Therapeutic drug monitoring (TDM) of anti-TNF therapies Infliximab and Adalimumab
The most promising indication for TDM is the assessment and management of treatment failure (reactive testing). In this setting TDM helps identify patients likely to benefit from dose intensification. The clinical utility of pre-emptive testing to maintain drug levels within a therapeutic range is currently uncertain, although its use in this setting is increasing in clinical practice. Other indications for TDM are detailed at the end of this document.
There are a number of commercially available assays for anti-TNF TDM and significant inter assay variation is observed, particularly with measurement of anti-drug antibodies. A therapeutic range requires correlation of drug concentration with patient outcome, and this has not been robustly defined. For any assay this range is likely to depend upon disease activity, the goal of therapy (higher for mucosal healing and histological healing compared to clinical remission) and the timing of the test (higher during induction compared to maintenance phases of treatment). In addition there are a number of uncertainties regarding the measurement and interpretation of anti-drug antibodies (ADA) In particular the clinical utility of measuring total rather than free antibodies is currently not clear. Data from the PANTS study will help define a therapeutic range and clarify the uncertainties regarding anti-TNF antibody measurement.
Assessment of anti-TNF treatment failure[i]
In the setting of primary non-response or loss of response it is important to first establish that symptoms are caused by active inflammation, rather than non-inflammatory conditions (e.g intestinal strictures, irritable bowel syndrome, bacterial overgrowth or bile salt malabsorption). CRP, calprotectin or endoscopic assessment may be required to confirm the presence of active disease.
Timing of sampling
| Drug | When to test |
| Infliximab | Trough level i.e. ideally immediately before the next dose is due |
| Adalimumab | As close to a trough level as possible. |
Laboratory Methods used at Exeter Clinical Laboratory
Exeter Clinical Laboratory uses ELISA assays to measure the following:
The kits used are manufactured by Immundiagnostik. Kit inserts are available on their web page http://www.immundiagnostik.com/en/home.html
***New Calibration for Infliximab in use from 1st April 2019***
For full details please see https://www.exeterlaboratory.com/test/infliximab-drug-levels/
Comparison between different laboratory methods used to measure antiTNF drug and antibody levels.
It is established that there are variations between different laboratory methods used to measure antiTNF drug and antibody levels. Please note, that the assays used measure total anti-drug antibodies rather than free anti-drug antibodies.
Measuring range
The measuring ranges of the assays are as follows:
Drug Level Assays
| Analyte | Lower limit of measuring range | Upper limit of measuring range |
Units
|
| Infliximab drug levels | 0.8 | 45 | mg/L |
| Adalimumab drug levels | 0.8 | 45 | mg/L |
We have demonstrated that the infliximab drug level assay measures the biosimilar drugs in the same manner as the parent drug.
Anti-drug antibody Assays
| Analyte | Cut-off for positive result | Upper limit of measuring range |
Units
|
| Total anti-Infliximab antibody levels | 10 | 400 | AU/mL |
| Total anti-Adalimumab antibody levels | 6 | 200 | AU/mL |
AU/mL = arbitrary units per mL
For all positive results, the centiles for anti-drug antibodies to infliximab and adalimumab is as follows:
Total Infliximab Antibodies
Total Antibody concentration >/= 10 AU/mL is positive. The distribution for 3760 positive results is as follows:
| Centile range | Total Infliximab antibodies (AU/mL) |
| 0 – 5th | 10 – 11 |
| 6th – 25th | 12 – 18 |
| 26th – 50th | 19 – 40 |
| 51st – 75th | 41 -115 |
| 76th – 95th | 116 – 400 |
| >95th | >400 |
The top of the measuring range is 400 AU/mL. This corresponds to the 95th Centile
Total Adalimumab Antibodies
Total Antibody concentration >/= 6 AU/mL is positive. The distribution for 600 positive results is as follows:
| Centile range | Total Adalimumab antibodies (AU/mL) |
| 0 – 5th | 10 – 11 |
| 6th – 25th | 12 – 21 |
| 26th – 50th | 22 – 57 |
| 51st – 75th | 58 – 199 |
| >76th | >200 |
The top of the measuring range is 200 AU/mL. This corresponds to the 76th Centile
Limitations of testing
It is not possible to provide a therapeutic range for antiTNF drug level monitoring, as the evidence does not yet exist which correlates drug concentration with patient outcome. We anticipate that the PANTS study will contribute to the derivation of this evidence.
