Development of an effective predictive screening tool for prostate cancer using the ClarityDX machine learning platform.

The current prostate cancer (PCa) screen test, prostate-specific antigen (PSA), has a high sensitivity for PCa but low specificity for high-risk, clinically significant PCa (csPCa), resulting in overdiagnosis and overtreatment of non-csPCa. Early identification of csPCa while avoiding unnecessary biopsies in men with non-csPCa is challenging. We built an optimized machine learning platform (ClarityDX) and showed its utility in generating models predicting csPCa. Integrating the ClarityDX platform with blood-based biomarkers for clinically significant PCa and clinical biomarker data from a 3448-patient cohort, we developed a test to stratify patients' risk of csPCa; called ClarityDX Prostate. When predicting high risk cancer in the validation cohort, ClarityDX Prostate showed 95% sensitivity, 35% specificity, 54% positive predictive value, and 91% negative predictive value, at a ≥ 25% threshold. Using ClarityDX Prostate at this threshold could avoid up to 35% of unnecessary prostate biopsies. ClarityDX Prostate showed higher accuracy for predicting the risk of csPCa than PSA alone and the tested model-based risk calculators. Using this test as a reflex test in men with elevated PSA levels may help patients and their healthcare providers decide if a prostate biopsy is necessary.

Clinical analysis of EV-Fingerprint to predict grade group 3 and above prostate cancer and avoid prostate biopsy.

BACKGROUND: There is an unmet clinical need for minimally invasive diagnostic tests to improve the detection of grade group (GG) ≥3 prostate cancer relative to prostate antigen-specific risk calculators. We determined the accuracy of the blood-based extracellular vesicle (EV) biomarker assay (EV Fingerprint test) at the point of a prostate biopsy decision to predict GG ≥3 from GG ≤2 and avoid unnecessary biopsies. METHODS: This study analyzed 415 men referred to urology clinics and scheduled for a prostate biopsy, were recruited to the APCaRI 01 prospective cohort study. The EV machine learning analysis platform was used to generate predictive EV models from microflow data. Logistic regression was then used to analyze the combined EV models and patient clinical data and generate the patients' risk score for GG ≥3 prostate cancer. RESULTS: The EV-Fingerprint test was evaluated using the area under the curve (AUC) in discrimination of GG ≥3 from GG ≤2 and benign disease on initial biopsy. EV-Fingerprint identified GG ≥3 cancer patients with high accuracy (0.81 AUC) at 95% sensitivity and 97% negative predictive value. Using a 7.85% probability cutoff, 95% of men with GG ≥3 would have been recommended a biopsy while avoiding 144 unnecessary biopsies (35%) and missing four GG ≥3 cancers (5%). Conversely, a 5% cutoff would have avoided 31 unnecessary biopsies (7%), missing no GG ≥3 cancers (0%). CONCLUSIONS: EV-Fingerprint accurately predicted GG ≥3 prostate cancer and would have significantly reduced unnecessary prostate biopsies.

Building predictive disease models using extracellular vesicle microscale flow cytometry and machine learning.

Extracellular vesicles (EVs) are highly abundant in human biofluids, containing a repertoire of macromolecules and biomarkers representative of the tissue of origin. EVs released by tumours can communicate key signals both locally and to distant sites to promote growth and survival or impact invasive and metastatic progression. Microscale flow cytometry of circulating EVs is an emerging technology that is a promising alternative to biopsy for disease diagnosis. However, biofluid-derived EVs are highly heterogeneous in size and composition, making their analysis complex. To address this, we developed a machine learning approach combined with EV microscale cytometry using tissue- and disease-specific biomarkers to generate predictive models. We demonstrate the utility of this novel extracellular vesicle machine learning analysis platform (EVMAP) to predict disease from patient samples by developing a blood test to identify high-grade prostate cancer and validate its performance in a prospective 215 patient cohort. Models generated using the EVMAP approach significantly improved the prediction of high-risk prostate cancer, highlighting the clinical utility of this diagnostic platform for improved cancer prediction from a blood test.

Enrichment and ratiometric detection of circulating tumor cells using PSMA- and folate receptor-targeted magnetic and surface-enhanced Raman scattering nanoparticles.

