Gene Therapy: Medicine of the 21st Century
As scientists are unravelling the mystery of genes and pitching it as a ‘cure all’ medicine of the future, Nancy Singh traces the baby steps taken in the world of gene therapy.
The year is 3050. Enter a patient suffering from partial blindness. The doctor enters his ‘gene-lab’ and after selecting the appropriate vector introduces ‘normal’ genes that replace the unhealthy ones and the patient regains full vision. Sounds like a scene straight out of a sci-fi movie? Today – yes, but perhaps tomorrow a fact. As they say, today’s fiction is tomorrow’s reality.
Says Sanjeev Saxena, Chairman and CEO of Actis Biologics, a company engaged in gene therapy research, “Gene therapy or nanotechnology may today sound like sci-fi just as the thought of prevention or treatment of smallpox or polio would have to people in the last century. But science has progressed and we will see all the current research in gene therapy becoming a reality soon and India will be hopefully in the lead.”
Gradually surfacing from the world of theoretical fantasies to the realm of actuality, gene therapy has come a long way indeed to be popularly known as the ‘Medicine of 21st Century.’
It is estimated that as of today around 1,340 gene therapy clinical trials have been completed, or are ongoing or been approved worldwide. Gene therapy is considered as treatment for common diseases, as well as cystic fibrosis, Severe Combined Immunodeficiency (SCID), haemophilia, muscular dystrophies, and so on.
“For terminal systemic disorders such as paralysis or Parkinson’s Disease, gene therapy has had reduced success, but for localised states like disc regeneration or spinal fusion, gene therapy can be an extremely powerful tool,” opines Dr Farzana Farzeh, Professor & Chair, Molecular Medicine, Kings College, London & President, International Society of Cell & Gene Therapy for Cancer.
Gene therapy received a significant push after the human genome project. Scientists have recognised exact alterations in the DNA sequences that play causative roles in an array of common diseases that include type 1 and type 2 diabetes, bipolar disorder, schizophrenia, inflammatory bowel disease, glaucoma, and rheumatoid arthritis. Clinical trials and research have also expanded to incorporate cardiovascular, neurological, and pulmonary disorders, cancer, infectious diseases such as AIDS, and monogenic disorders like haemophilia and cystic fibrosis. “Short-term therapy markets such as cardiovascular diseases and cancer will probably be the first to reap financial benefits from gene therapy,” says Dr Sara Collins, Cork Cancer Research Centre, Cork, Ireland.
However, worldwide experts say that gene therapy’s current status is similar to that of monoclonal antibodies 15 years ago, and this market is estimated to be worth over $20 billion.
Gene Therapy for Cancer
Around 70 per cent of more than 400 clinical gene therapy studies initiated are targeted at cancer. Scientists are using multi-pronged gene therapy strategies to fight cancer. One approach is to directly target cancer cells to annihilate them or stop their growth. In another, researchers replace altered or missing genes with healthy ones. “For instance, gene p53 may cause cancer: substituting ‘working’ copies of those genes may be used to treat cancer,” explains Dr Farzeh.
Researchers are also studying methods to enhance the body’s immune response to cancer. Here, gene therapy is used to stimulate the body’s natural ability to attack cancer cells.
In one technique that is currently under investigation, researchers obtain a small blood sample from a patient and insert genes that cause each cell to produce T-cell Receptors (TCR). The TCR recognises and attaches to certain molecules that are present on the surface of the tumour cells. Finally, the TCRs activate the white blood cells to attack and kill the tumour cells.
Research is currently on to insert genes into cancer cells to make them more sensitive to radiation therapy, chemotherapy or other treatment forms. In other studies, researchers are investigating removing healthy blood-forming stem cells from the body, then inserting a gene that will make these cells highly resistant to the side effects of drugs.
Another well-known strategy is inserting ‘suicide genes’ into a patient’s cancer cells. A pro-drug (an inactive form of a toxic drug) is then administered to the patient. The pro-drug is activated in cancer cells that contain these ‘suicide genes’, which leads to the destruction of those cancer cells. Other research is focused on the use of gene therapy to prevent cancer cells from angiogenesis.
Says Dr Adrian Thrasher, Professor of Paediatric Immunology, Molecular Immunology Unit, Institute of Child Health, UK, “Most early clinical trials have been primarily designed to study safety, applicability and toxicity. Several phase I and II studies have shown partial remission of tumours and, in exceptional cases, complete remission, although complete cure has not yet been shown.”
The driver of growth for gene therapy in cancer has been the substantial increase in the understanding of the pathogenesis and various gene expressions on these cancer cells.
Most recently, researchers at the University of Kentucky have created a mouse resistant to cancer. The research has been published in the October 2007 edition of the journal Cancer Research.
This breakthrough originates from a discovery by UK’s College of Medicine’s Professor of Radiation Medicine Dr Vivek Rangnekar and his team of researchers who hit upon a tumour-suppressor gene called ‘par-4’ in the prostate. Dr Rangnekar serves as the Associate Director, Translational Research, at the Markey Cancer Centre. The study was funded by several donations from the National Institutes of Health.
The team found that this ‘par-4’ gene kills cancer cells, but not normal cells. There are very few such molecules known, giving it a potentially therapeutic application. This study is unique in that the mice born carrying this gene are not developing tumours. They grow normally without any defects, and in fact actually live a few months longer than the control animals, indicating no toxic side effects. While originally discovered in the prostate, par-4 is not limited to this location. The gene is expressed in every cell type that the researchers looked at.
To further investigate the prospective therapeutic benefits of this gene, Dr Rangnekar’s team in October 2007 implanted the gene into the egg of a mouse. That egg was then introduced into a surrogate mother. The results were quite promising. The mouse itself does not express many copies of this gene, but the infants do. Hence, scientists have been able to transfer this activity to generations in the mouse.
The potential application for humans is that through Bone Marrow Transplant (BMT), the par-4 molecule could be used to fight cancer cells in patients without any damaging side effects of chemotherapy or radiation therapy.
The next logical step is of course to apply it to humans. “But there needs to be much more work done before any sort of human trial starts,” cautions Dr Farzeh. Nevertheless, a significant step has been taken in the right direction.
Gene Therapy for Immunodeficiency
As of now, the only claim to fame for gene therapy is its undisputed success in Severe Combined Immunodeficiency (SCID) or ‘bubble syndrome’ as it is commonly known.
This is a severe form of heritable immunodeficiency that affects about 1 in 1,00,000 live births. Since 1999, gene therapy has managed to restore the immune systems of at least 17 children (in a US, UK and French trial) with two forms (X-SCID and ADA-SCID) of the disorder.
The first ever gene therapy trials were started in 1990 by Dr William French Anderson in the US. The patient was a four-year-old girl called Ashanti. In her case, the disease was caused by the absence of the enzyme Adenosine Deaminase (ADA). This deficiency prevented her body from producing lymphocytes. The most common treatment for SCID is BMT, which requires matched donors.
Of late, gene therapy has proved useful. Transduction of the missing gene to haematopoietic stem cells by using viral vectors is being tested in ADA-SCID and X-linked SCID. In 2000, the first gene therapy ‘success’ resulted in SCID patients with a functional immune system. These trials were terminated when it was discovered that two out of 10 patients in one trial had developed leukaemia resulting from the insertion of the gene carrying retrovirus near an oncogene. Till 2007, four of the 10 patients are believed to have developed leukaemia. Work is currently on to focus on correcting the gene without triggering an oncogene. In trials of ADA-SCID, no leukaemia cases have yet been reported.
Further trials were initiated in which bone marrow cells or umbilical cord blood cells were used as targets. The modification of the stem cells present did result in the long-term production of a small number of ADA-positive lymphocytes. However, the ADA levels produced by the cells were low and it is not clear whether the patients would survive without concurrent ADA-PEG treatment.
In 2002, there was a major breakthrough in ADA gene therapy. It resulted from the use of a technique known as non-myeloblative conditioning, in which bone marrow in the SCID patient is partially killed in order to give the modified stem cells the chance to proliferate. Another important factor was that none of the children in this trial had been treated with ADA-PEG. It is believed that enzyme treatment may have contributed to the lack of success in previous trials.
