Antiangiogenic drugs as a novel anticancer therapeutic strategy. Which are the more promising agents? What are the clinical developments and indications?

The emerging low-dose therapy for advanced cancers

Generally minute doses of drugs have been prescribed in biotherapies, homeopathy, immunization and vaccinations for centuries. Now the use of low doses of drugs is on the rise to combat serious diseases such as advanced cancers around the world. This new therapeutic approach to address solid tumors and other advanced diseases is a departure from the conventional use of maximum dose protocol. A small dose of the prescribed drug is frequently administered in a continuous fashion, at regular intervals, either as a standard treatment or as a maintenance therapy for a long time. However, this new treatment method lacks any standard for drug quantization, dose fractionation, repetition frequency and duration of a treatment course for an individual patient. This paper reviews literature about metronomic therapy and discusses hormesis: both phenomena occur in low dose ranges. Better mathematical models, computer simulations, process optimization and clinical trials are warranted to fully exploit the potential of low dose metronomic therapy to cure chronic and complicated diseases. New protocols to standardize metronomic dosimetry will answer the age old questions related to hormesis and homeopathy. It appears that this new low-dose metronomic therapy will have far reaching effects in curing chronic diseases throughout the world.

Ligand- and cell-specific effects of signal transduction pathway inhibitors on progestin-induced vascular endothelial growth factor levels in human breast cancer cells

We evaluated the signaling pathways involved in regulating vascular endothelial growth factor (VEGF), a potent angiogenic growth factor, in response to natural and synthetic progestins in breast cancer cells. Inhibition of the phosphoinositide-3'-kinase (PI3-kinase) signaling pathway or the specificity protein-1 (SP-1) transcription factor abolished both progesterone- and medroxyprogesterone acetate (MPA)-induced VEGF secretion from BT-474 and T47-DCO)cells. Inhibitors of the MAPK kinase 1/2/MAPK and N-terminal jun kinase/MAPK signaling pathways blocked both progesterone- and MPA-induced VEGF secretion in BT-474 cells. However, these inhibitors blocked only progesterone-, but not MPA-induced VEGF secretion in T47-DCO cells. Inhibitors of PI3-kinase or SP-1 blocked both progesterone- and MPA-induced increases in VEGF mRNA levels in T47-DCO cells. The proximal SP-1 sites within the VEGF promoter were critical for progestin-dependent induction of VEGF. In contrast, MAPK inhibitors did not block the progesterone- or MPA-induced increases in VEGF mRNA in T47-DCO cells, suggesting that MAPK inhibitors decreased progesterone-induced VEGF secretion in T47-DCO cells by blocking posttranscriptional mechanisms. The MAPK kinase/ERK/MAPK-independent induction of VEGF mediated by MPA was associated with the PRB [progesterone receptor (PR) B] isoform of the PR in T47-DCO cells. None of the inhibitors tested reduced basal PR levels or abrogated PR-dependent gene expression from a reporter plasmid, indicating that loss of PR function cannot explain any of the observed effects. Because the PI3-kinase signaling pathway and SP-1 transcription factor play critical roles in progestin-dependent VEGF induction, these may be useful targets for developing antiangiogenic therapies to prevent progression of progestin-dependent human breast cancers.

Inhibition of HUVEC tubulogenesis by hederacolchiside-A1 is associated with plasma membrane cholesterol sequestration and activation of the Ha-Ras/MEK/ERK cascade

PURPOSE

Neoangiogenesis is critical to cancer proliferation and metastasis and constitutes an attractive target for cancer therapy. It has previously been demonstrated that hederacolchiside-A1 (HCol-A1), a triterpenoid saponin from Hedera colchica Koch, has antimelanoma potential. The goal of this study was to evaluate, in vitro, if in addition to its tumoricidal effect on melanoma cells, HCol-A1 might affect endothelial cell network formation.

METHODS

We investigated whether HCol-A1 affects matrigel-induced tubulogenesis and inhibits the viability (WST-1 assay) of human umbilical vein endothelial cells (HUVECs). To provide structure-activity relationships (SAR), studies were conducted on HCol-A1, oleanolic acid and hederacolchiside A (HCol-A), a triterpenoid saponin which possess the same sugar sequence as Hcol-A1. Plasma membrane cholesterol sequestration was studied by labelling with [3H]cholesterol and assayed with HCol-A1-cholesterol complexes. HCol-A1 signalling was investigated using immunoassays.

RESULTS

In contrast to HCol-A and oleanolic acid, HCol-A1 inhibited matrigel-induced angiogenesis at micromolar concentration. Plasma membrane cholesterol sequestration was found to be critical for this activity. Activation of the Ras/MEK/ERK cascade appears to be one of the mechanisms by which Hcol-A1 affects HUVEC network formation. The predominant activation of the Ha-Ras isoform, which decreases HUVEC-tolerance to apoptosis, might contribute to the high susceptibility of this cell line to HCol-A1.

