1459CHAPTER 190 Principles of Medical Virology

viral DNA genomes; alternatively, they can use reverse transcription of

RNA followed by PCR to detect a DNA product in clinical samples as a

means to detect viral RNA sequences. Multiple primers can be used in a

multiplex reaction to detect multiple pathogens. The process of nucleic

acid isolation, reverse transcription, and PCR has been automated, and

high-throughput instruments measure the HIV load in serum samples.

HSV-1 DNA can be measured in cerebrospinal fluid as a rapid assay for

HSV encephalitis. These methods have also been transferred to rapid

assays for point-of-care detection of viral genomes.

Viral Antigens Viral antigens can be detected by immunologic

methods such as immunofluorescence and enzyme immunosorbent

assay (EIA). Immunofluorescence involves fixation and permeabilization of cells or tissues from clinical specimens and reaction with

either (1) an antiviral antibody conjugated to a fluorophore (direct

immunofluorescence) or (2) an antiviral antibody followed by an

anti-immunoglobulin antibody conjugated to a fluorophore (indirect

immunofluorescence), with detection of the fluorophore by fluorescence microscopy in either case.

The EIA entails the immobilization of an antiviral antibody on a

substrate such as a microtiter well, incubation of the patient’s sample in

the well, and further incubation with an antibody linked to an enzyme.

The bound enzyme is then measured by production of a colored substrate that can be read spectrophotometrically.

Hemagglutination Some viruses have the ability to cross-link and

agglutinate red blood cells of specific species, a process called hemagglutination. Viral titer is measured by the inverse of the last dilution of

the sample that causes hemagglutination.

Quantitative Assays of Viruses Viruses can be quantified in

terms of virion particle numbers and/or infectivity. The number of

virion particles in a sample can be determined by negative staining and

observation by EM. The numbers of viral DNA genomes can be determined by PCR, and RNA genomes can be determined by reverse transcriptase PCR (RT-PCR), as described above. Alternatively, purified

viral particles can be quantified biochemically by spectrophotometric

assays that measure viral protein.

The number of infectious particles can be quantified by an endpoint

dilution assay in which the virus is diluted until only one-half of cultures are infected; this concentration is designated the tissue culture

infectious dose for 50% of cultures, or TCID50. An alternative assay can

determine at what dose one-half of experimental animals die of viral

disease (lethal dose for 50% of test animals, or LD50). A more quantitative assay of infectivity is the plaque assay. A plaque is an area of visualized localized CPE. In the plaque assay, dilutions of the virus sample are

placed on cells attached to a culture dish, and after adsorption of the

virus to cells, the cells are overlaid with semisolid medium or medium

containing antibody, which prevents virus diffusion through the

medium. Virus then spreads only cell to cell, causing a restricted area

of CPE—a plaque—on the cellular monolayer. The number of plaques

formed by each dilution of virus defines the titer in plaque-forming

units (PFUs) per volume of virus stock.

For viruses that infect humans, the ratio of viral particles to infectious units, or the particle-to-PFU ratio, is always much greater than

1—usually 10–1000. This result signifies a large excess of particles that

are defective and/or that do not score as infectious in laboratory assays.

Thus, for experimental purposes, following input virus particles, either

visually or biochemically, does not guarantee that the observer is following the real infection pathway. Accordingly, clinical preparations of

viruses used for vaccines, vaccine vectors, gene therapy vectors, and

oncolytic viruses need to be defined precisely and specifically in terms

of particles versus infectious units for accurate and safe dosing. As an

example, a recent adenovirus-based COVID vaccine was quantified on

the basis of spectrophotometric measurement of purified virions. After

the trial was initiated, lower than expected reactogenicity led to a reexamination of the vaccine dose. An excipient discovered in the vaccine

was found to cause errors in spectrophotometric measurement that led

to an overestimate of the virus concentration. Parallel measurements

of viral genomes with RT-PCR allowed a more accurate measurement

of the vaccine vector batches, and the dose was revised to one-half of

the original level. This example illustrates the importance of precise

measurements of viral particles and infectious particles in clinical

preparations of viruses.

DETECTION OF VIRUS-SPECIFIC

ANTIBODIES

The presence of virus-specific antibodies provides evidence of prior

infection with a virus or prior exposure to viral antigens through

immunization; thus, antibody tests are extremely important clinically.

The most common tests for antibodies are the enzyme-linked immunosorbent assay (ELISA) and the Western blot or immunoblot assay.

An ELISA involves the immobilization of viral antigen on a substrate

such as a microtiter well, its incubation with the patient’s serum, and

further incubation with an antibody to human IgG coupled to an

enzyme. The amount of bound antibody is measured by detection

of a colored product made by the bound enzyme. The Western blot

assay involves the resolution of viral proteins in a polyacrylamide gel,

their transfer to a membrane, incubation with the patient’s serum, and

further incubation with antibody to human IgG coupled to an enzyme.

Proteins with bound antibodies are detected as a colored product made

by the bound antibody. The Western blot detects antigen of a specific

size and therefore is more specific than ELISA. For example, HIV

serologic testing involves high-throughput ELISA screening followed

by a Western blot assay to confirm the specificity of any positive ELISA

result.

In a hemagglutination inhibition assay, antibodies specific for viral

surface proteins are detected by their ability to block hemagglutination.

