Vascular targeting agents as cancer therapeutics

Synthesis and biological evaluation of vinylogous combretastatin A-4 derivatives

Stereospecific syntheses of the Z-E and E-Z vinylogues of combretastatin A-4, and two B-ring related analogues, were achieved through a Suzuki-Miyaura coupling. As compared to CA4, the derivative with a phenyl moiety has shown increased potency in its ability to inhibit tubulin polymerisation.

Anti-vascular tumor therapy: recent advances, pitfalls and clinical perspectives

Anti-vascular tumor therapy represents a promising new strategy for cancer treatment. Anti-vascular treatment may be divided in anti-angiogenic and vascular targeting therapy. Whereas anti-angiogenic drugs aim on the inhibition of new vessel formation, vascular targeting compounds are designed to selectively destruct preexisting tumor blood vessels leading to secondary tumor cell death. Both anti-angiogenic drugs and vascular targeting agents have proven effective anti-tumoral activity in numerous preclinical studies over the last decade. In vivo, a combination with anti-vascular tumor therapy enhances the effects of other treatment modalities as chemo- and radiotherapy. Phase I clinical studies revealed a number of well-tolerated candidates. As monotherapy, however, anti-angiogenic treatment lacked efficacy in randomized clinical studies so far. In contrast, combination of anti-angiogenic therapy with chemotherapy was highly effective in an encouraging, large randomized phase III trial on metastatic colorectal cancer. This review will outline recent advances in the preclinical and clinical development of anti-vascular therapy with focus on vascular targeting. Conceptual differences between anti-angiogenic and vascular targeting therapies will be discussed with emphasis on specific problems and pitfalls in the conversion into the clinic.

Combination of the vascular targeting agent ZD6126 with boron neutron capture therapy

PURPOSE

The aim of this study was to evaluate the antitumor efficacy of the vascular targeting agent ZD6126 (N-acetylcochinol-O-phosphate) in the rodent squamous cell carcinoma (SCC) VII carcinoma model, in combination with boron neutron capture therapy (BNCT).

METHODS AND MATERIALS

Sodium borocaptate-(10)B (BSH, 125 mg/kg, i.p.) or l-p-boronophenylalanine-(10)B (BPA, 250 mg/kg, i.p.) was injected into SCC VII tumor-bearing mice, and 15 min later, ZD6126 (100 mg/kg, i.p.) was administered. Then, the (10)B concentrations in tumors and normal tissues were measured by prompt gamma-ray spectrometry. On the other hand, for the thermal neutron beam exposure experiment, SCC VII tumor-bearing mice were continuously given 5-bromo-2'-deoxyuridine (BrdU) to label all proliferating (P) cells in the tumors, followed by treatment with a (10)B-carrier and ZD6126 in the same manner as the above-mentioned (10)B pharmacokinetics analyses. To obtain almost similar intratumor (10)B concentrations during neutron exposure, thermal neutron beam irradiation was started from the time point of 30 min after injection of BSH only, 90 min after BSH injection for combination with ZD6126, 120 min after the injection of BPA only, and 180 min after BPA injection for combination with ZD6126. Right after irradiation, the tumors were excised, minced, and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labeling (quiescent [Q] cells) was determined using immunofluorescence staining for BrdU. Meanwhile, the MN frequency in total (P + Q) tumor cells was determined from the tumors that were not pretreated with BrdU. The clonogenic cell survival assay was also performed in mice given no BrdU.

RESULTS

Pharmacokinetics analyses showed that combination with ZD6126 greatly increased the (10)B concentrations in tumors after 60 min after BSH injection and after 120 min after BPA injection. The concentrations of (10)B from BSH in normal tissues were also raised by combination with ZD6126, although not so clearly as those in tumors. Combination with ZD6126 had almost no effect on the concentrations of (10)B from BPA in normal tissues. The clonogenic surviving fractions of total tumor cells and the MN frequencies of both total and Q tumor cells were reduced and increased by combination with ZD6126, respectively, whether BSH or BPA was employed. However, the degrees of these changes in the clonogenic surviving fractions and the MN frequencies were more obviously observed in tumors from BSH-injected mice than from BPA-injected mice, and in Q tumor cells than in total tumor cells regardless of the employed (10)B-carrier.

CONCLUSIONS

Combination with ZD6126 was regarded as more promising in BSH-BNCT than BPA-BNCT, and more effective for enhancing the sensitivity of the Q tumor cells than that of the total tumor cells. This resulted in the decrease in the extended difference in the sensitivity between the total and Q tumor cells caused by the use of (10)B-carrier for BNCT.

