McKenna's Pharmacology for Nursing, 2e

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P A R T 2  Chemotherapeutic agents

AGENTS FOR INFLUENZA A AND RESPIRATORY VIRUSES

Pharmacokinetics Amantadine is slowly absorbed from the gastrointesti- nal (GI) tract, reaching peak levels in 4 hours. Excretion occurs unchanged through the urine, with a half-life of 15 hours. Oseltamivir is readily absorbed from the GI tract, extensively metabolised in the urine and excreted in the urine with a half-life of 6 to 10 hours. Ribavirin, an inhaled drug, is slowly absorbed through the respiratory tract. It is metabolised at the cellular level and is excreted in the faeces and urine with a half-life of 9.5 hours. It is teratogenic and is rated pregnancy category X. Zanamivir must be delivered by a Diskhaler device, which comes with every prescription. It is absorbed through the respiratory tract and excreted unchanged in the urine with a half-life of 2.5 to 5.1 hours. Contraindications and cautions Because of its renal clearance, amantadine must be used at reduced doses and with caution in people who have any renal impairment to avoid altered metabolism and excretion of the drug . Amantadine is embryotoxic in animals and crosses into breast milk. Therefore, it should be used during pregnancy and breastfeeding only if the benefits clearly outweigh the risks to the fetus or neonate. People with renal dysfunction who are taking oseltamivir require reduced doses and close monitoring to avoid altered metabolism and excretion of the drug .

Influenza A and other respiratory viruses, including influenza B and respiratory syncytial virus (RSV), invade the respiratory tract and cause the signs and symptoms of respiratory “flu”. Vaccines have been developed (see Chapter 18) to stimulate immunity against influenza A and RSV. Preventing the viral infection is the best option, but if people do develop a viral infection, some drug therapies are available. Agents for influenza A and respiratory viruses include amantadine ( Symmetrel ), oseltamivir ( Tamiflu ), ribavirin ( Virazide ) and zanam- ivir ( Relenza ). These drugs are described in detail in Table 10.1. Therapeutic actions and indications The exact mechanism of action of drugs that combat influenza A and respiratory viruses is not known. The belief is that these agents prevent shedding of the viral protein coat and entry of the virus into the cell (Figure 10.2). This action prevents viral replication, causing viral death. These agents for influenza A and respiratory viruses are especially important for health- care workers and other high-risk individuals and for reducing the severity of infection if it occurs. See Table 10.1 for usual indications specific to each antiviral drug. Oseltamivir is the only antiviral agent that has been shown to be effective in treating avian flu.

TABLE 10.1

DRUGS IN FOCUS Agents for influenza A and respiratory viruses

Drug name

Dosage/route

Usual indications

amantadine (Symmetrel)

Adult and paediatric >10 years: 100 mg PO b.d. Paediatric 5–9 years: 100 mg PO daily.

Treatment of Parkinson’s disease; treatment and prevention of respiratory virus infections Treatment and prevention of uncomplicated influenza for person who is symptomatic for <2 days; only antiviral agent effective in treatment of avian flu Used in combination with interferon alfa-2b as an oral drug for the treatment of chronic hepatitis C in children and adults who relapse after interferon-alpha therapy Treatment of influenza A, respiratory syncytial virus (RSV), and herpes virus infections; treatment of children with RSV; has undergone testing for use in several other viral conditions Approved in 1999 for treatment and prevention of uncomplicated influenza infections in adults and in children >7 years of age who have had symptoms for <2 days

oseltamivir (Tamiflu)

Adult: 75 mg PO b.d. for 5 days (treatment); 75 mg/day PO for 7 days (prevention) Paediatric (1–12 years): 30–75 mg b.d. PO for 5 days (treatment); 30–75 mg/day for 7 days (prevention)

ribavirin (Rebetol, Virazide)

Adult <65 kg: 800 mg/day PO; 65–85 kg: 1000 mg/day PO; 86–105 kg 1400 mg/day PO; >105 kg: 1400 mg/day PO

zanamivir (Relenza)

2 inhalations b.d. for 5 days