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.

Characterization of P-glycoprotein transport and inhibition in vivo.

The P-glycoprotein is an energy-dependent efflux pump capable of decreasing the intracellular concentration of a broad range of chemotherapeutic agents. [99mTc]Sestamibi, a P-glycoprotein transport substrate, is a sensitive probe of P-glycoprotein function both in vitro and in vivo. A human tumor model in nude mice was evaluated to determine whether [99mTc]Sestamibi could detect in vivo differences in P-glycoprotein expression and P-glycoprotein modulation by the reversal agent SDZ PSC 833. Differential [99mTc]Sestamibi accumulation based upon P-glycoprotein expression was demonstrated in xenografts in vivo. Dose-dependent inhibition of P-glycoprotein function was achieved with SDZ PSC 833. Administration of the reversal agent increased [99mTc]Sestamibi accumulation in the xenografts expressing P-glycoprotein. These observations show that [99mTc]Sestamibi as capable of detecting the modulation of P-glycoprotein in a solid tumor model by the reversal agent SDZ PSC 833.

Novel hexakis(areneisonitrile)technetium(I) complexes as radioligands targeted to the multidrug resistance P-glycoprotein.

Transport substrates and modulators of the human multidrug resistance (MDR1) P-glycoprotein (Pgp) are generally lipophilic cationic compounds, many with substituted aryl moieties. We sought to synthesize aromatic technetium-isonitrile complexes to enable functional detection in vivo of Pgp expression in tissues. A series of substituted aromatic isonitrile analogs were synthesized from their corresponding amines by reaction with dichlorocarbene under phase transfer-catalyzed conditions, and the non-carrier-added hexakis(areneisonitrile)Tc-99m(I) complexes were produced by reaction with pertechnetate in the presence of sodium dithionite. Cellular accumulation in vitro, whole body biodistribution, and the imaging properties of these lipophilic, monocationic organometallic complexes were determined in Chinese hamster lung fibroblasts expressing MDR Pgp, in normal rats, and in rabbits, respectively. For this initial series, verapamil (50 microM), the classical Pgp modulator, significantly enhanced cellular accumulation or displaced binding of Tc complexes of 1b, 1d, 1h, 2a, 2d, 3a, and 3b, indicative of targeted interactions with Pgp. Most complexes, despite their modestly high lipophilicity, were excluded by the blood/brain barrier, and several complexes displayed simultaneously high hepatobiliary and renal excretion in vivo, consistent with the physiological expression pattern of Pgp in these tissues. Selected Tc- and Re-areneisonitrile complexes of this class have potential applicability to the functional imaging and modulation, respectively, of MDR Pgp in human tissues.

Effect of mitochondrial and plasma membrane potentials on accumulation of hexakis (2-methoxyisobutylisonitrile) technetium(I) in cultured mouse fibroblasts.

Hexakis(2-methoxyisobutylisonitrile) technetium(I) (Tc-MIBI) is representative of a class of 99mTc-based lipophilic cationic myocardial perfusion imaging agents. To test the hypothesis that the mechanism of cellular uptake may involve distribution across biologic membranes in response to membrane potential, Tc-MIBI net uptake and retention were determined in cultured mouse BALB/c 3T3, NIH 3T3, and v-src transformed NIH 3T3 fibroblasts as well as in cultured chick embryo heart cells. Isovolumic depolarization of plasma membrane potentials with 130 mM K 20 mM Cl buffer decreased Tc-MIBI net cell uptake in all preparations. In BALB/c 3T3 cells, depolarizing mitochondrial membrane potential with valinomycin in high K buffer or with the protonophore CCCP inhibited net uptake and retention of Tc-MIBI while hyperpolarizing mitochondrial and plasma membrane potentials with the K+/H+ exchanger nigericin increased Tc-MIBI net uptake. These results indicated that net cellular uptake and retention of Tc-MIBI in fibroblasts were determined by both mitochondrial and plasma membrane potentials; the gamma-emitting properties of Tc-MIBI may therefore raise the possibility of monitoring membrane potential in vivo.

Enhancement by tetraphenylborate of technetium-99m-MIBI uptake kinetics and accumulation in cultured chick myocardial cells.

