ANTI VIRAL AND ANTIRETROVIRAL DRUGS

Presented By-Krushangi Shah,Nirma University.

Viruses

• Viruses are the smallest infective agents, consisting essentially of nucleic acid (either RNA or DNA) enclosed in a protein coat or capsid.

• A virus cannot replicate on its own. It must attach to and enter a host cell. It then uses the host cell’s energy to synthesize protein, DNA, and RNA. Viruses are difficult to kill because they live inside the cells. Any drug that kills a virus may also kill cells

Some important examples of viruses and the diseases they cause are as follows:

DNA viruses• Pox viruses (smallpox)• Herpes viruses (chickenpox,

herpes etc.)• Adenoviruses (sore throat,

conjunctivitis)• Hepadnaviruses (serum

hepatitis)• Papillomaviruses (warts)

RNA viruses•Orthomyxoviruses (influenza)•Paramyxoviruses (measles, mumps)•Rubella virus (German measles)•Rhabdoviruses (rabies)•Picornaviruses (colds, meningitis, poliomyelitis)•Retroviruses (AIDS, T-cell leukaemia)•Arenaviruses (meningitis, Lassa fever)•Arboviruses (arthropod-borne encephalitis, yellow fever)

Replication in viruses• Replication in DNA viruses

Viral DNA enters the host cell nucleus, where transcription into mRNA by the host cell RNA polymerase occurs. Translation of the mRNA into virus-specific proteins takes place. These proteins are enzymes that then synthesise more viral DNA, as well as proteins for the viral coat and envelope. After assembly of coat proteins around the viral DNA, complete virions are released by budding or after host cell lysis.

• Replication in RNA virusesEnzymes within the virion synthesise its mRNA from the viral RNA template, or sometimes the viral RNA serves as its own mRNA. This is translated by the host cell into various enzymes, including RNA polymerase and also into structural proteins of the virion. Assembly and release of virions occurs as explained above. With these viruses, the host cell nucleus is usually not involved in viral replication, although some RNA viruses (e.g. orthomyxoviruses) replicate exclusively within the host nuclear compartment.

• Replication in retrovirusesThe virion in retroviruses contains a reverse transcriptase enzyme (virus RNA-dependent DNA polymerase), which makes a DNA copy of the viral RNA. This DNA copy is integrated into the genome of the host cell, and it is then termed a provirus. The provirus DNA is transcribed into both new viral genome RNA as well as mRNA for translation in the host into viral proteins, and the completed viruses are released by budding. Many retroviruses can replicate without killing the host cell.

Classification of antiviral drugs according to their therapeutic uses

• Anti-herpes virus agents Acyclovir, Famcyclovir, Gancyclovir, Idoxuridine, Foscarnet, Fomivirsen, Pencyclovir, Trifluridine, Tromantadine, Valacyclovir, Valgancyclovir, Vidarabine, Cidofovir, Docosanol

• Anti-influenza AgentsAmantadine, Oseltamivir, Peramivir, Rimantadine, Zanamivir

• Other antiviral agentsFomivirsen, Enfuvirtide, Imiquimod, Interferon, Ribavirin, Viramidine

• Antiretroviral Agents NRTIs: Zidovudine, Didanosine, Stavudine, Zalcitabine, Lamivudine, Abacavir,

Tenofovir NNRTIs: Nevirapine, Efavirenz, Delavirdine Protease Inhibitors: Saquinavir, Indinavir, Atazanavir, Ritonavir, Nelfinavir,

Amprenavir, Lopinavir, Tipranavir

CLASSIFICATION OF ANTIVIRAL DRUGS ACCORDING TO THE MECHANISM OF ACTIONThe viral growth cycle Selective inhibitors

1) Attachment2) Penetration

- Antiviral antibodies(gamma globulin)

3) Uncoating - Amantadine, Rimantadine - Interferon

4) Early translation(early mRNA and protein synthesis)

- fomivirsen

5) Transcription(viral genome replication)

Inhibitors of DNA-polymerase-Acyclovir, Gancyclovir, Famcyclovir, Cidofovir- Vidarabine, Idoxuridine -FoscarnetInhibitors of RNA-dependant DNA-polymerase(reverse transcriptase inhibitors)-Zidovudine, Stavudine, Lamivudine- Didanosine, Zelcitabine

6) Late translation(late mRNA an protein synthesis)

- Ribavirin -Interferons

7) Posttranslational modifications(proteolytic cleavage)

Protease inhibitors- Saquinavir, Indinavir, Ritonavir

8) Assembly(packaging of viral nucleic acids)

-Interferons -Rifampin

9) Release(virion is releasing from cell) (budding)

-Antiviral antibodies- Cytotoxic T lymphocytesNeuraminidase Inhibitors-Zanamivir, Oseltamivir

Mechanism of action of antiviral drugs

Inhibition of penetration of host cell• Amantadine and also Rimantadine

inhibit uncoating and are effective against influenza A virus.

