Pharmacokinetic evaluation of tissue distribution of pirfenidone and its metabolites for idiopathic pulmonary fibrosis therapy.

Pirfenidone is the first and only clinically used anti-fibrotic drug for the treatment of idiopathic pulmonary fibrosis (IPF). It was reported previously that pirfenidone metabolites (5-hydroxypirfenidone and 5-carboxypirfenidone) also have anti-fibrotic effects. The present study evaluated the distribution of pirfenidone and its metabolites in the lung, liver and kidney tissues in rats. The time course for the different concentrations of pirfenidone, 5-hydroxypirfenidone and 5-carboxypirfenidone in the lung tissue following oral administration (30 mg/kg) to rats was lower than that in plasma, and the area under the drug concentration-time curve (AUC) ratios of lung/plasma for pirfenidone, 5-hydroxypirfenidone and 5-carboxypirfenidone were 0.52, 0.40 and 0.61, respectively. In in vitro transport experiments, the basolateral-to-apical transport of pirfenidone and its metabolites through the model of lung epithelial cell (Calu-3) monolayers was not significantly different from their apical-to-basolateral transport. In binding experiments, the binding rate of these drugs to the lung tissue was lower than that to the plasma protein. These findings suggest that the low distribution of pirfenidone and its metabolites in the lungs was based on their low affinities with lung tissue and not the transport characteristics of lung epithelial cells. On the other hand, the AUC ratios of liver/plasma for pirfenidone and 5-carboxypirfenidone were 2.3 and 6.5 and the AUC ratios of kidney/plasma were 1.5 and 20, respectively. The binding rates to the liver and kidney tissues were higher than those to the plasma protein. These results suggest that high concentrations of these drugs were found in the liver and kidney tissues.