IMMUNIZATION AGAINST VIRAL DISEASES

Viral vaccines are among the most effective biomedical and public health

measures that have been implemented: millions of deaths have been prevented by their use. These vaccines are safe because extensive protocols

have been developed for monitoring vaccine safety both before and after

licensure. Historically, viral vaccines were based on either inactivated

virus or live attenuated viruses, as exemplified by the Salk polio vaccine

and the Sabin live attenuated polio vaccine, respectively. Both of these

vaccines were quite successful, offering individual advantages. Further

vaccine types have been developed, including those based on recombinant proteins, viral vectors, and, most recently, mRNA. For each virus,

the optimal antigen and immunization strategy must be developed on

the basis of the virus-specific immune correlates, antibodies, or T cells

needed for immunologic protection against infection and disease. These

concepts are discussed in greater detail in Chap. 123.

ANTIVIRAL THERAPEUTICS

■ ANTIVIRAL DRUGS

Viruses replicate in human cells and use much of the host cell’s machinery. Therefore, antiviral drugs must target virus-specific events to

optimize safety. Viral targets for drugs have been identified in studies

of the mechanisms of viral infection and replication (Chap. 191). Many

of the most successful antiviral drugs target viral enzymes; examples

include the anti-HSV drugs that target the virus DNA polymerases and

thymidine kinase (Chap. 191) and the HIV drugs that target the virus

reverse transcriptase, protease, and integrase (Chap. 202).

■ VIRUSES AS THERAPEUTICS

Viruses have been engineered for a number of medical purposes,

including gene delivery and tumor cell killing. As described above,

viruses have been developed as vaccines and vaccine vectors. For

example, vesicular stomatitis virus–based vectors have been employed

as Ebola vaccines. Adenovirus-based vectors have been used as

AIDS vaccine vectors and are now being used as COVID-19 vaccine

vectors. Viral recombinants, including those of retroviruses and

adeno-associated viruses, have been approved as vectors for delivery

of genes to cells for treatment of single-gene defects. Retroviruses

integrate into the cell’s chromosomes and are maintained with stable

expression of the transgene, although some concerns have arisen

1460 PART 5 Infectious Diseases

about possible activation of neighboring promoters and adverse effects

due to that activation. Adeno-associated viruses are not integrated

but are stably maintained and capable of durable expression of the

transgene. Adenoviruses and herpesviruses are also being tested as

gene therapy vectors. Finally, an attenuated strain of HSV expressing

granulocyte-macrophage colony-stimulating factor has been approved

for treatment of melanoma because of its oncolytic and immunotherapeutic properties. Many additional studies are assessing viruses for

use as vectors and for immunotherapeutic and oncolytic applications.

SUMMARY

As obligate intracellular parasites, viruses enter host cells, replicate,

and spread in the form of progeny viruses. Injury to the host cell

resulting from viral entry may lead to tissue and organ damage.

Basic knowledge of the mechanisms underlying infection by and replication of viruses that infect humans is the foundation for medical

studies of viral pathogenesis, viral vaccines, antiviral drugs, and the use

of viruses as therapeutics. A broad knowledge of all viruses is essential

to our preparedness for the next viral epidemic or pandemic.

■ FURTHER READING

Helenius A: Virus entry: Looking back and moving forward. J Mol

Biol 43:1853, 2018.

Howley PM et al (eds): Fields Virology: Vol. 1: Emerging Viruses,

7th ed. Philadelphia, Wolters Kluwer/Lippincott Williams & Wilkins

Health, 2020.

Knipe DM, Howley PM (eds): Fields Virology, 6th ed. Philadelphia,

Wolters Kluwer/Lippincott Williams & Wilkins Health, 2013.

Knipe DM et al: Ensuring vaccine safety. Science 370:1274, 2020.

Ksiazek TG et al: A novel coronavirus associated with severe acute

respiratory syndrome. N Engl J Med 348:1953, 2003.

Voysey M et al: Safety and efficacy of the ChAdOx1 nCoV-19 vaccine

(AZD1222) against SARS-CoV-2: An interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK. Lancet

397:99, 2021.

Zhou P et al: A pneumonia outbreak associated with a new coronavirus of probable bat origin. Nature 579:270, 2020.

Most antiviral drugs inhibit viral DNA or RNA replication, but other

activities, such as virus entry, viral RNA transcription, cleavage of proteins by the viral protease, virus uncoating after infection, and virus

release from cells, are all targeted by different licensed antiviral agents.

Inhibition of viral replication does not eliminate the virus in the cell;

host cell immune responses are important for viral clearance. Antiviral drugs usually do not eradicate latent viral infections, but instead

usually inhibit viral replication; thus, when treatment is stopped, the

virus can reactivate and replicate again. Resistance to antiviral agents

due to mutations in viral proteins is not uncommon and is more

common for RNA viruses with a higher mutation rate than for DNA

viruses. This difference may explain the observation that drug-resistant

DNA viruses are a greater problem in immunocompromised patients,

whereas drug-resistant RNA viruses can be found in healthy persons

as well. Patients may harbor a mixture of drug-resistant and drugsensitive viruses that is dynamic and changes under pressure from

the drug. Combination therapy with more than one antiviral agent,

each with a different mechanism of action, may be more effective than

191 Antiviral Chemotherapy,

Excluding Antiretroviral

Drugs

Jeffrey I. Cohen, Eleanor Wilson

monotherapy, particularly against RNA viruses, which may be present

as mixtures with different resistance patterns. Antiviral testing can

be performed in patients who do not respond to antiviral drugs or

whose response diminishes. For some viruses, such testing involves the

sequencing of selected viral genes; however, in many cases, it involves

the growth of virus in the presence of different concentrations of the

drug, which is a laborious, time-consuming process. Response to antiviral therapy has traditionally been assessed clinically, but quantitative

PCR has been useful in monitoring the response to therapy for viruses

that circulate in the blood (e.g., cytomegalovirus [CMV], hepatitis B

and C viruses [HBV and HCV, respectively]). Systemic therapy with

antivirals is usually more effective than topical therapy but is more

commonly associated with side effects.

