Previously, this blog has discussed the use of drug monitoring in inflammatory bowel disease (Drug levels for inflammatory bowel disease | gutsandgrowth). A good review of this topic has been published recently (Clin Gastroenterol Hepatol 2012; 10: 1079-87).
Some of the useful pointers:
- Factors that influence clearance of TNF antagonists are reviewed:
- Antidrug antibodies (ADA) increase clearance and worsen outcomes
- Use of concomitant immunosuppressives reduces the likelihood of ADA formation and increase drug concentration. In the SONIC trial, use of azathioprine was associated with trough infliximab (IFX) levels of 3.5 μg/mL compared with 1.6 μg/mL with monotherapy. Also, ADA was reduce: 0.9% compared with 14.6% in the monotherapy group. [Other studies though have found variable effects of cotherapy.]
- Low serum albumin and high CRP are associated with increased drug clearance
- Individuals with high body size and males are more likely to have increased drug clearance.
- Better assays for measurement of IFX and adalimumab (ADL) are now available.
- Currently a trial evaluating trough levels is underway: Trough Level Adapted Infliximab Treatment (TAXIT). With this study, the accepted target range for trough levels is 3-7 μg/mL. Levels >7 μg/mL are considered supratherapeutic and allows for a prolongation of dosing interval. Preliminary data confirm that trough levels inversely correlate with CRP.
- Proposed algorithm in individuals with loss of response & positive ADA. If ADAs present at high titer, then switch to different TNF antagonist. If low titer, could either switch or attempt drug escalation.
- With IFX, when antibodies to infliximab (ATIs) are present, the likelihood of responding to increased dose is less than 20% whereas changing to different TNF antagonist has about an 90% response (in patients who were previous responders).
- Proposed algorithm in individuals with loss of response & negative ADA. If subtherapeutic trough levels (IFX ❤ μg/mL, ADL <8 μg/mL, or certolizumab <27.5 μg/mL), then dose escalation is worthwhile. If drug levels are therapeutic, then dose escalation will not be effective.
- With IFX, more than 85% of patients will respond to drug escalation when the trough level is subtherapeutic. This is much more favorable than switching agents.
- One other issue with ADAs is that they may be transient is some patients. Perhaps one-fourth of ATIs may be transient which may explain why some individuals with ATIs may still respond to dose escalation.
These points give several reasons why drug monitoring is useful in individuals with loss of response and may help determine whether patients responding to therapy may be able to prolong dose intervals. At the same time, when an individual is not responding to therapy, it is also important to determine if in fact active inflammation is present with objective markers and to consider alternative explanations for GI symptoms (eg. Clostridium difficile infection, irritable bowel, bacterial overgrowth, etc.).
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