• Amantadine CNS effects: insomnia, nervousness, lightheadedness.GI effects: anorexia, nausea, others.

• RimantadineSame spectrum of activity, mechanism of action, and indications as amantadine. Fewer CNS adverse effects. Causes GI upset.

• Gammaglobulins neutralize viruses.

Antiviral mechanism of action: Inhibition of nucleic acid synthesis I

• Acyclovir a guanosine derivative selectively inhibits viral DNA polymerase; minimal unwanted effects. Used to suppress replication of:– HSV-1(oral herpes), HSV-2(genital herpes), VZV (chickenpox or shingles),

To some extend EBV and CMV. • Gancyclovir, also a guanosine derivative is phosphorylated and then

incorporated into viral DNA, suppressing its replication; used in cytomegalovirus infections; especially CMV retinitis in AIDS patients. CMV retinitis– Ophthalmic form surgically implanted.– Ocular injection (fomivirsen).Dose-Limiting Toxicities

• Gancyclovir and zidovudine– Bone marrow toxicity.

• Foscarnet and cidofovir– Renal toxicity.

Antiviral mechanism of action: Inhibition of nucleic acid synthesis II

• Vidarabine, an adenosine derivative, is a relatively selective inhibitor of viral DNA polymerase; effective against Herpes simplex and Varicella zoster; can have serious unwanted effects.

• Ribavirin is similar to guanosine and is thought to interfere with synthesis of viral DNA; can inhibit many DNA and RNA viruses. Given orally, or oral or nasal inhalation.Inhalation form (Virazole) used for hospitalized infants with RSV (respiratory syncytial virus) infections.

Antiviral mechanism of action: Inhibition of nucleic acid synthesis III

• Foscarnet inhibits viral DNA polymerase by attaching to the pyrophosphate binding site; approved for CMV infections.

• Zidovudine, an analogue of thymidine, inhibits reverse transcriptase and is relatively effective in HIV infection.

• Zalcitabine and didanosine are reverse transcriptase inhibitors used in HIV infection.

Neuraminidase inhibitors

• Zanamivir and oseltamivir inhibits neuraminidase which is essential for virus replication. They are used against influenza A and B. They are for avian and swine influenza (H5N1 and H1N1 respectively) which are influenza A subtypes. Zanamivir is used by inhalation; oseltamivir orally.

• oseltamivir: causes nausea & vomiting

• Zanamivir: causes diarrhea, nausea, sinusitis

Immunomodulators

• Interferons induce, in the host cells’ ribosomes, enzymes which inhibit viral mRNA; Interferon has broad spectrum anti-viral activity HSV 1 & 2, HZV, HPV (DNA viruses), influenza, chronic hepatitis, common cold, breast cancer, lung cancer, Karposi’s sarcoma (cancer associated with AIDS) (RNA viruses).

• ADRs: Flue like symptoms, neurotoxicity, myelosuppression etc.• Pavilizumab is a monoclonal

antibody against fusion proteins of respiratory syncytial virus.

• Imiquimod is an immune response modulator used topically against genital and perianal warts.

The human immunodeficiency virus (HIV) and AIDS

• Infection with HIV results in the acquired immune deficiency syndrome (AIDS). In 1997 it was estimated that nearly 30 million adults world-wide were infected with HIV, with the number of HIV infections increasing by five every minute. The epidemic is overwhelmingly centred on sub-Saharan Africa, where about 7% of the population is infected.

The interaction of HIV with the host's immune system

• The interaction of HIV with the host's immune system is complex. Cells of the immune system have the equivalent of “name badges” that identify them. The surface glycoprotein CD4 is the name badge of a particular group of helper T lymphocytes; it also occurs on macrophages and dendritic cells.

