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Mechanism-based pharmacokinetic modelling to describe the effect of protein binding on the pharmacokinetics of solifenacinAshley Strougo1,2,, Walter Krauwinkel1, Meindert Danhof2, Jan Freijer11 Exploratory Development Department, Astellas Pharma Europe, The Netherlands; 2Division of Pharmacology, LACDR, Leiden University, The Netherlands

[email protected]


 Solifenacin succinate is a muscarinic receptor antagonist used for the symptomatic treatment of overactive bladder (OAB).

 The parent compound extensively binds to α1-acid glycoprotein (AGP).


Develop a mechanism-based model that considers plasma protein binding, plasma volume and body composition and holds improved properties for extrapolation and prediction.


  • Model structure
  • Law of mass action applied to describe the reversible solifenacin-AGP, solifenacin-albumin and solifenacin-VBC binding
  • VBC positioned outside of the plasma

Table 1: Data overview

Absorption compartment


Central compartment





Peripheral compartment


Total, free, AGP (range 30 - 166 mg/dL) and albumin (range 2.5 - 5.1 g/dL) concentration available; *Model validation


  • Model equations
  • NONMEM VI and library ADVAN4 were used
  • Parameters were defined as follows:
  • Model results


  • Visual Predictive Check


Figure 1: Relationship between free fraction and plasma proteins. Symbols: observed data; red line: population prediction; salmon shade: 90% confidence interval of the population prediction


  • Conclusion
  • The developed mechanism-based PK model:
  • Adequately describes the PK of solifenacin in different sub-populations
  • Explains considerable part of the inter-individual variability
  • Constitutes a theoretical framework that could be also used to explore and quantify the effect of protein binding on the PK of other compounds
  • Holds much-improved properties for extrapolation and prediction by considering differences in body composition, plasma volume, AGP and albumin plasma concentrations

Figure 2: Visual predictive check. Symbols: observed data; red line: population prediction; salmon shade: 90% confidence interval of the population prediction; dark grey shade: 90% of the population predicted based only on covariates; light grey shade: 90% of the population including random-effects


1 Smulders et al.(2007),Pharmacol Sci 103: 67 - 74

2 Kuipers et al.(2006), Pharmacol Sci 102: 405 -412

3 Krauwinkel et al.(2005), Int J Pharmcol Ther 43:227-238

4 Wilkison et al. (1983), Drug Met Rewviews 14:427-465