ANTIVIRAL DRUGS FOR HERPESVIRUS

INFECTIONS

■ ACYCLOVIR, VALACYCLOVIR, FAMCICLOVIR,

AND PENCICLOVIR

Acyclovir is an analogue of deoxyguanosine and is phosphorylated to

the monophosphate form by viral thymidine kinase in cells infected

with herpes simplex virus (HSV) or varicella-zoster virus (VZV).

Cellular protein kinases further phosphorylate the drug to the active

triphosphate form, which inhibits viral DNA polymerase; the drug is

incorporated into viral DNA to terminate its replication. Valacyclovir,

a valine ester of acyclovir, is absorbed much better than acyclovir; its

rapid conversion to acyclovir in the liver and intestine results in plasma

acyclovir levels approximately four times higher than are attained with

oral acyclovir. Acyclovir and valacyclovir are approved by the U.S.

Food and Drug Administration (FDA) for treatment of initial episodes of genital herpes, recurrent genital herpes, varicella, and zoster

(Table 191-1). Valacyclovir is also approved for treatment of herpes

labialis (cold sores), for suppression of recurrences of genital herpes,

and for reduction of transmission of genital HSV. The doses of acyclovir and valacyclovir used for treating VZV infections are higher than

those used for HSV infections since VZV is less susceptible to inhibition by these drugs. Both drugs exhibit poor activity against CMV.

Intravenous acyclovir is used for severe disease requiring hospitalization; oral acyclovir or valacyclovir is used for outpatient therapy; and

topical acyclovir, penciclovir, and docosanol are approved for treatment of orolabial herpes but are much less effective than the oral drugs.

Acyclovir is excreted by the kidneys. Thus the dose of acyclovir

or valacyclovir needs to be reduced with renal insufficiency. Central

nervous system (CNS) side effects that occur with IV acyclovir or oral

valacyclovir are more common with the higher drug levels seen in

persons with renal insufficiency. Reversible renal insufficiency due to

crystallization of the drug in renal tubules can occur with IV acyclovir,

especially in persons who are dehydrated. Headache, nausea, rash, and

diarrhea have been reported with acyclovir. Mutations in the HSV or

VZV thymidine kinase or, less commonly, in viral DNA polymerase

can result in resistance to acyclovir or valacyclovir. Viruses lacking

thymidine kinase activity are also resistant to famciclovir and ganciclovir. Acyclovir- and valacyclovir-resistant HSV and VZV are rare in

immunocompetent persons. Resistant virus is treated with foscarnet

or, less commonly, cidofovir. Mucosal disease due to resistant virus

in immunocompromised persons is sometimes treated with topical

foscarnet, trifluridine, or cidofovir.

Famciclovir is a diacetyl ester of penciclovir that is converted to penciclovir in the intestine and liver. Penciclovir is a guanosine analogue

that is less potent than acyclovir, but, because of its longer intracellular

half-life, its activity is similar to that of acyclovir. Penciclovir is phosphorylated by HSV and VZV thymidine and cellular kinases and has

activity similar to that of acyclovir for HSV and VZV infections. Famciclovir is approved for treatment of zoster, suppression of genital herpes, and treatment of recurrent mucocutaneous herpes in patients with

HIV infection. Famciclovir is excreted by the kidneys, and the dose is

adjusted for renal insufficiency. Side effects are uncommon and can

include headache, nausea, and diarrhea. Resistance due to mutations

in viral thymidine kinase or DNA polymerase can occur.

1461CHAPTER 191 Antiviral Chemotherapy, Excluding Antiretroviral Drugs

■ GANCICLOVIR AND VALGANCICLOVIR

Ganciclovir is a deoxyguanosine analog that is phosphorylated by UL97

protein kinase in cells infected with CMV and converted to its active

form, ganciclovir triphosphate, by cellular protein kinases. Ganciclovir

triphosphate inhibits both viral DNA polymerase and incorporation of

guanosine triphosphate into viral DNA. Valganciclovir is a valine ester

of ganciclovir and is converted to ganciclovir in the liver and intestine.

Valganciclovir has much better oral bioavailability than ganciclovir;

plasma levels of oral valganciclovir and IV ganciclovir are similar.

Ganciclovir and valganciclovir are used for treatment and prevention

of CMV disease in immunocompromised patients and are approved for

prevention of CMV infection in transplant recipients and for treatment

of CMV retinitis. Ganciclovir is effective against HSV, VZV, human

herpesvirus type 6 (HHV-6), and herpes B virus. This drug is excreted

by the kidneys, and dose adjustment is required in renal insufficiency.

Ganciclovir therapy often results in neutropenia and thrombocytopenia after 1 week. Less commonly, ganciclovir has been associated with

