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TAMIFLU
The most frequently prescribed flu medicine
Oseltamivir is a prescription antiviral agent that inhibits the spread of influenza (flu) virus between cells in the body by stopping the virus from chemically reducing ties with its host cell. It is the first orally active neuraminidase inhibitor developed for commercial use by C.U. Kim and X. Chen from the US based firm Gilead Sciences. The drug, which is sold under the trade name Tamiflu, is the most widely prescribed flu medication to date.
Tamiflu is readily absorbed from the GIT after oral administration of oseltamivir phosphate and is extensively converted chiefly by hepatic esterases to become active metabolite. Plasma concentrations of the active metabolite are measurable within 30 min, reach near maximal levels in 2-3 hrs post-dose, and substantially exceed (more than 20 times) those of the prodrug. At least 75% of an oral dose reaches the systemic circulation as the active metabolite. Plasma concentrations of active metabolite are proportional to the dosage and are unaffected by co-administration with food.
As recent as October 2009[update], only 52 out of over 10,000 samples of the prevalent 2009 pandemic H1N1 (swine) flu tested worldwide have shown resistance to oseltamivir as compared with the 99.6% of the 2008 seasonal H1N1 flu strains tested which are also resistant to oseltamivir. Since its development in 1999, Tamiflu has been used to treat and prevent Influenza A and Influenza B in more than 50 million people. Other than its effectiveness there is no evidence for efficacy of Tamiflu in any illness caused by agents other than influenza viruses types A and B.
Tamiflu is available in capsule and powder form. Each capsule of Tamiflu contains oseltamivir phosphate 98.5 mg equivalent to oseltamivir 75 mg. The powder for oral suspension when reconstituted with water to a concentration of 1.2% contains oseltamivir 12 mg/ml.
Mechanism of Action
Oseltamivir phosphate is a prodrug of oseltamivir carboxylate (OC), a potent and selective inhibitor of influenza A and B virus neuraminidase enzymes. Viral neuraminidase is necessary both for viral entry into uninfected cells and for the release of recently formed virus particles from infected cells, and the further spread of infectious virus.
Oseltamivir carboxylate inhibits neuraminidase of influenza viruses of both types A and B. Concentrations of the OC required to inhibit the enzyme activity by 50% (IC50) are in the low nanomolar range. The OC also inhibits influenza virus infection and replication in vitro and inhibits influenza virus replication and pathogenicity in vivo.
More than ninety percent of the absorbed oseltamivir is primarily eliminated by conversion to the active metabolite. The active metabolite is not further metabolized and is eliminated in the urine.
Tamiflu as Prophylaxis of Influenza
The recommended oral dose of Tamiflu for prophylaxis of influenza following close contact with an infected individual is 75 mg once daily for 10 days. Therapy should begin within 2 days of exposure.
· Prophylaxis during a community outbreak of influenza is 75 mg once daily. Safety and efficacy have been demonstrated for up to 6 six weeks. The duration of protection last for as long as dosing is continued.
· Children weighing >40 kg, who are able to swallow capsules, may also receive prophylaxis with a 75 mg capsule once daily.
· The recommended prophylactic oral dose of Tamiflu for children ≥1 year of age is:
o ≤15 kg: 30 mg once daily
o >15-23 kg: 45 mg once daily
o >23-40 kg: 60 mg once daily
o >40 kg: 75 mg once daily
Adverse Drug Reaction
The most frequently reported adverse reaction to Tamiflu medication were nausea and vomiting, usually occurring during the first dose. Other possible but rarely except for rarely occurring include skin and subcutaneous tissue disorder, hypersensitivity reactions, Stevens-Johnson Syndrome, face edema, hepatitis and elevated liver enzymes, convulsion and delirium (symptoms such as altered level of consciousness, confusion, abnormal behavior, delusions, hallucinations, agitation, anxiety, nightmares),
Dosage of Tamiflu
Generally, dosage for treatment of Influenza should begin within the 1st or 2nd day of onset of symptoms.
· Tamiflu capsules in adults and adolescents ≥ 13 yrs is 75 mg twice daily, for 5 days.
· Adults and adolescents ≥ 13 years of age that are unable to swallow capsules may receive a dose of 75 mg Tamiflu suspension twice daily for 5 days.
· For children weighing >40 kg who are to swallow capsules, may also receive treatment with a 75-mg capsule twice daily.
A dosing syringe marked with 30 mg, 45 mg and 60 mg dosing levels is provided for oral suspension. It is recommended that Tamiflu powder for oral suspension be constituted by a pharmacist prior to dispensing to the patient
Doses of up to 1000 mg of Tamiflu have been well tolerated. So far, there have been no documented reports with Tamiflu overdose; however, the anticipated symptoms of acute overdose would be nausea, with or without accompanying emesis.
Individuals who can not take Tamiflu
- Patients with hypersensitivity to oseltamivir phosphate or any of its components
- Children under one year old.
Special Precautions
· Observe for convulsion and delirium like neuropsychiatric events during Tamiflu administration in patients with influenza, predominately in children and adolescents that may result in accidental injury.
· Dose adjustment is recommended for patients with CrCl 10-30 ml/min for the treatment and prophylaxis of influenza.
· For patients with hereditary fructose intolerance since a bottle of 30 g Tamiflu powder for oral suspension has 25.713 g of sorbitol, way above the tolerable daily limit of solbitol intake.
Pregnancy Category (US FDA)
Category C: Studies in animals have revealed adverse effects on the fetus (teratogenic or embryocidal or other) and there are no controlled studies in women or studies in women and animals are not available. Drugs should be given only if the potential benefit justifies the potential risk to the fetus
Storage
Capsules
- Below 30°C., 4 years validity
- Below 25°C., 5 years validity
Powder for Oral Suspension
- Below 25°C.
- After reconstitution, store suspension at room temperature for 10 days or in a refrigerator (2-8°C) for 17 days.
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