Discovery of 4-Aryl-4H-chromenes as a New Series of Apoptosis Inducers Using a Cell- and Caspase-based High-Throughput Screening Assay. 1. Structure−Activity Relationships of the 4-Aryl Group

By applying a novel cell- and caspase-based HTS assay, 2-amino-3-cyano-7-(dimethylamino)-4-(3-methoxy-4,5-methylenedioxyphenyl)-4H-chromene (1a) has been identified as a potent apoptosis inducer. Compound 1a was found to induce nuclear fragmentation and PARP cleavage, as well as to arrest cells at the G(2)/M stage and to induce apoptosis as determined by the flow cytometry analysis assay in multiple human cell lines (e.g. Jurkat, T47D). Through structure-activity relationship (SAR) studies of the 4-aryl group, a 4- and 7-fold increase in potency was obtained from the screening hit 1a to the lead compounds 2-amino-4-(3-bromo-4,5-dimethoxyphenyl)-3-cyano-7-(dimethylamino)-4H-chromene (1c) and 2-amino-3-cyano-7-(dimethylamino)-4-(5-methyl-3-pyridyl)-4H-chromene (4e), with an EC(50) of 19 and 11 nM in the caspase activation assay in T47D breast cancer cells, respectively. The 2-amino-4-aryl-3-cyano-7-(dimethylamino)-4H-chromenes also were found to be highly active in the growth inhibition MTT assay, with GI(50) values in the low nanomolar range for compound 1c. Significantly, compound 1c was found to have a GI(50) value of 2 nM in the paclitaxel resistant, p-glycoprotein overexpressed, MES-SA/DX5 tumor cells. Functionally, compound 1c was found to be a potent inhibitor of tubulin polymerization and to effectively inhibit the binding of colchicine to tubulin. These results confirm that the cell-based caspase activation assay is a powerful tool for the discovery of potent apoptosis inducers and suggest that the 4-aryl-4H-chromenes have the potential to be developed into future anticancer agents.

Neuropilin-1 regulates attachment in human endothelial cells independently of vascular endothelial growth factor receptor-2

Neuropilin-1 (NRP-1) is a type 1 membrane protein that binds the axon guidance factors belonging to the class-3 semaforin family. In endothelial cells, NRP-1 serves as a co-receptor for vascular endothelial growth factor (VEGF) and regulates VEGF receptor 2 (VEGFR-2)-dependent angiogenesis. Although gene-targeting studies documenting embryonic lethality in NRP-1 null mice have demonstrated a critical role for NRP-1 in vascular development, the activities of NRP-1 in mature endothelial cells have been incompletely defined. Using RNA interference-mediated silencing of NRP-1 or VEGFR-2 in primary human endothelial cells, we confirm that NRP-1 modulates VEGFR-2 signaling-dependent mitogenic functions of VEGF. Importantly, we now show that NRP-1 regulates endothelial cell adhesion to extracellular matrix proteins independently of VEGFR-2. Based on its dual role as an enhancer of VEGF activity and a mediator of endothelial cell adhesiveness described here, NRP-1 emerges as a promising molecular target for the development of antiangiogenic drugs.

Efficacy of combined antiangiogenic and vascular disrupting agents in treatment of solid tumors

PURPOSE

To evaluate the antitumor efficacy of a vascular targeting strategy that combines an agent that disrupts established tumor blood vessels (ZD6126) with one that interferes with new vessel formation (ZD6474) in models of human renal cell carcinoma (Caki-1) and Kaposi's sarcoma (KSY-1).

METHODS AND MATERIALS

Caki-1 and KSY-1 xenograft-bearing nude mice were treated with ZD6126 and ZD6474 either as single agents or in combination when the tumors reached a size of approximately 200 mm(3). ZD6126 therapy consisted of three doses of 100 mg/kg administered 1, 3, and 5 days after the tumor reached the starting size. ZD6474 was administered daily (25 mg/kg) on Days 1-5. In the combination studies, ZD6474 treatment began immediately after the first dose of ZD6126. The tumor response to treatment was evaluated using a regrowth delay endpoint.

RESULTS

Significant tumor growth delays were observed in both tumor models with either agent with the treatment regimen used. In the Caki-1 and KSY-1 models, respectively, ZD6126 treatment resulted in a tumor growth delay of 23 and 26 days and ZD6474 produced a tumor growth delay of 24.5 and 14.5 days. When ZD6126 and ZD6474 were combined, the tumor growth delays increased to 55 (Caki-1) and 86 (KSY-1) days. In the KSY-1 model, the combination therapy also resulted in 3 of 8 long-term tumor-free survivors.

