Vascular targeted therapies in oncology

A review on various targeted anticancer therapies

Translational oncology aims to translate laboratory research into new anticancer therapies. Contrary to conventional surgery, chemotherapy, and radiotherapy, targeted anticancer therapy (TAT) refers to systemic administration of drugs with particular mechanisms that specifically act on well-defined targets or biologic pathways that, when activated or inactivated, may cause regression or destruction of the malignant process, meanwhile with minimized adverse effects on healthy tissues. In this article, we intend to first give a brief review on various known TAT approaches that are deemed promising for clinical applications in the current trend of personalized medicine, and then we will introduce our newly developed approach namely small molecular sequential dual targeting theragnostic strategy as a generalized class of TAT for the management of most solid malignancies, which, after optimization, is expected to help improve overall cancer treatability and curability.

A pharmacokinetic and safety study of single dose intravenous combretastatin A4 phosphate in Chinese patients with refractory solid tumours

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• Three pharmacokinetic and safety studies for combretastatin A4 phosphate (CA4P), the first vascular disrupting agent, have been conducted in Western countries. • The maximum tolerated dose (MTD) was approximately 60-68 mg m(-2). • CA4P-related grade 3 or 4 adverse events were tumour pain, dyspnoea, hypoxia and syncope in patients who received doses ≥ 50 mg m(-2).

WHAT THIS STUDY ADDS

• This is the first pharmacokinetic and safety study conducted in East Asian patients. • There appeared to be a trend that Chinese patients metabolized CA4 more rapidly and had greater neurotoxicity than patients in Western countries. • We observed favourable clinical responses in patients with refractory nasopharyngeal carcinoma. • CA4P-induced acute renal failure was seen in one dehydrated Chinese patient.

AIMS

This trial was conducted to evaluate the safety and pharmacokinetics of combretastatin A4 phosphate (CA4P) given intravenously as a single dose to Chinese patients with refractory solid tumours. METHODS Twenty-five patients were treated with single doses of CA4P according to a dose escalation scheme: 5, 10, 20, 33, 50, 65 and 85 mg m(-2) infused intravenously over 30 min.

RESULTS

CA4P was generally well tolerated at ≤ 65 mg m(-2). Transient, moderate increases in the heart rate-corrected QT interval occurred at all doses. CA4P produced a transient dose-dependent increase in neural and gastrointestinal toxicities. Acute renal failure occurred in one dehydrated patient who had also taken paracetamol. There were seven episodes of dose-limiting toxicity at doses ≥65 mg m(-2), including two episodes of reversible ataxia at 85 mg m(-2).For CA4, at 50 mg m(-2),mean (SD) peak plasma concentration (C(max) was 0.99 (0.33) mM, area under the curve from time zero to time of last quantifiable concentration (AUC(0,t)) was 1.42 (0.30) mM h and terminal elimination half-life (t(1/2)was 1.81 (0.61) h. At 65 mg m-2,C(max) was 1.73 (0.62) mM,AUC(0,t) was 3.19 (1.47) mM h and t (1/2) was 1.90 (0.61) h [corrected]One patient with nasopharyngeal carcinoma had an obvious clinical response with central necrosis in the metastatic lung mass. CONCLUSION Doses ≤ 65 mg m(-2) given as 30 min infusions define the maximum tolerated dose in East Asian patients, and doses in the range of 50-65 mg m(-2) have been selected for further studies.

Cytotoxicity of VEGF(121)/rGel on vascular endothelial cells resulting in inhibition of angiogenesis is mediated via VEGFR-2

Background

The fusion protein VEGF₁₂₁/rGel composed of the growth factor VEGF₁₂₁ and the plant toxin gelonin targets the tumor neovasculature and exerts impressive anti-vascular effects. We have previously shown that VEGF₁₂₁/rGel is cytotoxic to endothelial cells overexpressing VEGFR-2 but not to endothelial cells overexpressing VEGFR-1. In this study, we examined the basis for the specific toxicity of this construct and assessed its intracellular effects in vitro and in vivo.

Methods

We investigated the binding, cytotoxicity and internalization profile of VEGF₁₂₁/rGel on endothelial cells expressing VEGFR-1 or VEGFR-2, identified its effects on angiogenesis models in vitro and ex vivo, and explored its intracellular effects on a number of molecular pathways using microarray analysis.

