antiviral and anti fungal

1 Jcelimpin Jmmission (1/24/12) Special Thanks to: Carla Medina for the recordings ^^

Antiviral and Antifungal Agents Celia R. Ravelo, MD, DPPS

Objectives 1. To know the mechanism of action of different antiviral

drugs 2. To know the application on what virus the drugs are

utilized 3. To know the different adverse effects of the these drugs

I. Viral Life Cycle

Virus-cell adsorption (binding, attachment) o The virus has to attach on the host cell o The binding depends on the receptors present on

the different cells of the body

Virus-cell fusion (entry, penetration) o Once they are attached they will fuse into the

host cell membrane

Uncoating (decapsidation) o Then eventually they will uncoat and release the

viral genome GENOME REPLICATION

Early transcription

Early translation

Replication of the viral genome o the uncoated viral genome will undergo

replication, transcription and translation o different proteins necessary for the virus will be

formed o the proteins will form proteases

Late transcription

Late translation

Virus assembly o The proteins will form proteases and they will

form together o This called the viral assembly

Release o Once the mature virus has been formed and it

has already matured, it will then be released into the circulation where it will infect other cells

Viral Life Cycle

- The virus will attach on the receptors - Through the process of endocytosis it will enter the cell

- Inside the endosome there would be uncoating and releases of viral genome

- The viral genome will be replicated, transcribed and translated

- A new virus will then be formed through the process of assembly and maturation

- It will then be released through the process of budding

Reverse Transcriptase - The HIV viruses is an RNA virus - But since it has a reverse transcriptase it is then reversed

into a DNA virus - The HIV DNA genome is then integrated into the host DNA

to produce different RNA and proteins - The HIV virus uses the host DNA genome to make its own

RNA and proteins for its own formation and maturation

Viral Genome Replication

Requirements:

DNA viruses –Deoxyribonucleoside triphosphate

RNA Viruses – Ribonucleoside triphosphate

Pathways for deoxyribonucleoside triphosphates synthesis

Salvage pathway – uses the enzyme thymidine kinase De Novo pathway - uses the enzyme thymidylate

kinase

Nucleoside triphosphates - are incorporated into new viral genomes by a viral or cellular polymerase.

HSV – phosphorylation of nucleosides via the salvage pathway by a viral thymidine kinase;

- a viral DNA polymerase then adds deoxyribonucleoside triphosphates to the growing DNA genome II. Pharmacologic Classes of Agents

- Anti-HIV drugs - Anti-Herpes drugs

o Drugs used against CMV - Anti-Influenza drugs

A. ANTI-HIV DRUGS

Human Immunodeficiency Virus (HIV)

- HIV contains glycoproteins on its surface

o Gp41 o Gp120

- Both of them are necessary in order to attach to the T-cell - Only the T-cell has the chemokine receptors (CCR5 and

CXR4) and CD4 Receptors


HIV Life Cycle

- Gp120 of HIV will attach into the chemokine and CD4

receptors of the host cell - The gp41 will then be exposed, injecting itself into the host

cell membrane - There will then be fusion of the viral membrane and the

host membrane - Penetration will follow, there would then be a release of

nucleocapsid into the cytoplasm of the host cell - Once it is released, the reverse transcriptase enzyme

(connected to the viral genome) would now be active - Reverse transcriptase enzyme will transcribe the viral RNA

into viral DNA - The integrase enzyme would integrate the viral DNA into

the host cell DNA - The RNA will then be transcribed - Then there would be synthesis of different proteins in order

to assemble the different viral parts - The proteases would cleave each protein and through the

process of assembly different proteins will come together to form an mature virus

- Through the process of budding the mature virus will be released from the host cell

*Note: Penetration allows the nucleocapsid -- the genetic core -- of the virus to be injected directly into the cell's cytoplasm HIV The process by which HIV's RNA is converted to DNA is called reverse transcription

1. Inhibitors of viral entry - Inhibits the attachment of the virus into the host cell

a. Enfuvirtide Peptide

First drug that acts by inhibiting viral entry to be approved by the FDA

Structurally similar to a segment of gp41 o gp41 - HIV protein that mediates membrane

fusion - You can still take this drug even though you are already

infected with HIV

- Because inside you are replicating viruses, and these viruses in order for them to attack other T cells they need attachment

