1. In mature (non-growing) capillaries the vessel wall is composed of an endothelial cell lining, a basement membrane, and a layer of cells called pericytes which partially surround the endothelium. The pericytes are contained within the same basement membrane as the endothelial cells and occassionally make direct contact with them.
2. Angiogenic factors (the black triangles) bind to endothelial cell receptors and initiate the sequence of angiogenesis. When the endothelial cells are stimulated to grow, they secrete proteases which digest the basement membrane surrounding the vessel. The junctions between endotelial cells are altered, cell projections pass through the space created, and the newly formed sprout grows towards the source of the stimulus.
3. Continued capillary sprout growth is dependent upon several processes: the stimulus for growth (angiogenic factors, hypoxia, etc.) must be maintained; the endothelial cells must secrete the proteases required to break down the adjacent tissue; the cells themselves must be capable of movement/migration; and endothelial cell division must take place to provide the necessary number of cells (this takes place at a site behind the growth front of the sprout). Neighboring blind-ended sprouts then join together to form a capillary loop which later matures into a vessel like the one from which it arose.
One of the most inexpensive, relatively quick and highly visual assays is the chorioallantoic membrane (CAM) of the developing chick embryo (See Illustration below). The CAM assay is widely used to assess the activity of drugs of varoius drug formulations (liposomes, polymers) at different stages of development. If a formulation inhibits angiogenesis on the CAM, it indicates that sufficient drug is being released from the polymer to produce the desired biological effect; it also indicates that systemic drug levels are not high enough to cause toxicity to the embryo (the embryos die if drug release is too rapid). In addition, tumors can also be grown on the CAM to allow testing of formulations against the upregulated vasculature of a tumor. As such, the assay provides us with a fair amount of information in a short period of time.
 Positive regulators |
 Negative regulators |
| Fibroblast growth factors | Thrombospondin-1 |
| Placental growth factor | Angiostatin |
| Vascular endothelial growth factor | Interferon alpha |
| Transforming growth factors | Prolactin 16-kd fragment |
| Angiogenin | Metallo-proteinase inhibitors |
| Interleukin-8 | Platelet factor 4 |
| Hepatocyte growth factor | Genistein |
| Granulocyte colony-stimulating factor | Placental proliferin-related protein |
| Platelet-derived endothelial cell growth factor | Transforming growth factor beta? |
Abnormal angiogenesis occurs when the body loses its control of angiogenesis, resulting in either excessive or insufficient blood vessel growth. For instance, conditions such as ulcers, strokes, and heart attacks may result from the absence of angiogenesis normally required for natural healing. On the contrary, excessive blood vessel proliferation may favour tumor growth and spreading, blindness, and arthritis.
Accumulating evidences
indicate that progressive tumor growth is dependent on angiogenesis. Most tumors in humans persist in situ
for a long period of time (from months to years) in an avascular, quiescent status. In this phase the tumor
may contain few million cells. When a subgroup of cells within the tumor switches to an angiogenic phenotype
by changing the local equilibrium between positive and negative regulators of angiogenesis,
tumor starts to grow rapidly and becomes clinically detectable.
Interestingly, not only cancer cells but also inflammatory cells that infiltrate the tumor and the extracellular matrix can be a source of angiogenesis factors.
New blood vessels will provide nutrients to proliferating cancer cells, thus favouring tumor growth. On the other hand, new capillaries provide a port of entry for anti-neoplastic drugs, thus allowing the chemotherapeutic treatment of cancer patients. Unfortunately, leaky blood vessels cause an increase of interstitial pressure that limit drug diffusion within the tumor and favour tumor cell dissemination in the blood stream. Eventually, this will induce tumor spreading and the appearance of tumor metastases.
Clinical applications of research in tumor angiogenesis have taken three major directions:
Different positive and negative regulators of angiogenesis have been identified so far. They are produced by tumor cells as well as by infiltrating inflammatory cells. Most of them are stored in the extracellular matrix in a bioactive form. Also, receptors for angiogenesis factors have been demonstrated on the endothelial cell surface and the signal transduction mechanisms responsible for the induction of the angiogenic phenotype are currently being investigated.
Nine different inhibitors of angiogenesis are under investigation in clinical trials in patients with advanced cancer:
Even though no definitive conclusions can be drawn, antiangiogenic therapy combined with conventional anticancer therapies may represent an useful tool in the future care of patients with cancer.
Quantitation of microvessel density in tumor specimens has been perfomed. The evaluation of blood vessel density in a tumor may help to predict the risk of metastases or recurrence. Recent observations have confirmed this hypothesis for different types of tumors, including breast cancer.