Results

Incubation of PAE/VEGFR-2 and PAE/VEGFR-1 cells with ¹²⁵I-VEGF₁₂₁/rGel demonstrated binding specificity that was competed with unlabeled VEGF₁₂₁/rGel but not with unlabeled gelonin. Assessment of the effect of VEGF₁₂₁/rGel on blocking tube formation in vitro revealed a 100-fold difference in IC₅₀ levels between PAE/VEGFR-2 (1 nM) and PAE/VEGFR-1 (100 nM) cells. VEGF₁₂₁/rGel entered PAE/VEGFR-2 cells within one hour of treatment but was not detected in PAE/VEGFR-1 cells up to 24 hours after treatment. In vascularization studies using chicken chorioallantoic membranes, 1 nM VEGF₁₂₁/rGel completely inhibited bFGF-stimulated neovascular growth. The cytotoxic effects of VEGF₁₂₁/rGel were not apoptotic since treated cells were TUNEL-negative with no evidence of PARP cleavage or alteration in the protein levels of select apoptotic markers. Microarray analysis of VEGF₁₂₁/rGel-treated HUVECs revealed the upregulation of a unique "fingerprint" profile of 22 genes that control cell adhesion, apoptosis, transcription regulation, chemotaxis, and inflammatory response.

Conclusions

Taken together, these data confirm the selectivity of VEGF₁₂₁/rGel for VEGFR-2-overexpressing endothelial cells and represent the first analysis of genes governing intoxication of mammalian endothelial cells by a gelonin-based targeted therapeutic agent.

Investigation of protein induction in tumour vascular targeted strategies by MALDI MSI

Characterising the protein signatures in tumours following vascular-targeted therapy will help determine both treatment response and resistance mechanisms. Here, mass spectrometry imaging and MS/MS with and without ion mobility separation have been used for this purpose in a mouse fibrosarcoma model following treatment with the tubulin-binding tumour vascular disrupting agent, combretastatin A-4-phosphate (CA-4-P). Characterisation of peptides after in situ tissue tryptic digestion was carried out using Matrix-Assisted Laser Desorption/Ionisation-Mass Spectrometry (MALDI-MS) and Matrix-Assisted Laser Desorption/Ionisation-Ion Mobility Separation-Mass Spectrometry Imaging (MALDI IMS-MSI) to observe the spatial distribution of peptides. Matrix-Assisted Laser Desorption/Ionisation-Ion Mobility Separation-Tandem Mass Spectrometry (MALDI-IMS-MS/MS) of peaks was performed to elucidate any pharmacological responses and potential biomarkers. By taking tumour samples at a number of time points after treatment gross changes in the tissue were indicated by changes in the signal levels of certain peptides. These were identified as arising from haemoglobin and indicated the disruption of the tumour vasculature. It was hoped that the use of PCA-DA would reveal more subtle changes taking place in the tumour samples however these are masked by the dominance of the changes in the haemoglobin signals.

Classification and toxicities of vascular disrupting agents

Vascular disrupting agents (VDAs) are an exciting new group of targeted therapies under active clinical research in many solid tumors, in particular, lung cancer. Small-molecule VDAs are the focus of current clinical research, and consist of the flavonoids and the tubulin-binding agents. Toxicities of single-agent VDAs are characterized by acute, transient, and generally noncumulative side effects including headaches, nausea and vomiting, tumor pain, hypertension, and tachycardia. Flavonoid agents can also cause infusion site pain, visual disturbances, electrocardiac abnormalities, and symptoms consistent with an acute release of serotonin. Tubulin-binding agents can result in cardiac ischemia, abdominal pain, neuromotor abnormalities and cerebellar ataxia, and acute hemodynamic changes. Clinical trials investigating VDAs in combination with traditional chemotherapy have also shown the potential for significant pharmacologic and adverse toxicity interactions. Further research will need to focus on pharmacokinetic and pharmacodynamic parameters to optimize dosing schedules, determine effective combinations with chemotherapy, and minimize toxicities associated with VDAs.

Temporal aspects of the action of ASA404 (vadimezan; DMXAA)

IMPORTANCE OF THE FIELD

Tumor vascular disrupting agents (tumor VDAs) act by selective induction of tumor vascular failure. While their action is distinct from that of antiangiogenic agents, their clinical potential is likely to reside in improving the efficacy of combination therapy.

AREAS COVERED IN THIS REVIEW

This review describes the preclinical development, clinical trial and mode of action of ASA404, a flavonoid class tumor VDA. This class has a unique dual action, simultaneously disrupting vascular endothelial function and stimulating innate tumor immunity. This review covers the early development of ASA404, through to Phase III trial.

