Combretastatins Novel Vascular targeting Drugs for Improving Anticancer therapy

Stilbene Glycosides in Pinus cembra L. Bark: Isolation, Characterization, and Assessment of Antioxidant Potential and Antitumor Activity on HeLa Cells

Stilbenes are plant secondary metabolites with remarkable antidiabetic, anti-inflammatory, antimicrobial, antioxidant, antitumor, and neuroprotective properties. As these compounds are valuable constituents in healthcare products and promising drug candidates, exploring new sources of stilbenes is essential for therapeutic advancement. The present study reports the isolation of two stilbene glycosides, resveratroloside and pinostilbenoside, from Pinus cembra L. bark. Their antioxidant activity and cytotoxic effects against HeLa cells were evaluated in comparison to the raw bark extract. The structures of resveratroloside and pinostilbenoside were confirmed by nuclear magnetic resonance (NMR) and mass spectrometry (MS) data analyses. Antioxidant activity was assessed by 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging and reducing power assays. Cell viability, apoptosis, cell proliferation, and cell cycle assays were used to evaluate the cytotoxic potential against HeLa cells. Resveratroloside and pinostilbenoside exhibited lower activity as free radical scavengers and reducing agents. However, they showed greater efficacy in reducing viability and suppressing proliferation in human cervical carcinoma HeLa cells. Given the promising findings of our study, the therapeutic potential of resveratroloside and pinostilbenoside should be further investigated.

Preparation of truncated tissue factor antineuropilin-1 monoclonal antibody conjugate and identification of its selective thrombosis in tumor blood vessels

In recent decades, selectively inducing tumor vascular thrombosis, followed by necrosis of tumor tissues has been a promising and potential anticancer strategy. In this report, we prepared a kind of vascular targeting drug that consists of anti-neuropilin-1 monoclonal antibody (anti-NRP-1 mAb) and truncated tissue factor (tTF). Anti-NRP-1 mAb could guide tTF to the surface of tumor vascular endothelial cells and lead to subsequent vascular embolization. This vascular targeting drug, which is also one of the antibody drug conjugates, was generated using a coupling method with water-soluble 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysulfosuccimide. Afterwards, in-vitro and in-vivo assays were performed to characterize its potential coagulation ability and antitumor activity. In-vitro experiments indicated that tTF-anti-NRP-1 monoclonal antibody (tTF-mAb) retained both the targeting activity of anti-NRP-1 mAb and the procoagulant activity of tTF. Live imaging system was used to assess its biodistribution and tumor-binding capability, which also yielded promising results. Furthermore, in-vivo studies showed that tTF-mAb was capable of significantly inducing tumor vascular thrombosis and inhibiting tumor growth in nude mice bearing subcutaneous xenografts, and histopathologic changes were rarely observed in normal organs.

Tumors Resistant to Checkpoint Inhibitors Can Become Sensitive after Treatment with Vascular Disrupting Agents

Immune therapy improves cancer outcomes, yet many patients do not respond. This pre-clinical study investigated whether vascular disrupting agents (VDAs) could convert an immune unresponsive tumor into a responder. CDF1 mice, with 200 mm³ C3H mammary carcinomas in the right rear foot, were intraperitoneally injected with combretastatin A-4 phosphate (CA4P), its A-1 analogue OXi4503, and/or checkpoint inhibitors (anti-PD-1, PD-L1, or CTLA-4 antibodies), administered twice weekly for two weeks. Using the endpoint of tumor growth time (TGT5; time to reach five times the starting volume), we found that none of the checkpoint inhibitors (10 mg/kg) had any effect on TGT5 compared to untreated controls. However, CA4P (100 mg/kg) or OXi4503 (5–50 mg/kg) did significantly increase TGT5. This further significantly increased by combining the VDAs with checkpoint inhibitors, but was dependent on the VDA, drug dose, and inhibitor. For CA4P, a significant increase was found when CA4P (100 mg/kg) was combined with anti-PD-L1, but not with the other two checkpoint inhibitors. With OXi4503 (50 mg/kg), a significant enhancement occurred when combined with anti-PD-L1 or anti-CTLA-4, but not anti-PD-1. We observed no significant improvement with lower OXi4503 doses (5–25 mg/kg) and anti-CTLA-4, although 30% of tumors were controlled at the 25 mg/kg dose. Histological assessment of CD4/CD8 expression actually showed decreased levels up to 10 days after treatment with OXi4503 (50 mg/kg). Thus, the non-immunogenic C3H mammary carcinoma was unresponsive to checkpoint inhibitors, but became responsive in mice treated with VDAs, although the mechanism remains unclear.

Combretastatins: An Overview of Structure, Probable Mechanisms of Action and Potential Applications

Combretastatins are a class of closely related stilbenes (combretastatins A), dihydrostilbenes (combretastatins B), phenanthrenes (combretastatins C) and macrocyclic lactones (combretastatins D) found in the bark of Combretum caffrum (Eckl. & Zeyh.) Kuntze, commonly known as the South African bush willow. Some of the compounds in this series have been shown to be among the most potent antitubulin agents known. Due to their structural simplicity many analogs have also been synthesized. Combretastatin A4 phosphate is the most frequently tested compounds in preclinical and clinical trials. It is a water-soluble prodrug that the body can rapidly metabolize to combretastatin A4, which exhibits anti-tumor properties. In addition, in vitro and in vivo studies on combretastatins have determined that these compounds also have antioxidant, anti-inflammatory and antimicrobial effects. Nano-based formulations of natural or synthetic active agents such as combretastatin A4 phosphate exhibit several clear advantages, including improved low water solubility, prolonged circulation, drug targeting properties, enhanced efficiency, as well as fewer side effects. In this review, a synopsis of the recent literature exploring the combretastatins, their potential effects and nanoformulations as lead compounds in clinical applications is provided.

