July 9, 2001
I have never been a great believer in therapeutic drug monitoring, but I might have to change my opinion about it. The idea behind therapeutic drug level monitoring is that by determining drug levels after the administration of the antiretroviral therapy, one then can try to adjust dosing of the medications to increase the potency and virologic success. This can be done by increasing the dose when necessary and adjusting the levels down when they are high to reduce the toxicity of the medications.
This French study (ATHENA) enrolled 147 patients randomized to have their drug levels monitored frequently and the dosing of the medications adjusted based on those levels (with a delay between the test and the intervention of approximately four weeks) vs. no drug monitoring. Doses could be adjusted up, if the levels were found to be insufficient, or down if they were too high. They presented the data on nelfinavir and indinavir monitoring. Clinicians who enrolled patients in this study received not only the drug level value, but also some advice of what to do about it.
In the case of indinavir, dosing was adjusted if necessary. At the end of the 48-week follow up, more patients stayed on indinavir if they had their drugs levels monitored than if they did not. The reason for this was that the patients who had their drug levels monitored had less episodes of toxicity (less kidney stones). There were no differences in virologic failure between the arms. The rate of limiting toxicity in the non-monitored indinavir group was quite high (more than 30%), which made some people question the applicability of the trial.
For nelfinavir the story was different. Dietary recommendations were given first and then adjustment of the dosing of this drug was done. In this case more patients also stayed on nelfinavir if they had their levels monitored than if they did not. But here what was driving this result was not a decrease in toxicity, rather a decrease in virologic failures in those patients who had their levels monitored.
The study is important because it shows nicely how drug monitoring can be used in clinical practice to improve virologic and potentially clinical outcome in our patients. It also shows that the application of the information provided by drug level monitoring is very drug-specific and the benefits obtained with this approach will be different with different drugs -- something important to keep in mind in future clinical trials.
I think therapeutic drug monitoring is here to stay. We need to figure out a way this information can be provided to the clinician in a more timely manner. Four weeks' delay for a lab test result seems like an eternity in this era of immediate gratification.
I have been very skeptical about TDM, but studies like this one are starting to convince me.
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