Oseltamivir

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Oseltamivir
Clinical data
Pregnancy
category
Routes of
administration		oral
ATC code
Legal status
Legal status
  • AU: S4 (Prescription only)
  • UK: POM (Prescription only)
  • US: ℞-only
Pharmacokinetic data
Bioavailability75%
MetabolismHepatic, to GS4071
Elimination half-life6–10 hours
ExcretionRenal (GS4071)
Identifiers
  • ethyl (3R,4R,5S)-5-amino-4-acetamido-3- (pentan-3-yloxy)cyclohex-1-ene-1-carboxylate
CAS Number
PubChem CID
DrugBank
CompTox Dashboard (EPA)
Chemical and physical data
FormulaC16H28N2O4
Molar mass312.4 g/mol g·mol−1
3D model (JSmol)
  • CCOC(=O)C1=C[C@@H](OC(CC)CC)[C@H](NC(=O)C)[C@@H](N)C1

Oseltamivir (INN) (Template:PronEng) is an antiviral drug that slows the spread of non-resistant strains of the influenza virus between cells in the body. It is used in the treatment and prophylaxis of Influenzavirus A and Influenzavirus B infection. Like zanamivir, oseltamivir is a neuraminidase inhibitor. It acts as a transition-state analogue inhibitor of influenza neuraminidase, preventing progeny virions from detaching from infected cells.

Oseltamivir was the first orally active neuraminidase inhibitor commercially developed. It is a prodrug, which is hydrolysed hepatically to the active metabolite, the free carboxylate of oseltamivir (GS4071). It was developed by US-based Gilead Sciences and is currently marketed by Hoffmann–La Roche (Roche) under the trade name Tamiflu. In Japan, it is marketed by Chugai Pharmaceutical Co., which is more than 50% owned by Roche. Oseltamivir is generally available by prescription only.[1]

Roche estimates that 50 million people have been treated with oseltamivir.[2][needs update] The majority of these have been in Japan, where an estimated 35 million have been treated.[3] Since June 2009, Roche has been forced to allow other companies to develop competing drugs to Tamiflu, after much speculation about Roche's so-called 'monopoly' of Tamiflu in the UK.[citation needed]

Clinical usage

Indications and dosage

Oseltamivir is indicated for the treatment and prevention of infections due to influenza A and B virus in people at least one year of age. The usual adult dosage for treatment of influenza is 75 mg twice daily for 5 days, beginning within 2 days of the appearance of symptoms and with decreased doses for children and patients with renal impairment. Oseltamivir may be given as a preventive measure either during a community outbreak or following close contact with an infected individual. Standard prophylactic dosage is 75 mg once daily for patients aged 13 and older, which has been shown to be safe and effective for up to six weeks. The importance of early treatment is that the NA protein inhibition is more effective within the first 48 hours. If the virus has replicated and infected many cells the effectiveness of this medication will be severely diminished, especially over time.[4][5]

The standard recommended dose incompletely suppresses viral replication in at least some patients with H5N1 avian influenza, increasing the risk of viral resistance and rendering therapy less effective.[6] Accordingly, it has been suggested that higher doses and longer durations of therapy should be used for treatment of patients with the H5N1 virus.[6][7]

Clinical trials for an increased dosage began in May 2007. All avian influenza cases in Indonesia, Thailand, and Vietnam were inducted into the trial. The trial also include 100 cases of severe seasonal influenza from each of those countries and the United States. Half received the current standard dose, and half received a double dose, but for the standard length of time.[8]

A study recommend the use of oseltamivir for children with avian influenza, based on experience with one patient.[9]

Co-administration with probenecid

It has been suggested that co-administration of oseltamivir with probenecid could extend a limited supply of oseltamivir. Probenecid reduces renal excretion of the active metabolite of oseltamivir. One study showed that 500 mg of probenecid given every six hours doubled both the peak plasma concentration (Cmax) and the half-life of oseltamivir, increasing overall systemic exposure (AUC) by 150 percent.[10] Although the evidence for this interaction comes from a study by Roche, it was publicised only in October 2005 by a doctor who had reviewed the data.[11] Probenecid was used in similar fashion during World War II to extend limited supplies of penicillin. It is still used to increase penicillin concentrations in serious infections.

Dosage forms

Oseltamivir is marketed by Roche under the trade name Tamiflu, as capsules (containing oseltamivir phosphate 98.5 mg equivalent to oseltamivir 75 mg) and as a powder for oral suspension (oseltamivir phosphate equivalent to oseltamivir 12 mg/ml).

