EUROPEAN REGULATORS CURDLE PLANS FOR GOAT MILK HUMAN ANTITHROMBIN
On February 22, 2006, the European Medicines Agency refused marketing authorization for ATryn®, a recombinant form of human antithrombin. The ATryn® application had been the first submitted to a US or European regulatory authority requesting approval of a recombinant therapeutic protein produced in the milk of genetically modified animals.
The strategy of producing therapeutic proteins in transgenic animals’ milk arose during the mid-1980s when the fledgling biotech industry faced the challenge of synthesizing properly folded and glycosylated proteins. While vats of mammalian cells offered one tactic, the cultures often produced low yields. To increase efficiency, researchers considered the mammary glands of transgenic cows, goats, and rabbits.
In the early 1990s, Genzyme began a program to produce therapeutic proteins in the milk of transgenic goats. The company spun its operation off as a separate company, Genzyme Transgenics, which was later named GTC Biotherapeutics. The company’s scientists used microinjection to insert a genetic construct into the nucleus of a one-cell embryo. In the case of ATryn®, the construct included a human antithrombin gene under the control of a beta casein promoter.
GTC marketed its technology platform to pharmaceutical companies. Although the company’s production method worked, the question remained whether regulatory agencies would approve therapeutics synthesized by transgenic animals. This uncertainty nurtured the ‘pharma’ industry’s hesitancy about producing recombinant drugs in milk. Nobody seemed keen on being the first to try the approval process.
To test the regulatory waters, GTC sought approval for the sale of antithrombin to a small target population. The company aimed at subjects with an inherited antithrombin deficiency, who need the protein to prevent damaging blood clot formation during high-risk situations, such as major surgery.
After testing the anticoagulant in experimental models, GTC investigated the effects of the drug on people with congenital antithrombin deficiency who were undergoing surgery (five subjects) or giving birth (nine subjects). Since either event presents risks of bleeding, these individuals cannot take the anti-clotting drugs that they normally use.
GTC included the results of the clinical investigations in the ATryn® marketing application. The European Medicines Agency rejected the application, because GTC had performed studies with five surgery patients rather than the recommended twelve, and because the agency wants the company to carry out additional studies to assess whether patients developed antibodies in response to ATryn® treatment. GTC announced that it would appeal the decision, emphasizing that the agency had not based its negative ruling on the fact that the drug had been produced in transgenic goats.
The microinjection technique used by GTC in its ATryn® project tends to generate embryos that give rise to transgenic animals at a rate of one to five percent. To produce its newer drugs in transgenic animals, GTC uses the more efficient somatic cell nuclear transfer method. This technique can also produce clones.
Clone, Clone on the Range
Cloning came of age in 1996 with the birth of Dolly the sheep, the world’s first mammal cloned from an adult cell. Researchers at Scotland’s Roslin Institute produced Dolly using somatic cell nuclear transfer. In this process, the nucleus of a somatic cell from the animal to be cloned is injected into an enucleated oocyte and the embryo is transferred to the uterus of a surrogate mother. Theoretically, nuclear transfer can be used to make an unlimited number of copies of one animal.
After Dolly, nuclear transfer enabled the cloning of cattle, goats, and pigs. The era of livestock cloning had arrived. Proponents of livestock cloning claim that it benefits consumers by providing reliably high quality food products, while farmers can select and propagate animals with superior genetics without risking gene reshuffling associated with sexual reproduction. The technology’s supporters also claim that cloning may allow the replacement of grain-fed livestock with grass-fed livestock. Since grass does not require the quantities of fertilizers and pesticides required for growing grain, cloning may benefit the environment.
Although some view livestock cloning as an extension of centuries-old selected breeding practices, critics have voiced social, ethical, and religious concerns. For example, animal welfare groups claim that livestock cloning poses unnecessary health risks to farm animals. Opponents of livestock cloning also suggest that seemingly healthy clones and their progeny may harbor subtle defects that could make their food products unsafe to eat.
The Food and Drug Administration has pondered the safety of food products obtained from the offspring of clones. In 2001, the FDA’s Center for Veterinary Medicine requested the National Research Council to identify any safety concerns that cloned animals might present to food, animals, and the environment. In its 2002 report, the NRC concluded that "there is no current evidence that food products derived from adult somatic cell clones or their progeny present a food safety concern." The NRC committee recommended the collection of additional information about the composition of food products to ensure that cloned animal products do not differ from those of normal animals in ways that might affect human health.
Two years later, the NRC published a second report on the safety of cloned animals in the food supply. Again, the organization could find no scientific evidence that cloning produced an unintended compositional change that posed health risks in humans.
The University of Connecticut’s Xiangzhong Yang led a team of US and Japanese researchers in an investigation of potential compositional changes induced by cloning. They compared the produce of two beef and four dairy clones generated by nuclear transfer with that from normal animals of similar age and breed. Meat was analyzed against more than 100 quality criteria, while milk was analyzed for protein, fat, lactose, and urea nitrogen.
Published in the May 3, 2005, issue of the Proceedings of the National Academy of Sciences, their study indicated that the composition of meat and milk from somatic animal clones did not differ significantly from that of genetically matched comparators or industry breed comparators. Although they found that meat and dairy products met industry standards, Yang and colleagues cautioned that their pilot study should provide guidelines for more conclusive studies with larger numbers of clones with different genetic backgrounds.
The FDA has been cautious as well. In 2003, the Center for Veterinary Medicine published a draft summary of its science-based review of the risks that may arise in livestock cloning. The agency could find no evidence that consumption of edible products from clones of cattle, pigs, sheep, or goats poses a greater risk than consumption of those products from their non-clone counterparts. Nevertheless, the FDA requested producers to abstain from placing edible products from clones into the food supply until the agency has concluded its safety evaluation.
For years, the FDA has promised a final policy and, several times, approval seemed imminent. Most recently, The Washington Post reported in October 2005 that the agency was expected to rule soon that milk from cloned animals and meat from their offspring are safe to eat. Yet, in response to written questions, Stephen F. Sundlof, director of the Center for Veterinary Medicine, informed The Washington Post that the FDA "really can’t provide a reliable estimate on the time frame" for releasing a policy. With the future of a new food industry at stake, the agency thoughtfully chews on safety issues.
European Medicines Agency (2006) Q&A on Recommendation for Refusal of Marketing Application for ATRYN, February 23, 2006. Available at: http://www.emea.eu.int/.
Gillis J (2005) Clone-generated milk, meat may be approved. The Washington Post, page A01, October 6, 2005.
National Research Council (2002) Animal Biotechnology: Science-Based Concerns. Available at: http://darwin.nap.edu/.
National Research Council (2004) Safety of Genetically Engineered Foods: Approaches to Assessing Unintended Health Effects. Available at: http://darwin.nap.edu/.