October 8, 2015
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Mechanism Of Action

Zanamivir is an antiviral drug [see Microbiology].


Absorption and Bioavailability

Pharmacokinetic studies of orally inhaled zanamivir indicate that approximately 4% to 17% of the inhaled dose is systemically absorbed. The peak serum concentrations ranged from 17 to 142 ng/mL within 1 to 2 hours following a 10-mg dose. The area under the serum concentration versus time curve (AUC∞) ranged from 111 to 1,364 ng•h/mL.


Zanamivir has limited plasma protein binding ( < 10%).


Zanamivir is renally excreted as unchanged drug. No metabolites have been detected in humans.


The serum half-life of zanamivir following administration by oral inhalation ranges from 2.5 to 5.1 hours. It is excreted unchanged in the urine with excretion of a single dose completed within 24 hours. Total clearance ranges from 2.5 to 10.9 L/h. Unabsorbed drug is excreted in the feces.

Impaired Hepatic Function

The pharmacokinetics of zanamivir have not been studied in patients with impaired hepatic function.

Impaired Renal Function

After a single intravenous dose of 4 mg or 2 mg of zanamivir in volunteers with mild/moderate or severe renal impairment, respectively, significant decreases in renal clearance (and hence total clearance: normals 5.3 L/h, mild/moderate 2.7 L/h, and severe 0.8 L/h; median values) and significant increases in half-life (normals 3.1 h, mild/moderate 4.7 h, and severe 18.5 h; median values) and systemic exposure were observed. Safety and efficacy have not been documented in the presence of severe renal insufficiency. Due to the low systemic bioavailability of zanamivir following oral inhalation, no dosage adjustments are necessary in patients with renal impairment. However, the potential for drug accumulation should be considered.

Pediatric Patients

The pharmacokinetics of zanamivir were evaluated in pediatric subjects with signs and symptoms of respiratory illness. Sixteen subjects, aged 6 to 12 years, received a single dose of 10 mg zanamivir dry powder via DISKHALER. Five subjects had either undetectable zanamivir serum concentrations or had low drug concentrations (8.32 to 10.38 ng/mL) that were not detectable after 1.5 hours. Eleven subjects had Cmax median values of 43 ng/mL (range: 15 to 74) and AUC∞ median values of 167 ng•h/mL (range: 58 to 279). Low or undetectable serum concentrations were related to lack of measurable PIFR in individual subjects [see Use In Specific Populations, Clinical Studies].

Geriatric Patients

The pharmacokinetics of zanamivir have not been studied in subjects older than 65 years [see Use In Specific Populations].

Gender, Race, and Weight

In a population pharmacokinetic analysis in patient trials, no clinically significant differences in serum concentrations and/or pharmacokinetic parameters (V/F, CL/F, ka, AUC0-3, Cmax, Tmax, CLr, and % excreted in urine) were observed when demographic variables (gender, age, race, and weight) and indices of infection (laboratory evidence of infection, overall symptoms, symptoms of upper respiratory illness, and viral titers) were considered. There were no significant correlations between measures of systemic exposure and safety parameters.


Mechanism of Action

Zanamivir is an inhibitor of influenza virus neuraminidase affecting release of viral particles.

Antiviral Activity

The antiviral activity of zanamivir against laboratory and clinical isolates of influenza virus was determined in cell culture assays. The concentrations of zanamivir required for inhibition of influenza virus were highly variable depending on the assay method used and virus isolate tested. The 50% and 90% effective concentrations (EC50 and EC90) of zanamivir were in the range of 0.005 to 16.0 μM and 0.05 to > 100 μM, respectively (1 μM = 0.33 mcg/mL). The relationship between the cell culture inhibition of influenza virus by zanamivir and the inhibition of influenza virus replication in humans has not been established.


