ATP-dependent drug transport by human P-glycoprotein (Pgp, ABCB1) involves a coordinated communication between its drug-binding site (substrate site) and the nucleotide binding/hydrolysis domain (ATP sites). It has been demonstrated that the two ATP sites of Pgp play distinct roles within a single catalytic turnover; whereas ATP binding or/and hydrolysis by one drives substrate translocation and dissociation, the hydrolytic activity of the other resets the transporter for the subsequent cycle (Sauna, Z. E., and Ambudkar, S. V. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 2515-2520; Sauna, Z. E., and Ambudkar, S. V. (2001) J. Biol. Chem. 276, 11653-11661). Trapping of ADP (or 8-azido-ADP) and vanadate (ADP.Vi or 8-azido-ADP.Vi) at the catalytic site, following nucleotide hydrolysis, markedly reduces the affinity of Pgp for its transport substrate [125I]iodoarylazidoprazosin ([125I]IAAP), resulting in dissociation of the latter. Regeneration of the [125I]IAAP site requires an additional round of nucleotide hydrolysis. In this study, we demonstrate that certain thioxanthene-based allosteric modulators, such as cis-(Z)-flupentixol and its closely related analogs, induce regeneration of [125I]IAAP binding to vanadate-trapped (or fluoroaluminate-trapped) Pgp without any further nucleotide hydrolysis. Regeneration was facilitated by dissociation of the trapped nucleotide and vanadate. Once regenerated, the substrate site remains accessible to [125I]IAAP even after removal of the modulator from the medium, suggesting a modulator-induced relaxation of a constrained transition state conformation. Consistent with this, limited trypsin digestion of vanadate-trapped Pgp shows protection by cis-(Z)-flupentixol of two Pgp fragments (approximately 60 kDa) recognizable by a polyclonal antiserum specific for the NH2-terminal half. No regeneration was observed in the Pgp mutant F983A that is impaired in modulation by flupentixols, indicating involvement of the allosteric modulator site in the phenomenon. In summary, the data demonstrate that in the nucleotide-trapped low affinity state of Pgp, the allosteric site remains accessible and responsive to modulation by flupentixol (and its closely related analogs), which can reset the high affinity state for [125I]IAAP binding without any further nucleotide hydrolysis.