Substrate and inhibitor properties of H17 as novel modulator of transmembrane efflux pump activities have been characterized in an in situ absorption model. Poor substrate properties towards P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP) have been demonstrated. In competition with a MRP substrate H17 proved to have strong MRP-inhibiting properties. The profile of a strong inhibitor with poor substrate properties makes H17 a perspective hopeful candidate for effective therapies of transmembrane efflux pump activities.