Modulation of gemcitabine (2',2'-difluoro-2'-deoxycytidine) pharmacokinetics, metabolism, and bioavailability in mice by 3,4,5,6-tetrahydrouridine

Nucleotide metabolism in cancer cells fuels a UDP-driven macrophage cross-talk, promoting immunosuppression and immunotherapy resistance

Many individuals with cancer are resistant to immunotherapies. Here, we identify the gene encoding the pyrimidine salvage pathway enzyme cytidine deaminase (CDA) among the top upregulated metabolic genes in several immunotherapy-resistant tumors. We show that CDA in cancer cells contributes to the uridine diphosphate (UDP) pool. Extracellular UDP hijacks immunosuppressive tumor-associated macrophages (TAMs) through its receptor P2Y6. Pharmacologic or genetic inhibition of CDA in cancer cells (or P2Y6 in TAMs) disrupts TAM-mediated immunosuppression, promoting cytotoxic T cell entry and susceptibility to anti-programmed cell death protein 1 (anti-PD-1) treatment in resistant pancreatic ductal adenocarcinoma (PDAC) and melanoma models. Conversely, CDA overexpression in CDA-depleted PDACs or anti-PD-1-responsive colorectal tumors or systemic UDP administration (re)establishes resistance. In individuals with PDAC, high CDA levels in cancer cells correlate with increased TAMs, lower cytotoxic T cells and possibly anti-PD-1 resistance. In a pan-cancer single-cell atlas, CDAhigh cancer cells match with T cell cytotoxicity dysfunction and P2RY6high TAMs. Overall, we suggest CDA and P2Y6 as potential targets for cancer immunotherapy.

Advances in antitumor activity and mechanism of natural steroidal saponins: A review of advances, challenges, and future prospects

BACKGROUND

Cancer, the second leading cause of death worldwide following cardiovascular diseases, presents a formidable challenge in clinical settings due to the extensive toxic side effects associated with primary chemotherapy drugs employed for cancer treatment. Furthermore, the emergence of drug resistance against specific chemotherapeutic agents has further complicated the situation. Consequently, there exists an urgent imperative to investigate novel anticancer drugs. Steroidal saponins, a class of natural compounds, have demonstrated notable antitumor efficacy. Nonetheless, their translation into clinical applications has remained unrealized thus far. In light of this, we conducted a comprehensive systematic review elucidating the antitumor activity, underlying mechanisms, and inherent limitations of steroidal saponins. Additionally, we propose a series of strategic approaches and recommendations to augment the antitumor potential of steroidal saponin compounds, thereby offering prospective insights for their eventual clinical implementation.

PURPOSE

This review summarizes steroidal saponins' antitumor activity, mechanisms, and limitations.

METHODS

The data included in this review are sourced from authoritative databases such as PubMed, Web of Science, ScienceDirect, and others.

RESULTS

A comprehensive summary of over 40 steroidal saponin compounds with proven antitumor activity, including their applicable tumor types and structural characteristics, has been compiled. These steroidal saponins can be primarily classified into five categories: spirostanol, isospirostanol, furostanol, steroidal alkaloids, and cholestanol. The isospirostanol and cholestanol saponins are found to have more potent antitumor activity. The primary antitumor mechanisms of these saponins include tumor cell apoptosis, autophagy induction, inhibition of tumor migration, overcoming drug resistance, and cell cycle arrest. However, steroidal saponins have limitations, such as higher cytotoxicity and lower bioavailability. Furthermore, strategies to address these drawbacks have been proposed.

CONCLUSION

In summary, isospirostanol and cholestanol steroidal saponins demonstrate notable antitumor activity and different structural categories of steroidal saponins exhibit variations in their antitumor signaling pathways. However, the clinical application of steroidal saponins in cancer treatment still faces limitations, and further research and development are necessary to advance their potential in tumor therapy.

Translational PK-PD/TD modeling of antitumor effects and peripheral neuropathy in gemcitabine and nab-paclitaxel chemotherapy from xenograft mice to patients for optimal dose and schedule

PURPOSE

Gemcitabine and nab-paclitaxel (GnP) treatment, the standard first-line chemotherapy for unresectable pancreatic cancer, often causes peripheral neuropathy (PN). To develop alternative dosing strategies to avoid severe PN, understanding the relationship between pharmacokinetics (PK) and pharmacodynamics/toxicodynamics (PD/TD) is necessary. We established a PK-PD/TD model of GnP treatment to develop an optimal dose schedule.