Myocellular uptake and retention of technetium-99m-hexakis (2-methoxyisobutylisonitrile) (Tc-MIBI), a lipophilic cationic myocardial perfusion and viability imaging agent, is dependent on both mitochondrial and plasma-membrane potentials. To test for enhancement of uptake kinetics by lipophilic anions, cultured chick heart cells were exposed to tetraphenylborate (TPB), which produced a concentration-dependent maximal 15-fold increase in Tc-MIBI uptake kinetics (at 3 x 10(-5) M) and enhanced peak accumulation of Tc-MIBI from 165.4 +/- 26.3 to 705.6 +/- 61.3 fmoles/mg protein.nMo (P less than 0.001). Carbonyl cyanide-m-chloro phenylhydrazone (CCCP; 10(-5) M), a mitochondrial uncoupler, rapidly depleted cellular content of Tc-MIBI in the presence of TPB (10(-5) M) from 300.0 +/- 30.0 to 42.5 +/- 1.9 fmole/mg protein.nMo (p less than 0.001). TPB enhanced both uptake rates and net accumulation of Tc-MIBI at all buffer Ko concentrations between 130 mM and 0.54 mM. Tc-MIBI influx rates allowed estimation of plasma-membrane potential as a function of Ko in the presence of valinomycin with a slope of -67 mV/decade (r = -0.99). The results further support a potential-dependent mechanism for cell uptake of Tc-MIBI and suggest a rational approach for increasing tissue extraction fraction in vivo.

Interspecies variation in biodistribution of technetium (2-carbomethoxy-2-isocyanopropane)6+.

The cationic complex technetium (2-carbomethoxy-2-isocyanopropane)6+ (99mTc-CPI) contains terminal ester groups that were included to provide a pathway for in-vivo metabolism of this compound, thereby enhancing its performance as a myocardial perfusion agent. Biodistribution studies of the compound demonstrated myocardial accumulation in rabbit, guinea pig, and chick, but not in rat and mouse. Radiochemical analysis by HPLC after in-vitro incubation of 99mTc-CPI in blood plasma from the various species confirmed enzymatic hydrolysis to numerous new compounds. Rat and mouse serum produced complete hydrolysis of this agent after incubation for less than 5 sec at 25 degrees C or rates greater than 500 times those observed in human, rabbit, guinea pig and chick serum. Chemical synthesis and isolation of the monohydrolyzed species with subsequent biodistribution studies in guinea pig and rabbit confirmed that this neutral lipophilic complex did not accumulate in heart tissue. It is concluded that varying rates of enzymatic in-vivo hydrolysis produce the interspecies biodistribution differences and may account for the moderate myocardial clearance relative to other isonitrile complexes.

Expression of recombinant human multidrug resistance P-glycoprotein in insect cells confers decreased accumulation of technetium-99m-sestamibi.

UNLABELLED: The multidrug-resistant P-glycoprotein is a M(r) 170,000 plasma membrane protein encoded by the mammalian multidrug resistance gene (MDR) which appears to function as an efflux transporter of a variety of potent chemotherapeutic agents. METHODS: To directly demonstrate that 99mTc-sestamibi is recognized by the human P-glycoprotein, we overexpressed recombinant human MDR1 P-glycoprotein in host Sf9 insect cells using a baculoviral vector and correlated expression of the gene product with 99mTc-sestamibi accumulation. RESULTS: In parental Sf9 cells and in wild-type baculoviral infected (control) cells, 99mTc-sestamibi accumulation asymptotically approached a plateau of 650 fmoles (mg protein)-1 (nMo)-1 and 337 fmoles (mg protein)-1 (nMo)-1, respectively. In MDR1 baculoviral infected cells, P-glycoprotein expression was maximal at 72 hr postinfection, while 99mTc-sestamibi accumulation was reduced to 12 fmole (mg protein)-1 (nMo)-1. Verapamil (500 microM), the classical MDR modulator, produced an approximately 300% enhancement of 99mTc-sestamibi accumulation in Sf9 cells expressing MDR1 P-glycoprotein, but only a 50% enhancement in parental Sf9 cells, consistent with verapamil-induced inhibition of P-glycoprotein-mediated 99mTc-sestamibi efflux. CONCLUSIONS: These data demonstrate that the recombinant protein is transiently expressed in a functional state capable of drug transport in Sf9 cell membranes and that 99mTc-sestamibi is a transport substrate recognized by the human MDR1 P-glycoprotein. Technetium-99m-sestamibi may prove useful for functionally characterizing P-glycoprotein expression in human tumors in vivo.

Hexakis(carbomethoxyisopropylisonitrile) technetium(I), a new myocardial perfusion imaging agent: binding characteristics in cultured chick heart cells.

Cellular kinetics and binding characteristics of hexakis(carbomethoxyisopropylisonitrile) technetium(I) (Tc-CPI), a new cationic, highly lipophilic myocardial perfusion imaging agent, were evaluated in chick embryo heart cells grown in monolayer culture. Myocytes showed uptake of Tc-CPI to a plateau level with a half-time (t1/2) of 4.1 +/- 0.7 min (mean +/- s.e.m.; n = 6); t1/2 appeared independent of extracellular Tc-CPI concentration. Plateau level Tc-CPI uptakes (10(-16) to 10(-11) mole Tc-CPI/mg cell protein) were a linear function of extracellular Tc-CPI concentration (range: 10(-13)M to 10(-8)M, respectively). Tracer 99mTc-CPI uptake (binding) was not competitively displaced by carrier 99Tc-CPI. Uptake was temperature-sensitive; however, several inhibitors of cationic membrane transport (ouabain, amiloride, bumetanide, and verapamil) showed no significant effect. Extreme alkalinization of external load solution (pHo approximately 8.5) partially inhibited Tc-CPI uptake; however, intracellular pH changes showed no effect. Washout from contractile preparations could be described by a two component system: a fast component (myocytes) with a t1/2 approximately 4.5 min and a slow component (fibroblasts) with a t1/2 approximately 40 min. Cell fractionation experiments showed most of the activity to be associated with the cell membrane fraction. The data do not demonstrate a specific mechanism for uptake of Tc-CPI; however, results suggest binding to myocytes in a manner proportional to the delivery of the complex to the extracellular spaces. Such properties would be desirable for a myocardial perfusion imaging agent.

Subcellular distribution and analysis of technetium-99m-MIBI in isolated perfused rat hearts.

To address the apparent discrepancy between cultured cells and whole heart preparations, Langendorff-perfused rat hearts loaded with hexakis (2-methoxyisobutyl isonitrile) technetium (I) (99mTc-MIBI) were fractionated by a standard differential centrifugation method and fractional contents of 99mTc-MIBI were correlated with the mitochondrial marker, malate dehydrogenase (MDH), and mitochondrial substrates. The "cytosolic" fraction nominally contained 89% +/- 3% of total 99mTc-MIBI, but also contained 91% +/- 1% of total MDH activity by this method. Chromatographic analysis of activity in the "cytosolic" fraction demonstrated greater than 95% of the agent was present as the original free cationic complex; binding to a small molecular weight cytosolic protein was not involved in localization. Addition of the mitochondrial uncoupler CCCP (5 microM) to both "mitochondrial" and "cell fragment" pellets released up to 84% +/- 8% of 99mTc-MIBI content and addition of the mitochondrial substrate succinate (10 microM) in the presence of rotenone (1 microM) enhanced 99mTc-MIBI content by up to 139% +/- 52% over the control. These correlative data from rat hearts indicate that approximately 90% of 99mTc-MIBI activity in vivo is associated with mitochondria in an energy-dependent manner as a free cationic complex, but migrates during fractionation/centrifugation.

Effect of metabolic inhibition on technetium-99m-MIBI kinetics in cultured chick myocardial cells.

Cellular uptake characteristics of hexakis(methoxyisobutylisonitrile)technetium(I) ([99mTc]MIBI), a myocardial perfusion imaging agent, were evaluated in cultured chick embryo heart cells. Myocyte net uptake of 99mTc-MIBI approached a plateau with a half-time of 9.3 +/- 1.5 min (mean +/- s.e.m.; n = 10). Tracer [99mTc]MIBI showed apparent competitive displacement by carrier [99Tc]MIBI at relatively high molar ratios ([99mTc]MIBI/[99Tc]MIBI) indicating a low affinity cellular retention process (apparent KD approximately 7 x 10(-5)). Metabolic inhibition induced by pre-incubation of cells for 2.5 hr in rotenone (10 microM), iodoacetate (1 mM), or both metabolic inhibitors together reduced 1-min [99mTc] MIBI uptake to 74.1% +/- 8.0% (p less than 0.05), 6.2% +/- 3.4% (p less than 0.01), and 10.1% +/- 3.6% of control (p less than 0.01), respectively (n = 11-12). Half-maximal inhibitory concentration of iodoacetate was approximately 5 microM. Iodoacetate inhibition of [99mTc]MIBI uptake kinetics was time-dependent; no significant effect on [99mTc]MIBI uptake was seen during the first 60 min of metabolic inhibition despite significant depletion of ATP content determined on the same preparations (control ATP: 40.2 nmoles/mg protein versus iodoacetate incubation: 2.8 nmoles/mg protein; p less than 0.01). However, prolonged metabolic blockade did eventually depress 1-min [99mTc]MIBI uptake. These data indicate that a late component of myocardial cell injury can depress [99mTc]MIBI cellular uptake.

Myocardial imaging with technetium-99m CPI: initial experience in the human.

The hexakis(isonitrile)technetium(I) analog [99mTc]carbomethoxyisopropyl isonitrile (CPI) has high myocardial uptake and rapid lung and liver clearance in most animal species. To evaluate [99mTc]CPI as a myocardial imaging agent in the human, we evaluated this tracer in three normals and in six patients with coronary artery disease (CAD). In normals, [99mTc]CPI cleared quickly from the lungs and accumulated in the liver and heart. The liver activity peaked at 10-15 min and cleared through the hepatobiliary system. Planar images were of excellent technical quality with high myocardial to background ratios as early as 10 min after injection. Myocardial activity fell gradually to 76.1 +/- 2.9 (s.d.)% of initial activity by 60 min after injection. In six patients with CAD, myocardial defects were present on planar images up to 2 hr after exercise and injection. In one out of six patients, the defect was not seen 3 hr after injection. In five of the six patients, normal perfusion patterns were observed 1 hr after reinjection of CPI at rest (4 hr after the initial injection). In one patient who developed spontaneous angina prior to reinjection, the perfusion defects persisted. The repeat study 3 days later with injection of [99mTc]CPI at rest was normal. Technetium-99m CPI appears to have excellent physical and biologic properties for use in association with myocardial imaging with exercise.

Human pharmacokinetics and radiopharmacological studies of the myocardial perfusion agent: technetium (2-carbomethoxy-2-isocyano-propane)6+.

The myocardial perfusion agent technetium (2-carbomethoxy-2-isocyano-propane)6+ (99mTc-CPI) is unique from other cationic technetium isonitrile complexes in that it exhibits moderate washout from the heart and rapid hepatobiliary clearance in animal models and human volunteers. Dynamic imaging and HPLC analysis were performed in humans and guinea pigs to outline the pharmacological basis of its pharmacokinetics. Enzymatic hydrolysis of the terminal ester groups in blood was found to occur at a moderate rate producing new species that have been shown not to accumulate in heart tissue. However, after extraction by the heart, liver or kidneys, the 99mTc-CPI complex undergoes metabolism at a much slower rate than observed in the blood. Differences in hydrolysis rate and products obtained indicate separate mechanisms of hydrolysis occurring in blood and other organs. It is proposed that the heart washout occurring after hydrolysis produces a neutral compound which is no longer retained by the negative cytosolic and mitochondrial membrane potentials in myocardial tissue.

Comparative effects of neutral dipolar compounds and lipophilic anions on technetium 99m-hexakis (2-methoxyisobutyl isonitrile) accumulation in cultured chick ventricular myocytes.

RATIONALE AND OBJECTIVES: Non-flow-dependent myocellular accumulation and uptake kinetics of the myocardial perfusion and viability imaging agent, hexakis (2-methoxyisobutyl isonitrile) technetium 99m(I) (Tc-SESTAMIBI), are thermodynamically driven by large negative sarcolemmal and mitochondrial membrane potentials, and can be enhanced by addition of the lipophilic anion, tetraphenylborate (TPB). To further understand the general properties required of a co-administered compound for increasing the kinetic response of Tc-SESTAMIBI to membrane potential, a systematic appraisal of additional candidate lipid-soluble anions and neutral dipolar compounds was undertaken. METHODS: Each compound was biologically tested for its ability to enhance Tc-SESTAMIBI accumulation in a cultured heart cell model, and electronic dipole moments were evaluated using semi-empirical molecular orbital calculations. RESULTS: Of this series, phloretin (100 microM), TPB (10 microM), and to a lesser degree, 8-anilino-1-naphthalene sulfonate (100 microM) enhanced myocellular accumulation of Tc-SESTAMIBI. Phloretin enhancement was pH-dependent, showing maximal effect at pH 7.4, and was not additive to the augmentation induced by TPB. A series of additional lipid soluble anions and structural analogues of phloretin were without effect. CONCLUSION: Although selected compounds enhanced Tc-SESTAMIBI accumulation, overall, no direct relationship of dipole moment to biologic enhancement was demonstrated.

Comparison of I-123 IMP cerebral uptake and MR spectroscopy following experimental carotid occlusion.

Both I-123 IMP scintigraphy and MRI have been suggested as sensitive detectors of changes shortly after acute cerebral infarction. We compared the uptake of N-isopropyl I-123 p-iodoamphetamine (IMP) and MR spectroscopy of the brain after internal carotid artery ligation. Thirteen gerbils were lightly anesthetized with ether. After neck dissection, an internal carotid artery was occluded. After 2.8 hours, 100 muCi I-123 IMP was injected intravenously into the 13 experimental animals plus three controls. Seven gerbils remained asymptomatic while six developed hemiparesis. At 3 hours after ligation, the animals were killed. The brains were bisected and T1 and T2 relaxation times were determined for the right and left hemispheres by MR spectroscopy immediately after dissection. I-123 IMP uptake was then determined in the samples. Interhemispheric differences in uptake for I-123 IMP were 0.1 +/- 1.7% (SEM) in the control, 33.5 +/- 10% in the asymptomatic and 54.6 +/- 9.7% in the symptomatic animals. Significant differences were seen with I-123 IMP in 6/7 asymptomatic and 6/6 symptomatic animals. In conclusion, I-123 is more sensitive than T1 or T2 relaxation times for the detection of cerebral perfusion abnormalities. Prolongation in T1 and T2 relaxation times correlates closely with increased brain tissue water content and the development of symptoms, indicators of structural brain damage and probable infarction.

Comparative myocardial uptake characteristics of hexakis (alkylisonitrile) technetium(I) complexes. Effect of lipophilicity.

Hexakis (alkylisonitrile) technetium(I) complexes are a new class of cationic, lipophilic myocardial perfusion imaging agents. To further evaluate the effect of lipophilicity on myocardial uptake characteristics, the authors systematically synthesized and tested Tc-isonitrile complexes of varying lipophilicity in both cellular and whole animal systems. In chick heart cells in monolayer culture, cellular plateau level uptake in general correlated with lipophilicity of the complexes (determined by reverse phase high performance liquid chromatography) (r = .71) as well as with scintigraphic intensity of imaged rabbit hearts (r = .91). Exceptions to this trend indicated that additional factors such as size of the complex and form of the terminal alkyl chain branching also may have influenced uptake. The data indicated that neither the lipophilic properties nor the cation charge alone were sufficient to predict myocardial uptake. In addition, intravenous injection of complexes into rabbits showed optimal myocardial images with agents of intermediate lipophilicity. Results indicated that, following intravenous administration, complexes of low lipophilicity yielded suboptimal myocardial images because of low heart cell uptake, whereas complexes of high lipophilicity yielded poor relative myocardial visualization because of excessive binding to additional organs and compartments.

Detection of adriamycin-induced cardiotoxicity in cultured heart cells with technetium 99m-SESTAMIBI.

Adriamycin, a broad-spectrum cytotoxic agent useful in cancer chemotherapy, is limited by a dose-dependent cardiomyopathy mediated in part by disruption of mitochondrial energetics. Hexakis(2-methoxyisobutyl isonitrile)technetium(I) (99mTc-SESTAMIBI) is a gamma-emitting radiopharmaceutical with myocellular accumulation properties dependent on mitochondrial membrane potential. To test the hypothesis that 99mTc-SESTAMIBI could monitor Adriamycin-induced alterations in cardiac energetics, cultured chick heart cells were treated with Adriamycin and 99mTc-SESTAMIBI tracer kinetics were determined. Concentration- and time-dependent depression of 99mTc-SESTAMIBI accumulation was evident within 60 min of treatment. The apparent Ki for acute Adriamycin inhibition of tracer accumulation was 82 microM. After 24 h of treatment, Adriamycin concentrations as low as 0.1 microM demonstrated detectable inhibitory effects. The apparent Ki for this subchronic Adriamycin inhibition of 99mTc-SESTAMIBI accumulation was 18 microM. Subchronic concentration-dependent increases in adriamycin-induced myocellular injury as reflected by lactate dehydrogenase (LDH) release correlated inversely with decreases in 99mTc-SESTAMIBI accumulation. These data further support a contribution from altered mitochondrial energetics to Adriamycin-induced injury and establish a pharmacological foundation for pursuing the possibility of noninvasive imaging of chronic Adriamycin cardiotoxicity in cancer patients using 99mTc-SESTAMIBI.

