ANTIVIRALS
Outline

Copyright, Purdue Research Foundation, 1996

Quick Review

RNA Viruses:

Pcornaviridae: polioviruses (poliomyelitis), coxsackieviruses and echoviruses (aseptic meningitis), and rhinoviruses (common cold)
Reoviridae: rotaviruses (diarrhea)
Togaviridae: various mosquito-borne encephalitis viruses (encephalitis), yellow fever virus (yellow fever), and rubella virus (rubella)
Orthomyxoviridae: influenza viruses (influenza)
Paramyxoviridae: parainfluenza viruses (croup, pneumonia, bronchitis), mumps virus (mumps), measles virus (measles), and respiratory syncytial virus (bronchiolitis, pneumonia)
Rahabdoviridae: rabies virus (rabies)
Coronaviridae: coronoviruses (respiratory illnesses)
Bunyaviridae: California encephalitis viruses (encephalitis)
Retroviridae: human T-cell lymphotropic viruses (leukemia / lymphoma; HTLV-1, HTLV-II), human immunodeficiency virus types 1 and 2 (HIV-1, HIV-2) (acquired immunodeficiency syndrome)

DNA Viruses:

Papovaviridae: papilloma viruses (warts, progressive multifocal leukoencephalopathy)
Adenoviridae: adenoviruses (acute respiratory diseases, keratitis)
Herpesviridae [HSV]: herpes simplex types 1 and 2 ("cold" sores, keratitis, genital infections, encephalitis), varicella-zoster (chickenpox, shingles), cytomegalovirus (cytomegalic inclusion disease, chorioretinitis, pneumonitis), Epstein-Barr virus (infectious mononucleosis, association with Burkitt's lymphoma), and human herpesvirus type 6 and 7 (unknown disease association)
Chordopoxviridae: variola virus (smallpox)

Tidbits

Transforming viruses --Viruses capable of transforming cells
- adenoviruses, papovaviruses, retroviruses, herpesviruses
Most viral infections acute
Chronic viral infections examples:
- hepatitis B and non-A/non-B, warts, molluscum contagiosum.
Abbreviations
- HSV - herpes simplex virus
- VZV - Varicella-Zoster Virus
- CMV - Cytomegalovirus

Ref: AMADE90,5:2

Problem with treating viral infections

Most replication precedes signs
Major dependence on host enzymes

Process unique to viruses

Host cell-surface receptors
To which viruses attach and that probably mediate absorption
Viral-encoded enzymes
- herpesvirus-induced thymidine kinases
- HIV-induced reverse transcriptase
- Differences important in MOA of --
  - amantadine Symmetrel^R
  - acyclovir Zovirax^R
  - zidovudine Retrovir^R

General Treatment of Viral Infections

symptomatic and supportive
especially for respiratory infections
Example: bed rest, analgesics, antipyretics

Acquired Resistance to Antiviral Drugs

Key examples
- Viral thymidine kinase (TK)
  -- decrease or loss of TK (important and major example)
- Viral DNA polymerase
  -- alterations in structure or activity or decreased activity
Can be produced by repeated passage in culture
Cross resistance reported for --
- TK-mutants
  - idoxuridine, cytarabine, acyclovir, and BVDU
- DNA polymerase mutants
  - Exist, but not well defined

Overview of Drugs

Protein synthesis inhibitor
- None
Protease inhibitors
- saquinavir (Hoffman LaRoche, FDA approved 12/95)
- ritonavir (Abbott Labs., FDA approved 3/96)
- indinavir (Merck & Co, FDA approved 3/96)
- nelfinavir (Agouron Inc, FDA approval expected '97)
Inhibit early viral processes
- Amantadine, Symmetrel^R
- Rimantadine
Nucleic acid synthesis inhibitor
- Acyclovir, Zovirax^R
- Ganciclovir, Cytovene^R
- Idoxuridine, Herplex^R, Stoxil^R
- Ribavarin, Virazole^R
- Vidarabine, Ara-A^R
- Fluridine, Viroptic^R
Reverse transcriptase inhibitors
- Zidovudine, Retrovir^R (AZT, Azidothymidine)
- Zalcitabine, HIVID^R (dideoxycytidine, ddC)
- Didanosine, Videx^R, ddI
- Stavudine, Zerit^R, d4T