The presence of circulating tumor cells (CTCs) in a patient's bloodstream is a hallmark of metastatic cancer. The detection and analysis of CTCs is a promising diagnostic and prognostic strategy as they may carry useful genetic information from their derived primary tumor, and the enumeration of CTCs in the bloodstream has been known to scale with disease progression. However, the detection of CTCs is a highly challenging task owing to their sparse numbers in a background of billions of background blood cells. To effectively utilize CTCs, there is a need for an assay that can detect CTCs with high specificity and can locally enrich CTCs from a liquid biopsy. We demonstrate a versatile methodology that addresses these needs by utilizing a combination of nanoparticles. Enrichment is achieved using targeted magnetic nanoparticles and high specificity detection is achieved using a ratiometric detection approach utilizing multiplexed targeted and non-targeted surface-enhanced Raman Scattering Nanoparticles (SERS-NPs). We demonstrate this approach with model prostate and cervical circulating tumor cells and show the ex vivo utility of our methodology for the detection of PSMA or folate receptor over-expressing CTCs. Our approach allows for the mitigation of interference caused by the non-specific uptake of nanoparticles by other cells present in the bloodstream and our results from magnetically trapped CTCs reveal over a 2000% increase in targeted SERS-NP signal over non-specifically bound SERS-NPs.

Quantitative in vivo whole genome motility screen reveals novel therapeutic targets to block cancer metastasis.

Metastasis is the most lethal aspect of cancer, yet current therapeutic strategies do not target its key rate-limiting steps. We have previously shown that the entry of cancer cells into the blood stream, or intravasation, is highly dependent upon in vivo cancer cell motility, making it an attractive therapeutic target. To systemically identify genes required for tumor cell motility in an in vivo tumor microenvironment, we established a novel quantitative in vivo screening platform based on intravital imaging of human cancer metastasis in ex ovo avian embryos. Utilizing this platform to screen a genome-wide shRNA library, we identified a panel of novel genes whose function is required for productive cancer cell motility in vivo, and whose expression is closely associated with metastatic risk in human cancers. The RNAi-mediated inhibition of these gene targets resulted in a nearly total (>99.5%) block of spontaneous cancer metastasis in vivo.

Non-interferometric photoacoustic remote sensing microscopy.

Elasto-optical refractive index modulation due to photoacoustic initial pressure transients produced significant reflection of a probe beam when the absorbing interface had an appreciable refractive index difference. This effect was harnessed in a new form of non-contact optical resolution photoacoustic microscopy called photoacoustic remote sensing microscopy. A non-interferometric system architecture with a low-coherence probe beam precludes detection of surface oscillations and other phase-modulation phenomenon. The probe beam was confocal with a scanned excitation beam to ensure detection of initial pressure-induced intensity reflections at the subsurface origin where pressures are largest. Phantom studies confirmed signal dependence on optical absorption, index contrast and excitation fluence. In vivo imaging of superficial microvasculature and melanoma tumors was demonstrated with ~2.7±0.5 µm lateral resolution.

Enhanced Detection of Cancer Biomarkers in Blood-Borne Extracellular Vesicles Using Nanodroplets and Focused Ultrasound.

The feasibility of personalized medicine approaches will be greatly improved by the development of noninvasive methods to interrogate tumor biology. Extracellular vesicles shed by solid tumors into the bloodstream have been under recent investigation as a source of tumor-derived biomarkers such as proteins and nucleic acids. We report here an approach using submicrometer perfluorobutane nanodroplets and focused ultrasound to enhance the release of extracellular vesicles from specific locations in tumors into the blood. The released extracellular vesicles were enumerated and characterized using micro flow cytometry. Only in the presence of nanodroplets could ultrasound release appreciable levels of tumor-derived vesicles into the blood. Sonication of HT1080-GFP tumors did not increase the number of circulating tumor cells or the metastatic burden in the tumor-bearing embryos. A variety of biological molecules were successfully detected in tumor-derived extracellular vesicles, including cancer-associated proteins, mRNAs, and miRNAs. Sonication of xenograft HT1080 fibrosarcoma tumors released extracellular vesicles that contained detectable RAC1 mRNA with the highly tumorigenic N92I mutation known to exist in HT1080 cells. Deep sequencing serum samples of embryos with sonicated tumors allowed the identification of an additional 13 known heterozygous mutations in HT1080 cells. Applying ultrasound to HT1080 tumors increased tumor-derived DNA in the serum by two orders of magnitude. This work is the first demonstration of enhanced extracellular vesicle release by ultrasound stimulation and suggests that nanodroplets/ultrasound offers promise for genetic profiling of tumor phenotype and aggressiveness by stimulating the release of extracellular vesicles. Cancer Res; 77(1); 3-13. ©2016 AACR.