Influenza viruses with reduced susceptibility to zanamivir have been selected in cell culture by multiple passages of the virus in the presence of increasing concentrations of the drug. Genetic analysis of these viruses showed that the reduced susceptibility in cell culture to zanamivir is associated with mutations that result in amino acid changes in the viral neuraminidase or viral hemagglutinin or both. Resistance mutations selected in cell culture which result in neuraminidase amino acid substitutions include E119G/A/D and R292K. Mutations selected in cell culture in hemagglutinin include: K68R, G75E, E114K, N145S, S165N, S186F, N199S, and K222T.

In an immunocompromised patient infected with influenza B virus, a variant virus emerged after treatment with an investigational nebulized solution of zanamivir for 2 weeks. Analysis of this variant showed a hemagglutinin substitution (T198I) which resulted in a reduced affinity for human cell receptors, and a substitution in the neuraminidase active site (R152K) which reduced the enzyme's activity to zanamivir by 1,000-fold. Insufficient information is available to characterize the risk of emergence of zanamivir resistance in clinical use.


Cross-resistance has been observed between some zanamivir-resistant and some oseltamivir-resistant influenza virus mutants generated in cell culture. However, some of the in cell culture zanamivir-induced resistance mutations, E119G/A/D and R292K, occurred at the same neuraminidase amino acid positions as in the clinical isolates resistant to oseltamivir, E119V and R292K. No trials have been performed to assess risk of emergence of cross-resistance during clinical use.

Influenza Vaccine Interaction Trial

An interaction trial (n = 138) was conducted to evaluate the effects of zanamivir (10 mg once daily) on the serological response to a single dose of trivalent inactivated influenza vaccine, as measured by hemagglutination inhibition titers. There was no difference in hemagglutination inhibition antibody titers at 2 weeks and 4 weeks after vaccine administration between zanamivir and placebo recipients.

Influenza Challenge Trials

Antiviral activity of zanamivir was supported for infection with influenza A virus, and to a more limited extent for infection with influenza B virus, by Phase I trials in volunteers who received intranasal inoculations of challenge strains of influenza virus, and received an intranasal formulation of zanamivir or placebo starting before or shortly after viral inoculation.

Clinical Studies

Treatment Of Influenza

Adults and Adolescents

The efficacy of RELENZA 10 mg inhaled twice daily for 5 days in the treatment of influenza has been evaluated in placebo-controlled trials conducted in North America, the Southern Hemisphere, and Europe during their respective influenza seasons. The magnitude of treatment effect varied between trials, with possible relationships to population-related factors including amount of symptomatic relief medication used.

Populations Studied

The principal Phase III trials enrolled 1,588 subjects aged 12 years and older (median age 34 years, 49% male, 91% Caucasian), with uncomplicated influenza-like illness within 2 days of symptom onset. Influenza was confirmed by culture, hemagglutination inhibition antibodies, or investigational direct tests. Of 1,164 subjects with confirmed influenza, 89% had influenza A and 11% had influenza B. These trials served as the principal basis for efficacy evaluation, with more limited Phase II studies providing supporting information where necessary. Following randomization to either zanamivir or placebo (inhaled lactose vehicle), all subjects received instruction and supervision by a healthcare professional for the initial dose.

Principal Results: The definition of time to improvement in major symptoms of influenza included no fever and self-assessment of “none” or “mild” for headache, myalgia, cough, and sore throat. A Phase II and a Phase III trial conducted in North America (total of over 600 influenza-positive subjects) suggested up to 1 day of shortening of median time to this defined improvement in symptoms in subjects receiving zanamivir compared with placebo, although statistical significance was not reached in either of these trials. In a trial conducted in the Southern Hemisphere (321 influenza-positive subjects), a 1.5-day difference in median time to symptom improvement was observed. Additional evidence of efficacy was provided by the  European trial.

Other Findings

There was no consistent difference in treatment effect in subjects with influenza A compared with influenza B; however, these trials enrolled smaller numbers of subjects with influenza B and thus provided less evidence in support of efficacy in influenza B.

In general, subjects with lower temperature (e.g., 38.2°C or less) or investigator-rated as having less severe symptoms at entry derived less benefit from therapy.

