Adenoviral-mediated overexpression of human equilibrative nucleoside transporter 1 (hENT1) enhances gemcitabine response in human pancreatic cancer

Nucleoside transporters and immunosuppressive adenosine signaling in the tumor microenvironment: Potential therapeutic opportunities

Adenosine compartmentalization has a profound impact on immune cell function by regulating adenosine localization and, therefore, extracellular signaling capabilities, which suppresses immune cell function in the tumor microenvironment. Nucleoside transporters, responsible for the translocation and cellular compartmentalization of hydrophilic adenosine, represent an understudied yet crucial component of adenosine disposition in the tumor microenvironment. In this review article, we will summarize what is known regarding nucleoside transporter's function within the purinome in relation to currently devised points of intervention (i.e., ectonucleotidases, adenosine receptors) for cancer immunotherapy, alterations in nucleoside transporter expression reported in cancer, and potential avenues for targeting of nucleoside transporters for the desired modulation of adenosine compartmentalization and action. Further, we put forward that nucleoside transporters are an unexplored therapeutic opportunity, and modulation of nucleoside transport processes could attenuate the pathogenic buildup of immunosuppressive adenosine in solid tumors, particularly those enriched with nucleoside transport proteins.

Challenges and Opportunities Associated With Platelets in Pancreatic Cancer

Pancreatic cancer is one of the most common malignant tumors in the digestive system with a poor prognosis. Accordingly, better understanding of the molecular mechanisms and innovative therapies are warranted to improve the prognosis of this patient population. In addition to playing a crucial role in coagulation, platelets reportedly contribute to the growth, invasion and metastasis of various tumors, including pancreatic cancer. This narrative review brings together currently available evidence on the impact of platelets on pancreatic cancer, including the platelet-related molecular mechanisms of cancer promotion, pancreatic cancer fibrosis, immune evasion, drug resistance mechanisms, thrombosis, targeted platelet therapy, combined radiotherapy and chemotherapy treatment, platelet combined with nanotechnology treatment and potential applications of pancreatic cancer organoids. A refined understanding of the role of platelets in pancreatic cancer provides the foothold for identifying new therapeutic targets.

Tailored adjuvant gemcitabine versus 5-fluorouracil/folinic acid based on hENT1 immunohistochemical staining in resected pancreatic ductal adenocarcinoma: A biomarker stratified prospective trial

BACKGROUND

The study aimed to evaluate the clinical outcomes of tailored adjuvant chemotherapy according to human equilibrative nucleoside transporter 1 (hENT1) expression in resected pancreatic ductal adenocarcinoma (PDA).

METHODS

Patients who underwent pancreatectomy for PDA were enrolled prospectively. According to intra-tumoral hENT1 expression, the high hENT1 (≥50%) group received gemcitabine and the low hENT1 (<50%) group received 5-fluorouracil plus folinic acid (5-FU/FA). The propensity score-matched control consisted of patients who received hENT1-independent adjuvant chemotherapy. The primary outcome was recurrence free survival (RFS) and the secondary outcomes were overall survival (OS) and toxicities.

RESULTS

Between May 2015 and June 2017, we enrolled 44 patients with resected PDA. During a median follow-up period of 28.5 months, the intention-to-treat population showed much longer median RFS [22.9 (95% CI, 11.3-34.5) vs. 10.9 (95% CI, 6.9-14.9) months, P = 0.043] and median OS [36.2 (95% CI, 26.5-45.9) vs. 22.1 (95% CI, 17.7-26.6) months, P = 0.001] compared to the controls. Among 5 patients in the low hENT1 group who discontinued treatment, 2 patients receiving 5-FU/FA discontinued treatment due to drug toxicities (febrile neutropenia and toxic epidermal necrolysis).

CONCLUSION

Tailored adjuvant chemotherapy based on hENT1 staining provides excellent clinical outcomes among patients with resected PDA.

CLINICAL TRIAL REGISTRATION

clinicaltrials.gov identifier: NCT02486497.

Review of clinical and emerging biomarkers for early diagnosis and treatment management of pancreatic cancer: towards personalised medicine

Background:

Pancreatic cancer is the 12th most commonly diagnosed cancer and the 3rd leading cause of cancer mortality and accounts for approximately 2·7% of all newly diagnosed cancer cases and 6·4% of all cancer mortalities in Canada. It has a very poor survival rate mainly due to the difficulty of detecting the disease at an early stage. Consequently, in the advancement of disease management towards the concept of precision medicine that takes individual patient variabilities into account, several investigators have focused on the identification of effective clinical biomarkers with high specificity and sensitivity, capable of early diagnosis of symptomatic patients and early detection of the disease in asymptomatic individuals at high risk for developing pancreatic cancer.

Materials and methods:

We searched several databases from August to December 2020 for relevant studies published in English between 2000 and 2020 and reporting on biomarkers for the management of pancreatic cancer. In this narrative review paper, we describe 13 clinical and emerging biomarkers for pancreatic cancers used in screening for early detection and diagnosis, to identify patients’ risk for metastatic disease and subsequent relapse, to monitor patient response to specific treatment and to provide clinicians the possibility of prospectively identifying groups of patients who will benefit from a particular treatment.

Conclusions:

Current and emerging biomarkers for pancreatic cancer with high specificity and sensitivity has the potential to account for individual patient variabilities, for early detection of disease before the onset of metastasis to improve treatment outcome and patients’ survival, help screen high-risk populations, predict prognosis, provide accurate information of patient response to specific treatment and improve patients monitoring during treatment. Thus, the future holds promise for the use of effective clinical biomarkers or a panel of biomarkers for personalised patient-specific targeted medicine for pancreatic cancer.