Overview of Life Cycle of a DNA VIrus (e.g., herpes)

Attachment
Penetration by pinocytosis
Uncoating and transfer of viral DNA to nucleus
Early transcription into viral mRNA
Early translation of viral mRNA into enzymes for viral DNA synthesis
- DRUG ACTIONS -- INHIBITION OF DNA POLYMERASE
  - acyclovir
  - vidarabine
Synthesis of viral DNA and late transcription of viral mRNA
- DRUG ACTIONS -- INHIBITION OF TRANSLATION OF VIRAL mRNA
  - Interferon
Late translation of mRNA into viral structural proteins
Assembly of virus particles in nucleus
Budding from nucleus and release of virions

Amantadine (Symmetrel^R)

Structure and chemistry

Tricyclic amine
Rimantidine is newer related drug

MOA in Parkinson's disease
- nicotinic cholinergic receptor blocked -- MOA in Parkinson's disease
Blocks acidification -- important at two stages
- M2 membrane protein (an ion channel) responsible
- Entry -- Inhibits fusion of viral membrane and endosome membrane
- Exit -- Inhibits late stage assembly and release of virus
Serial passage can create resistant strains

Pharmacokinetics

PO
Well absorbed
Human: Dose 200 mg gives 0.3 to 0.6 mcg/mL peak concentration
Eliminated unchanged in urine

Adverse Effects

Dose related and appear above 1-5 mcg/mL
CNS toxicity
- 1-5% patients wtih normal renal function
- nervousness, confusion, hallucinations, seizures, and coma

Uses

Highly selective
Influenza A viruses (H1 N1, H2, N2, and H3 N2)
NOT clinically active versus influenza B viruses
Prophylactically in midst of influenza epidemic
- 200 mg/day -- 70-80% effective
- 100 mg/day -- protect children and elderly.
Therapeutically
- Accelerates recovery from acute signs and fever
- Main problem -- diagnosis
Widely used in Parkinson's disease
Successful in experimental infections in turkeys and horses -- (Brander91)

AMADE90,5:10 5:11. major reference

Acyclovir (Zovirax^R)

Structure and chemistry

Acyclic purine nucleoside analogue of guanosine

Mechanism of action

AMADE90,5:7
Relatively large margin of safety
- Best example of selective antiviral drug!
Prodrug -- must be activated
Activated by herpesvirus thymidine kinase
- Activation -- conversion of acyclovir to acyclovir monophosphate (acyc-MP)
- binds 200 times more strongly than to host TK
- phosphorylates 3 million times faster than host TK
- Rate of conversion 30 to 120-fold faster in infected cells
- THEREFORE -- primarily activated in infected cells
Acyclo-MP to acyclo -acyclo-Diphosphate then to TriphosphateP by host enzymes
Viral DNA polymerase -- inhibited more than host enzyme
- Acyclo-TP incorporated into DNA
  - No 3'-hydroxyl so chain terminated
  - Viral DNA polymerase more incorporatin than host polymerase
  - Terminated chain remains bound to polymerase which is inhibited
- Therefore, more terminated chains in virus than host leading to more DNA polymerase inhibition

Pharmacokinetics

PO and IV
Widely distributed, including CSF
Probenecid increases half-life by 20%

Adverse effects

Few side effects inhumans
Nausea with PO
Tissue irritation if extravasation on IV

Therapeutic application

Essentially only for herpes viruses (especially H.simplex type 1
AMADE90,5:7
Rx of Choice -- herpes simplex encephalitis