These diseases either induce normal blood vessels to grow into them at accelerated rates (as a source of oxygen and nutrients to facilitate further growth) or are caused by inappropriate blood vessel growth in an area where it can be harmful (i.e. in the cornea or cartilage). These are serious conditions as is illustrated by the following statistics:
Cancer is not a single disease, nor is it a simple one; rather it is a family of at least 100 diseases with a similar origin, progression and treatment strategy. Cancer results from an abnormal, rapid growth of cells. These cells divide and multiply unchecked, often becoming tumors that invade healthy tissue. They also quickly adapt to changes in their environment and are capable of developing mechanisms of resistance to accepted treatment regimes. For these reasons, the focus of much current research is on new approaches to the disease and agents that work along new biological pathways.
Cancer is a serious disease. One in three people will be struck with cancer in their lifetime (an estimated 1.2 million in the United States in 1995) and of these people 50% to 60% will die from the disease. Every minute there is another death from cancer in the United States. It is not surprising that in North America cancer is the second leading cause of death (1 in 5 deaths) next to cardiovascular diseases and is projected to be the leading cause of death by the year 2000. Actually, in 1994 cancer was the leading cause of death in British Columbia, Canada.
The market for cancer treatments in the United States is currently $1.65 billion and is growing by 10% per annum. The factors contributing to this growth are the social need for more effective treatments, the increasing understanding of the disease and the positive climate at the FDA and other regulatory agencies (promising cancer treatments are given greater priority by these agencies). Paclitaxel sales in 1994 were reported by BMS to be between $300 and $400 million U.S.
In the coming years, the two areas where growth will be greatest in the cancer market are in angiogenesis treatments and cytotoxic agents (agents that kill cancer cells) that work along new pathways. Angiotech is developing promising technologies in each of these areas. (Sources - American Association of Cancer Research, "Recent Progress and Future Opportunities in Cancer Research", 1995; Find/SVP, Inc., "The Market for Cancer Therapeutics and Diagnostics - A Market Intelligence Report", 1992)
The term "arthritis" means joint inflammation. Like cancer, arthritis is an umbrella term covering a range of over 100 conditions of differing causes (most of which are unknown) and characteristics. Osteoarthritis results from wear and tear on joints and is tied to the aging process. It is the most prevalent form of arthritis. Rheumatoid arthritis ("RA"), by contrast, is a systemic disease which causes fever, joint inflammation and pain. These symptoms are caused by the ingrowth of pannus tissue (synoviocytes and new blood vessels) which destroys the joint cartilage and behaves like a localized malignancy.
Arthritis is also not a disease to be treated lightly. Arthritis afflicts 1 in 7 people (400 million people worldwide - 50 million people in Europe and 40 million people in the United States), and 2 in 100 people are struck with RA. For RA, there are 150,000 new cases diagnosed each year in the United States. These (and existing cases) result in 3.5 million office visits and 100,000 hospital admissions annually (14% of all prescriptions are for arthritis), and 1 in 3 persons will cease employment within five years from the initial diagnosis. In women alone, RA is responsible for $8.9 billion U.S. in lost earnings annually. In advanced stages of RA, the mortality rate over a five year time period is rivals that of many malignancies.
The 1996 world market for arthritis treatments is estimated to be $15.9 billion U.S. (United States market is projected to be $5.37 billion) and the market is projected to expand at an annual rate of 7.5 % to the year 2000. The majority of current sales are in nonsteroidal anti-inflammatory drugs ("NSAID's") (projected sales of $8.4 billion) and over-the-counter medications ("OTC's") (projected sales of $4.2 billion). The major market opportunity in arthritis, however, is in disease modifying agents as opposed to palliative treatments such as NSAID's and OTC's. The reasons for this anticipated shift are (i) NSAID's and OTC's only alleviate the symptoms without altering the progression of the disease, (ii) some studies have suggested that certain NSAID's may actually hasten the progression of the disease and (iii) the treatment philosophy of rheumatologists is shifting as physicians attempt to intervene earlier with disease modifying agents to prevent irreparable joint damage. Various results have shown that paclitaxel has significant potential as a disease modifying agent in RA and is developing various paclitaxel formulations to address this disease. (Sources - Dr. Ernest Brahn and Dr. Stephen Oliver, "National Institutes of Health Care Curriculum on Women's Health: Rheumatoid Arthritis"; Frost and Sullivan, Inc., "Report on the World Arthritis Treatment Product Markets", 1993)
A limiting factor in the clinical utility of many pharmaceutics is drug delivery. Many agents that show promising biological activity often do not progress to commercialization because of problems of unwanted toxicity, bioavailability, solubility, the necessity for local, sustained drug concentrations, cost of administration and other factors which for the most part are tied to drug delivery. The activity of other agents can be improved or enhanced by effective drug delivery designed to address the problems identified above. For these reasons, and also because agents that are coming off-patent can have their useful life extended through reformulation, the interest in drug delivery within the pharmaceutical industry has been growing steadily. The drug delivery market includes oral formulations, transdermal techniques, implants (including polymeric delivery), liposomes and other novel systemic treatments (again including polymeric delivery) and other novel drug carrier products such as photodynamic therapy. Each of these different technologies will have applicability for different agents and product applications.
