Transcript Anti HIV agents

Anti HIV agents
Some facts about HIV:
• HIV – the Human Immunodeficiency Virus is the
retrovirus that causes AIDS
• Discovered independently by Luc Montagnier of
France and Robert Gallo of the US in 1983-84
• HIV belongs to the retrovirus subfamily
lentivirus.
• Former names of the virus include:
– Human T cell lymphotrophic virus (HTLV-III)
– Lymphadenopathy associated virus (LAV)
– AIDS associated retrovirus (ARV)
• One million people infected in US, 30 million
worldwide are infected.
• Leading cause of death of men aged 25-44
and 4th leading cause of death of women in
this age group in the US.
Methods of transmission:
– Sexual transmission, presence of STD increases
likelihood of transmission.
– Exposure to infected blood or blood products.
– Use of contaminated clotting factors by
hemophiliacs.
– Sharing contaminated needles (IV drug users).
– Transplantation of infected tissues or organs.
– Mother to fetus, perinatal transmission variable,
dependent on viral load and mother’s CD4 count.
Transmission
Characteristics of the virus
• Icosahedral (20 sided), enveloped virus of the
lentivirus subfamily of retroviruses.
• Retroviruses transcribe RNA to DNA.
• Two viral strands of RNA found in core surrounded by
protein outer coat.
– Outer envelope contains a lipid matrix within which
specific viral glycoproteins are embedded.
– These knob-like structures responsible for binding to target
cell.
Characteristics of the virus:
HIV
• The outer shell of the virus is
known as the Viral enevlope.
Embedded in the viral envelope
is a complex protein known as
env which consists of an outer
protruding cap glycoprotein (gp)
120, and a stem gp14. Within
the viral envelope is an HIV
protein called p17(matrix), and
within this is the viral core or
capsid, which is made of
another viral protein p24(core
antigen).
Viral Replication
• First step, HIV attaches to susceptible host cell.
– Site of attachment is the CD4 antigen found on a variety of
cells
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helper T cells
macrophages
monocytes
B cells
microglial brain cells
intestinal cells
– T cells infected later on.
Life Cycle:
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(a) HIV (red) attaches to two cell-surface receptors
(the CD4 antigen and a specific chemokine receptor).
(b) The virus and cell membrane fuse, and the virion
core enters the cell.
(c) The viral RNA and core proteins are released from
the virion core and are then actively transported to
the nucleus.
(d) The viral RNA genome is converted into doublestranded DNA through an enzyme unique to viruses,
reverse transcriptase (red dot).
(e) The double-stranded viral DNA moves into the
cell nucleus.
(f) Using a unique viral enzyme called integrase, the
viral DNA is integrated into the cellular DNA.
(g) Viral RNA is synthesized by the cellular enzyme
RNA polymerase II using integrated viral DNA as a
template. Two types of RNA transcripts shorter
spliced RNA (h) and full-length genomic RNA (j) are
produced.
(h) Shorter spliced RNAs are transported to the
cytoplasm and used for the production of several
viral proteins that are then modified in the Golgi
apparatus of the cell (i).
(j) Full-length genomic RNAs are transported to the
cytoplasm (k).
(l) New virion is assembled and then buds off.
(m) Mature virus is released.
Latency and manifestation of symptoms:
• After a period of latency lasting up to 10 years viral
replication is triggered and occurs at high rate.
• CD4 cell may be destroyed in the process, body
attempts to replace lost CD4 cells, but over the
course of many years body is unable to keep the
count at a safe level.
• Destruction of large numbers of CD4 cause
symptoms of HIV to appear with increased
susceptibility to opportunistic infections, disease and
malignancy.
Clinical Latency Period:
• HIV continues to reproduce, CD4 count gradually
declines from its normal value of 500-1200.
• Once CD4 count drops below 500, HIV infected
person at risk for opportunistic infections.
• The following diseases are predictive of the
progression to AIDS:
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persistent herpes-zoster infection (shingles)
oral candidiasis (thrush)
oral hairy leukoplakia
Kaposi’s sarcoma (KS)
AIDS
• CD4 count drops below 200 person is considered to have
advanced HIV disease
• If preventative medications not started the HIV infected
person is now at risk for:
– Pneumocystis carinii pneumonia (PCP)
– cryptococcal meningitis
– toxoplasmosis
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Mycobacterium avium
Cytomegalovirus infections
lymphoma
dementia
Most deaths occur with CD4 counts below 50.
Anti- HIV Drug Targets
Three types of drugs are
currently in clinical use:
1. nucleoside and
nucleotide reverse
transcriptase (RT)
inhibitors
2. non-nucleoside reverse
transcriptase inhibitors
3. protease inhibitors (PIs)
Nucleoside and Nucleotide Analogs
• Nucleoside analogs (NRTI) act as chain terminators
or inhibitors at the substrate binding site of RT
• NRTI’s must be phosphorylated (three steps) to their
5’-triphosphate form to become active inhibitors.
• Nucleotide analogs (NtRTI) already contain a
phosphate group and only go through 2 steps to
become active.
• The 5’-triphosphate of the NRTI’s compete with the
2’-deoxynucleoside’s 5’-triphosphate for binding to
reverse transcriptase leading to viral DNA chain
termination.
Nucleoside Analogs
• There are currently 7 FDAapproved NRTI’s and one
nucleotide analog.
• The first anti-HIV drug approved
was the NRTI known as AZT or
Zidovudine (1987).
