Monitoring Drug Levels - The AIDS InfoNet

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New Mexico AIDS InfoNet

Fact Sheet Number 418

MONITORING DRUG LEVELS

WHAT IS THERAPEUTIC DRUG MONITORING?
HOW CAN TDM HELP?
DOES TDM WORK FOR ALL HIV DRUGS?
DIFFICULTIES WITH TDM
WHEN CAN TDM HELP?
FUTURE DIRECTIONS

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WHAT IS THERAPEUTIC

DRUG MONITORING?

Therapeutic Drug Monitoring, or TDM, means testing a patient’s

blood to check the levels of a medication they are taking. Drug

levels that are too high sometimes cause serious side effects.

Levels that are too low might allow HIV to multiply and develop

resistance.

TDM is not generally used or available yet in the United States.

Researchers do not agree on its benefits.

HOW CAN TDM HELP?

Even when people take the same dose of a drug, blood levels can

be very different. Several factors can affect drug levels:

Food effects: more or less drug can be absorbed depending
on how much food is in the stomach, or what kind of food
Metabolism: how fast the drugs are broken down in the body
Drug interactions: some drugs affect the metabolism of other
drugs, and can raise or lower their levels.
Smoking and drinking habits
Herbal and other supplements. For example, garlic capsules
can reduce blood levels of saquinavir.
Hepatitis or other liver disease or damage, or kidney problems
Pregnancy
Age: children and adolescents process drugs differently than
adults.

If the viral load isn’t going down far enough, it might be

because drug levels are too low. A doctor might be able to increase

the dose and bring HIV under control.

If a patient is having serious side effects, it might be because

drug levels are too high. If they are, a smaller dose might still

control HIV but relieve some side effects.

DOES TDM WORK

FOR ALL HIV DRUGS?

TDM might work well for protease inhibitors and for non-nucleoside

reverse transcriptase inhibitors (non-nukes.) Research shows that

blood levels of these types of drugs affect their ability to control

HIV and to cause side effects.

The nucleoside analog reverse transcriptase inhibitors (nukes)

are a different case. They must be processed inside individual

cells before they’re active against HIV. Therefore, the blood

level of these drugs is less important than the amount inside

cells. This is very difficult to measure. We don’t yet know whether

the drug levels of the nukes affect how well they work, or the

side effects they cause. TDM is not yet being used with the nukes.

DIFFICULTIES WITH

TDM

1. TDM is not ready for use with the “nuke” drugs.

As noted above, blood levels of the nukes aren’t as important

as the levels inside HIV-infected cells. The technology for measuring

these levels is not very good yet.

2. There isn’t just one target blood level for each drug.

The “best” amount of drug for a patient depends on how

resistant their virus is to that drug. The more resistant the

virus, the higher the blood level of drug needed to control it.

3. It’s difficult to measure drug levels accurately.

With the current technology, repeated tests can give very different

results.

4. Adherence is very important.

Missing doses of a drug can make more of a difference than any

other factor in how well HIV is controlled, and can throw off

the results of TDM.

5. TDM might not make any difference.

Many doctors use a dose of ritonavir to boost blood levels. It

might not be possible to increase the blood levels any further,

even with an increased dose.

6. Lowering blood levels won’t decrease all side effects.

Some side effects aren’t linked to the amount of drug in the body.

Higher doses probably cause more stomach problems with the protease

inhibitors, or kidney stones with indinavir. However, the hypersensitivity

reaction to abacavir is unrelated to the dose of the drug. TDM

won’t reduce this side effect.

7. TDM is expensive.

Because it’s still experimental, it’s difficult to get reimbursement

for TDM.

WHEN CAN TDM HELP?

TDM could provide useful information in several situations:

For pregnant patients
For children and teen-aged patients
To assess interactions between HIV medications
To assess interactions with non-HIV medications such as birth
control pills, methadone, or TB medications
For patients with high levels of side effects
When treatments fail to control HIV even though patients
are taking all their doses

FUTURE

DIRECTIONS

Research will continue on several key questions related to TDM:

1. For each drug, what is the relationship between drug levels

and viral control? How sensitive is it?

2. For each drug side effect, how do drug levels affect the

amount or severity of side effects?

3. How can drug levels of the nukes, inside the cells, be measured

more accurately?

Revised January 19, 2002