Targeting tumour vasculature: the development of combretastatin A4

Synergy between vascular targeting agents and antibody-directed therapy

PURPOSE

Tumor heterogeneity necessitates the use of combined therapies. We have shown that combining antibody-directed therapy with antivascular agents converts a subcurative to a curative treatment. The purpose of this study was to investigate, by radioluminographic and microscopic techniques, the regional effects of the two complementary therapies.

METHODS AND MATERIALS

Nude mice bearing colorectal tumors were injected with 125I-labeled anti-carcinoembryonic antigen antibody, and images were obtained for antibody distribution and modeling studies using radioluminography. For therapy studies, the mice were given radioimmunotherapy alone (131I-A5B7 anti-carcinoembryonic antigen antibody), the antivascular agent combretastatin A-4 3-0-phosphate (200 mg/kg), or both. Extra mice were used to study the regional tumor effects of these therapies over time: relevant histochemical procedures were performed on tissue sections to obtain composite digital microscopic images of apoptosis, blood vessels, perfusion, hypoxia, and morphology.

RESULTS

Antibody distribution, modeling, and immunohistochemistry showed how radioimmunotherapy (7.4 MBq/40 microg antibody) effectively treated the outer, well-oxygenated tumor region only. Combretastatin A-4 3-0-phosphate treated the more hypoxic center, and in doing so altered the relationship between tumor parameters.

CONCLUSION

The combined complementary therapies produced cures by destroying tumor regions with different pathophysiologies. Relating these regional therapeutic effects to the relevant tumor parameters microscopically allows optimization of therapy and improved translation to clinical trials.

Combretastatin A4 phosphate has primary antineoplastic activity against human anaplastic thyroid carcinoma cell lines and xenograft tumors

Anaplastic thyroid carcinoma (ATC) is a fatal malignancy the clinical outcome of which is unaltered by current therapeutic modalities. A recent phase 1 clinical trial of combretastatin A4 phosphate (CA4P) produced a long-lasting total remission in a patient with ATC. CA4P is a tubulin-binding agent derived from the African bush willow, Combretum caffrum, which possesses tumor vascular-targeting activity. In order to discriminate primary antineoplastic effects from tumor antivascular activity, we evaluated CA4P cytotoxicity in eight human ATC cell lines and compared it to paclitaxel, another tubulin-binding agent with significant clinical activity. CA4P displayed significant cytotoxicity against the ATC cell lines, comparable to that of paclitaxel, and these effects were longer lasting in two cell lines compared to the duration of paclitaxel. We further investigated the effects of CA4P on xenograft tumors from four ATC cell lines injected in athymic nude mice. Significantly lower tumor weights were observed in animals treated with CA4P compared to those treated with vehicle alone. Continuous monitoring of xenograft tumor volumes from one of the ATC cell lines also revealed a significantly lower rate of tumor growth in the CA4P-treated mice compared to those receiving vehicle alone. These results suggest that antitumoral effects of CA4P can be consequent to a combination of primary antineoplastic effects as well as the potential destruction of tumor vasculature.

An antibody-calmodulin fusion protein reveals a functional dependence between macromolecular isoelectric point and tumor targeting performance

PURPOSE

Human monoclonal antibodies are promising agents for the development of improved anticancer therapeutics, because, unlike low-molecular-weight chemotherapeutic agents, they can selectively localize to solid tumors. In particular, the scFv(L19) antibody fragment, specific for the EDB domain of fibronectin, a marker of angiogenesis, has demonstrated an impressive tumor targeting performance in a variety of tumor-bearing animals and in patients with cancer. The purpose of this study was to develop a tumor pretargeting strategy, based on a novel anti-EDB fusion protein.

METHODS AND MATERIALS

We have fused the scFv(L19) to calmodulin, a small acidic protein for which specific binding peptides with a dissociation constant in the picomolar range are available. The resulting fusion protein has been expressed in mammalian cells and purified to homogeneity, before being characterized by quantitative biodistribution analysis in mice bearing the F9 murine teratocarcinoma.