- This drug will prevent the virus from infecting other T cells in the body

Enfuvirtide: Mode of Action

- The gp120, after binding into the CD4 or chemokine

receptors, will expose the gp41 (Fig. a) - It is the gp41 that will inject itself into the host cell

membrane (Fig. b) - Enfuvirtide insert itself into gp41 particles (fig. f),

preventing gp41 from injecting itself into the host cell - The viral outer layer will not fuse with outer host cell

membrane

* Notes from Ppt The native gp41 protein is trapped in the virion in a

conformation that prevents its ability to fuse membranes or to bind T-20. Binding of HIV to its cellular receptors triggers a conformational change in gp41 that exposes the fusion-active segment (fusion peptide), heptad repeat region, and a second heptad repeat region mimicked by T-20. The gp41 then refolds, so that the segments mimicked by T-20 bind to the first set of heptad repeats. If the fusion peptide has properly inserted into the host cell membrane, this refolding brings the virion envelope and the cell membrane into close proximity, allowing membrane fusion to occur (by mechanisms that remain poorly understood). When T-20 is present, however, the drug binds to the first set of heptad repeats and prevents the refolding process, thereby preventing fusion of the HIV envelope with the host cell membrane

Administered parenterally, typically by twice daily

subcutaneous injections o Since it is a peptide it can’t be given orally


Adverse effects o Not yet well recognized o Common: Irritation at the site of injection

2. Inhibitor of Viral Genome Replication a. Inhibitor of Reverse Transcriptase

o It is important to inhibit this enzyme since it is the one responsible for transcription of viral RNA to viral DNA that will be integrated into the host cell genome

Inhibit Viral polymerases or reverse transcriptase(human herpesviruses, the retrovirus HIV, and the hepadna virus HBV)

Nucleoside analogue

Non-nucleoside inhibitor of DNA polymerase or reverse transcriptase

All nucleoside analogues must be activated by phosphorylation, usually to the triphosphate form, in order to exert their effect

Nucleoside analogues inhibit polymerases by competing with the natural triphosphate substrate; incorporated into the growing DNA chain, where they often terminate elongation.

Either or both of these features—enzyme inhibition and incorporation into DNA—can be important for antiviral Role of RT (reverse transcriptase)

2 main categories of nucleoside analogues o Antiherpesvirus agent o Anti-HIV agents o 2 anti-HIV agents (adefovir and lamivudine)

and a third drug, entecavir are also approved for use agai*nst hepatitis B virus

*Note:

Reverse transcriptase (RT) - DNA polymerase that can copy both DNA and RNA

Copies the RNA retrovirus genome into double-stranded DNA after the virus enters a new cell

2 Groups of Drugs that Inhibit Reverse Transcriptase

NRTI – Nucleotide Reverse Transcriptase Inhibitors

NNRTI – Non-Nucleotide Reverse Transcriptase Inhibitors

Nucleoside Reverse Transcriptase Inhibitor (NRTI)

Zidovudine

Lamivudine

Zidovudine (AZT)

Nucleoside analogue with an altered sugar moiety

o A nucleoside has to be converted to a nucleotide, so this drug needs to undergo 3 phosphorylation inside the cell

As with acyclovir, AZT is an obligatory chain-terminator

Excellent substrate for cellular thymidine kinase which phosphorylates AZT to AZT monophosphate

o This drug will enter the cell as nucleoside and by

the action of the cellular thymidyllate kinase it will be phosphorylated

o Thus ultimately becoming a Zidovudine triphosphate

o The triphosphate form will the one to attach to the reverse transcriptase

o The Reverse transcriptase will attach the nucleotide into the protein chain of the viral DNA

Nucleotide is needed for the formation of the protein chain of the viral DNA

o Since the drug is a false triphosphate, the growth of DNA will not progress and at the same time

o Since the drug attaches to the active site of the enzyme, the said enzyme will be paralyzed

o Thus the RNA genome of HIV will not progress into DNA, thereby breaking the process of the viral life cycle

More potent inhibitor of HIV RT but it also affects the human DNA polymerases (reason for having adverse effects)

Adverse effect: bone marrow suppression neutropenia anemia

The limited clinical effectiveness of AZT, and problems with its toxicity and resistance, have led to the development of other anti-HIV drugs and to the use of combination chemotherapy for HIV


Lamivudine (3TC)

L-stereoisomer biologic nucleosides

Contains sulfur atom in its five-membered ring Weakly inhibits mitochondrial DNA polymerase

therefore less toxic

Resistance to 3TC develops quickly in patients so it is used in combination with other anti-HIV drugs

Non- Nucleoside Reverse Transcriptase Inhibitors (NNRTI)

