Hunt For Viable Vectors Leads To Jobs For Gene Therapy Researchers

Therapy Researchers Sidebar: Summary Table of Vectors PROGRESS: Forrest Anthony says some FDA concerns have been allayed. Despite safety questions from federal agencies, the gene therapy industry is hotly pursuing new vectors. Since 1990, dozens of biotech and gene therapy companies have hired life scientists and launched research programs to find safe, effective carriers to shuttle therapeutic genes into the body. According to the Pasteur Institute, a nonprofit research organization in Paris, 216 gene therapy protocols with human patients-treating diseases such as cystic fibrosis, cancer, and AIDS-are under way worldwide. In the vast majority, therapeutic genes are packed into disarmed viruses, which shuttle the genetic payload into a cell's chromosomes. These viral vectors require the expertise of virologists, biochemists, and molecular biologists, among others. Meanwhile, nonviral vectors-for example, synthetic molecules that tote therapeutic genes-call for biophysicists and analytical chemists. Gene therapy researchers agree that finding-or creating-viable vectors is of utmost importance to patients and investigators. "There isn't much future to gene therapy without viable vectors," remarks Didier Trono, head of infectious diseases research at the Salk Institute for Biological Studies in La Jolla, Calif. "This is where work needs to be done." Four types of vectors share the gene therapy spotlight: retroviruses, such as HIV; adenoviruses, which normally cause upper respiratory infections; adeno-associated viruses, which are harmless viruses that need adenoviruses to replicate; and a variety of synthetic vectors, such as lipid-DNA complexes, in which scientists attach useful genes to fatty molecules that ferry the genes into a cell. Each vector has pros and cons (see accompanying table). Summary Table of Vectors VECTORSAFETYEFFICIENCYPERSISTENCE RetrovirusPotential Insertional Mutagenesis High High AdenovirusPotential Immunogenicity High VeryLow Adeno-Associated VirusUnknownModerateModerate Lipid-DNAUnknownModerateLow Retroviruses easily integrate into a cell's genome, offering long-term expression of therapeutic genes. But these vectors typically infect only dividing cells, which limits their use. What's more, researchers and Food and Drug Administration (FDA) officials worry that retroviral vectors might revert back to pathogenic wild viruses or jump into a host's genome in a dangerous spot, causing harmful mutations. Adenoviral vectors are efficient. However, they alert a host's immune cells, which sometimes can render repeat administrations fruitless. They also can be toxic. Synthetic vectors generally are considered safe. But these must be created from scratch, and scientists are still learning about their efficiency. CHALLENGES: Lawrence Cohen notes vectors' strengths, weaknesses Speaking at the annual meeting of the Biotechnology Industry Association (BIO) last month, Lawrence Cohen, vice president of gene therapy development at Somatix Therapy Corp., summed up the challenges companies face when trying to commercialize vectors. "The successful application of gene transfer to gene therapy . . . is going to require a broad technology platform because each of these vectors has its own strengths and its own weaknesses, and [a given vector] will be most applicable to a certain disease type." For example, Cohen stated, chronic disease might be best treated by genes delivered from a long-lasting retroviral vector. Chemotherapy, on the other hand, might be enhanced with short-lived gene therapy designed to maximize treatment. Last year, Somatix shifted its focus from strictly retroviral vectors to all four systems. The public, Alameda, Calif.-based company has a partnership with Bristol-Myers Squibb Co. in New York to pursue gene therapy against cancer; clinical trials of a vaccine against melanoma are ongoing. Somatix also is conducting animal experiments on gene therapy for Parkinson's disease. Viral vector research is particularly promising for Somatix, Cohen notes. In the gene therapy field, "there was an initial feeling that vectors were entities unto themselves," he remarks, noting that molecular geneticists and other gene scientists were in great demand. More than 100 of the company's 120 staff members do R&D in gene therapy. "But I think the better you understand the virus itself, the better you can modify it to do what you want it to do," Cohen says. Thus, basic virologists also receive a warm welcome at Somatix and other companies. INEVITABLE: Didier Trono says safety concerns are unavoidable at this stage. When scientists first began looking for ways to slip therapeutic genes into cells, viruses were a natural choice. "With viruses, nature does three-fourths of the job for us," explains Trono. After all, he says, the microbes have evolved to successfully invade host cells. While viral vectors showed promise, some scientists grew disenchanted with their tendency to cause immune system reaction or toxic side effects. Synthetic, or nonviral, vectors emerged as an alternative. "I think nonviral applications have gained some attention recently, primarily because of problems associated with [adenovirus] vectors," contends Seng Cheng, director of gene transfer at Genzyme Corp., a publicly held biotech company in Cambridge, Mass. Cohen