RESULTS

Surprisingly, we have found that the fusion of scFv(L19) to calmodulin completely abrogated the tumor targeting ability of the antibody in vivo, although both scFv(L19) and calmodulin moieties within the fusion protein retained unaltered binding affinities toward their respective ligand. Furthermore, a systematic analysis of 13 derivatives of scFv(L19) recently produced in our laboratories showed that the 10 derivatives that retain the tumor targeting ability of the parental antibody have isoelectric points (pI) between 5.0 and 9.0, whereas scFv(L19)-calmodulin (pI = 4.49) and two other derivatives of scFv(L19) with pI >9.0 were unable to target tumors in vivo.

CONCLUSIONS

Because the EDB domain of fibronectin is a component of the modified extracellular matrix, predominantly located at the abluminal side of tumor blood vessels, our data suggest that extreme pI values of antibody-based pharmaceuticals may inhibit protein extravasation, perhaps by virtue of electrostatic interactions with endothelial cells and/or components of the extracellular matrix.

Exploiting the differential production of angiogenic factors within the tumor microenvironment in the design of a novel vascular-targeted gene therapy-based approach to the treatment of cancer

PURPOSE

The aim of this study is to explore a novel strategy through which the differential production of pro-angiogenic cytokines within the tumor microenvironment can be exploited as a means of selectively killing the vascular endothelial cells upon which the survival and growth of a tumor depend.

METHODS AND MATERIALS

Adenoviral vectors encoding a chimeric cell surface receptor composed of the extracellular domain of the vascular endothelial growth factor (VEGF) receptor Flk-1/KDR fused in frame to the membrane spanning and cytoplasmic domain of Fas were constructed and used to transduce primary human endothelial cells in vitro. The apoptotic response of these cells induced upon ligation of the chimeric receptor with VEGF was determined by measuring caspase-3 activation, AnnexinV-FITC binding, and the release of glucose-6-phosphate dehydrogenase.

RESULTS

The chimeric Flk-1/Fas protein is stable and expressed at high levels on the surface of adenovirally transduced cells. Upon the addition of exogenous VEGF, these cells undergo rapid apoptosis.

CONCLUSIONS

Receptor/Fas chimeras that recognize and bind pro-angiogenic cytokines represent a novel means by which the signal transduction events normally triggered in vascular endothelial cells upon the binding of angiogenic cytokines may be redirected toward the induction of apoptotic cell death. It is proposed that these constructs will prove of value in the further development of safe and effective vascular-targeted gene therapy-based approaches to the treatment of cancer.

Recent advances in angiogenesis, anti-angiogenesis and vascular targeting

Angiogenesis, the development of new blood vessels, has become a major focus of research. This has been stimulated by the therapeutic opportunities offered by the ability to manipulate the vasculature in pathologies such as cancer. Here, we present an overview of recent advances in angiogenesis. Especially noteworthy is the large volume of information from developmental studies, particularly those that involve transgenic and gene knockout mice. We also discuss the increasing repertoire of drugs with which to manipulate angiogenesis and new endothelial-specific genes with which to target the vasculature.

The influence of combretastatin A-4 and vinblastine on interstitial fluid pressure in BT4An rat gliomas

The influence of combretastatin A-4 disodium phosphate (CA-4, 50mg/kg intraperitoneally (i.p.)) and vinblastine (2mg/kg i.p.) on interstitial fluid pressure (IFP) was assessed in BT4An rat gliomas implanted subcutaneously in the neck. Furthermore the growth inhibitory effect of vinblastine and the distribution of fluorescence-conjugated vinblastine (BODIPY-vinblastine) were investigated. Tumors at different volumes were compared. Whereas CA-4 had no major influence on IFP, independent of tumor size, vinblastine increased the IFP in neoplasms above 8 cm(3) (P=0.03). Vinblastine yielded a significant tumor response only in tumors below 2.1 cm(3) (P=0.03). The distribution of BODIPY-vinblastine was heterogeneous and comparable despite tumor volume differences. We conclude that the influence of vinblastine on IFP is more pronounced than that of CA-4 in BT4An neck tumors, and that vinblastine may reduce subsequent drug delivery to solid tumors by increasing the IFP.

