Antiviral Drugs: Introduction

Antiviral Drugs: Introduction

Compared with the number of drugs available to treat bacterial infections, the number of antiviral drugs is very small. The major reason for this difference is the difficulty in obtaining selective toxicity against viruses; their replication is intimately involved with the normal synthetic processes of the cell. Despite the difficulty, several virus-specific replication steps have been identified that are the site of action of effective antiviral drugs
Another limitation of antiviral drugs is that they are relatively ineffective because many cycles of viral replication occur during the incubation period when the patient is well. By the time the patient has a recognizable systemic viral disease, the virus has spread throughout the body and it is too late to interdict it. Furthermore, some viruses, e.g., herpesviruses, become latent within cells, and no current antiviral drug can eradicate them.

Another potential limiting factor is the emergence of drug-resistant viral mutants. At present, this is not of major clinical significance. Mutants of herpesvirus resistant to acyclovir have been recovered from patients, but they do not interfere with recovery.

Inhibition of Early Events

Amantadine (-adamantanamine, Symmetrel) is a three-ring compound (Figure 35–1) that blocks the replication of influenza A virus. It prevents replication by inhibiting uncoating of the virus by blocking the "ion channel" activity of the matrix protein (M2 protein) in the virion. Absorption and penetration occur normally, but transcription by the virion RNA polymerase does not because uncoating cannot occur. This drug specifically inhibits influenza A virus; influenza B and C viruses are not affected.
Despite its efficacy in preventing influenza, it is not widely used in the United States because the vaccine is preferred for the high-risk population. Furthermore, most isolates have become resistant to amantadine. The main side effects of amantadine are central nervous system alterations such as dizziness, ataxia, and insomnia. Rimantadine (Flumadine) is a derivative of amantadine and has the same mode of action but fewer side effects.

Enfuvirtide (Fuzeon) is a synthetic peptide that binds to gp41 on the surface of the virion, thereby blocking the entry of human immunodeficiency virus (HIV) into the cell. It is the first of a new class of anti-HIV drugs known as "fusion inhibitors," i.e., they prevent the fusion of the viral envelope with the cell membrane.

Maraviroc (Selzentry) blocks the binding of HIV to CCR-5—an important coreceptor for those strains of HIV that use CCR-5 for entry into the cell. The drug binds to CCR-5 and blocks the interaction of gp120, an HIV envelope protein, to CCR-5 on the cell surface.

Inhibition of Viral Nucleic Acid Synthesis

Inhibitors of Herpesviruses

Nucleoside Inhibitors

These drugs are analogues of nucleosides that inhibit the DNA polymerase of one or more members of the herpesvirus family. For example, acyclovir inhibits the DNA polymerase herpes simplex virus types 1 and 2 (HSV-1 and -2) and varicella-zoster virus but not cytomegalovirus (CMV).

Acyclovir—Acyclovir (acycloguanosine, Zovirax) is a guanosine analogue that has a three-carbon fragment in place of the normal sugar, ribose, that has five carbons (Figure 35–1). The term "acyclo" refers to the fact that the three-carbon fragment does not have a sugar ring structure (a = without, cyclo = ring).

Acyclovir is active primarily against HSV-1 and -2 and varicella-zoster virus (VZV). It is relatively nontoxic, because it is activated preferentially within virus-infected cells. This is due to the virus-encoded thymidine kinase, which phosphorylates acyclovir much more effectively than does the cellular thymidine kinase. Because only HSV-1, HSV-2, and VZV encode a kinase that efficiently phosphorylates acyclovir, the drug is active primarily against these viruses. It has no activity against CMV. Once the drug is phosphorylated to acyclovir monophosphate by the viral thymidine kinase, cellular kinases synthesize acyclovir triphosphate, which inhibits viral DNA polymerase much more effectively than it inhibits cellular DNA polymerase. Acyclovir causes chain termination because it lacks a hydroxyl group in the 3' position.

To recap, the selective action of acyclovir is based on two features of the drug. (1) Acyclovir is phosphorylated to acyclovir monophosphate much more effectively by herpesvirus-encoded thymidine kinase than by cellular thymidine kinase. It is therefore preferentially activated in herpesvirus-infected cells and much less so in uninfected cells, which accounts for its relatively few side effects. (2) Acyclovir triphosphate inhibits herpesvirus-encoded DNA polymerase much more effectively than it does cellular DNA polymerase. It therefore inhibits viral DNA synthesis to a much greater extent than cellular DNA synthesis (Figure 35–2).

