Download HIV Protease Inhibitors -- Background Information

HIV Protease Inhibitors -- Background Information
Bioengineering and Environmental Health
Acquired Immune Deficiency Syndrome (AIDS). AIDS appeared as a clinical entity in
1981 and, within two years, it became clear that the causative agent was likely to be a
virus. Two viruses have been implicated in AIDS. The first is human immunodeficiency
virus I, or HIV-1, which is responsible for most cases of the disease worldwide. HIV-2 is
now a serious concern in sub-Saharan Africa and is spreading into Asia. It is estimated
that in excess of 22 million people are HIV positive (note that being HIV positive does
not mean that you have AIDS; it is, however, a necessary pre-requisite). Over 90% of
those who are HIV positive live in areas of the world where they either do not have
access to or cannot afford the various therapies available to slow the development of the
syndrome. In the US, the number of people who are HIV positive is approximately 1
million. People become exposed to HIV mainly by unsafe sex with HIV positive persons
of either sex, sharing contaminated needles during recreational drug use, or by
transfusions with virally contaminated blood.
HIV selectively attacks white blood cells called CD4-positive T-cells and, with less
efficiency, macrophages and neurons. T-cells are components of the immune system that
are critical to our ability to fight off diseases such as tuberculosis, parasitic infections,
fungal infections and infections by viruses. These immune cells also help prevent certain
cancers, such as Kaposis sarcoma, non-Hodgkins lymphoma and Burkitts lymphoma as
well as primary malignancies of the brain. When the virus interacts with a T-cell, it
triggers a cascade of events that results in the death of the cell. The exact mechanism(s)
for cell death are not known precisely but one proposal is that the interaction causes a
natural process called programmed cell death, or apoptosis, to be turned on at the wrong
time. Another proposal is that the virus marks the T-cell for destruction by other white
blood cells. By either or both mechanisms, the initial symptom following HIV exposure
is a flu like syndrome accompanied by a sharp drop in CD4 T-cells. Typically, the levels
of CD4 T-cells drop from the normal level of about 1,200 cells per ์l to less than 400.
There is then a period of rebound, followed by a slow reduction in T-cell count to the
point that the infected person cannot fight off opportunistic infections and endogenous
cancers. At this time, the person has full blown AIDS. Finally, until recently, all patients
with AIDS eventually died. They could live with their disease for 2-15 years, but AIDS
was essentially a death sentence. Interestingly, a small fraction of the population carries a
specific mutation in a chemokine receptor that makes them immune to developing AIDS,
even though they are HIV positive.
The life cycle of HIV. To design a drug to treat a disease, it is of great value to know as
much as possible about the biology of the disease. Many years ago, it was believed that
most cancers were caused by viruses (actually, few cancers have a viral origin). As a
consequence, the public health community around the world had invested much effort
toward probing the how viruses get into cells, replicate, and transform cells to
malignancy. This platform of information on retroviruses (RNA viruses) became
particularly valuable as the story slowly unfolded that the retrovirus, HIV, was the likely
cause of AIDS.
HIV infects T-cells and macrophages by docking with a cell surface receptor protein
called CD4 (macrophages have less CD4 than T-cells and are less easily infected). The
component of the virus that snares the receptor is the glycoprotein (a protein decorated
with carbohydrates) termed gp120. The virus is engulfed and thereby introduces its single
stranded RNA genome (two copies are carried per virus) into the cytosol of the target cell.
The viral RNA has a closely associated protein called reverse transcriptase (RT), a
polymerase that copies RNA into a complementary DNA (cDNA) strand. The RNA
parental genome is degraded and a second replication event by RT results in the initially
single stranded DNA genome being made into a duplex. The duplex DNA is shuttled
from the cytosol of the cell into the nucleus, where the enzyme HIV integrase facilitates
its random insertion into the genome of the host T-cell (or macrophage). The cDNA
inserted into the host genome is called a provirus.