Efavirenz – first anti-HIV drug to be taken OD

Nevirapine

Delavirdine o Note that since they are non-nucleoside

they are in nucleotide form already o They don’t need to be phosphorylated by

the kinases o It will be inserted directly into the reverse

transcriptase *Approved in combination with other antiretroviral drugs for the treatment of HIV-1 infection.

b. Inhibitor of Integrase Enzyme

o Integrase enzyme is the one responsible for integrating HIV pro-viral DNA to the host cell DNA to produce the viral mRNA

o Drug: __________

3. Inhibitor of Viral Maturation

a. HIV Protease Inhibitors o HIV protease are the one responsible for cleaving

different protein that were synthesized for the assembly of a new virus

o The inhibitors will prevent this assembly, thus preventing the formation of a new virus

Saquinavir

Ritonavir

Amprenavir Indinavir

Nelfinavir

Lopinavir

Atazanavir Tipranavir

Darunavir

HIV Proteases as Target for anti-HIV drugs

First, it is essential for HIV replication

Second, a point mutation is sufficient to inactivate the enzyme

Third, the substrates of HIV protease are conserved and somewhat unusual, suggesting both specificity and a starting point for drug design.

Fourth, HIV protease—unlike the human proteases most closely related to it—is a symmetric dimer of two identical subunits, each of which contributes to the active site, again suggesting both specificity and a starting point for drug design.

Fifth, the enzyme can be easily overexpressed and assayed, and its crystal structure has been solved.

Common Toxicities of Antiretroviral Drugs

B. ANTI-INFLUENZA

1. Inhibitor of Viral Uncoating (Exclusive for Influenza A viruses)

Amantadine

Rimantadine


- Amantadine and Rimantadine act on the “uncoating” of

the viral core - If the viral core will not be uncoated then the viral genome

will not move out, thus replication of these genome will cease

- Uncoating o The uncoating happens within the endosome o There is a M2 channel on the surface of the

endosome o M2 is important for the acidification of the

endosome and ultimately the viral matrix o If acidity is achieved the matrix will be disrupted

or endosome will be ruptured, releasing now the viral genome

- Action of Amantadine and Rimantadine o Both of them block the M2 channel o If M2 channels are blocked the acidification of

the endosome will cease o The viral matrix and the endosome will not

rupture, therefore the viral genome will not be released

- Amantadine and Rimantadine only differ in their P’kinetics o Rimantadine has longer half life, metabolized in

the liver o Amantadine is the only drug utilized in the

management of Parkinson’s disease o Adverse effect is more common in amantadine

Notes from ppt

The early endosome contains an H+-ATPase that acidifies

the endosome by pumping protons from the cytosol into the endosome. A low pH-dependent conformational change in the viral

envelope hemagglutinin (HA) protein triggers fusion of the viral membrane with the endosomal membrane. HA binding alone is not sufficient to cause viral uncoating, however. In addition, protons from the low-pH endosome must enter the virus through M2, a pH-gated proton channel in the viral envelope that opens in response to acidification. The entry of protons through the viral envelope causes dissociation of matrix protein from the influenza virus ribonucleoprotein (RNP), releasing RNP and thus the genetic material of the virus into the host cell cytosol. Amantadine and rimantadine block M2 ion channel function and thereby inhibit acidification of the interior of the virion, dissociation of matrix protein, and uncoating. NA, neuraminidase; ADP, adenosine diphosphate. Adverse Effects:

- Amantadine GIT

o Nausea, vomiting, diarrhea, anorexia

CNS o Lightheadedness o Headache o difficulty concentrating

likely due to its effects on host ion channel

Note: adverse effects are more common in CNS

Used also for treatment of Parkinson's disease

- Rimantadine

Analogue of amantadine Similar antiviral mechanism

Lacks adverse effects as seen in amantadine especially the neurological

o Less adverse effect o Reason why patients prefer this drug

Prophylactic agent in settings where there is a large population at risk from influenza morbidity (e.g., nursing homes)

2. Inhibitor of Viral Release

Zanamivir o Poor oral bioavailability o Administered by inhalation

Oseltamivir o Oral availability is approximately 75% o When taken prophylactically, oseltamivir

reduces the number of flu cases in susceptible populations (e.g., nursing home residents)

o Both oseltamivir and zanamivir reduce the duration of flu symptoms in patients who are already infected with the virus. However, this reduction is only 1 day on average, and even this modest effect requires that the drugs be taken within 2 days of the onset of symptoms.

o Oseltamivir is effective in preventing human mortality due to H5N1 avian influenza (“bird flu”)