METHODS

A mouse xenograft model of human pancreatic cancer was generated to measure drug concentrations in the plasma and tumor, antitumor effects, and PN after GnP treatment. The Simeoni tumor growth inhibition model with tumor concentrations and empirical indirect response models were used for the PD and TD models, respectively. Clinical outcomes were predicted with reported population estimates of PK parameters in cancer patients.

RESULTS

The PK-PD/TD model simultaneously described the observed tumor volume and paw withdrawal frequency in the von Frey test. For the standard GnP regimen, the model predicted clinical overall response (75.1%), which was overestimated compared to that in a recent phase II study (42.1%) but lower than the observed disease control rate (96.5%). Model simulation showed that dose reduction to less than 40% GnP dose was not effective; a change of dose schedule from every week for 3 weeks to every 2 weeks was a more favorable approach than dose reduction to 60% every week.

CONCLUSION

The PK-PD/TD model-based translational approach provides a guide for optimal dose determination to avoid severe PN while maintaining antitumor effects during GnP chemotherapy. Further research is needed to enhance its applicability and potential for combination chemotherapy regimens.

Synthesis and anticancer evaluation of acetylated-lysine conjugated gemcitabine prodrugs

Gemcitabine is an antimetabolite drug approved for the treatment of various cancers. However, its use is limited due to several issues such as stability, toxicity and drug resistance. Herein, we present the design and synthesis of a series of gemcitabine prodrugs with modifications on the 4-N-amino group by employing an acetylated l- or d-lysine moiety masked by different substitutions. Prodrugs 1-3 and 6-8 showed up to 2.4 times greater anticancer activity than gemcitabine in A549 lung cells, while they exhibited potent activity against BxPC-3 pancreatic cells with IC50 values in the range of 7-40 nM. Moreover, prodrugs 2-3 and 7-8 were found to be less potent against CTSL low expression Caco-2 cells and at least 69-fold less toxic towards human normal HEK-293T cells compared to gemcitabine, leading to improved selectivity and safety profiles. Further stability studies showed that representative prodrugs 2 and 7 exhibited enhanced metabolic stability in human plasma, human liver microsomes and cytidine deaminase. Prodrug 1 can be cleaved by tumor cell-enriched CTSL to release parent drug gemcitabine. Overall, these results demonstrated that acetylated lysine conjugated gemcitabine prodrugs could serve as promising leads for further evaluation as new anticancer drugs.

Exploring the Inclusion Complex of an Anticancer Drug with β-Cyclodextrin for Reducing Cytotoxicity Toward the Normal Human Cell Line by an Experimental and Computational Approach

The toxicity of any drug against normal cells is a health hazard for all humans. At present, health and disease researchers from all over the world are trying to synthesize designer drugs with diminished toxicity and side effects. The purpose of the present study is to enhance the bioavailability and biocompatibility of gemcitabine (GEM) by decreasing its toxicity and reducing deamination during drug delivery by incorporating it inside the hydrophobic cavity of β-cyclodextrin (β-CD) without affecting the drug ability of the parent compound (GEM). The newly synthesized inclusion complex (IC) was characterized by different physical and spectroscopic techniques, thereby confirming the successful incorporation of the GEM molecule into the nanocage of β-CD. The molecular docking study revealed the orientation of the GEM molecule into the β-CD cavity (-5.40 kcal/mol) to be stably posed for ligand binding. Photostability studies confirmed that the inclusion of GEM using β-CD could lead to better stabilization of GEM (≥96%) for further optical and clinical applications. IC (GEM-β-CD) and GEM exhibited effective antibacterial and antiproliferative activities without being metabolized in a dose-dependent manner. The CT-DNA analysis showed sufficiently strong IC (GEM-β-CD) binding (Ka = 8.1575 × 1010), and this interaction suggests that IC (GEM-β-CD) may possibly exert its biological effects by targeting nucleic acids in the host cell. The newly synthesized biologically active IC (GEM-β-CD), a derivative of GEM, has pharmaceutical development potentiality.