CONCLUSION

These results indicate that statistically significant antitumor efficacy can be achieved using a treatment strategy that combines a therapy that targets the established tumor blood vessels with one that interferes with the process of angiogenesis.

Targeting the tumor vascular compartment to improve conventional cancer therapy

The fact that a single blood vessel can support the life of thousands of tumor cells has been known for a long time. However, therapeutic strategies that aim to impair vascular development in tumors are only slowly emerging in the clinics. Nevertheless, the accumulation of data from many successful preclinical studies of the effects of a variety of drugs that target tumor vasculature provides clues that should help rationalize future treatment modalities for human tumors. Indeed, the 'old' view of an immature and non-functional vascular network within tumors has evolved and, in this article, we will show that the concept of tumor heterogeneity should be extended to the vascular compartment. In addition, we will review recent data documenting that both mature and immature vessels coexist within tumors and, importantly, that their relative density responds to a dynamic process that evolves with time and treatments.

Angiogenesis in colorectal cancer: therapeutic implications and future directions

This article discusses the therapeutic implications and future directions of angiogenesis in colorectal cancer.

Antiangiogenic gene therapy of cancer: recent developments

With the role of angiogenesis in tumor growth and progression firmly established, considerable effort has been directed to antiangiogenic therapy as a new modality to treat human cancers. Antiangiogenic agents have recently received much widespread attention but strategies for their optimal use are still being developed. Gene therapy represents an attractive alternative to recombinant protein administration for several reasons. This review evaluates the potential advantages of gene transfer for antiangiogenic cancer therapy and describes preclinical gene transfer work with endogenous angiogenesis inhibitors demonstrating the feasibility of effectively suppressing and even eradicating tumors in animal models. Additionally, we describe the advantages and disadvantages of currently available gene transfer vectors and update novel developments in this field. In conclusion, gene therapy holds great promise in advancing antiangiogenesis as an effective cancer therapy and will undoubtedly be evaluated in human clinical trials in the near future.

Highlights on endoglin (CD105): from basic findings towards clinical applications in human cancer

Antibody targeting of tumor-associated vasculature is a promising therapeutic approach in human cancer; however, a specific cell membrane marker for endothelial cells of tumor vasculature has not been discovered yet. Endoglin (CD105) is a cell-surface glycoprotein most recently identified as an optimal indicator of proliferation of human endothelial cells. The finding that CD105 is over-expressed on vascular endothelium in angiogenetic tissues has prompted several pre-clinical studies designed to get a deeper understanding on the role of CD105 in angiogenesis, and to evaluate the most appropriate clinical setting(s) to utilize CD105 as a therapeutic target. In this review, the foreseeable clinical applications of CD105 in human cancer are discussed.

In vivo videomicroscopy reveals differential effects of the vascular-targeting agent ZD6126 and the anti-angiogenic agent ZD6474 on vascular function in a liver metastasis model

Metastases require a functional blood supply for progressive growth. Thus, therapies that target metastatic vasculature have potential clinical utility. The effects of the vascular-targeting agent (VTA), ZD6126, and the anti-angiogenic agent, ZD6474, on vascular development and function within metastases were compared in an experimental liver metastasis model. Ras-transformed PAP2 fibroblasts were injected into the mesenteric veins of SCID mice to produce a control liver metastasis burden of approximately 40% at 14 days. Mice given a single dose of ZD6126 (200 mg/kg, i.p.) on day 13 were examined 24 h later. Histology revealed a significant reduction in metastatic burden, associated with extensive tumor necrosis, increased tumor cell apoptosis and a reduction in tumor-associated vasculature. In vivo videomicroscopy (IVVM) revealed disrupted, non-functional vascular channels within metastases, with no blood flow. Mice given ZD6474 on days 4 to 10 (50 mg/kg daily, oral gavage) were examined on day 11. Histology revealed a lower metastatic burden, significant reductions in metastasis size and vasculature, and a significant increase in tumor cell apoptosis. IVVM revealed extensive reductions in vascularity and blood flow within metastases. Neither ZD6126 nor ZD6474 treatment affected surrounding normal liver tissue. This study shows that both agents can reduce experimental liver metastasis with no apparent effect on normal vasculature. However, these reductions were attained through distinct effects on the metastatic vasculature. Understanding differences in the modes of action of VTAs and anti-angiogenic agents will be important in optimizing their clinical application and in developing appropriate combination strategies.