Chapter 9, Section 4

Robert G. Gish, MD

There are many references in this section to clinical trials. If you are unfamiliar with the clinical trial process, it might be helpful to review the definitions of the different phases of clinical trials in the Glossary. As you read about potential therapies currently in development, keep in mind that many new drugs or treatments that appear promising in the laboratory, or in phase I and early phase II trials are withdrawn from development because of unexpected side effects and/or lack of effectiveness. In general, for every 100 drugs taken through phase I testing, only one will eventually be approved by the Food and Drug Administration for use in a non-trial setting.

Interferons

Pegylated Interferon

Scientists have recently learned how to attach molecules called polyethylene glycol (peg) to different sites on the interferon molecule. The result is pegylated interferon or peginterferon. Pegylated interferon is not cleared by the kidneys as quickly as standard alpha-2a and 2b interferon. Peginterferon remains active in the body for a much longer period.^1-9 In theory, long-acting pegylated interferons deliver a more constant interferon dose than standard interferons. Because the drug is cleared from the body slowly, pegylated interferon can be given once a week. Although the activity of interferon is decreased by the attachment of peg molecules to it, the longer duration of action counteracts the reduction in immune activity. The effects of weekly pegylated interferon on the immune system and HCV are an enhanced version of those produced by standard interferon taken thrice weekly.

The Food and Drug Administration (FDA) approved PegIntron^® (pegylated interferon alfa-2b) as single agent therapy for treatment of chronic hepatitis C in January 2001. It was approved for use in combination with ribavirin (Rebetol^®) in August 2001. Pegasys^® (pegylated interferon alfa-2a) has also been FDA-approved for use as monotherapy or in combination with ribavirin. The sustained response rates with these two pegylated interferons in combination with ribavirin are approximately 53-55%.^10,11 The populations studied in the two trials that led to FDA approval of each of the pegylated interferon combination therapies differed. Therefore, differences in sustained viral response rates will only be defined if a large head-to-head clinical trial involving both combinations is conducted. Emerging data show overall response rates with pegylated interferon plus ribavirin in the 60% range if patients are adherent and compliant with optimal dosing. Among people with HCV genotype 1, response rates of nearly 50-60% have been observed when patients took weight-based PegIntron^® plus ribavirin at full dose (optimal dosing based on a person's weight for the prescribed duration of treatment).¹²

Two studies have evaluated the use of pegylated interferon in patients with cirrhosis and compensated liver disease. One trial enrolled only patients with cirrhosis. This study found a 29% sustained response rate with pegylated interferon alpha-2a compared to a 6% response rate with non-pegylated interferon alpha-2a. In the other international, multi-center trial, 28% of the patients had cirrhosis. This group of patients had an overall sustained response rate of 39% with pegylated interferon alpha-2a compared to a 19% sustained response rate for non-pegylated interferon alpha-2a.^13,14

Other Interferons

Albumin-interferon alpha (Albuferon™) is a long-acting interferon. The connection of interferon alpha with the naturally occurring protein albumin keeps active interferon molecules circulating in the body for an extended period of time (a prolonged half-life). Phase I/II clinical trial data demonstrated Albuferon™ is well tolerated, has a prolonged half-life, and is biologically active in adults with chronic hepatitis C.¹⁵ Phase III studies are being planned.

Possible treatment advances that may be possible with the successful development of longer acting interferons include:

longer intervals between interferon dosing
improved sustained viral response rates
fewer treatment side effects

Therapies That Modulate The Immune Response

Vaccines

Vaccine research has historically focused on preventing infection. Recently, vaccine research has taken a new direction. Scientists are now attempting to develop vaccines to either protect people from chronic infection or modify the course of chronic infection. A preliminary study conducted in chimpanzees found an experimental HCV vaccine led to the production of antibodies and inflammatory T cells (immune cells) against the virus. The experimental vaccine contained recombinant HCV envelope proteins (proteins found on the outside of the virus). The antibody and T cell responses observed in the chimpanzees prevented chronic HCV infection in the majority of the animals tested in this very small study.¹⁶ Recent advances in recombinant protein technology, novel vaccine adjuvants, and DNA-based vaccines are providing essential tools for the development of HCV vaccines.