Increased renal elimination of endogenous and synthetic pyrimidine nucleosides in concentrative nucleoside transporter 1 deficient mice

Concentrative nucleoside transporters (CNTs) are active nucleoside influx systems, but their in vivo roles are poorly defined. By generating CNT1 knockout (KO) mice, here we identify a role of CNT1 in the renal reabsorption of nucleosides. Deletion of CNT1 in mice increases the urinary excretion of endogenous pyrimidine nucleosides with compensatory alterations in purine nucleoside metabolism. In addition, CNT1 KO mice exhibits high urinary excretion of the nucleoside analog gemcitabine (dFdC), which results in poor tumor growth control in CNT1 KO mice harboring syngeneic pancreatic tumors. Interestingly, increasing the dFdC dose to attain an area under the concentration-time curve level equivalent to that achieved by wild-type (WT) mice rescues antitumor efficacy. The findings provide new insights into how CNT1 regulates reabsorption of endogenous and synthetic nucleosides in murine kidneys and suggest that the functional status of CNTs may account for the optimal action of pyrimidine nucleoside analog therapeutics in humans.

Biological evaluation of novel gemcitabine analog in patient-derived xenograft models of pancreatic cancer

Gemcitabine (Gem) has been a standard first-line drug for pancreatic cancer (PCa) treatment; however, Gem's rapid metabolism and systemic instability (short half-life) limit its clinical outcome. The objective of this study was to modify Gem into a more stable form called 4-(N)-stearoyl-gemcitabine (4NSG) and evaluate its therapeutic efficacy in patient-derived xenograft (PDX) models from PCa of Black and White patients.Methods 4NSG was synthesized and characterized using nuclear magnetic resonance (NMR), elemental analysis, and high-performance liquid chromatography (HPLC). 4NSG-loaded solid lipid nanoparticles (4NSG-SLN) were developed using the cold homogenization technique and characterized. Patient-derived pancreatic cancer cell lines labeled Black (PPCL-192, PPCL-135) and White (PPCL-46, PPCL-68) were used to assess the in vitro anticancer activity of 4NSG-SLN. Pharmacokinetics (PK) and tumor efficacy studies were conducted using PDX mouse models bearing tumors from Black and White PCa patients.Results 4NSG was significantly stable in liver microsomal solution. The effective mean particle size (hydrodynamic diameter) of 4NSG-SLN was 82 ± 6.7 nm, and the half maximal inhibitory concentration (IC50) values of 4NSG-SLN treated PPCL-192 cells (9 ± 1.1 µM); PPCL-135 (11 ± 1.3 µM); PPCL-46 (12 ± 2.1) and PPCL-68 equaled to 22 ± 2.6 were found to be significantly lower compared to Gem treated PPCL-192 (57 ± 1.5 µM); PPCL-135 (56 ± 1.5 µM); PPCL-46 (56 ± 1.8 µM) and PPCL-68 (57 ± 2.4 µM) cells. The area under the curve (AUC), half-life, and pharmacokinetic clearance parameters for 4NSG-SLN were 3-fourfold higher than that of GemHCl. For in-vivo studies, 4NSG-SLN exhibited a two-fold decrease in tumor growth compared with GemHCl in PDX mice bearing Black and White PCa tumors.Conclusion 4NSG-SLN significantly improved the Gem's pharmacokinetic profile, enhanced Gem's systemic stability increased its antitumor efficacy in PCa PDX mice bearing Black and White patient tumors.

Design, synthesis and antitumor activity study of a gemcitabine prodrug conjugated with a HDAC6 inhibitor

Gemcitabine, as a first-line antitumor drug, has attracted extensive attention. However the occurrence of drug resistance limits its clinical utilization. In this paper, a gemcitabine prodrug GZ was designed and synthesized by conjugation of gemcitabine with a newly reported HDAC6 selective inhibitor pentadecanoic acid. GZ displayed high cytotoxicity to nine cancer cell lines with IC₅₀ values in the low micromolar range. In vivo, GZ displayed superior antitumor activity to gemcitabine in a 4T1 tumor xenograft model without obvious pathological damage to important organs of mice. Our study showed that compound GZ is a potential gemcitabine prodrug, which is worthy of further antitumor activity exploration.