• AZT was discovered as a
treatment of AIDS during a
screening process for the
identification of effective AIDS
treatments4.
• Antiviral selectivity due to
higher affinity for HIV RT than
human DNA polymerases.
Non-Nucleoside Analogs
• Non-nucleoside analog reverse transcriptase
inhibitors (NNRTI’s) inhibit viral DNA replication by
binding at the allosteric non-bonding site of RT,
causing a conformational change of the active site.
• NNRTI’s do not require bioactivation by kinases.
• Three NNRTI’s are currently approved for clinical use
in combination therapy: nevirapine, delavirdine, and
efavirenz
Non-Nucleoside Analogs
Delavirdine
Benzoxazinone
Nevirapine
Protease Inhibitors
• During the reproduction cycle of HIV a specific
protease is needed to process GAG and POL
polyproteins into mature HIV components.
• If protease is missing noninfectious HIV is
produced.
• HIV protease inhibitors are specific to HIV
protease because it differs significantly from
human protease.
• The 6 PI’s currently approved for clinical use were
all designed by using structure-based drug design
methods.
FYI: Structural Genes
• Three main structural genes:
– Group Specific Antigen (Gag)
– Envelope (Env)
– Polymerase (Pol)
Group Specific Antigen (Gag)
• Located in nucelocapsid of virus.
• Icosahedral capsid surrounds the internal
nucleic acids made up of p24 andp15.
• p17 lies between protein core and envelope
and is embedded in the internal portion of the
envelope.
• Two additional p55 products, p7 and p9, are
nucleic acid binding proteins closely
associated with the RNA.
HIV Protease
• The crystal structure of
HIV protease was first
obtained at Merck
Laboratories.
• HIV protease is a 99
amino acid aspartyl
protease that functions as
a homodimer with one
active site.
• The active sites of
protease are hydrophobic.
Protease Inhibitors
• HIV PI’s target the peptide
linkages in the gag and
gag-pol polyproteins
which must be cleaved by
protease.
• All approved PI’s contain a
hydroxyethylene bond
instead of a normal
peptide bond.
• The hydroxyethylene
bond makes PI’s nonscissile substrate analogs
for HIV protease
Protease Inhibitors
• ABT-378 or lopinavir
was approved in 2000
for use in combination
with ritonavir (a PI)
(Kaletra)
• Ritonavir strongly
inhibits the
metabolism of ABT378
Some Alternative Therapies
• Virus adsorption inhibitors – interfere with
virus binding to cell surface by shielding the
positively charged sites on the gp-120
glycoprotein
– Polyanionic compounds
• Viral coreceptor antagonists – compete for
binding at the CXCR4 (X4) and CCR5 (R5)
coreceptors
– bicyclams and ligands
Virus Adsorption Inhibitors
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Cosalane was originally developed as
an anti-cancer agent by researchers
at Purdue University and the U.S.
National Cancer Institute8.
Cosalane was developed from a
chemical known as ATA
(aurintricarboxylic acid), which has
long been known to have
anti-HIV activity8.
ATA is a mixture of different
polymers. Chemists took one of the
low molecular weight components of
ATA, and attached it to a steroid
molecule in order to target the
substance more effectively to the
surface of viruses and of cells.
The result was cosalane.
Cosalane binds to the HIV gp-120
protein.
Viral Coreceptor Antagonists
• Bicyclams are a type of viral
coreceptor antagonist.
• They are very specific and
potent X4 coreceptor
antagonists.
• Bicyclams belong to a
class of macrocyclic
polyamines consisting of
two cyclam units linked by
an aliphatic bridge
• Bicyclams with an aromatic
linker apparently had higher
antiviral activity10.
• One such compound is
AMD3100.
Combination Therapy
• Monotherapy created virus resistance to the
individual drug. Some combination therapies
increase the time it takes for the virus to become
resistant.
• Combinations of a PI or NNRTI with one or two
NRTI’s is often recommended.
• Combination therapy may reduce individual drug
toxicity by lowering the dosage of each drug
Combination Therapy
• The combination of drugs chosen is based
on the history of each individual patient
and synergistic drug interactions.
• Some drugs compete with each other for
binding sites or enzymes.
– Example: zidovudine and stavudine
• both nucleoside analogs compete for the same
kinase. Stavudine is not phosphorylated because
zidovudine is preferred
Combination Therapy and Drug
Resistance
• Some drug combinations can restore
sensitivity of the virus to drugs it was
previously resistant to.
– Example: lamivudine and zidovudine
• The HIV M184V mutation is resistant to lamivudine but
restores sensitivity to zidovudine resistant virus
mutants
Drug Toxicity and Side Effects
• All available antiretroviral drugs are toxic.
• Side effects of nucleoside analogs are lactic
acidosis and severe hepatomegaly with
steatosis (enlarged fatty liver).
• Other side effects of anti-HIV drugs include
pancreatitis, myopathy, anemia, peripheral
neuropathy, nausea, and diarrhea.
Reducing Drug Toxicity
• The use of combination therapy:
– Combining agents with favorable synergistic
properties allows a decrease in dose or dosing
frequency
– Ritonavir alone cause gastrointestinal side effects
but when used in combination with other PI’s it
can be administered at a lower dose.
Conclusions
• An effective anti-HIV therapy is still needed.
• Several possible targets are being studied and
tested.
• The area of anti-HIV drugs has more room for
growth and the future for the discovery of
new effective drugs is promising.