Impact of RUNX2 on drug-resistant human pancreatic cancer cells with p53 mutations

Background

Despite the remarkable advances in the early diagnosis and treatment, overall 5-year survival rate of patients with pancreatic cancer is less than 10%. Gemcitabine (GEM), a cytidine nucleoside analogue and ribonucleotide reductase inhibitor, is a primary option for patients with advanced pancreatic cancer; however, its clinical efficacy is extremely limited. This unfavorable clinical outcome of pancreatic cancer patients is at least in part attributable to their poor response to anti-cancer drugs such as GEM. Thus, it is urgent to understand the precise molecular basis behind the drug-resistant property of pancreatic cancer and also to develop a novel strategy to overcome this deadly disease.

Review

Accumulating evidence strongly suggests that p53 mutations contribute to the acquisition and/or maintenance of drug-resistant property of pancreatic cancer. Indeed, certain p53 mutants render pancreatic cancer cells much more resistant to GEM, implying that p53 mutation is one of the critical determinants of GEM sensitivity. Intriguingly, runt-related transcription factor 2 (RUNX2) is expressed at higher level in numerous human cancers such as pancreatic cancer and osteosarcoma, indicating that, in addition to its pro-osteogenic role, RUNX2 has a pro-oncogenic potential. Moreover, a growing body of evidence implies that a variety of miRNAs suppress malignant phenotypes of pancreatic cancer cells including drug resistance through the down-regulation of RUNX2. Recently, we have found for the first time that forced depletion of RUNX2 significantly increases GEM sensitivity of p53-null as well as p53-mutated pancreatic cancer cells through the stimulation of p53 family TAp63/TAp73-dependent cell death pathway.

Conclusions

Together, it is likely that RUNX2 is one of the promising molecular targets for the treatment of the patients with pancreatic cancer regardless of their p53 status. In this review article, we will discuss how to overcome the serious drug-resistant phenotype of pancreatic cancer.

The Role of Platelet-Derived ADP and ATP in Promoting Pancreatic Cancer Cell Survival and Gemcitabine Resistance

Platelets have been demonstrated to be vital in cancer epithelial-mesenchymal transition (EMT), an important step in metastasis. Markers of EMT are associated with chemotherapy resistance. However, the association between the development of chemoresistance, EMT, and the contribution of platelets to the process, is still unclear. Here we report that platelets regulate the expression of (1) human equilibrative nucleoside transporter 1 (hENT1) and (2) cytidine deaminase (CDD), markers of gemcitabine resistance in pancreatic cancer. Human ENT1 (hENT1) is known to enable cellular uptake of gemcitabine while CDD deactivates gemcitabine. Knockdown experiments demonstrate that Slug, a mesenchymal transcriptional factor known to be upregulated during EMT, regulates the expression of hENT1 and CDD. Furthermore, we demonstrate that platelet-derived ADP and ATP regulate Slug and CDD expression in pancreatic cancer cells. Finally, we demonstrate that pancreatic cancer cells express the purinergic receptor P2Y₁₂, an ADP receptor found mainly on platelets. Thus ticagrelor, a P2Y₁₂ inhibitor, was used to examine the potential therapeutic effect of an ADP receptor antagonist on cancer cells. Our data indicate that ticagrelor negated the survival signals initiated in cancer cells by platelet-derived ADP and ATP. In conclusion, our results demonstrate a novel role of platelets in modulating chemoresistance in pancreatic cancer. Moreover, we propose ADP/ATP receptors as additional potential drug targets for treatment of pancreatic cancer.

Predictive role of human equilibrative nucleoside transporter 1 in patients with pancreatic cancer treated by curative resection and gemcitabine-only adjuvant chemotherapy

The predictive roles of human equilibrative nucleoside transporter 1 (hENT-1) in patients who undergo curative resection and adjuvant chemotherapy with gemcitabine alone have not been established. The present study retrospectively analyzed the clinical data from 101 consecutive patients who underwent curative resection and who received gemcitabine adjuvant chemotherapy for the treatment of pancreatic cancer at Kanagawa Cancer Center (Yokohama, Japan) between 2005 and 2014. The associations between the hENT-1 status and the survival and clinicopathological features of the patients were investigated. Of the 101 patients, 60 patients (59.4%) had high levels of hENT-1 expression. A significant association was observed between hENT-1 status and sex; however, for all the other clinicopathological parameters, including tumor and node stages, no differences were observed between the high and low hENT-1 expression groups. The median follow-up period of the present study was 67.3 months. Between the high and low hENT-1 expression groups, there was a statistically significant difference in the 5-year overall survival (OS) rates following surgery (20.6 and 8.9%, respectively; P=0.019). In addition, a significant difference was observed in the recurrence-free survival (RFS) rates at 5 years following surgery (P=0.049). hENT-1 status was one of the important predictive factors for OS and RFS in patients with pancreatic cancer who underwent curative resection followed by adjuvant chemotherapy with gemcitabine. Adjuvant chemotherapy with gemcitabine alone may be insufficient, particularly in patients with certain relevant risk factors.

Human equilibrative nucleoside transporter 1 gene expression is associated with gemcitabine efficacy in advanced leiomyosarcoma and angiosarcoma

Background:

The expression of human equilibrative nucleoside transporter 1 (hENT1), the major gemcitabine transporter into cells, has been thoroughly investigated as a predictive marker of response to gemcitabine in pancreatic cancer and biliary tract cancers. Since gemcitabine is widely used in the treatment of leiomyosarcoma and angiosarcoma, we investigated the correlation between hENT1 expression and gemcitabine efficacy in these sarcoma subtypes.

Methods:

We retrospectively identified 71 patients affected by advanced angiosarcoma (26) or leiomyosarcoma (45) treated within five Italian referral centres for sarcoma; among them, 49 patients (15 angiosarcoma, 34 leiomyosarcoma) were treated with gemcitabine. All tumour samples were analysed for hENT1 expression by real-time PCR. Median 2^–ΔCt value was used as the cutoff to dichotomise patients into ‘high’ expression and ‘low’ expression groups. Kaplan–Meier analysis was performed to estimate progression-free survival (PFS) and overall survival (OS).