Both of them inhibit influenza virus neuramidase, causing newly synthesized virions to attached to the host cell


Neuramidase o An enzyme that reduces the stickiness of the

virus from the host cell o It is needed for the release of the new virus

from the host cell

Zanamivir and Oseltamivir inhibit the neuramidase o Thus the virus will remain attach to the host

cell o The newly formed virus will not be able to

infect other host cells

Zanamivir and Oseltamivir o Both of them has the same mechanism of

action their difference lies at the P’kinetics

Zanamivir o Poor bioavailability o So it is only given via Aerosol

Oseltamivir o With oral preparation o Used in H1N1 virus o It has to be taken within 2 days of the onset

of symptoms o Lifesaving drug – endemic regions

3. Antiviral Drug with Unknown Mechanisms of Action

Fomivirsen o An antisense oligonucleotide. that target specific

RNAs o If the viral RNA is an mRNA, binding of the

oligonucleotide should prevent the synthesis of the protein encoded by the mRNA.

o Fomivirsen is the first FDA-approved oligonucleotide drug

o Designed to bind to an mRNA that encodes IE2, a gene-regulatory protein of CMV

o More potent than ganciclovir against CMV, with activity at submicromolar concentrations.

o Approved for treatment of ophthalmic CMV disease and is used mainly in CMV retinitis

o Administered intravitreally

o Antisense oligonucleotide o Antisense oligonucleotides target specific RNAs o It is utilized for the treatment of CMV o The drug is approved for treatment of

ophthalmic CMV disease and it is used mainly in CMV retinitis

Ribavirin o “Broad-spectrum antiviral” that exhibits activity

against many viruses in vitro and efficacy against several in vivo

o Approved only in aerosol form (in effect, topical application to the lungs) for severe respiratory syncytial virus (RSV) infection, and only in combination with an interferon for chronic hepatitis C virus (HCV) infection.

o Ribavirin is converted to a monophosphate by cellular adenosine kinase and is known to inhibit cellular inosine monophosphate dehydrogenase, thereby lowering cellular GTP pools

o Inhibition of viral RNA polymerase could represent a second possible selective mechanism for ribavirin action

o Interestingly, both ribavirin diphosphate and ribavirin triphosphate have inhibitory activity against the RNA polymerase from certain viruses.

o A third possible mechanism also involves viral RNA polymerase.

o It has been approved only in aerosol form (in effect, topical application to the lungs) for severe respiratory syncytial virus (RSV) infection

o It is also approved only in combination with an interferon for chronic hepatitis C virus (HCV) infection

C. Anti-Herpes Agents o Phosphorylation of drug by viral kinases leads to

inhibition of DNA synthesis in virus-infected cells

1. Inhibitor of Viral Replication Nucleoside and Nucleotide Analogue

Acyclovir Valacyclovir

Ganciclovir

Valganciclovir

Penciclovir

Famciclovir Cidofovir

a. Acyclovir

Against HSV and VZV o It most common drug against HSZ and VZV

because it is the cheapest drug

Nucleoside analogue

Has high therapeutic index

A guanosine analog that acts as an antimetabolite It is the only approved drug for neonatal encephalitis

It has long bioavailability


Acyclovir Mode of Action

- Acyclovir once it enters the virally infected cell it will be phosphorylated

- It will then be doubly phosphorylated by cellular kinase o Note that it only requires the virally infected

cellular kinase o This is also the reason for the resistance to

Acyclovir o Some viruses can mutate and will no longer

produce the kinase needed by the Acyclovir - Once Acyclovir is phosphorylated and eventually become

Acyclovir triphosphate the herpes DNA polymerase will now insert it into the herpes simplex genome

- If the Acyclovir triphosphate is inserted into the growing

DNA of herpes virus, the next nucleotide will not be inserted, thereby terminating the growth of the viral DNA

- Acyclovir triphosphate will also paralyze the DNA polymerase, it can no longer carry another nucleotide to be inserted into the growing viral DNA

Notes from ppt

The acyclovir molecules enter the cell and are converted to acyclovir monophosphate by the HSV enzyme thymidine kinase (TK). Enzymes in the human cell add two more phosphates to eventually form the active drug acyclovir triphosphate. The acyclovir triphosphate competes with 2-deoxyguanosine triphosphate (dGTP) as a substrate for viral DNA polymerase, as well as acting as a chain terminator. In actual infection, the HSV releases its naked capsid that

delivers DNA to the human nucleus; the active drug acyclovir triphosphate exerts its action on the viral DNA located in the nucleus.

