The use of encapsulated living cells provides added possibilities for complex therapeutics. Cellular therapeutics uses living cells to deliver ameliorating biochemicals (whether natural or engineered) or to serve as full-scale replacements for defective tissues. Although blood transfusions have been performed successfully on a routine basis since the 1930s (and peripatetically for a thousand years before), the real birth of complex cellular therapeutics can be dated to the first human bone marrow transplant in 1969 by E. Donnal Thomas as part of a defined treatment regime against leukemia. This technological innovation paved the way for a host of potential cellular and tissue therapeutics. Today, infusing or implanting living cells—in a free state or in polymeric capsules—into people afflicted with a variety of diseases is under intense study in animal models and in promising human clinical trials.

Cell transplants
Simple transplantation of bone marrow cells was the first and, so far, the most widely used and effective of the cellular therapies, opening the door to all of the others. Since the late 1960s, bone marrow cells have been used to replace the chemotherapy-destroyed marrow of patients afflicted with various cancers. These marrow cells can be derived either autologously (from the patient before chemotherapy) or from allogeneic (nonself) tissue donors. Unless they are from an identical twin, almost all other forms of cell, tissue, or organ transplants are usually allogeneic (and in some cases, involve xenotransplantation—the use of biological implants from totally different species).

Luckily, in the case of bone marrow transplantation, the body often can adapt completely to the new allogeneic cells because they have, in effect, replaced the patient’s immune system with one that perforce recognizes the transplant as being part of itself. (In fact, the need for tissue typing for bone marrow transplants is primarily to prevent graft-versus-host rejection disorder rather than vice versa). Lifetime use of immunosupressive drugs is the usual result of non autologous transplantation, unless the immune system can be otherwise tricked into accepting the new cells. One such “trick” has been used in concert with pancreatic islet cell transplants. Marrow cells from the same donor, when also transplanted, can “educate” the body to accept the islet cells by modifying the host immune system. But barring such tricks, simple cellular transplantation is fraught with the same problems as organ transplantation.

Alternative methods
To avoid the problems of simple transplantation, there are currently two main viable approaches. The first is the use of embryonic or cord blood stem cells for transplants. Such cells can adapt to a variety of mature tissues after transplant and have the added value of generally being accepted by the host immune system without rejection or the use of drugs. The investigation and use of stem cells to treat disease is a burgeoning field with tremendous promise, whether the cells are derived from fetal sources (with all the concomitant ethical implications) or sequestered from adults (see box, “Naked came the cell”).

The second and more “traditional” way to eliminate the risk of rejection or the need to use problematic antirejection drugs is to encapsulate “naked” cells in polymeric substances. Encapsulation can open the door to the immune-protected use of cells from various species. It could also increase the possibility of using several genetic and cellular engineering techniques, including the creation of artificial organs.

Figure 1. Encapsulated cells are protected by a membrane or capsular matrix that allows nutrients, waste, and therapeutic products to pass freely but serves as a barrier to the host immune system.

Encapsulated cells
The use of encapsulated cells is a fairly simple concept. The principle is to develop a capsule with sufficient permeability that nutrients and oxygen can reach the transplanted cells, and appropriate cellular products (insulin in the case of islet cells, for example) can be released into the bloodstream or to adjacent tissues. At the same time, the capsular material must be restrictive enough to exclude immune cells and antibodies that would cause rejection and destroy the implant (Figure 1). Such encapsulation can be achieved using polymers.

As can be expected, research into various capsular materials is almost as important as studying the requirements and kinds of cells that can be appropriately transplanted. The first encapsulated cells were developed as far back as the 1960s, when T.M.S. Chang and colleagues first reported the development of semipermeable aqueous microencapsulation of cells. The vision of using these cells for therapeutic purposes was present from the start.

Several polymeric systems are currently in use or under development. The requirements for success (both clinical and regulatory) include chemical definability, the ability to validate structure, stability, resistance to protein absorption, lack of toxicity, permeability to oxygen and nutrients as well as to the released therapeutic compounds, and resistance to antibodies or cellular attack.

Several polymeric encapsulation systems have been developed or are currently being tested in clinical trials. These include polysaccharide hydrogels, chitosan, calcium or barium alginate, and a layered matrix of alginate and polylysine. Novocell, Inc., has developed a photopolymerizable poly(ethylene glycol) (PEG) polymer to encapsulate individual cells or cell clusters. Various polyacrylates are also being tested, including polymers such as hydroxyethyl methacrylate methyl methacrylate.

One novel approach being used by Mauro Ferrari, director of Ohio State University’s new biomedical engineering center, is to use photolithography techniques adapted from the semiconductor industry to encapsulate living cells in silicon capsules that have pores only a few nanometers wide. (Typically, antibodies require a pore larger than 18 nm to pass through.) The initial goal is to use pig islet cells to control diabetes. At the University of Illinois, bioengineer Tejal Desai is taking a similar approach to create silicon nanocapsules that might ultimately implant new neurons in the brain.

Many research groups are examining the use of biocompatible semipermeable membranes to surround the encapsulated cells, sometimes within a capillary device, to create a miniature artificial organ, such as one that would include functional pancreas or liver cells. This is often called macroencapsulation. The value of such an artificial organ and organized encapsulation lies in better simulating the natural environment of the cells in vivo as well as more efficiently protecting them from rejection and allowing easy surgical removal or replacement of the entire ensemble, if required.