Results:

We found a significant association between high hENT1 expression levels and favourable outcome in terms of PFS and OS compared to cases with low hENT1 expression in leiomyosarcoma treated with gemcitabine (PFS: 6.8 vs 3.2 months, P=0.004; OS: 14.9 vs 8.5 months, P=0.007). In addition, hENT1 overexpression correlated with a significant improvement in PFS (9.3 vs 4.5 months; P=0.02) and OS (20.6 vs 10.8 months; P=0.001) in angiosarcoma patients treated with gemcitabine.

Conclusions:

Our study suggests that higher hENT1 expression are associated to gemcitabine efficacy both in patients with advanced leiomyosarcoma and angiosarcoma.

Enhancing Drug Efficacy and Therapeutic Index through Cheminformatics-Based Selection of Small Molecule Binary Weapons That Improve Transporter-Mediated Targeting: A Cytotoxicity System Based on Gemcitabine

The transport of drug molecules is mainly determined by the distribution of influx and efflux transporters for which they are substrates. To enable tissue targeting, we sought to develop the idea that we might affect the transporter-mediated disposition of small-molecule drugs via the addition of a second small molecule that of itself had no inhibitory pharmacological effect but that influenced the expression of transporters for the primary drug. We refer to this as a “binary weapon” strategy. The experimental system tested the ability of a molecule that on its own had no cytotoxic effect to increase the toxicity of the nucleoside analog gemcitabine to Panc1 pancreatic cancer cells. An initial phenotypic screen of a 500-member polar drug (fragment) library yielded three “hits.” The structures of 20 of the other 2,000 members of this library suite had a Tanimoto similarity greater than 0.7 to those of the initial hits, and each was itself a hit (the cheminformatics thus providing for a massive enrichment). We chose the top six representatives for further study. They fell into three clusters whose members bore reasonable structural similarities to each other (two were in fact isomers), lending strength to the self-consistency of both our conceptual and experimental strategies. Existing literature had suggested that indole-3-carbinol might play a similar role to that of our fragments, but in our hands it was without effect; nor was it structurally similar to any of our hits. As there was no evidence that the fragments could affect toxicity directly, we looked for effects on transporter transcript levels. In our hands, only the ENT1-3 uptake and ABCC2,3,4,5, and 10 efflux transporters displayed measurable transcripts in Panc1 cultures, along with a ribonucleoside reductase RRM1 known to affect gemcitabine toxicity. Very strikingly, the addition of gemcitabine alone increased the expression of the transcript for ABCC2 (MRP2) by more than 12-fold, and that of RRM1 by more than fourfold, and each of the fragment “hits” served to reverse this. However, an inhibitor of ABCC2 was without significant effect, implying that RRM1 was possibly the more significant player. These effects were somewhat selective for Panc cells. It seems, therefore, that while the effects we measured were here mediated more by efflux than influx transporters, and potentially by other means, the binary weapon idea is hereby fully confirmed: it is indeed possible to find molecules that manipulate the expression of transporters that are involved in the bioactivity of a pharmaceutical drug. This opens up an entirely new area, that of chemical genomics-based drug targeting.

Immunohistochemical hENT1 expression as a prognostic biomarker in patients with resected pancreatic ductal adenocarcinoma undergoing adjuvant gemcitabine-based chemotherapy

BACKGROUND

Human equilibrative nucleoside transporters (hENTs) are transmembranous proteins that facilitate the uptake of nucleosides and nucleoside analogues, such as gemcitabine, into the cell. The abundance of hENT1 transporters in resected pancreatic ductal adenocarcinoma (PDAC) might make hENT1 a potential biomarker of response to adjuvant chemotherapy. The aim of this study was to see whether hENT1 expression, as determined by immunohistochemistry, was a suitable predictive marker for subsequent treatment with gemcitabine-based adjuvant chemotherapy.

METHODS

A systematic review was performed, searching databases from January 1997 to January 2016. Articles pertaining to hENT1 immunohistochemical analysis in resected PDAC specimens from patients who subsequently underwent adjuvant gemcitabine-based chemotherapy were identified. Eligible studies were required to contain survival data, reporting specifically overall survival (OS) and disease-free survival (DFS) with associated hazard ratios (HRs) stratified by hENT1 status.

RESULTS

Of 42 articles reviewed, eight were suitable for review, with seven selected for quantitative meta-analysis. The total number of patients included in the meta-analysis was 770 (405 hENT1-negative, 365 hENT1-positive). Immunohistochemically detected hENT1 expression was significantly associated with both prolonged DFS (HR 0·58, 95 per cent c.i. 0·42 to 0·79) and OS (HR 0·52, 0·38 to 0·72) in patients receiving adjuvant gemcitabine but not those having fluoropyrimidine-based adjuvant therapy.

CONCLUSION

Expression of hENT1 is a suitable prognostic biomarker in patients undergoing adjuvant gemcitabine-based chemotherapy.

Metabolism, Biochemical Actions, and Chemical Synthesis of Anticancer Nucleosides, Nucleotides, and Base Analogs

Nucleoside, nucleotide, and base analogs have been in the clinic for decades to treat both viral pathogens and neoplasms. More than 20% of patients on anticancer chemotherapy have been treated with one or more of these analogs. This review focuses on the chemical synthesis and biology of anticancer nucleoside, nucleotide, and base analogs that are FDA-approved and in clinical development since 2000. We highlight the cellular biology and clinical biology of analogs, drug resistance mechanisms, and compound specificity towards different cancer types. Furthermore, we explore analog syntheses as well as improved and scale-up syntheses. We conclude with a discussion on what might lie ahead for medicinal chemists, biologists, and physicians as they try to improve analog efficacy through prodrug strategies and drug combinations.