WHAT THE READER WILL GAIN

The reader will gain insight into the sequence of ASA404-induced changes in tumor tissue. Early events include increased vascular permeability, increased endothelial apoptosis and decreased blood flow, while later effects include the induction of serotonin, tumor necrosis factor, other cytokines and chemokines, and nitric oxide. This cascade of events induces sustained reduction of tumor blood flow, induction of tumor hypoxia and increased inflammatory responses. The reader will also gain an appreciation of how the potentiation of radiation and chemotherapeutic effects by ASA404 in murine tumors shaped the development of combination clinical trials.

TAKE HOME MESSAGE

Although there are species differences in ASA404 activity, many features of its action in mice translate to human studies. The future of ASA404 as an effective clinical agent will rely on the development of an appreciation of its ability to optimize the complex interaction between tumor vasculature and tumor immunity during therapy.

A perspective on vascular disrupting agents that interact with tubulin: preclinical tumor imaging and biological assessment

The tumor microenvironment provides a rich source of potential targets for selective therapeutic intervention with properly designed anticancer agents. Significant physiological differences exist between the microvessels that nourish tumors and those that supply healthy tissue. Selective drug-mediated damage of these tortuous and chaotic microvessels starves a tumor of necessary nutrients and oxygen and eventually leads to massive tumor necrosis. Vascular targeting strategies in oncology are divided into two separate groups: angiogenesis inhibiting agents (AIAs) and vascular disrupting agents (VDAs). The mechanisms of action between these two classes of compounds are profoundly distinct. The AIAs inhibit the actual formation of new vessels, while the VDAs damage and/or destroy existing tumor vasculature. One subset of small-molecule VDAs functions by inhibiting the assembly of tubulin into microtubules, thus causing morphology changes to the endothelial cells lining the tumor vasculature, triggered by a cascade of cell signaling events. Ultimately this results in catastrophic damage to the vessels feeding the tumor. The rapid emergence and subsequent development of the VDA field over the past decade has led to the establishment of a synergistic combination of preclinical state-of-the-art tumor imaging and biological evaluation strategies that are often indicative of future clinical efficacy for a given VDA. This review focuses on an integration of the appropriate biochemical and biological tools necessary to assess (preclinically) new small-molecule, tubulin active VDAs for their potential to be clinically effective anticancer agents.

Diffusion-weighted MR imaging allows monitoring the effect of combretastatin A4 phosphate on rabbit implanted VX2 tumor model: 12-day dynamic results

OBJECTIVES

To investigate the 12-day dynamic characteristics of tumor response to intravenous administration of CA4P in rabbit VX2 tumor models.

METHODS

Study protocol was approved by local ethical committee for animal care and use. Sixteen rabbits with 32 tumors on bilateral legs were randomly divided into treated and control groups. Conventional and DWI images were acquired before and 24 h, 4 days, 8 days and 12 days after treatment. The dynamic changes of tumor on images were correlated with histological results. ADCs were compared among and between groups at different time points.

RESULTS

The tumors in treated group grew slower than those in control group. In treaded group, the mean ADC decreased slightly at 24 h point due to cell edema caused by ischemia. Then, it increased significantly at 4 days and 8 days because of progressive central necrosis. Finally, peripheral tumor proliferation caused a second decrease of ADC at 12 days. The significant difference of ΔADC% between the two groups at 24 h, 4 days and 8 days indicated that the change of ADC in treated group was really caused by CA4P.

CONCLUSION

The dynamic histological changes of tumor caused by CA4P as reflected exactly by diffusion-weighted MR imaging indicate a noninvasive measure for monitoring tumor vascular targeting treatment.

The unique characteristics of tumor vasculature and preclinical evidence for its selective disruption by Tumor-Vascular Disrupting Agents

The vasculature of solid tumors is fundamentally different from that of normal vasculature and offers a unique target for anti-cancer therapy. Direct vascular-targeting with Tumor-Vascular Disrupting Agents (Tumor-VDAs) is distinctly different from anti-angiogenic strategies, and offers a complementary approach to standard therapies. Tumor-VDAs therefore have significant potential when combined with chemotherapy, radiotherapy, and angiogenesis-inhibiting agents. Preclinical studies with the different Tumor-VDA classes have demonstrated key tumor-selective anti-vascular and anti-tumor effects.