CONCLUSION

Since cholesterol sequestration affects cell confluence-dependent remodelling of endothelial membranes and vascular endothelial growth factor receptor-2 activity, these results raise the possibility that Hcol-A1 might slow-down cancer proliferation and metastasis in vivo by inhibiting critical aspects of neoangiogenesis. Further in vivo studies are needed to verify this hypothesis.

A clinical model of dermal wound angiogenesis

Full-thickness dermal biopsies were performed in healthy volunteers to establish the range of angiogenic responses in wound healing in a normal population. Four-millimeter punch biopsies were made in the forearms of 15 healthy volunteers. Each wound was evaluated microscopically 4-5 times per week for 2 weeks. A semiquantitative wound scoring system to evaluate the neovasculature at the wound periphery was investigated. A vascular score was calculated for each wound at each observation. Two independent observers analyzed the microscopic wound images using the scoring system. At the end of the 14-day period, repeat biopsies were performed on some of the volunteers, and the granulation tissue was stained with anti-CD31. The Kaplan-Meier method was used to estimate the distribution of the time to reach predetermined target average vascular scores. A mixed-effects regression model indicated that time, age, and observer were predictors for the average vascular score outcome. The pattern and time course for wound neovascularization was highly reproducible in this group of healthy volunteers, and the assay was feasible and well tolerated. This wound angiogenesis model may be useful for monitoring the effects of antiangiogenic agents on normal wound neovascularization.

Tumor angiogenesis as a therapeutic target

Angiogenesis - the formation of new blood vessels within a tumor (or many other tissue types) - has become a hotbed of pharmacological research as well as industrial drug discovery. This is the result of the efforts of a generation of scientists elucidating the complex (patho)physiological, biochemical and molecular events accompanying angiogenesis. It is estimated that >300 drug candidates are currently in various stages of testing, and it is, therefore, impossible to capture all of this in a brief review. Therefore, the emphasis here is on relatively advanced projects that are either in preclinical or clinical development, thus neglecting, to a large extent, the many exciting avenues being pursued in both academic and biotechnology laboratories. Although the potential of the approaches described cannot be overestimated, it is also important to note that there is still no drug on the market that achieves clinical benefit based on a selective modulation or inhibition of angiogenesis.

Clinical significance of determination of surrogate markers of angiogenesis in breast cancer

Compelling experimental and clinical data support the concept that breast carcinoma, as most of the other solid tumors, needs to develop the angiogenic phenotype for invasiveness, progression and metastasis. Several studies have determined intratumoral microvessel density by panendothelial markers and immunohistochemical techniques, with most of them showing that the degree of vascularity is associated with prognosis of the patients operated of early-stage invasive breast cancer. More recently, certain angiogenic peptides have been assessed in human breast cancer: vascular endothelial growth factor (VEGF), platelet derived-endothelial cell growth factor (PD-ECGF, also known as thymidine phosphorylase, TP) and fibroblast growth factor family (FGFs). Among these, the most studied is VEGF, which appears to be a powerful prognostic indicator. Little data are available on the clinical significance of naturally occurring antiangiogenic factors, with few studies reporting on interleukin-12 and thrombospondins. In vivo techniques for dynamic assessment of tumor blood network are presently under extensive research, in particular for monitoring activity of inhibitors of angiogenesis. The methods of assessment of angiogenic activity and the results of published clinical studies in peer reviewed Journals with a computerized overview of literature will be presented. Overall, the results of the reported studies suggest that human breast cancer is an angiogenic-dependent tumor for which antiangiogenic therapy represents a promising novel antitumoral therapeutic strategy.

New functions for non-collagenous domains of human collagen type IV. Novel integrin ligands inhibiting angiogenesis and tumor growth in vivo

Collagen type IV is a major component of the basal lamina of blood vessels. Six genetically distinct collagen type IV chains have been identified and are distributed in a tissue-specific manner. Here we define a novel function for soluble non-collagenous (NC1) domains of the alpha2(IV), alpha3(IV), and alpha6(IV) chains of human collagen type IV in the regulation of angiogenesis and tumor growth. These NC1 domains were shown to regulate endothelial cell adhesion and migration by distinct alpha(v) and beta(1) integrin-dependent mechanisms. Systemic administration of recombinant alpha2(IV), alpha3(IV), and alpha6(IV) NC1 domains potently inhibit angiogenesis and tumor growth, whereas alpha1(IV), alpha4(IV), and alpha5(IV) showed little if any effect. These findings suggest that specific NC1 domains of collagen type IV may represent an important new class of angiogenesis inhibitors.

The role of the vascular phase in solid tumor growth: a historical review

Angiogenesis is a biological process by which new capillaries are formed from pre-existing vessels. It occurs in both physiological conditions such as embryo development, cyclically in the female genital system and during wound repair, and pathological conditions, such as arthritis, diabetic retinopathy and tumors. In solid tumor growth, a specific critical turning point is the transition from the avascular to the vascular phase. Having developed an intrinsic vascular network, the neoplastic mass is able to grow indefinitely (unlike all the other forms, tumor angiogenesis is not limited in time) both in situ and at distant sites (metastasis) in so far as an intrinsic vascular network enables its cells to enter the vascular bed and colonize other organs. Tumor angiogenesis depends mainly on the release by neoplastic cells of growth factors specific for endothelial cells and able to stimulate growth of the host's blood vessels. This review describes its history as traced by the main contributions to the international medical literature and their contents. The specific new paradigm discussed here has been gaining general approval and considerable confirmation, thanks to its possible applications, as recently highlighted by the introduction of anti-angiogenic substances in adjuvant tumor management.

