This episode is about the TDM of biologics, a therapeutic class that only really emerged into widespread use about 20 years ago – but that as a class represents a breakthrough for many therapeutic challenges. This area has some differences compared to traditional TDM of small molecules, and although it’s established for some biologics, the role for TDM in the space is, in a sense, still being determined. Florian has consequently titled this episode ‘Tomorrow never knows’ – a great song and a fitting title.   Our guests today are Annick de Vries and Murray Barclay, and both have made important contributions in the space. Once again, our guests coincided on their favourite Beatle – one that we haven’t heard mentioned in previous episodes!   In this episode we hear about: the differences between small molecule drugs and biologics, and the implications this has for TDM of the latter how disease severity may influence pharmacodynamics, and ultimately how we might ultimately select treatment targets despite challenges in obtaining measurements of disease activity anti-drug antibodies and the role of TDM there disease dependent clearance and whether concentrations of biologics might possibly be a marker of disease activity   Eight Drugs a Week is supported by the International Association of Therapeutic Drug Monitoring and Clinical Toxicology (IATDMCT). The personal views of the hosts and guests do not necessarily reflect the position of the Association.   To join IATDMCT, visit https://www.iatdmct.org/member-join Subscribe to Eight Drugs a Week on your preferred podcast platform.   About our guests: Annick de Vries is director of Diagnostic Services at Sanquin in Amsterdam, The Netherlands. Sanquin is a not-for-profit that provides blood products nationwide, but also develop specialized pharmaceutical products and provide diagnostic services. She is the current chair of the IATDMCT Biologics Committee.   Murray Barclay is a clinical pharmacologist and a gastroenterologist at Christchurch Hospital in New Zealand, and a Clinical Professor at the University of Otago. Murray was the previous chair of the Biologics Committee.