Combretastatins: more than just vascular targeting agents?

Several prodrugs of the naturally occurring combretastatins have undergone extensive clinical evaluation as vascular targeting agents (VTAs). Their increased selectivity toward endothelial cells together with their innate ability to rapidly induce vascular shutdown and inhibit tumor growth at doses up to 10-fold less than the maximum tolerated dose led to the clinical evaluation of combretastatins as VTAs. Tubulin is well established as the molecular target of the combretastatins and the vast majority of its synthetic derivatives. Furthermore, tubulin is a highly validated molecular target of many direct anticancer agents routinely used as front-line chemotherapeutics. The unique vascular targeting properties of the combretastatins have somewhat overshadowed their development as direct anticancer agents and the delineation of the various cell death pathways and anticancer properties associated with such chemotherapeutics. Moreover, the ongoing clinical trial of OXi4503 (combretastatin-A1 diphosphate) together with preliminary preclinical evaluation for the treatment of refractory acute myelogenous leukemia has successfully highlighted both the indirect and direct anticancer properties of combretastatins. In this review, we discuss the development of the combretastatins from nature to the clinic. The various mechanisms underlying combretastatin-induced cell cycle arrest, mitotic catastrophe, cell death, and survival are also reviewed in an attempt to further enhance the clinical prospects of this unique class of VTAs.

Synthesis and biochemical evaluation of benzoylbenzophenone thiosemicarbazone analogues as potent and selective inhibitors of cathepsin L

Upregulation of cathepsin L in a variety of tumors and its ability to promote cancer cell invasion and migration through degradation of the extracellular matrix suggest that cathepsin L is a promising biological target for the development of anti-metastatic agents. Based on encouraging results from studies on benzophenone thiosemicarbazone cathepsin inhibitors, a series of fourteen benzoylbenzophenone thiosemicarbazone analogues were designed, synthesized, and evaluated for their inhibitory activity against cathepsins L and B. Thiosemicarbazone inhibitors 3-benzoylbenzophenone thiosemicarbazone 1, 1,3-bis(4-fluorobenzoyl)benzene thiosemicarbazone 8, and 1,3-bis(2-fluorobenzoyl)-5-bromobenzene thiosemicarbazone 32 displayed the greatest potency against cathepsin L with low IC50 values of 9.9 nM, 14.4 nM, and 8.1 nM, respectively. The benzoylbenzophenone thiosemicarbazone analogues evaluated were selective in their inhibition of cathepsin L compared to cathepsin B. Thiosemicarbazone analogue 32 inhibited invasion through Matrigel of MDA-MB-231 breast cancer cells by 70% at 10 μM. Thiosemicarbazone analogue 8 significantly inhibited the invasive potential of PC-3ML prostate cancer cells by 92% at 5 μM. The most active cathepsin L inhibitors from this benzoylbenzophenone thiosemicarbazone series (1, 8, and 32) displayed low cytotoxicity toward normal primary cells [in this case human umbilical vein endothelial cells (HUVECs)]. In an initial in vivo study, 3-benzoylbenzophenone thiosemicarbazone (1) was well-tolerated in a CDF1 mouse model bearing an implanted C3H mammary carcinoma, and showed efficacy in tumor growth delay. Low cytotoxicity, inhibition of cell invasion, and in vivo tolerability are desirable characteristics for anti-metastatic agents functioning through an inhibition of cathepsin L. Active members of this structurally diverse group of benzoylbenzophenone thiosemicarbazone cathepsin L inhibitors show promise as potential anti-metastatic, pre-clinical drug candidates.

Targeting tumour hypoxia to improve outcome of stereotactic radiotherapy

BACKGROUND

Hypoxia is a characteristic feature of solid tumours that significantly reduces the efficacy of conventional radiation therapy. In this study we investigated the role of hypoxia in a stereotactic radiation schedule by using a variety of hypoxic modifiers in a preclinical tumour model.

MATERIAL AND METHODS

C3H mammary carcinomas were irradiated with 3 × 15 Gy during a one-week period, followed three days later by a clamped top-up dose to produce a dose response curve; the endpoint was tumour control. The hypoxic modifiers were nimorazole (200 mg/kg), nicotinamide (120 mg/kg) and carbogen (95% O2 + 5% CO2) breathing, OXi4503 (10 mg/kg), and hyperthermia (41.5°C; 1 h).

RESULTS

The radiation dose controlling 50% of clamped tumours (TCD50) following 3 × 15 Gy was 30 Gy. Giving nimorazole or nicotinamide+ carbogen prior to the final 15 Gy fraction non-significantly (χ(2)-test; p < 0.05) reduced this TCD50 to 20-23 Gy; when administered with each 3 × 15 Gy fraction these values were significantly reduced to ≤ 2.5 Gy. Injecting OXi4503 or heating after irradiating significantly reduced the TCD50 to 9-12 Gy regardless of whether administered with one or all three 15 Gy fractions. Combining OXi4503 and heat with the final 15 Gy had a significantly larger effect (TCD50 = 2 Gy).

CONCLUSIONS

Clinically relevant modifiers of hypoxia effectively enhanced an equivalent stereotactic radiation treatment confirming the importance of hypoxia in such schedules.

Therapeutic potential of using the vascular disrupting agent OXi4503 to enhance mild temperature thermoradiation

PURPOSE

The response of tissues to radiation with mild temperature hyperthermia is dependent on the interval between the two modalities. This study investigated the effect that the vascular disrupting agent OXi4503 had on this time-interval interaction.

METHODS

The normal right rear foot of female CDF1 mice or foot-implanted C3H mammary carcinomas were locally irradiated (230 kV X-rays) and heated (41.5 °C for 60 min) by foot immersion in a water bath. OXi4503 (50 mg/kg) was injected intraperitoneally 1.5 h before irradiating. Irradiation was performed either in the middle of the heating period (simultaneous treatment) or at 1 or 4 h prior to starting the heating (sequential treatments). Response was the percentage of mice showing local tumour control at 90 days or skin moist desquamation between days 11-23. From the radiation dose response curves the dose producing tumour control (TCD(50)) or moist desquamation (MDD50) in 50% of mice was calculated.

RESULTS

The TCD(50) and MDD50 values for radiation alone were 54 Gy and 29 Gy, respectively. Simultaneously heating the tissues enhanced radiation response, the respective TCD(50) and MDD50 values being significantly (chi-square test, p < 0.05) reduced to 33 Gy and 14 Gy. A smaller enhancement was obtained with a sequential treatment in both tissues. OXi4503 enhanced the radiation response of tumour and skin. Combined with radiation and heat, the only effect was in tumours where OXi4503 prevented the decrease in sensitisation seen with the sequential treatment.

CONCLUSION

Combining OXi4503 with a sequential radiation and heat treatment resulted in a 1.4-fold therapeutic gain.

Combination of vascular disrupting agents and ionizing radiation

Vascular disrupting agents (VDAs) are a relatively new class of drugs that target tumor vasculature and induce tumor blood flow shutdown and subsequent necrosis in the tumor core. The first generation of these agents is actively evaluated in clinical trials, whereas new molecules are developed in order to enhance efficacy and to overcome resistance mechanisms. VDA used as a single agent only cause a moderate tumor growth delay. So, strategy aiming at combining VDA to conventional cancer treatments is undergoing extensive investigations. A special emphasis has been put on combination with chemotherapeutic agents. Besides, numerous preclinical studies have also clearly established that the association of VDA to radiotherapy can improve antitumor treatment and may lead to a therapeutic gain. However, up to date, there is a lack of clinical trials evaluating such combinations, whereas it would be of great interest since radiotherapy is widely used as anticancer treatment.

Vascular effects of plinabulin (NPI-2358) and the influence on tumour response when given alone or combined with radiation

PURPOSE

This study investigated the anti-tumour effects of the novel vascular disrupting agent plinabulin (NPI-2358) when given alone or combined with radiation.

MATERIALS AND METHODS

Foot implanted C3H mammary carcinomas or leg implanted KHT sarcomas were used, with plinabulin injected intraperitoneally. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) measurements were made with gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) on a 7-tesla magnet. Treatment response was assessed using regrowth delay (C3H tumours), clonogenic survival (KHT sarcomas) or histological estimates of necrosis for both models.

RESULTS

Plinabulin (7.5 mg/kg) significantly reduced the initial area under curve (IAUC) and the transfer constant (K(trans)) within 1 hour after injection, reaching a nadir at 3 h, but returning to normal within 24 h. A dose-dependent decrease in IAUC and K(trans), was seen at 3 h. No significant anti-tumour effects were observed in the C3H tumours until doses of 12.5 mg/kg were achieved, but started at 1.5 mg/kg in the KHT sarcoma. Irradiating tumours 1 h after injecting plinabulin enhanced response in both models.

CONCLUSIONS

Plinabulin induced a time- and dose-dependent decrease in tumour perfusion. The KHT sarcoma was more sensitive than the C3H tumour to the anti-tumour effects of plinabulin, while radiation response was enhanced in both models.

Non-invasive imaging of combretastatin activity in two tumor models: Association with invasive estimates

INTRODUCTION

The efficacy of the vascular disrupting agent combretastatin A-4 phosphate (CA4P) depends on several factors including tumor size, nitric oxide level, interstitial fluid pressure, and vascular permeability. These factors vary among tumor types. The aim of this study was to investigate all these factors in two tumor models that respond differently to CA4P.

MATERIAL AND METHODS

Mice bearing C3H mammary carcinomas or KHT sarcomas (200 to 800 mm(3)) were intraperitoneally injected with CA4P (100 mg/kg). Tumor size and the effect of a nitric oxide inhibitor nitro-L-arginine (NLA) administered intravenously were evaluated by necrotic fraction histologically assessed at 24 hours. Interstitial fluid pressure (IFP) was measured using the wick-in-needle technique, and vascular characteristics were assessed with dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI).

RESULTS

Initial necrotic fraction was about 10% in both tumor models at 200 mm(3), but only increased significantly with tumor size in the C3H mammary carcinoma. In this tumor, CA4P significantly induced further necrosis by about 15% at all sizes, but in the KHT tumor, the induced necrotic fraction depended on tumor size. For both tumor types, NLA with CA4P significantly increased necrotic fraction above that for each drug alone. CA4P significantly decreased IFP in all tumors except in the 800 mm(3) C3H tumor, which had an initially non-significant lower value. Interstitial volume estimated by DCE-MRI increased in all groups, except the 800 mm(3) C3H tumors. DCE-MRI vascular parameters showed different initial characteristics and general significant reductions following CA4P treatment.

CONCLUSIONS

Both tumor models showed differences in all factors before treatment, and in their response to CA4P. Perfusion and permeability as estimated by DCE-MRI play a central role in the CA4P response, and interstitial volume and IFP seemed related. These factors may be of clinical value in the planning of CA4P treatments.