Despite encouraging preliminary research, it will probably take many years to develop an effective HCV vaccine. Some of the many challenges of HCV vaccine research are described below.^17-21

HCV is difficult to grow in a laboratory setting.

Vaccine development begins in the research laboratory where potential vaccine components are studied in animals and living cells. In the past, HCV was found only in humans and chimpanzees. Scientists at the University of Alberta, Canada have recently developed a mouse model that supports HCV replication.²² An additional animal model called the Trimera mouse has been developed by XTL Pharmaceuticals. The discovery of these animal models is an important breakthrough in HCV research. The animal models will be key tools in future HCV vaccine research and should dramatically advance future drug development.

HCV is highly susceptible to mutation. This characteristic of the virus makes it difficult to provide long-term, antibody-based immunity. Thus, an effective vaccine must stimulate T cells, immune system partners to the antibody-producing B cells. T cells interact directly and indirectly with HCV-infected cells and other immune cells.

The underlying concept behind vaccination is that a vaccine will stimulate the immune system to respond to a specific infectious agent leading to elimination of the agent or limitation of its damaging activities. Immune system responses are highly specific. A specific antibody will react only with the agent that stimulated its production. This highly specific interaction is often described as being similar to a lock and key. HCV is known to mutate frequently, meaning the virus frequently makes small changes in its molecular structure. These small changes may make the virus unrecognizable to specific antibodies against the virus. Therefore, developing a vaccine to stimulate the production of antibodies that will continue to recognize the virus long-term and provide long-term protection is challenging.

HCV can avoid detection by the human immune system.

The immune system has a highly developed surveillance system that is used to detect the presence of any substance foreign to the body (such as viruses and bacteria). The detection of an "invader" leads to a complex series of immune response that are intended to eliminate the invader. Thus, the detection of a foreign substance is the first step in the immune response. HCV appears to have the ability to escape detection by the immune system. The ability to avoid detection allows the virus to flourish with little disruption by the immune system.

HCV-neutralizing immune cells (specific CD4 and CD8 T cells) are not efficiently produced in all persons.

As noted above, immune reactions are highly specific. Researchers have found that the cellular immune response to HCV varies from person to person. This may be partially responsible for the fact that some people clear HCV on their own while others do not. This variability could also be a factor in potential treatments and vaccines.

HCV can easily become resistant to treatments. Resistant viruses may be spread to other persons. The development of treatment resistance has not yet been shown for HCV, but it remains a serious concern as more therapies are developed.

As noted earlier, HCV is prone to mutations. These mutations can result in the emergence of resistance to specific treatments for HCV. The evolution of HCV strains that are resistant to certain treatments is problematic for both vaccine and new drug development.

Several companies are working on developing therapeutic HCV vaccines. Results from a small phase IIa vaccine trial involving 23 people found 87% (20/23) of the patients exhibited either improvement (10/23) or stabilization (10/23) of their liver fibrosis after three years of vaccine treatments.²³ The vaccine being studied is made from HCV envelope proteins (the proteins on the outside of the virus). Despite promising early data, a clinically available HCV vaccine remains many years away.

Immune Globulin Preparations

Approximately 15-40% of adults infected with HCV clear the virus through a naturally occurring immune process, but 60-85% do not. Some studies suggest that up to 55% of children may spontaneously clear the virus. While we do not yet know exactly how the immune system spontaneous clears HCV, we have discovered some of the mechanisms that allow HCV to persist in the body. HCV has the ability to rapidly change its genetic structure (mutate). This helps the virus survive by allowing it to escape detection and recognition by B cells and T cells. B cells are immune cells that produce antibodies called globulins. T cells are immune cells that interact directly with infectious agents and infected cells.