Notes from ppt The acyclovir triphosphate competes with 2-deoxyguanosine triphosphate (dGTP) as a substrate for viral DNA polymerase, as well as acting as a chain terminator. In actual infection, the HSV releases its naked capsid that delivers DNA to the human nucleus; the active drug acyclovir triphosphate exerts its action on the viral DNA located in the nucleus.

Resistance vs Acyclovir HSV

Absence of partial production of viral thymidine kinase

Altered thymidine kinase substrate specificity

Altered viral DNA polymerase VZV

Mutation in VZV thymidine kinase

Mutation in viral DNA polymerase

A vaccine is produced for VZV o Since it has no subspecies or different strain, a

vaccine was produced o This is also the reason why VZV can be

eradicated by immunization

Adverse effects

ORAL: nausea, diarrhea, rash, headache, renal insufficiency, neurotoxicity

TOPICAL: mucosal irritation and transient burning to genital lesions

IV: renal insufficiency

b. Valacyclovir A prodrug form of acyclovir

Has approximately fivefold greater oral bioavailability than acyclovir

Rapidly converted to acyclovir after oral administration

The active metabolite acyclovir can easily go into the circulation

c. Penciclovir and famciclovir

Famciclovir is the diacetyl 6-deoxy analogue of penciclovir Penciclovir is the active form

MOA same as Acyclovir

Used for treatment of shingles, cold sores and HSV infections

It should be given at the early phase of the illness, what it do is that it reduces the duration of the illness


d. Ganciclovir

A nucleoside analogue

Originally synthesized as a derivative of acyclovir, with the intention of developing another anti-HSV drug

Much more potent than acyclovir against CMV

First antiviral drug approved for use against CMV

More closely resembles deoxyguanine accounting for its toxicity

Used only for more serious infection

Viral protein kinase called UL97 phosphorylates ganciclovir (30-fold increase in the amount of phosphorylated ganciclovir in infected versus uninfected cells)

Ganciclovir triphosphate inhibits CMV DNA polymerase more potently than it does cellular DNA polymerases

Selective against CMV at two steps: phosphorylation and DNA polymerization.

More toxic than acyclovir

Toxicity: bone marrow suppression, especially neutropenia

It is a problem among pregnant individual o It can cause fetal abnormality

It is also a problem among immunocompromised patient

e. Valganciclovir

Prodrug of ganciclovir For better bioavailabilty

With same mechanism of action

f. Cidofovir Also known as hydroxyphosphonylmethoxypropylcytosine

(HPMPC),

Phosphonate-containing acyclic cytosine analogue o It means that this drug is already a

monophosphate Can be considered a nucleotide rather than a nucleoside

analogue

Mimics deoxycytidine monophosphate;( thus, in effect, it is already phosphorylated)

Does not require viral kinases for its phosphorylation

Active against kinase-deficient viral mutants that are resistant to ganciclovir

Enters cells efficiently

Further phosphorylated (twice) by cellular enzymes to yield an analogue of dCTP, which inhibits herpesvirus DNA polymerases more potently than cellular DNA polymerases

Cidofovir Mecahnism of Action

- This drug is a phosphate - It will then be phosphorylated by the host cell kinase - This will then be inserted into the growing viral genome - Eventually it will also inhibit the viral polymerase enzyme

Approved for use in the treatment of CMV retinitis in patients with HIV/AIDS

Long intracellular half-life Cleared in the kidneys

Most prominent toxicity: Nephrotoxicity o To prevent the toxicity it must be administered

with probenecid o Probenecid inhibits a proximal tubule anion

transporter and thereby decreases cidofovir excretion

Two related phosphonate-containing drugs:

Tenofovir and Adefovir - acyclic deoxyadenosine monophosphate analogues

o Tenofovir - approved as an anti-HIV drug in 2001, can be administered just once each day an important advantage for HIV-infected individuals who must comply with complex combination chemotherapy regimens

o Adefovir - approved as an anti-HBV drug in 2002

The mechanisms of action of these drugs against their respective viruses are similar to that of cidofovir against CMV

Non-Nucleoside DNA polymerase Inhibitor

Foscarnet

a. Foscarnet

Inhibits both DNA and RNA polymerases encoded by a wide variety of viruses

Relatively broad spectrum of activity in vitro (including against HIV), but clinically it is used for certain serious HSV and CMV infections where therapy with acyclovir or ganciclovir has not succeeded

Differs from nucleoside analogues in that it does not require activation by cellular or viral enzymes: rather, foscarnet inhibits viral DNA polymerase directly by mimicking the pyrophosphate product of DNA polymerization

Selectivity results from the increased sensitivity of viral DNA polymerase relative to cellular enzymes;

Selectivity is not as high as acyclovir's

It inhibits cell division at concentrations not much higher than its effective antiherpesvirus concentration.