Recent Development of Prodrugs of Gemcitabine

Gemcitabine is a nucleoside analog that has been used widely as an anticancer drug for the treatment of a variety of conditions, including ovarian, bladder, non-small-cell lung, pancreatic, and breast cancer. However, enzymatic deamination, fast systemic clearance, and the emergence of chemoresistance have limited its efficacy. Different prodrug strategies have been explored in recent years, seeking to obtain better pharmacokinetic properties, efficacy, and safety. Different drug delivery strategies have also been employed, seeking to transform gemcitabine into a targeted medicine. This review will provide an overview of the recent developments in gemcitabine prodrugs and their effectiveness in treating cancerous tumors.

Single Diastereomers of the Clinical Anticancer ProTide Agents NUC-1031 and NUC-3373 Preferentially Target Cancer Stem Cells In Vitro

A 3'-protected route toward the synthesis of the diastereomers of clinically active ProTides, NUC-1031 and NUC-3373, is described. The in vitro cytotoxic activities of the individual diastereomers were found to be similar to their diastereomeric mixtures. In the KG1a cell line, NUC-1031 and NUC-3373 have preferential cytotoxic effects on leukemic stem cells (LSCs). These effects were not diastereomer-specific and were not observed with the parental nucleoside analogues gemcitabine and FUDR, respectively. In addition, NUC-1031 preferentially targeted LSCs in primary AML samples and cancer stem cells in the prostate cancer cell line, LNCaP. Although the mechanism for this remains incompletely resolved, NUC-1031-treated cells showed increased levels of triphosphate in both LSC and bulk tumor fractions. As ProTides are not dependent on nucleoside transporters, it seems possible that the LSC targeting observed with ProTides may be caused, at least in part, by preferential accumulation of metabolized nucleos(t)ide analogues.

Design, synthesis, and evaluation of liver-specific gemcitabine prodrugs for potential treatment of hepatitis C virus infection and hepatocellular carcinoma

Many successful anti-viral and anti-cancer drugs are nucleoside analogs, which disrupt RNA and/or DNA synthesis. Here, we present liver-specific prodrugs of the chemotherapy drug gemcitabine (2',2'-difluorodeoxycytidine) for the treatment of hepatitis C virus (HCV) infection and hepatocellular carcinoma. The prodrugs were synthesized by introducing aromatic functional moieties to the cytosine 4-NH₂ group of gemcitabine via amide bonds. The chemical modification was designed to i) enable passive diffusion across cellular membrane, ii) protect the prodrugs from inactivating deamination by cellular enzymes, and iii) allow release of active gemcitabine after amide hydrolysis by high levels of carboxylesterases in the liver. We found that many of our prodrugs exhibited similar toxicity as gemcitabine toward liver- and kidney-derived cancer cell lines but were 24- to 620-fold less cytotoxic than gemcitabine in breast- and pancreas-derived cancer cells, respectively. The prodrugs also inhibited an HCV replicon with IC₅₀ values ranging from 10 nM-1.7 μM. Moreover, many of the prodrugs had therapeutic index values of >10,000 and have synergetic effects when combined with other Food and Drug Administration-approved anti-HCV small molecule drugs. These characteristics support the development of gemcitabine prodrugs as liver-specific therapeutics.

Mechanochemical activation of disulfide-based multifunctional polymers for theranostic drug release

Drug delivery systems responsive to physicochemical stimuli allow spatiotemporal control over drug activity to overcome limitations of systemic drug administration. Alongside, the non-invasive real-time tracking of drug release and uptake remains challenging as pharmacophore and reporter function are rarely unified within one molecule. Here, we present an ultrasound-responsive release system based on the mechanochemically induced 5-exo-trig cyclization upon scission of disulfides bearing cargo molecules attached via β-carbonate linker within the center of a water soluble polymer. In this bifunctional theranostic approach, we release one reporter molecule per drug molecule to quantitatively track drug release and distribution within the cell in real-time. We use N-butyl-4-hydroxy-1,8-naphthalimide and umbelliferone as fluorescent reporter molecules to accompany the release of camptothecin and gemcitabine as clinically employed anticancer agents. The generality of this approach paves the way for the theranostic release of a variety of probes and drugs by ultrasound.

A theranostic approach for the mechanochemically induced release of drugs is presented to track drug release and uptake in real-time.

Small Molecular Gemcitabine Prodrugs for Cancer Therapy

Gemcitabine as a pyrimidine nucleoside analog anticancer drug has high efficacy for a broad spectrum of solid tumors. Gemcitabine is activated within tumor cells by sequential phosphorylation carried out by deoxycytidine kinase to mono-, di-, and triphosphate nucleotides with the last one as the active form. But the instability, drug resistance and toxicity severely limited its utilization in clinics. In the field of medicinal chemistry, prodrugs have proven to be a very effective means for elevating drug stability and decrease undesirable side effects including the nucleoside anticancer drug such as gemcitabine. Many works have been accomplished in design and synthesis of gemcitabine prodrugs, majority of which were summarized in this review.

Enhancing efficacy of gemcitabine in pancreatic patient-derived xenograft mouse models

Gemcitabine (Gem), a nucleoside analog, is a preferred choice of treatment for pancreatic cancer (PCa) and often used in combination therapy against wide range of solid tumors. It is known to be rapidly inactivated in blood by cytidine deaminase. The objective of the study was to improve the systemic stability and anticancer activity of modified Gem termed 4-N-stearoylGem (4NSG) In this study, the IC50 values of 4NSG treated MiaPaCa-2 and primary pancreatic cancer (PPCL-46) cultures were significantly lower when compared with gemcitabine hydrochloride (GemHCl) treated cultures. In acute toxicity study, liver enzyme level of aspartate aminotransferase (AST) of the control mice was not significantly different from AST levels of 4NSG and GemHCl treated mice. However, alanine aminotransferase (ALT) level of control mice (67 ± 5 mUnits/mL) was significantly lower compared with ALT levels of GemHCl (232 ± 28 mUnits/mL) and that of 4NSG (172 ± 22 mUnits/mL) (p < 0.0001). More importantly, ALT level of 4NSG was lower than ALT level of GemHCl (p < 0.05). Although ALT levels were elevated, pathological images of liver and kidney tissues of control, GemHCl and 4NSG treated mice revealed no architectural changes and no significant change in mice weight was observed during treatment. The bioavailability (AUC) of 4NSG was 3-fold high and significantly inhibited the tumor growth as compared with equivalent dose of GemHCl. Immunohistochemical staining revealed that 4NSG significantly inhibited the expression vascular endothelial growth factor (VEGF) receptor. The study is unique because it established, for the first time, enhanced anticancer activity of 4NSG against pancreatic patient-derived xenograft (PDX) mouse model and PPCL-46 cells compared with Gem. 4SGN enhanced pharmacokinetic profile and improved the therapeutic efficacy of the standard-of-care Gem. Lastly, 4GSN showed a remarkable tumor growth inhibition and revealed significant antiangiogenic activity in 4GSN treated pancreatic PDX tumor.

Pharmacokinetics of gemcitabine and its amino acid ester prodrug following intravenous and oral administrations in mice

Gemcitabine is an intravenously administered anti-cancer nucleoside analogue. Systemic exposure following oral administration of gemcitabine is limited by extensive first-pass metabolism via cytidine deaminase (CDA) and potentially by saturation of nucleoside transporter-mediated intestinal uptake. An amino acid ester prodrug of gemcitabine, 5'-l-valyl-gemcitabine (V-Gem), was previously shown to be a substrate of the intestinally expressed peptide transporter 1 (PEPT1) and stable against CDA-mediated metabolism. However, preliminary studies did not evaluate the in vivo oral performance of V-Gem as compared to parent drug. In the present study, we evaluated the pharmacokinetics and in vivo oral absorption of gemcitabine and V-Gem following intravenous and oral administrations in mice. These studies revealed that V-Gem undergoes rapid systemic elimination (half-life < 1 min) and has a low oral bioavailability (<1%). Most importantly, the systemic exposure of gemcitabine was not different following oral administration of equimolar doses of gemcitabine (gemcitabine bioavailability of 18.3%) and V-Gem (gemcitabine bioavailability of 16.7%). Single-pass intestinal perfusions with portal blood sampling in mice revealed that V-Gem undergoes extensive activation in intestinal epithelial cells and that gemcitabine undergoes first-pass metabolism in intestinal epithelial cells. Thus, formulation of gemcitabine as the prodrug V-Gem does not increase systemic gemcitabine exposure following oral dosing, due, in part, to the instability of V-Gem in intestinal epithelial cells.

