Mechanistic PK/PD modelling for signal transduction modulators. Application to TGF-beta RI antagonists.

Objectives: To develop a mechanistic PK/PD Model for a new Type I Receptor TGF-b Kinase Inhibitor using human xenografts.

Methods: Human xenografts (MX1-breast and Calu6-NSCLC) were implanted subcutaneously to nude mice. Experiments started 7 to 10 days after tumor implantation. Two different type of experiments were performed: (i) the PK/PD experiment providing information about the plasma levels of the Type I Receptor TGF-b Kinase inhibitor and the percentage change (inhibition) with respect to baseline of phospho-SMAD2,3 (PSMAD) in tumor, and (ii) the tumor growth experiment where the kinetics of tumor growth was followed during 25 to 30 days after the first drug administration. In both experiments drug or saline was administered orally in a single dose or in a multiple dosing design in a range from 10 to 300 mg/kg.

An indirect response model was used to relate the predicted plasma concentrations with the observed inhibition in PSMAD. The model assumes the existence of factors within the tumor cell responsible of the synthesis and degradation of PSMAD.

Tumor size (TS) in animals receiving saline did not reach a plateau and therefore a variant of the Gompertz model allowing for the switch from an exponential to a linear growth was used. Tumor growth inhibition observed in the animals receiving the new compound was linked to the inhibition of PSMAD through a delay in the propagation of the inhibitory signal which was modelled as a chain of transit compartments, and quantified by the mean signal propagation time (MSPT) parameter.

Results: Drug disposition was best described with a two compartment model. Dose and time did not show significant effects (P>0.05) on the kinetics in plasma. The new compound showed very similar PSMAD effects for the two cell lines. Estimates of IC₅₀ (mM) were 0.79 and 0.70 for Calu6-NSCLC and MX1-breast, respectively. The model predicted a complete inhibition of PSMAD at high drug concentrations, and a very rapid turnover rate [t_1/2 (min) = 18.6 (Calu6) and 32.0 (MX1)]. MSPT was estimated in 6.17 days for Calu6-NSCLC, which means that the drug will reach its steady-state effects after three weeks of continuous administration. For MX1 the estimate of MSPT was 28.7 days.

Conclusion: The integrated model was validated externally, and provided a tool to investigate different experimental scenarios as well as giving insights regarding the mechanisms of signal transduction in the cascade of events associated to the TGF-b membrane receptor.