calls nonviral vectors "one of the Holy Grails of gene therapy." Like synthetic drugs, nonviral vectors appeal to researchers because they potentially offer a great level of control. By crafting these vectors in the lab, chemists theoretically can mix and match the most desirable properties of viral vectors into safe complexes. Nonviral vectors typically begin with plasmids, or closed loops of therapeutic genes to be delivered. Some scientists hope to inject this "naked DNA" directly into the body. Others want to combine these plasmids with fatty molecules that can travel across a cell's membrane. For example, researchers can enclose a plasmid inside a lipid envelope. They also attach various molecules that naturally target a specific cell's nucleus. Like a homing pigeon, the concoction travels to a given destination in the body and delivers its genetic message. So far, Genzyme's most efficient lipid vectors are not as effective as their viral counterparts, Cheng says. However, he adds, the vectors are improving. Speaking at the BIO meeting, Cheng reported that lipid vectors appear able to deliver genes to lung, heart, and other tissues. This January, Genzyme launched a clinical study to test its top lipid-DNA complex on cystic fibrosis. "I think we've only scratched the surface of new structures for lipids," Cheng maintains. The search for structures offers new jobs to scientists. "We're really looking to attract more chemists into this area," he says. Genzyme, one of the largest biotech companies, employs more than 100 full-time researchers in gene therapy. Cohen notes that lipid biochemists are in demand: "We need people who understand the association of lipids and their association with polymers like DNA." Other gene-therapy companies pursuing nonviral gene vectors include GeneMedicine Inc., a publicly held firm in The Woodlands, Texas; Vical Inc., a public company in San Diego; and Megabios Corp., a private company in Burlingame, Calif. At GeneMedicine, researchers are exploring a range of molecules to guide and help express therapeutic genes in certain tissues. "We have a multidisciplinary approach," says Alain Rolland, director of gene delivery at GeneMedicine. He explains that he prefers the terms "gene delivery system" and "gene expression system" over "vector," which is less precise. Rolland reports that GeneMedicine uses a lot of molecular biologists-who clone a variety of genes and work on DNA promoters-and chemists, who essentially design novel molecules. "We develop ways of assembling different molecules with plasmid-based expression systems," he points out. "We look for formulation scientists trained in liposomal formulations, microsphere preparations, and pharmaceutical science." Given gene therapists' enthusiasm, one might forget the scolding companies got from the National Institutes of Health in December. Back then, an NIH-appointed panel accused biotech companies of overselling results to win investor and public approval of gene therapy (K.S. Brown, The Scientist, Feb. 19, 1996, page 1). The panel cautioned that without substantial results, investor enthusiasm might fizzle. So far, that hasn't happened, according to Carl L. Gordon, an analyst with Mehta and Isaly, a health care investment firm in New York. "To the pharmaceutical industry, gene therapy is very promising," Gordon says. Partnering with or purchasing biotech firms, the giant pharmaceutical companies have spent at least $1 billion on gene-therapy ventures (K.S. Brown, The Scientist, Nov. 13, 1995, page 1). "[These companies] are looking to the future, 10 years in advance," says Gordon. "And as long as those guys are interested, investors are interested. The investment community remains positive." FDA also is open-minded, contends Forrest Anthony, president of Avid Corp., a private, Camden, N.J.-based company whose subsidiary, Quality Biotech Inc., conducts biosafety testing of gene therapy vectors. "A lot of the FDA's initial concerns [about vector safety] have been allayed," contends Anthony. "Still, there are concerns about retroviruses recombining or transforming cells and causing problems." Philip Noguchi, director of FDA's division of cellular and gene therapies, says the agency is worried that randomly integrating therapeutic genes might interfere with a cell's growth cycle and lead to tumors. "At this point, it is a theoretical concern," Noguchi states. "But it is one that we need to watch out for. It doesn't mean gene therapy can't be done. But it means we need to be cautious." Trono agrees. "[Concern] is something that, at this stage, we cannot avoid," he says. But he notes that no evidence for retroviral vector-induced mutations have been found in human patients. "And now people are trying to engineer retroviral vectors that will integrate into specific parts of the genome where there is no danger," Trono reports. In the future, researchers hope to create vectors from lentiviruses, which do not cause human disease. At this point, only time will tell which type of vector becomes the choice for any disease. "It is a new area of discovery," stresses Rolland. For biologists and chemists, he adds, "it is certainly a field of opportunity." Kathryn S. Brown is a freelance science writer based in Columbia, Mo.

Hunt For Viable Vectors Leads To Jobs For Gene Therapy Researchers

Interested in reading more?

Become a Member of