Once the HIV genome is in the proviral state, it can be transcribed into RNA, an event
that is promoted by a transcription factor known as NF-์B. The process of transcription
produces an RNA copy of one DNA strand. Early in the replication cycle some of the
transcript is exported to the cytosolic compartment where it is translated into a long
polyprotein -- i.e., a tandemly arranged, covalently connected, series of proteins. These
are cleaved by the HIV protease to give rise to the various proteins of the virus. Another
portion of the RNA transcript is exported to the cytosol where it becomes the genome of
the next generation of viruses. The RNA and the proteins co-mingle and matured viruses
begin to bud from the cell. Those viruses then go and infect other cells. An infected Tcell eventually dies, so there is a steady reduction in the number of T-cells and, therefore,
immune competence.
Treatments for AIDS. Coming up with a treatment for AIDS is complicated by numbers
-- there are billions of viruses produced per day even in persons who are not showing the
symptoms of the disease. In addition, the virus hides out in T-cells that are themselves
often hidden away in out-of-the-way places in the body, primarily in the lymph nodes.
Moreover, the virus is really only vulnerable to todays therapies when it is invading cells
or replicating -- when it is in the provirus state, it is invulnerable to the countermeasures
the body or conventional pharmacology can throw at it. Finally, the virus is error prone in
its replication and that fact, combined with its enormous proliferative capacity, makes it
evolve quickly in the host to drug resistant forms.
The obstacles indicated above notwithstanding, some therapies have been known for a
decade to slow the spread of the virus and thus extend the lives of people with AIDS.
The bold and underlined words in the previous section are the principal targets of these
therapies. The earliest therapeutics were the HIV reverse transcriptase inhibitors. These
are agents such as AZT (3'-azidothymidine; Glaxo-Wellcome) that are selectively
incorporated into the growing viral DNA molecule but, once incorporated, they are not
extendable by RT. Thus, they block viral replication. There are no integrase inhibitors on
the market, but some are in the pipeline and represent opportunities for the future.
Protease inhibitors have only been on the market for several years, but they have shown
remarkable success, especially when used in combination with RT inhibitors, to block the
polyprotein cleavage reaction that is essential for production of mature viral proteins. It
is also important to note that many efforts are underway to develop a vaccine against
HIV. Vaccine trials are underway in Thailand, or will be soon, through the efforts of a
company named VaxGen. There is an economic risk for companies that emphasize
vaccines, although immunization would be without question the way to curb this
epidemic. The risk is owed to the fact that the virus mutates quickly and hence the virus
may evolve to a form against which the vaccine is ineffective.
There are several protease inhibitors1 on the market, including Saquinavir (Roche),
Ritonavir (Abbott) and Indinavir (Merck). Several years ago market for therapies aimed
at HIV specifically was currently slightly ~$600 million (which is about half the cost of
AIDS, with the balance of the cost directed toward treatment of the cancers and infections
that attend the disease). The protease inhibitors have not been on the market long enough
for their full market potential to have been realized. One estimate predicts that 275,000
HIV positive and AIDS patients in the US in this year, 2000, will produce a market of
$755 million divided among about six competing protease inhibitors. Each inhibitor will
cost the patient about $4,000-6,000 per year; they must be used in a triple combination
chemotherapeutic regimen (which would include typically two RT inhibitors) costing the
patient a total of about $12-15,000 per year. The upper estimate of the annual cost is
$25,000. The doses of the protease inhibitors used to achieve a pharmacologically active
level of drug are high: typically several grams per day. Because the drugs must be given
frequently, and several different drugs must be taken, patients take between 9 and 36 pills
per day. Patient compliance with such a complicated regimen is problematic. The side
effects of AIDS therapy may also be severe. The question for our enterprise is the
following. Can we develop a means to make Western AIDS therapy work in the East?
Can we make protease inhibitors cheaply enough to make this therapy feasible? What are
the possibilities for vaccination? Are there other drugs that will work, and if they are less
effective, will they be effective enough to have an impact on this disease?
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1
Science, 272: 1876-1890 (1990).