and reviews the clinical trial findings to date.

The use of vascular biomarkers and imaging studies in the early clinical development of anti-tumour agents targeting angiogenesis

Recent findings about the pathomechanisms of tumour angiogenesis have led to new therapeutic options in the treatment of malignant tumours. During the development of anti-angiogenic drugs, reporting ranged from healing cancer to completely ineffective drugs. In 2003 the first anti-angiogenic drug was approved. Several anti-angiogenic drugs are still in the clinical phase of development. In contrast to identifying the maximal tolerable dose, determination of the optimal biological dose--reaching biological activity at lower doses--has become the main target in the early development of anti-angiogenic agents. This has been evaluated by different biomarker techniques. As a new standard in anti-tumour treatment, a better understanding of imaging in the treatment monitoring for anti-angiogenic agents is important. Studies of tumour angiogenesis by tissue sampling rely on invasive procedures, adequate sampling and painstaking estimation of histological microvessel density. Attempts to develop wound healing assays to correlate angiogenesis in wounds with angiogenesis in tumour have been made but are still considered invasive and correlation of healthy with malignant tissue is still of limited validity. Several soluble markers of tumour angiogenesis were detected in various malignant diseases and were evaluated for assessing their use as surrogate markers in tumour angiogenesis. Further, soluble markers were investigated for visualizing them as imaging tools. Combining both, new soluble markers and imaging techniques, developing anticancer drugs and monitoring of therapy success becomes a dynamic process in which finally the patients' individual response can be achieved soon. Time-consuming delays for anatomically based restaging procedures can be avoided. Characterization of soluble biomarkers as well as different imaging techniques such as ultrasound, computed tomography (CT), magnetic resonance imaging and positron emission tomography combined with or without CT are reviewed in this manuscript.

Tyrosine kinase inhibitors of vascular endothelial growth factor receptors in clinical trials: current status and future directions

Angiogenesis plays a central role in the process of tumor growth and metastatic dissemination. The vascular endothelial growth factor (VEGF) family of peptide growth factors and receptors are key regulators of this process. Agents directed either against VEGF or VEGF receptors (VEGFRs) have been developed. The tyrosine kinase inhibitors of VEGFRs are low-molecular-weight, ATP-mimetic proteins that bind to the ATP-binding catalytic site of the tyrosine kinase domain of VEG-FRs, resulting in blockade of intracellular signaling. Several of these agents are currently in different phases of clinical development. Large randomized phase III trials have demonstrated the efficacy of sunitinib and sorafenib in the treatment of patients affected by gastrointestinal stromal tumors and renal cancer refractory to standard therapies, respectively. Positive results also have been reported with the combination of ZD6474 and chemotherapy in previously treated non-small cell lung cancer patients. For other agents, such as vatalanib, contrasting outcomes in metastatic colorectal cancer patients have been reported: the final results of these trials are expected in 2006. However, several key questions remain to be addressed, regarding the choice of an adequate dose or schedule, the presence of "off-target" effects, the safety of long-term administration, and the research of new clinical end points or methodological approaches for the optimal clinical development of these agents.

A physiologic imaging pilot study of breast cancer treated with AZD2171

BACKGROUND

This pilot study combined physiologic imaging, microcomputed tomography, and histologic tumor evaluation with a xenograft model of breast cancer to identify surrogates likely to correlate with response to AZD2171, an inhibitor of the vascular endothelial growth factor (VEGF) receptor tyrosine kinases.

EXPERIMENTAL DESIGN

MCF-7 cells transfected with vector (MCF-7neo) or VEGF (MCF(VEGF)) were implanted in the right and left mammary fat pads of 75 athymic mice. Treatment with AZD2171 (5 mg/kg/d) or vehicle control was initiated once tumors were established. Positron emission tomography with [11C]carbon monoxide to measure blood volume, [18F]fluoromethane to measure perfusion, and [18F]fluorodeoxyglucose to measure glucose utilization was done at baseline, and after 24 hours, 72 hours, and 4 weeks of treatment. After imaging, tumors were analyzed for microvessel density, proliferation, and VEGF expression.

