The ways of science are sometimes unfathomable: in the 1960s, American researchers set to work to invent a new remedy for cancer - the result was a well-effective drug for AIDS.
Zidovudine is now part of the combination therapy of HIV infection along with other drugs.
The pharmacological action of zidovudine is based on the inhibition of nucleosidic reverse transcriptase, an enzyme that requires HI viruses for their multiplication and pathogenicity.
The human immunodeficiency virus (HIV), which causes the disease AIDS (Acquired Immune Deficiency Syndrome), belongs to the so-called retroviruses. It uses RNA as its genome, not DNA, as is the case with many other life forms, such as humans. In order for HIV to integrate into the human cell and, as is common practice with viruses, to cause it to multiply its genome and thus help it multiply, it requires reverse transcriptase:
It transcribes viral RNA into DNA (usually reversed in biology, hence the term "reverse"), which is then integrated into the normal course of cell metabolism and used to read new proteins and thus new viruses and to make.
Zidovudine is converted intracellularly into its active form zidovudine triphosphate and then has a high affinity for retroviral reverse transcriptase, including HIV-1 and HIV-2. However, it must be remembered that other transcriptases that perform their work in normal cell metabolism are also blocked by the drug with lower affinity, which accounts for much of the side effects.
As an antimetabolite, zidovudine triphosphate is incorporated into the DNA of the provirus instead of a thymidine component and blocks further production at this site. The involved reverse transcriptase is blocked. In this way, zidovudine inhibits only the HI viruses newly entering a host cell - what is already integrated into the cell genome, however, remains unaffected. Therefore, the remedy must always be used in combination with others in order to be able to attack the viral infection in a truly comprehensive manner.
The use of zidovudine usually takes place in the context of a broad-based HIV therapy, the so-called HAART (Highly active anti-retroviral therapy) instead.
After approximately six months of sole zidovudine therapy, the development of resistance on the part of the HI viruses usually occurs, which mutates in several steps and makes their reverse transcriptase insensitive to the drug. In combination with other drugs, this resistance development is made very difficult for the viruses because they are attacked from several sides at the same time.
Usually a triple combination is used, usually two nucleosidic reverse transcriptase inhibitors with a non-nucleoside reverse transcriptase inhibitor or with a protease inhibitor.
The therapy must be closely monitored, especially the viral load and the CD4 cell count are important markers for the direct success of the therapy. Initially, such a treatment was only launched at the time of the onset of the disease AIDS, but nowadays people are more and more inclined to start therapy in earlier stages of infection.
Zidovudine has some side effects that can occur during a long-lasting therapy.
The fact that it should be originally developed as a chemotherapeutic for cancerous tumors, already shows that some side effects must agree with those of chemotherapy: damage to the bone marrow is here, which already two to four weeks after the start of therapy in anemia and usually from from the sixth to the eighth week as neutropenia, ie white blood cell waste.
Neurotoxic effects include, for example, headache (in 50 percent of those treated), insomnia and mental changes. Long-term therapy can also cause muscle pain. Gastrointestinal disturbances, fever and rash may also occur.
Some interactions are also to be considered, among other things, aspirin and morphine may inhibit the degradation of zidovudine in the liver and thus ensure increased drug concentrations. Of course, other cytotoxic or bone marrow-suppressing drugs exacerbate the side effects of zidovudine.