Despite cost and regulatory hurdles, inroads from the bench, the desk, and even the greenhouse are advancing cell-based medicine
Canavan is a freelance writer based in New York
A recent front-page New York Times article focused on the promise of stem cell-based therapy for cardiovascular disease, but it did nothing to advocate the cell source in bone marrow. Marrow transplants are expensive, and harvesting the stem cells is an arduous procedure at best. But the article, and many others in the popular press, might well lead people to believe that the only option is embryonic stem cells (ESCs). They’re cheaper, plentiful, and a lot easier to obtain, assuming you’re not in the United States, that is. The problem is the controversy. Further, ESC therapy is still a very blue-sky proposition. Many scientists are convinced that adult stem cells are the way to go, but again, there’s the problem of where to get them. Marc Beer, president and CEO of ViaCell Inc., Cambridge, Mass., suggests an alternative.
“Cell therapy is going to be enabled by using a noncontroversial source of stem cells that we can obtain in high volume that can demonstrate good plasticity, the ability of those cells to give rise to different
tissues.” That source is umbilical cord blood. The past several years have seen critical breakthroughs in the understanding of cord blood dynamics, the most interesting of which is the documentation of pluripotency in cord-blood-derived stem cells, called unrestricted somatic stem cells (USSCs). This gave rise to several different lineages in vivo [G. Kogler et al., Journal of Experimental Medicine, vol. 200, pp. 123-135 (2004)], one of those being the cardiomyocyte.
As for how USSCs are used, Beer is not following the typical therapeutic method in which a particular progenitor cell, a hematopoietic cell for example, is isolated in culture. “We keep the USSC in an unrestricted state,” Beer says. “Then we inject it and allow the cell to endogenously do what it should do.”
Several studies have shown that the injected cells migrate to the damaged tissue and endogenous signals coax the USSCs to differentiate. In mice, this method demonstrated new human cardiomyocytes in residence at the point of infarct, and porcine models showed an improvement in left ventricle ejection fraction, a cardiac standard. “In the cardiology model, I think that the real need is an allogeneic, off-the-shelf cell that can be preloaded into a catheter.” According to Beer, the USSCs should be naïve enough to attain tolerance, requiring little, and eventually no, systemic immune suppression.
Successful cell therapy requires two things: One is a bountiful, accessible cell source. Another is a good donor match to avoid rejection, possibly a naïve USSC or a sibling
donor, or ideally, the donor is the recipient. To achieve these ends, John Fraser, vice president of research and technology at MacroPore Biosurgery Inc., San Diego, encourages
|A Potato a Day . . .
The cells used in cell-based drug delivery do not necessarily come from people. A new study demonstrates that human hepatitis B vaccine (HBV) can be expressed in, of all things, a potato [Y. Thanavala et al., Proc. Natl. Acad. Sci. U.S.A., vol. 102, pp. 3378-3382 (2005)]. Lead investigator Yasmin Thanavala, PhD, an immunologist at the Roswell Park Cancer Institute, Buffalo, N.Y., recalls her initial efforts in spud-dose development: “Actually, the first experiment that worked, I was absolutely staggered by it. That a viral antigen from a human virus, expressed in a plant, should then evoke an immune response faithful to that which was provoked by the real antigen? But indeed it did.”
The goal of this experiment was indeed novel, but the technique used is well known to plant biologists: The gene of interest (hepatitis B surface antigen, or HBsAg) is inserted into a plasmid of Agrobacterium tumefaciens, which is used to infect the plant of interest (i.e., Solanum tuberosum cultivar “Frito-Lay 1607”). Top expressers of the gene are selected, cloned, and propagated. But why use plants? “Plant delivery systems have much to offer [given] the way we are going to make vaccines, especially in developing countries,” says Thanavala. The need is profound: The global caseload for HBV is currently 350 million, and approximately 750,000 patients die annually from HBV-associated liver failure; this frequency persists though a safe, effective, injectable vaccine already exists. The vaccine is not used globally because it requires cold storage and a significant amount of money. Thus, the potato.
