Overexpression of the multidrug resistance (MDR1) P-glycoprotein (Pgp) correlates with cancer chemotherapeutic failure. Lipophilic cationic radiopharmaceuticals such as 99mTc-sestamibi, 99mTc-tetrofosmin and 99Tc-furifosmin (Tc-Q12) have been validated as transport substrates for the MDR1 Pgp and may enable functional imaging of the MDR phenotype in cancer by observing enhanced washout rates of the tracers in those tumor areas expressing Pgp. To further explore and optimize the Pgp recognition properties of Schiff base phosphine mixed-ligand complexes of the Tc-Q series of nonreducible (Tc(III) cations, a variety of Tc-Q complexes were synthesized and tested in vitro for recognition as transport substrates by the human MDR1 Pgp.
Methods: Tracer assays with human drug-sensitive KB-3-1 epidermal carcinoma and MDR KB-8-5 cells expressing nonimmunodetectable and modest levels of MDR1 Pgp, respectively, were used to screen and pharmacologically characterize 37 novel 99mTc-Q analogs.
Results: The ideal agent should have low nonspecific binding, high distinction in net uptake between drug-sensitive cells and MDR tumor cells, and high enhancement of uptake in resistant cells after treatment with an MDR modulator, indicating selective blockade of Pgp-mediated efflux of the radiotracer. Three analogs, trans-[5,5'-(1,2-ethanediyldiimino)bis(2-OEt-2-Me-4-penten-3 -one)]bis[dimethyl(3-OMe-1-propyl)phosphine]99mTc(III) (99mTc-Q63) and two trans-[bis(methyl-bis(3-OMe-1-propyl)phosphine)] analogs (99mTc-Q57 and 99mTc-Q58) displayed transport distinctions between drug-sensitive and MDR cell lines that were equal to or greater than all previously available agents. Cyclosporin A, an MDR modulator, had no significant effect in KB-3-1 cells for these 99mTc-complexes but enhanced tracer accumulations in KB-8-5 cells with IC50 values of approximately 1 microM. In contrast, the non-MDR agents methotrexate and cisplatin had no effect on accumulation of 99mTc-Q complexes and 99mTc-sestamibi in KB-8-5 cells.
Conclusion: Technetium-99m-Q57, 99mTc-Q58 and 99mTc-Q63 are avid transport substrates recognized by the human MDR1 Pgp, and have enhanced in vitro properties that may enable functional imaging of Pgp in vivo with improved signal-to-noise ratios and tissue contrast compared to currently available agents.