Imaging pulmonary P-glycoprotein activity with aerosolized [^99mTc]Tc-sestamibi in mice

Adenosine triphosphate-binding cassette (ABC) transporters are expressed throughout the lung epithelium, where they may influence the pulmonary disposition of inhaled therapeutics and contribute to transporter-mediated drug-drug interactions. Imaging approaches using radiolabeled transporter-specific probe substrates potentially offer a means to assess drug-transporter interactions directly in the lung. [^99mTc]Tc-sestamibi, a widely used myocardial perfusion tracer, is a known substrate of P-glycoprotein (P-gp/ABCB1) and multidrug resistance-associated protein 1 (MRP1/ABCC1). Here, we evaluated the suitability of [^99mTc]Tc-sestamibi for imaging transporter activity in the lungs. We measured pulmonary elimination kinetics by dynamic planar gamma scintigraphy following intratracheal aerosolization of [^99mTc]Tc-sestamibi in wild-type and MRP1-deficient (hABCC1^(-/-)) mice, with and without co-administration of the P-gp inhibitor tariquidar. P-gp inhibition significantly reduced the pulmonary elimination rate constant (k_E) of [^99mTc]Tc-sestamibi (1.14 ± 0.32 h^-1 in wild-type mice versus 0.56 ± 0.36 h^-1 in tariquidar-treated wild-type mice; P = 0.0056). In contrast, MRP1 deficiency did not alter pulmonary elimination of [^99mTc]Tc-sestamibi (0.93 ± 0.23 h^-1), and combined MRP1 deficiency with P-gp inhibition (0.61 ± 0.24 h^-1) produced no additional effect beyond P-gp inhibition alone. These findings indicate that pulmonary disposition of [^99mTc]Tc-sestamibi is primarily governed by P-gp, with minimal contribution from MRP1 in mice, supporting its potential use as an imaging probe of pulmonary P-gp activity. Nevertheless, given possible species differences in transporter abundance, cellular localization, or function, clinical studies are required to evaluate the translatability of these results to humans.