Synthesis and Biological Evaluation of Triazolyl 13α-Estrone-Nucleoside Bioconjugates

2′-Deoxynucleoside conjugates of 13α-estrone were synthesized by applying the copper-catalyzed alkyne–azide click reaction (CuAAC). For the introduction of the azido group the 5′-position of the nucleosides and a propargyl ether functional group on the 3-hydroxy group of 13α-estrone were chosen. The best yields were realized in our hands when the 3′-hydroxy groups of the nucleosides were protected by acetyl groups and the 5′-hydroxy groups were modified by the tosyl–azide exchange method. The commonly used conditions for click reaction between the protected-5′-azidonucleosides and the steroid alkyne was slightly modified by using 1.5 equivalent of Cu(I) catalyst. All the prepared conjugates were evaluated in vitro by means of MTT assays for antiproliferative activity against a panel of human adherent cell lines (HeLa, MCF-7 and A2780) and the potential inhibitory activity of the new conjugates on human 17β-hydroxysteroid dehydrogenase 1 (17β-HSD1) was investigated via in vitro radiosubstrate incubation. Some protected conjugates displayed moderate antiproliferative properties against a panel of human adherent cancer cell lines (the protected cytidine conjugate proved to be the most potent with IC₅₀ value of 9 μM). The thymidine conjugate displayed considerable 17β-HSD1 inhibitory activity (IC₅₀ = 19 μM).

Fasting cycles potentiate the efficacy of gemcitabine treatment in in vitro and in vivo pancreatic cancer models

Background/aims

Pancreatic cancer (PC) is ranked as the fourth leading cause of cancer-related deaths worldwide. Despite recent advances in treatment options, a modest impact on the outcome of the disease is observed so far. Short-term fasting cycles have been shown to potentiate the efficacy of chemotherapy against glioma. The aim of this study was to assess the effect of fasting cycles on the efficacy of gemcitabine, a standard treatment for PC patients, in vitro and in an in vivo pancreatic cancer mouse xenograft model.

Materials and Methods

BxPC-3, MiaPaca-2 and Panc-1 cells were cultured in standard and fasting mimicking culturing condition to evaluate the effects of gemcitabine. Pancreatic cancer xenograft mice were subjected to 24h starvation prior to gemcitabine injection to assess the tumor volume and weight as compared to mice fed ad libitum.

Results

Fasted pancreatic cancer cells showed increased levels of equilibrative nucleoside transporter (hENT1), the transporter of gemcitabine across the cell membrane, and decreased ribonucleotide reductase M1 (RRM1) levels as compared to those cultured in standard medium. Gemcitabine was more effective in inducing cell death on fasted cells as compared to controls. Consistently, xenograft pancreatic cancer mice subjected to fasting cycles prior to gemcitabine injection displayed a decrease of more than 40% in tumor growth.

Conclusion

Fasting cycles enhance gemcitabine effect in vitro and in the in vivo PC xenograft mouse model. These results suggest that restrictive dietary interventions could enhance the efficacy of existing cancer treatments in pancreatic cancer patients.

Development of a metabolites risk score for one-year mortality risk prediction in pancreatic adenocarcinoma patients

PURPOSE

Survival among patients with adenocarcinoma pancreatic cancer (PDCA) is highly variable, which ranges from 0% to 20% at 5 years. Such a wide range is due to tumor size and stage, as well other patients' characteristics. We analyzed alterations in the metabolomic profile, of PDCA patients, which are potentially predictive of patient's one-year mortality.

EXPERIMENTAL DESIGN

A targeted metabolomic assay was conducted on serum samples of patients diagnosed with pancreatic cancer. Statistical analyses were performed only for those 27 patients with information on vital status at follow-up and baseline clinical features. Random Forest analysis was performed to identify all metabolites and clinical variables with the best capability to predict patient's mortality risk at one year. Regression coefficients were estimated from multivariable Weibull survival model, which included the most associated metabolites. Such coefficients were used as weights to build a metabolite risk score (MRS) which ranged from 0 (lowest mortality risk) to 1 (highest mortality risk). The stability of these weights were evaluated performing 10,000 bootstrap resamplings.

RESULTS

MRS was built as a weighted linear combination of the following five metabolites: Valine (HR = 0.62, 95%CI: 0.11-1.71 for each standard deviation (SD) of 98.57), Sphingomyeline C24:1 (HR = 2.66, 95%CI: 1.30-21.09, for each SD of 20.67), Lysine (HR = 0.36, 95%CI: 0.03-0.77, for each SD of 51.73), Tripentadecanoate TG15 (HR = 0.25, 95%CI: 0.01-0.82, for each SD of 2.88) and Symmetric dimethylarginine (HR = 2.24, 95%CI: 1.28-103.08, for each SD of 0.62), achieving a very high discrimination ability (survival c-statistic of 0.855, 95%CI: 0.816-0.894). Such association was still present even after adjusting for the most associated clinical variables (confounders).

CONCLUSIONS

The mass spectrometry-based metabolomic profiling of serum represents a valid tool for discovering novel candidate biomarkers with prognostic ability to predict one-year mortality risk in patients with pancreatic adenocarcinoma.