Adverse effects

Common adverse drug reactions (ADRs) associated with oseltamivir therapy (occurring in over 1% of clinical trial participants) include: nausea, vomiting, diarrhea, abdominal pain, and headache. Rare ADRs include: hepatitis and elevated liver enzymes, rash, allergic reactions including anaphylaxis, and Stevens-Johnson syndrome.[4][5]

Various other ADRs have been reported in postmarketing surveillance including: toxic epidermal necrolysis, cardiac arrhythmia, seizure, confusion, aggravation of diabetes, and haemorrhagic colitis.[4]

Neurological effects

There are concerns that oseltamivir may cause dangerous psychological, neuropsychiatric side effects including self harm in some users. These dangerous side effects occur more commonly in children than in adults.[12] This stems from cases in Japan, where the drug is most heavily prescribed, consuming 60% of the world's production[13]. Concern has focused on teenagers, but problems have also been reported in children and adults.

In March 2007, Japan's Health Ministry warned that oseltamivir should not be given to those aged 10 to 19. The Ministry had previously decided, in May 2004, to change the literature accompanying oseltamivir to include neurological and psychological disorders as possible adverse effects, including: impaired consciousness, abnormal behavior, and hallucinations.[13]

According to Japan's Health Ministry, between 2004 and March 2007, fifteen people aged 10 to 19 have been injured or killed by jumps or fallen from buildings after taking oseltamivir, and one 17-year-old died after he jumped in front of a truck.[14] A renewed investigation of the Japanese data was completed in April 2007. It found that 128 patients had been reported to behave abnormally after taking oseltamivir since 2001. Forty-three of them were under 10 years old, 57 patients were aged 10 to 19, and 28 patients were aged 20 or over. Eight people, including five teens and three adults, had died from these actions.[15][16][14]

In October 2006, Shumpei Yokota, a professor of pediatrics at Yokahama City University, released the results of research involving around 2,800 children which found no difference in the behavior between those who took oseltamivir and those who did not. Chugai Pharmaceutical Co. (which produces oseltamivir in Japan) gave Yokota's department 10 million yen (about US$105,000) over five years.[17]

To determine whether to lift the 2007 ban, a research team from the Japanese Health, Labour and Welfare Ministry studied 10,000 children under the age of 18 who had been diagnosed with influenza since 2006. The study was finalised in April 2009. Taking into account all degrees of abnormal behaviour, including minor behavioural problems such as incoherent speech, the study found that children who took oseltamivir were 54 per cent more likely to exhibit abnormal behaviour than those who did not take the drug. When the team limited its analysis to children who had displayed serious abnormal behaviour that led to injury or death, it found those who had taken oseltamivir were 25 per cent more likely to behave unusually.[18]

In November 2006, the United States Food and Drug Administration (FDA) amended the warning label to include the possible side effects of delirium, hallucinations, or other related behavior.[19] This went further than the FDA's previous pronouncement, from a year before, that there was insufficient evidence to claim a causal link between oseltamivir use and the deaths of 12 Japanese children (only two were from neurological problems, although more have died since then).[20] The change to a more cautionary stance was attributed to 103 new reports that the FDA received of delirium, hallucinations and other unusual psychiatric behavior, mostly involving Japanese patients, received between August 29, 2005 and July 6, 2006. This was an increase from the 126 similar cases logged between the drug's approval in 1999 and August 2005.[21]

Roche points out that oseltamivir has been used to treat 50 million people since 1999, and states that influenza may itself cause psychological problems.

In March 2007, the European Medicines Agency said that the benefits of oseltamivir outweighed the costs, but that it would closely monitor reports from Japan.

In April 2007, South Korea issued a safety warning against prescribing oseltamivir to teenagers except in special cases.[22]

Mode of action

Oseltamivir is a neuraminidase inhibitor, serving as a competitive inhibitor towards sialic acid, found on the surface proteins of normal host cells. By blocking the activity of the neuraminidase, Oseltamivir prevents new viral particles from being released by infected cells.[4]

Resistance

As with other antivirals, resistance to the agent was expected with widespread use of oseltamivir, though the emergence of resistant viruses was expected to be less frequent than with amantadine or rimantadine. The resistance rate reported during clinical trials up to July 2004 was 0.33% in adults, 4.0% in children, and 1.26% overall. Mutations conferring resistance are single amino acid residue substitutions in the neuraminidase enzyme.[7]

Seasonal flu

According to the CDC, Tamiflu may not be able to treat Flu type A, the most common influenza virus in 2008. Doctors are being warned to watch out for it so they can attempt to use other treatments if Tamiflu doesn't work.[23]