“We first thought that the banana would be our ultimate choice, but as the idea matured we thought, the intended strategy is not to have people running around buying an apple or banana and eating it.” The idea is to produce vaccine cheaply, at a dependable concentration, in a form that can be easily transported, without, as Thanavala puts it, “A lot of high-falutin’ technology.” And the vaccine cannot be consumed in random quantities, but has to be given like a drug. “People will be administered vaccine in a powdered form that will be capsuled, so the plant will be processed, minimal processing, like freeze drying, common food technology.”
Thanavala and colleagues have already shown that their plant-expressed HBV antigen survives the process intact, and the first human trials, just concluded, show that it works. Moving forward, optimization of the immune response (currently at 40%) will involve the addition of a mucosal adjuvant in the form of a component unfortunately already well known in the developing world: cholera toxin.
researchers to think fat. “The theoretical advantage of using adipose tissue as opposed to other tissue is that you can get lots of it quite easily. It can be used autologously, and it can be turned around quickly. You can get the tissue out and back into the patient in about an hour.”
Historically, adipose tissue was thought to be quiescent, but it is now clear that fat
click the image to enlarge
A hematoxylin and eosin stain shows human adipose tissue before it has been processed and before regenerative cells have been isolated. (Source: MacroPore Biosurgery Inc.)
expands. That expansion is accommodated by the wide range of cell types that make up any tissue, including smooth muscle cells, endothelial cells, and anything involved in angiogenesis. “You get a lot of vascular elements,” says Fraser, “and some progenitor cells that have potential for disorders where blood flow is an issue, [such as] heart attack, stroke, etc.”
As is the case with cord blood, adipose-derived cells (ADCs) will be used as a mélange of progenitor types rather than trying to mimic the narrowly defined activity of a therapeutic drug. “On one level, yes, it makes sense. You give patients drugs that are substantially pure and are the same, well-defined chemical compound, but the body’s not like that. You have [myocardial infarction], for example, [and] you’ve got many different cells interacting for the purposes of trying to heal the heart.” Selected ADC cells could be grown as a homogeneous population, and that would fit what regulatory bodies want, but, Fraser asserts, “It doesn’t fit what the body wants.”
ADCs may be what the body wants, but Jane Lebkowski, PhD, senior vice president of regenerative medicine at Geron Corp., Menlo Park, Calif., doesn’t think the market will buy it. “I see it as a very expensive alternative. When you’re looking at processing one person’s cells for that one person, individualized cell therapy, your lot size is one. Given all the regulatory considerations, that makes it hugely expensive. Part of the beauty of ESCs is that a researcher can grow them up into large numbers and can do batch processing: small scale means a lot size of one million.”
Lebkowski concedes that the theoretical cost for Geron’s ESCs could rival that of ADCs when considered for postoperative uses: systemic immunosuppression can be very expensive. “We’re looking at ways of giving these cells without immunosuppression, and without using something as elaborate as nuclear transfer. For instance, you can also generate ‘tolerizing’ cells from ESCs, dendritic cells, that when co-administered with a graft, would lead to tolerance.” This technical advance is high on Lebkowski’s wish list, but for now, regulatory issues are more pressing for those advancing the cause of ESCs. The month of March saw a flurry of press releases addressing a single regulatory issue. Geron; Advanced Cell Technology, Worcester, Mass.; the Roslin Institute, Edinburgh, UK; and a laboratory at the University of California at San Diego all announced the development of a potentially contaminant-free method for cell culture, contaminants defined as any nonhuman animal product, be it a feeder layer or nutrient. In discussing the various methods, Lebkowski acknowledged that the innovations were driven purely by US Food and Drug Administration (FDA) considerations.