Multiparametric MRI biomarkers for measuring vascular disrupting effect on cancer

Solid malignancies have to develop their own blood supply for their aggressive growth and metastasis; a process known as tumor angiogenesis. Angiogenesis is largely involved in tumor survival, progression and spread, which are known to be significantly attributed to treatment failures. Over the past decades, efforts have been made to understand the difference between normal and tumor vessels. It has been demonstrated that tumor vasculature is structurally immature with chaotic and leaky phenotypes, which provides opportunities for developing novel anticancer strategies. Targeting tumor vasculature is not only a unique therapeutic intervention to starve neoplastic cells, but also enhances the efficacy of conventional cancer treatments. Vascular disrupting agents (VDAs) have been developed to disrupt the already existing neovasculature in actively growing tumors, cause catastrophic vascular shutdown within short time, and induce secondary tumor necrosis. VDAs are cytostatic; they can only inhibit tumor growth, but not eradicate the tumor. This novel drug mechanism has urged us to develop multiparametric imaging biomarkers to monitor early hemodynamic alterations, cellular dysfunctions and metabolic impairments before tumor dimensional changes can be detected. In this article, we review the characteristics of tumor vessels, tubulin-destabilizing mechanisms of VDAs, and in vivo effects of the VDAs that have been mostly studied in preclinical studies and clinical trials. We also compare the different tumor models adopted in the preclinical studies on VDAs. Multiparametric imaging biomarkers, mainly diffusion-weighted imaging and dynamic contrast-enhanced imaging from magnetic resonance imaging, are evaluated for their potential as morphological and functional imaging biomarkers for monitoring therapeutic effects of VDAs.

Antivascular effects of electrochemotherapy: implications in treatment of bleeding metastases

Solid tumors of various etiologies can be treated efficiently by electrochemotherapy (ECT), a combined use of electroporation (EP) and chemotherapeutic drugs, such as bleomycin and cisplatin. EP alone and ECT in particular, induce a profound reduction in tumor blood flow, which contributes to the antitumor effect. After EP and ECT, the time course of blood flow changes and follows the same two-phase pattern. The first rapid and short-lived vasoconstriction phase is followed by the second much longer-lived phase resulting from disrupted cytoskeletal structures and a compromised barrier function of the microvascular endothelium. In the case of ECT, however, tumor vascular endothelial cells are also affected by the chemotherapeutic drug, which leads to irrecoverable damage to tumor vessels and to a further decrease in tumor blood flow within hours after application of ECT. Tumor cells surviving the direct effects of ECT are consequently exposed to lack of oxygen and nutrients and are pushed into the secondary cascade of induced cell death. Clinically, the antitumor effectiveness of ECT has been proven extensively in the treatment of melanoma metastases, with 70-80% complete responses. The antivascular effects of ECT were also exploited for palliative treatment of bleeding melanoma metastases, with immediate cessation of bleeding and very good antitumor effectiveness. The antivascular effect of ECT is of utmost importance for translation of ECT into the treatment of deep-seated tumors, especially in well vascularized organs, such as the liver, where it prevents bleeding of the treated area.

Support of a free radical mechanism for enhanced antitumor efficacy of the microtubule disruptor OXi4503

Unlike normal blood vessels, the unique characteristics of an expanding, disorganized and leaky tumor vascular network can be targeted for therapeutic gain by vascular disrupting agents (VDAs), which promote rapid and selective collapse of tumor vessels, causing extensive secondary cancer cell death. A hallmark observation following VDA treatment is the survival of neoplastic cells at the tumor periphery. However, comparative studies with the second generation tubulin-binding VDA OXi4503 indicate that the viable rim of tumor tissue remaining following treatment with this agent is significantly smaller than that seen for the lead VDA, combretastatin. OXi4503 is the cis-isomer of CA1P and it has been speculated that this agent's increased antitumor efficacy may be due to its reported metabolism to orthoquinone intermediates leading to the formation of cytotoxic free radicals. To examine this possibility in situ, KHT sarcoma-bearing mice were treated with either the cis- or trans-isomer of CA1P. Since both isomers can form quinone intermediates but only the cis-isomer binds tubulin, such a comparison allows the effects of vascular collapse to be evaluated independently from those caused by the reactive hydroxyl groups. The results showed that the cis-isomer (OXi4503) significantly impaired tumor blood flow leading to secondary tumor cell death and >95% tumor necrosis 24h post drug exposure. Treatment with the trans-isomer had no effect on these parameters. However, the combination of the trans-isomer with combretastatin increased the antitumor efficacy of the latter agent to near that of OXi4503. These findings indicate that while the predominant in vivo effect of OXi4503 is clearly due to microtubule collapse and vascular shut-down, the formation of toxic free radicals likely contributes to its enhanced potency.