Topical acyclovir is effective in the treatment of primary genital herpes and reduces the frequency of recurrences while it is being taken. However, it has no effect on latency or on the rate of recurrences after treatment is stopped. Acyclovir is the treatment of choice for HSV-1 encephalitis and is effective in preventing systemic infection by HSV-1 or VZV in immunocompromised patients. It is not effective treatment for HSV-1 recurrent lesions in immunocompetent hosts.

Acyclovir-resistant mutants have been isolated from HSV-1- and VZV-infected patients. Resistance is most often due to mutations in the gene encoding the viral thymidine kinase. This results in reduced activity of or the total absence of the virus-encoded thymidine kinase.

Acyclovir is well-tolerated and causes few side effects—even in patients who have taken it orally for many years to suppress genital herpes. Intravenous acyclovir may cause renal or central nervous system toxicity.

Derivatives of acyclovir with various properties are now available. Valacyclovir (Valtrex) achieves a high plasma concentration when taken orally and is used in herpes genitalis and in herpes zoster. Penciclovir cream (Denavir) is used in the treatment of recurrent orolabial herpes simplex. Famciclovir (Famvir) when taken orally is converted to penciclovir and is used to treat herpes zoster and in herpes simplex infections.

Ganciclovir—Ganciclovir (dihydroxypropoxymethylguanine, DHPG, Cytovene) is a nucleoside analogue of guanosine with a four-carbon fragment in place of the normal sugar, ribose (Figure 35–1). It is structurally similar to acyclovir but is more active against CMV than is acyclovir. Ganciclovir is activated by a CMV-encoded phosphokinase in a process similar to that by which HSV activates acyclovir. Isolates of CMV resistant to ganciclovir have emerged, mostly due to mutations in the UL97 gene that encodes the phosphokinase.

Ganciclovir is effective in the treatment of retinitis caused by CMV in AIDS patients and may be useful in other disseminated infections, such as colitis and esophagitis, caused by this virus. The main side effects of ganciclovir are leukopenia and thrombocytopenia as a result of bone marrow suppression. Valganciclovir, which can be taken orally, is also effective against CMV retinitis.

Cidofovir—Cidofovir (hydroxyphosphonylmethoxypropylcytosine, HPMPC, Vistide) is a nucleoside analogue of cytosine that lacks a ribose ring. It is useful in the treatment of retinitis caused by CMV and in severe human papillomavirus infections. It may be useful in the treatment of severe molluscum contagiosum in immunocompromised patients. Kidney damage is the main side effect.

Vidarabine—Vidarabine (adenine arabinoside, ara-A) is a nucleoside analogue with arabinose in place of the normal sugar, ribose. On entering the cell, the drug is phosphorylated by cellular kinases to the triphosphate, which inhibits the herpesvirus-encoded DNA polymerase more effectively than the cellular DNA polymerase. Vidarabine is effective against HSV-1 infections such as encephalitis and keratitis but is less effective and more toxic than acyclovir.

Iododeoxyuridine—Iododeoxyuridine (idoxuridine, IDU, IUDR) is a nucleoside analogue in which the methyl group of thymidine is replaced by an iodine atom (Figure 35–1). The drug is phosphorylated to the triphosphate by cellular kinases and incorporated into DNA. Because IDU has a high frequency of mismatched pairing to guanine, it causes the formation of faulty progeny DNA and mRNA. However, because IDU is incorporated into normal cell DNA as well as viral DNA, it is too toxic to be used systemically. It is clinically useful in the topical treatment of keratoconjunctivitis caused by herpes simplex virus, but in the United States, trifluorothymidine (see below) is the drug of choice.

Trifluorothymidine—Trifluorothymidine (trifluridine, Viroptic) is a nucleoside analogue in which the methyl group of thymidine contains three fluorine atoms instead of three hydrogen atoms. Its mechanism of action is probably similar to that of IDU. Like IDU, it is too toxic for systemic use but is clinically useful in the topical treatment of keratoconjunctivitis caused by herpes simplex virus.
Nonnucleoside Inhibitors

These drugs inhibit the DNA polymerase of herpesviruses by mechanisms distinct from the nucleoside analogues described above. Foscarnet is the only approved drug in this class at this time.