Major drawbacks to foscarnet use include its lack of oral bioavailability and its poor solubility; renal impairment is its major dose-limiting toxicity

D. DRUGS THAT MODULATE IMMUNE SYSTEM

Immunization o Active o Passive

Interferons

Imiquimod

a. Immunization Active and passive immunization inhibit viral infection by

providing antibodies against viral envelope proteins

Antibodies then block the attachment and penetration of virions into cells and increase virion clearance

Some antibodies are directly virucidal, causing virions to be destroyed or inactivated before the virus can interact with its receptor(s) on target cells


Active o It involves challenging our body with inactivated

organism or a particle of an organism o The weakened particle will boost our immune

system to develop antibodies so that the next time we encounter those antigen, we are already immune to them

Passive o Antibodies are given o Given to new born, gainst tetanus toxin

b. Interferon Proteins that were produced in response to virus infection

and that could inhibit replication of the same or other viruses

o Note that it has no specificity o No specific virus is targeted

Two major types of interferons

Type I interferons - interferon a and interferon ß, which are produced by many cell types and interact with the same cell-surface receptor

Type II interferons - interferon γ, which is typically produced by cells of the immune system, especially T cells, and interacts with a separate receptor

Interaction of interferons with their receptors induces a series of signaling events that activate and/or induce the expression of proteins that combat virus infections

One relatively well-understood example of such a protein is a protein kinase, called PKR, which is activated by double-stranded RNA.

PKR phosphorylates a component of the host translational machinery, thereby turning off protein synthesis and thus the production of virus in infected cells

Used as a therapeutic agent in the treatment of: HCV

HBV

condyloma acuminata (which is caused by certain HPVs

Kaposi's sarcoma (which is caused by Kaposi's sarcoma-associated herpesvirus [KSHV]

c. Imiquimod

Approved for reatment of certain diseases caused by HPVs Interacts with the Toll-like receptors TLR7 and TLR8 to

boost innate immunity, including the secretion of interferons.

Toll-like receptors are cell surface proteins that recognize pathogen-associated molecular patterns. Activation of Toll-like receptors induces intracellular signaling events that are important for defense against pathogens

It enhances our body to produce interferon

ANTIVIRAL DRUGS SUMMARY HIV CMV HSV Influenza

A Influenza A and B

Zidovudine Didanosine Zalcitabine Stavudine Lamivudine Nevirapine Delavirdine Efavirenz Saquinavir Ritonavir Indinavir Nelfinavir Amprenavir Lopinavir Abacavir

Ganciclovir Foscarnet Cidofovir Fomivirsen

Acyclovir Valaciclovir Penciclovir Famciclovir Idoxuridine Trifluridine Brivudi

Amantadine Rimantadine

Oseltamivir Zanamivir


Anti-Fungal Agents Main Target Site of Anti-Fungal Drugs

- Cell membrane - Cell wall - Nucleic Acid Synthesis

Sites of action of anti-fungal drugs

a. Inhibitor of Fungal Nucleic Acid Synthesis: Flucytosine

Flucytosine is selectively taken up by fungal cells via cytosine-specific permeases that are expressed only in fungal membranes

Lacking these transporters, mammalian cells are protected

Inside the fungal cell, the enzyme cytosine deaminase

converts flucytosine to 5-fluorouracil (5-FU)

Subsequent reactions convert 5-FU to 5-fluorodeoxyuridylic acid (5-FdUMP), which is a potent inhibitor of thymidylate synthase

Inhibition of thymidylate synthase results in inhibition of DNA synthesis and cell division

Fungistatic under most circumstances

Fungi and bacteria in the intestine can convert flucytosine into 5-fluorouracil, which can cause adverse effects in host cells

o The adverse effect is not directly given by flucystosine because the adverse effects comes from the 5FU that is secreted by the fungi and bacteria inside the intestine

Used in combination with amphotericin B to treat systemic mycoses

Used singly, resistance emerges rapidly due to mutations in fungal cytosine permease or cytosine deaminase

Can kill aspergillus when combined with amphotericin B

The mechanism of this synergistic interaction appears to involve enhancement of flucytosine uptake by fungal cells due to amphotericin-induced damage to the fungal plasma membrane

As a single agentactionis limited to o candidiasis, cryptococcosis, and chromomycosis o has large volume of distribution, with excellent

penetration into the central nervous system (CNS), eyes, and urinary tract

Dose-dependent adverse effects: bone marrow suppression leading to leukopenia and thrombocytopenia, nausea, vomiting, diarrhea, and hepatic dysfunction

Contraindicated during pregnancy

b. Inhibitor of fungal Mitosis: Griseofulvin

Binds to tubulin and a microtubule-associated protein disrupting assembly of the mitotic spindle.