Zidovudine, an anti-viral drug, resensitizes gemcitabine-resistant pancreatic cancer cells to gemcitabine by inhibition of the Akt-GSK3β-Snail pathway

Pancreatic cancer is one of the most difficult malignancies to treat owing to the rapid acquisition of resistance to chemotherapy. Gemcitabine, a first-line treatment for pancreatic cancer, prolongs patient survival by several months, and combination treatment with gemcitabine and other anti-cancer drugs in the clinic do not show any significant effects on overall survival. Thus, identification of a drug that resensitizes gemcitabine-resistant pancreatic cancer to gemcitabine and a better understanding of the molecular mechanisms of gemcitabine resistance are critical to develop new therapeutic options for pancreatic cancer. Here, we report that zidovudine resensitizes gemcitabine-resistant pancreatic cancer to gemcitabine as shown by screening a compound library, including clinical medicine, using gemcitabine-resistant cells. In analyzing the molecular mechanisms of zidovudine effects, we found that the epithelial-to-mesenchymal transition (EMT)-like phenotype and downregulation of human equilibrative nucleoside transporter 1 (hENT1) are essential for the acquisition of gemcitabine resistance, and zidovudine restored these changes. The chemical biology investigations also revealed that activation of the Akt-GSK3β-Snail1 pathway in resistant cells is a key signaling event for gemcitabine resistance, and zidovudine resensitized resistant cells to gemcitabine by inhibiting this activated pathway. Moreover, our in vivo study demonstrated that co-administration of zidovudine and gemcitabine strongly suppressed the formation of tumors by gemcitabine-resistant pancreatic cancer and prevented gemcitabine-sensitive pancreatic tumors from acquiring gemcitabine-resistant properties, inducing an EMT-like phenotype and downregulating hENT1 expression. These results suggested that co-treatment with zidovudine and gemcitabine may become a novel therapeutic strategy for pancreatic cancer by inhibiting chemoresistance-specific signaling.

Nucleoside transporter proteins as biomarkers of drug responsiveness and drug targets

Nucleoside and nucleobase analogs are currently used in the treatment of solid tumors, lymphoproliferative diseases, viral infections such as hepatitis and AIDS, and some inflammatory diseases such as Crohn. Two gene families are implicated in the uptake of nucleosides and nucleoside analogs into cells, SCL28 and SLC29. The former encodes hCNT1, hCNT2, and hCNT3 proteins. They translocate nucleosides in a Na⁺ coupled manner with high affinity and some substrate selectivity, being hCNT1 and hCNT2 pyrimidine- and purine-preferring, respectively, and hCNT3 a broad selectivity transporter. SLC29 genes encode four members, being hENT1 and hENT2 the only two which are unequivocally implicated in the translocation of nucleosides and nucleobases (the latter mostly via hENT2) at the cell plasma membrane. Some nucleoside-derived drugs can also interact with and be translocated by members of the SLC22 gene family, particularly hOCT and hOAT proteins. Inter-individual differences in transporter function and perhaps, more importantly, altered expression associated with the disease itself might modulate the transporter profile of target cells, thereby determining drug bioavailability and action. Drug transporter pharmacology has been periodically reviewed. Thus, with this contribution we aim at providing a state-of-the-art overview of the clinical evidence generated so far supporting the concept that these membrane proteins can indeed be biomarkers suitable for diagnosis and/or prognosis. Last but not least, some of these transporter proteins can also be envisaged as drug targets, as long as they can show “transceptor” functions, in some cases related to their role as modulators of extracellular adenosine levels, thereby providing a functional link between P1 receptors and transporters.

Defective hCNT1 transport contributes to gemcitabine chemoresistance in ovarian cancer subtypes: overcoming transport defects using a nanoparticle approach

Nucleoside analogs are used as chemotherapeutic options for the treatment of platinum-resistant ovarian cancers. Human concentrative nucleoside transporter 1 (hCNT1) is implicated in sensitizing solid tumors to nucleoside analogs although its role in determining drug efficacy in ovarian cancers remains unclear. Here we examined the functional expression of hCNT1 and compared its contributions toward gemcitabine efficacy in histological subtypes of ovarian cancer. Radioactivity analysis identified hCNT1-mediated (3)H-gemcitabine transport in ovarian cancer cells to be significantly reduced compared with that of normal ovarian surface epithelial cells. Biochemical and immunocytochemical analysis identified that unlike normal ovarian cells which expressed high levels of hCNT1 at the apical cell surface, the transporter was either diminished in expression and/or mislocalized in cell lines of various subtypes of ovarian cancer. Retroviral expression of hCNT1 selectively rescued gemcitabine transport in cell lines representing serous, teratocarcinoma, and endometrioid subtypes, but not clear cell carcinoma (CCC). In addition, exogenous hCNT1 predominantly accumulated in intracytoplasmic vesicles in CCC suggesting defective cellular trafficking of hCNT1 as a contributing factor to transport deficiency. Despite diminution of hCNT1 transport in the majority of ovarian cancers and apparent trafficking defects with CCC, the chemotherapeutic efficacy of gemcitabine was broadly enhanced in all subtypes when delivered via engineered nanoparticles (NPs). Additionally, by bypassing the transport requirement, the delivery of a gemcitabine-cisplatin combination in NP formulation increased their synergistic interactions. These findings uncover hCNT1 as a putative determinant for nucleoside analog chemoresistance in ovarian cancer and may help rationalize drug selection and delivery strategies for various histological subtypes of ovarian cancer.

Modeling interactions between Human Equilibrative Nucleoside Transporter-1 and other factors involved in the response to gemcitabine treatment to predict clinical outcomes in pancreatic ductal adenocarcinoma patients

Background

Pancreatic ductal adenocarcinoma (PDAC) is an extremely aggressive malignancy, characterized by largely unsatisfactory responses to the currently available therapeutic strategies. In this study we evaluated the expression of genes involved in gemcitabine uptake in a selected cohort of patients with PDAC, with well-defined clinical-pathological features.

Methods

mRNA levels of hENT1, CHOP, MRP1 and DCK were evaluated by means of qRT-PCR in matched pairs of tumor and adjacent normal tissue samples collected from PDAC patients treated with gemcitabine after surgical tumor resection. To detect possible interaction between gene expression levels and to identify subgroups of patients at different mortality/progression risk, the RECursive Partitioning and Amalgamation (RECPAM) method was used.

Results

RECPAM analysis showed that DCK and CHOP were most relevant variables for the identification of patients with different mortality risk, while hENT1 and CHOP were able to identify subgroups of patients with different disease progression risk. Conclusion: hENT1, CHOP, MRP1 and DCK appear correlated to PDAC, and this interaction might influence disease behavior.