CNS symptoms, particularly at high plasma drug levels. Mutations in

TABLE 191-1 Antiviral Drugs for Herpesvirus Treatment and Prophylaxis in Adults

DISEASE DRUG ROUTE ADULT DOSE COMMENTS

Orolabial herpes, primary

episode

Acyclovir

Valacyclovir

Famciclovir

Oral

Oral

Oral

400 mg tid × 7–10 d

1 g bid × 7–10 d

500 mg bid or 250 mg tid × 7–10 d

Reduces duration of fever, lesions, and

virus shedding

Orolabial herpes, recurrence Acyclovir

Valacyclovir

Famciclovir

Oral

Oral

Oral

400 mg 5 times daily × 5 d

2 g bid × 1 d

1500 mg × 1 d

Reduces duration of lesions by 1–2 d if given

during prodrome

Orolabial herpes, suppression Acyclovir

Valacyclovir

Famciclovir

Oral

Oral

Oral

400 mg bid

500 mg or 1 g once daily

500 mg bid

In patients with >6 recurrences per year,

reduces number of recurrences by ~50%

and increases time to first recurrence

Genital herpes, primary episode Acyclovir

Valacyclovir

Famciclovir

Oral

Oral

Oral

400 mg tid or 200 mg 5 times daily × 7–10 d

1 g bid × 7–10 d

250 mg tid × 7–10 d

Reduces duration of symptoms, genital

lesions, and virus shedding by 2, 4, and 7 d,

respectively

Genital herpes, recurrence Acyclovir

Valacyclovir

Famciclovir

Oral

Oral

Oral

800 mg tid × 2 d or 400 mg tid × 5 d

500 mg bid × 3 d or 1 g daily × 5 d

500 mg once, then 250 mg bid × 2 d

Reduces duration of symptoms, genital

lesions, and virus shedding by 1–2 d

Genital herpes suppression Acyclovir

Valacyclovir

Famciclovir

Oral

Oral

Oral

400 mg bid

250 mg bid

500 mg to 1 g daily

In patients with >6 recurrences per year,

reduces recurrence rates from 80–85%

to 25–30%, reduces virus shedding and

transmission

HSV encephalitis Acyclovir IV 10–15 mg/kg q8h × 14–21 d Reduces mortality and sequelae

HSV keratitis Acyclovir

Trifluridine

Vidarabine

Topical

Topical

Topical

3% ophthalmic ointment, 5 times daily

1% ophthalmic solution, 1 drop q2h when

awake (9 drops daily max)

3% ointment, ½-inch ribbon 5 times daily

Shortens duration of disease; acyclovir better

tolerated, especially with prolonged treatment

Mucocutaneous herpes in

immunocompromised patient

Acyclovir

Valacyclovir

Famciclovir

IV

Oral

Oral

5 mg/kg q8h × 7–14 d

500 mg to 1 g bid × 7–10 d

500 mg bid × 7–10 d

IV acyclovir reduces time to healing, duration

of pain, and duration of virus shedding

Varicella Acyclovir

Valacyclovir

Oral

Oral

20 mg/kg (800 mg max) 5 times daily × 5 d

20 mg/kg (1 g max) tid × 5 d

Has modest effect on symptoms, reduces

fever duration by 1 day

Zoster Acyclovir

Valacyclovir

Famciclovir

Oral

Oral

Oral

800 mg 5 times daily × 7 d

1 g tid × 7 d

500 mg tid × 7 d

Reduces time for last new lesion formation,

virus shedding, and pain duration

Varicella or zoster,

disseminated

Acyclovir IV 10 mg/kg q8h × 7 d Reduces time for last new lesion formation

and virus shedding; reduces cutaneous

dissemination

Cytomegalovirus disease Ganciclovir

Valganciclovir

Foscarnet

Cidofovir

IV

Oral

IV

IV

5 mg/kg q12h × 14–21 d, then 5 mg/kg daily

(maintenance dose)

900 mg bid × 14–21 d, then 90 mg daily

(maintenance dose)

60 mg/kg q8h × 14–21 d, then 90–120 mg daily

(maintenance dose)

5 mg/kg once weekly twice, then every other

week

Neutropenia and thrombocytopenia common

after 1 week

Levels and side effects similar to ganciclovir

Nephrotoxicity, electrolyte abnormalities; give

with additional saline

Nephrotoxicity; give with probenecid and

saline

Oral acyclovir reduces the duration of pain and other symptoms,

time to healing, and shedding in patients with their first episode of

genital herpes when treatment is begun within 6 days of infection.

Acyclovir, valacyclovir, and famciclovir are all effective for treatment

of primary and recurrent genital and orolabial herpes as well as for

suppressive therapy for these conditions. Topical acyclovir cream

reduces shedding and time to healing by 1–2 days if given within 1

day of symptom onset in persons with recurrent genital or orolabial

herpes. Oral acyclovir or valacyclovir reduces the severity of varicella

when given within 1 day of onset of the rash. Oral acyclovir, famciclovir, or valacyclovir shortens the duration of pain and rash associated with zoster if given within 3 days of onset. Oral valacyclovir is

more effective than oral acyclovir and is generally preferred since it

has better oral bioavailability and does not need to be given as frequently. Suppressive valacyclovir therapy for genital herpes reduces

transmission to uninfected partners by 50%. Intravenous acyclovir is used for herpes encephalitis and disseminated HSV or VZV

disease.

1462 PART 5 Infectious Diseases

CMV UL97 protein kinase or, less commonly, UL54 viral DNA polymerase can result in resistance to ganciclovir or valganciclovir. CMV

with mutations in protein kinase is usually sensitive to foscarnet and

cidofovir, while CMV with mutations in both protein kinase and DNA

polymerase is usually sensitive only to foscarnet. Mutations are more

common among persons who are highly immunocompromised and

who have been taking the drug for a long time. Resistant virus is treated

with foscarnet or cidofovir.

Ganciclovir and valganciclovir are used for treating severe CMV

infections in immunocompromised patients, including colitis, pneumonitis, retinitis, and encephalitis. Induction therapy, given two or

three times daily, is usually followed by less frequently administered

maintenance therapy. Oral valganciclovir has activity similar to that

of intravenous ganciclovir. Ganciclovir and valganciclovir are used for

prevention of CMV infection in transplant recipients when given either

preemptively (on the basis of viremia) or prophylactically. Ganciclovir

reduces developmental delay in infants with congenital CMV disease

involving the CNS and reduces hearing loss in infants with asymptomatic congenital CMV infection. Ganciclovir and valganciclovir are used

for treatment of HHV-6 encephalitis, HHV-8–associated Castleman

disease in patients with poorly controlled HIV infection, and severe

HSV or VZV disease when acyclovir is unavailable.

