Blood tests are critical in the medical field for a variety of reasons. Specifically, blood tests can help doctors evaluate how well the organs are working, diagnose diseases, and assess how well your blood is clotting. In addition, a blood test can also ensure the amount of medicine you are taking is both safe and effective.
For patients suffering from inflammatory bowel disease (IBD), routine blood draws can help physicians understand how long infliximab (IFX) is present in the patient’s body before they clear the drug. By monitoring clearance and half-life value, physicians will be able to provide an accurate dosing interval, allowing the drug to be present long enough and at a high enough exposure to treat the patient’s condition.
What is Therapeutic Drug Monitoring (TDM)?
Therapeutic drug monitoring is “the measurement of specific drugs and/or their breakdown products at timed intervals to maintain a relatively constant concentration of the medication in the blood.” The purpose of therapeutic drug monitoring is to enhance drug efficacy, reduce toxicity and/or assist with diagnosis.
When a drug enters the body, it is absorbed into the blood or administered as IV directly into the blood, and distributed throughout the body. Once the drug reaches a peak level, the drug levels will begin to fall steadily as it’s eliminated from the body. Without routine blood draws, physicians will not have an accurate indication of how fast (or slowly) the drug is cleared from the patient, causing the patient to lose response to the drug or if the drug clears slowly potentially cause adverse events.
Timing Over Dosing
When following the standard infliximab label, patient clearance and half-life are not taken into account. According to research, “the standard infliximab maintenance dosing is 5 mg/kg every 8 weeks and has shown to be inadequate to consistently achieve sufficient drug exposure to minimize loss of response.”
Timing can be more important than dosing, as it allows the drug to be dosed before it is completely eliminated from the body. In some cases, patients are encouraged to take more of a drug. However, increasing the dose doesn’t always fix the problem with patients with a short half-life and low troughs.
Increasing the dose for small molecules often works well because the dosing interval is quite short. However, for biologics which typically have longer dosing intervals (2 to 8 weeks), increasing the dose may not work because half-life (the time to reduce a drug concentration by half) is not altered by the dose.
The fact that increasing the dose can be readily seen with T3 output. If we consider a 70 kg patient who is getting 5 mg/kg IFX every 8 weeks and who has a measured trough of 2 ug/mL, but we would prefer a trough of 7 ug/mL, we can see the impact of increasing the dose below.
|Effective Half-Life (days)
|Days to Target
|Increase in Duration (Days)
|Increase in Amount if IFX (mg)
This suggests that doubling the IFX dose only maintains a concentration above 7 for an additional 11 days — not much of a benefit.
For patients to achieve sufficient drug exposure during infliximab therapy, it is essential that modifiable factors for regulating drug levels are taken into account, which can be identified through routine blood draws.
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Baysient’s web-based applications, iDose and Time To Target (T3), provide multiple dosing regimens that can allow patients to attain and sustain a therapeutic drug trough level, improving their long-term drug response.
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