Efficacy in Meningitis¹

Survival at -- Acyclovir Vidarabine

one months 87 57

six months 81 46

Overall 72 46

¹Herpes simplex encephalitis
PO in Immunocompetent patients
- primary Herpes genitalis
  - PO effective, but
  - NOT reduce rate of asymptomatic viral shedding
- Herpes labialis
  - PO reduces pain and healing time, but
  - not affect new lesion development.
- Herpes zoster
  - PO and IV reduce pain and accelerate healing
- Varicella-zoster infections -- PO form
ImmunoCOMPROMISED adults and children
- IV form for varicella-zoster infections
Topical
- Mucotaneous herpesvirus infections
- May be useful for primary genital herpes, less for recurrent (PO better)
- Herpes simplex keratitis

Efficacy in Meningitis¹
Survival at --	Acyclovir	Vidarabine
one months	87	57
six months	81	46
Overall	72	46
¹Herpes simplex encephalitis

Ganciclovir (Cytovene^R, (DHPG))

Conceptually similar to acyclovir
Lower margin of safety than acyclovir
Highly efficacious and important drug
See full notes for more detail

Mechanism of action

Prodrug
Activated drug incorporated into BOTH host and viral DNA
Predict toxicity by rapidly dividing tissues
- bone marrow, GI mucosa, and gonads (at all doses tested)

Uses

Potent and broad spectrum vs herpes viruses, especially CMV
FDA approved for CMV retinitis in immuncompromised patients

AMADE90,5:7-5:8

Ribavarin, (Virazole^R)

Effective against BOTH DNA and RNA viruses
Prodrug that must be phosphorylated
Phosphorylated derivatives inhibit viral nucleic acid synthesis
Teratogenic in experimental animals
Generally low toxicity

Idoxuridine

Herplex^R, Stoxil^R
Related drug: Trifluridine, Viroptic^R

First generation antiviral
NOT ACCEPTABLE FOR PARENTERAL USE!
Thymidine analogue
Prodrug phosphorylated by thymidine kinases to active triphosphates which inhibit DNA synthesis of virus and host.
Bone marrow toxicity if systemic
Herpes simplex keratitis. Trifluridine may be superior.

AMADE90,p5:5

Zidovudine

Retrovir^R, Apo-Zidovudine^R, Novo-AZT^R
(AZT, Azidothymidine, 3'-Azido-3'-deoxythymidine)

Overview

First and most important drug for palliation of AIDS
Active against HIV-1 and other mammalian retroviruses

Mechanism of action

Decreases reverse transcriptase activity in cultured cells at 0.013 mcg/mL
Inhibits replication of HIV-1 virus
- Exogenously infected cells -- at 0.02 to 1.3 mcg/mL
- Requires higher concentrations in chronically infected cells
Some activity against human lymphotrophic virus type 1 (HTLV-1)
Prodrug that must be phosphorylated
- Phosphorylated to triphosphate by cellular enzymes
- inhibits viral RNA-directed DNA polymerase (transcriptase)
  - Triphosphate binds more strongly to viral than eukaryotic DNA polymerase
- DNA chain termination
  - azidothymidine triphosphate can be incorporated into DNA
  - Nucleotides cannot be added to modified 3'-position
- Mammalian DNA polymerases
  - Alpha-DNA polymerase relatively resistant to incorporating azido-TP
  - Gamma DNA polymerase in mitochondria does incorporate -- source of toxicity? (Rang et al. 95, p38)
Activity enhanced by --
- acyclovir, interferon, dideoxyadenosine, granulocyte-macrophage colony stimulating factor, neutralizing antibody, and other newer drugs
Activity decreased by --
- thymidine and ribivarin
- (?) competition for phosphorylation, thus decreasing activation(?)

Resistant strains

Isolated from AIDs patients treated 6 months or more
Still sensitive to dideoxycytidine and foscarnet

Pharmacokinetics

PO -- F=60-65% (60-80% [Rang'95,p750])
Peak concentration 30 to 90 minutes.
Wide distribution
Usual dosage is 200 mg q4h!! continuously!!