Combretastatin A4 phosphate: a novel vascular disrupting agent

Combretastatin A4 phosphate (CA4P) is the lead compound of a relatively new class of agents termed vascular disrupting agents that target existing tumor blood vessels. Rapid tumor blood flow shutdown has been demonstrated in preclinical models and patients by various techniques such as dynamic contrast-enhanced MRI, perfusion computed tomography and PET scans following CA4P infusion. CA4P typically induces rapid tumor necrosis in the center of the tumor and leaves a rim of viable cells in the periphery. In oncology, CA4P does not appear to be that active by itself, but may be more efficacious when combined with chemotherapy, antiangiogenic therapy and radiation therapy. Studies are currently underway, which combine CA4P with antiangiogenic agents. Side effects have included hypertension, tumor pain and occasional cardiovascular toxicity, without any significant myelosuppression or disabling systemic symptoms. The utility of CA4P for conditions other than cancer, which involves neovascularization such as macular degeneration, is also being explored.

Imaging tumour physiology and vasculature to predict and assess response to heat

The vascular supply of tumours and the tumour microenvironment both play an important role when tumours are treated with hyperthermia. Blood flow is one of the major vehicles by which heat is dissipated thus the vascular supply will influence the ability to heat the tumour. It also influences the type of microenvironment that exists within tumours, and it is now well-established that cells existing in areas of oxygen deficiency, nutrient deprivation and acidic conditions are more sensitive to the effect of hyperthermia. The vascular supply and microenvironment are also affected by hyperthermia. In general, mild heat temperatures transiently improve blood flow and oxygenation, while higher hyperthermia temperatures cause vascular collapse and so increase the adverse microenvironmental conditions. Being able to image these vascular and microenvironmental parameters both before and after heating will help in our ability to predict and assess response. Here we review the various techniques that can be applied to supply this information, especially using non-invasive imaging approaches.

The Novel Vascular Disrupting Agent ANG501 Induces Cell Cycle Arrest and Enhances Endothelial Cell Sensitivity to Radiation

The efficacy of approaches in which vascular disrupting agents (VDA) are used in combination with conventional chemotherapy and/or radiation therapy in the treatment of cancer might be improved if there were a better understanding of the cellular and molecular changes induced in normal and malignant cells as a result of VD A exposure. Toward this goal, murine endothelial cells were treated in vitro with ANG501, a novel stilbene VDA developed in our laboratory, and alterations in gene expression determined by genome-wide microarray analysis and subsequently confirmed by Western blot analysis. Among the genes that were shown to be induced upon brief exposure to non-cytotoxic doses of ANG501 were several involved in the control of cell cycle progression and apoptosis, including p21Wafl and the heat shock/stress proteins hsp25, hsp70 and anti-B-crystallin. Reflecting such induction, functional studies confirmed that normal cell cycling is temporarily inhibited following treatment with ANG501 such that the majority of cells accumulate at the radiation-sensitive G2/M phase of the cell cycle at 6 hr. The effects were transient and by 24 hr normal cell cycling had largely resumed. Combination experiments confirmed that endothelial cells treated 6 hr previously with ANG501 were more readily killed by radiation. Importantly, significant effects were evident at clinically relevant radiation doses. Taken together these findings emphasize the need to consider the radiosensitizing activity of VD As when developing therapies in which these promising compounds are used in combination with radiation.

Vascular disrupting agents: a novel mechanism of action in the battle against non-small cell lung cancer

Targeting vasculature, essential in oxygen and nutrient supply, represents a new frontier in the treatment of cancer. Apart from angiogenesis inhibitors that compromise the formation of new blood vessels, a second class of vascular disrupting agents (VDAs) targets endothelial cells and pericytes of the already established tumor vasculature, resulting in tumor ischemia and necrosis. VDAs have been divided into two types: ligand-directed VDAs and small molecules. Ligand-directed VDAs consist of targeting and effector moieties that are linked together. Their clinical efficacy appears limited because of cost and a lack of specificity and toxicity. Small molecules include two classes: the synthetic flavonoids, which work through induction of local cytokine production, and the tubulin-binding agents. The aim of this review is to discuss the hypothesized molecular mechanisms of action of VDAs and their early preclinical and clinical results, emphasizing ASA404, combretastatin A-4 disodium phosphate, ABT-751, and NPI-2358, reported in the treatment of non-small cell lung cancer, which is the leading cause of cancer death worldwide, and also to discuss future developments in this cancer population.

A review and update of the current status of the vasculature-disabling agent combretastatin-A4 phosphate (CA4P)

Vascular-disrupting strategies impair a tumor's blood vessel network, which is essential for tumor progression and metastasis. Vascular-disrupting agents (VDAs) cause a rapid and selective vascular shutdown in tumors to produce extensive secondary neoplastic cell death due to ischemia. A lead agent in this therapeutic strategy is the tubulin depolymerizing agent combretastatin-A4 phosphate (CA4P). Used alone, CA4P induces extensive necrosis in a wide variety of preclinical cancer models and significant blood flow reductions in the patient tumors. Preclinical and clinical data further indicate that CA4P can effectively be combined with chemotherapy or radiotherapy. Finally, the potential of combining VDAs with antiangiogenic therapies has shown considerable promise in preclinical models and such combinations are now beginning to be evaluated in patients.

Efficacy of selected natural products as therapeutic agents against cancer

With emerging sophistication in the exploration of ocean environment, a number of marine bioactive products have been identified with promising anticancer activity. Many of these are in active phase I or phase II clinical trials or have been terminated because of adverse side effects, mainly hematological in nature. Nonetheless, the information derived has aided enormously in providing leads for laboratory synthesis with modifications in the parent structure affecting compound solubility, absorption, and toxicity, resulting in less severe toxicity while achieving maximum efficacy in smaller doses. We describe herein, a few of the compounds obtained from marine and terrestrial sources [bryostatin 1 ( 1), dolastatin 10 ( 2), auristatin PE ( 3), and combretastatin A4 ( 4)] that have been extensively investigated in our laboratory and continue to be investigated for their sensitization effects with other cytotoxic agents in several different site-specific tumors employing murine models or human subjects.

