Sunday, April 30, 2006

More information on Tamiflu and Relenza

Two antiviral drugs are currently available that have been shown in laboratory settings to be effective against avian flu: Tamiflu (Oseltamivir) and
Relenza (Zanamivir known as Neuraminidase Inhibitors
From: Therapeutics Letter(The International Society of Drug Bulletins) of  contains an assessment and synthesis of published (and whenever possible peer-reviewed) publications up to November 1, 2000.
  • Two drugs were approved in 2000 in Canada for treatments of influenza A and B. Both selectively inhibit neuraminidase, a surface enzyme of the influenza virus. Oral oseltamivir (Tamiflu®) is indicated for patients aged 18 and over and inhaled zanamivir (Relenza®) for patients aged 12 and over. Begin treatment within 36 hours of first symptoms. Both drugs are indicated for influenza A and B, but clinical evidence of efficacy for B is limited.
  • Evidence of efficacy:
    Zanamivir and oseltamivir have not been compared to each other or to amantadine. In placebo-controlled trials, the primary outcome was defined as the time to the first 24 hours with absent or mild symptoms. This outcome measure does not necessarily reflect a patient’s full experience of the flu; 32% of participants had moderate to severe symptom recurrence after this endpoint.9 Only 50–60% of patients in clinical trials were influenza positive and in regular clinical care that proportion will likely be lower. Since there is no practical way to detect influenza positive patients prior to treatment, the assessment of efficacy is based on all trial participants. In pooled data from 2 RCTs zanamivir reduced the primary outcome by a median of 0.9 days10,11 and oseltamivir by 0.8 days.12,13 Symptom severity was not significantly different between zanamivir and placebo.10 Symptom severity was not adequately reported apart from duration for oseltamivir.12,13 No significant difference was seen in use of acetaminophen, cough syrup or antibiotics with either drug versus placebo. Serious complications leading to hospitalization or mortality were rare and not different. Minor complications were not adequately defined and were not consistent across trials. Neither oseltamivir nor zanamivir have been specifically tested in high-risk groups, such as immunocompromised patients.
  • Evidence of harm:
    Zanamivir has been associated with bronchospasm in patients with and without pre-existing lung disease. As a result of these reports the manufacturer has changed the drug’s labelling and sent a warning letter to physicians in both Canada and the US.
    Oseltamivir’s most common adverse effects are nausea and vomiting. Pooling the two trials – nausea: 14% oseltamivir and 5% placebo  (ARI=9%, NNH=11), vomiting: 11% oseltamivir and 3% placebo (ARI=8%, NNH=13).12,13
  • Dose and cost:
    • Both drugs should be started as in the RCTs within 36 hours of the first onset of symptoms. Zanamivir  is supplied as a powder for inhalation via a Diskhaler, 10mg twice daily for 5 days. Cost for 5-day course, $38. Oseltamivir  is in tablet form, 75mg twice daily for 5 days. Cost for 5-day course, $45


Prevention: Vaccination in cohort studies lowered rates of hospitalization, serious morbidity and mortality in patients over 60. Amantadine is a second-line preventive agent.
Treatment: Antiviral treatment at the onset of symptoms shows that amantadine reduces fever by one day and oseltamivir and zanamivir reduce duration of flu symptoms by 0.8 – 0.9 day.
RCTs(Random Controlled Double Blind Studies) are needed to determine whether any prevention or treatment of influenza reduces complications leading to hospitalization or mortality.

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