The first patient was a two-year-old Palestinian child named Salsabil who had never received ADA-PEG therapy. The new treatment seems to have cured her condition and she is enjoying a comparatively normal life. Her body is producing antibodies that even managed to fight chicken pox, which would almost certainly have killed her months earlier.
Gene therapy for the Heart
To date, most gene therapy studies are accomplished in the laboratory and the earliest experiments seem promising for treatment of cardiovascular diseases in the future. A case in point is the use of gene therapy to help increase blood flow to ischemic tissue.
The body’s first response to decreased blood flow to the heart is to grow small new ‘collateral’ vessels to help blood flow around the blockage. For unspecified reasons, this process of angiogenesis eventually switches off.
There are some angiogenic proteins in the body that are known to help trigger new blood vessel growth. These include the endothelial growth factors, Vascular Endothelial Growth Factor (VEGF), Hepatocyte Growth Factor (HGF) and Fibroblast Growth factor (FGF).
In gene therapy trials, scientists have used a variety of different ways to deliver the genes for VEGF-1, VEGF-2 and FGF4 into the hearts of patients suffering from advanced myocardial ischemia. After gene therapy, patients reported less severe angina. Similarly, after gene delivery of VEGF to patients with limb ischemia, the blood supply improved and leg sores healed better. In fact, gene therapy has prevented below-knee amputation in some patients for whom it had been recommended.
Researchers at Johns Hopkins have successfully transferred a gene for the ‘G protein’ to cells of the AV node in pigs having atrial fibrillation, which has resulted in a therapeutic slowing of the heart rate. In Germany, scientists transported a gene in the heart muscle of rabbits and rats that was shown to increase the heart’s ability to contract forcefully. The gene transfers in these two animal studies were done by transfecting the target cells with a virus carrying the desired DNA.
Gene therapy has also been a success in preventing re-blockage or re-occlusion of coronary artery bypass grafts and in maintaining arteries open after angioplasty. Though gene therapy looks very promising, it still needs improvement before it becomes a routine treatment in the clinic for cardiovascular diseases, experts point out.
But several indispensable concepts of genetic therapy, by virtue of successful trials, have now been validated. Says Dr David Klatzmann, Director, Biotherapy Centre, Pierre & Marie Curie University, France, “If the field continues to advance briskly over the next few years, we may be able to apply genetic therapy to problems such as coronary artery disease, cardiomyopathy and certain cardiac arrhythmias.”
Obstacles to Overcome
The possibilities are limitless, but so are the challenges. Things are not as hunky-dory as theory would make it seem. Difficulties arise when the genes get out of the controlled laboratory environment and have to be tested in practical waters.
Gene therapy has suffered some grave setbacks in the past few years and its success graph has been considerably slower than many people, particularly investors, had anticipated.
The first Blow: Immune Response The industry as a whole suffered in 1999 when 18-year-old Jesse Gelsinger died from organ failure just four days after initiating a gene therapy trial at the University of Pennsylvania. He had a rare liver disorder, and died of complications from an inflammatory response shortly after receiving a dose of experimental adenovirus vector. His death halted all gene therapy trials in the US as it raised many questions concerning the safety of experimental gene therapy treatments. Dr Ramani Iyer, Chief Scientific Officer, Actis Biologics, argues, “I don’t understand why there is such a big hue and cry when these patients really required the drug and had no other recourse.”
The Next Setback: Wrong Location Researchers tested a gene therapy treatment to restore the function of a crucial gene, gamma c, to cells of the immune system, in children with X-linked SCID. Initially, this treatment appeared very successful, restoring immune function for seven out of 10 children. But, two years later, two of the children developed leukaemia.
The virus that was used to deliver the newly transferred gamma c gene had stitched itself into the wrong place, interrupting the function of a gene that normally helps regulate the rate at which cells divide, and activated an oncogene.
The Most Recent Bad News – Targeted Genetics
One patient died in a Phase 1/2 trial of gene therapy drug candidate tgAAC4 for treatment of rheumatoid arthritis, from a fungal infection (Histoplasmosis) due to suppressed immune response. tgAAC4 works as a local anti-TNF-alpha gene therapy treatment, which is delivered only to the affected joint(s) of arthritis patients.