Inhibition of proliferative retinopathy by the anti-vascular agent combretastatin-A4

Retinal neovascularization occurs in a variety of diseases including diabetic retinopathy, the most common cause of blindness in the developed world. There is accordingly considerable incentive to develop drugs that target the aberrant angiogenesis associated with these conditions. Previous studies have shown that a number of anti-angiogenic agents can inhibit retinal neovascularization in a well-characterized murine model of ischemia-induced proliferative retinopathy. Combretastatin-A4 (CA-4) is an anti-vascular tubulin-binding agent currently undergoing clinical evaluation for the treatment of solid tumors. We have recently shown that CA-4 is not tumor-specific but elicits anti-vascular effects in nonneoplastic angiogenic vessels. In this study we have examined the capacity of CA-4 to inhibit retinal neovascularization in vivo. CA-4 caused a dose-dependent inhibition of neovascularization with no apparent side effects. The absence of vascular abnormalities or remnants of disrupted neovessels in retinas of CA-4-treated mice suggests an anti-angiogenic mechanism in this model, in contrast to the anti-vascular effects observed against established tumor vessels. Importantly, histological and immunohistochemical analyses indicated that CA-4 permitted the development of normal retinal vasculature while inhibiting aberrant neovascularization. These data are consistent with CA-4 eliciting tissue-dependent anti-angiogenic effects and suggest that CA-4 has potential in the treatment of nonneoplastic diseases with an angiogenic component.

Effect of p53 status on tumor response to antiangiogenic therapy

The p53 tumor suppressor gene is inactivated in the majority of human cancers. Tumor cells deficient in p53 display a diminished rate of apoptosis under hypoxic conditions, a circumstance that might reduce their reliance on vascular supply, and hence their responsiveness to antiangiogenic therapy. Here, we report that mice bearing tumors derived from p53(-/-) HCT116 human colorectal cancer cells were less responsive to antiangiogenic combination therapy than mice bearing isogenic p53(+/+) tumors. Thus, although antiangiogenic therapy targets genetically stable endothelial cells in the tumor vasculature, genetic alterations that decrease the vascular dependence of tumor cells can influence the therapeutic response of tumors to this therapy.

Natural products in cancer chemotherapy: past, present and future

Natural products have been the mainstay of cancer chemotherapy for the past 30 years. However, the quickening pace of (aberrant) gene identification, and the new technologies of combinatorial chemistry and high-throughput screening, should provide access to a wide range of new, totally synthetic drugs. Will these new approaches sound the death knell for therapies based on natural products? In reality, natural products are likely to provide many of the lead structures, and these will be used as templates for the construction of novel compounds with enhanced biological properties.

A novel cyclooxygenase-inhibitory stilbenolignan from the seeds of Aiphanes aculeata

[structure: see text] Aiphanol (1), a novel stilbenolignan, along with isorhapontigenin (2), piceatannol (3), and luteolin, were isolated by bioassay-guided fractionation from the seeds of Aiphanes aculeata Willd. (Arecaceae). The structure of compound 1 was elucidated by spectroscopic methods. Compound 1 is based on an unprecedented stilbenolignan skeleton in which a stilbene moiety is linked with a phenylpropane unit through a dioxane bridge. Compounds 1 and 2 exhibited significant inhibitory activities against cyclooxygenases-1 and -2.

Combretastatin-A4 disrupts neovascular development in non-neoplastic tissue

Combretastatin-A4 phosphate (cis-CA-4) is a tubulin-binding agent currently undergoing clinical trials as an anti-tumour drug. We have investigated whether CA-4 functions as a tumour-specific anti-vascular agent using the hyperplastic thyroid as a novel in vivo model of neovascularization. CA-4 elicited pathological changes in normal tissue, manifested as the induction of multiple, discrete intravascular thrombi. These vascular-damaging effects indicate that CA-4P does not function as a tumour-specific agent but targets neovasculature irrespective of the primary angiogenic stimulus.