Vascular damaging agents

To provide a comprehensive overview on vascular targeting agents and the application of radiobiological principles in pre-clinical and clinical studies, we completed a comprehensive review of published medical studies on vascular targeting agents using Pub Med. Vascular targeting agents are now divided into vascular disrupting agents (VDAs), which target the pre-existing tumour vasculature, and angiogenesis inhibitors (AIs), which prevent the formation of new blood vessels. Modest success has been seen when VDAs and AIs are used as single agents and therefore combination therapies that can work in a complimentary and synergistic manner, targeting both the tumour cells and endothelial cells, are needed. Radiobiological principles have been used to increase our understanding of these agents, and can explain the increased efficacy of combination treatments. In particular, the alteration of the tumour microenvironment by AIs and VDAs can lead to enhanced efficacy when combined with chemotherapy or radiotherapy, with phase II/III trials showing encouraging results. The optimal use and scheduling of AIs and VDAs remains to be determined. Further understanding of the mechanisms of action of these potentially very exciting anti-neoplastic agents is urgently required.

Pathophysiologic Effects of Vascular-Targeting Agents and the Implications for Combination with Conventional Therapies

A functional vascular supply is critical for the continued growth and development of solid tumors. It also plays a major role in metastatic spread of tumor cells. This importance has led to the concept of targeting the vasculature of the tumor as a form of cancer therapy. Two major types of vascular-targeting agent (VTA) have now emerged: those that prevent the angiogenic development of the neovasculature of the tumor and those that specifically damage the already established tumor vascular supply. When used alone neither approach readily leads to tumor control, and so, for VTAs to be most successful in the clinic they will need to be combined with more conventional therapies. However, by affecting the tumor vascular supply, these VTAs should induce pathophysiologic changes in variables, such as blood flow, pH, and oxygenation. Such changes could have negative or positive influences on the tumor response to more conventional therapies. This review aims to discuss the pathophysiologic changes induced by VTAs and the implications of these effects on the potential use of VTAs in combined modality therapy.

Effect of the second-generation vascular disrupting agent OXi4503 on tumor vascularity

PURPOSE

As first-generation small-molecule vascular disrupting agents (VDA) have begun to enter clinical trials, second-generation agents are under active development. One such agent is the combretastatin A4 disodium phosphate (CA4P) analogue OXi4503 (CA1P).

EXPERIMENTAL DESIGN

C3H/HeJ mice bearing KHT sarcomas were treated with CA4P and OXi4503 and the effect on tumor vasculature was determined by evaluating the extent of vascular shutdown (Hoechst-33342 vessel staining) and tumor perfusion inhibition (dynamic contrast-enhanced magnetic resonance imaging). Dynamic contrast-enhanced magnetic resonance imaging and tumor necrosis end points also were used to examine the pathophysiologic tumor effects following repeated exposures to these agents.

RESULTS

Single doses of either agent (CA4P, 100 mg/kg; OXi4503, 25 mg/kg) resulted in an 80% to 90% reduction in tumor perfusion 4 hours after treatment. Whereas recovery in tumor perfusion was observed 48 hours posttreatment, this recovery was significantly slower in mice treated with OXi4503. Tumors re-treated with either VDA 72 hours after the first drug exposure showed a similar reduction and recovery in tumor perfusion. Histologic evidence showed the presence of a smaller viable rim after exposure to OXi4503 than that observed after CA4P treatment. Furthermore, the extent of recovery of tumor necrosis 72 hours after drug treatment was less for OXi4053.

CONCLUSIONS

The present studies show that the second-generation VDA OXi4503 possesses significant antivascular effects in solid tumors. Importantly, the vasculature of tumors of mice that had received an initial dose this agent was as responsive to a subsequent treatment.

Combretastatin A4 phosphate: background and current clinical status

Combretastatin A4 phosphate (CA4P) represents the lead compound in a group of novel tubulin depolymerising agents being developed as vascular targeting agents (VTAs). VTAs are drugs that induce rapid and selective vascular dysfunction in tumours. CA4P is a water-soluble prodrug of the cis-stilbene CA4 originally isolated from the tree Combretum caffrum. Preclinical studies have shown that CA4P induces blood flow reductions and subsequent tumour cell death in a variety of preclinical models. Moreover, this activity has been linked to its ability to rapidly alter the morphology of immature endothelial cells by disrupting their tubulin cytoskeleton. Phase I clinical trials have established a maximum tolerated dose in the range 60-68 mg/m2 and in addition have established that significant changes to tumour perfusion can be achieved across a wide range of doses. The dose-limiting toxicities include tumour pain, ataxia and cardiovascular changes. The maximum tolerated dose was independent of schedule, indicating the absence of cumulative toxicity. Although unexpected from preclinical studies, some evidence of clinical response was seen using CA4P as a single modality. Based on the Phase I data, combination studies of CA4P with established therapies are in progress and should determine whether the exciting preclinical data obtained when VTAs are used in combination with cytotoxic chemotherapy, radiation, radioimmunotherapy and even antiangiogenic agents, can be translated into man.