In December 2000, for example, Neurotech S.A. announced that it is cooperating with the Foundation Fighting Blindness to move toward clinical trials to examine the use of its encapsulated cell therapy (ECT) for protection against blindness caused by a progressive loss of photoreceptors and the degeneration of retinal pigment epithelial cells. Neurotech purchased the ECT technology from the American company CytoTherapeutics, Inc., early last year. The ECT proprietary device uses a polymembrane (acrylonitrile-co-vinyl chloride) to form a tubular structure 1 mm in diameter and 5–7 cm in length, sealed at either end by methacrylate resin. The membrane allows compounds that are smaller than 100,000 M_r to permeate, but larger molecules are excluded, thereby protecting the cells from immunological attack.

Similarly, Cell Based Delivery, Inc., has developed the proprietary ImPACT (Implantable Protein fACTory) system for the cell-based delivery of therapeutics. Treatments for various cardiovascular diseases, hormonal growth deficiency, musculoskeletal disease, and solid tumors are in preclinical trials with animals.

Prospects and promises
Diabetes is one of the most significant areas of current research for encapsulation of cells—specifically the islet cells of the pancreas that produce insulin. Although Type I insulin-dependent diabetes is controllable with daily injections, this is seldom as effective as the normal metabolic control governed by functional islet cells. Individuals with Type I diabetes have a high risk of diabetes-related medical complications. Encapsulation of islets has the potential to prevent rejection of both allogeneic and xenotransplanted islets.

Unfortunately, although the various microencapsulation methods provide the cells safe harbor against antibody or cell-based immune system attack, they can be permeable to the small-molecule cytokines secreted by cells of the immune system and thus liable to apoptosis—cells committing suicide within their otherwise protected environment. Shimon Efrat and colleagues at Tel Aviv University are attempting to genetically engineer islet cells with “a group of genes from adenoviruses that produce a variety of proteins that counteract apoptosis.” Although studies are initially being performed in rodent systems, the researchers believe that there is no reason they will not be able to “translate what [they] learn in the mouse to human beta cells.”

Encapsulated cells (using ECT technology and otherwise) are also being studied for the treatment of Parkinson’s disease, Huntington’s disease, and amyotrophyic lateral sclerosis (ALS or Lou Gehrig’s disease), various ophthalmic disorders, as well as managing chronic pain.

Using encapsulated cells that have been genetically engineered for drug production is a fairly recent development. Following promising antitumor effects seen in the nude mouse, researchers in Germany conducted Phase I studies in 1999 showing the feasibility of using encapsulated cells to enable the localized delivery of highly toxic chemotherapies that could not be used under normal circumstances. Cells were transfected with cytochrome P-450 CYT2B1, which can enzymatically activate ifosfamide, converting it to its active toxic components, phosphomustard and acrolein.

Encapsulated transformed cells were well tolerated during tumor injection, and subsequent chemotherapy was well tolerated, increasing the possibility of using chemotherapies that were previously too toxic in a localized and localizable fashion. Implantable microcapsules are being studied for use in cellular-based gene therapy for hemophilia, lysomal storage diseases, and growth hormone deficits such as dwarfism, as well as central nervous system diseases such as ALS and parkinsonism.

Cellular futures
The future of cellular therapies appears bright. S. A. Edwards, in a report for the Business Communications Co., Inc., identifies more than 40 different companies that are involved in various cell and tissue therapies. The future of encapsulated cells appears especially promising, although their use is far from being a routine medical procedure, and the results of clinical trials and laboratory studies will determine how widely applicable this technology becomes. In addition, it is likely that significant follow-up studies will be needed to determine the long-term viability of such treatments.

Several cellular transplant technologies are on the horizon that might supplement or replace ECT, even as it comes into its own. These include methods being developed to eliminate the problem of host rejection. Among these options is the development of less troublesome antirejection drugs and “humanized” animal donors whose cells would be genetically engineered to be readily acceptable to the individual human immune system. And ultimately, the greatest promise is the development of totipotent immortalized stem cells, which would eliminate many of the moral qualms associated with stem cell research and provide a reliable and highly plastic source of cells for direct transplantation, whether naturally or genetically engineered. But until such therapies become available, it seems likely that more and better forms of encapsulation will be required to provide the best and most eclectic mix that cellular therapeutics has to offer.

For further information

1. Artificial Cells & Organ Research Centre (McGill University), www.artcell.mcgill.ca, provides detailed information about the center and numerous pertinent links (accessed March 2001).
2. Chang, T.M.S. Artificial Cell Biotechnology for Medical Applications; Blood Purif. 2000, 18, 91–96. Available on the Web at www.artcell.mcgill.ca/2000ISBP.htm. Also, Artificial Cells, www.artcell.mcgill.ca/artcell.htm (both accessed March 2001).
3. Edwards, S. A. Cell Therapy and Tissue Engineering: Emerging Products. http://buscom.com/biotech/B119.html (accessed March 2001).
4. Neurotech S.A. ECT Web site. www.neurotech.fr/technos/encap-cell-d.htm (accessed March 2001).

Mark S. Lesney is a senior editor of Modern Drug Discovery. Send your comments or questions regarding this article to mdd@acs.org or the Editorial Office by fax at 202-776-8166 or by post at 1155 16th Street, NW; Washington, DC 20036.