Multimodality Raman and photoacoustic imaging of surface-enhanced-Raman-scattering-targeted tumor cells.

A multimodality Raman and photoacoustic imaging system is presented. This system has ultralow background and can detect tumor cells labeled with modified surface-enhanced-Raman-scattering (SERS) nanoparticles in vivo. Photoacoustic imaging provides microvascular context and can potentially be used to guide magnetic trapping of circulating tumor cells for SERS detection in animal models.

Porphyrin Nanodroplets: Sub-micrometer Ultrasound and Photoacoustic Contrast Imaging Agents.

A novel class of all-organic nanoscale porphyrin nanodroplet agents is presented which is suitable for multimodality ultrasound and photoacoustic molecular imaging. Previous multimodality photoacoustic-ultrasound agents are either not organic, or not yet demonstrated to exhibit enhanced accumulation in leaky tumor vasculature, perhaps because of large diameters. In the current study, porphyrin nanodroplets are created with a mean diameter of 185 nm which is small enough to exhibit the enhanced permeability and retention effect. Porphyrin within the nanodroplet shell has strong optical absorption at 705 nm with an estimated molar extinction coefficient >5 × 10(9) m(-1) cm(-1) , allowing both ultrasound and photoacoustic contrast in the same nanoparticle using all organic materials. The potential of nanodroplets is that they may be phase-changed into microbubbles using high pressure ultrasound, providing ultrasound contrast with single-bubble sensitivity. Multispectral photoacoustic imaging allows visualization of nanodroplets when injected intratumorally in an HT1080 tumor in the chorioallantoic membrane of a chicken embryo. Intravital microscopy imaging of Hep3-GFP and HT1080-GFP tumors in chicken embryos determines that nanodroplets accumulated throughout or at the periphery of tumors, suggesting that porphyrin nanodroplets may be useful for enhancing the visualization of tumors with ultrasound and/or photoacoustic imaging.

Validating tyrosinase homologue melA as a photoacoustic reporter gene for imaging Escherichia coli.

To understand the pathogenic processes for infectious bacteria, appropriate research tools are required for replicating and characterizing infections. Fluorescence and bioluminescence imaging have primarily been used to image infections in animal models, but optical scattering in tissue significantly limits imaging depth and resolution. Photoacoustic imaging, which has improved depth-to-resolution ratio compared to conventional optical imaging, could be useful for visualizing melA-expressing bacteria since melA is a bacterial tyrosinase homologue which produces melanin. Escherichia coli-expressing melA was visibly dark in liquid culture. When melA-expressing bacteria in tubes were imaged with a VisualSonics Vevo LAZR system, the signal-to-noise ratio of a 9×dilution sample was 55, suggesting that ∼20 bacteria cells could be detected with our system. Multispectral (680, 700, 750, 800, 850, and 900 nm) analysis of the photoacoustic signal allowed unmixing of melA-expressing bacteria from blood. To compare photoacoustic reporter gene melA (using Vevo system) with luminescent and fluorescent reporter gene Nano-lantern (using Bruker Xtreme In-Vivo system), tubes of bacteria expressing melA or Nano-lantern were submerged 10 mm in 1% Intralipid, spaced between <1 and 20 mm apart from each other, and imaged with the appropriate imaging modality. Photoacoustic imaging could resolve the two tubes of melA-expressing bacteria even when the tubes were less than 1 mm from each other, while bioluminescence and fluorescence imaging could not resolve the two tubes of Nano-lantern-expressing bacteria even when the tubes were spaced 10 mm from each other. After injecting 100-µL of melA-expressing bacteria in the back flank of a chicken embryo, photoacoustic imaging allowed visualization of melA-expressing bacteria up to 10-mm deep into the embryo. Photoacoustic signal from melA could also be separated from deoxy- and oxy-hemoglobin signal observed within the embryo and chorioallantoic membrane. Our results suggest that melA is a useful photoacoustic reporter gene for visualizing bacteria, and further work incorporating photoacoustic reporters into infectious bacterial strains is warranted.