Immunostimulatory effects of human recombinant interleukin-12 on peripheral blood mononuclear cells from normal dogs

Interleukin-12 (IL-12) plays a pivotal role in regulating cellular immune responses involving autoimmunity, infectious disease, and cancer. Human recombinant (hr) IL-12 is being evaluated for therapy of human cancer. We investigated the potential of hrIL-12 to activate canine peripheral blood mononuclear cells (PBMC) using proliferation and cytotoxicity as readouts. Human rIL-12 caused increased proliferation of PBMC, and enhanced lysis of allogeneic canine tumor targets mediated by PBMC from normal dogs in vitro. In addition, antibody-dependent cellular cytotoxicity (ADCC) mediated by canine PBMC was enhanced by hrIL-12. These results indicate that hrIL-12 is recognized by canine immune cells, triggering a number of immune responses in canine PBMC, that may be important for immunotherapy of canine cancer. Information from this investigation provides impetus for evaluation of the effects of hrIL-12 on PBMC from tumor-bearing dogs and should be helpful in the development of hrIL-12 as an immune cell activator in vivo in the dog.

The rationale and future potential of angiogenesis inhibitors in neoplasia

Malignant tumours are angiogenesis-dependent diseases. Several experimental studies suggest that primary tumour growth, invasiveness and metastasis require neovascularisation. Tumour-associated angiogenesis is a complex multistep process under the control of positive and negative soluble factors. A mutual stimulation occurs between tumour and endothelial cells by paracrine mechanisms. Angiogenesis is necessary, but not sufficient, as the single event for tumour growth. There is, however, compelling evidence that acquisition of the angiogenic phenotype is a common pathway for tumour progression, and that active angiogenesis is associated with other molecular mechanisms leading to tumour progression. Experimental research suggests that it is possible to block angiogenesis by specific inhibitory agents, and that modulation of angiogenic activity is associated with tumour regression in animals with different types of neoplasia. The more promising angiosuppressive agents for clinical testing are: naturally occurring inhibitors of angiogenesis (angiostatin, endostatin, platelet factor-4 and others), specific inhibitors of endothelial cell growth (TNP-470, thalidomide, interleukin-12 and others), agents neutralising angiogenic peptides (antibodies to fibroblast growth factor or vascular endothelial growth factor, suramin and analogues, tecogalan and others) or their receptors, agents that interfere with vascular basement membrane and extracellular matrix [metalloprotease (MMP) inhibitors, angiostatic steroids and others], antiadhesion molecules antibodies such as antiintegrin alpha v beta 3, and miscellaneous drugs that modulate angiogenesis by diverse mechanisms of action. Antiangiogenic therapy is to be distinguished from vascular targeting. Gene therapy aimed to block neovascularisation is also a feasible anticancer strategy in animals bearing experimental tumours. Antiangiogenic therapy represents one of the more promising new approaches to anticancer therapy and it is already in early clinical trials. Because angiosuppressive therapy is aimed at blocking tumour growth indirectly, through modulation of neovascularisation, antiangiogenic agents need to be developed and evaluated as biological response modifiers. Therefore, adequate and well designed clinical trials should be performed for a proper evaluation of antiangiogenic agents, by determination and monitoring of surrogate markers of angiogenic activity.

Clinical significance of angiogenic factors in breast cancer

Growth, progression, and metastasis of breast cancer, as well as of most of the other tumors, are angiogenesis-dependent processes. Several pro-angiogenic growth factors and endogenous inhibitors of angiogenesis have been identified and sequenced, and experimental studies suggest that angiogenic activity of a tumor may result from downregulation of inhibitors of angiogenesis or up-regulation of endothelial growth factors. The mechanisms leading to the alteration of the balance between positive and negative modulators of angiogenesis are only partially known. We are at the beginning of research to identify the more active angiogenic factors in human breast cancer, and little information is presently available on their clinical significance. Preliminary results suggest that among the known angiogenic peptides, both vascular endothelial growth factor (VEGF) and platelet-derived endothelial cell growth factor / thymidine phosphorylase (PD-ECGF/TP) have promising prognostic and, perhaps, predictive value. No data are available on the clinical value of co-determination of positive and negative regulators of angiogenesis to look at the angiogenic balance of each single tumor. Only a few studies have assessed the role of endogenous inhibitors of angiogenesis in human breast cancer, with results available only on thrombospondin-1 and -2 (TSP-1, -2). Finally, the determination of some integrins such as alpha6 and alphavbeta3 and of some other endothelial-adhesion molecules seems to be of potential prognostic value. Recognizing which are the more biologically active positive and negative angiogenic factors is the key for the identification not only of new prognostic markers but also of targets for antiangiogenic therapy in human breast cancer.