■ FOSCARNET

Foscarnet is a pyrophosphate analogue that directly inhibits herpesvirus DNA polymerases by blocking the pyrophosphate binding site in

the enzyme. Foscarnet does not require additional phosphorylation

(unlike acyclovir, cidofovir, or ganciclovir) in virus-infected cells for

its activity. This drug is approved for treatment of CMV retinitis and

mucocutaneous acyclovir-resistant HSV disease. It is also used to treat

ganciclovir-resistant CMV and acyclovir-resistant VZV. Foscarnet is

given intravenously and is excreted by the kidneys; dose adjustment

is required in renal insufficiency. Up to one-third of patients receiving

foscarnet develop nephrotoxicity with elevated levels of creatinine and

blood urea nitrogen, and proteinuria. Renal tubular acidosis and interstitial nephritis also have been reported. Renal insufficiency is more

common among persons who are dehydrated, given other nephrotoxic

drugs, or given high doses or rapid infusions of foscarnet. Administering IV saline before and after each foscarnet dose and giving the drug

over an adequate period can reduce nephrotoxicity. Renal insufficiency

is often reversible after treatment when the drug is stopped. Other side

effects include hypomagnesemia and hypocalcemia, which can be associated with arrhythmias, paresthesias, and seizures. Other metabolic

abnormities include hypokalemia, hypophosphatemia, or hyperphosphatemia. Foscarnet can also cause headache, fever, rash, diarrhea,

acute dystonia, tremors, hemorrhagic cystitis, genital ulcerations,

anemia, and abnormal liver function values. Mutations in CMV DNA

polymerase (UL54) or in HSV or VZV DNA polymerase can result

in resistance to foscarnet. CMV, HSV, and VZV can become resistant

to foscarnet; some strains of CMV are resistant to foscarnet, ganciclovir, and cidofovir; and HSV can become resistant to acyclovir and

foscarnet. Foscarnet is typically used to treat CMV retinitis, HHV-6

encephalitis, or drug-resistant severe CMV, HSV, or VZV infections

in immunocompromised patients. Topical foscarnet has been used to

treat acyclovir-resistant mucosal infections due to HSV.

■ CIDOFOVIR

Cidofovir is an analogue of deoxycytidine monophosphate and is

phosphorylated in cells to its active diphosphate form. The diphosphate form of cidofovir competes with deoxycytidine triphosphate for

incorporation into herpesvirus DNA. The drug inhibits replication

of all human herpesviruses as well as poxviruses, papillomaviruses,

polyomaviruses, and adenoviruses. Cidofovir is approved for treatment

of CMV retinitis in patients with AIDS; it is also used for treatment

of infections caused by CMV exhibiting ganciclovir resistance due to

mutations in UL97 protein kinase and of those caused by HSV or VZV

displaying mutations in thymidine kinase. Because cidofovir is excreted

by the kidneys, dose adjustment is required in renal insufficiency.

About one-fifth of patients receiving cidofovir develop nephrotoxicity,

and the drug is associated with metabolic acidosis and glucosuria.

Cidofovir therapy is preceded by at least 1 L of saline, and probenecid is given 3 h before, 2 h after, and 8 h after each dose to reduce

nephrotoxicity. An additional 1 L of saline is recommended during

treatment or immediately thereafter. About one-fourth of patients

receiving cidofovir develop neutropenia; additional side effects include

ocular hypotony, uveitis, iritis, headache, nausea, vomiting, diarrhea,

and rash. Mutations in CMV DNA polymerase (UL54) or HSV DNA

polymerase can result in resistance to cidofovir. Some strains of

CMV exhibiting ganciclovir resistance due to mutations in viral DNA

polymerase are resistant to cidofovir, whereas many CMV and HSV

strains exhibiting foscarnet resistance due to mutations in DNA polymerase may retain sensitivity to cidofovir. Cidofovir is typically used

to treat ganciclovir- and/or foscarnet-resistant severe CMV disease or

acyclovir- and/or foscarnet-resistant HSV disease in immunocompromised patients. Cidofovir has been used as preemptive therapy against

CMV infection in transplant recipients. It has also been used to treat

severe adenovirus infections, adenovirus or BK virus hemorrhagic cystitis, BK nephropathy, and severe molluscum contagiosum, although

controlled studies have not been performed. Topical cidofovir has

been used to treat acyclovir-resistant HSV mucosal infections and

anogenital warts.

■ LETERMOVIR

Letermovir is a dihydroquinazolin that inhibits the CMV DNA terminase complex (UL51, UL59), which is required for cleavage and packaging of CMV into nucleocapsids. The drug has no activity against other

human herpesviruses. Letermovir is approved for prophylaxis of CMV

infection and disease in adult CMV-seropositive recipients of an allogeneic hematopoietic stem cell transplant. Letermovir is metabolized

by the liver and excreted in the feces; dose adjustment is not required if

the creatinine clearance rate (CrCl) is >10 mL/min. The dose of letermovir must be decreased in persons taking cyclosporine. Letermovir

therapy results in reduced levels of voriconazole and increased levels

of sirolimus, tacrolimus, cyclosporine, and other drugs metabolized

by CYP2C8 or transported by OAT1B1/3. Side effects of letermovir

include headache, nausea, diarrhea, and peripheral edema. Letermovir

does not cause nephrotoxicity and is not myelosuppressive. Resistance

to letermovir occurs more frequently in vitro than resistance to ganciclovir or foscarnet, and clinically significant letermovir resistance due

to mutations in UL56 in patients with CMV disease has been reported;

resistance may be less common when the drug is used for prophylaxis in patients with low or undetectable CMV levels. When given to

CMV-seropositive patients, starting a median of 8 days after hematopoietic stem cell transplantation and continuing for 14 weeks, letermovir

reduced the incidence of clinically significant CMV infection by 38%

compared with placebo. While anecdotes describe the use of letermovir

for treatment of CMV disease, resistance may develop quickly.