Disrupting tumour blood vessels

Low-molecular-weight vascular-disrupting agents (VDAs) cause a pronounced shutdown in blood flow to solid tumours, resulting in extensive tumour-cell necrosis, while they leave the blood flow in normal tissues relatively intact. The largest group of VDAs is the tubulin-binding combretastatins, several of which are now being tested in clinical trials. DMXAA (5,6-dimethylxanthenone-4-acetic acid) - one of a structurally distinct group of drugs - is also being tested in clinical trials. A full understanding of the action of these and other VDAs will provide insights into mechanisms that control tumour blood flow and will be the basis for the development of new therapeutic drugs for targeting the established tumour vasculature for therapy.

Vascular targeting agents as cancer therapeutics

Vascular targeting agents (VTAs) for the treatment of cancer are designed to cause a rapid and selective shutdown of the blood vessels of tumors. Unlike antiangiogenic drugs that inhibit the formation of new vessels, VTAs occlude the pre-existing blood vessels of tumors to cause tumor cell death from ischemia and extensive hemorrhagic necrosis. Tumor selectivity is conferred by differences in the pathophysiology of tumor versus normal tissue vessels (e.g., increased proliferation and fragility, and up-regulated proteins). VTAs can kill indirectly the tumor cells that are resistant to conventional antiproliferative cancer therapies, i.e., cells in areas distant from blood vessels where drug penetration is poor, and hypoxia can lead to radiation and drug resistance. VTAs are expected to show the greatest therapeutic benefit as part of combined modality regimens. Preclinical studies have shown VTA-induced enhancement of the effects of conventional chemotherapeutic agents, radiation, hyperthermia, radioimmunotherapy, and antiangiogenic agents. There are broadly two types of VTAs, small molecules and ligand-based, which are grouped together, because they both cause acute vascular shutdown in tumors leading to massive necrosis. The small molecules include the microtubulin destabilizing drugs, combretastatin A-4 disodium phosphate, ZD6126, AVE8062, and Oxi 4503, and the flavonoid, DMXAA. Ligand-based VTAs use antibodies, peptides, or growth factors that bind selectively to tumor versus normal vessels to target tumors with agents that occlude blood vessels. The ligand-based VTAs include fusion proteins (e.g., vascular endothelial growth factor linked to the plant toxin gelonin), immunotoxins (e.g., monoclonal antibodies to endoglin conjugated to ricin A), antibodies linked to cytokines, liposomally encapsulated drugs, and gene therapy approaches. Combretastatin A-4 disodium phosphate, ZD6126, AVE8062, and DMXAA are undergoing clinical evaluation. Phase I monotherapy studies have shown that the agents are tolerated with some demonstration of single agent efficacy. Because efficacy is expected when the agents are used with conventional chemotherapeutic drugs or radiation, the results of Phase II combination studies are eagerly awaited.

Combretastatin A4 phosphate

Combretastatin A4 phosphate (CA4P) is a water-soluble prodrug of combretastatin A4 (CA4). The vascular targeting agent CA4 is a microtubule depolymerizing agent. The mechanism of action of the drug is thought to involve the binding of CA4 to tubulin leading to cytoskeletal and then morphological changes in endothelial cells. These changes increase vascular permeability and disrupt tumor blood flow. In experimental tumors, anti-vascular effects are seen within minutes of drug administration and rapidly lead to extensive ischemic necrosis in areas that are often resistant to conventional anti-cancer treatments. Following single-dose administration a viable tumor rim typically remains from which tumor regrowth occurs. When given in combination with therapies targeted at the proliferating viable rim, enhanced tumor responses are seen and in some cases cures. Results from the first clinical trials have shown that CA4P monotherapy is safe and reduces tumor blood flow. There has been some promising demonstration of efficacy. CA4P in combination with cisplatin is also safe. Functional imaging studies have been used to aid the selection of doses for phase II trials. Both dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and positron emission tomography can measure the anti-vascular effects of CA4P in humans. This review describes the background to the development of CA4P, its proposed mechanism of action, the results from the first clinical trials with CA4P and the role of imaging techniques in its clinical development.

Clostridia in cancer therapy

During the past decade, the search for an effective system for the selective delivery of high therapeutic doses of anti-cancer agents to tumours has explored a variety of ingenious and increasingly complex biological systems. These systems are most often based on gene therapy and use viral vectors as the delivery vehicle. Invariably, such systems have been found wanting with respect to a lack of tumour specificity, poor levels of transgene expression and inefficient distribution of the vector throughout the tumour mass. By contrast, the ability of intravenously injected clostridial spores to infiltrate, then selectively germinate in, the hypoxic regions of solid tumours seems to be a totally natural phenomenon, which requires no fundamental alterations and is exquisitely specific.

Combretastatin family member OXI4503 induces tumor vascular collapse through the induction of endothelial apoptosis

The mechanism of tumor cell killing by OXI4503 was investigated by studying vascular functional and morphological changes post drug administration. SCID mice bearing MHEC5-T hemangioendothelioma were given a single dose of OXI4503 at 100 mg/kg. Tumor blood flow, measured by microsphere fluorescence, was reduced by 50% at 1 hr, and reached a maximum level 6-24 hr post drug treatment. Tumor vascular permeability, measured by Evan's blue and hemoglobin, increased significantly from 3 hr and peaked at 18 hr. The elevated tumor vessel permeability was accompanied by an increase in vascular endothelial growth factor (VEGF) from 1 hr post drug treatment. Immunohistochemical staining for CD31 and laminin showed that tumor blood vessels were affected as early as 3 hr but more prominent from 6 hr. From 12 hr, the vessel structure was completely destroyed. Histopathological and double immunohistochemical staining showed morphological change and induction of apoptosis in endothelial cells at 1-3 hr, followed by tumor cell necrosis from 6-72 hr. There were no statistically significant changes of Evan's blue and hemoglobin contents in liver tissue over the time course. These results suggest that OXI4503 selectively targets tumor blood vessels, and induces blood flow shutdown while it enhances tumor blood vessel permeability. The early induction of endothelial cell apoptosis leads to functional changes of tumor blood vessels and finally to the collapse of tumor vasculature, resulting in massive tumor cell necrosis. The time course of the tumor vascular response observed with OXI4503 treatment supports this drug for development as a stand alone therapy, and also lends support for the use of the drug in combination with other cancer therapies.