In 2008, the World Health Organization announced that preliminary results from experiments with Canadian Influenza A virus subtype H1N1 showed that 8 out of 81 samples were resistant to oseltamivir.[24] Tamiflu-resistant strains have also appeared in the EU, which remain sensitive to zanamivir.[25][26]

In the 2007-2008 US flu season, resistance of influenza A(H1N1) was 12.3%, in preliminary data of 2008-2009 resistance reached 98.5%.[27]

Swine flu concerns

A study published in the 2009 June Issue of Nature Biotechnology emphasized the need for augmentation of oseltamivir (Tamiflu) stockpiles with additional antiviral drugs including zanamivir (Relenza) based on an evaluation of the performance of these drugs in the scenario that the 2009 H1N1 'Swine Flu' neuraminidase (NA) were to acquire the tamiflu-resistance (His274Tyr) mutation which is currently widespread in seasonal H1N1 strains.[28] Five cases of swine flu resistance to Oseltamivir were diagnosed, to July 2009.[29]

H3N2

Mutant H3N2 influenza A virus isolates resistant to oseltamivir were found in 18% of a group of 50 Japanese children treated with oseltamivir.[30] This rate was similar to another study where resistant isolates of H1N1 influenza virus were found in 16.3% of another cohort of Japanese children.[7] Several explanations were proposed by the authors of the studies for the higher-than-expected resistance rate detected. First, children typically have a longer infection period, giving a longer time for resistance to develop. Second, Kiso et al. claim to have used more rigorous detection techniques than previous studies.[30]

H5N1 avian influenza

High-level resistance has been detected in one girl suffering from H5N1 avian influenza in Vietnam. She was being treated with oseltamivir at time of detection.[31][32] de Jong et al. (2005) describe resistance development in two more Vietnamese patients suffering from H5N1, and compare their cases with six others. They suggest that the emergence of a resistant strain may be associated with a patient's clinical deterioration. They also note that the recommended dosage of oseltamivir does not always completely suppress viral replication, a situation that could favor the emergence of resistant strains. Moscona (2005) gives a good overview of the resistance issue, and says that personal stockpiles of Tamiflu could lead to under-dosage and thus the emergence of resistant strains of H5N1.[33]

Resistance is of concern in the scenario of an influenza pandemic (Wong and Yuen 2005), and may be more likely to develop in avian influenza than seasonal influenza due to the potentially longer duration of infection by novel viruses. Kiso et al. suggest that "a higher prevalence of resistant viruses should be expected" during a pandemic.[30]

Resistance mechanism

The genetic sequence for the neuraminidase enzyme is highly conserved across virus strains. This means that there are relatively few variations, and there is also evidence that variations that do occur tend to be less "fit." Thus, mutations that convey resistance to oseltamivir may also tend to cripple the virus by giving it an otherwise less-functional enzyme. The lack of variation in neuraminidase gives two advantages to oseltamivir and zanamivir, the drugs that target that enzyme. First, these drugs work on a broader spectrum of influenza strains. Second, the development of a robust, resistant virus strain appears to be less likely.[7] It is worth noting that the oseltamivir-resistant strains detected by Kiso et al. all appeared within individual children after treatment with oseltamivir – the children did not catch the resistant strains in human-to-human or bird-to-human transmission.[30]

In 2007, Japanese investigators detected neuraminidase-resistant Influenza B virus strains in individuals who had not been treated with these drugs. The prevalence was 1.7%.[34]

Pandemic fears

Oseltamivir was widely used during the H5N1 avian influenza epidemic in Southeast Asia in 2005. In response to the epidemic, various governments – including those of the United Kingdom, Canada, Israel, United States and Australia – stockpiled quantities of oseltamivir in preparation for a possible pandemic. Although large, the quantities stockpiled would not have been sufficient to protect the entire population of these countries.[citation needed]

In late October 2005, Roche announced that it was suspending shipments to pharmacies in the United States and Canada until the North American seasonal flu outbreak began, to address concerns about private stockpiling and to preserve supplies for seasonal influenza. It said that, when distribution resumes in Canada, the remaining available drug will be saved for use in high-risk settings like long-term care facilities and hospitals.[35] Sales were suspended in Hong Kong as well, and on November 8, 2005, also in China. Roche said it would instead send all supplies to China's health ministry.[36]

On November 9, 2005, Vietnam became the first country to be granted permission by Roche to produce a generic version of oseltamivir.[37] The week before, Thai authorities said they would begin producing generic oseltamivir, claiming that Roche had not patented Tamiflu in Thailand.[38] The first Thai generic oseltamivir was produced in February 2006 and was to have been available to the public in July 2006.

In December 2005, Roche also signed a sublicense for complete oseltamivir production with China's Shanghai Pharmaceuticals, and by March 2006 a sublicense had also been granted to India's Hetero.[39][40] In June 2006, the Chinese government gave Shanghai Pharmaceuticals permission to proceed, based upon tests of the domestic production. The company said it planned to market the drug by the end of the month.[41]

In late May 2006, the World Health Organization asked Roche to be ready to ship an emergency stockpile of oseltamivir to Indonesia if needed. The alert was in response to suspected human-to-human transmission within a family and was planned to last for two weeks.[42]

In December 2008, the Indian drug company, Cipla won its case in India's court system allowing it to manufacture a cheaper generic version of Tamiflu, called Antiflu. In May 2009, Cipla won approval from the World Health Organization certifying that its drug Antiflu was as effective as Tamiflu, and Antiflu is included in the World Health Organization list of prequalified medicinal products.[43]

On April 24, 2009, with three cases of swine flu identified in Texas, Governor Rick Perry requested 37,430 doses of Tamiflu from the Centers for Disease Control and Prevention.[44][45]

U.S. Government policy and oseltamivir

In November 2005, U.S. President George W. Bush requested that Congress fund US$7.1 billion in emergency spending for flu pandemic preparedness (the Senate had already passed a US$8.1 billion bill). Bush's plan included US$1.4 billion for government purchases of antiviral drugs.

Personal stockpiling of oseltamivir

Marketing display used at festivals features a person living in a hermetically-sealed environment

A short supply of oseltamivir prompted some individuals to stockpile the drug. Several American states, including Massachusetts and Colorado, issued advisories strongly discouraging this practice. Production has now caught up with current demand (see below), so some (but not all) of the practical and ethical issues surrounding personal stockpiling are reduced. It is possible that shortages could again be encountered if a global influenza pandemic actually arose.[citation needed]

In the New England Journal of Medicine, Anne Moscona (2005) argues that the use of personal stockpiles of oseltamivir could result in the administration of low dosages, allowing for the development of drug-resistant virus strains.[33] Many stockpilers will only have ten 75 mg pills (the current recommended dosage for oseltamivir), but this may be insufficient for the treatment of H5N1.[6]

Another argument against individual stockpiling is that limited drugs should be kept for more strategic deployment, that is, to hard-hit areas, to people in critical roles (e.g., healthcare and government workers), to people vulnerable to seasonal flu, or to people who actually have come down with avian influenza. Ethical arguments are sometimes made as to whether affluent people or nations should have preferred access to antiviral medications. Illegal importation might divert the drug from poorer countries where the risk of avian influenza is actually higher. A counter argument is that it is difficult to justify prohibition of individual stockpiling, when some of the same arguments are pertinent to corporate stockpiling, which is both allowed and encouraged.[46]

A third argument is that it would be difficult for home users to determine whether illegally-imported Tamiflu is counterfeit. This is genuinely a potential problem, but, in the face of a shortage, some individuals might be willing to face such a risk. In December 2005, 53 packages of counterfeit Tamiflu tablets were intercepted by the US Customs Service in South San Francisco. The packages were labeled "Generic Tamiflu". Roche officials know of only one instance of counterfeit Tamiflu appearing outside of the United States: incorrectly-labelled tablets found in Holland, which contained only Vitamin C and lactose. However, sophisticated criminals could produce convincing fake packaging in the future.[47]

Finally, a fourth purported problem is that the H5N1 virus can be reliably diagnosed only in a small number of labs around the world; therefore, there is no way for home users to know whether flu-like symptoms are the result of avian flu or a more benign ailment. This argument lacks face validity, since treatment must begin before such tests results would be available anyway.[citation needed]

An argument in favor of individual stockpiling is that Roche is on the record as saying that without more orders, they may have to actually curtail production.[48] Individual stockpiling could bring market forces to play, maintaining production capacity and allowing the total supply on hand to be higher in case demand again outstrips production in the future, for instance, during a sudden influenza outbreak.