Foscarnet—Foscarnet (trisodium phosphonoformate, Foscavir), unlike the previous drugs, which are nucleoside analogues, is a pyrophosphate analogue. It binds to DNA polymerase at the pyrophosphate cleavage site and prevents removal of the phosphates from nucleoside triphosphates (dNTP). This inhibits the addition of the next dNTP and, as a consequence, the extension of the DNA strand. Foscarnet inhibits the DNA polymerases of all herpesviruses, especially HSV and CMV. Unlike acyclovir, it does not require activation by thymidine kinase. Foscarnet also inhibits the reverse transcriptase of HIV. It is useful in the treatment of retinitis caused by CMV, but ganciclovir is the treatment of first choice for this disease. Foscarnet is also used to treat patients infected with acyclovir-resistant mutants of HSV-1 and VZV.

Inhibitors of Retroviruses

Nucleoside Inhibitors

The selective toxicity of azidothymidine, dideoxyinosine, dideoxycytidine, d4T, and 3TC is based on their ability to inhibit DNA synthesis by the reverse transcriptase of HIV to a much greater extent than they inhibit DNA synthesis by the DNA polymerase in human cells. The effect of these drugs on the replication of HIV is depicted in Figure 45–3.

Azidothymidine—Azidothymidine (zidovudine, Retrovir, AZT) is a nucleoside analogue that causes chain termination during DNA synthesis; it has an azido group in place of the hydroxyl group on the ribose (Figure 35–1). It is particularly effective against DNA synthesis by the reverse transcriptase of HIV and inhibits the growth of the virus in cell culture. The main side effects of AZT are bone marrow suppression and myopathy.

Dideoxyinosine—Dideoxyinosine (didanosine, Videx, ddI) is a nucleoside analogue that causes chain termination during DNA synthesis; it is missing hydroxyl groups on the ribose. The administered drug ddI is metabolized to ddATP, which is the active compound. It is effective against DNA synthesis by the reverse transcriptase of HIV and is used to treat patients with AIDS who are intolerant of or resistant to AZT. The main side effects of ddI are pancreatitis and peripheral neuropathy.

Dideoxycytidine—Dideoxycytidine (zalcitabine, Hivid, ddC) is a nucleoside analogue that causes chain termination during DNA synthesis; it is missing hydroxyl groups on the ribose. The administered drug ddC is metabolized to ddCTP, which is the active compound. It is effective against DNA synthesis by the reverse transcriptase of HIV and is used to treat patients who are intolerant of or resistant to AZT. The main side effects of ddC are the same as those of ddI but occur less often.

Stavudine—Stavudine (d4T, Zerit) is a nucleoside analogue that causes chain termination during DNA synthesis. It inhibits DNA synthesis by the reverse transcriptase of HIV and is used to treat patients with advanced AIDS who are intolerant of or resistant to other approved therapies. The molecular name of stavudine is didehydrodideoxythymidine. The main side effect is peripheral neuropathy.

Lamivudine—Lamivudine (3TC, Epivir) is a nucleoside analogue that causes chain termination during DNA synthesis by the reverse transcriptase of HIV. When used in combination with AZT, it is very effective both in reducing the viral load and in elevating the CD4 cell count. The molecular name of lamivudine is dideoxythiacytidine. Lamivudine is also used in the treatment of chronic hepatitis B. It is one of the best-tolerated of the nucleoside inhibitors, but side effects such as neutropenia, pancreatitis, and peripheral neuropathy do occur.

Abacavir—Abacavir (Ziagen) is a nucleoside analogue of guanosine that causes chain termination during DNA synthesis. It is available through the "expanded access" program to those who have failed currently available drug regimens. Abacavir is used in combination with either a protease inhibitor or AZT plus lamivudine.

Tenofovir—Tenofovir (Viread) is an acyclic nucleoside phosphonate that is an analogue of adenosine monophosphate. It is a reverse transcriptase inhibitor that acts by chain termination. It is approved for use in patients who have developed resistance to other reverse transcriptase inhibitors and in those who are starting treatment for the first time. It should be used in combination with other anti-HIV drugs.