No consistent treatment effect was demonstrated in subjects with underlying chronic medical conditions, including respiratory or cardiovascular disease [see WARNINGS AND PRECAUTIONS].

No consistent differences in rate of development of complications were observed between treatment groups.

Some fluctuation of symptoms was observed after the primary trial endpoint in both treatment groups.

Pediatric Patients

The efficacy of RELENZA 10 mg inhaled twice daily for 5 days in the treatment of influenza in pediatric patients has been evaluated in a placebo-controlled trial conducted in North America and Europe, enrolling 471 subjects, aged 5 to 12 years (55% male, 90% Caucasian), within 36 hours of symptom onset. Of 346 subjects with confirmed influenza, 65% had influenza A and 35% had influenza B. The definition of time to improvement included no fever and parental assessment of no or mild cough and absent/minimal muscle and joint aches or pains, sore throat, chills/feverishness, and headache. Median time to symptom improvement was 1 day shorter in subjects receiving zanamivir compared with placebo. No consistent differences in rate of development of complications were observed between treatment groups. Some fluctuation of symptoms was observed after the primary trial endpoint in both treatment groups.

Although this trial was designed to enroll children aged 5 to 12 years, the product is indicated only for children aged 7 years and older. This evaluation is based on the combination of lower estimates of treatment effect in 5- and 6-year-olds compared with the overall trial population, and evidence of inadequate inhalation through the DISKHALER in a pharmacokinetic trial [see Use In Specific Populations, Pharmacokinetics].

Prophylaxis Of Influenza

The efficacy of RELENZA in preventing naturally occurring influenza illness has been demonstrated in 2 post-exposure prophylaxis trials in households and 2 seasonal prophylaxis trials during community outbreaks of influenza. The primary efficacy endpoint in these trials was the incidence of symptomatic, laboratory-confirmed influenza, defined as the presence of 2 or more of the following symptoms: oral temperature ≥ 100°F/37.8°C or feverishness, cough, headache, sore throat, and myalgia; and laboratory confirmation of influenza A or B by culture, PCR, or seroconversion (defined as a 4-fold increase in convalescent antibody titer from baseline).

Household Prophylaxis Trials

Two trials assessed post-exposure prophylaxis in household contacts of an index case. Within 1.5 days of onset of symptoms in an index case, each household (including all family members aged 5 years and older) was randomized to RELENZA 10 mg inhaled once daily or placebo inhaled once daily for 10 days. In the first trial only, each index case was randomized to RELENZA 10 mg inhaled twice daily for 5 days or inhaled placebo twice daily for 5 days. In this trial, the proportion of households with at least 1 new case of symptomatic laboratory-confirmed influenza was reduced from 19.0% (32 of 168 households) for the placebo group to 4.1% (7 of 169 households) for the group receiving RELENZA.

In the second trial, index cases were not treated. The incidence of symptomatic laboratory-confirmed influenza was reduced from 19.0% (46 of 242 households) for the placebo group to 4.1% (10 of 245 households) for the group receiving RELENZA.

Seasonal Prophylaxis Trials

Two seasonal prophylaxis trials assessed RELENZA 10 mg inhaled once daily versus placebo inhaled once daily for 28 days during community outbreaks. The first trial enrolled subjects aged 18 years or older (mean age: 29 years) from 2 university communities. The majority of subjects were unvaccinated (86%). In this trial, the incidence of symptomatic laboratory-confirmed influenza was reduced from 6.1% (34 of 554) for the placebo group to 2.0% (11 of 553) for the group receiving RELENZA.

The second seasonal prophylaxis trial enrolled subjects aged 12 to 94 years (mean age: 60 years) with 56% of them older than 65 years. Sixty-seven percent of the subjects were vaccinated. In this trial, the incidence of symptomatic laboratory-confirmed influenza was  reduced from 1.4% (23 of 1,685) for the placebo group to 0.2% (4 of 1,678) for the group receiving RELENZA.

Last reviewed on RxList: 5/12/2015
This monograph has been modified to include the generic and brand name in many instances.

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