■ TRIFLURIDINE AND VIDARABINE

Trifluridine is a thymidine analogue that is incorporated into viral DNA

and inhibits its synthesis. Vidarabine is approved for topical therapy of

herpes keratitis and has also been used topically to treat acyclovir-resistant mucosal HSV infections. Trifluridine is active against acyclovir-resistant HSV, CMV, and vaccinia virus. Vidarabine is an adenosine analogue

that is incorporated into viral DNA and inhibits viral DNA polymerase.

Both trifluridine and vidarabine are used for topical therapy only.

■ INVESTIGATIONAL AND OTHER AGENTS

Brincidofovir is a phospholipid conjugate of cidofovir that is rapidly

taken up by cells and converted into cidofovir. It is active against

herpesviruses (including most strains of ganciclovir-resistant CMV),

poxviruses, adenovirus, and polyomaviruses. It does not cause nephrotoxicity and is not myelosuppressive. Diarrhea is the most common side

effect. The drug has been associated with intestinal toxicity and acute

graft-versus-host disease of the gastrointestinal tract. The drug did not

meet its primary endpoints in trials for adenovirus disease or CMV

prophylaxis. Clinical trials of oral brincidofovir have been discontinued, although it is still being developed for treatment of smallpox.

1463CHAPTER 191 Antiviral Chemotherapy, Excluding Antiretroviral Drugs

It  is  available for patients with serious adenovirus or poxvirus infections as part of the expanded access program. An IV formulation,

which, it is hoped, will cause less gastrointestinal toxicity, is being

tested for adenovirus viremia.

Maribavir is a benzimidazole that inhibits the CMV UL97 protein

kinase and reduces the egress of viral particles from the nucleus. The

drug is active against most strains of ganciclovir- and foscarnet-resistant

CMV. In phase 3 trials, maribavir was unsuccessful in preventing CMV

disease in transplant recipients; it is currently being tested as therapy

for CMV infections refractory to treatment with other antiviral agents.

Pritelivir inhibits the helicase–primase complex required for replication of HSV. This drug has reduced viral shedding in patients with recurrent genital herpes and is being tested for use against acyclovir-resistant

HSV mucocutaneous infection. Pritelivir is available as an expanded

access drug for acyclovir-resistant HSV infection.

Amenamevir is a helicase–primase inhibitor under development for

HSV and VZV infections.

ANTIVIRAL DRUGS FOR RESPIRATORY

VIRUS INFECTIONS

■ INFLUENZA

Neuraminidase Inhibitors Oseltamivir, zanamivir, and peramivir

are neuraminidase inhibitors that inhibit cleavage of sialic acid, which

is required for the release of influenza virus from infected cells and its

spread to other cells.

Oseltamivir phosphate is an oral prodrug that is cleaved by esterases

in the liver, gastrointestinal tract, and blood to oseltamivir carboxylate,

the more active form. It is approved for treatment of uncomplicated

influenza A or B disease when given ≤48 h after symptom onset

and for prophylaxis of influenza A and B in persons ≥1 year of age

(Table 191-2). Oseltamivir is much less active against influenza B

than against influenza A. The drug is excreted by the kidneys, and the

dose is adjusted in renal insufficiency. The most common side effects

are nausea, abdominal pain, and vomiting. Although CNS side effects

have been reported, particularly in children, it is unclear whether they

are due to the drug or to influenza virus infection itself. Resistance to

oseltamivir can develop as a result of mutations in the viral neuraminidase or in the hemagglutinin. Oseltamivir-resistant virus has been

transmitted from person to person. Resistance has been reported in

~15% of healthy children and ~1% of adults; resistance is more common among immunocompromised persons.

Zanamivir is approved for treatment of uncomplicated influenza A

and B in adults and children ≥7 years of age who have had symptoms

for ≤2 days and for prophylaxis in persons ≥5 years of age. Because

zanamivir has poor oral bioavailability, it is given as a powder through

an inhaler. Thus, use of the drug can be difficult for young children

and some elderly patients. Inhalation of zanamivir may cause bronchospasm, particularly in persons with underlying lung disease; it is

not recommended for persons with asthma, chronic obstructive pulmonary disease, or other airway disease. Zanamivir is more active than

oseltamivir against influenza B. It is also active against some isolates of

influenza virus that are resistant to oseltamivir; resistance to zanamivir

is less common than that to oseltamivir.

Peramivir is approved for treating uncomplicated influenza in

patients ≥2 years of age who have had symptoms for ≤2 days. Because

of its long half-life, it is given as a single IV dose. Peramivir is highly

active against both influenza A and B. The drug is excreted by the kidneys, and the dose is adjusted in renal insufficiency. The most common

side effect is diarrhea. While peramivir-resistant virus is rare in healthy

persons, peramivir-resistant virus has been isolated from immunocompromised persons.

Oseltamivir, zanamivir, and peramivir are effective for treatment

of uncomplicated influenza A and B, including disease caused by

avian influenza viruses (e.g., H5N1, H7N9, and H9N2). None of the

neuraminidase inhibitors is approved by the FDA for complicated

influenza or for persons requiring hospitalization for the disease. While

not licensed for the treatment of persons with complicated disease,

inpatients, and pregnant women, oseltamivir is considered the drug of

choice in these settings. The efficacy of zanamivir is similar to that of

oseltamivir in hospitalized patients. Treatment is most effective when

begun within 2 days of symptom onset and should be started as early

as possible; such early treatment reduces symptoms by ~1 day in persons with uncomplicated disease. For persons with influenza requiring

hospitalization and with pneumonia, treatment with oseltamivir or

zanamivir is recommended even later. Treatment may reduce the risk

of complications and death in hospitalized patients with influenza.

Oseltamivir and zanamivir (but not peramivir) are approved for

prophylaxis of influenza, especially in institutions where outbreaks can

be severe, and for prophylaxis in persons who have been exposed to the

virus, are at high risk for disease complications, and have not recently

been vaccinated. The efficacy of oseltamivir and zanamivir for prophylaxis is estimated to be ~70–90%. For persons at institutions, prophylaxis is given for at least 2 weeks and for up to 1 week after outbreaks

resolve. For other high-risk persons, prophylaxis is given within 2 days

of exposure and continued for 1 week after exposure. Since neuraminidase inhibitors reduce virus release from cells, they should not be

given 2 days before or within 2 weeks after receipt of live, attenuated

influenza vaccine. Resistance has been reported during treatment with

oseltamivir or peramivir, especially in immunocompromised persons;

oseltamivir-resistant viruses are usually sensitive to zanamivir.

TABLE 191-2 Antiviral Drugs for Respiratory Virus Treatment and Prophylaxis in Adults

DISEASE DRUG ROUTE ADULT DOSE COMMENTS

Influenza A, B Oseltamivir Oral Treatment: 75 mg bid × 5 d

Prophylaxis: 75 mg/d

Shortens duration of symptoms by 1 d when given within 2 d of onset; reduces

complications; considered drug of choice for patients with complications of

influenza

Influenza A, B Zanamivir Inhaled Treatment: 10 mg bid × 5 d

Prophylaxis: 10 mg/d

Shortens duration of symptoms by 1–2 d when given within 2 d of onset;

requires patient training for use; can cause bronchospasm; not recommended

for persons with asthma or chronic obstructive pulmonary disease

Influenza A, B Peramivir IV 600 mg once Shortens duration of symptoms by 1–2 d when given within 2 d of onset

Influenza A, B Baloxavir Oral 40 mg once; if >80 kg, 80 mg

once

Shortens duration of symptoms by 1 d when given within 2 d of onset; active

against virus resistant to neuraminidase inhibitors

Influenza A Amantadine Oral Treatment: 100 mg bid × 5 d

Prophylaxis: 200 mg/d

Most influenza virus strains are resistant; use only if virus is known

to be sensitive.

Influenza A Rimantadine Oral Treatment: 100 mg bid × 5 d

Prophylaxis: 200 mg/d

Most influenza virus strains are resistant; use only if virus is known

to be sensitive.

Respiratory

syncytial virus

Ribavirin Inhaled Aerosol from reservoir

containing 20 mg/mL for

12−18 h/d × 3–6 d

Reduces severity of symptoms in hospitalized infants with lower respiratory

tract disease; anecdotal reports of reduced progression to lower respiratory

tract disease and mortality in stem cell transplant patients

SARS-CoV-2 Remdesivir IV 200 mg on day 1, then 100 mg

qd × 4 d

Reduces duration of hospitalization in some studies. Duration of treatment

extended up to 10 days if no improvement.

1464 PART 5 Infectious Diseases

Baloxavir Baloxavir inhibits the cap-dependent endonuclease that

is important in initiating synthesis of influenza virus mRNA. This drug

is approved by the FDA as a single oral dose for postexposure prophylaxis of influenza and for treatment of uncomplicated influenza in

persons ≥12 years of age who have had symptoms for ≤48 h. Baloxavir

inhibits influenza A and B viruses, including avian strains and strains

that are resistant to neuraminidase inhibitors. The drug’s efficacy is

similar to that of the neuraminidase inhibitors in persons with uncomplicated influenza and reduces symptoms by ~1 day. In addition,

baloxavir exhibits efficacy similar to that of oseltamivir for reducing

symptoms in high-risk patients. However, its effectiveness in patients

hospitalized with complications of influenza is unknown. Reduced

sensitivity of influenza virus to baloxavir has been associated with

mutations in the viral polymerase acidic protein after one dose. The

incidences of nausea and vomiting are lower with baloxavir than with

oseltamivir. Levels of the drug are lower if it is taken with dairy products, polyvalent cation-containing laxatives or antacids, or oral supplements containing calcium, iron, magnesium, selenium, or zinc. Since

baloxavir reduces virus replication, it should not be given 2 days before

or within 2 weeks after receipt of live, attenuated influenza vaccine.

Adamantanes Amantadine and rimantadine inhibit the influenza

virus’s M2 protein and its uncoating and membrane fusion. While these

drugs are active against influenza A, resistance is widespread and can

develop rapidly; thus, the adamantanes are not recommended as treatment or prophylaxis for influenza unless the virus is known to be sensitive.