Also inhibit fungal RNA and DNA synthesis

Accumulates in keratin precursor cells and binds tightly to keratin in differentiated cell that allows new growth of skin, hair, or nail to be free of dermatophyte infection.

Fungistatic under most circumstances.

Therapeutic uses: topical antifungal medications

Griseofulvin can be used to treat fungal infection of the skin, hair, and nail due to Trichophyton, Microsporum, and Epidermophyton.

Not effective against yeast (such as Pityrosporum) and dimorphic fungi.

It is important to continue treatment until the infected skin, hair, or nail is completely replaced by normal tissue

c. Inhibitor of Ergosterol biosynthesis

Squalene epoxidase Inhibitor Inhibitors of 14a- Sterol Demethylase


- Ergosterol is synthesized in fungal cells from acetyl CoA building blocks

- One of the intermediates, squalene, is converted to lanosterol by the

- 14a-Sterol demethylase, a cytochrome P450 enzyme not expressed in mammalian cells, catalyzes the first step in the conversion of lanosterol to the unique fungal sterol ergostero

- Imidazoles and triazoles inhibit 14a-sterol demethylase and thereby prevent the synthesis of ergosterol, which is the principal sterol in fungal membranes. Fluconazole and voriconazole are two representative triazoles action of squalene epoxidase

- Allylamines and benzylamines inhibit the action of squalene epoxidase

1. Squalene epoxidase Inhibitor: Terbinafine and

Naftifine

Squalene is converted to lanosterol by the action of squalene epoxidase

Prevent the formation of lanosterol, which is a precursor for ergosterol

Promote accumulation of the toxic metabolite squalene in the fungal cell, making them fungicidal under most circumstances

Divided into allylamines and benzylamines based on their chemical structures: terbinafine and naftifine are allylamines, whereas butenafine is a benzylamine.

Terbinafine

Available in both oral and topical formulations

When taken orally, the drug is 99% protein-bound in the plasma and it undergoes first-pass metabolism in the liver

Oral bioavailability of terbinafine is 40%.

Elimination half-life is extremely long, approximately 300 hours, because terbinafine accumulates extensively in the skin, nails, and fat.

Oral form is used in the treatment of onychomycosis, tinea corporis, tinea cruris, tinea pedis, and tinea capitis.

Not recommended in patients with renal or hepatic failure and in pregnant women

Very rarely, the oral form of terbinafine can lead to hepatotoxicity, Stevens-Johnson syndrome, neutropenia, and exacerbation of psoriasis or subacute cutaneous lupus erythematosus.

Liver function enzymes should be monitored during the treatment course.

Plasma levels of terbinafine are increased by coadministration with cimetidine (a cytochrome P450 inhibitor) and decreased by coadministration with rifampin (a cytochrome P450 inducer)

Topical terbinafine is available in cream or spray form and is indicated for tinea pedis, tinea cruris, and tinea corporis.

Naftifine

broad-spectrum antifungal activity

Available topically as a cream or gel Effective in tinea corporis, tinea cruris, and tinea

pedis

Butenafine A benzylamine

Topical antifungal agent with a mechanism of action and spectrum of antifungal activity similar to that of the allylamine

More effective than topical azole agents against common dermatophytes

2. Inhibitors of 14a- Sterol Demethylase Imidazoles and Triazoles

Results in decreased ergosterol synthesis and accumulation of 14a-methyl sterols disrupting the tightly packed acyl chains of the phospholipids in fungal membrane

Destabilization of the fungal membrane leads to dysfunction of membrane-associated enzymes, including those in the electron transport chain, and may ultimately lead to cell death.