Pancreatic biomarkers: Could they be the answer?

Pancreatic ductal adenocarcinoma (PDA) is known for its poor prognosis. Most of the patients are diagnosed with advanced stages, when no curative treatment is available. Currently, despite extensive clinical research on PDA, the median overall survival remains short. Diagnosis delay and primary chemo-resistance due to its intrinsic biological nature may explain the challenges to improve our results. Our knowledge about the molecular biology of PDA has exponentially increased during the last decades and its use for the development of biomarkers could help to reach better results in the clinical setting. These biomarkers could be the clue for the improvement in PDA clinical research by earlier detection strategies with diagnostic biomarkers, and by an individualization of treatment approach with prognostic and predictive biomarkers. This review summarizes the current knowledge about the molecular biology of PDA and the status of the most important prognostic and predictive biomarkers.

Do hENT1 and RRM1 predict the clinical benefit of gemcitabine in pancreatic cancer?

Gemcitabine is a nucleoside analog that is indicated in the treatment of pancreatic cancer. In order to provide a better use of this drug, the search for immunohistological markers is a hot topic in the field of pancreatic cancer. In particular, the use of nucleoside transporter hENT1 and the intracellular target of gemcitabine RRM1 are current subjects for discussion. We have analyzed the majority of studies of hENT1 and RRM1 on pancreatic cancer, and will discuss the further directions that might be followed in order to integrate these proteins in routine clinical practice. The data that is currently available would benefit from the completion of well-designed randomized trials in order to confirm the clinical value of hENT1 and RRM1 as biomarkers in pancreatic cancer patients.

dCK expression correlates with 5-fluorouracil efficacy and HuR cytoplasmic expression in pancreatic cancer: a dual-institutional follow-up with the RTOG 9704 trial

Deoxycytidine kinase (dCK) and human antigen R (HuR) have been associated with response to gemcitabine in small studies. The present study investigates the prognostic and predictive value of dCK and HuR expression levels for sensitivity to gemcitabine and 5-fluorouracil (5-FU) in a large phase III adjuvant trial with chemoradiation backbone in pancreatic ductal adenocarcinoma (PDA). The dCK and HuR expression levels were determined by immunohistochemistry on a tissue microarray of 165 resected PDAs from the Radiation Therapy Oncology Group (RTOG) 9704 trial. Association with overall survival (OS) and disease-free survival (DFS) status were analyzed using the log-rank test and the Cox proportional hazards model. Experiments with cultured PDA cells were performed to explore mechanisms linking dCK and HuR expression to drug sensitivity. dCK expression levels were associated with improved OS for all patients analyzed from RTOG 9704 (HR: 0.66, 95% CI [0.47-0.93], P = 0.015). In a subset analysis based on treatment arm, the effect was restricted to patients receiving 5-FU (HR: 0.53, 95% CI [0.33-0.85], P = 0.0078). Studies in cultured cells confirmed that dCK expression rendered cells more sensitive to 5-FU. HuR cytoplasmic expression was neither prognostic nor predictive of treatment response. Previous studies along with drug sensitivity and biochemical studies demonstrate that radiation interferes with HuR's regulatory effects on dCK, and could account for the negative findings herein based on the clinical study design (i.e., inclusion of radiation). Finally, we demonstrate that 5-FU can increase HuR function by enhancing HuR translocation from the nucleus to the cytoplasm, similar to the effect of gemcitabine in PDA cells. For the first time, in the pre-treatment tumor samples, dCK and HuR cytoplasmic expression were strongly correlated (chi-square P = 0.015). This dual-institutional follow up study, in a multi-institutional PDA randomized clinical trial, observed that dCK expression levels were prognostic and had predictive value for sensitivity to 5-FU.

Biomarkers in pancreatic cancer: diagnostic, prognostic, and predictive

Pancreatic cancer is the fourth leading cause of cancer-related death in the United States. There has been minimal progress with regard to cancer-specific outcomes in recent decades. Although effective therapies will undoubtedly change the natural history of the disease, effective biomarkers are a promising tool that will likely have a positive impact and will undoubtedly have an important role in the management of patients with pancreatic ductal adenocarcinoma (PDA) in the future. At present, serum CA-19-9 (carbohydrate antigen 19-9) is the only Food and Drug Administration-approved biomarker for PDA, and it has utility as a prognostic marker and as a marker of disease recurrence. There has been a recent explosion in the pancreatic cancer biomarker field with more than 2000 biomarker studies implicating thousands of informative genes as candidate biomarkers. In this review, we summarize the literature on CA-19-9 in PDA and highlight the most promising investigational biomarkers. Distinctions are made between diagnostic biomarkers (detection of disease), prognostic biomarkers (provide information on prognosis and recurrence pattern), and predictive biomarkers (predict treatment response).

Concentrative nucleoside transporter 1 (hCNT1) promotes phenotypic changes relevant to tumor biology in a translocation-independent manner

Nucleoside transporters (NTs) mediate the uptake of nucleosides and nucleobases across the plasma membrane, mostly for salvage purposes. The canonical NTs belong to two gene families, SLC29 and SLC28. The former encode equilibrative nucleoside transporter proteins (ENTs), which mediate the facilitative diffusion of natural nucleosides with broad selectivity, whereas the latter encode concentrative nucleoside transporters (CNTs), which are sodium-coupled and show high affinity for substrates with variable selectivity. These proteins are expressed in most cell types, exhibiting apparent functional redundancy. This might indicate that CNTs have specific roles in the physiology of the cell beyond nucleoside salvage. Here, we addressed this possibility using adenoviral vectors to restore tumor cell expression of hCNT1 or a polymorphic variant (hCNT1S546P) lacking nucleoside translocation ability. We found that hCNT1 restoration in pancreatic cancer cells significantly altered cell-cycle progression and phosphorylation status of key signal-transducing kinases, promoted poly-(ADP-ribose) polymerase hyperactivation and cell death and reduced cell migration. Importantly, the translocation-defective transporter triggered these same effects on cell physiology. Moreover, this study also shows that restoration of hCNT1 expression is able to reduce tumor growth in a mouse model of pancreatic adenocarcinoma. These data predict a novel role for a NT protein, hCNT1, which appears to be independent of its role as mediator of nucleoside uptake by cells. Thereby, hCNT1 fits the profile of a transceptor in a substrate translocation-independent manner and is likely to be relevant to tumor biology.