Veterinary use

There have been anecdotal reports of oseltamivir reducing disease severity and hospitalization time in canine parvovirus infection. The drug may limit the ability of the virus to invade the crypt cells of the small intestine and decrease gastrointestinal bacteria colonization and toxin production.[49]

Physical properties

Aqueous solubility of oseltamivir in form of phosphate salt is 588 mg/ml at 25°C.[50]

Chemical synthesis

Main article: Oseltamivir total synthesis

The current production method features a number of reaction steps, two of which involve potentially hazardous azides. A reported azide-free Roche synthesis of tamiflu is summarised graphically below:

synthesis of tamiflu
synthesis of tamiflu

Production shortage/shikimic acid

In early 2005, Roche announced a production shortage. (See Pandemic Fears, above). However, 2006, Roche said that production was about to reach 400-million treatment courses annually, that "capacity was well in excess of total government orders placed to date," and that "the supply shortage no longer exists." Total government orders between 2005 and 2007 were estimated to be around 200 million treatment doses. In fact, Roche CEO William Burns said that a shortage of orders could cause Roche to reduce production in the future. Roche attributes production increases in part to its agreements with 15 external contractors in 9 countries.[46][48][51]

While current demand for seasonal influenza treatment and pandemic stockpiling are being met, it is unclear what the situation would be if a pandemic actually started. Doctors are now testing a doubling of the standard dose with the hopes that it would cut H5N1 influenza virus death rate.[52] If this became the new standard, it would decrease the effective supply.

According to Roche, the major bottleneck in oseltamivir production is the availability of shikimic acid, which cannot be synthesised economically and is only effectively isolated from Chinese star anise, an ancient cooking spice; the herb is also used in Traditional Chinese Medicine. Although most autotrophic organisms produce shikimic acid, the isolation yield is low. A shortage of star anise is one of the key reasons why there is a worldwide shortage of Tamiflu (as of 2005). Star anise is grown in four provinces in China and harvested between March and May. It is also produced in Lang Son province, Vietnam. The shikimic acid is extracted from the seeds in a ten-stage process. Thirteen grams of star anise make 1.3 grams of shikimic acid, which can be made into 10 oseltamivir 75 mg capsules. Ninety percent of the harvest is already used by Roche in making oseltamivir.

Some academic experts[who?] and other drug companies are disputing the difficulty of producing shikimic acid by means other than star anise extraction. An alternative method for production of the acid involves fermentation of genetically-modified bacteria. Recently, biosynthetic pathways in Escherichia coli have been enhanced to allow the organism to accumulate enough shikimic acid to be used commercially.[53][54][55] Canadian generic drug company Apotex is attempting to modify oseletamivir to use a synthetic alternative to shikimic acid. Other potential sources of shikimic acid include the sweetgum and ginkgo trees. Quinic acid, derived from the bark of the cinchona tree, is a potential alternative base material for the production of oseltamivir. In addition, Aminoshikimic acid, biosynthesized via fermentation of genetically-modified bacteria (Guo and Frost)[56], is a very promising alternative starting material for the production of oseltamivir.

The multistep synthesis above shows that although the major bottleneck for Roche may be the availability of shikimic acid, production of oseltamivir is very involved. Increasing production volume (by Roche or others) would require construction of extensive new facilities (which may not be amenable to scaleup and, even if identical on paper, may not necessarily produce acceptable yields), and even if current facilities could handle a larger feedstock quantity, there would be a delay in production as the material makes it down the pipeline (~6 months or so). Producing large amounts of Tamiflu not only takes months to complete, but is also hazardous. Some of the steps in the synthesis require careful handling and relatively mild reaction conditions, as they involve the use of potentially explosive azide chemistry. Roche has explored ways to speed up production (Chimia 2004, 58, 621). It has developed an azide-free allylamine route from the epoxide to Tamiflu. It has also crafted routes that don’t rely on (–)-shikimic acid: a Diels-Alder-based one that uses furan and ethyl acrylate as starting materials and another that relies on catalytic hydrogenation of an isophthalic acid derivative followed by enzymatic desymmetrization. In addition, Frost and Guo at Michigan State University has developed a microbial synthesis of Aminoshikimic acid, which could reduce the need for azide chemistry if used as a starting material.[57]

See also

References

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  52. ^ "Doctors test double Tamiflu dose to cut H5N1 deaths". Retuers. 2007-03-28. Retrieved 2009-07-29.
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  57. ^ Frost, John. and Guo, Jiantao. (2007). "Synthesis of oseltamivir carboxylates, US patent 2007/0190621 A1". {{cite journal}}: Cite journal requires |journal= (help)CS1 maint: multiple names: authors list (link)

Further reading

  • Pollack, Andrew. Is Bird Flu Drug Really So Vexing? Debating the Difficulty of Tamiflu [News article]. The New York Times (Accessed on November 5, 2005 at http://www.nytimes.com/2005/11/05/business/05tamiflu.html)
  • Schwartz, Nelson . Oct 31, 2005. Rumsfeld's growing stake in Tamiflu: Defense Secretary, ex-chairman of flu treatment rights holder, sees portfolio value growing. Fortune (Accessed on Nov 28, 2005 at http://money.cnn.com/2005/10/31/news/newsmakers/fortune_rumsfeld/?cnn=yes)
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External links

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