Adverse effects

Prophylactive use -- minor, reversible adverse effects
Higher dose, long term use -- effects significant and serious
Hematologic
- Current rates less than these as doses have been reduced
- Granulocytopenia and anemia in up to 45% of patients
  - Risk factors increase probability
- Anemia may occur --
  - 2 to 4 weeks into therapy
  - 6 weeks into therapy more common
  - 30% require transfusions.
  - Granulocytopenia occurs after 6 to 8 weeks
  - Monitor hematologic parameters q2w.
- Other occur sometimes
  - Gastrointestinal disturbances, paresthesia, skin rash, insomnia, fever, headaches, abnormalities of liver function
  - Myopathy
  - Confusion, anxiety, depression, and flu-like syndrome
- Drug interaction -- increased risk
  - Drugs that inhibit glucuronyl transferase
    - acetaminophen, aspirin, indomethacin, probenecid Probenecid has action here in addition to effect on renal tubular secretion.
  - Renal excretion -- Probenecid slows

Clinical uses

Patients with AIDS -- reduces
- opportunistic infections, e.g., Pneumocystis carinii pneumonia
- stabilizes weight
- reduces HIV-associated thrombocytopenia
- stabilizes HIV-associated dementia
- reduces viral burden
HIV positive patients before onset of AIDS
- Delays progession of disease
  - by 12-18 months
- May not increase median survival time beyond typical 3 yrs from diagnosis
HIV-positive mothers
- reduces risk of transmission to fetus by 66%
After specific exposure to HIV virus
- Recommend giving zidovudine, but benefit not proven
Final value -- not certain

ref: Rang et al'95 p 750

Zalcitabine

HIVID^R (dideoxycytidine, ddC)

New drug
Similar conceptually to zidovudine
Reverse transcriptase inhibitor
Activated in T cell different path than zidovudine
Dose related neuropathy
- Can worsen after stop therapy
Used in combination with zidovudine in late AIDS

New Products / Approaches for AIDS

Nucleoside analogs
- 3TC (Glaxo Wellcome Inc, FDA approved around '95)
  - Synergistic with zidovudine
  - sustained antiviral effect
  - Combo is new "first-line" therapy for AIDS
- didanosine, Videx^R, ddI
- zalcitabine (ddC)
- stavudine, Zerit^R, d4T
Protease inhibitors [Neergaard, J&C 3/96]
- Long term usefulness still not known
- saquinavir (Hoffman LaRoche, FDA approved 12/95)
- ritonavir (Abbott Labs., FDA approved 3/96)
- indinavir (Merck & Co, FDA approved 3/96)
- nelfinavir (Agouron Inc, FDA approval expected '97)
Combinations -- exciting current development [Neergaard, J&C 3/96]
Use of these with nucleoside analogs decreases blood HIV by up to 98%
In 3/96 recommended therapy to postpone AIDS includes--
- zidovudine
- 3TC
- ritonavir
Other combinations being tested, some 4 drug mixtures

New class (nature not defined in article)

Drugs in testing stage
nevaripine
delvaripine

Interferons

Family of inducible proteins of at least 3 types

Produced by B and T lympocytes, macrophages and fibroblasts in response to viruses and cytokines
- Alpha-interferon -- smallest at 20,000 Daltons
- Beta-interferon -- ~28,000 Daltons
Cytokines produced by T lymphocytes in response to antigens, including many infectious agents
- Gamma-interferon -- ~70,000 Daltons