Assessment of pharmacodynamic vascular response in a phase I trial of combretastatin A4 phosphate

PURPOSE

Clinical evaluation of novel agents that target tumor blood vessels requires pharmacodynamic end points that measure vascular damage. Positron emission tomography (PET) was used to measure the effects of the vascular targeting agent combretastatin A4 phosphate (CA4P) on tumor and normal tissue perfusion and blood volume.

PATIENTS AND METHODS

Patients with advanced solid tumors were enrolled onto part of a phase I, accelerated-titration, dose-escalation study. The effects of 5 to 114 mg/m2 CA4P on tumor, spleen, and kidney were investigated. Tissue perfusion was measured using oxygen-15 (15O)-labeled water and blood volume was measured using 15O-labeled carbon monoxide (C15O). Scans were performed immediately before, and 30 minutes and 24 hours after the first infusion of each dose level of CA4P. All statistical tests were two sided.

RESULTS

PET data were obtained for 13 patients with intrapatient dose escalation. Significant dose-dependent reductions were seen in tumor perfusion 30 minutes after CA4P administration (mean change, -49% at >or= 52 mg/m2; P =.0010). Significant reductions were also seen in tumor blood volume (mean change, -15% at >or= 52 mg/m2; P =.0070). Although by 24 hours there was tumor vascular recovery, for doses >or= 52 mg/m2 the reduction in perfusion remained significant (P =.013). Thirty minutes after CA4P administration borderline significant changes were seen in spleen perfusion (mean change, -35%; P =.018), spleen blood volume (mean change, -18%; P =.022), kidney perfusion (mean change, -6%; P =.026), and kidney blood volume (mean change, -6%; P =.014). No significant changes were seen at 24 hours in spleen or kidney.

CONCLUSION

CA4P produces rapid changes in the vasculature of human tumors that can be assessed using PET measurements of tumor perfusion.

Combretastatin A4 phosphate has tumor antivascular activity in rat and man as demonstrated by dynamic magnetic resonance imaging

PURPOSE

Combretastatin A4 phosphate (CA4P) is a novel vascular targeting agent. Dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) studies were performed to examine changes in parameters related to blood flow and vascular permeability in tumor and normal tissue after CA4P treatment.

MATERIALS AND METHODS

Changes in kinetic DCE-MRI parameters (transfer constant [Ktrans] and area under contrast medium-time curve [AUC]) over 24 hours after treatment with CA4P were measured in 18 patients in a phase I trial and compared with those obtained in the rat P22 carcinosarcoma model, using the same imaging technique. Rats were treated with 30 mg/kg of CA4P; patients received escalating doses from 5 to 114 mg/m2.

RESULTS

A similar pattern and time course of change in tumor and normal tissue parameters was seen in rats and humans. Rat tumor Ktrans was reduced by 64% 6 hours after treatment with CA4P (30 mg/kg). No significant reductions in kidney or muscle parameters were seen. Significant reductions were seen in tumor Ktrans in six of 16 patients treated at >or= 52 mg/m2, with a significant group mean reduction of 37% and 29% at 4 and 24 hours, respectively, after treatment. The mean reduction in tumor initial area under the gadolinium-diethylenetriamine pentaacetic acid concentration-time curve (AUC) was 33% and 18%, respectively, at these times. No reduction was seen in muscle Ktrans or in kidney AUC in group analysis of the clinical data.

CONCLUSION

CA4P acutely reduces Ktrans in human as well as rat tumors at well-tolerated doses, with no significant changes in kidney or muscle, providing proof of principle that this drug has tumor antivascular activity in rats and humans.

Structure-activity and crystallographic analysis of benzophenone derivatives-the potential anticancer agents

Compounds 1-5, structurally related to combretastatin A-4 showed excellent cytotoxic activities against a panel of human cancer cell lines including multi-drug resistant cell lines. The X-ray three-dimensional structural analysis shows that proton donor in B ring may be required for cytotoxic activity, with intermolecular hydrogen bonding playing an important role.

Targeting the tumor blood vessel network to enhance the efficacy of radiation therapy

It has been well established that the vascularization of solid tumors is a prerequisite if a clinically relevant size is to be reached. For progressive tumor growth, the vessel network must continuously expand to satisfy the neoplastic cells' nutritional needs and waste product removal requirements. This utter reliance of the tumor on its vasculature provides a logical target for new approaches to cancer therapy. Indeed, there currently exists a great deal of enthusiasm for the development of interventions that compromise the growth and/or function of the tumor neovasculature. Two primary directions are being pursued. Inhibitors of angiogenesis seek to interrupt the angiogenic process to prevent new vessel formation. Antivascular approaches aim to cause direct damage to the tumor endothelium and thus lead to extensive secondary neoplastic cell death. The application of such strategies as adjuvants to conventional radiation treatments offers unique opportunities to develop more effective cancer therapies.