Researchers are exploring the possible uses of antibody preparations (immune globulin) to treat HCV.²⁴ Current development is focused on liver transplantation. Investigators theorize that immune globulins may prevent transplanted livers from being infected with HCV when they are placed in HCV-positive persons. If the use of immune globulin preparations is successful in this setting, they may be tested for prevention of HCV infection after accidental exposure to blood or body secretions.

Civacir™ is an antibody preparation targeted specifically to HCV. It is currently in clinical trials for the prevention of hepatitis C infection of transplanted livers among HCV-infected transplant patients. HepeX-C is another HCV-specific antibody preparation current being studied for the prevention of re-infection in HCV-positive people undergoing liver transplantation.

Other Therapies That Modulate the Immune System

Isatoribine (ANA245) is one of a new class of drugs that regulate a person's natural immunity. It is a nucleoside antiviral drug. The drug was found to be safe and well tolerated in a small, phase I clinical trial. Although testing the effectiveness of a drug is not the goal of a phase I trial, the data suggest that ANA245 is active among adults with HCV. Isatoribine is believed to stimulate the immune system by interacting with specific immune cell receptors (specifically, toll-like receptor 7 or TLR7). People are currently being enrolled for a phase I/II clinical trial of isatoribine. Another trial was recently announced using ANA971, a precursor of isatoribine. Animal studies suggest the ingestion of ANA971 results in higher levels of active isatoribine than ingestion of isatoribine itself.

Cytokines such as gamma interferon, interleukin-10 (an anti-fibrosis agent), interleukin-2, and interleukin-12 have not shown any significant antiviral or antifibrotic activity.

Products Derived From Thymus Extracts

The thymus gland is a small structure located in the chest under the breastbone. It is an important part of the immune system. Immune cells called lymphocytes are formed in the bone marrow. Lymphocytes that travel to the thymus gland to mature become T lymphocytes or T cells. T cells are the primary actors involved in the cellular immune response, which is particularly important in battling viral infections.

Thymosin fraction 5 and thymalfasin (Zadaxin^® [SciClone Pharmaceuticals], also known as thymosin alpha-1) are two special types of proteins called cytokines that are derived from the thymus gland. Cytokines are produced by many cell types in the body. They cause specific reactions, many of which are important for immune function. Thymosin fraction 5 and thymalfasin appear to be able to change a person's response to HCV infection.²⁵ They may be able to prevent a chronic infection, or slow or halt disease progression.

Thymalfasin stimulates the immune system. Abnormally low levels of thymalfasin have been found in people chronically infected with the hepatitis B virus (HBV).²⁶ Initial studies in HBV-infected animals and humans suggest that thymalfasin and thymosin factor 5 increase the rate of clearance of HBV DNA (the genetic material of the virus).^27-29 Blood tests conducted during these studies showed that significant numbers of both animals and people converted from hepatitis B surface antigen (a marker for the presence of HBV) positive to negative during treatment. However, another study showed no difference in HBV clearance when thymalfasin was compared to a placebo (an inactive substance). Therefore, the results from these studies are inconclusive. Small, preliminary studies have shown the addition of thymalfasin to interferon therapy for HCV may enhance treatment response.^30,31 Two large phase III studies of thymalfasin in combination with Pegasys^® (without ribavirin) are currently in progress for the treatment of HCV-infected people who were non-responders to previous interferon plus ribavirin therapy. One of the studies involves people with cirrhosis. The outcomes of these studies will clarify whether there is a role for thymalfasin in the treatment of chronic hepatitis C. Thymalfasin has been approved for the treatment of patients with viral hepatitis infections in more than 34 countries.

HCV Enzyme Inhibitors

The total genetic blueprint of any living organism is called its genome. The genome contains the specific information that makes a tree a tree, a virus a virus, and a human a human. The genome is made up of individual genes. Each gene has the blueprint or code for a specific protein. The types of proteins made by an organism determine how it lives, functions, and survives.

