In vivo and in vitro multitracer analyses of P-glycoprotein expression-related multidrug resistance

Cancer-Associated Membrane Protein as Targeted Therapy for Bladder Cancer

Bladder cancer (BC) recurrence is one of the primary clinical problems encountered by patients following chemotherapy. However, the mechanisms underlying their resistance to chemotherapy remain unclear. Alteration in the pattern of membrane proteins (MPs) is thought to be associated with this recurrence outcome, often leading to cell dysfunction. Since MPs are found throughout the cell membrane, they have become the focus of attention for cancer diagnosis and treatment. Identifying specific and sensitive biomarkers for BC, therefore, requires a major collaborative effort. This review describes studies on membrane proteins as potential biomarkers to facilitate personalised medicine. It aims to introduce and discuss the types and significant functions of membrane proteins as potential biomarkers for future medicine. Other types of biomarkers such as DNA-, RNA- or metabolite-based biomarkers are not included in this review, but the focus is mainly on cell membrane surface protein-based biomarkers.

Pharmacogenetic analysis of advanced non-small-cell lung cancer patients treated with first-line paclitaxel and carboplatin chemotherapy

BACKGROUND

Genetic polymorphisms contribute toward interindividual variations in drug response. We investigated the effects of genetic polymorphisms on the clinical outcome of advanced non-small-cell lung cancer patients with first-line paclitaxel and carboplatin.

MATERIALS AND METHODS

A total of 194 non-small-cell lung cancer patients were prospectively enrolled from January 2010 to January 2013. We genotyped 11 polymorphisms in seven genes involved in the glycolysis pathway and the related pharmacokinetic/pharmacodynamic pathway. Genetic associations with PET-SUV, survival outcome, and toxicity were analyzed, and in-vitro drug transport activity was measured in the oocyte system.

RESULTS

Patients with the c.334 T>G and c.699 G>A homozygous variant in SLCO1B3 showed a higher incidence of grade 3/4 anemia (P=0.002). Transport activities of oocyte that overexpress the SLCO1B3 c.699 G>A variant showed a significantly decreased uptake of paclitaxel compared with the wild-type expressing oocytes. In addition, patients with GG/GA/AA genotypes of ABCB1, c.2677 T>G/A locus showed inferior progression-free survival (hazard ratio=1.49, P=0.017) compared with other genotypes. The GA genotype of HIF1A, c.1834 G>A locus was associated with inferior progression-free survival compared with the GG genotype (hazard ratio=2.47, P=0.008).

CONCLUSION

This study showed that the SLCO1B3 c.699 G>A polymorphism may predict anemia and ABCB1, HIF1A polymorphism are highly predictive for worse survival in advanced NSCLC with first-line paclitaxel and carboplatin.

¹⁸F-fluorodeoxyglucose and ¹¹C-methionine positron emission tomography in relation to methyl-guanine methyltransferase promoter methylation in high-grade gliomas

INTRODUCTION

Methylation status of the methyl-guanine methyltransferase (MGMT) promoter is associated with a favorable response to a DNA alkylating agent in high-grade gliomas. We analyzed PET scans of patients with high-grade gliomas to determine whether the MGMT methylation status affects the tumor metabolic characteristics.

PATIENTS AND METHODS

Twenty-three patients with high-grade glioma, who were initially examined with 11C-methionine (MET) and 18F-fluorodeoxyglucose (FDG) PET, were retrospectively enrolled. MET and FDG PET images were coregistered to each other and quantitative uptake of MET or FDG was assessed using tumor-to-normal uptake ratio of the cortex (TNR). TNRs for MET and FDG PET were compared between the two groups classified by MGMT promoter methylation status.

RESULTS

Maximum TNR(FDG) of the MGMT methylated group was significantly higher than that of the MGMT unmethylated group (1.80±0.90 vs. 1.29±0.19; P=0.02). The MGMT methylated group also showed a trend for increased mean TNRFDG compared with the unmethylated group (0.85±0.21 vs. 0.72±0.11; P=0.10). There was no significant difference in TNR(MET) between the groups. In subgroup analyses with WHO grade 3 and 4, a trend for higher maximum TNR(FDG) was found in the MGMT methylated group compared with the unmethylated group.

CONCLUSION

The MGMT methylated group showed higher glucose metabolism compared with the unmethylated group, whereas MET uptake did not show a significant difference. This suggests that MGMT methylation in high-grade gliomas could affect the tumor glucose metabolism. Thus, MGMT methylation status can cause a discrepancy in the prognostic prediction of high-grade gliomas by FDG PET, especially in patients scheduled for DNA alkylating chemotherapeutics.

Positron emission tomography diagnostic imaging in multidrug-resistant hepatocellular carcinoma: focus on 2-deoxy-2-(18F)Fluoro-D-Glucose

Hepatocellular carcinoma (HCC) is one of the most lethal cancers worldwide. Surgical resection and liver transplantation are still the best options for treatment. Nevertheless, as the number of patients who may benefit from these therapies is limited, alternative therapies have been developed, including chemotherapy. However, partly due to the expression of multidrug resistance (MDR) proteins, it has been found that HCC is a highly chemoresistant tumor. The major family of MDR proteins is the ATP-binding cassette (ABC) transporter superfamily, which includes P-glycoprotein (Pgp) and MDR-associated protein 1 (MRP1). Positron emission tomography using the radiolabeled analog of glucose, 2-deoxy-2-((18)F)fluoro-D-glucose ([(18)F]FDG), has been used in diagnostic imaging of various types of tumors. Clinical studies are inconsistent but experimental studies have shown that [(18)F]FDG uptake is associated with tumor grade and is inversely proportional to Pgp expression in HCC. These studies unveil that [(18)F]FDG can be a substrate of Pgp, although that relationship remains unclear. This review sums up the relationship between MDR expression in HCC, and [(18)F]FDG uptake by tumor cells, showing that this radiopharmaceutical may provide a useful tool for the study of chemoresistance in HCC, and that the use of this marker may contribute to the therapeutic choice on this highly aggressive tumor.

[(18)F]FDG is not transported by P-glycoprotein and breast cancer resistance protein at the rodent blood-brain barrier

INTRODUCTION

Transport of 2-[(18)F]fluoro-2-deoxy-d-glucose ([(18)F]FDG) by the multidrug efflux transporters P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) at the blood-brain barrier (BBB) may confound the interpretation of [(18)F]FDG brain PET data. Aim of this study was to assess the influence of ABCB1 and ABCG2 at the BBB on brain distribution of [(18)F]FDG in vivo by performing [(18)F]FDG PET scans in wild-type and transporter knockout mice and by evaluating changes in [(18)F]FDG brain distribution after transporter inhibition.

METHODS

Dynamic small-animal PET experiments (60min) were performed with [(18)F]FDG in groups of wild-type and transporter knockout mice (Abcb1a/b((-/-)), Abcg2((-/-)) and Abcb1a/b((-/-))Abcg2((-/-))) and in wild-type rats without and with i.v. pretreatment with the known ABCB1 inhibitor tariquidar (15mg/kg, given at 2h before PET). Blood was sampled from animals from the orbital sinus vein at the end of the PET scans and measured in a gamma counter. Brain uptake of [(18)F]FDG was expressed as the brain-to-blood radioactivity concentration ratio in the last PET time frame (Kb,brain).

RESULTS

Kb,brain values of [(18)F]FDG were not significantly different between different mouse types both without and with tariquidar pretreatment. The blood-to-brain transfer rate constant of [(18)F]FDG was significantly lower in tariquidar-treated as compared with vehicle-treated rats (0.350±0.025mL/min/g versus 0.416±0.024mL/min/g, p=0.026, paired t-test) but Kb,brain values were not significantly different between both rat groups.

CONCLUSION

Our results show that [(18)F]FDG is not transported by Abcb1 at the mouse and rat BBB in vivo. In addition we found no evidence for Abcg2 transport of [(18)F]FDG at the mouse BBB.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Our findings imply that functional activity of ABCB1 and ABCG2 at the BBB does not need to be taken into account when interpreting brain [(18)F]FDG PET data.

Influence of P-Glycoprotein Inhibition or Deficiency at the Blood-Brain Barrier on (18)F-2-Fluoro-2-Deoxy-D-glucose ( (18)F-FDG) Brain Kinetics

The fluorinated D-glucose analog (18)F-2-fluoro-2-deoxy-D-glucose ((18)F-FDG) is the most prevalent radiopharmaceutical for positron emission tomography (PET) imaging. P-Glycoprotein's (P-gp, MDR1, and ABCB1) function in various cancer cell lines and tumors was shown to impact (18)F-FDG incorporation, suggesting that P-gp function at the blood-brain barrier may also modulate (18)F-FDG brain kinetics. We tested the influence of P-gp inhibition using the cyclosporine analog valspodar (PSC833; 5 μM) on the uptake of (18)F-FDG in standardized human P-gp-overexpressing cells (MDCKII-MDR1). Consequences for (18)F-FDG brain kinetics were then assessed using (i) (18)F-FDG PET imaging and suitable kinetic modelling in baboons without or with P-gp inhibition by intravenous cyclosporine infusion (15 mg kg(-1) h(-1)) and (ii) in situ brain perfusion in wild-type and P-gp/Bcrp (breast cancer resistance protein) knockout mice and controlled D-glucose exposure to the brain. In vitro, the time course of (18)F-FDG uptake in MDR1 cells was influenced by the presence of valspodar in the absence of D-glucose but not in the presence of high D-glucose concentration. PET analysis revealed that P-gp inhibition had no significant impact on estimated brain kinetics parameters K 1, k 2, k 3, V T , and CMRGlc. The lack of P-gp effect on in vivo (18)F-FDG brain distribution was confirmed in P-gp/Bcrp-deficient mice. P-gp inhibition indirectly modulates (18)F-FDG uptake into P-gp-overexpressing cells, possibly through differences in the energetic cell level state. (18)F-FDG is not a P-gp substrate at the BBB and (18)F-FDG brain kinetics as well as estimated brain glucose metabolism are influenced by neither P-gp inhibition nor P-gp/Bcrp deficiencies in baboon and mice, respectively.

The strong in vivo anti-tumor effect of the UIC2 monoclonal antibody is the combined result of Pgp inhibition and antibody dependent cell-mediated cytotoxicity

P-glycoprotein (Pgp) extrudes a large variety of chemotherapeutic drugs from the cells, causing multidrug resistance (MDR). The UIC2 monoclonal antibody recognizes human Pgp and inhibits its drug transport activity. However, this inhibition is partial, since UIC2 binds only to 10-40% of cell surface Pgps, while the rest becomes accessible to this antibody only in the presence of certain substrates or modulators (e.g. cyclosporine A (CsA)). The combined addition of UIC2 and 10 times lower concentrations of CsA than what is necessary for Pgp inhibition when the modulator is applied alone, decreased the EC50 of doxorubicin (DOX) in KB-V1 (Pgp+) cells in vitro almost to the level of KB-3-1 (Pgp-) cells. At the same time, UIC2 alone did not affect the EC50 value of DOX significantly. In xenotransplanted severe combined immunodeficient (SCID) mice co-treated with DOX, UIC2 and CsA, the average weight of Pgp+ tumors was only ∼10% of the untreated control and in 52% of these animals we could not detect tumors at all, while DOX treatment alone did not decrease the weight of Pgp+ tumors. These data were confirmed by visualizing the tumors in vivo by positron emission tomography (PET) based on their increased 18FDG accumulation. Unexpectedly, UIC2+DOX treatment also decreased the size of tumors compared to the DOX only treated animals, as opposed to the results of our in vitro cytotoxicity assays, suggesting that immunological factors are also involved in the antitumor effect of in vivo UIC2 treatment. Since UIC2 binding itself did not affect the viability of Pgp expressing cells, but it triggered in vitro cell killing by peripheral blood mononuclear cells (PBMCs), it is concluded that the impressive in vivo anti-tumor effect of the DOX-UIC2-CsA treatment is the combined result of Pgp inhibition and antibody dependent cell-mediated cytotoxicity (ADCC).

¹⁸FDG a PET tumor diagnostic tracer is not a substrate of the ABC transporter P-glycoprotein

2-[(18)F]fluoro-2-deoxy-d-glucose ((18)FDG) is a tumor diagnostic radiotracer of great importance in both diagnosing primary and metastatic tumors and in monitoring the efficacy of the treatment. P-glycoprotein (Pgp) is an active transporter that is often expressed in various malignancies either intrinsically or appears later upon disease progression or in response to chemotherapy. Several authors reported that the accumulation of (18)FDG in P-glycoprotein (Pgp) expressing cancer cells (Pgp(+)) and tumors is different from the accumulation of the tracer in Pgp nonexpressing (Pgp(-)) ones, therefore we investigated whether (18)FDG is a substrate or modulator of Pgp pump. Rhodamine 123 (R123) accumulation experiments and ATPase assay were used to detect whether (18)FDG is substrate for Pgp. The accumulation and efflux kinetics of (18)FDG were examined in two different human gynecologic (A2780/A2780AD and KB-3-1/KB-V1) and a mouse fibroblast (3T3 and 3T3MDR1) Pgp(+) and Pgp(-) cancer cell line pairs both in cell suspension and monolayer cultures. We found that (18)FDG and its derivatives did not affect either the R123 accumulation in Pgp(+) cells or the basal and the substrate stimulated ATPase activity of Pgp supporting that they are not substrates or modulators of the pump. Measuring the accumulation and efflux kinetics of (18)FDG in different Pgp(+) and Pgp(-) cell line pairs, we have found that the Pgp(+) cells exhibited significantly higher (p⩽0.01) (18)FDG accumulation and slightly faster (18)FDG efflux kinetics compared to their Pgp(-) counterparts. The above data support the idea that expression of Pgp may increase the energy demand of cells resulting in higher (18)FDG accumulation and faster efflux. We concluded that (18)FDG and its metabolites are not substrates of Pgp.

PET and SPECT radiotracers to assess function and expression of ABC transporters in vivo

Adenosine triphosphate-binding cassette (ABC) transporters, such as P-glycoprotein (Pgp, ABCB1), breast cancer resistance protein (BCRP, ABCG2) and multidrug resistance-associated proteins (MRPs) are expressed in high concentrations at various physiological barriers (e.g. blood-brain barrier, blood-testis barrier, blood-tumor barrier), where they impede the tissue accumulation of various drugs by active efflux transport. Changes in ABC transporter expression and function are thought to be implicated in various diseases, such as cancer, epilepsy, Alzheimer's and Parkinson's disease. The availability of a non-invasive imaging method which allows for measuring ABC transporter function or expression in vivo would be of great clinical use in that it could facilitate the identification of those patients that would benefit from treatment with ABC transporter modulating drugs. To date three different kinds of imaging probes have been described to measure ABC transporters in vivo: i) radiolabelled transporter substrates ii) radiolabelled transporter inhibitors and iii) radiolabelled prodrugs which are enzymatically converted into transporter substrates in the organ of interest (e.g. brain). The design of new imaging probes to visualize efflux transporters is inter alia complicated by the overlapping substrate recognition pattern of different ABC transporter types. The present article will describe currently available ABC transporter radiotracers for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) and critically discuss strengths and limitations of individual probes and their potential clinical applications.

A comparative small-animal PET evaluation of [11C]tariquidar, [11C]elacridar and (R)-[11C]verapamil for detection of P-glycoprotein-expressing murine breast cancer

PURPOSE

One important mechanism for chemoresistance of tumours is overexpression of the adenosine triphosphate-binding cassette transporter P-glycoprotein (Pgp). Pgp reduces intracellular concentrations of chemotherapeutic drugs. The aim of this study was to compare the suitability of the radiolabelled Pgp inhibitors [(11)C]tariquidar and [(11)C]elacridar with the Pgp substrate radiotracer (R)-[(11)C]verapamil for discriminating tumours expressing low and high levels of Pgp using small-animal PET imaging in a murine breast cancer model.

METHODS

Murine mammary carcinoma cells (EMT6) were continuously exposed to doxorubicin to generate a Pgp-overexpressing, doxorubicin-resistant cell line (EMT6AR1.0 cells). Both cell lines were subcutaneously injected into female athymic nude mice. One week after implantation, animals underwent PET scans with [(11)C]tariquidar (n = 7), [(11)C]elacridar (n = 6) and (R)-[(11)C]verapamil (n = 7), before and after administration of unlabelled tariquidar (15 mg/kg). Pgp expression in tumour grafts was evaluated by Western blotting.

RESULTS

[(11)C]Tariquidar showed significantly higher retention in Pgp-overexpressing EMT6AR1.0 compared with EMT6 tumours: the mean ± SD areas under the time-activity curves in scan 1 from time 0 to 60 min (AUC(0-60)) were 38.8 ± 2.2 min and 25.0 ± 5.3 min (p = 0.016, Wilcoxon matched pairs test). [(11)C]Elacridar and (R)-[(11)C]verapamil were not able to discriminate Pgp expression in tumour models. Following administration of unlabelled tariquidar, both EMT6Ar1.0 and EMT6 tumours showed increases in uptake of [(11)C]tariquidar, [(11)C]elacridar and (R)-[(11)C]verapamil.

CONCLUSION

Among the tested radiotracers, [(11)C]tariquidar performed best in discriminating tumours expressing high and low levels of Pgp. Therefore [(11)C]tariquidar merits further investigation as a PET tracer to assess Pgp expression levels in solid tumours.

Pgp inhibition by UIC2 antibody can be followed in vitro by using tumor-diagnostic radiotracers, 99mTc-MIBI and 18FDG

P-glycoprotein (Pgp, ABCB1) is one of the active efflux pumps that are able to extrude a large variety of chemotherapeutic drugs from the cells, causing the phenomenon of multidrug resistance. It has been shown earlier that the combined application of a class of Pgp modulators (e.g. cyclosporine A and SDZ PSC 833) used at low concentrations and UIC2 antibody is a novel, specific, and effective way of blocking Pgp function (Goda et al., 2007). In the present work we study the UIC2 antibody mediated Pgp inhibition in more detail measuring the accumulation of tumor diagnostic radiotracers, 2-[(18)F]fluoro-2-deoxy-d-glucose ((18)FDG) and [(99m)Tc]hexakis-2-methoxybutyl isonitrile ((99m)Tc-MIBI), into Pgp(+) (A2780AD) and Pgp(-) (A2780) human ovarian carcinoma cells. Co-incubation of cells with UIC2 and cyclosporine A (CSA, 2μM) increased the binding of UIC2 more than 3-fold and reverted the rhodamine 123 (R123), daunorubicin (DNR) and (99m)Tc-MIBI accumulation of the Pgp(+) 2780AD cells to approx. the same level as observed in Pgp(-) cells. Similarly, 50μM paclitaxel (Pacl) increased UIC2 binding, and consequently reinstated the uptake of R123, DNR and (99m)Tc-MIBI into the Pgp(+) cells. Blocking Pgp by combined treatments with CSA+UIC2 or Pacl+UIC2 also decreased the glucose metabolic rate of the A2780AD Pgp(+) cells measured in (18)FDG accumulation experiments suggesting that the maintenance of Pgp activity requires a considerable amount of energy. Similar treatments of the A2780 Pgp(-) cells did not result in significant change in the R123, DNR, (99m)Tc-MIBI and (18)FDG accumulation demonstrating that the above effects are Pgp-specific. Thus, combined treatment with the UIC2 antibody and Pgp modulators can completely block the function of Pgp in human ovarian carcinoma cells and this effect can be followed in vitro by using tumor-diagnostic radiotracers, (99m)Tc-MIBI and (18)FDG.

Evaluation of morphine effect on tumour angiogenesis in mouse breast tumour model, EATC

Breast cancer is the leading cause of death among women, and morphine is used to relieve the pain of patients with cancer. The data on the effects of morphine on tumour growth and angiogenesis are contradictory. We determined in mouse breast cancer model whether analgesic doses of morphine would affect tumour angiogenesis, and then the correlation between microvessel density (MVD), Doppler sonography (DS) and 99mTc-Tetrofosmin (TF) uptake. Ehrlich ascites tumour cell xenografts, Pgp-negative tumour were divided into two groups: (a) Morphine sulphate [0.714 mg/kg/day (equivalent to 50 mg per day for a 70 kg human)], (b) no-morphine. For the determination of angiogenesis in mice tumour tissue, TF scintigraphy, microvessel density and DS were done. MVD was significantly different between groups (49.4±1.8 vs. 41.8±1.9, morphine and no-morphine groups, respectively, P<0.001). A strong correlation was found between late uptakes of mass at scintigraphy and degree of angiogenesis in histopathologic examination (r=0.52, P<0.01). There was statistically significant inverse correlation between degree of angiogenesis in histopathologic examination and washout ratio of TF (r=0.40, P<0.05). The higher values for angiogenesis are related to higher TF reuptake. There was no statistically significant correlation between DS and TF. A strong correlation was found between MVD and grade of DS (r=0.51, P<0.01). Our preclinical mice study indicates that morphine at clinically relevant doses stimulates angiogenesis, and angiogenesis triggered of morphine is demonstrated with MVD and DS, but not TF. However, uptake and washout of TF are compared with immunohistochemically assessed morphine-stimulated angiogenesis in tumour tissue.

18F-FDG uptake in lung, breast, and colon cancers: molecular biology correlates and disease characterization

It is hoped that in the not too distant future, noninvasive imaging-based molecular interrogation and characterization of tumors can improve our fundamental understanding of the dynamic biologic behavior of cancer. For example, the new dimension of diagnostic information that is provided by (18)F-FDG PET has led to improved clinical decision making and management changes in a substantial number of patients with cancer. In this context, the aim of this review is to bring together and summarize the current data on the correlation between the underlying molecular biology and the clinical observations of tumor (18)F-FDG accumulation in 3 major human cancers: lung, breast, and colon.

Glucose-induced alteration of accumulation of organotechnetium complexes accumulation in Pgp-negative tumor-bearing mice

The biologic and microenvironmental factors determining (99m)Tc sestamibi (MIBI) and (99m)Tc tetrofosmin (TF) uptake in breast tumors are incompletely understood, especially in P-glycoprotein (Pgp)-negative tumors. We analyzed the influence of glucose administration on the uptake and retention of MIBI and TF in Pgp-negative tumor-bearing mice in vivo. Twenty (20) mice bearing Ehrlich ascites tumor cell (EATC) xenografts were divided into four groups: (1) MIBI, (2) MIBI+glucose, (3) TF, and (4) TF+glucose. Glucose was administered (5.0 g/kg body weight) intraperitoreally (i.p.) 1 hour before scintigraphy. There were significant differences between the E-UPR MIBI and MIBI+glucose groups (p = 0.009) and minor differences in L-UPR between these groups (p = 0.04). There was a significant inverse correlation between E-UPR of MIBI and glucose levels (r = 0.71, p = 0.02). Comparing the four groups, the highest E-UPR was obtained in the MIBI group (p = 0.006). Other parameters were not different in the MIBI and MIBI+glucose groups and in the TF and TF+glucose groups. Increased blood glucose level affected the MIBI uptake of tumor tissue, particularly for E-UPR. We suggest that these findings were due to basically decreased blood flow and secondarily decreased extracellular pH. However, glucose administration did not affect TF.

Daunorubicin and doxorubicin inhibit the [(11)C]choline accumulation in cancer cells

We studied how very short (10-40min) incubation with anthracycline derivatives modifies the accumulation of PET tumor-diagnostic radiotracers in cancer cells. The human ovarian A2780 and A2780AD, human B lymphoid JY, human epidermoid KB-3-1 and KB-V-1, and smooth muscle DDT1 MF-2 cells were pre-incubated with daunorubicin and doxorubicin, and the uptake of [(18)F]FDG and [(11)C]choline was measured. Anthracycline treatment decreased remarkably the [(11)C]choline accumulation in a concentration dependent manner, while it did not modify significantly the [(18)F]FDG uptake of the cells.

Evaluation of (64)Cu(DO3A-xy-TPEP) as a potential PET radiotracer for monitoring tumor multidrug resistance

In this study, we evaluated the potential of (64)Cu(DO3A-xy-TPEP) (DO3A-xy-TPEP = (2-(diphenylphosphoryl)ethyl)diphenyl(4-((4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecan-1-yl)methyl)benzyl)phosphonium) as a PET (positron emission tomography) radiotracer for noninvasive monitoring of multidrug resistance (MDR) transport function in several xenografted tumor models (MDR-negative: U87MG; MDR-positive: MDA-MB-435, MDA-MB-231, KB-3-1, and KB-v-1). It was found that (64)Cu(DO3A-xy-TPEP) has a high initial tumor uptake (5.27 +/- 1.2%ID/g at 5 min p.i.) and shows a steady uptake increase between 30 and 120 min p.i. (2.09 +/- 0.53 and 3.35 +/- 1.27%ID/g at 30 and 120 min p.i., respectively) in the MDR-negative U87MG glioma tumors. (64)Cu(DO3A-xy-TPEP) has a greater uptake difference between U87MG glioma and MDR-positive tumors (MDA-MB-231: 1.57 +/- 0.04, 1.00 +/- 0.17, and 0.93 +/- 0.15; MDA-MB-435: 1.15 +/- 0.19, 1.12 +/- 0.20, and 0.81 +/- 0.11; KB-3-1: 1.45 +/- 0.31, 1.43 +/- 0.16, and 1.08 +/- 0.19; and KB-v-1: 1.63 +/- 0.47, 1.81 +/- 0.31, and 1.14 +/- 0.22%ID/g at 30, 60, and 120 min p.i., respectively) than (99m)Tc-Sestamibi. Regardless of the source of MDR, the overall net effect is the rapid efflux of (64)Cu(DO3A-xy-TPEP) from tumor cells, which leads to a significant reduction of its tumor uptake. It was concluded that (64)Cu(DO3A-xy-TPEP) is more efficient than (99m)Tc-Sestamibi as the substrate for MDR P-glycoproteins (MDR Pgps) and multidrug resistance-associated proteins (MRPs), and might be a more efficient radiotracer for noninvasive monitoring of the tumor MDR transport function. (64)Cu(DO3A-xy-TPEP) and (99m)Tc-Sestamibi share almost identical subcellular distribution patterns in U87MG glioma tumors. Thus, it is reasonable to believe that (64)Cu(DO3A-xy-TPEP), like (99m)Tc-Sestamibi, is able to localize in mitochondria due to the increased plasma and mitochondrial transmembrane potentials in tumor cells.

In vivo evaluation in rodents of [(123)I]-3-I-CO as a potential SPECT tracer for the serotonin 5-HT(2A) receptor

INTRODUCTION

[(123)I]-(4-fluorophenyl)[1-(3-iodophenethyl)piperidin-4-yl]methanone ([(123)I]-3-I-CO) is a potential single photon emission computed tomography tracer with high affinity for the serotonin 5-HT(2A) receptor (K(i)=0.51 nM) and good selectivity over other receptor (sub)types. To determine the potential of the radioligand as a 5-HT(2A) tracer, regional brain biodistribution and displacement studies will be performed. The influence of P-glycoprotein blocking on the brain uptake of the radioligand will also be investigated.

METHODS

A regional brain biodistribution study and a displacement study with ketanserin were performed with [(123)I]-3-I-CO. Also, the influence of cyclosporin A (50 mg/kg) on the brain distribution of the radioligand was investigated. For the displacement study, ketanserin (1 mg/kg) was administered 30 min after injection of [(123)I]-3-I-CO.

RESULTS

The initial brain uptake of [(123)I]-3-I-CO was quite high, but a rapid wash-out of radioactivity was observed. Cortex-to-cerebellum binding index ratios were low (1.1 - 1.7), indicating considerable aspecific binding and a low specific 'signal' of the radioligand. Tracer uptake was reduced to the levels in cerebellum (a 60% reduction) after ketanserin displacement. Administration of cyclosporin A resulted in a doubling of the brain radioactivity concentration.

CONCLUSIONS

Although [(123)I]-3-I-CO showed adequate brain uptake and could be displaced by ketanserin, high aspecific binding to brain tissue was responsible for very low cortex-to-cerebellum binding index ratios, possibly limiting the potential of the radioligand as a serotonin 5-HT(2A) receptor tracer. We also demonstrated that [(123)I]-3-I-CO is probably a weak substrate for the P-glycoprotein efflux transporter.

(99m)Tc-MIBI imaging for prediction of therapeutic effects of second-generation MDR1 inhibitors in malignant brain tumors

The aim of this study was to explore whether (99m)Tc-methoxyisobutylisonitrile ((99m)Tc-MIBI) is suitable to elucidate multidrug resistance and prediction of potentiation of antitumor agents by second-generation MDR1 inhibitors (PSC833, MS-209) in malignant brain tumors in rat. Malignant tumor cells (RG2 and C6 gliomas, Walker 256 carcinoma) were incubated with low dose vincristine (VCR) to induce multidrug resistance. MTT assay demonstrated a significant increase of surviving fractions in VCR-resistant sublines compared to those of drug-naive cells. Reverse transcriptase polymerase chain reaction revealed higher expression of MDR1 mRNA in VCR-resistant cells than drug-naive cells in each line. Volume distribution (V(d)) of (99m)Tc-MIBI was negatively correlated with MDR1 mRNA expression among drug-naive and VCR-resistant cells. MDR1 inhibitors decreased surviving fractions and increased V(d) of (99m)Tc-MIBI significantly in VCR-resistant sublines, whereas MDR1 mRNA expression was unchanged. These findings indicate that (99m)Tc-MIBI efflux was functionally suppressed by MDR1 inhibitors. Autoradiographic images of (99m)Tc-MIBI revealed higher uptake in drug-naive cells at basal ganglia compared with VCR-resistant cells at the opposite basal ganglia of rats. Oral administration of the second-generation MDR1 inhibitors significantly increased (99m)Tc-MIBI accumulation of both tumors. Therapeutic effects of VCR with or without the MDR1 inhibitors were also evaluated autoradiographically using (14)C-methyl-L-methionine ((14)C-Met) and MIB-5 index. (14)C-Met uptake and MIB-5 index of both tumors treated with VCR following the MDR1 inhibitor treatment significantly decreased compared with tumors treated with VCR alone. Analysis of (99m)Tc-MIBI accumulation is considered informative for detecting MDR1-mediated drug resistance and for monitoring the therapeutic effects of MDR1 inhibitors in malignant brain tumors.

64Cu-Labeled Triphenylphosphonium and Triphenylarsonium Cations as Highly Tumor-Selective Imaging Agents

This report presents synthesis and evaluation of the 64Cu-labeled triphenylphosphonium (TPP) cations as new radiotracers for imaging tumors by positron emission tomography. Biodistribution properties of 64Cu-L1, 64Cu-L2, 64Cu-L3, and 99mTc-Sestamibi were evaluated in athymic nude mice bearing U87MG human glioma xenografts. The most striking difference is that 64Cu-L1, 64Cu-L2, and 64Cu-L3 have much lower heart uptake (<0.6% ID/g) than 99mTc-Sestamibi ( approximately 18% ID/g) at >30 min p.i. Their tumor/heart ratios increase steadily from approximately 1 at 5 min p.i. to approximately 5 at 120 min p.i. The tumor/heart ratio of 64Cu-L3 is approximately 40 times better than that of 99mTc-Sestamibi at 120 min postinjection. Results from in vitro assays show that 64Cu-L1 is able to localize in tumor mitochondria. The tumor is clearly visualized in the tumor-bearing mice administered with 64Cu-L1 as 30 min postinjection. The 64Cu-labeled TPP/TPA cations are very selective radiotracers that are able to provide the information of mitochondrial bioenergetic function in tumors by monitoring mitochondrial potential in a noninvasive fashion.

Na+/Ca2+ exchanger inhibitors modify the accumulation of tumor-diagnostic PET tracers in cancer cells

AIM

To establish the effects of Na(+)/Ca(2+) exchanger (NCX) blockers on 2-[(18)F]fluoro-2-deoxy-D-glucose ((18)FDG) and (11)C-choline accumulation in different cancer cells.

METHODS

The tumor cells were incubated with NCX inhibitors, and the uptakes of (18)FDG and (11)C-choline were measured. Flow cytometric measurements of intracellular Ca(2+) and Na(+) concentrations were carried out. The presence of the NCX antigen in the cancer cells was proved by Western blotting, flow cytometry and confocal laser scanning microscopy.

RESULTS

The NCX is expressed at a noteworthy level in the cytosol and on the cytoplasmic membrane of the examined cells. Incubation of the cells with three chemically unrelated NCX blockers (bepridil, KB-R7943 or 3',4'-dichlorobenzamil hydrochloride) resulted in an increase in the intracellular Ca(2+) concentration, with a simultaneous decrease in the intracellular Na(+) concentration. The treatment with the NCX inhibitors increased the energy consumption of the tumor cells by 50-100%. Thapsigargin abolished the NCX-induced (18)FDG accumulation in the cells. The NCX blockers applied decreased the (11)C-choline accumulation of all the investigated cancer cells by 60-80% relative to the control.

CONCLUSION

A possible masking effect of NCX medication must be taken into consideration during the diagnostic interpretation of PET scans.

Complete Inhibition of P-glycoprotein by Simultaneous Treatment with a Distinct Class of Modulators and the UIC2 Monoclonal Antibody

P-glycoprotein (Pgp) is one of the active efflux pumps that are able to extrude a large variety of chemotherapeutic drugs from the cells, causing multidrug resistance. The conformation-sensitive UIC2 monoclonal antibody potentially inhibits Pgp-mediated substrate transport. However, this inhibition is usually partial, and its extent is variable because UIC2 binds only to 10 to 40% Pgp present in the cell membrane. The rest of the Pgp molecules become recognized by this antibody only in the presence of certain substrates or modulators, including vinblastine, cyclosporine A (CsA), and SDZ PSC 833 (valspodar). Simultaneous application of any of these modulators and UIC2, followed by the removal of the modulator, results in a completely restored steady-state accumulation of various Pgp substrates (calcein-AM, daunorubicin, and 99mTc-hexakis-2-methoxybutylisonitrile), indicating near 100% inhibition of pump activity. Remarkably, the inhibitory binding of the antibody is brought about by coincubation with concentrations of CsA or SDZ PSC 833 approximately 20 times lower than what is necessary for Pgp inhibition when the modulators are applied alone. The feasibility of such a combinative treatment for in vivo multidrug resistance reversal was substantiated by the dramatic increase of daunorubicin accumulation in xenotransplanted Pgp+ tumors in response to a combined treatment with UIC2 and CsA, both administered at doses ineffective when applied alone. These observations establish the combined application of a class of modulators used at low concentrations and of the UIC2 antibody as a novel, specific, and effective way of blocking Pgp function in vivo.

Paclitaxel modifies the accumulation of tumor-diagnostic tracers in different ways in P-glycoprotein-positive and negative cancer cells

AIM

To study how paclitaxel treatment modifies the accumulation of tumor-diagnostic radiotracers in P-glycoprotein (P-gp) positive and negative cancer cells.

METHODS

The accumulations of different P-gp substrates, including rhodamine 123, daunorubicin and [(99m)Tc]hexakis-2-methoxybutyl isonitrile ((99m)Tc-MIBI), were measured in P-gp-positive (A2780AD) and P-gp-negative human ovarian carcinoma cells (A2780) and JY human lymphoid B cells. The uptakes of the tumor-diagnostic tracers (11)C-choline and 2-[(18)F]fluoro-2-deoxy-d-glucose ((18)FDG) were measured in the same cell lines. The P-gp expression and function were demonstrated by flow-cytometry.

RESULTS

The (18)FDG measurements revealed that the glucose metabolic rate was significantly higher (p<0.01) in the P-gp-positive A2780AD cells than in the P-gp-negative cells. Paclitaxel (1-70microM) increased the (18)FDG uptake (up to 200%) of both P-gp-positive and P-gp-negative cells, whereas it did not modulate their (11)C-choline uptake. Paclitaxel reinstated the (99m)Tc-MIBI accumulation of the A2780AD cells (to 1500% of the control) in a concentration-dependent manner, while it increased the uptake of the P-gp-negative cells to a lesser extent (to a maximum of 200% of the control).

CONCLUSION

Paclitaxel modifies the uptake of tumor-diagnostic tracers in both P-gp-dependent and independent manners. Interpretation of the multifactorial effects of paclitaxel may promote a correct in vivo diagnosis of P-gp-positive and P-gp-negative tumors.

Alternating current electrical stimulation enhanced chemotherapy: a novel strategy to bypass multidrug resistance in tumor cells

Background

Tumor burden can be pharmacologically controlled by inhibiting cell division and by direct, specific toxicity to the cancerous tissue. Unfortunately, tumors often develop intrinsic pharmacoresistance mediated by specialized drug extrusion mechanisms such as P-glycoprotein. As a consequence, malignant cells may become insensitive to various anti-cancer drugs. Recent studies have shown that low intensity very low frequency electrical stimulation by alternating current (AC) reduces the proliferation of different tumor cell lines by a mechanism affecting potassium channels while at intermediate frequencies interfere with cytoskeletal mechanisms of cell division. The aim of the present study is to test the hypothesis that permeability of several MDR1 over-expressing tumor cell lines to the chemotherapic agent doxorubicin is enhanced by low frequency, low intensity AC stimulation.

Methods

We grew human and rodent cells (C6, HT-1080, H-1299, SKOV-3 and PC-3) which over-expressed MDR1 in 24-well Petri dishes equipped with an array of stainless steel electrodes connected to a computer via a programmable I/O board. We used a dedicated program to generate and monitor the electrical stimulation protocol. Parallel cultures were exposed for 3 hours to increasing concentrations (1, 2, 4, and 8 μM) of doxorubicin following stimulation to 50 Hz AC (7.5 μA) or MDR1 inhibitor XR9576. Cell viability was assessed by determination of adenylate kinase (AK) release. The relationship between MDR1 expression and the intracellular accumulation of doxorubicin as well as the cellular distribution of MDR1 was investigated by computerized image analysis immunohistochemistry and Western blot techniques.

Results

By the use of a variety of tumor cell lines, we show that low frequency, low intensity AC stimulation enhances chemotherapeutic efficacy. This effect was due to an altered expression of intrinsic cellular drug resistance mechanisms. Immunohistochemical, Western blot and fluorescence analysis revealed that AC not only decreases MDR1 expression but also changes its cellular distribution from the plasma membrane to the cytosol. These effects synergistically contributed to the loss of drug extrusion ability and increased chemo-sensitivity.

Conclusion

In the present study, we demonstrate that low frequency, low intensity alternating current electrical stimulation drastically enhances chemotherapeutic efficacy in MDR1 drug resistant malignant tumors. This effect is due to an altered expression of intrinsic cellular drug resistance mechanisms. Our data strongly support a potential clinical application of electrical stimulation to enhance the efficacy of currently available chemotherapeutic protocols.

In vitro andin vivo evaluation of WK-X-34, a novel inhibitor of P-glycoprotein and BCRP, using radio imaging techniques

Overexpression of the multidrug resistance proteins P-glycoprotein (Pgp) and breast cancer resistance protein (BCRP) results in treatment failure of many malignancies including ovarian cancer. Dual inhibition of Pgp and BCRP may restore the sensitivity of resistant cells to anticancer drugs. We report the synthesis and characterization of a novel anthranilic-acid based Pgp and BCRP modulator, WK-X-34. In vitro inhibition of Pgp activity was evaluated using 99mTc-Sestamibi and daunorubicin accumulation in Pgp overexpressing human ovarian cancer cells (A2780/Adr) and its sensitive counterpart (A2780/wt). Interaction with BCRP was examined with a mitoxantrone-efflux assay in BCRP-overexpressing MCF7/mx cells, with flow cytometry. Interactions with the multidrug resistance associated proteins (MRP) were evaluated in transfected MRP1, MRP2 and MRP3 cell lines, using a 5-CFDA efflux assay. In vivo 99mTc-Sestamibi imaging of human ovarian cancer xenografts was used to evaluate the in vivo efficacy of WK-X-34 in mice. Daunorubicin accumulation in A2780/Adr cells was inhibited by WK-X-34 at nanomolar concentrations (IC50: 82.1 +/- 6 nM). WK-X-34 inhibited mitoxantrone accumulation in BCRP-overexpressing cells at micromolar concentrations (IC50 = 26.5 +/- 4.6 microM), whereas WK-X-34 did not significantly alter 5-CFDA accumulation in MRP transfected cells. In vivo, uptake of 99mTc-Sestamibi was significantly increased in A2780/Adr xenograft tumors, brain and intestine (AUCs(0-4h) 136%, 147% and 138%; p < 0.05) in mice dosed with WK-X-34 (20 mg/kg i.p.). WK-X-34 selectively modulates Pgp and BCRP in vitro and in vivo in multidrug resistant ovarian cancer cells, and thus may have potential utility in the treatment of multidrug resistant tumors.

Synergistic effect of bromocriptine and tumor necrosis factor-α on reversing hepatocellular carcinoma multidrug resistance in nude mouse MDR1 model of liver neoplasm

AIM: To investigate the effect of bromocriptine (BCT) and tumor necrosis factor-α (TNF-α ) on hepatocellular carcinoma (HCC) multidrug resistance (MDR) in nude mouse MDR model of liver neoplasm.

METHODS: Human hepatocarcinoma cell line HepG₂, drug resistant hepatocarcinoma cell line HepG₂/adriamycin (ADM) and hepatocarcinoma cell line transfected with TNF-α gene HepG₂/ADM/TNF were injected into the liver of nude mice via orthotopic implantation and MDR model of liver neoplasm in vivo was established (HepG₂, ADM, TNF, BCT groups). Among these groups, BCT group and TNF group were treated with BCT through gastric canal. Each group was divided into control group and chemotherapy group. Size and weight of the tumor were measured. Furthermore, tumor histological character and growth of the nude mice were observed and their chemosensitivity was tested. MDR-associated genes and proteins (MRP, LRP) of implanted tumors were detected by immunohistochemistry, reverse transcriptase polymerase chain reaction, and apoptosis rate of hepatocarcinoma cells was detected by TUNEL assay.

RESULTS: The nude mouse model of each cell line was inoculated successfully. The tumor growth rate and weight were significantly different among groups. After chemotherapy, abdominal cavity tumor growth inhibition rate was higher in BCT group (67%) compared to ADM and TNF groups, and similar to HepG₂ group (54%). MDR1 and LRPmRNA could be detected in all groups, but TNF-αwas detected only in TNF and BCT groups. Furthermore, MDR1 and LRP protein expression of tumors in TNF and BCT groups was low similar to HepG2 group. The apoptosis rate of hepatocarcinoma cells was much higher in BCT group than in other groups with TUNEL assay.

CONCLUSION: BCT and TNF-α can reverse HCC MDR in nude mouse MDR1 model of liver neoplasm.

The role of 99mTc-tetrofosmin scintigraphy for staging patients with laryngeal cancer

Diagnosis and staging of laryngeal cancer is currently based on physical examination, endoscopy, and imaging techniques such as computed tomography (CT) and/or magnetic resonance (MR) and histology. While imaging techniques have a pivotal role for defining the size of the primary tumor, they are less accurate for defining metastatic involvement of regional lymph nodes, especially if lymph nodes are smaller than 10-15 mm. The aim of this study was to comparatively assess the relevance of (99m)Tc-tetrofosmin scintigraphy for the staging of laryngeal tumors versus the CT scan. We evaluated the sensitivity of imaging with (99m)Tc-tetrofosmin in 28 consecutively enrolled patients with squamous cell laryngeal carcinoma. Total-body scintigraphy with 99mTc-tetrofosmin was performed preoperatively, and the results were compared to CT images of the neck and mediastinum. CT and (99m)Tc-tetrofosmin scintigraphy were equally sensitive (96%) in identifying the primary tumor. While CT was more sensitive for detecting metastatic lymph nodes (100% versus 50%), (99m)Tc-tetrofosmin scintigraphy was more specific (100% versus 56%; p < 0.04). The overall diagnostic capabilities of the two techniques for detecting lymph node metastases were comparable (Youden Index: J = 0.56 for CT and J = 0.50 for (99m)Tc-tetrofosmin scintigraphy). (99m)Tc-tetrofosmin scintigraphy is a useful complement to CT for staging laryngeal tumors, especially for detecting metastatic lymph nodes and distant metastases.

Role of drug efflux transporters in the brain for drug disposition and treatment of brain diseases

The blood-brain barrier (BBB) serves as a protective mechanism for the brain by preventing entry of potentially harmful substances from free access to the central nervous system (CNS). Tight junctions present between the brain microvessel endothelial cells form a diffusion barrier, which selectively excludes most blood-borne substances from entering the brain. Astrocytic end-feet tightly ensheath the vessel wall and appear to be critical for the induction and maintenance of the barrier properties of the brain capillary endothelial cells. Because of these properties, the BBB only allows entry of lipophilic compounds with low molecular weights by passive diffusion. However, many lipophilic drugs show negligible brain uptake. They are substrates for drug efflux transporters such as P-glycoprotein (Pgp), multidrug resistance proteins (MRPs) or organic anion transporting polypeptides (OATPs) that are expressed at brain capillary endothelial cells and/or astrocytic end-feet and are key elements of the molecular machinery that confers the special permeability properties to the BBB. The combined action of these carrier systems results in rapid efflux of xenobiotics from the CNS. The objective of this review is to summarize transporter characteristics (cellular localization, specificity, regulation, and potential inhibition) for drug efflux transport systems identified in the BBB and blood-cerebrospinal fluid (CSF) barrier. A variety of experimental approaches available to ascertain or predict the impact of efflux transport on brain access of therapeutic drugs also are described and critically discussed. The potential impact of efflux transport on the pharmacodynamics of agents acting in the CNS is illustrated. Furthermore, the current knowledge about drug efflux transporters as a major determinant of multidrug resistance of brain diseases such as epilepsy is reviewed. Finally, we summarize strategies for modulating or by-passing drug efflux transporters at the BBB as novel therapeutic approaches to drug-resistant brain diseases.

Effects of miltefosine on membrane permeability and accumulation of [99mTc]-hexakis-2-methoxyisobutyl isonitrile, 2-[18F]fluoro-2-deoxy-d-glucose, daunorubucin and rhodamine123 in multidrug-resistant and sensitive cells

Miltefosine is a phospholipid analog that exhibits antineoplastic activity against breast cancer metastases, but its mechanism of action remains uncertain. The aim of this study was to investigate the transport mechanism for the removal of miltefosine and [99mTc]-hexakis-2-methoxyisobutyl isonitrile (99mTc-MIBI) from multidrug-resistant cells. The P-glycoprotein pump function, cell viability, and 99mTc-MIBI and 2-[18F]fluoro-2-deoxy-D-glucose (18FDG) uptakes were measured in NIH 3T3 (3T3) and NIH 3T3MDR1 G185 (3T3MDR1) mouse fibroblasts and human lymphoid B JY cells. Miltefosine treatment increased the permeability and fluidity of these tumor cells in a concentration-dependent manner. The multidrug-sensitive cells were 3-4 times more sensitive to miltefosine than the multidrug-resistant ones. The extent of 99mTc-MIBI accumulation in the P-glycoprotein-expressing cells increased in the presence of miltefosine, whereas the rhodamine123 and daunorubicin uptakes of the cells did not change significantly. In the 3T3MDR1 cells verapamil reinstated the rhodamine123 and daunorubicin accumulation, but not the 99mTc-MIBI uptake. Cyclosporin A reinstated the uptakes of 99mTc-MIBI, daunorubicin and rhodamine123 by the 3T3MDR1 cells. In a concentration-dependent manner miltefosine decreased the extents of 99mTc-MIBI, rhodamine123, daunorubicin and 18FDG accumulation in the JY and 3T3 cells. Our findings indicate a common transport mechanism for 99mTc-MIBI and miltefosine, which is distinct from that for rhodamine123 and daunorubicin in MDR cells.

Biphasic accumulation kinetics of [99mTc]-hexakis-2-methoxyisobutyl isonitrile in tumour cells and its modulation by lipophilic P-glycoprotein ligands

AIM

To study the accumulation and washout kinetics of [99mTc]-hexakis-2-methoxyisobutyl isonitrile (99mTc-MIBI) in MDR positive and MDR negative tumour cells and how this is modified by lipophilic P-glycoprotein ligands.

METHODS

The tumour cells were incubated in the presence and absence of the ligands and the uptakes of 99mTc-MIBI, rhodamine 123 and 2-[18F]fluoro-2-deoxy-D-glucose (18FDG) were measured.

RESULTS

The accumulation of 99mTc-MIBI in the tumour cells followed biphasic kinetics. Verapamil and cyclosporin A increased the membrane fluidity and significantly enhanced the 99mTc-MIBI uptake of the MDR negative cells, while the rhodamine 123 uptake was not affected. Verapamil significantly increased the uptake of rhodamine 123 and 18FDG but did not modify that of 99mTc-MIBI in the MDR positive cells. Cyclosporin A significantly increased the 18FDG uptake of the MDR positive and negative tumour cells; these effects were ouabain-sensitive. Depolarization of the cytoplasmic membrane, acidification of the extracellular medium and the administration of CCCP decreased the accumulation of 99mTc-MIBI and rhodamine 123 uptake in the tumour cells.

CONCLUSIONS

Lipophilic P-glycoprotein ligands modified the biphasic accumulation kinetics of the 99mTc-MIBI uptakes of MDR negative and positive tumour cells in different and complex ways and could therefore mask the P-glycoprotein pump-dependent changes in tracer accumulation.

Imaging Multidrug Resistance P-glycoprotein Transport Function Using MicroPET with Technetium-94m-Sestamibi

The best characterized mechanism of multidrug resistance (MDR) in cancer involves the MDR1 efflux transporter P-glycoprotein (Pgp). The positron-emitting radiotracer hexakis(2-methoxyisobutylisonitrile)-(94m)Tc ((94m)Tc-MIBI) was synthesized and validated in cell transport studies as a substrate for MDR1 Pgp. In vivo small-scale PET imaging and biodistribution studies of mdr1a/1b (-/-) gene deleted and wild-type mice demonstrated the use of (94m)Tc-MIBI to detect Pgp function. The reversal effect of a Pgp modulator was shown in tissue distribution studies of KB 3-1 (Pgp-) and KB 8-5 (Pgp+) tumor-bearing nude mice. The current (94m)Tc-MIBI experiments parallel previous studies employing (99m)Tc-MIBI, showing essentially identical performance of the two technetium radiotracers and providing biological validation of (94m)Tc-MIBI for PET imaging of multidrug resistance.