Schedule dependence of combretastatin A4 phosphate in transplanted and spontaneous tumour models

Tubulin depolymerizing drugs that selectively disrupt tumour-associated vasculature have recently been identified. The lead drug in this class, combretastatin A4 phosphate (CA4P), has just completed Phase I clinical trial. Previous studies have focussed on the effects of single drug doses and have demonstrated little or no retardation of tumour growth when CA4P is used alone, but significant benefit when it is combined with conventional treatment. We have investigated the effects of multiple daily or twice daily dosing with CA4P on the vascular function, cell survival and growth of syngeneic and spontaneous breast cancers in mice. In both transplanted and spontaneous tumours significant growth retardation is observed if CA4P is administered daily (10 doses x 50 mg/kg), whereas no significant effects are seen if the same total dose (500 mg/kg) is administered as a single bolus injection. This effect is attributed, at least in part, to anti-proliferative effects on the tumour and endothelial cells, which retard the revascularisation and repopulation of the tumour core that is initially necrosed by the drug treatment. Further investigation of dose scheduling showed that the initial anti-vascular effects of CA4P are enhanced by administering the drug in 2 equal doses separated between 2 and 6 hr. The twice daily dosing schedule (25 mg/kg twice a day) produced increased growth retardation compared to the 50 mg/kg once a day schedule in the transplanted CaNT tumour. It did not do so in the spontaneous T138 tumour model. These studies indicate that the potential anti-tumour activity of CA4P when used as a single agent in clinical trials may be enhanced when used in multiple dose schedules.

Synthesis and biological evaluation of 2-indolyloxazolines as a new class of tubulin polymerization inhibitors. Discovery of A-289099 as an orally active antitumor agent

A series of indole containing oxazolines has been discovered as a result of structural modifications of the lead compound A-105972. The compounds exert their anticancer activity through inhibition of tubulin polymerization by binding at the colchicine site. A-289099 was identified as an orally active antimitotic agent active against various cancer cell lines including those that express the MDR phenotype. The anticancer activity, pharmacokinetics, and an efficient and enantioselective synthesis of A-289099 are described.

The biology of the combretastatins as tumour vascular targeting agents

The tumour vasculature is an attractive target for therapy. Combretastatin A-4 (CA-4) and A-1 (CA-1) are tubulin binding agents, structurally related to colchicine, which induce vascular-mediated tumour necrosis in animal models. CA-1 and CA-4 were isolated from the African bush willow, Combretum caffrum, and several synthetic analogues are also now available, such as the Aventis Pharma compound, AVE8062. More soluble, phosphated, forms of CA-4 (CA-4-P) and CA-1 (CA-1-P) are commonly used for in vitro and in vivo studies. These are cleaved to the natural forms by endogenous phosphatases and are taken up into cells. The lead compound, CA-4-P, is currently in clinical trial as a tumour vascular targeting agent. In animal models, CA-4-P causes a prolonged and extensive shut-down of blood flow in established tumour blood vessels, with much less effect in normal tissues. This paper reviews the current understanding of the mechanism of action of the combretastatins and their therapeutic potential.

Clostridium spores for tumor-specific drug delivery

Insufficient blood supply of rapidly growing tumors leads to the presence of hypoxia, a well-known feature in solid tumors. Hypoxia is known to decrease the efficiency of currently used anti-cancer modalities like surgery, chemotherapy and radiotherapy. Therefore, hypoxia seems to be a major limitation in current anti-cancer therapy. The use of non-pathogenic clostridia to deliver toxic agents to the tumor cells takes advantage of this unique physiology. These strictly anaerobic, Gram-positive, spore-forming bacteria give, after systemic administration, a selective colonization of hypoxic/necrotic areas within the tumor. Moreover, they can be genetically modified to secrete therapeutic proteins like cytosine deaminase or tumor necrosis factor-alpha. The specificity of this protein delivery system can be further increased when expression is controlled by the use of a radio-inducible promoter, leading to increased spatial and temporal regulation of protein expression. This approach of bacterial vector systems to target protein expression to the tumor can be considered very safe since bacteria can be eliminated at any moment by the addition of proper antibiotics. The Clostridium-based delivery system thus presents an alternative therapeutic modality to deliver anti-tumor agents specifically to the tumor site. This high selectivity offers a major advantage in comparison with the classical gene therapy systems.

Tumor vasculature is targeted by the combination of combretastatin A-4 and hyperthermia

BACKGROUND AND PURPOSE

Combretastatin A-4 disodium phosphate (CA-4) enhances thermal damage in s.c. BT(4)An rat gliomas. We currently investigated how CA-4 and hyperthermia affect the tumor microenvironment and neovasculature to disclose how the two treatment modalities interact to produce tumor response.

METHODS

By confocal microscopy and immunostaining for von Willebrand factor, we examined the extent of vascular damage subsequent to CA-4 (50 mg/kg) and hyperthermia (waterbath 44 degrees C, 60 min). The influence on tumor oxygenation was assessed using interstitial pO(2)-probes (Licox system) and by immunostaining for pimonidazole. We examined the direct effect of CA-4 on the tumor cell population by flow cytometry (cell cycle distribution) and immunostaining for beta-tubulin (cytoskeletal damage).

RESULTS

Whereas slight vascular damage was produced by CA-4 in the BT(4)An tumors, local hyperthermia exhibited moderate anti-vascular activity. In tumors exposed to CA-4 3 h before hyperthermia, massive vascular damage ensued. CA-4 reduced the pO(2) from 36.1 to 17.6 mmHg (P=0.01) in the tumor base, and tumor hypoxia increased slightly in the tumor center (pimonidazole staining). Extensive tumor hypoxia developed subsequent to hyperthermia or combination therapy. Despite a profound influence on beta-tubulin organization in vitro, CA-4 had no significant effect on the cell cycle distribution in vivo.

CONCLUSION

Our results indicate that the anti-vascular activity exhibited by local hyperthermia can be augmented by previous exposure to CA-4.