Highly active antiretroviral therapy

• The advent of highly active antiretroviral therapy (HAART) has changed the prognosis, in countries that can afford it. HAART is a combination of three antiviral drugs including at least one protease inhibitor. There are 20anti-HIV drugs available; six nucleoside reverse transcriptase inhibitors, four non-nucleoside reverse transcriptase inhibitors, one nucleotide reverse transcriptase inhibitor, eight protease inhibitors, and one fusion inhibitor. Most are discussed below.

• With a HAART regime, HIV replication is inhibited -its presence in the plasma being reduced to undetectable levels-and patient survival is prolonged; but the regime is complex, difficult to adhere to and may well have to be lifelong. It is also extremely expensive-which effectively prevents its use in developing countries. In any case, with the high mutation rate of the virus, resistance is an important problem. HIV has certainly not yet been outsmarted.

Anti-AIDS Drugs Reverse Transcriptase Inhibitors

• Nucleoside:AbacavirDidanosine Lamivudine Stavudine ZalcitabineZidovudine

• Non-nucleoside : Neviparine Delavirdine Efavirenz Loviride

• Nucleotide: Tenefovir

Adefovir

• Protease Inhibitors:IndinavirRitonavirSaquinavirNelfinavirAmprenavir

• Fusion Inhibitor Enfuvirtide

Zidovudine (Azidothymidine, AZT)

• Zidovudine is an analogue of thymidine. In retroviruses-such as the HIV virus-it is an active inhibitor of reverse transcriptase. It is phosphorylated by cellular enzymes to the triphosphate form, in which it competes with equivalent cellular triphosphates which are essential substrates for the formation of proviral DNA by viral reverse transcriptase (viral RNA-dependent DNA polymerase); its incorporation into the growing viral DNA strand results in chain termination.

• Mammalian alpha DNA polymerase is relatively resistant to the effect. However, gamma DNA polymerase in the host cell mitochondria is fairly sensitive to the compound and this may be the basis of unwanted effects.

Pharmacokinetic aspects of zidovudine

• Given orally, the bioavailability of zidovudine is 60-80% and the peak plasma concentration occurs at 30 minutes. It can also be given intravenously. Its half-life is 1 hour, and the intracellular half-life of the active triphosphate is 3 hours. • Zidovudine enters mammalian cells by passive diffusion and

in this is unlike most other nucleosides which require active uptake. The drug passes into the CSF and brain. Most of the drug is metabolized to inactive glucuronide in the liver, only 20% of the active form being excreted in the urine.

Unwanted effects of zidovudine

• Anaemia and neutropenia are common, particularly with long-term administration. Administration of erythropoietin and GM-CSF may alleviate these problems.

• Other unwanted effects include gastrointestinal disturbances, paraesthesia, skin rash, insomnia, fever, headaches, abnormalities of liver function and, more particularly, myopathy. Confusion, anxiety, depression and a flu-like syndrome are also reported. The prophylactic use of the drug, short-term, in fit individuals, after specific exposure to the virus, is associated with only minor, reversible unwanted effects.

Resistance to the antiviral action of zidovudine

• Because of rapid mutation the virus is a constantly moving target, thus the therapeutic response wanes with long-term use, particularly in late-stage disease. Furthermore, resistant strains can be transferred between individuals. Other factors which underlie the loss of efficacy of the drug are decreased activation of zidovudine to the triphosphate and increased virus load due to reduction in immune mechanisms.

Didanosine(dideoxyinosine, ddl)

• Didanosine is a synthetic purine dideoxynucleoside analogue. It is phosphorylated in the host cell to the triphosphate, dideoxyadenosine, in which form it acts as a chain terminator and inhibitor of the viral reverse transcriptase. It is used to treat AIDS.

• Pharmacokinetic aspects of didanosineDidanosine is given orally, rapidly absorbed and is actively secreted by the kidney tubules. The plasma half-life is 30 minutes but the intracellular half-life is more than 12 hours. The cerebrospinal fluid/plasma ratio is 0.2.

Unwanted effects of didanosine

• The main unwanted effect -occurring in >30% of patients-is dose-related pain and sensory loss in the feet. Dose-related pancreatitis occurs in 5-10% of patients and has been fatal in a few cases. Headache and gastrointestinal disturbance are also common and insomnia, skin rashes, bone marrow depression (less marked than with AZT) and alterations of liver function have been reported.

