April 13, 2003
Tenofovir belongs to a group of drugs called nucleotide analogues, examples of which also include the following:
All three drugs are used to treat different viral infections. What they all have in common is the potential to cause some degree of kidney damage.
The kidneys filter the blood, flushing waste into the urine and reabsorbing important substances. When kidney damage occurs, these organs may not work properly, which can cause waste to build up and/or cause the body to lose valuable nutrients.
Tenofovir users can develop higher-than-normal levels of creatinine in their blood and urine, suggesting kidney damage. Laboratory tests may also detect less-than-normal phosphorus levels in the blood, another consequence of kidney damage.
Questions remain as to what proportion of tenofovir users is at risk for kidney damage. To try to answer this question, researchers in Vancouver, British Columbia, collected and analyzed data from patients attending HIV clinics and who were in an expanded access program for tenofovir. A research team member made a presentation about this study at CAHR.
The study team compared data from 322 patients who used tenofovir to data collected from 430 other patients who used the nucleoside analogue abacavir (ABC, Ziagen). What makes the researchers' work interesting is how they assess kidney damage. The Vancouver researchers consulted kidney specialists, who told them that creatinine levels greater than 1.5 times each patient's normal values should suggest that kidney damage was occurring. This way of reporting kidney damage is different from that seen in clinical trials of tenofovir. In those trials, attention was drawn to creatinine levels if they were so high they were judged to be "severe" or "potentially life-threatening." This can also be described as grade 3 or grade 4 elevations in creatinine.
In analyzing the data, the Vancouver researchers found that tenofovir users were about three times more likely than abacavir users to develop higher-than-normal levels of creatinine in their blood. Another factor linked to having this problem was low CD4+ counts, roughly fewer than 150 cells. All in all, about 7 percent of tenofovir users in the Vancouver study developed some degree of kidney damage over an average of six months. A total of six patients had to stop taking tenofovir because of injured kidneys.
After listening to the presentation, another researcher from Vancouver in the audience announced that a patient in that city had been recently hospitalized and later died as a result of complications from severe kidney damage related to tenofovir use.
These findings are preliminary and further analysis is needed to take into account other factors that may have had an impact on the results. For instance, there are a number of other drugs that can affect the health of the kidneys. Did tenofovir users in this study take some of these other drugs (see below)? If so, could these drugs have intensified tenofovir-related damage? Examples of drugs that can cause kidney damage and dysfunction may include the following:
About 10 percent of tenofovir users and about 20 percent of abacavir users in this study had previously used the protease inhibitor indinavir (Crixivan) which is also processed by the kidneys. However, exposure to indinavir was not a factor in the development of the kidney problems seen in this study.
Another factor for kidney damage was having a low CD4+ cell count. It was not clear how or why this could increase the risk of kidney damage. Some speculations: It is possible that people with low CD4+ counts were more ill than other patients and therefore more susceptible to drug side effects. Also, patients with low CD4+ cells could be exposed to more drugs (both for HIV and related complications), again increasing the risk for toxicity. However these theories are unproven.
Tenofovir may cause kidney damage by injuring the energy-producing parts of kidney tubules, called mitochondria, as does the nucleotide analogue adefovir. Ways of preventing tenofovir-related kidney damage were not mentioned in the presentation. However, if tenofovir does damage mitochondria, then perhaps a first step in testing ways to prevent this damage is to conduct test-tube experiments with kidney cells and antioxidants such as: