Functional imaging of multidrug-resistant P-glycoprotein with an organotechnetium complex.

The multidrug-resistant P-glycoprotein (Pgp), a M(r) 170,000 plasma membrane protein encoded by the mammalian multidrug resistance gene (MDR1), appears to function as an energy-dependent efflux pump. Many of the drugs that interact with Pgp are lipophilic and cationic at physiological pH. We tested the hypothesis that the synthetic gamma-emitting organotechnetium complex, hexakis(2-methoxyisobutylisonitrile)technetium(I) ([99mTc]SESTAMIBI), a lipophilic cationic radiopharmaceutical, could be a suitable Pgp transport substrate capable of functional imaging of the MDR phenotype. The cellular pharmacological profile of [99mTc]SESTAMIBI transport was examined in Chinese hamster V79 lung fibroblasts and the 77A and LZ derivative cell lines which express modestly low, intermediate, and very high levels of Pgp, respectively. Steady-state contents of [99mTc]SESTAMIBI in V79, 77A, and LZ cells were 10.0 +/- 0.5 (SEM) (n = 9), 3.6 +/- 0.5 (n = 8), and 0.4 +/- 0.02 (n = 9) fmol.(mg protein)-1 (nMo)-1, respectively, consistent with enhanced extrusion of the imaging agent by Pgp-enriched cells. Maximal doses (> 100 microM) of the multidrug-resistant reversal agents verapamil and cyclosporin A enhanced [99mTc]SESTAMIBI accumulation in V79, 77A, and LZ cells by approximately 10-, 25-, and 200-fold, respectively. The median effective concentration values for tracer accumulation in the presence of verapamil in V79, 77A, and LZ cells were 4, 100, and 200 microM, and those for cyclosporin A were 0.9, 3, and > 25 microM, respectively. Pgp-mediated [99mTc]SESTAMIBI transport occurred against its electrochemical gradient and was found to be ATP dependent displaying an apparent Km of 50 microM. Carrier-added [99Tc]SESTAMIBI was 11- to 13-fold less toxic in multidrug-resistant cells, and inhibited photolabeling of Pgp by [125I]iodoaryl azidoprazosin in a concentration-dependent manner; half-maximal displacement was observed at approximately 100- to 1000-fold molar excess [99Tc]SESTAMIBI. Exploiting the favorable gamma emission properties of 99mTc, functional expression of Pgp was successfully imaged in human tumor xenographs in nude mice with pharmacologically inert tracer quantities of [99mTc]SESTAMIBI. Functional imaging with these organotechnetium complexes may provide a novel mechanism to rapidly characterize Pgp expression in human tumors in vivo, target reversal agents in vivo, and ultimately provide a means to direct patients to specific cancer therapies.

Benzquinamide inhibits P-glycoprotein mediated drug efflux and potentiates anticancer agent cytotoxicity in multidrug resistant cells.

We have previously shown that efflux of cytotoxic drugs from multidrug resistant (MDR) cell lines can be quantitated at the single cell level using a sensitive fluorescence microscopy technique. Based on the structure of compounds which inhibited the efflux of Rhodamine-123 (Rho-123) using this methodology, we hypothesized that the antiemetic, antihistaminic agent benzquinamide (BZQ) would interfere with P-glycoprotein (P-gp) mediated drug transport and potentiate the effects of anticancer agents in MDR cell lines. We show that BZQ interferes with P-gp mediated drug efflux and increases drug accumulation in MDR cells using Rho-123 as a fluorescent probe. BZQ increases the cytotoxicity of chemotherapeutic agents to both human and hamster MDR cell lines in vitro. A slight increase in cytotoxicity to chemotherapeutic agents is also observed in the parental cell lines with BZQ. BZQ increases [3H]daunorubicin accumulation and inhibits the binding of [125I]iodoaryl azidoprazosin to the P-gp in MDR cells. BZQ is a new agent to increase the cytotoxic effects of anticancer agents in MDR cells and may ultimately prove useful as an adjunct in cancer chemotherapy.

Isolation and characterization of Drosophila multidrug resistance gene homologs.

Mammalian multidrug-resistant cell lines, selected for resistance to a single cytotoxic agent, display cross-resistance to a broad spectrum of structurally and functionally unrelated compounds. These cell lines overproduce a membrane protein, the P-glycoprotein, which is encoded by a member(s) of a multigene family, termed mdr or pgp. The amino acid sequence of the P-glycoprotein predicts an energy-dependent transport protein with homology to a large superfamily of proteins which transport a wide variety of substances. This report describes the isolation and characterization of two Drosophila homologs of the mammalian mdr gene. These homologs, located in chromosomal sections 49EF and 65A, encode proteins that share over 40% amino acid identity to the human and murine mdr P-glycoproteins. Fly strains bearing disruptions in the homolog in section 49EF have been constructed and implicate this gene in conferring colchicine resistance to the organism. This work sets the foundation for the molecular and genetic analysis of mdr homologs in Drosophila melanogaster.