Finding novel pharmaceuticals in the systems biology era using multiple effective drug targets, phenotypic screening and knowledge of transporters: where drug discovery went wrong and how to fix it

Despite the sequencing of the human genome, the rate of innovative and successful drug discovery in the pharmaceutical industry has continued to decrease. Leaving aside regulatory matters, the fundamental and interlinked intellectual issues proposed to be largely responsible for this are: (a) the move from 'function-first' to 'target-first' methods of screening and drug discovery; (b) the belief that successful drugs should and do interact solely with single, individual targets, despite natural evolution's selection for biochemical networks that are robust to individual parameter changes; (c) an over-reliance on the rule-of-5 to constrain biophysical and chemical properties of drug libraries; (d) the general abandoning of natural products that do not obey the rule-of-5; (e) an incorrect belief that drugs diffuse passively into (and presumably out of) cells across the bilayers portions of membranes, according to their lipophilicity; (f) a widespread failure to recognize the overwhelmingly important role of proteinaceous transporters, as well as their expression profiles, in determining drug distribution in and between different tissues and individual patients; and (g) the general failure to use engineering principles to model biology in parallel with performing 'wet' experiments, such that 'what if?' experiments can be performed in silico to assess the likely success of any strategy. These facts/ideas are illustrated with a reasonably extensive literature review. Success in turning round drug discovery consequently requires: (a) decent systems biology models of human biochemical networks; (b) the use of these (iteratively with experiments) to model how drugs need to interact with multiple targets to have substantive effects on the phenotype; (c) the adoption of polypharmacology and/or cocktails of drugs as a desirable goal in itself; (d) the incorporation of drug transporters into systems biology models, en route to full and multiscale systems biology models that incorporate drug absorption, distribution, metabolism and excretion; (e) a return to 'function-first' or phenotypic screening; and (f) novel methods for inferring modes of action by measuring the properties on system variables at all levels of the 'omes. Such a strategy offers the opportunity of achieving a state where we can hope to predict biological processes and the effect of pharmaceutical agents upon them. Consequently, this should both lower attrition rates and raise the rates of discovery of effective drugs substantially.

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 ³H-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 ³H-gemcitabine uptake observed in cells with only functional hCNT3, suggesting that equilibrative nucleoside transporters caused significant efflux of ³H-gemcitabine. Efflux assays confirmed that dilazep could inhibit the majority of ³H-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.

Overcoming nucleoside analog chemoresistance of pancreatic cancer: a therapeutic challenge

Clinical refractoriness to nucleoside analogs (e.g., gemcitabine, capecitabine) is a major scientific problem and is one of the main reasons underlying the extremely poor prognostic state of pancreatic cancer. The drugs' effects are suboptimal partly due to cellular mechanisms limiting their transport, activation, and overall efficacy. Nonetheless, novel therapeutic approaches are presently under study to circumvent nucleoside analog resistance in pancreatic cancer. With these new approaches come additional challenges to be addressed. This review describes the determinants of chemoresistance in the gemcitabine cytotoxicity pathways, provides an overview of investigational approaches for overcoming chemoresistance, and discusses new challenges presented. Understanding the future directions of the field may assist in the successful development of novel treatment strategies for enhancing chemotherapeutic efficacy in pancreatic cancer.

Diagnostic, prognostic, and predictive biomarkers in pancreatic cancer

Serum CA 19-9 is the only FDA approved biomarker recommended for use in the routine management of pancreatic ductal adenocarcinoma (PDA). Over 2,000 biomarker studies related to pancreatic cancer appear in the literature, highlighting the need to discover and develop improved tests. Diagnostic biomarkers have implications for early detection of PDA, prognostic markers predict patient survival and recurrence patterns, and predictive markers can help personalize treatment regimens.

Gemcitabine-loaded innovative nanocarriers vs GEMZAR: biodistribution, pharmacokinetic features and in vivo antitumor activity

INTRODUCTION

Gemcitabine, an anticancer drug, is a nucleoside analog deoxycytidine antimetabolite, which acts against a wide range of solid tumors. The limitation of gemcitabine is its rapid inactivation by the deoxycytidine deaminase enzyme following its in vivo administration.

AREAS COVERED

One of the most promising new approaches for improving the biopharmaceutical properties of gemcitabine is the use of innovative drug delivery devices. This review explains the current status of gemcitabine drug delivery, which has been under development over the past 5 years, with particular emphasis on liposomal delivery. In addition, the use of novel supramolecular vesicular aggregates (SVAs), polymeric nanoparticles and squalenoylation were treated as interesting innovative approaches for the administration of the nucleoside analog.

EXPERT OPINION

Different colloidal systems containing gemcitabine have been realized, with the aim of providing important potential advancements through traditional ways of therapy. A possible future commercialization of modified gemcitabine is desirable, as was true in the case of liposomal doxorubicin (Doxil(®), Caely(®)).

