Nucleoside/Nucleotide Reverse Transcriptase Inhibitors (NRTIs)

Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) are a class of drugs known as “antiretrovirals,” and which are used in the treatment of HIV infection, hepatitis B (HPB), tumors, and also certain cancers. These agents work by inhibiting the normal functioning of an enzyme known as reverse transcriptase, which is an essential element in the reproductive processes of retroviruses. By interfering with the natural mechanisms of these enzymes, the replication of retroviruses responsible for HIV, cancer, and other conditions is blocked.

Specifically, nucleoside/nucleotide reverse transcriptase inhibitors work by altering the DNA replication process. When a virus infects a host cell, it looks for a way to replicate itself, using the host cell’s replication processes. In some cases, the virus copies its genetic code directly into the host cell’s DNA. But in the case of tumors and HIV, the viral genetic code is in the form of RNA. Before it can be coped into the host cell’s DNA, it must be modified into DNA. The reverse transcriptase enzyme is the trigger mechanism for converting viral RNA into DNA that is compatible with the host cell. Nucleotides and nucleosides are the fundamental elements used by the reverse transcriptase enzymes to complete the transformation to DNA.

Nucleoside/nucleotide reverse transcriptase inhibitors actually contain imperfect versions of these nucleotides and nucleosides. When reverse transcriptase enzymes incorporate these faulty building blocks during the RNA-DNA conversion process, the resultant DNA is also faulty, and unable to reproduce. As a result, the genetic material of the virus cannot be assimilated into the host cell’s DNA, and the cell cannot produce new viruses.

The following is a list of the generic and trade names of nucleoside/nucleotide reverse transcriptase inhibitors on the market:

· Abacavir, also called ABC, is sold under the trade name Ziagen
· Adefovir, also known as bis-POM PMPA, is sold under the names Preveon and Hepsera (this agent does not have FDA approval for the treatment of HIV)
· Didanosine, also called ddl, is sold under the trade names Videx and Videx EC
· Emtricitabine, also called FTC, is sold under the trade name Emtriva, which was previously known as Coviracil
· Lamivudine, also called 3TC, is sold under the trade name Epivir
· Stavudine, also called d4T, is sold under the trade names Zerit and Zerit XR
· Tenofovir, also known as tenofovir disopoxil fumarate, is sold under the trade name Viread
· Zidovudine, also called AZT, ZDV, and azidothymidine, is sold under the trade name Retrovir
· Aprictabine, also called ATC, is in Phase III clinical trials and is expected to be released in 2011
· Zalcitabine, also called ddC and dideoxycytidine, previously sold under the trade name Hivid, but has been recalled by the manufacturer

Most NRTIs are taken as pills, although some are available as liquids. Therapy with these agents is usually prolonged, often exceeding a year and much longer in HIV patients, and relapse may occur once the medication is stopped. Some patients develop resistance to an agent after prolonged therapy, and a new agent must be substituted.

Adverse effects

NRTIs are generally well tolerated and the advent of adverse side effects during NRTI use is rare. When they do occur, side effects may include headache, dizziness, back or abdominal pain, sore throat, fever, diarrhea, and a generally feeling of fatigue or weakness. In very rare cases, NRTI use has been associated with kidney or liver failure, as well as a condition known as lactic acidosis, where unhealthy levels of lactic acid accumulate in the blood. Patients experiencing any of the following symptoms should contact their doctor for testing: severe or prolonged muscle pain; heartbeat irregularities or abnormally rapid heartbeat; nausea; severe abdominal pain; persistent dizziness; pale stools; or yellowing of the whites of the eyes or skin.

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