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What does the transition state of the
neuraminidase-catalyzed reaction look like?
• Note that a key step in the previous mechanism is the loss of an alkoxy
(sugar) moiety from the position (C2) next to the carboxylate group.
• This creates a carbocation (Sn1 process), which is sp2 hybridized.
• The initial substrate is sp3 hybridized at C2
• Thus it was decided to try to synthesize and test compounds which had
a double bond to C2, with the prospect of identifying something which
bound tighter than the substrate and which could, therefore, function as
an effective inhibitor.
Chemical evolution of neuraminidase inhibitors
• Note that lower Ki values correspond to more active inhibitors
• The final product, Relenza (Zanamivir), has a positively
charged guanidinium cation (southern end of molecule)
• Thus, it is too polar to be absorbed orally and must be
administered by inhalation.
Further chemical evolution of neuraminidase inhibitors
While the C5 substituent (glycerol side chain) bound to a
polar pocket, some of the more recent analogs have
shown there is also a hydrophobic pocket, which can
bind more hydrophobic C5 side chains, such as the
tertiary amide side chain of I.
• Note that the actual reaction intermediate (lower left) involves a resonancestabilized carbocation (with oxygen providing electrons to stabilize the C2
carbocation.
• Thus it was decided that the double bond of Relenza (Zanamivir) might be in a
somewhat incorrect position for optimal binding.
• To prepare a stable compound with the double bond in the optimal position, they
had to replace the oxygen of the six-membered ring with a carbon (called a carbon
isostere).
•Note the improvement in activity as the double bond position is altered.
Further chemical evolution of neuraminidase inhibitors
• Thus a series of compounds was prepared having an
appropriately placed C=C and an adjacent hydrophobic
group (in this case a substituted ether linkage).
• The most active of these (above) has a branched side
chain (3-pentyl side chain) on the ether.
• Due to the improvement in inhibitory activity, it was
possible to remove the highly polar guanidinium side chain
and replace it with a slightly less polar amine side chain.
Evolution leading to the final product, Tamiflu.
• The carboxylic acid of GS 4071 is still too polar.
• Thus, they replaced the acid with an ester, which can be
hydrolyzed by esterases.
• Compounds related to II (above) are currently in clinical
trials. These seem to show a further improvement in
selectivity against viruses.
Other Drugs to Treat Influenza
Antiviral Drugs
• amantadine (Symmetrel): used
prophylactically against influenza A in
high-risk individuals.
• rimantidine (Flumadine): used for
treatment and prophylaxis of influenza
A.
Mechanism of Action of
Amantadine and Rimantadine
• These agents were discovered by random
screening and are now known to interfere with a
viral ion channel called matrix (M2) protein.
• This causes an inhibition of uncoating of the virus.
• At high concentrations, they also buffer the pH of
the endosomes and prohibit the acidic
environment needed for Hemagglutinin (HA) to
fuse the viral membrane with that of the
endosome.
What is HSV?
• HSV = Herpes Simplex Virus
• HSV-1 is the ‘usual’ cause of cold sores
• HSV-2 is the ‘usual’ cause of genital
herpes
• Both types look the same under the
microscope and share about 50% of
their DNA.
What is the difference
between HSV-1 and HSV-2?
• Both types infect the body’s mucosal surfaces, usually
mouth or genitals, and then establish latency in the nervous
system.
• For both types, at least two-thirds of the infected people
have no symptoms, or symptoms too mild to notice.
• However, both types can recur and spread, even after a
period in which there were no symptoms.
HSV-1 and HSV-2
http://www.healthscout.com/ani
mation/68/21/main.html
Link
Link
• HSV spends much of its time hiding in a
latent form.
• The virus reproduces efficiently in
epithelial cells and it hides in nerve cells.
• HSV-1 usually establishes latency in the
trigeminal ganglion, a collection of nerve
cells near the ear. Then it tends to recur on
the lower lip or face.
HSV-2 usually establishes latency in sacral
ganglion at the base of the spine. From there, it
recurs in the genital area.
• For a virus, the herpes genome includes an
extremely large number of genes (at least 74
genes, or open reading frames ORFs).
• About half of these genes code for proteins
that are involved in evading host defenses.
HSV-1
• One can have HSV-1 both genitally and orally.
• HSV-1 is usually mild, especially when it infects the
lips, face, or genitals.
• However, HSV-1 can recur in the eye, causing ocular
herpes, which can lead to blindness, and can even
spread spontaneously to the brain, causing herpes
encephalitis, which can lead to death.
HSV-2
• 22% of adult Americans have HSV-2
• Like HSV-1, HSV-2 symptoms are usually mild,
so mild, in fact, that two-thirds of infected people
don’t know they have it.
• HSV-2 rarely causes complications or spreads to
other parts of the body.
• Oral HSV-2 infections are rare. But even when
an infection does occur, recurrent oral outbreaks
are uncommon.