Connexin-26 is a key factor mediating gemcitabine bystander effect

Gemcitabine is a nucleoside analogue with anticancer activity. Inside the cell, it is sequentially phosphorylated to generate the active drug. Phosphorylated nucleoside analogues have been shown to traffic through gap junctions. We investigated the participation of gap junctional intercellular communication (GJIC) as a possible mechanism spreading gemcitabine cytotoxicity in pancreatic tumors. Immunohistochemical analysis of pancreatic cancer biopsies revealed increased connexin 26 (Cx26) content but loss of connexins 32 (Cx32) and 43 (Cx43) expression. Cx26 abundance in neoplastic areas was confirmed by Cx26 mRNA in situ hybridization. Heterogeneity on the expression levels and the localization of Cx26, Cx32, and Cx43 were identified in pancreatic cancer cells and found to be associated with the extent of GJIC, and correlated with gemcitabine bystander cytotoxic effect. The abundance of Cx26 at the contact points in tumoral regions prompted us to study the involvement of Cx26 in the GJIC of gemcitabine toxic metabolites and their influence on the antitumoral effects of gemcitabine. Knockdown of Cx26 led to decreased GJIC and reduced gemcitabine bystander killing whereas overexpression of Cx26 triggered increased GJIC and enhanced the gemcitabine cytotoxic bystander effect. Gemcitabine treatment of mice bearing tumors, with a high GJIC capacity, resulted in a significant delay in tumor progression. Interestingly, gemcitabine administration in mice bearing tumors that overexpress Cx26 triggered a dramatic tumor regression of 50% from the initial volume. This study shows that Cx26 participates in the gap junction-mediated bystander cytoxic effect of gemcitabine and provides evidence that upregulation of Cx26 improves gemcitabine anticancer efficacy.

Prognostic roles of human equilibrative transporter 1 (hENT-1) and ribonucleoside reductase subunit M1 (RRM1) in resected pancreatic cancer

BACKGROUND

Pancreatic adenocarcinoma is a malignancy with a dismal prognosis. Previous studies have suggested that in pancreatic cancer, human equilibrative nucleoside transporter 1 (hENT-1) and ribonucleoside reductase subunit M1 (RRM1) expression may have prognostic value as well as predictive value with sensitivity to gemcitabine. This study investigated the prognostic value of hENT-1 and RRM1 expression in resected pancreatic cancer.

METHODS

Eighty-four patients who underwent pancreaticoduodenectomy from 2000 to 2005 were included in this study. Patients were followed for a median of 60 months (range, 44-110). Total RNA was isolated from macrodissected paraffin-embedded tumors. hENT-1 and RRM1 expression levels in tumors were evaluated by quantitative reverse transcription-polymerase chain reaction (QRT-PCR), normalized to 2 reference genes, and expressed as ΔCt (low ΔCt means high expression). Univariate and multivariable prognostic factors for overall survival (OS) and progression-free survival (PFS) were identified via Cox proportional hazards analysis.

RESULTS

Univariate analysis identified hENT-1, overall stage, lymphovascular invasion, perineural invasion, and adjuvant therapy as prognostic factors for both PFS and OS. Multivariate analysis confirmed the association of low expression of hENT-1 (ΔCt > 0.2027) (P = .007), perineural invasion (P = .021), and lack of adjuvant treatment (P < 0.001) with worse OS. Multivariate analysis also confirmed the association of low expression of hENT-1 (ΔCt > 0.5391) with worse PFS (P = .016) in addition to overall stage (P = .013), perineural invasion (P = .042), and lack of adjuvant treatment (hazard ratio 2.31, P = .029). RRM1 expression was not associated with OS or PFS in the current cohort.

CONCLUSIONS

Low expression of hENT-1 was associated with worse OS and PFS in patients with resected pancreatic adenocarcinoma independent of gemcitabine therapy.

Interdependence of gemcitabine treatment, transporter expression, and resistance in human pancreatic carcinoma cells

Gemcitabine is widely used as first-line chemotherapeutic drug in the treatment of pancreatic cancer. Our previous experimental chemotherapy studies have shown that treatment of human pancreatic carcinoma cells with 5-fluorouracil (5-FU) alters the cellular transporter expression profile and that modulation of the expression of multidrug resistance protein 5 (MRP5; ABCC5) influences the chemoresistance of these tumor cells. Here, we studied the influence of acute and chronic gemcitabine treatment on the expression of relevant uptake and export transporters in pancreatic carcinoma cells by reverse transcription-polymerase chain reaction (RT-PCR), quantitative RT-PCR, and immunoblot analyses. The specific role of MRP5 in cellular gemcitabine sensitivity was studied by cytotoxicity assays using MRP5-overexpressing and MRP5-silenced cells. Exposure to gemcitabine (12 nM for 3 days) did not alter the messenger RNA (mRNA) expression of MRP1, MRP3, MRP5, and equilibrative nucleoside transporter 1 (ENT1), whereas high dosages of the drug (20 microM for 1 hour) elicited up-regulation of these transporters in most cell lines studied. In cells with acquired gemcitabine resistance (up to 160 nM gemcitabine), the mRNA or protein expression of the gemcitabine transporters MRP5 and ENT1 was upregulated in several cell lines. Combined treatment with 5-FU and gemcitabine caused a 5- to 40-fold increase in MRP5 and ENT1 expressions. Cytotoxicity assays using either MRP5-overexpressing (HEK and PANC-1) or MRP5-silenced (PANC1/shMRP5) cells indicated that MRP5 contributes to gemcitabine resistance. Thus, our novel data not only on drug-induced alterations of transporter expression relevant for gemcitabine uptake and export but also on the link between gemcitabine sensitivity and MRP5 expression may lead to improved strategies of future chemotherapy regimens using gemcitabine in pancreatic carcinoma patients.

Human nucleoside transporters: biomarkers for response to nucleoside drugs

This review describes recent advances in developing human nucleoside transporters (hNTs) as biomarkers to predict response to nucleoside analog drugs with clinical activity. Understanding processes that contribute to drug response or lack thereof will provide strategies to potentiate efficacy or avoid toxicities of nucleoside analog drugs. hNT abundance, evaluated by immunohistochemical methods, has shown promise as a predictive marker to assess clinical drug response that could be used to identify patients who would most likely benefit from nucleoside analog drug treatment.