Multi-wavelength photoacoustic imaging of inducible tyrosinase reporter gene expression in xenograft tumors.

Photoacoustic imaging is an emerging hybrid imaging technology capable of breaking through resolution limits of pure optical imaging technologies imposed by optical-scattering to provide fine-resolution optical contrast information in deep tissues. We demonstrate the ability of multi-wavelength photoacoustic imaging to estimate relative gene expression distributions using an inducible expression system and co-register images with hemoglobin oxygen saturation estimates and micro-ultrasound data. Tyrosinase, the rate-limiting enzyme in melanin production, is used as a reporter gene owing to its strong optical absorption and enzymatic amplification mechanism. Tetracycline-inducible melanin expression is turned on via doxycycline treatment in vivo. Serial multi-wavelength imaging reveals very low estimated melanin expression in tumors prior to doxycycline treatment or in tumors with no tyrosinase gene present, but strong signals after melanin induction in tumors tagged with the tyrosinase reporter. The combination of new inducible reporters and high-resolution photoacoustic and micro-ultrasound technology is poised to bring a new dimension to the study of gene expression in vivo.

Detection of circulating tumor cells using targeted surface-enhanced Raman scattering nanoparticles and magnetic enrichment.

While more than 90% of cancer deaths are due to metastases, our ability to detect circulating tumor cells (CTCs) is limited by low numbers of these cells in the blood and factors confounding specificity of detection. We propose a magnetic enrichment and detection technique for detecting CTCs with high specificity. We targeted both magnetic and surface-enhanced Raman scattering (SERS) nanoparticles to cancer cells. Only cells that are dual-labeled with both kinds of nanoparticles demonstrate an increasing SERS signal over time due to magnetic trapping.

RNA biomarker release with ultrasound and phase-change nanodroplets.

Microbubbles driven by ultrasound are capable of permeabilizing cell membranes and allowing biomarkers or therapeutics to exit from or enter cancer cells, respectively. Unfortunately, the relatively large size of microbubbles prevents extravasation. Lipid-based perfluorobutane microbubbles can be made seven-fold smaller by pressurization, creating 430-nm nanodroplets. The present study compares microbubbles and nanodroplets with respect to their ability to enhance miR-21 and mammaglobin mRNA release from cultured ZR-75-1 cells. Mammaglobin mRNA and miR-21 release increased with escalating concentrations of nanodroplets up to, respectively, 25- and 42-fold with 2% nanodroplets (v/v), compared with pre-ultrasound levels, whereas cell viability decreased to 62.4%. Sonication of ZR-75-1 cells incubated with microbubbles or nanodroplets caused relatively similar levels of cell death and miR-21 release, suggesting that nanodroplets are similar to microbubbles in enhancing cell permeability, but may be more advantageous because of their smaller size, which may allow extravasation through leaky tumor vasculature.

Human concentrative nucleoside transporter 3 transfection with ultrasound and microbubbles in nucleoside transport deficient HEK293 cells greatly increases gemcitabine uptake.

Gemcitabine is a hydrophilic clinical anticancer drug that requires nucleoside transporters to cross plasma membranes and enter cells. Pancreatic adenocarcinomas with low levels of nucleoside transporters are generally resistant to gemcitabine and are currently a clinical problem. We tested whether transfection of human concentrative nucleoside transporter 3 (hCNT3) using ultrasound and lipid stabilized microbubbles could increase gemcitabine uptake and sensitivity in HEK293 cells made nucleoside transport deficient by pharmacologic treatment with dilazep. To our knowledge, no published data exists regarding the utility of using hCNT3 as a therapeutic gene to reverse gemcitabine resistance. Our ultrasound transfection system--capable of transfection of cell cultures, mouse muscle and xenograft CEM/araC tumors--increased hCNT3 mRNA and (3)H-gemcitabine uptake by >2,000- and 3,400-fold, respectively, in dilazep-treated HEK293 cells. Interestingly, HEK293 cells with both functional human equilibrative nucleoside transporters and hCNT3 displayed 5% of (3)H-gemcitabine uptake observed in cells with only functional hCNT3, suggesting that equilibrative nucleoside transporters caused significant efflux of (3)H-gemcitabine. Efflux assays confirmed that dilazep could inhibit the majority of (3)H-gemcitabine efflux from HEK293 cells, suggesting that hENTs were responsible for the majority of efflux from the tested cells. Oocyte uptake transport assays were also performed and provided support for our hypothesis. Gemcitabine uptake and efflux assays were also performed on pancreatic cancer AsPC-1 and MIA PaCa-2 cells with similar results to that of HEK293 cells. Using the MTS proliferation assay, dilazep-treated HEK293 cells demonstrated 13-fold greater resistance to gemcitabine compared to dilazep-untreated HEK293 cells and this resistance could be reversed by transfection of hCNT3 cDNA. We propose that transfection of hCNT3 cDNA using ultrasound and microbubbles may be a method to reverse gemcitabine resistance in pancreatic tumors that have little nucleoside transport activity which are resistant to almost all current anticancer therapies.

