Imaging reversal of multidrug resistance in living mice with bioluminescence:: MDR1 P-glycoprotein transports coelenterazine

Coelenterazine is widely distributed among marine organisms, producing bioluminescence by calcium-insensitive oxidation mediated by Renilla lucif erase (RIuc) and calcium-dependent oxidation mediated by the photoprotein aequorin. Despite its abundance in nature and wide use of both proteins as reporters of gene expression and signal transduction, little is known about mechanisms of coelenterazine transport and cell permeation. Interestingly, coelenterazine analogues share structural and physiochemical properties of compounds transported by the multidrug resistance MDR1 P-glycoprotein (Pgp). Herein, we report that living cells stably transfected with a codon-humanized RIuc show coelenterazine-mediated bioluminescence in a highly MDR1 Pgp-modulated manner. In Pgp-expressing RIuc cells, low baseline bioluminescence could be fully enhanced (reversed) to non-Pgp matched control levels with potent and selective Pgp inhibitors. Therefore, using coelenterazine and noninvasive bioluminescence imaging in vivo, we could directly monitor tumor-specific Pgp transport inhibition in living mice. While enabling molecular imaging and high-throughput screening of drug resistance pathways, these data also raise concern for the indiscriminate use of RIuc and aequorin as reporters in intact cells or transgenic animals, wherein Pgp-mediated alterations in coelenterazine permeability may impact results.