Marketed under the trade name Videx, didanosine (DDI) is a nucleotide antagonist originally used in the treatment of HIV/AIDS following AZT administration. Serious side effects associated with use of DDI, however, hamper drug therapy based on this pharmaceutical.

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Better known to biochemists as 2',3'-dideoxyinosine or 9-(2,3-didesoxy-beta-para-ribofuranosyl)-1,9-dihydro-6H-purin-6-one, DDI is a synthetic purine nucleoside. It acts as a reverse transcriptase inhibitor, preventing further reproduction, but not affecting existing viruses. Often, didanosine is used in combination with other agents, including a protease inhibitor, to treat infections with the human immunodeficiency virus. Rapidly metabolized in the patient's liver, the half-life of elimination after ingestion is between 1 and 2 hours. Functioning as an antagonist of nucleotides, this organic compound appears to prevent uncontrolled replication of the HIV/AIDS viruses. Doctors often prescribe it to patients with advanced infections who have deteriorated under AZT treatment. An additional effect of DDI treatment is an increase in the number of CD4 helper white blood cells, from the depressed counts associated with advanced stages of this viral scourge. Unfortunately, didanosine often produces serious side effects including pancreas inflammation, painful nerve damage, and occasionally death.

In its pure white crystalline powder form, DDI is composed of 10 carbon, 12 hydrogen, 4 nitrogen, and 3 oxygen atoms, and has a molecular weight of 236.2. An aqueous solution of didanosine has a pH of about 6.0. The therapeutic chemical becomes unstable in more acidic solutions, with 10 percent decomposing to hypoxanthine in less than 2 minutes at pH 3. This partially explains the mechanism by which the active ingredients enter the blood stream through the gastrointestinal lining following ingestion. As the second antiretroviral agent approved by the U.S. Food and Drug Administration (FDA) for the treatment of HIV/AIDS, didanosine was labeled in 1991 only for use following prolonged AZT treatments. More recently, the FDA approved DDI as a front-line therapy against the deadly viruses.