Antivirals can be used prophylactically to prevent a virus infection in a person who has not yet been infected but who will be in contact with others who are infected, in much the same way as most vaccines are used. Prophylactic protection with a chemical antiviral is more rapid in onset than that induced by vaccines, since some antiviral protection would be anticipated within 30 minutes of drug administration. Most antivirals are administered by mouth and rapid adsorption of drug and spread to all tissues of the body is often achieved. To maintain active levels of drug in the target organ redosing is required, perhaps twice daily. Clinical use is less convenient if oral absorption is poor, and if the drug has to be given by intravenous infusion or, in the case of respiratory infections, by aerosol or nasal spray. However, it could be argued that with respiratory infection direct application of a drug to the nose and airways could have medical advantages.
Two anti-herpes prodrugs and more recently an anti-influenza neuramini-dase inhibitor have been studied and licensed, which are inactive themselves but are converted by enzymes in the patient to the active antiviral (prodrugs).
It should be remembered that when drug prophylaxis is discontinued the patient becomes susceptible to virus infection unless a subclinical infection has occurred in the first instance, giving the patient some immunity to reinfection with the same virus.
An example of prophylaxis is the use of amantadine or the neuraminidase inhibitors to prevent spread of influenza A virus within families. The drugs are given to the family members who are still well, but in contact with an ill person perhaps in their own family. Under these circumstances nearly 90% protection can be achieved.
Most often antivirals are used therapeutically, being administered either after infection or even after the first clinical signs of the disease are noted. In this situation further progression of the disease may stop and/or the virus infection may resolve more rapidly. Therapy is the favoured mode with HIV-1-infected patients, although the drug may be given before overt clinical signs of disease both to delay the time before early symptoms occur and also to lessen the chance of drug resistance occurring. There is still active debate about when to initiate chemotherapy, early in the disease or later. Aciclovir may be used to prevent recurrent herpes infections and this is a therapeutic use because the patient is already infected with the virus. The three anti-influenza drugs can all be used to ameliorate clinical symptoms and also to prevent secondary complications of influenza, but have to be given to the patient within 48 hours of onset of symptoms. It should be appreciated that even experienced clinicians are still on a learning curve with the prevention and treatment of most viral infections using antivirals.
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