The DNA vector had the potential to cause systemic immune suppression. However, preliminary testing results from three tissue sites revealed that the level of vector DNA from tgAAC94 present in the patient’s system was too low to have a systemic effect or cause suppression of the immune system. Also, the patient who died in this study was on systemic arthritis medications, which are known to cause immune system suppression. Hence, there are still expectations that the FDA may allow the trial to continue.
There are many pre-clinical technical hurdles as these involve manufacturing under high-level bio-safety precautions and things get tougher keeping in mind strong FDA oversight on its clinical applications. “International standards for drug testing are rigorous, time-consuming and expensive, but they are crucial to protect patients,” says Dr Iyer.
An additional roadblock for the technology is the short-lived nature of the therapeutic DNA that is introduced into cells and the human body’s immune system response.
However, with more researchers joining the bandwagon and many promising drugs in clinical trials expected to surface in the market, the potential applications for gene therapy are increasing.
Lack of appropriate finances is also another stumbling block, especially if you consider the fact that such large-scale studies involve billions of dollars.
Almost 75 per cent of research in gene therapy is mainly at academic institutional level and they too require appropriate and timely grants to flourish. While there are companies who have a strong base for gene therapy research and future products, they generally aren’t money-spinning operations yet. Celera, for example, reported a $20 million loss in early 2004. But their future products hold promise.
After 12 unprofitable years, Avigen Inc decided to stop funding its experimental gene therapy platform and focus its limited funds on developing traditional pharmaceuticals. It agreed to sell its AAV gene therapy assets to Genzyme in December 2005 for an upfront cash payment of $12 million, with additional milestone payments and royalty payments on all products developed under this portfolio, including the current Parkinson’s disease programme.
However not all news is bad news. Some companies have received notable grants that infuse a feeling of optimism. Celladon, developing gene therapy for congestive heart failure, received $30 million in a Series B round. Ceregene, focusing on several prevalent central nervous system diseases, such as Parkinson’s, Alzheimer’s, and ALS, received $32 million. Virxsys, developing a lentiviral vector gene delivery technology for HIV/AIDS, raised a total of almost $52 million in 2005/2006 financings.
Experience is the best teacher, they say. This statement can be best applicable to gene therapy. Dr Iyer agrees, “Today, the innovators need to capitalise on the significant progress that has been achieved in the discovery area, be it the choice of vectors, manufacturing, RCV Analysis, etc.”
He suggests, “No drug is either 100 per cent good or 100 per cent bad— it comes down to a clear risk-benefit analysis for a particular patient population for a given indication. Hence, focus initially on a patient population in whom the highest possible benefit may be shown. This may entail compromising on ‘blockbuster’ market potential, but may prove worthwhile in the long run. Finally, be willing to go the extra mile on safety issues and work closely with the regulator.”
Despite the troubles, if companies singlehandedly pursue their current lines of studies, applications for gene therapy are nearly limitless.
There is huge unmet need in the world of incurable diseases and this is where gene therapists are aggressively working hard trying to bridge the supply-demand gap even if that means reaching the unknown. The high prevalence of untreatable diseases drives the demand for this new treatment.
The moments of agony have not been without their share of ecstasy. This market has witnessed both devastating clinical failures as well as incredible breakthroughs for the treatment of severe diseases. On the positive side, the risks historically associated with gene therapy are lessening, largely through scientific advances in gene delivery.
Several products are in the pipeline with the regulatory approval hoped for in the next three years. “The market does face huge challenges in terms of patient recruitment, or the development of clinical success, but when these are overcome, the results will pave the way to a new era of medicine,” predicts Saxena.
The bottom line is that any kind of biomedical research depends on its relevance. Regardless of the excitement gene therapy can lead to, the field is yet in its infancy. Several clinical trials of gene therapy have been completed or are under way. They can provide information that cannot be concluded from tests in animals. Although this therapy has been theoretically good in principle, it has proved cumbersome to demonstrate its efficacy in practice. Thus, the success rate in clinical trials has been relatively low. “Such results can reduce the enthusiasm for genetic approaches, but the field is still new and the pace and surprises of new discoveries are amazing,” says Dr Farzeh.
The gene therapy market is currently going through an exciting transition phase from infancy to its adolescence and gradually taking slow steps to lay the foundation of the medicine for generations yet to come.
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