The HCV genome contains the code for ten building blocks that make up the "house" the virus lives in and the machinery needed for the virus to reproduce and make more virus particles (replication). New virus particles infect other liver cells and can infect other people.³² The "machinery" proteins of HCV act primarily as enzymes, which are needed for viral replication (reproduction) and processing other proteins. Enzymes are specialized proteins that are necessary for various chemical reactions. Several HCV enzymes (called proteases, helicases, and polymerases) are the targets of new anti-HCV therapies. If the function of one or more of these enzymes can be interrupted, the replication and damage caused by HCV may also be interrupted. The three-dimensional structures of several HCV enzymes have been recently determined.³³ It is important to recognize there are many barriers to overcome in the development of effective HCV enzyme inhibitors. Such barriers include the need for inhibitors to have activity against a broad range of virus types and the potential development of resistance to the inhibitors.

Several pharmaceutical companies are currently developing molecules that may act as HCV enzyme inhibitors. Boehringer Ingelheim is developing a protease inhibitor (BILN 2061) that has been targeted to HCV genotype 1. Preliminary data showed a significant decrease in viral load after only two doses. Viral levels were not significantly affected by the medication among people with HCV genotypes 2 and 3.³⁴ The future development of this medication appears to hinge on finding a drug that is nontoxic to key organ system.

Abbott Laboratories, Vertex Pharmaceuticals, Idenix Pharmaceuticals, Schering-Plough, and Schering (Germany) have each developed substances as potential HCV enzyme inhibitors. The development of these medications may eventually progress to full-scale human studies. In preliminary testing, a substance called nm107 (Idenix Pharmaceuticals) caused a significant reduction in HCV levels in chimpanzees, leading to a proposal to proceed to clinical trials in humans.

Vertex Pharmaceuticals has developed a compound called merimepodib or VX 497. While the drug does not target an HCV-specific enzyme, it inhibits an enzyme normally found in human cells. The product of this enzyme is needed for HCV replication. Testing of merimepodib in combination with pegylated interferon plus ribavirin is moving forward to phase II/III clinical trials after having met safety standards in earlier studies.

I personally believe that as new enzyme inhibitors are developed, they will initially be used in addition to pegylated interferon-based therapy. In my opinion, pegylated interferon will be the main stay of HCV therapy for at least the next 5 years.

Anti-Sense Oligonucleotides

The genome of a living organism exists in the form of either RNA (ribonucleic acid) or DNA (deoxyribonucleic acid), depending upon the type of organism. The HCV genome is made of RNA; the human genome is made of DNA. Because HCV "borrows" the protein-making machinery of human cells during replication, the blueprint for HCV's specific proteins must be "read" or translated into DNA before HCV proteins can be produced. Anti-sense oligonucleotides are small pieces of DNA or RNA molecules that are designed to block the "reading" (translation) of viral RNA.³⁵ Isis Pharmaceuticals has developed an anti-sense oligonucleotide known as ISIS 14803. This substance has been shown to have anti-HCV activity in several pre-clinical studies and in a phase I clinical trial.^36,37 Temporary, asymptomatic increases in serum ALT levels up to 1,000 IU/mL have been seen in some people taking ISIS 14803.³⁷ It is unclear whether these increases are due to the drug's anti-HCV activity or another mechanism.

Ribozymes And Short Interfering RNAs

At one time, scientists believed all enzymes were proteins. However approximately 20 years ago, researchers discovered that certain RNA molecules can act as enzymes. These specialized RNA molecules are called ribozymes. They act by binding to and cutting (cleaving) specific sites of larger RNA molecules.³⁸ Anti-HCV ribozymes have been developed in the laboratory. However, there were serious side effects when these molecules were administered to animals. Because of the toxicity of the anti-HCV ribozymes developed to date, further development has been halted for the time being. It is unclear whether ribozymes may have a role in therapy for chronic hepatitis C at some time in the future.

During the replication process of many viruses, including HCV, two strands of RNA come together to form a double-stranded RNA molecule (dsRNA). An enzyme called DICER binds to and cuts (cleaves) dsRNA. Short interfering RNA (siRNA) molecules are small pieces of dsRNA produced when larger lengths of dsRNA are cleaved (see Figure 1).

