Therapeutic Drug Monitoring of the Subcutaneous Formulations of Infliximab and Vedolizumab—Current Knowledge and Future Directions

Therapeutic drug monitoring of the intravenous formulations of infliximab in particular, but also vedolizumab, has become an important means of optimising these agents to minimise primary and secondary loss of response. More recently subcutaneous formulations of both infliximab and vedolizumab have become widely available. These new molecules offer patients the convenience of self-administration, and also have pharmacokinetic benefits via maintaining high drug levels, reducing the risk of the development of immunogenicity. It took many years before recommended therapeutic target ranges for intravenous biologics were agreed on, and it is now clear that target levels for the subcutaneous formulations are different, and further research is required before optimal drug levels are confirmed. This narrative review summarises the current literature of therapeutic drug monitoring of subcutaneous infliximab and vedolizumab, acknowledging that this evidence base is presently incomplete. We also aim to provide clinicians with some practical recommendations for the use of TDM with these formulations in clinical practice today. In summary, we recommend performing TDM of the IV formulations prior to switching and then measuring drug levels at 8 weeks after switching to SC infliximab and at 16 weeks for SC vedolizumab. We suggest provisional target drug levels obtained from post hoc analyses of >14 μg/mL for SC infliximab and 25-35 μg/mL for SC vedolizumab. We recommend performing reactive TDM in cases of loss of response to SC therapy. In conclusion we offer suggested areas for future research.