Another reason for the increase in interest in drug delivery is the fact that drug delivery technologies are for the most part less expensive and less time consuming to develop. An original pharmaceutical product can cost as much as U.S. $360 million and 7 to 12 years to develop, while in comparison a new formulation for an existing drug can be developed for as little as U.S. $10 million over a period of only 3 to 5 years.
In a recent article on drug delivery, the estimated 1994 worldwide drug delivery market was approximately U.S. $4 billion. It was predicted that the market for drug delivery would expand at an annual rate of 20% to between U.S. $10 and U.S. $15 billion by the turn of the century. Of this market, parenteral drug delivery (implants) and liposomes and related technologies represent approximately 20% of the market at present and are expected to expand to between 30 and 35% of the total market by 1998. The reasons for this projected growth are the need to increase efficacy (by increasing local drug concentrations) and decrease toxicity (by decreasing systemic drug levels), while at the same time using sustained release to reduce the need for repeated interventions. At least 30 different products could be developed in polymeric drug delivery. These include formulations that enhance the performance of existing drugs, expand the uses of existing agents (i.e. the paclitaxel microspheres in arthritis), improve the performance of existing medical therapies such as stents and allow new treatments such as the surgical paste.
Reduced vision and blindness can be caused by a number of conditions which are characterized by inappropriate blood vessel growth or neovascularization. These include corneal neovascularization caused by contact lens wear, corneal infections, graft rejection, burns, trauma and inflammation, neovascular glaucoma (development of new blood vessels in the iris) and diabetic retinopathy (development of new blood vessels from the surface of the optic nerve in the retina). Treatments options exist but they invariably result in surgical intervention.
Neovascular diseases of the eye are the world's leading cause of blindness. Of the various problems caused by inappropriate blood vessel growth into the eye, diabetic retinopathy and neovascular glaucoma are two of the most serious conditions. In the United States there are approximately 12 million people that suffer from diabetes. Of these people, 5 million suffer from diabetic retinopathy. In addition, there are at least 2 million cases of neovascular glaucoma in the United States.
The total United States market for ophthalmic pharmaceutical products is estimated to be $3.19 billion in 1995 and the growth rate to the year 2000 is projected to be 8.9%. One of the major opportunities in this market will be the delivery of agents to the eye for long term release. (Source - Frost and Sullivan, Inc., "Report on the United States Ophthalmic Markets")
| Drug | Phase of Trial | Sponsor |
| TNP-470 | Phase II | TAP Pharmaceuticals, Inc., Deerfield, Wis. |
| Squalamine | Phase I | Magainin Pharmaceuticals, Inc., Plymouth Meeting, Pa. |
| Vitaxin | Phase I | Ixsys, Inc. San Diego, Calif. |
| Thalidomide | Phase II against Kaposi's sarcoma, breast, prostate and primary brain cancers |
EntreMed,
Inc., Rockville, Md. |
| RhuMab, VEGF | Phase II | Genentech,
Inc. South San Francisco, Calif. |
| SU5416 | Phase I | Sugen, Inc. Redwood City, Calif. |
| Marimastat | Phase III against pancreas, lung, gastric and breast cancers and glioma |
British Biotech, Inc. Annapolis, Md. |
| Bay 12-9566 | Phase III against lung and pancreas cancers |
Bayer
Corp., West Haven, Conn. |
| AG3340 | Phase III against lung and prostate cancers |
Agouron
Pharmaceuticals, Inc., LaJolla, Calif. |
| Col-3 | Phase I | CollaGenex
Pharmaceuticals, Inc., Newton, Pa. |
| CM101 | Phase I | Carbomed, Brentwood, Tenn. |
adapted from Prof. Goo-Bo Jeong, College of Medicine, Chungbuk National University, South Korea (1998) and the Italian Association of Cancer Research.