Theoretical Analysis of Interplay of Therapeutic Protein Drug and Circulating Soluble Target: Temporal Profiles of 'Free' and 'Total' Drug and Target

To systemically investigate, for a therapeutic protein with a circulating soluble target, how the interplay of target dynamics and drug pharmacokinetics defines the 'total' and 'free' drug and target temporal profiles. By extending the established rapid-binding target-mediated drug disposition (TMDD) pharmacokinetic model to circulating soluble targets, the temporal profiles of 'total' and 'free' drug and target were simulated with varying binding affinity (K(D)), target baseline (R(ss)), target turnover, and drug dose level. Two sets of published experimental data were compared with the simulated results. Binding to a circulating soluble target could lead to a divergence of the 'free' drug from the 'total' drug. Simulations show this divergent magnitude determined by K(D) and R(ss), with the temporal profile being defined by target turnover and drug dose level. As divergence proceeds, starting at the distribution phase, 'free' drug would decline faster but eventually parallel 'total' drug at the terminal phase, giving rise to a steeper distribution phase and comparable terminal half-life, relative to the 'total' form. The model also allows for estimation of the dynamic change of 'total' and 'free' target in response to the treatment of a therapeutic protein drug, facilitating dose level and regimen design to achieve desired 'free' target suppression. Model predictions compared favorably with two sets of published experimental data. Theoretical analyses identified key variables governing the different temporal profiles of 'total' and 'free' drug and target. The rapid-binding TMDD model reasonably captured the features of the interplay of drug pharmacokinetics and target dynamics for two reported cases.