Mechanisms of gemcitabine oral absorption as determined by in situ intestinal perfusions in mice

Gemcitabine is a widely used chemotherapeutic drug that is administered via intravenous infusion due to a low oral bioavailability of only 10%. This low oral bioavailability is believed to be the result of gemcitabine's low intestinal permeability and oral absorption, followed by significant presystemic metabolism. In the present study, we sought to define the mechanisms of gemcitabine intestinal permeability, the potential for saturation of intestinal uptake, and the transporter(s) responsible for mediating the oral absorption of drug using in situ single-pass intestinal perfusions in mice. Concentration-dependent studies were performed for gemcitabine over 0.5-2000 μM, along with studies of 5 μM gemcitabine in a sodium-containing buffer ± thymidine (which can inhibit concentrative (i.e., CNT1 and CNT3) and equilibrative (i.e., ENT1 and ENT2) nucleoside transporters) or dilazep (which can inhibit ENT1 and ENT2), or in a sodium-free buffer (which can inhibit CNT1 and CNT3). Our findings demonstrated that gemcitabine was, in fact, a high-permeability drug in the intestine at low concentrations, that jejunal uptake of gemcitabine was saturable and mediated almost exclusively by nucleoside transporters, and that jejunal flux was mediated by both high-affinity, low-capacity (K_m = 27.4 µM, V_max = 3.6 pmol/cm²/s) and low-affinity, high-capacity (K_m = 700 µM, V_max = 35.9 pmol/cm²/s) transport systems. Thus, CNTs and ENTs at the apical membrane allow for gemcitabine uptake from the lumen to enterocyte, whereas ENTs at the basolateral membrane allow for gemcitabine efflux from the enterocyte to portal venous blood.

Synthesis of Gemcitabine-Threonine Amide Prodrug Effective on Pancreatic Cancer Cells with Improved Pharmacokinetic Properties

To investigate the amino acid transporter-based prodrug anticancer strategy further, several amino acid-conjugated amide gemcitabine prodrugs were synthesized to target amino acid transporters in pancreatic cancer cells. The structures of the synthesized amino acid-conjugated prodrugs were confirmed by ¹H-NMR and LC-MS. The pancreatic cancer cells, AsPC1, BxPC-3, PANC-1 and MIAPaCa-2, appeared to overexpress the amino acid transporter LAT-1 by conventional RT-PCR. Among the six amino acid derivatives of gemcitabine, threonine derivative of gemcitabine (Gem-Thr) was more effective than free gemcitabine in the pancreatic cancer cells, BxPC-3 and MIAPaCa-2, respectively, in terms of anti-cancer effects. Furthermore, Gem-Thr was metabolically stable in PBS (pH 7.4), rat plasma and liver microsomal fractions. When Gem-Thr was administered to rats at 4 mg/kg i.v., Gem-Thr was found to be successfully converted to gemcitabine via amide bond cleavage. Moreover, the Gem-Thr showed the increased systemic exposure of formed gemcitabine by 1.83-fold, compared to free gemcitabine treatment, due to the significantly decreased total clearance (0.60 vs. 4.23 mL/min/kg), indicating that the amide prodrug approach improves the metabolic stability of gemcitabine in vivo. Taken together, the amino acid transporter-targeting gemcitabine prodrug, Gem-Thr, was found to be effective on pancreatic cancer cells and to offer an efficient potential means of treating pancreatic cancer with significantly better pharmacokinetic characteristics than gemcitabine.

Advances in new drug therapies for the management of sickle cell disease

Introduction

Sickle cell disease (SCD) is an orphan disease in the United States, but is highly prevalent worldwide. Only two drugs, hydroxyurea and L-glutamine, are approved for this disease. With an improved understanding of the pathophysiology of SCD as well as the success of several recently approved drugs for other orphan diseases, there is an increased interest in the development of drugs for SCD.

Areas covered

This review summarizes published studies of drug therapies and ongoing trials of novel agents.

Expert opinion

The development of drugs with different mechanisms of action offers opportunities for combination and individualized therapy in SCD. In addition to acute pain crisis, the evaluation of other SCD-related complications, exercise capacity, patient reported outcomes and validated surrogate endpoints are necessary to advance drug development. It is important to involve sites in sub-Saharan Africa and India, which have the highest burden of SCD, in trials of novel therapies.

Fetal Hemoglobin Induction by Epigenetic Drugs

Fetal hemoglobin (HbF) inhibits the root cause of sickle pathophysiology, sickle hemoglobin polymerization. Individuals who naturally express high levels of HbF beyond infancy thus receive some protection from sickle complications. To mimic this natural genetic experiment using drugs, one guiding observation was that HbF is increased during recovery of bone marrow from extreme stress. This led to evaluation and approval of the cytotoxic (cell killing) drug hydroxyurea to treat sickle cell disease. Cytotoxic approaches are limited in potency and sustainability, however, since they require hematopoietic reserves sufficient to repeatedly mount recoveries from stress that destroys their counterparts, and such reserves are finite. HbF induction even by stress ultimately involves chromatin remodeling of the gene for HbF (HBG), therefore, a logical alternative approach is to directly inhibit epigenetic enzymes that repress HBG-implicated enzymes include DNA methyltransferase 1, histone deacetylases, lysine demethylase 1, protein arginine methyltransferase 5, euchromatic histone lysine methyltransferase 2 and chromodomain helicase DNA-binding protein 4. Clinical proof-of-principle that this alternative, noncytotoxic approach can generate substantial HbF and total hemoglobin increases has already been generated. Thus, with continued careful attention to fundamental biological and pharmacologic considerations (reviewed herein), there is potential that rational, molecular-targeted, safe and highly potent disease-modifying therapy can be realized for patients with sickle cell disease, with the accessibility and cost-effective properties needed for world-wide effect.

Sensitive analysis and pharmacokinetic study of a novel gemcitabine carbamate prodrug and its active metabolite gemcitabine in rats using LC-ESI-MS/MS

FY363 is a new chemical entity of gemcitabine analog, which has been shown to have a significant inhibitory effect on cell proliferation in a variety of tumor cell lines in vitro. As a carbamate derivative, FY363 would be converted to the active metabolite gemcitabine through enzyme action in vivo. In order to clarify the exposure of FY363 prototype and its metabolite gemcitabine in vivo after administration of FY363, a sensitive and specific liquid chromatography tandem mass spectrometry (LC-MS/MS) was developed and validated to simultaneously determine FY363 and gemcitabine in rat plasma after liquid-liquid extraction with ethyl acetate. Chromatographic separation was achieved on a highly stable polar column of Synergi 4u Polar-RP 80A (4 μm, 4.6 × 250 mm) which has a unique ether - phenyl bonded phase. Gradient elution was accomplished with mobile phase system consisting of 5 mM ammonium formate buffer containing 0.1% formic acid and mixed organic solvents containing methanol-acetonitrile (3:2, v/v). Multiple reaction monitoring transitions were performed on triple quadrupole mass spectrometric detection in positive-ion mode with an electrospray ionization source. The calibration curves showed good linearity (r > 0.99) over the established concentration range of 1.0-1000 ng/mL both for FY363 and gemcitabine. The assay was validated to be selective, robust and reproducible. This well validated method was successfully applied to demonstrate the pharmacokinetic behavior and the metabolic transformation of FY363 in rats. Results revealed that about 20% of FY363 were converted into its active metabolite gemcitabine in rats by comparing the exposure of gemcitabine after the FY363 administration with that after direct gemcitabine administration at equimolar dose.