Transmission of HSV-2
• In the first year of HSV-2 infection, people shed the virus
from the genital area about 6 to 10% of those days when
they are asymptomatic. This decreases over time and can
also be further lessened by the use of oral medication. Sex
should be avoided in the presence of symptomatic lesions.
• Having a previous HSV-1 infection seems to
provide some immunity to an HSV-2
infection. This is probably the reason that
oral HSV-2 infections are rare, given the
studies which show that a significant
proportion of the population practices oral
sex.
How severe an infection?
• HSV is a lifelong illness
• But HSV-2 usually produces only mild symptoms or signs or
no symptoms at all.
• However, HSV-2 can cause recurrent painful genital sores in
many adults, and HSV-2 infection can be severe in people
with suppressed immune systems.
• Another factor is how long a person has had the infection. It
seems to decrease in severity over time, for reasons which
are unclear.
Symptoms
• If signs and symptoms occur during the first
episode, they can be quite pronounced. The
first episode usually occurs within two
weeks after the virus is transmitted, and the
sores typically heal within two to four
weeks.
• Other signs and symptoms during the
primary episode may include a second crop
of sores, or flu-like symptoms, including
fever and swollen glands.
Is there a cure?
• There is no treatment that can cure
herpes, but antiviral medications can
shorten and prevent outbreaks during
the period of time the person takes the
medication.
Vaccines?
• No vaccine for HSV-2 is currently
available.
DNA Virus
• Recall that HSV is a DNA virus (influenza was an RNA
virus)
• In general, more drugs are available to treat DNA viruses
than for RNA viruses (excluding those used to treat HIV).
• Most of the drugs available for treatment of DNA viruses
have been developed against herpesviruses.
• Diseases include cold sores, genital herpes, chickenpox,
shingles, mononucleosis, etc.
Antiviral Chemotherapy for HSV
• There are several prescription antiviral
medications for controlling herpes outbreaks,
include acyclovir (Zovirax), valacyclovir
(Valtrex), famcyclovir (Famvir), and
pencyclovir.
• Acyclovir was the original and prototypical
member of this class
• Valacyclovir and famcyclovir are prodrugs of
acyclovir and pencyclovir respectively, with
improved oral bioavailability.
Chemotherapy for HSV
O
O
HN
N
N
H2N
N
N
H2N
N
HN
N
H2N
HO
O
O
O
O
Acyclovir (Zovirax)
Valacyclovir (Valtrex),
O
N
HN
H2N
O
N
HN
N
N
HO
H2N
N
N
AcO
OH
OAc
pencyclovir
Famcyclovir (Famvir),
Gertrude B. Elion won the Nobel prize in Physiology or Medicine
in 1988, partly for the discovery of acyclovir.
Drugs Discovered by
Gertrude B. Elion Include:
• 6-mercaptopurine (Purinethol), the first treatment for
leukemia.
• Azathioprine (Imuran), the first immuno-suppressive
agent, used for organ transplants.
• Allopurinol (Zyloprim), for gout.
• Pyrimethamine (Daraprim), for malaria.
• Trimethoprim (Septra), for meningitis, septicemia, and
bacterial infections of the urinary and respiratory tracts.
• Acyclovir (Zovirax), for viral herpes.
Mechanism of Action of
Antivirals to treat HSV
• Both acyclovir and pencyclovir work by
interfering with viral replication, effectively
slowing the replication rate of the virus, and
providing a greater opportunity for the
immune response to intervene.
• All drugs in this class depend on the activity
of the viral thymidine kinase to convert the
drug to a monophosphate form and
subsequently interfere with viral DNA
replication.
Acyclovir (ZOVIRAX)
• Discovered by random compound screening
and introduced into the market in 1981.
• It was the first non-toxic herpes drug to be
used systemically.
• It is used for the treatment of infections due to
both HSV-1 and HSV-2.
The Mechanism of Acyclovir
• The genome of the Herpes viridae code for the
production of viral DNA polymerase (rather than using
the host cell DNA polymerase).
• The drug interferes with the action of the viral DNA
polymerase, with high selectivity over that of the host
enzyme.
• Additionally, to be activated the drug must first be
phosphorylated. This is achieved by viral thymidine
kinase, which is less selective than cellular thymidine
kinase.
• Aciclovir interferes with DNA synthesis, but must first become activated.
•To become activated, Aciclovir must be phosphorylated (3x)
• However, Aciclovir itself is not a good substrate for mammalian kinases,
thus it relies on the viral thymidine kinase to become phosphorylated the
first time.
• This is good, since the drug cannot interfere with DNA synthesis in cells
that are not infected with the virus, thus reducing the toxicity of the drug.
• The second and third phosphorylations, however, are performed by the
cellular thymidylate kinase.
•Aciclovir triphosphate is mistaken for
deoxyguanosine triphosphate.
• However, since it lacks the 3’-OH
group, it cannot be linked to the
adjacent residue in the ‘usual’
fashion.
http://www.dnalc.org/ddnalc/reso
urces/sangerseq.html
• The mechanism of action of Acyclovir is
analogous to that utilized by Fred
Sanger’s method of DNA sequencing.