Not completely selective for the fungal P450 enzyme, however, and they can also inhibit hepatic P450 enzymes


Azoles

Antifungal Activity:

B. Dermatitidis

Cryptococcus neoformans

H. capsulatum, Coccidioides species,

P. brasiliensis,

dermatophytes, and most Candida species

Generally fungistatic rather than fungicidal against susceptible organisms

2 Types of Azoles

Imidazoles

ketoconazole, clotrimazole, miconazole, econazole, butoconazole, oxiconazole, sertaconazole, and sulconazole

Triazoles

itraconazole, fluconazole, voriconazole, terconazole, and posaconazole; one additional member of this class, ravuconazole, is currently in clinical trials

Fluconazole

Most widely used antifungal drug Hydrophilic triazole that is available in both oral and

intravenous formulations

The bioavailability of oral fluconazole is nearly 100%, and, unlike ketoconazole and itraconazole, its absorption is not influenced by gastric pH.

Once absorbed, fluconazole diffuses freely into CSF, sputum, urine, and saliva. Fluconazole is excreted primarily by the kidneys.

low adverse-effect profile

excellent CSF penetration

drug of choice for systemic candidiasis and cryptococcal meningitis

also the drug of choice for coccidioidal meningitis Not effective against aspergillosis

d. Polyene antifungal agents

Amphotericin B

Nystatin

Both are derived from Streptomyces sp.

Amphotericin B

Binding to ergosterol produces channels or pores that alter fungal membrane permeability and allow for leakage of essential cellular contents, leading ultimately to cell death

Destabilize fungal membranes by generating toxic free radicals upon oxidation of the drug

Affinity to ergosterol is 500 times greater than its affinity for cholesterol

Concentration of membrane-associated ergosterol in a given fungal species determines whether it is fungicidal or fungistatic for that species

Resistance is attributable to a decrease in the ergosterol content of the fungal membraen

Highly insoluble

Supplied as a buffered deoxycholate colloidal suspension

o Given via IV

Poorly absorbed from the gastrointestinal tract; administered intravenously

In the bloodstream, more than 90% of the drug binds rapidly to tissue sites

Low CSF, vitreous humor and amniotic fluid penetration

Intrathecal therapy may be necessary for treatment of serious meningeal disease

o To target CNS infection Notes from ppt

For decades, this drug provided the only effective treatment vs systemic mycoses .Its therapeutic and toxic effects are related to its affinity for plasma membrane sterols.

Adverse Effects

Immediate systemic reactions

Renal effects

Hematologic effects Systemic Reactions

Cytokine storm - elicits release of tumor necrosis factor-alpha (TNF-a) and interleukin-1 (IL-1) from cells of the host immune system fever chills hypotension within the first several hours after drug

administration loss of consciousness

May be minimized by decreasing the rate of drug administration or by pretreatment with antipyretic agents (e.g., acetaminophen, nonsteroidal anti-inflammatory drugs [NSAIDs], or hydrocortisone)

Renal Toxicity

Mechanism of renal toxicity is unknown but may be related to amphotericin-mediated vasoconstriction of afferent arterioles leading to renal ischemia

Often the limiting factor in determining the extent of the therapeutic response to amphotericin B

Discontinue therapy temporarily if the blood urea nitrogen (BUN) exceeds 50 mg/dL or the serum creatinine exceeds 3 mg/dL.


Renal tubular acidosis

Cylindruria (the presence of renal cell casts in the urine)

Hypokalemia

Hematologic Toxicity

Anemia - secondary to decreased production of erythropoietin.

Renal and hematologic toxicities of amphotericin B are cumulative and dose-related

Therapeutic measures that can minimize these toxicities:

o avoidance of other nephrotoxic drugs, such as aminoglycosides and cyclosporine

o maintenance of euvolemia to provide adequate renal perfusion

Nystatin

Structural relative of amphotericin B

Acts by binding ergosterol and causing pore formation in fungal cell membranes

Used topically to treat candidiasis involving the skin, vaginal mucosa, and oral mucosa

e. Glucan Synthesis Inhibitor: Echinocandins - Inhibitor of fungal cell wall synthesis

Noncompetitively inhibit the synthesis of ß-(1,3)-D-glucans

Disrupts cell wall integrity resulting in osmotic stress, lysis of the fungal cell, and ultimately fungal cell death

caspofungin micafungin Anidulafungin

all are semisynthetic lipopeptides derived from natural products (not oral)

Fungicidal against Candida species, including Candida glabrata and Candida krusei

Fungistatic against Aspergillus species Currently available only in parenteral form because they

are insufficiently bioavailable for oral use

Summary of Anti-Fungal Drugs and their MOA

antiviral and anti fungal

Documents

viral life

viral dna

viral dna

host cell

host cell

acyclovir

ophthalmic

viral membrane