Nonnucleoside Inhibitors

Unlike the drugs described above, the drugs in this group are not nucleoside analogues and do not cause chain termination. The nonnucleoside reverse transcriptase inhibitors (NNRTI) act by binding near the active site of the reverse transcriptase and inducing a conformational change that inhibits the synthesis of viral DNA. NNRTIs should not be used as monotherapy because resistant mutants emerge rapidly. Strains of HIV resistant to one NNRTI are usually resistant to others as well. NNRTIs are typically used in combination with one or two nucleoside analogues.

Nevirapine—Nevirapine (Viramune) is usually used in combination with zidovudine and didanosine. There is no cross-resistance with the nucleoside inhibitors of reverse transcriptase described above. The main side effect of nevirapine is a severe skin rash (Stevens-Johnson syndrome). Nevirapine is a member of a class of drugs called the dipyridodiazepinones; its precise name is beyond the scope of this book.

Delavirdine—Delavirdine (Rescriptor) is effective in combination with either zidovudine or zidovudine plus didanosine. Delavirdine is a member of a class of drugs called bisheteroarylpiperazines; its precise name is beyond the scope of this book.

Efavirenz—Efavirenz (Sustiva) in combination with zidovudine plus lamivudine was more effective and better tolerated than the combination of indinavir, zidovudine, and lamivudine. The most common side effects are referable to the central nervous system, such as dizziness, insomnia, and headaches. Efavirenz is a member of a class of drugs called benzoxazin-2-ones; its precise name is beyond the scope of this book.

Inhibitors of Hepatitis B Virus

Adefovir

Adefovir (Hepsera) is a nucleotide analogue of adenosine monophosphate that inhibits the DNA polymerase of hepatitis B virus. It is used for the treatment of chronic active hepatitis caused by this virus.

Entecavir

Entecavir (Baraclude) is a guanosine analogue that inhibits the DNA polymerase of hepatitis B virus (HBV). It has no activity against the DNA polymerase (reverse transcriptase) of HIV. It is approved for the treatment of adults with chronic HBV infection.

Lamivudine

Lamivudine is described under section "Inhibitors of Retroviruses."

Telbivudine

Telbivudine (Tyzeka) is a thymidine analogue that inhibits the DNA polymerase of HBV but has no effect on the reverse transcriptase of HIV. It is useful in the treatment of chronic HBV infection.

Inhibitors of Other Viruses

Ribavirin

Ribavirin (Virazole) is a nucleoside analogue in which a triazole-carboxamide moiety is substituted in place of the normal purine precursor aminoimidazole-carboxamide (Figure 35–1). The drug inhibits the synthesis of guanine nucleotides, which are essential for both DNA and RNA viruses. It also inhibits the 5' capping of viral mRNA. Ribavirin aerosol is used clinically to treat pneumonitis caused by respiratory syncytial virus in infants and to treat severe influenza B infections. Ribavirin is also used in combination with -interferon (peg-interferon) for the treatment of hepatitis C.

Inhibition of Integrase

Raltegravir (Isentress) is an integrase inhibitor, i.e., it blocks the HIV-encoded integrase that mediates the integration of the newly synthesized viral DNA into host cell DNA.

Inhibition of Cleavage of Precursor Polypeptides

Members of the protease inhibitor (PI) class of drugs, such as saquinavir (Invirase, Fortovase), indinavir (Crixivan), ritonavir (Norvir), lopinavir/ritonavir (Kaletra), azatanavir (Reyataz), tipranavir (Aptivus), amprenavir (Agenerase), darunavir (Prezista), and nelfinavir (Viracept) inhibit the protease encoded by HIV (Figure 35–3). The protease cleaves the gag and pol precursor polypeptides to produce several nucleocapsid proteins and enzymatic proteins, e.g., reverse transcriptase, required for viral replication. These inhibitors contain peptide bonds that bind to the active site of the viral protease, thereby preventing the protease from cleaving the viral precursor. These drugs inhibit production of infectious virions but do not affect the proviral DNA and therefore do not cure the infection. The effect of protease inhibitors on the replication of HIV is depicted in Figure 45–3.