Zalcitabine(dideoxycytidine, ddC)• Zalcitabine, a synthetic nucleoside analogue, is used in

combination with zidovudine for the therapy of AIDS. It is a reverse transcriptase inhibitor and it is activated in the T cell by a different phosphorylation pathway from zidovudine. It is given orally, its plasma half-life is 20 minutes, its intracellular half-life is nearly 3 hours and its cerebrospinal fluid/plasma ratio is ~0.2.

• Unwanted effects of zalcitabineThe most important unwanted effect is a dose-related neuropathy (which can increase for several weeks after the drug has been stopped). Other unwanted effects include gastrointestinal disturbances, headache, mouth ulcers, nail changes, edema of the lower limbs and general malaise. Skin rashes occur but may resolve spontaneously. Pancreatitis has been reported.

Nevirapine• Mechanism Of Action:

– Direct inhibitor of reverse transcriptase without intracellular phosphorylation.

– NNRTIs bind to the Reverse transcriptase near the catalytic site and cause its denaturation.

– More potent on HIV-1 than Zidovudine but not HIV-2.– Cross resistance among themselves but not with others.

• Kinetics:– Administered Orally. Plasma half-life – 20 min. Conc. in CSF – 45% of that in

plasma. Metabolism – Metabolized in the liver and metabolite is excreted in the urine (CYP3A4).

– Nevirapine can prevent mother-to-baby transmission of HIV if given to the parturient mother and the neonate

• Unwanted effects:– Skin rash(17%), Fever, Headaches, Lethargy.– If not monitored carefully: Stevens-Johnson syndrome or Toxic epidermal

necrosis.– Fulminant hepatitis (occasionally)

• Dose: 200 mg/day

Other reverse transcriptase inhibitors

• New nucleoside reverse transcriptase inhibitors now in use include Lamivudine and Stavudine. Non-nucleoside reverse transcriptase inhibitors now in use include nevirapine and delavirdine. All are given orally.

Protease inhibitors

• Host mRNAs code directly for functional proteins, but in HIV, the RNA is translated into biochemically inert polyproteins. A virus-specific protease then converts the polyprotein into various structural and functional proteins by cleavage at the appropriate positions. Since this protease does not occur in the host, it is a good target for chemotherapeutic intervention.

• Several protease inhibitors have now been developed, and their use, in combination with reverse transcriptase inhibitors, has transformed the therapy of AIDS. Examples of current protease inhibitors are: amprenavir, atazanavir, indinavir, fosemprenavir, nelfinavir, ritonavir and saquinavir.

• They are all given orally and all can cause gastrointestinal disorders: nausea, vomiting and diarrhoea. Raised concentrations of liver enzymes in the blood are reported with ritonavir and indinavir. Ritonavir can cause paraesthesia around the mouth, and in the hands and feet, and patients on indinavir may develop kidney stones.

Protease inhibitors(continued)

• All inhibit the cytochrome P450 enzymes (indinavir and saquinavir to a lesser extent than ritonavir) and can interact with other drugs handled by this system, and all can increase the plasma concentration of benzodiazepines. Preliminary evidence suggests that long-term use may lead to an unusual redistribution of cutaneous fat.

• Class-Specific Toxicities:

NRTI Mitochondrial DNA toxicity

NtRTI Proximal renal tubular dysfunction

NNRTI Hypersensitivity, Rash

PI Metabolic disorders

Fusion inhibitors

• Enfuvirtide inhibits the entrance of HIV virus into host cells. It is applied subcutaneously in combination with other anti-AIDS drugs.

Principles of HIV/AIDS therapy

• Monitor plasma viral load and CD4 count.• Start treatment before immunodeficiency becomes

evident.• Aim to reduce plasma viral concentration as much

as possible for as long as possible. • Use combinations of at least three drugs, e.g. two

reverse transcriptase inhibitors and one protease inhibitor.

• Change to a new regime if plasma viral concentration increases.

New Agents to Treat HIV Infection

PhaseReverse

TranscriptaseInhibitors

ProteaseInhibitors

Entry Inhibitors

Integrase/MaturationInhibitors;

Immunologics

2/3 Etravirine (TMC 125)

Darunavir MaravirocVicriviroc

MK-0518

2 BILR 355 BS TNX-355 GS-9137

1/2 AmdoxovirApricitabineRilpivirine (TMC 278)

Brecanavir AMD 070PRO 542

Bevirimat CTLA4-mAb

1 Fosalvudine PPL-100 PRO 140TAK 652

Pre-clinical 22 7 21 11

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