Molecular structure of zanamivir
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Zanamivir

Zanamivir is a neuraminidase inhibitor used in the treatment of and prophylaxis of both influenza A and influenza B. Zanamivir was the first neuraminidase inhibitor commercially developed. It is currently marketed by GlaxoSmithKline under the trade name Relenza®. more...

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Development

Zanamivir was discovered in 1989 by scientists at the Australian biotechnology company Biota Holdings, working in conjunction with the CSIRO and the Victorian College of Pharmacy. The development was part of Biota's ongoing program to develop antiviral agents through rational drug design. An outline of the successful rational design strategy used was published in a paper in Nature (von Itzstein et al., 1993).

The strategy relied on the availability of the crystal structure of influenza neuraminidase which was achieved by x-ray crystallography. It was known as far back as 1974 that 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (DANA), a sialic acid analogue, was an inhibitor of neuraminidase (Meindl et al., 1974). Using the crystal structure of neuraminidase and DANA as a starting point, the researchers employed a computer-aided process to attempt to design a molecule which better fitted (and therefore inhibited) the active site of neuraminidase. Zanamivir, a transition-state analogue inhibitor of neuraminidase, was the result.

In 1990, zanamivir was licensed to Glaxo (now GlaxoSmithKline) for exclusive worldwide development and marketing. In 1999, the product was approved for marketing in the US and subsequently has been registered by GSK in a total of 70 countries.

Limitations

Whilst zanamivir proved to be a potent and effective inhibitor of influenza neuraminidase and inhibitor of influenza virus replication in vitro and in vivo, this didn't necessarily translate into a successful clinical treatment for influenza. In clinical trials it was found that zanamivir was able to reduce the time to symptom resolution by 1.5 days provided therapy was started within 48 hours of the onset of symptoms.

A further limitation concerns the poor oral bioavailability of zanamivir. This meant that oral dosing was impossible limiting dosing to the parenteral routes. Zanamivir, therefore, is administered by inhalation - a route that was chosen for patient compliance with therapy. But even this route of administration is not acceptable to many in the community.

A troubled commercial venture

Biota, being only a small company, was not able to bring the drug to market by itself. Consequently, it was licensed to Glaxo (now GlaxoSmithKline) to complete development and to market internationally as Relenza, delivered via Glaxo's proprietary, and some would say cumbersome, Diskhaler inhalation device. The license agreement entitled Biota to receive a 7% royalty on Glaxo's sales of Relenza.

A combination of factors has resulted in the limited commercial success of zanamivir (Relenza). The relatively small effect on the timecourse of influenza symptoms, the inhalation dosage form, a less-than-ideal device, and high expense make it a difficult product to market well. And although zanamivir was the first neuraminidase inhibitor to the market, it had only a few months lead over the second entrant, oseltamivir (Tamiflu), with an oral formulation much preferred by patients. Faced with this competition, GSK effectively abandoned the product.

Read more at Wikipedia.org


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Zanamivir Is Effective as Prophylaxis Against Influenza
From American Family Physician, 6/15/01 by Jeffrey T. Kirchner

The influenza viruses are highly contagious, which partially explains the observation that 25 to 50 percent of all household contacts of an index patient will contract the "flu" during an outbreak. Postexposure prophylaxis with amantadine or rimantadine is often used and has proved effective in several studies. However, at least two studies found no protection from this therapy, partially because of the rapid emergence of viral resistance to these medications. Zanamivir is a member of a new class of influenza therapies known as neuraminidase inhibitors. Inhaled zanamivir is effective for the acute treatment of influenza and has been found to confer seasonal protection when administered as daily prophylaxis. Hayden and colleagues performed a randomized, double-blind, placebo-controlled, parallel group study of zanamivir for the treatment and prevention of influenza in families.

Families eligible for the study had to include at least one adult and one child who was five to 17 years of age. Excluded were persons younger than five years and those with hypersensitivity to zanamivir or any immunocompromised state, and those who were pregnant or breast-feeding. Influenza-like illness had to include at least two of the following symptoms: fever higher than 37.8[degrees]C (100[degrees]F), cough, headache, sore throat and myalgias. A diagnosis of influenza was confirmed by rapid viral isolation and serologic testing. Family members who were index cases of influenza were randomly assigned to receive 10 mg of inhaled zanamivir or placebo twice daily for five days. Healthy household members were given 10 mg of zanamivir or placebo once daily for 10 days. Treatments were started within 36 hours of the onset of symptoms. All participants recorded the presence or absence of influenza symptoms twice daily for 14 days. The index case patients were seen on days 1 and 5, and household contacts were seen on days 11 and 28, and were also screened by telephone on days 5 and 14. The primary end point of the study was the proportion of families with at least one initially healthy member in whom symptomatic, laboratory-confirmed influenza was diagnosed.

Of the 337 families that participated in the study, 163 index cases were randomly assigned to the zanamivir arm and 158 index cases were randomly assigned to the placebo arm. The comparable number of household contacts for each group was 414 and 423, respectively. The proportion of families with one or more household contacts in whom laboratory-confirmed influenza developed was 19 percent in the group receiving placebo and 4 percent in the group receiving zanamivir. In families where the index case was laboratory-confirmed, influenza developed in 29 percent of patients in the placebo group and 8 percent of patients in the zanamivir group. The proportion of families with at least one family member who had confirmed symptomatic influenza was 15 percent of patients in the placebo group and 2 percent of patients in the zanamivir group, which translates to an 84 percent rate of protection with the influenza therapy. In all patients who had laboratory-confirmed influenza, the duration of symptoms was 2.5 days shorter for those receiving zanamivir compared with those receiving placebo. The frequency of adverse events was low and similar in both groups.

The authors conclude that administration of inhaled zanamivir to the index case and healthy family members significantly decreases the incidence of infection in household contacts. Although not a substitute for vaccination, clinical benefits of zanamivir include a rapid onset of action, a high degree of protection and good tolerability.

JEFFREY T. KIRCHNER, D.O.

Hayden FG, et al. Inhaled zanamivir for the prevention of influenza in families. N Engl J Med November 2, 2000; 343:1282-9.

COPYRIGHT 2001 American Academy of Family Physicians
COPYRIGHT 2001 Gale Group

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