Z α1-antitrypsin transgenic animals were dosed with 10, 30 or 100 mg/kg GSK716 three times a day. Total concentrations of GSK716 were determined by LC-MS/MS (Table 2). (A) 100mg/kg GSK716 resulted in a mean 7-fold increase in circulating monomeric Z α1-antitrypsin levels demonstrating robust target engagement in the liver. The observed steady state total and free compound levels of GSK716 in the transgenic Z α1-antitrypsin mouse were well-predicted by the in silico PK model built on: (1) in vitro metabolic clearance data, (2) plasma protein binding data, (3) in vivo PK data from wild type mice and (4) a term comprising a 5μM circulating sink for drug with an affinity of 1.5 nM, representing the Z α1-antitrypsin within blood. The target free drug concentration was selected based on the observed potency in the in vitro secretion assays in which the total drug approximates to the free drug in the assay. Interestingly, 30mg/kg and 10mg/kg groups also gave significant, dose-dependent increases in circulating Z α1-antitrypsin despite free concentrations being below the cellular EC50 for secretion for much or all of the dosing period. Significance at p<0.05 by a student's t test is denoted by *, shown as pairwise comparisons (each animal is compared with itself, treated vs pre-treatment) and between groups (compound treated vs vehicle), with n= 7, 8 and 8 per group for day 6 dose level 10, 30 and 100mg/Kg respectively; day 15, n=10; day 21, n=5. Data presented as mean ± SD. 