Figure 1: Formation of siRNA

siRNA molecules bind with proteins to form a unit called the RNA-induced silencing complex (RISC). Through a series of complex interactions, RISC suppresses the expression of the gene it corresponds to in the viral genome. In other words, the gene from which the siRNA is derived is silenced. In theory, the ability to silence specific genes in the HCV viral genome could prevent viral replication. Recent studies have confirmed this theory. HCV-specific siRNAs have been shown to block HCV replication and protein expression.³⁹ These early findings suggest that RNA interference may have a role in treating people with chronic hepatitis C. It is yet to be determined whether siRNA molecules, which are relatively large compared to other molecules used to treat HCV, can be delivered in such a way that they are able to reach the site of viral replication inside infected cells.

Anti-Fibrotic Therapy

The liver damage caused by HCV is largely the result of an inflammatory response that leads to fibrosis. Doctors believe that long-term therapy (also known as maintenance therapy) may reduce liver inflammation and prevent the development of fibrosis among people who have not experienced viral clearance.

The phrases "long-term therapy" and "maintenance therapy" are often used interchangeably. However, there is a minor difference between these two terms. Maintenance therapy for hepatitis C refers to any form of treatment that extends beyond one year and is continued for an indefinite period of time. Long-term therapy also refers treatments that last longer than one year, but for which there is an expected stopping point.

Several important clinical trials are currently underway to determine the potential roles of long-term and maintenance therapies for the management of chronic hepatitis C. Brief descriptions of these trials are shown below. Researchers hope that long-term and/or maintenance therapies will effectively slow or prevent fibrosis progression among people who are complete non-responders, partial responders, or relapsers to prior HCV therapy. Exploration of this treatment strategy is also important for people who are not able to tolerate current therapies for one reason or another.

CoPILOT: Colchicine or PegIntron^® Long-Term Therapy

The CoPILOT study will evaluate the effects of colchicine and alpha 2b (PegIntron^®) on disease progression and fibrosis among people with advanced fibrosis or cirrhosis due to HCV. Participants are virologic non-responders to previous combined interferon plus ribavirin therapy. Colchicine is an anti-fibrotic drug that has been used in alcoholic hepatitis and cirrhosis.

HALT-C: Hepatitis C Antiviral Long-Term Treatment Against Cirrhosis

The goal of the HALT-C study is to determine if progression from fibrosis to cirrhosis or from cirrhosis to liver decompensation can be prevented or delayed. The study is designed for people with significant fibrosis who have been unable to clear virus with any prior form of interferon or interferon plus ribavirin. All participants are treated with five months of full dose alpha 2a (Pegasys^®) plus ribavirin. Those with undetectable viral loads at five months complete a full course of therapy. People with a detectable viral load after five months of treatment (non-responders) are assigned to receive either low dose interferon alone or no treatment. For additional information about HALT-C, see Chapter 9, Section 3 and Appendix III.

EPIC³: Evaluation of pegylated interferon alpha 2b (PegIntron^®) in Chronic Hepatitis C Cirrhosis

The EPIC³ study is similar to HALT-C in terms of the goals of the trial and the study participants. Patients responding to treatment at 12 weeks complete a full 48 weeks of therapy. Those who are not responding at 12 weeks are treated with long-term, low-dose pegylated interferon alpha 2b (PegIntron^®).

Several other anti-fibrotic compounds are in development. A large trial involving interferon gamma-1b (Actimmune^®) was recently halted due to lack of effectiveness among people with advanced fibrosis. (Personal communication, InterMune, Inc.) Results from a small pilot study suggest combination treatment with interferon gamma-1b (Actimmune^®) and interferon alfacon-1 (Infergen^®) may be beneficial among people whose disease has not responded to treatment with pegylated interferon plus ribavirin. A larger trial to study this regimen is planned.⁴⁰

Ribavirin Analogs

Ribavirin is one component of current standard therapy. It is described as a nucleoside-like antiviral drug. Its structure resembles that of nucleosides, the building blocks of the gene-carrying molecules DNA and RNA. Ribavirin is minimally effective against HCV when used as monotherapy and has a troublesome side-effect profile. Several pharmaceutical companies are currently involved in developing an improved version of ribavirin. The new compounds are chemically altered versions of ribavirin and are known collectively as ribavirin analogues.