FDA and stem cells
The FDA has great interest in what a manufacturer puts into a therapeutic, but is not interested in how it got there. “The FDA is actually quite receptive to the use of embryonic stem cells. There’s
no innate prohibition against it,” says Darin Weber, formerly of the FDA and now with The Biologics Consulting Group, Alexandria, Va. Any objections raised to an investigational new drug submission (IND) for such a product could only be the result of the patent holder going public. “If a company actually did submit an IND for using nuclear transfer (cloning), the FDA is prohibited by law from disclosing that. It’s confidential.” And the IND would proceed as long as the safety issues were fully addressed, and, specific to cell-based therapeutics, there was a full accounting of the cell’s lineage. “The issue of nuclear transfer is all upstream of the product. How you derive the material, those are moral, ethical issues that the FDA is not involved with. That’s not their job.”
Their job is approving new treatment options, but given the fact that Carticel from Genzyme Corp., Cambridge, Mass., autologous cultured chondrocytes for the treatment of damaged knee cartilage, is the only currently approved cell-based therapeutic, one might conclude the job takes quite a long time. Weber disagrees. “If there’s a significant unmet medical need, and a lot of these products are intended for the sickest patients, with Alzheimer’s disease, and so forth, if a product can be shown to be not only safe, but that it works, I think you are going to see some extremely short time frames” for approval.
Medical urgency aside, Weber says the success of cell-based therapy will initially depend on the marketplace. “Approvals in the near future will probably be in the cosmetic enhancement area. The beauty of that market, no pun intended, is that most patients getting those sorts of therapies are wealthy; they can do it outside of insurance so the whole issue of reimbursement goes away.” This is not an esoteric consideration. As seen with approved protein-based therapies, costs can be enormous, so much so that the only insurmountable barrier to great innovation may be the price.
|Reovirus Infection Means Runny Noses in Humans, Death in Tumors
Here’s a new way to kill a tumor: Don’t bother with poisons or radiation or a knife, just make it terminally ill. Give it a virus. Specifically, give it the lytic, tumor-specific reovirus, an infection that keeps on giving. Reovirus (respiratory enteric orphan virus) was first characterized in 1951 and is very common and rather benign, often causing little more than a runny nose. So why is it special? Matthew Coffey, PhD, chief scientific officer of Oncolytics Biotech Inc., Calgary, Canada, says the company discovered the virus is able to replicate only in cells with an activated Ras pathway. Ras, an oncogene, induces growth-factor-independent cell proliferation and is thought to be constitutively active in 30% of all tumor cells.
But reovirus is not targeting Ras, says Coffey. “The virus uses sialic acid residues expressed on [any] tissue, so we’re not seeing tissue tropism.” The virus is internalized into normal cells as well, but fails to replicate in non-Ras conditions before it’s detected by the cell’s defenses. The viral genome (double-stranded RNA) is recognized by normal cells as foreign, triggering the activation of the protein PKR. PKR then shuts down host translation of the viral genome. “When the Ras pathway is active, PKR is diverted such that it can no longer act as an off-switch of the translation machinery, and what we get is very effective viral replication which results in cell death.” Tumor cell death. And the process is contagious. “For every cell that’s infected, between 3,000 and 10,000 progeny virus then infect neighboring neoplastic tissue, which in turn propagates the virus. So, basically, it acts as an internal bioreactor.”
The purported efficacy of this process begs the question: if reovirus is so common, why would anyone ever die of Ras-dependent cancers? The answer lies in the largely fecal or oral route of transmission, which stems from bad water. “When you ingest contaminated water, you’re probably only taking in several hundred or several thousand viral particles. What we’re doing in clinical studies is exposing patients to starting doses of more than a million particles that have been highly purified.” Thus far, results from the first clinical trial have been promising, and the US Food and Drug Administration recently signed off on a second phase I trial for Coffey’s novel reovirus drug, Reolysin.
Filed Under: Drug Discovery