Biochemistry and biology of 2'-Fluoro-2'-deoxythymidine (FT), a putative highly selective substrate for thymidine kinase type 2 (TK2).

2'-Deoxy-2'-fluorothymidine (FT) is a bioisostere of both thymidine (TdR), in which F replaces H at C-2' in the ribosyl configuration, and methyluridine, in which F replaces OH at C-2' in the ribosyl configuration. Fluorine is bioisosteric with H with respect to atomic radius and is bioisosteric with OH with respect to polarity and H-bonding as an H acceptor. The consequences of this C-2' F for H substitution on cytotoxicity, nucleoside transporter affinity, phosphorylation by thymidine kinases (TK1, TK2), cell uptake and biodistribution of FT in a murine tumor model are now reported. FT toxicity against a bank of murine and human cells was seen only at very high (˜1 mM) concentrations, although the cellular uptake of [3H]FT in these cells was comparable to that of [3H]TdR over a 24 h period. Human equilibrative nucleoside transporters (hENT1, hENT2) displayed weaker affinity for FT than for TdR, but the concentrative transporters (hCNT1, hCNT2, hCNT3) had much higher affinities for FT. FT was phosphorylated by both mitochondrial thymidine kinase (TK2) (58 % of TdR) and cytosolic thymidine kinase (TK1) (39 % of TdR). Preliminary in vivo imaging with [18F]FT in mice bearing implanted KBALB and contralateral KBALB-STK tumors showed highly selective uptake, with a tumor:blood ratio of 33 in a small herpes simplex type 1 (HSV-1 TK) expressing tumor. In conclusion, [18F]FT appears to be a strong candidate for PET imaging of viral TK transgene imaging, based on its TK1:TK2 phosphorylation differential, its selective uptake by an HSV-TK expressing murine tumor model, its interaction with nucleoside transporters and its low toxicity.

Tyrosinase as a dual reporter gene for both photoacoustic and magnetic resonance imaging.

Reporter genes are useful scientific tools for analyzing promoter activity, transfection efficiency, and cell migration. The current study has validated the use of tyrosinase (involved in melanin production) as a dual reporter gene for magnetic resonance and photoacoustic imaging. MCF-7 cells expressing tyrosinase appear brown due to melanin. Magnetic resonance imaging of tyrosinase-expressing MCF-7 cells in 300 µL plastic tubes displayed a 34 to 40% reduction in T1 compared to normal MCF-7 cells when cells were incubated with 250 µM ferric citrate. Photoacoustic imaging of tyrosinase-expressing MCF-7 cells in 700 µm plastic tubes displayed a 20 to 57-fold increase in photoacoustic signal compared to normal MCF-7 cells. The photoacoustic signal from tyrosinase-expressing MCF-7 cells was significantly greater than blood at 650 nm, suggesting that tyrosinase-expressing cells can be differentiated from the vasculature with in vivo photoacoustic imaging. The imaging results suggest that tyrosinase is a useful reporter gene for both magnetic resonance and photoacoustic imaging.

Biodistribution and uptake of 3'-deoxy-3'-fluorothymidine in ENT1-knockout mice and in an ENT1-knockdown tumor model.

UNLABELLED: (18)F-3'-Deoxy-3'-fluorothymidine ((18)F-FLT) is a PET tracer that accumulates in proliferating tissues. The current study was undertaken to determine whether equilibrative nucleoside transporter 1 (ENT1) is important for (18)F-FLT uptake in normal tissues and tumors. METHODS: ENT1-knockout (ENT1(-/-)) mice were generated and compared with wild-type (ENT1(+/+)) mice using small-animal (18)F-FLT PET. In addition, ENT1(+/+) mice were also injected with the ENT1 inhibitor nitrobenzylmercaptopurine ribonucleoside phosphate (NBMPR-P) at 1 h before radiotracer injection, followed by (18)F-FLT small-animal PET. Tissues of interest were analyzed for thymidine kinase 1 and nucleoside transporters by immunoblotting and immunohistochemistry, respectively, and plasma thymidine levels were analyzed by liquid chromatography-mass spectrometry. Human lung carcinoma A549 cells were stably transfected with pSUPER-producing short-hairpin RNA against human ENT1 (hENT1) or a scrambled sequence with no homology to mammalian genes (A549-pSUPER-hENT1 and A549-pSUPER-SC, respectively). Cultured transfected cells were characterized for hENT1 transcript levels and (18)F-FLT uptake using real-time polymerase chain reaction and (3)H-FLT uptake assays, respectively. Transfected A549 cells were grown as xenograft tumors in NIH-III mice, which were analyzed by (18)F-FLT small-animal PET. RESULTS: Compared with noninjected ENT1(+/+) mice, ENT1(+/+) mice injected with NBMPR-P and ENT1(-/-) mice displayed a reduced percentage injected dose per gram (%ID/g) for (18)F-FLT in the blood (84 and 81%, respectively) and an increased %ID/g for (18)F-FLT in the spleen (188 and 469%, respectively) and bone marrow (266 and 453%, respectively). ENT1(-/-) mice displayed 1.65-fold greater plasma thymidine levels than did ENT1(+/+) mice. Spleen tissue from ENT1(+/+) and ENT1(-/-) mice displayed similar thymidine kinase 1 protein levels and significant concentrative nucleoside transporter 1 and 3 staining. Compared with A549-pSUPER-SC cells, A549-pSUPER-hENT1 cells displayed 0.45-fold hENT1 transcript levels and 0.68-fold (3)H-FLT uptake. Compared with A549-pSUPER-SC xenograft tumors, A549-pSUPER-hENT1 xenograft tumors displayed 0.76-fold %ID/g values (ex vivo gamma-counts) and 0.65-fold maximum standardized uptake values (PET image analysis) for (18)F-FLT uptake at 1 h after tracer injection. CONCLUSION: Loss of ENT1 activity significantly affected (18)F-FLT biodistribution in mice and (18)F-FLT uptake in xenograft tumors, suggesting that nucleoside transporters are important mediators of (18)F-FLT uptake in normal and transformed cells.

Predicting gemcitabine transport and toxicity in human pancreatic cancer cell lines with the positron emission tomography tracer 3'-deoxy-3'-fluorothymidine.

The abundance of human equilibrative nucleoside transporter 1 (hENT1) has recently been shown to be a predictive marker of benefit from gemcitabine therapy in patients with pancreatic cancer. Since hENT1 is also important for the uptake of positron emission tomography (PET) tracer 3'-deoxy-3'-fluorothymidine (FLT) in various cultured human cell lines, this study was undertaken to determine if FLT uptake predicts gemcitabine uptake and/or toxicity in a panel of human pancreatic cancer cell lines (Capan-2, AsPC-1, BxPC-3, PL45, MIA PaCa-2, and PANC-1). Capan-2 cells displayed the lowest levels of (1) extracellular nitrobenzylmercaptopurine ribonucleoside (NBMPR) binding, which represents cell-surface hENT1, (2) FLT and gemcitabine uptake during short (1-45s) and prolonged (1h) periods, and (3) gemcitabine sensitivity. Exposure to NBMPR (inhibits only hENT1) or dilazep (inhibits hENT1 and hENT2) reduced FLT and gemcitabine uptake and gemcitabine sensitivity, with dilazep having greater effects than NBMPR. Gemcitabine permeation was almost completely mediated, primarily by hENT1 and to a lesser extent by hENT2, whereas FLT permeation included a substantial component of passive diffusion. In five of six cell lines, correlations were observed between (1) FLT and gemcitabine initial rates of uptake, (2) gemcitabine uptake and gemcitabine toxicity, (3) FLT uptake and gemcitabine toxicity, and (4) ribonucleotide reductase subunit M1 expression and gemcitabine toxicity. FLT and gemcitabine uptake were comparable for predicting gemcitabine toxicity in the tested pancreatic cancer cell lines suggesting that FLT PET may provide clinically useful information about tumor gemcitabine transport capacity and sensitivity.

The role of human nucleoside transporters in uptake of 3'-deoxy-3'-fluorothymidine.

3'-Deoxy-3'-fluorothymidine (FLT) is a positron emission tomography (PET) tracer used to identify proliferating tumor cells. The purpose of this study was to characterize FLT transport by human nucleoside transporters (hNTs) and to determine the role of hNTs for FLT uptake in various human cancer cell lines. FLT binding to hNTs was monitored by the inhibitory effects of FLT on [(3)H]uridine uptake in yeast cells producing recombinant hNT proteins. hCNT1 displayed the lowest FLT K(i) value for inhibition of [(3)H]uridine uptake, followed by hCNT3, hENT2, hENT1, and hCNT2. [(3)H]FLT was efficiently transported in Xenopus laevis oocytes individually producing hENT1, hENT2, hCNT1, or hCNT3. [(3)H]FLT uptake in MCF-7, A549, U251, A498, MIA PaCa-2, and Capan-2 cells was inhibited at least 50% by the hENT1 inhibitor nitrobenzylmercaptopurine ribonucleoside (NBMPR). According to results of real-time polymerase chain reactions, hENT1 and hENT2 had the most abundant hNT transcripts in all cell lines. Cell lines also underwent 1) [(3)H]NBMPR equilibrium binding assays with or without 5-S-{2-(1-[(fluorescein-5-yl)thioureido]hexanamido)ethyl}-6-N-(4-nitrobenzyl)-5-thioadenosine, a membrane-impermeable NBMPR analog, to determine plasma membrane hENT1 levels, and 2) dose-response NBMPR inhibition of [(3)H]FLT uptake. MCF-7, A549, and Capan-2 cells displayed NBMPR IC(50) values that were smaller or equal to NBMPR K(d) values, suggesting that 50% inhibition of hENT1 reduced [(3)H]FLT uptake by at least 50%. A strong correlation between extracellular NBMPR binding sites/cell and [(3)H]FLT uptake was observed for all cell lines except MIA PaCa-2. These data suggest that plasma membrane hNTs (especially hENT1) are important determinants of cellular FLT uptake.

Mutation of Trp29 of human equilibrative nucleoside transporter 1 alters affinity for coronary vasodilator drugs and nucleoside selectivity.

hENT1 (human equilibrative nucleoside transporter 1) is inhibited by nanomolar concentrations of various structurally distinct coronary vasodilator drugs, including dipyridamole, dilazep, draflazine, soluflazine and NBMPR (nitrobenzylmercaptopurine ribonucleoside). When a library of randomly mutated hENT1 cDNAs was screened using a yeast-based functional complementation assay for resistance to dilazep, a clone containing the W29G mutation was identified. Multiple sequence alignments revealed that this residue was highly conserved. Mutations at Trp29 were generated and tested for adenosine transport activity and inhibitor sensitivity. Trp29 mutations significantly reduced the apparent V(max) and/or increased the apparent K(m) values for adenosine transport. Trp29 mutations increased the IC50 values for hENT1 inhibition by dipyridamole, dilazep, NBMPR, soluflazine and draflazine. NBMPR and soluflazine displayed remarkably similar trends, with large aromatic substitutions at residue 29 resulting in the lowest IC50 values, suggesting that both drugs could interact via ring-stacking interactions with Trp29. The W29T mutant displayed a selective loss of pyrimidine nucleoside transport activity, which contrasts with the previously identified L442I mutant that displayed a selective loss of purine nucleoside transport. W29T, L442I and the double mutant W29T/L442I were characterized kinetically for nucleoside transport activity. A helical wheel projection of TM (transmembrane segment) 1 suggests that Trp29 is positioned close to Met33, implicated previously in nucleoside and inhibitor recognition, and that both residues line the permeant translocation pathway. The data also suggest that Trp29 forms part of, or lies close to, the binding sites for dipyridamole, dilazep, NBMPR, soluflazine and draflazine.