Making Sense of Antisense and Interference
Making Sense of Antisense and Interference
MANAGED CARE November 2003. ©MediMedia USA
Treatments that interfere with protein synthesis at the cellular level will soon be debated in medical policy committee meetings.
Thomas Morrow, MD
In my travels around the country, I find that most pharmacy and medical directors are having a difficult time keeping up with all of the newly released products in the pharmaceutical arena. They appear to have a particularly difficult time with the biologics as these agents are used to treat many diseases that received little time and attention in medical and pharmacy school. Until recently, biologics were also off the pharmacy and medical director's radar due to their cost.
Prior columns have concentrated on newly released products. This column, however, will give a glimpse of the future.
Most biologic drugs currently available in the U.S. are proteins or polypeptides. Other cellular products and processes are starting to appear in the drug pipeline.
It is known that abnormal, insufficient, or overabundant proteins are a major factor in many, if not most, human diseases. Therefore, most current small molecules and biologic drugs target proteins. Some take aim at protein receptors that turn cellular function on or off.
Others, such as the monoclonal antibodies, bind to a protein to cause its destruction or inactivation. Still others such as insulin, Ceradase (alglucerase injection), or Fabrazyme (agalsidase beta) are simply protein or polypeptide replacements for deficient or abnormal molecules. But merely supplementing a protein or blocking its overproduction with bioengineered small molecules can lead to undesirable side effects or efficacy issues.
One really interesting area that will soon demand the attention of medical and pharmacy directors is the area of protein manufacture. With the ability to identify specific genes that affect disease, there is growing concern among clinicians that by altering one area of the gene, other areas that are essential to life may also be affected. To avoid this, researchers are targeting an intermediate process of gene replication — messenger RNA.
There are two methods that interfere with the manufacture of proteins in our cells — RNA interference and antisense.
We must first review RNA. There are numerous forms of RNA, each playing an extensive role in cellular activity. Recalling basic cellular metabolism, during protein manufacturing DNA unravels and a "messenger" is made that is a single-stranded compound called messenger RNA (mRNA). The mRNA is then used to code for the actual proteins made by the protein-creating machine, the ribosome. Geneticists and bioengineers often refer to this mRNA as the "sense" form.
Numerous experiments have shown that introducing a neutralizing "antisense" version of RNA can modify this process. Antisense RNA is a matched "mirror image" of the mRNA created by using the matching base pairs of the original RNA. When inserted into the cell, antisense binds to the mRNA and protein synthesis ceases.
Another strategy is to deter RNA metabolism by using a technique called RNA interference (RNAi). This process involves the use of a synthetically created double-stranded RNA known as dsRNA. This form of RNA does not normally occur in humans; however, viruses that use this RNA can infect humans.
Human cellular response
When faced with dsRNA, human cells silence gene expression. It is thought that the human cellular response to dsRNA is a protective action against double-stranded RNA viruses. This response appears to occur when the dsRNA is approximately 21 to 25 base pairs in length. This response is remarkably gene-specific, silencing only the gene that was used for the model of the dsRNA. Silencing appears to occur in several ways.
In some cases, it involves methylation of the gene, which may be permanent. In other cases, the silencing occurs at the post-transcriptional level later in the protein building process.
The result is the destruction of the corresponding mRNA before it can be used to trigger protein synthesis. This process has been used to study gene expression. Scientists turn off a specific DNA segment and observe what happens to the organism. This effect appears to be present in all species studied. In theory, the use of this same technique can be used as a therapy.
One possible disease to target would be cancer. Imagine taking a biopsy from a patient with a malignancy, analyzing the DNA, and finding specific mutated genes coding for a protein that stimulates the proliferation of cancer cells. Imagine that these genes are then used to map out a dsRNA that is injected back into the tumor, shutting down the factors thought to be the cause of the cancer cell's replication.
How might this be of interest to managed care professionals?
First, it is exciting science. Second, it is a rapidly developing field that will require deliberation at medical policy committee meetings. To demonstrate, there were nine articles written about RNA interference in 1998. That number jumped to 229 in 2002, according to the Web of Science database. The December 2002 edition of Science called RNA interference the "breakthrough of the year."
The first issue that MCOs will be asked to review will be genetic testing using RNAi or antisense technology. The new technology will also be used to develop a better understanding of our existing drug armamentarium.
Researchers have already demonstrated RNAi's ability to block viruses and certain diseases. In fact, there have been positive results demonstrated in hepatitis C virus, poliovirus, and human immunodeficiency virus.
Within two years, U.S. clinical trials of RNAi-based drugs will be initiated. But that is not all.
ISIS Pharmaceuticals is currently enrolling patients in two separate antisense phase III trials for Crohn's disease. The company has numerous antisense products in various stages of development for cancer, hepatitis C, ulcerative colitis, diabetes, multiple sclerosis, rheumatoid arthritis, and CMV retinitis. Sooner rather than later, one or more of these drugs may be in front of the pharmacy committee.
Keep informed, and remember to look for tomorrow's medicine!
Thomas Morrow, MD, is president of the National Association of Managed Care Physicians and vice president and medical director of Matria Health Care. He has 19 years of managed care experience at the payer or health plan level.
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