Population Pharmacokinetics of Tideglusib in Congenital and Childhood Myotonic Dystrophy Type 1: Influence of Demographic and Clinical Factors on Systemic Exposure

<b>Background</b>: GSK3β is an intracellular regulatory kinase that is dysregulated in multiple tissues in Type 1 myotonic dystrophy (DM-1). Tideglusib inhibits GSK3β activity in preclinical models of DM-1 and promotes cellular maturation, normalising aberrant molecular and behavioural phenotypes. It is currently in clinical development for the treatment of paediatric and adult patients affected by congenital and juvenile-onset DM-1. Here, we summarise the development of a population pharmacokinetic model and subsequent characterisation of influential demographic and clinical factors on the systemic exposure to tideglusib. The availability of a population PK model will allow further evaluation of age-and weight-related changes in drug disposition, supporting the dose rationale and implementation of a paediatric extrapolation plan. <b>Methods:</b> Given the sparse pharmacokinetic sampling scheme in patients receiving tideglusib, model development was implemented in two steps. First, data from Phase I studies in healthy elderly subjects (i.e., 1832 plasma samples, n = 54) were used to describe the population pharmacokinetics of tideglusib in adults. Then, pharmacokinetic model parameter estimates obtained from healthy subjects were used as priors for the evaluation of the disposition of tideglusib in adolescent and adult DM-1 patients (51 plasma samples, n = 16), taking into account demographic and clinical baseline characteristics, as well as food intake. Secondary pharmacokinetic parameters (AUC, C_max and T_max) were derived and summarised by descriptive statistics. <b>Results:</b> Tideglusib pharmacokinetics was described by a two-compartment model with first-order elimination and dose-dependent bioavailability. There were no significant differences in disposition parameters between healthy subjects and DM-1 patients. Body weight was a significant covariate on clearance and volume of distribution. Median AUC_(0-12) and C_max were 1218.1 vs. 3145.7 ng/mL∙h and 513.5 vs. 1170.9 ng/mL, following once daily doses of 400 and 1000 mg tideglusib, respectively. In addition, the time of food intake post-dose or the type of meal appeared to affect the overall exposure to tideglusib. No accumulation, metabolic inhibition, or induction was observed during the treatment period. <b>Conclusions:</b> Even though clearance was constant over the dose range between 400 and 1000 mg, a less than proportional increase in systemic exposure appears to be caused by the dose-dependent bioavailability, which reflects the solubility properties of tideglusib. Despite large interindividual variability in the tideglusib concentration vs. time profiles, body weight was the only explanatory covariate for the observed differences. This finding suggests that the use of weight-banded or weight-normalised doses should be considered to ensure comparable exposure across the paediatric population, regardless of age or body weight.