Interpretation of results – in the setting of active disease
The following table may be used to guide patient management in the setting of active disease i.e. primary non-response, or loss of response. Drug and ADA concentrations should be interpreted with the clinical status of the patient.
The lower limit of the therapeutic ranges is yet to be established, and will be derived from PANTS study data. Undetectable drug levels (i.e. < 0.8 mg/L) is however, certainly subtherapeutic. The true lower limit of the therapeutic range is likely to be greater than 0.8 mg/L.
| antiTNF drug level | Total anti-drug antibody (ADA) level | Interpretation | Considerations for patient management |
|
Sub-therapeutic Lower limit of therapeutic range is yet to be established, and will be derived from PANTS study data
|
Absent
|
Insufficient bioavailability of the drug due to non-adherence or non-immune mediated pharmacokinetic mechanisms.
|
Check for non-adherence (deliberate or accidental) Consider intensifying current anti-TNF drug: i.e. double the dose of anti-TNF or shorten the interval between doses. Suggest recheck drug levels and anti-drug antibodies in 8-16 weeks to assess response to intervention. |
|
Sub-therapeutic Lower limit of therapeutic range is yet to be established, and will be derived from PANTS study data
|
Low positive titre (likely < 50th Centile)
|
Insufficient bioavailability of the drug. Role of immunogenicity vs. non-immune pharmacokinetic mechanisms uncertain.
The higher the ADA concentration the less likely this is to be a transient phenomenon and suggests immune clearance of drug
|
Consider intensifying current anti-TNF drug: i.e. double the dose of anti-TNF or shorten the interval between doses.
Low concentrations of ADA may be transient and overcome with dose escalation. Suggest recheck drug levels and anti-drug antibodies in 8-16 weeks to assess response to intervention and to look for emerging immunogenicity. If patient not taking an immunomodulator then consider starting. If a low concentration of ADA is repeatedly detected in the presence of low drug levels switch to alternative anti-TNF therapy or out-of-class. |
|
Subtherapeutic Lower limit of therapeutic range is yet to be established, and will be derived from PANTS study data |
High positive titre (likely >50th Centile)
|
Insufficient bioavailability of the drug due to immunogenicity with functional anti-drug antibodies that increase drug clearance
|
Dose escalation with the same antiTNF is unlikely to lead to sustained benefit. Consider switching to second antiTNF or out-of-class. Start immunomodulator if not already taking one. |
|
Therapeutic
|
Absent
|
Consider non-TNF driven disease or symptoms caused by problems other than active IBD |
Consider non inflammatory causes for ongoing symptoms If presence of active disease confirmed then consider switching out of class |
|
Therapeutic
|
Positive
|
Uncertain significance. Consider transient antibodies, non-functional antibodies, or false positive antibody test
|
Recommended action uncertain Repeat measurements over time may clarify the cause. If results remain unchanged then a pharmacodynamic problem seems likely. Suggest switch out of class |
Identifying patients at lower risk of relapse after drug withdrawal
TDM is one factor that may help identify patients at lower risk of relapse after withdrawal of anti-TNF therapy. Patients in clinical and biochemical remission (CRP <3) despite an undetectable drug level (i.e. < 0.8 mg/L) with/without ADA may be at a lower risk of relapse. Prior to anti-TNF withdrawal it is recommended that immunomodulatory therapy is optimised whenever possible.
Contact Details
Please contact the RD&E Blood Sciences Duty Biochemist rde-tr.biochem@nhs.net to discuss any aspect of this further.
[i] Hendy P, et al. Frontline Gastroenterology 2016;7:122–128. doi:10.1136/flgastro-2014-100527
Specimen Labelling Procedure
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