Viramidine

Viramidine is described as a liver-targeting form of ribavirin. Viramidine is converted into ribavirin by an enzyme called adenosine deaminase (ADA). The liver is rich in ADA, which leads to a higher concentration of viramidine/ribavirin in the liver compared to other tissues. Experiments conducted in monkeys confirmed this preferential concentration of viramidine/ribavirin in the liver. A phase III study of viramidine in combination with pegylated interferon is currently underway. Phase II data suggest the antiviral effects of viramidine are equivalent to those of ribavirin. It also appears the toxic effects of viramidine on red blood cells are less than those associated with ribavirin.⁴¹

Other Ribavirin-Like Molecules

NM283 is a ribonucleoside being developed for the inhibition of HCV replication. In a one-week study in chimpanzees, NM283 treatment resulted in a significant reduction in HCV viral load. ⁴² A phase I/II clinical trial is currently underway.

Other Potential Treatments for Chronic HCV

UT 231-B is an antiviral substance called an iminosugar. Scientists state UT 231-B appears to interfere with the assembly of viruses including HCV. This action prevents viruses from making new infectious particles and thus blocks the virus from infecting other cells. UT 231-B is currently in phase Ib clinical testing.

Recent studies suggest the rate of liver cell death is abnormally high in several liver diseases including chronic hepatitis C. A series of chemical reactions and other activities in a cell ultimately leads to cell death. These processes are known as programmed cell death or apoptosis. Early phase clinical trials have been conducted with a new substance called IDN-6556. The drug inhibits the action of an enzyme named caspase. Caspase is an important enzyme in programmed cell death. Scientists believe blocking the action of caspase may reduce the rate of liver cell death. In a phase IIa study, researchers found IDN-6556 significantly decreased AST and ALT levels among people with chronic hepatitis C.⁴³ Additional studies will be conducted to further evaluate this new drug.

Several other compounds are in various stages of preclinical development and testing.

Future Non-Western Treatments

One goal of researchers and practitioners of complementary and alternative medicine (CAM) is to determine the role of CAM therapies in the management of hepatitis C. This is important for both people living with HCV and their health care providers. The role of herbal and other therapies in controlling arthralgia, myalgia, mental fogginess, and fatigue is clear to individual patients. However, research data are needed to support broad usage of these agents in symptom management across diverse populations. The possible anti-inflammatory role of herbal therapies to prevent or slow disease progression must also be explored. Carefully designed clinical trials may determine which therapies are most beneficial to specific subgroups of people with HCV. For instance, herbal therapy and acupuncture may benefit people with joint pain. Large-scale clinical studies are needed to obtain conclusive information about the efficacy of CAM therapies for these and other signs and symptoms of HCV. A series of anecdotes is not sufficient.

Clinical research may clarify whether the use of CAM approaches is safe and beneficial in combination with western therapies. Such studies may also determine if CAM therapies are useful to control side effects of western therapies.

Safety is an important issue since many CAM therapies have been anecdotally reported to cause side effects that may be serious. We need to determine the actual incidence of these reported side effects and document their severity with carefully designed clinical studies.

Proving the presence or absence of antiviral effects of non-western therapy is important. Some CAM practitioners claim to be able to cure HCV with a variety of therapies. But these claims are poorly documented. Scientifically sound studies are needed to discover which, if any, CAM agents have clinical benefit. Herbal remedies may actually decrease liver inflammation, the early component of liver disease that can lead to fibrosis and cirrhosis. Prevention of the development of cirrhosis would be of great benefit to people with chronic HCV who cannot be cured with interferon-based therapy.

Integrative medicine utilizes both western and CAM therapies. For information on the integrative medicine approach to hepatitis C management, see Chapter10.