Leukemia regression by vascular disruption and antiangiogenic therapy

Acute myelogenous leukemias (AMLs) and endothelial cells depend on each other for survival and proliferation. Monotherapy antivascular strategies such as targeting vascular endothelial growth factor (VEGF) has limited efficacy in treating AML. Thus, in search of a multitarget antivascular treatment strategy for AML, we tested a novel vascular disrupting agent, OXi4503, alone and in combination with the anti-VEGF antibody, bevacizumab. Using xenotransplant animal models, OXi4503 treatment of human AML chloromas led to vascular disruption in leukemia cores that displayed increased leukemia cell apoptosis. However, viable rims of leukemia cells remained and were richly vascular with increased VEGF-A expression. To target this peripheral reactive angiogenesis, bevacizumab was combined with OXi4503 and abrogated viable vascular rims, thereby leading to enhanced leukemia regression. In a systemic model of primary human AML, OXi4503 regressed leukemia engraftment alone and in combination with bevacizumab. Differences in blood vessel density alone could not account for the observed regression, suggesting that OXi4503 also exhibited direct cytotoxic effects on leukemia cells. In vitro analyses confirmed this targeted effect, which was mediated by the production of reactive oxygen species and resulted in apoptosis. Together, these data show that OXi4503 alone is capable of regressing AML by a multitargeted mechanism and that the addition of bevacizumab mitigates reactive angiogenesis.

In vivo functional differences in microvascular response of 4T1 and Caki-1 tumors after treatment with OXi4503

4T1 mouse mammary adenocarcinomas and Caki-1 human renal cell carcinomas grown in mouse dorsal window chambers were serially treated with the vascular disrupting agent (VDA) OXi4503 and their responses compared. The real-time in vivo response was assessed using spectral imaging of microvascular hemoglobin saturation. To our knowledge this is the first use of spectral imaging technology for investigation of vascular disrupting agents. Previous research showing tumor size dependence in the treatment response to VDAs suggested that for the size of tumors used in this study only a moderate response would be observed; however, the tumors unexpectedly had very different responses to treatment. Caki-1 tumors showed little permanent vessel damage and experienced transient vessel collapse with time-dependent oxygenation changes followed by recovery starting at 6 h after treatment. Caki-1 tumors did not manifest necrotic avascular regions even after repeated treatments. These results are consistent with those obtained using other imaging modalities and histology. In contrast, similarly sized 4T1 tumors showed extensive vessel disintegration, minor vascular collapse, and a drop in tumor oxygenation up to 6 h post-treatment, after which reperfusion of collapsed vessels and extensive vascular remodeling and neovascularization of the tumor rim occurred from 8-48 h. The completely disintegrated vessels did not recover and left behind avascular and apparently necrotic regions in the tumor core. Spectral imaging appears to be a useful technique for in vivo investigation of vascular disrupting agents. The differential responses of these two tumor-types suggest that further investigation of the mechanisms of action of VDAs and individual characterization of tumor VDA-responses may be needed for optimal clinical use of these agents.

An in vitro model that can distinguish between effects on angiogenesis and on established vasculature: actions of TNP-470, marimastat and the tubulin-binding agent Ang-510

In anti-cancer therapy, current investigations explore the possibility of two different strategies to target tumor vasculature; one aims at interfering with angiogenesis, the process involving the outgrowth of new blood vessels from pre-existing vessels, while the other directs at affecting the already established tumor vasculature. However, the majority of in vitro model systems currently available examine the process of angiogenesis, while the current focus in anti-vascular therapies moves towards exploring the benefit of targeting established vasculature as well. This urges the need for in vitro systems that are able to differentiate between the effects of compounds on angiogenesis as well as on established vasculature. To achieve this, we developed an in vitro model in which effects of compounds on different vascular targets can be studied specifically. Using this model, we examined the actions of the fumagillin derivate TNP-470, the MMP-inhibitor marimastat and the recently developed tubulin-binding agent Ang-510. We show that TNP-470 and marimastat solely inhibited angiogenesis, whereas Ang-510 potently inhibited angiogenesis and caused massive disruption of newly established vasculature. We show that the use of this in vitro model allows for specific and efficient screening of the effects of compounds on different vascular targets, which may facilitate the identification of agents with potential clinical benefit. The indicated differences in the mode of action between marimastat, TNP-470 and Ang-510 to target vasculature are illustrative for this approach.