■ RESPIRATORY SYNCYTIAL VIRUS

Ribavirin Ribavirin is an analogue of guanosine and inhibits replication of numerous RNA and DNA viruses. The drug inhibits viral

RNA synthesis and capping of viral mRNA and in some cases increases

the viral RNA mutation rate to lethal levels for some viruses. Ribavirin

inhibits replication of respiratory syncytial virus (RSV), influenza virus,

parainfluenza virus, and many other RNA viruses in vitro. While the

drug has been used to treat numerous viral infections, including Lassa

fever and hepatitis E, it is approved by the FDA only for use against RSV

and as a component of combination therapy for hepatitis C. Aerosolized

ribavirin is approved for treatment of hospitalized infants and young

children with severe lower respiratory tract infections due to RSV; it

is given for 18 h per day and is most effective when used early in the

course of these severe infections. Ribavirin is given in a generator that

yields an aerosol of particles small enough to reach the lower respiratory tract; the level of systemic absorption is low. The aerosolized form

of the drug can induce bronchospasm, sudden deterioration of respiratory function (especially in infants), and rash and can precipitate in

ventilators, interfering with their function. Ribavirin is mutagenic and

teratogenic in animals; accordingly, it is not recommended for use in

pregnant women, and the exposure of health care workers should be

minimized with personal protective equipment. In early studies, ribavirin reduced the shedding of RSV and the severity of symptoms in hospitalized infants with lower respiratory tract disease who were not on

mechanical ventilation, the duration of oxygen supplementation, and

the duration of time on mechanical ventilation in infants. More recent

analyses of the literature suggest that the efficacy of the drug in these

settings is much less certain, and the drug is not recommended for routine use by the American Academy of Pediatrics. In retrospective studies, ribavirin has been reported to reduce the risk of progression of RSV

from upper to lower respiratory tract disease in stem cell transplant

recipients and to reduce mortality rates in these patients. In a retrospective study, the outcome of treatment with oral ribavirin was similar to that

obtained with the aerosolized drug in hematopoietic stem cell transplant

recipients with RSV disease. Ribavirin has not been shown to affect the

clinical course of patients with parainfluenza and is not recommended

for their treatment. Ribavirin costs more than $25,000 per day.

Palivizumab Palivizumab, a humanized monoclonal antibody to RSV

F protein, is approved for prevention of lower respiratory tract disease

due to RSV in pediatric patients at high risk of RSV disease, including

premature infants and children with bronchopulmonary dysplasia.

■ SARS-COV-2 (SEE CHAP. 199)

Remdesivir is converted in cells to an adenosine triphosphate analogue

that inhibits the RNA-dependent RNA polymerase of several viruses.

The drug is approved by the FDA for treatment of persons ≥12 years of

age with SARS-CoV-2 requiring hospitalization; it shortens the duration of hospitalization in persons with lower respiratory tract disease.

While the results of studies with the drug vary, it is recommended by

the National Institutes of Health for patients with SARS-CoV-2 who

require supplemental oxygen while hospitalized. The drug is given

IV and is not recommended in persons with a GFR <30 mL/min.

Serum transaminase elevations have been reported in healthy persons

receiving remdesivir, and liver enzymes should be monitored before

and during treatment. Chloroquine inhibits the activity of remdesivir

in vitro; hydroxychloroquine or chloroquine phosphate should not be

given with remdesivir.

■ INVESTIGATIONAL AGENTS FOR RESPIRATORY

VIRUS INFECTIONS

Favipiravir (T705) inhibits viral RNA polymerases and is active against

influenza and other RNA viruses. It is approved for treatment of

emerging influenza viruses in Japan. Presatovir is an RSV fusion inhibitor that was ineffective in two trials of RSV disease. DAS181 (Fludase)

is a sialidase that cleaves sialic acid, a receptor for influenza A and B

and parainfluenza viruses; it did not improve the clinical outcomes of

patients with influenza, but in case reports transplant recipients with

parainfluenza have improved clinically with the drug. Laninamivir

octanoate inhibits the neuraminidase of influenza A and B viruses

and is approved for treating influenza in Japan. RSV604 interacts with

the RSV nucleocapsid and is undergoing phase 2 studies in transplant

recipients.

Molnupiravir is an oral ribonucleoside analog that inhibits replication of SARS-CoV-2. The drug reduced the risk of hospitalization

or death in patients with mild-to-moderate COVID-19 by ~50% in a

phase 3 clinical trial. AT-527 is an oral nucleotide prodrug that reduced

SARS-CoV-2 viral loads in patients hospitalized with COVID-19 in a

phase 2 clinical trial. PF-07321332 is an oral SARS-CoV-2 protease

inhibitor that is being tested in combination with low dose ritonavir in

a phase 2/3 clinical trial for prevention of COVID-19 infection.

ANTIVIRAL DRUGS FOR HUMAN

PAPILLOMAVIRUS AND POXVIRUS

INFECTIONS

Interferon α (IFN-α) inhibits replication of many RNA and DNA

viruses in vitro. IFN-α is approved by the FDA for intralesional treatment of external anogenital warts caused by human papillomavirus

(HPV). It is effective in resolving lesions in ~50% of cases, with a

recurrence rate of ~25%.

Imiquimod is a toll-like receptor 7 agonist that induces production

of IFN-α and other cytokines. It is approved as a topical cream for

treatment of external genital and perianal warts caused by HPV in

persons ≥12 years of age. This drug is effective in resolving lesions in

~40% of cases.

Tecovirimat is approved by the FDA for treatment of smallpox and

inhibits replication of monkeypox and vaccinia viruses. Resistance to

tecovirimat developed in a person treated with the drug for progressive

vaccinia.

INVESTIGATIONAL ANTIVIRAL

DRUGS FOR PICORNAVIRUS

Pocapavir inhibits picornaviruses by inhibiting virus uncoating and is

being developed to reduce poliovirus shedding; resistance to the drug

develops rapidly.

ANTIVIRAL DRUGS FOR HEPATITIS B

VIRUS INFECTION

Eight drugs of two classes are approved for the treatment of chronic

HBV infection in the United States. One class, the nucleos(t)ide

analogues, act as chain-terminating competitive inhibitors of HBV