Toxic side-effects are those caused by the effects of a drug, rather than by the immune system’s reaction to the drug. Often the nature of the side-effect will depend on the part of the body adversely affected by the drug or the route of excretion from the body. One instance of this is drugs that damage the bone marrow cells responsible for producing new blood cells, leading to blood abnormalities such as anaemia or neutropenia. Nausea, malaise, diarrhoea, and drowsiness are other common side-effects resulting from toxicity.
Perhaps the organ most vulnerable to toxicity is the liver, the principal organ of drug metabolism. The liver has very high concentrations of most drug-metabolising enzyme systems, relative to other organs. Many drugs can cause liver toxicity, particularly if taken in excess.
Certain drugs can also produce a toxic response by increasing the amount of substances normally present in the body. In HIV treatment, the most common effect of this type is raised cholesterol or triglycerides (hyperlipidemia syndrome), which may be caused by protease inhibitors (PIs) and some of the nucleoside reverse transcriptase (NRTI) drugs. Glucose and bilirubin levels are also elevated by some drugs and may produce laboratory evidence as well as clinical symptoms. Other toxic side-effects include reduced bone density and fat redistribution among people on antiretroviral therapy; kidney failure associated with tenofovir (Viread); and pancreatic failure associated with ddI (didanosine/Videx).
Toxic side-effects are usually dose-related. The greater the amount of drug taken, the greater the risk is of a negative side-effect. Factors that can affect drug metabolism are age, nutrition status, gender, and individual variation.
Some people may be vulnerable to unusual or unpredictable side-effects because of inherited conditions. The most common case of this is among people whose genetic make-up results in very low production of certain enzymes. People lacking the enzyme glucose-6-phosphate dehydrogenase (G6PD) are more likely to suffer toxicities from antibiotics such as dapsone, because the body cannot process the drug and toxic accumulation result. At-risk groups can be tested for these genetic deficiencies.
Another common genetic variation (polymorphism) affecting drug biotransformation is variability of N-acetyltransferase (NAT). This is one of the earliest polymorphisms discovered and has both clinical and toxicological consequences.
Virtually every drug carries the risk of side-effects in a proportion of patients. Additionally, the more drugs an individual takes, the greater the chances are that additional or synergistic toxicity or undesired drug/drug interactions will occur. Side-effects are sometimes referred to as adverse drug reactions (ADRs) or adverse effects (AEs). Much of the information regarding ADRs is gathered when a new drug is being tested in large clinical trials. However, the emergence of metabolic and fat disorders associated with protease inhibitors demonstrates that some side-effects take months or years to be identified.
In the age of highly active antiretroviral therapy (HAART), drug side-effects are a common cause of ill health among people who have an undetectable viral load or asymptomatic HIV disease. Even mild or moderate side-effects can significantly affect the quality of life.
It is not always clear whether unwanted side-effects are a direct result of antiretroviral therapy or of a strengthened immune response to infections. For example, people with HIV and hepatitis C (HCV) who take anti-HIV drugs are more likely to experience liver toxicity than people who do not have HCV. An increased susceptibility to drug-related side-effects could be caused by HCV. Increased susceptibility might also be the result of an improved immune response to HCV leading to liver inflammation.
Additionally, people with HIV infection (or any other immunodeficiency) often have a higher susceptibility to drug side-effects than those with a healthy immune system. For example, cotrimoxazole causes side-effects in HIV-positive people about 30% of the time versus 10% of the time in those who are HIV-negative. People with advanced HIV infection also seem more prone to experience peripheral neuropathy, pancreatitis, and fat wasting from nucleoside analogue use.