Uptake of cationic technetium complexes in cultured human carcinoma cells and human xenografts.

We evaluated lipophilicity, in vitro cell accumulation, and biodistribution of a series of 99mTc-ether isonitrile complexes to determine whether increased lipophilicity promotes extraction by tumor or enhances imaging properties of the radiopharmaceutical. Nine 99mTc-sestamibi analogs were synthesized and their lipophilicity was determined. Net cellular accumulation and membrane-potential-independent uptake were quantitatively compared in cultured human colon, breast, and lung carcinoma cells. The biodistribution of [99mTc-(2-methoxy-2-ethyl-isocyanopropane)6]+ (99mTc-MMBI) and [99mTc-(2-ethoxy-2-methyl-1-isocyanopropane)6]+ (99mTc-EIBI) was studied in nude mice using subcutaneous, subrenal capsule, and hepatic tumor xenografts. Accumulation of these compounds in colon cells correlated with increasing lipophilicity. Compared with 99mTc-sestamibi, 99mTc-EIBI exhibited (i) in colon cells both higher net accumulation and a higher specific/nonspecific uptake ratio; (ii) in all three cell lines higher membrane-potential-dependent accumulation; and (iii) in all subcutaneous tumor xenografts and in colon subrenal capsule and hepatic tumor xenografts higher tumor/background ratios.

A novel areneisonitrile Tc complex inhibits the transport activity of MDR P-glycoprotein.

P-glycoprotein (Pgp), the product of the multidrug resistance (MDR1) gene, has been an important cancer target for development of MDR modulators that act to inhibit Pgp efflux transport activity. From a series of novel substituted areneisonitrile analogues of Tc-sestamibi, a known Pgp transport substrate, emerged the hexakis(3,4,5-trimethoxyphenylisonitrile)Tc(I) complex (Tc-TMPI) as a potential modulator of Pgp. Tracer 99mTc-TMPI showed net cellular accumulation in inverse proportion to expression of Pgp and enhancement upon addition of classic MDR modulators. At pharmacological concentrations, the carrier-added 94Tc-TMPI complex showed potent inhibition of Pgp-mediated 99mTc-sestamibi transport (EC50, 1.1 +/- 0.2 microM) and displacement of a Pgp-specific photolabel in a concentration-dependent manner. We conclude that 99Tc-TMPI directly inhibited Pgp transport activity and serves as a convenient template for development of nonradioactive Re(I) analogues as novel MDR modulators.

Synthesis and biodistribution of 64Cu-labeled monocationic diiminedioxime copper(II) complexes.

Monocationic copper(II) complexes of three derivatives of the diiminedioxime ligand 2,10-dioximino-3,9-dimethyl-4,8-diazaundeca-3,8-diene were labeled with 64Cu in high radiochemical yield, and the biodistribution of the complexes was measured in mice. The concentration of the complexes in the heart was low, but the partition coefficients of the complexes are less than optimal for a myocardial perfusion agent. All three complexes are resistant to reduction by glutathione in vitro. This suggests that more lipophilic derivatives of these compounds merit further investigation as possible myocardial perfusion agents.

Comparison of uptake of 99mTc-alkylisonitriles in the rat 9L gliosarcoma tumor model.

The accumulation of three 99mTc(I) alkyl isonitriles was compared in vitro and in vivo using 9L gliosarcoma cells. In vitro, the uptake of 99mTc-EIBI and 99mTc-EPI was higher than that of 99mTc-MIBI. In vivo, however, there was no difference in the tumor concentration at 15 or 60 min postinjection and only a small difference at 24 h. The differences in uptake observed in vitro are apparently offset in vivo by differences in delivery of the tracers to the tumor.