2.1 Angiogenesis-Dependent Diseases
Angiogenesis-dependent diseases (i.e., those diseases which require or induce vascular growth) represent a significant portion of all diseases for which medical treatment is sought. For example, cancer remains the second leading cause of death in the United States, and accounts for over one-fifth of the total mortality. Briefly, cancer is characterized by the uncontrolled division of a population of cells which, most typically, leads to the formation of one or more tumors. Such tumors are also characterized by the ingrowth of vasculature which provide by blood circulation various factors that permit continued tumor growth. Although cancer is generally more readily diagnosed than in the past, many forms, even if detected early, are still incurable.
A variety of methods are presently utilized to treat cancer, including for example, various surgical procedures. If treated with surgery alone however, many patients (particularly those with certain types of cancer, such as breast, brain, colon and hepatic cancer) will experience recurrence of the cancer. Therefore, in addition to surgery, many cancers are also treated with a combination of therapies involving cytotoxic chemotherapeutic drugs (e.g., vincristine, doxorubicin, taxol, vinblastine, cisplatin, methotrexate, 5-FU, etc.) and/or radiation therapy. One difficulty with this approach, however, is that radiotherapeutic and chemotherapeutic agents are toxic to normal tissues, and often create life-threatening side effects. In addition, these approaches often have extremely high failure/remission rates.
In addition to surgical, chemo- and radiation therapies, others have attempted to utilize an individual's own immune system in order to eliminate cancerous cells. For example, some have suggested the use of bacterial or viral components as adjuvants in order to stimulate the immune system to destroy tumor cells. (See generally “Principles of Cancer Biotherapy,” Oldham (ed.), Raven Press, New York, 1987.) Such agents have generally been useful as adjuvants and as nonspecific stimulants in animal tumor models, but have not as of yet proved to be generally effective in humans.
One additional limitation of present methods is that local recurrence and local disease control remains a major challenge in the treatment of malignancy. In particular, a total of 630,000 patients annually (in the U.S.) have localized disease (no evidence of distant metastatic spread) at the time of presentation; this represents 64% of all those patients diagnosed with malignancy (this does not include nonmelanoma skin cancer or carcinoma in situ). For the vast majority of these patients, surgical resection of the disease represents the greatest chance for a cure and indeed 428,000 will be cured after the initial treatment—428,000. Unfortunately, 202,000 (or 32% of all patients with localized disease) will relapse after the initial treatment. Of those who relapse, the number who will relapse due to local recurrence of the disease amounts to 133,000 patients annually (or 21% of all those with localized disease). The number who will relapse due to distant metastases of the disease is 68,000 patients annually (11% of all those with localized disease). Approximately another 102,100 patients annually will die as a direct result of an inability to control the local growth of the disease. Examples of cancers which illustrate the problem patients are faced with are breast cancer and liver cancer.
Breast Cancer
This problem is particularly evident for breast cancer which is a disease which affects approximately 186,000 women annually in the U.S. and for which the mortality rate has remained unchanged for 50 years. Surgical resection of the disease through radical mastectomy, modified radical mastectomy, or lumpectomy still remains the mainstay of treatment for this disease. Unfortunately, 39% of those treated with lumpectomy alone will develop a recurrence of the disease, and surprisingly, so will 25% of those in which the resection margin is found to be clear of tumor histologically. As many as 90% of these local recurrences will occur within 2 cm of the previous excision site.
Liver Cancer
Over 1.2 million people died in 1999 from primary liver cancer, the majority of them in Asia. Primary liver cancer refers to liver cancer in which the initial cancerous cells are formed in the liver, rather than traveling to the liver from some other cancer site in the body. Patients with certain forms of hepatitis, including hepatitis C, a viral disease which causes inflammation of the liver, are known to be at great risk for primary liver cancer. The incidence of primary liver cancer is expected to increase dramatically in the United States, where estimates have indicated that more than 4 million people are now hepatitis C positive.
Over 70 percent of primary liver cancers are inoperable and are treated with radiation or chemotherapy. The currently available treatment options include the following:
Chemotherapy:
Chemotherapy seeks to control cancer by killing rapidly dividing cancer cells. However, a number of non-cancerous cells in the body, such as bone marrow cells, also rapidly divide and are, therefore, highly susceptible to being inadvertently killed by the chemotherapy. Thus, doses sufficient to eradicate the cancer often cause life-threatening side effects due to the destruction of non-cancerous cells.
Chemoembolization and Various Treatments Under Development:
For example, precutaenous ethanol injection is a painful procedure which only works for small tumors which are limited to smaller than 3-4 cm.
Transplantation:
Transplantation is also an available therapy which is expensive and limited by the availability of organs and which can still lead to a recurrence of the tumor. Also, there are recurring dangers associated with invasive surgery.
Moreover, chemotherapy may also damage the natural anti-tumor defense of the human body.
Uterine Fibroids
A relevant example of a non-cancerous tumor would be uterine fibroids, also known as leiomyomas. These are non-cancerous tumors composed of certain types of muscle fibers and fibrous connective tissue. The cause of uterine fibroids is unknown. Most patients with uterine fibroids do not initially have symptoms and remain untreated until the patient experiences abnormal bleeding, urinary frequency, pain, swelling and difficulty with fertility. Approximately 25 million women in the United States have uterine fibroids, and approximately 5.5 million of these women are symptomatic enough to seek treatment each year. Until now, women suffering from uterine fibroids have had few viable treatment options, including hysterectomy, myomectomy, and Medical Management and “Watch and Wait”. The therapies currently available for treating uterine fibroids have significant drawbacks including: temporary or permanent loss of fertility for women of child-bearing age, lengthy recovery periods, adverse psychological effects which may lead to early menopause, high costs, including costs of medications, surgical procedures, frequent and long hospital stays, discomfort and side effects from invasive surgical procedures and hormone therapy, and/or risk of recurrence of the fibroids.