• http://www.dnalc.org/view/15479Sanger-method-of-DNA-sequencing3D-animation-with-narration.html
• http://www.biostudio.com/demo_freema
n_dna_sequencing.htm
Valacyclovir
• Valaciclovir is an esterified version of
aciclovir, that has greater oral
bioavailability.
Penciclovir
• Penciclovir is used primarily as a cream
(it has poor oral bioavailability) to treat
herpesvirus infections.
• It is the active ingredient in the cold sore
medications Denavir and Fenistil..
Famciclovir
• Famciclovir is a di-esterified version of penciclovir
• The ester groups give the drug better oral
bioavailability
• To treat shingles, it is taken three times a day for
seven days.
• To treat genital herpes, it is taken twice a day for five
days.
Major members of the herpesvirus
group of the family Herpesviridae
• Herpes simplex viruses: HSV-1 and
HSV-2
• Cytomegalovirus (CMV)
• Varicella-zoster virus (VZV)
• Epstein-Barr virus (EBV)
Varicella-zoster virus (VZV)
• The Varicella zoster virus
(VZV) is one of the eight
herpes viruses known to
affect humans (and other
vertebrates). It commonly
causes chickenpox in
children and shingles later in
life.
Shingles
Treatment of Varicella Infection
• Acyclovir has been shown to reduce fever
and skin lesions in patients with varicella
and it use is recommended in healthy
patients over 18 years of age.
Acyclovir (ZOVIRAX)
Cytomegalovirus
• Cytomegalovirus (CMV) is a common
virus that infects most people during
their life. Most people are infected in
early childhood (under 3 years of age)
or in the teenage years. Usually there
are no symptoms of CMV infection.
Link
• Since most CMV infections are mild and
usually do not cause long-term
problems, most people don't even know
they are infected.
• However, CMV can cause problems in a
developing baby if the mother gets the
infection during pregnancy.
Cytomegalovirus
(CMV)
• After a person has a CMV infection, the virus stays in
the body but is not active. The virus can reactivate
months or years later, which occurs most often when
a person's immune system is weakened.
Cytomegalovirus
(CMV)
• Anyone with a weakened immune system is at risk
for problems with CMV infection. A weakened
immune system can be related to infection with the
human immunodeficiency virus (HIV) or medical
treatments.
• Medical treatments that can weaken the immune
system include: chemotherapy, radiation therapy,
steroids, and stem cell or organ transplantation.
Cytomegalovirus (CMV)
• A few people will have symptoms such as sore throat,
fever, headache and fatigue. People who have weakened
immune systems may develop severe symptoms, such as
pneumonia or infections of the eyes, liver, or intestinal
tract. People with HIV infection should be sure to let their
doctor know if they are having any painless blurring of
their vision, "floaters" in the eye, light flashes, areas of
blindness, or shortness of breath.
Treatment of CMV Infection
• Gangiclovir, a nucleoside analog of acyclovir,
inhibits CMV replication and reduces the severity
of CMV syndromes, such as retinitis and
gastrointestinal disease.
O
N
HN
H2N
O
O
N
HN
N
N
H2N
HO
N
N
O
OH
Deoxyguanosine
H2N
N
N
HO
HO
O
N
HN
O
OH
Acyclovir
Gangciclovir
Ganciclovir
Ganciclovir is used to treat or prevent cytomegalovirus
Gangciclovir for ocular use is marketed under the trade
name Vitrasert.
Valganciclovir
Valganciclovir is a more orally bioavailable version of this
drug.
Epstein-Barr
Virus (EBV)
• EBV is the etiologic agent of infectious mononucleosis
• Symptoms include fever, tender lymph nodes, sore throat,
mental fatigue
• Treatment is usually supportive
• Acyclovir can decrease replication of EBV in culture, but has
little impact on clinical illness.
Assigned Reading
• Wasik, Mitzi; Kachlic, Marlowe Djuric. A review of common
sexually transmitted diseases. Formulary (2009), 44(3),
78-85.
• De Clercq, Erik; Field, Hugh J. Antiviral prodrugs - the
development of successful prodrug strategies for antiviral
chemotherapy. British Journal of Pharmacology (2006),
147(1), 1-11.
Homework Questions:
1. Describe the methods used for diagnostic testing for HSV.
2. List the accepted treatment for this disease.
3. From one point of view, all antiviral nucleosides can be
considered prodrugs. Explain. What enzyme do they
require for activation?
4. What two undesirable properties of acyclovir (ACV)
prompted the search for a prodrug?
5. What protein is responsible for the improved
bioavailability of valaciclovir?
6. Gangiciclovir (GCV) has superior activity than acyclovir
against which virus?
7. Oseltamivir (Tamiflu) is also a prodrug. Which
functionality will be transformed in vivo? Why was this
functionality incorporated?