RESULTS

AZD2171 induced significant inhibition of tumor growth in established MCF-7(neo) xenografts and regression of established MCF-7(VEGF) xenografts. An acute decrease in blood flow was detected in MCF-7(VEGF) tumors at 24 hours (P = 0.05). Tumor blood volume was increased in the MCF-7(VEGF) tumors but correlated with tumor size; blood volume did not change with AZD2171 therapy. Glucose utilization correlated with tumor size and did not change with acute or chronic AZD2171 therapy. Unlike blood flow and blood volume, glucose utilization was similar in MCF-7neo and MCF-7(VEGF) tumors. Microvessel density and proliferation acutely decreased in MCF-7(VEGF) tumors but returned to baseline during chronic therapy.

CONCLUSIONS

[18F]Fluoromethane imaging may be a useful surrogate for biological activity of AZD2171 with changes identified within 24 hours of starting therapy.

Inhibitors of signal transduction protein kinases as targets for cancer therapy

Cancer development requires that tumour cells attain several capabilities, including increased replicative potentials, anchorage and growth-factor independency, evasion of apoptosis, angiogenesis and metastasis. Many of these processes involve the actions of protein kinases, which have emerged as key regulators of all aspects of neoplasia. Perturbed protein kinase activity is repeatedly found to be associated with human malignancies, making these proteins attractive targets for anti-cancer therapy. The last decade has witnessed an exponential increase in the development of specific small protein kinase inhibitors. Many of them are in clinical trials in patients with different types of cancer and some are successfully used in clinic. This review describes different approaches that are currently applied to develop such specific protein kinase inhibitors and provides an overview of protein kinase inhibitors that are currently in clinical trials or are administered in the clinic. Focus is directed on inhibitors against receptor tyrosine kinases and protein kinases participating in the signalling cascades.

AZD2171: a highly potent, orally bioavailable, vascular endothelial growth factor receptor-2 tyrosine kinase inhibitor for the treatment of cancer

Inhibition of vascular endothelial growth factor-A (VEGF) signaling is a promising therapeutic approach that aims to stabilize the progression of solid malignancies by abrogating tumor-induced angiogenesis. This may be accomplished by inhibiting the kinase activity of VEGF receptor-2 (KDR), which has a key role in mediating VEGF-induced responses. The novel indole-ether quinazoline AZD2171 is a highly potent (IC50 < 1 nmol/L) ATP-competitive inhibitor of recombinant KDR tyrosine kinase in vitro. Concordant with this activity, in human umbilical vein endothelial cells, AZD2171 inhibited VEGF-stimulated proliferation and KDR phosphorylation with IC50 values of 0.4 and 0.5 nmol/L, respectively. In a fibroblast/endothelial cell coculture model of vessel sprouting, AZD2171 also reduced vessel area, length, and branching at subnanomolar concentrations. Once-daily oral administration of AZD2171 ablated experimental (VEGF-induced) angiogenesis in vivo and inhibited endochondral ossification in bone or corpora luteal development in ovary; physiologic processes that are highly dependent upon neovascularization. The growth of established human tumor xenografts (colon, lung, prostate, breast, and ovary) in athymic mice was inhibited dose-dependently by AZD2171, with chronic administration of 1.5 mg per kg per day producing statistically significant inhibition in all models. A histologic analysis of Calu-6 lung tumors treated with AZD2171 revealed a reduction in microvessel density within 52 hours that became progressively greater with the duration of treatment. These changes are indicative of vascular regression within tumors. Collectively, the data obtained with AZD2171 are consistent with potent inhibition of VEGF signaling, angiogenesis, neovascular survival, and tumor growth. AZD2171 is being developed clinically as a once-daily oral therapy for the treatment of cancer.