As a result, there is a significant need for a uniformly efficacious therapy program for patients with, inter alia, breast cancer, liver cancer, pancreatic cancer and uterine fibroids.
Passive Embolization
One method that has been attempted for the treatment of tumors albeit with limited success is passive embolization. Briefly, blood vessels which nourish a tumor are deliberately blocked by injection of an embolic material into the vessels. A variety of materials have been attempted in this regard, including autologous substances such as fat, blood clot, and chopped muscle fragments, as well as artificial materials such as wool, cotton, steel balls, plastic or glass beads, tantalum powder, silicone compounds, radioactive particles, sterile absorbable gelatin sponge (Sterispon, Gelfoam), oxidized cellulose (Oxycel), steel coils, alcohol, lyophilized human dura mater (Lyodura), microfibrillar collagen (Avitene), collagen fibrils (Tachotop), polyvinyl alcohol sponge (PVA; Ivalon), Barium-impregnated silicon spheres (Biss) and detachable balloons. The size of tumor metastases may be temporarily decreased utilizing such methods, but tumors typically respond by causing the growth of new blood vessels into the tumor.
A related problem to cancer tumor formation is the development of cancerous blockages which inhibit the flow of material through body passageways, such as the bile ducts, trachea, esophagus, vasculature and urethra. One device, the stent, has been developed in order to hold open passageways which have been blocked by tumors or other substances. Representative examples of common stents include the Wallstent, Strecker stent, Gianturco stent, and the Palmaz stent. The major problem with stents, however, is that they do not prevent the ingrowth of tumor or inflammatory material through the interstices of the stent. If this material reaches the inside of a stent and compromises the stent lumen, it may result in blockage of the body passageway into which it has been inserted. In addition, presence of a stent in the body may induce reactive or inflammatory tissue (e.g., blood vessels, fibroblasts, white blood cells) to enter the stent tureen, resulting in partial or complete closure of the stent.
2.2 Therapeutic or Active Embolization
The administration of cytotoxic drug into the proximity of a tumor increases the ratio of tumor to normal tissue delivery. Such regional administration can be accomplished by directly delivering drugs into the tumor via its blood supply or into the body cavity where the particular tumor is located. Regional drug perfusion has the advantage of increasing peak drug concentrations to the target tissue but exposure is limited to the first pass of blood through the organ being perfused. The portion of the drug not taken up by the initial pass circulates systemically and is then taken up by the normal tissues.
Therapeutic vascular occlusions (embolizations) are techniques used to treat certain pathological conditions in situ. They are practiced generally by means of catheters making it possible, under imagery control, to position particulate occlusion agents (emboli) in the circulatory system. They can also concern the vessels of various processes: tumors, vascular malformations, hemorrhagic processes, etc. Notably in the case of tumors, vascular occlusion can suppress pain, limit blood loss on the surgical intervention to follow embolization or even bring on a tumoral necrosis and avoid the operation. In the case of vascular malformations, it enables the blood flow to the normal tissues to be normalized, aids in surgery and limits the risk of hemorrhage. In hemorrhagic processes, vascular occlusion produces a reduction of flow, which promotes cicatrization of the arterial opening(s).
Furthermore, depending on the pathological conditions treated, embolization can be carried out for temporary as well as permanent objectives.
Different types of emboli are known in the prior art. In particular, liquid agents (acrylic glues, gels, viscous suspensions, etc.) or particulate agents (miscellaneous polymers, dura mater, gelatin sponges, spheres, balloons, spirals, etc.) can be involved. The major disadvantages of the known liquid emboli reside in their toxicity to the tissues, which can generate necrosis phenomena, and in the risk of sticking of the catheters. Another limitation of liquid emboli is that they act only in a passive way and are not capable of being used for drug delivery.
The dual functions of the regional distribution of drugs to and the minimization of loss from the target site can be achieved with microspheres. When introduced via a regional artery, such microspheres are trapped within the vasculature of tissues, where they release their drug load. Such dual action is referred to as active embolization. Microspheres may be of either a solid or porous composition, and can be made to contain dispersed drug molecules either in solution or solid form (Zimmer and Kreuter, 1995). Microspheres have special application in treating tumors located within organs supplied by a single afferent arterial blood supply, for example, the liver (Chen et al. 1994). They are of most value in cases where the tumor in the target organ is the only region that requires therapy.
While studies relating to the use of microspheres for embolization drug-based therapy are known, relatively few studies have been performed with microspheres for use in gene therapy. For instance, glass beads are commonly used in the selective extraction of DNA from heterogenous mixtures via electrostatic attraction. While hydroxyapetite has also been utilized for purification of nucleic acids (Kumazawa et al. 1992) and hydroxyapetite spheres have been used for sustained release of doxorubicin by direct implantation into hepatic tumors via ultrasonic guidance (Kunieda et al., 1993), other studies reveal that this particular matrix may indeed be unsuitable for exposure to mammalian tissue (Dass et al., 1997a, 1997b). DNA has also been retained on PDB microspheres with diethylaminoethyl (DEAE) functional groups at the surface (Katz et al, 1990; Maa et al., 1990).
Thus, there is a demonstrated need for the further development of microspheres for gene delivery. The major desired advantage is the ability to specifically target the anticancer agent to the tumor vasculature by the blood flow. Further, the obstruction of tumor blood supply, with the simultaneous disruption in nutrient supply and waste removal, is perhaps the most desired result for destruction of the tumor cells.