Mechanism of action

Interferons relatively species specific
MOA may differ with interferon
Unexpected actions of interferons
- enhance cytotoxic activities of lymphocytes
- inhibit cell proliferation
- inhibit expression of cell surface antigens,
- inhibit phagocytic and tumoricidal activities of macrophages
Pharmacokinetics
- Given IV or IM
- Half-life -- 2-4 hr
- Not cross blood-brain barrier
Adverse effects
- Alpha-Interferon (Alpha-IFN) given IM or IV
  - influenza-like signs
    - fever, headache, malaise, myalgia
    - Alpha-IFN may be cause of these signs in illness
  - Bone marrow suppression -- common
    - thrombocytopenia
    - granulocytopenia
  - Neurotoxicity
  - See references for more signs
Uses
- Approved by FDA: Interferon alfa-2a ROFERON-A^R and Interferon alfa-2b Intron A^R
  - hairy-cell leukemia,
  - AIDS-related Kaposi's sarcoma
  - genital warts
  - hepatitis B
Interferon inducers
- Generally not found to be safe and effective
- Agents tried include
  - certain microorganisms (e.g., viruses, Rickettsia, Mycoplasma, coliform bacteria),
  - microbial extracts
  - certain dyes (e.g., methylene blue, acridince orange),
  - synthetic polymers.
AMADE90,5:3-5:4
References:
- GG8th90 Douglas, R.G., Jr, Antiviral Agents, Chapter 51, in Goodman & Gilman's The Pharmacological Basis of Therapeutics, 8th ed, eds. A.G. Gilman, T.W. Rall, A.S. Nies, P. Taylor, Pergamon Press, 1990.
- Prescott & Baggot, 1988 Antimicrobial Therapy in Veterinary Medicine, Blackwell Scientific Publications, Boston.
- Brander91: Brander, G.C., D.M. Pugh, R.J. Bywater, & W.L. Jenkins. 1991. Chapter 35, Antifungal and Antiviral agents. in Veterinary Applied Pharmacology & Therapeutics, 5th ed. . Bailliere Tindall, London.
- AMADEnn American Medical Association Drug Evaluations Subscription, 1990 to present. Date reflects date on page used in citation, e.g., AMADE90 indicates 1990.
- Rang95: Antiviral Drugs, Chapter 38 in Pharmacology, eds: HP Rang, MM Dale, JM Ritter, P Gardner, Churchill Livingstone, New York, 1995, pp. 744-755
- Heit/Riviere95-Adams95: Heit, M.C., and J.E. Riviere, 1995, Antifungal and Antiviral Drugs, Ch 44, in Veterinary Pharmacology and Therapeutics, 7th ed, ed. H. Richard Adams, Iowa State University Press, Ames, IA, pp. 855-869
- Neergaard, J&C'96: Neergaard, Lauran, 1996, AIDS faces swarm of antagonists, Lafayette Journal & Courier, March 25, page A4.
| Drug Groups | | top |
Gordon L. Coppoc, DVM, PhD
Professor of Veterinary Pharmacology
Head, Department of Basic Medical Sciences
School of Veterinary Medicine
Purdue University
West Lafayette, IN 47907-1246
Tel: 317-494-8633Fax: 317-494-0781
Email: coppoc@vet.purdue.edu

Last modified
12:00 AM on 4/21/96
GLC

ANTIVIRALS Outline

Quick Review

RNA Viruses:

DNA Viruses:

Tidbits

Problem with treating viral infections

Process unique to viruses

General Treatment of Viral Infections

Acquired Resistance to Antiviral Drugs

Overview of Drugs

Overview of Life Cycle of a DNA VIrus (e.g., herpes)

Amantadine (SymmetrelR)

Structure and chemistry

Pharmacokinetics

Adverse Effects

Uses

Acyclovir (ZoviraxR)

Structure and chemistry

Mechanism of action

Pharmacokinetics

Adverse effects

Therapeutic application

Ganciclovir (CytoveneR, (DHPG))

Mechanism of action

Uses

Ribavarin, (VirazoleR)

Idoxuridine

Zidovudine

Overview

Mechanism of action

Resistant strains

Pharmacokinetics

Adverse effects

Clinical uses

Zalcitabine

New Products / Approaches for AIDS

Interferons

Mechanism of action

Unexpected actions of interferons

Pharmacokinetics

Adverse effects

Uses

Interferon inducers

References:

ANTIVIRALS
Outline

Amantadine (Symmetrel^R)

Acyclovir (Zovirax^R)

Ganciclovir (Cytovene^R, (DHPG))

Ribavarin, (Virazole^R)