Fluorinated nucleosides as an important class of anticancer and antiviral agents

Fluorine-containing nucleoside analogs (NAs) represent a significant class of the US FDA-approved chemotherapeutics widely used in the clinic. The incorporation of fluorine into drug-like agents modulates lipophilic, electronic and steric parameters, thus influencing pharmacodynamic and pharmacokinetic properties of drugs. Fluorine can block oxidative metabolism of drugs and the formation of undesired metabolites by changing H-bonding interactions. In this review, we focus our attention on chemical fluorination reagents and methods used in the NAs field, including positron emission tomography radiochemistry. We briefly discuss both the cellular biology and clinical properties of FDA-approved and fluorine-containing nucleoside/nucleotide analogs in development as well as common resistance mechanisms associated with their use. Finally, we emphasize pronucleotide strategies used to improve therapeutic outcome of NAs in the clinic.

Oral tetrahydrouridine and decitabine for non-cytotoxic epigenetic gene regulation in sickle cell disease: A randomized phase 1 study

BACKGROUND

Sickle cell disease (SCD), a congenital hemolytic anemia that exacts terrible global morbidity and mortality, is driven by polymerization of mutated sickle hemoglobin (HbS) in red blood cells (RBCs). Fetal hemoglobin (HbF) interferes with this polymerization, but HbF is epigenetically silenced from infancy onward by DNA methyltransferase 1 (DNMT1).

METHODS AND FINDINGS

To pharmacologically re-induce HbF by DNMT1 inhibition, this first-in-human clinical trial (NCT01685515) combined 2 small molecules-decitabine to deplete DNMT1 and tetrahydrouridine (THU) to inhibit cytidine deaminase (CDA), the enzyme that otherwise rapidly deaminates/inactivates decitabine, severely limiting its half-life, tissue distribution, and oral bioavailability. Oral decitabine doses, administered after oral THU 10 mg/kg, were escalated from a very low starting level (0.01, 0.02, 0.04, 0.08, or 0.16 mg/kg) to identify minimal doses active in depleting DNMT1 without cytotoxicity. Patients were SCD adults at risk of early death despite standard-of-care, randomized 3:2 to THU-decitabine versus placebo in 5 cohorts of 5 patients treated 2X/week for 8 weeks, with 4 weeks of follow-up. The primary endpoint was ≥ grade 3 non-hematologic toxicity. This endpoint was not triggered, and adverse events (AEs) were not significantly different in THU-decitabine-versus placebo-treated patients. At the decitabine 0.16 mg/kg dose, plasma concentrations peaked at approximately 50 nM (Cmax) and remained elevated for several hours. This dose decreased DNMT1 protein in peripheral blood mononuclear cells by >75% and repetitive element CpG methylation by approximately 10%, and increased HbF by 4%-9% (P < 0.001), doubling fetal hemoglobin-enriched red blood cells (F-cells) up to approximately 80% of total RBCs. Total hemoglobin increased by 1.2-1.9 g/dL (P = 0.01) as reticulocytes simultaneously decreased; that is, better quality and efficiency of HbF-enriched erythropoiesis elevated hemoglobin using fewer reticulocytes. Also indicating better RBC quality, biomarkers of hemolysis, thrombophilia, and inflammation (LDH, bilirubin, D-dimer, C-reactive protein [CRP]) improved. As expected with non-cytotoxic DNMT1-depletion, platelets increased and neutrophils concurrently decreased, but not to an extent requiring treatment holds. As an early phase study, limitations include small patient numbers at each dose level and narrow capacity to evaluate clinical benefits.

CONCLUSION

Administration of oral THU-decitabine to patients with SCD was safe in this study and, by targeting DNMT1, upregulated HbF in RBCs. Further studies should investigate clinical benefits and potential harms not identified to date.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT01685515.

LC-MS/MS assay for the quantitation of the ribonucleotide reductase inhibitor triapine in human plasma

The ribonucleotide reductase inhibitor and radiosensitizer triapine (3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP), NSC 663249) is clinically being evaluated via the intravenous (IV) route for the treatment of cervical and vulvar cancer in combination with primary cisplatin chemoradiation. The need for a 2-h infusion and frequent administration of triapine is logistically challenging, prompting us to pursue oral (PO) administration. In support of the clinical trial investigating oral triapine in combination with chemoradiation, we developed and validated a novel LC-MS/MS assay for the quantification of triapine in 50μL human plasma. After protein precipitation, chromatographic separation of the supernatant was achieved with a Shodex ODP2 column and an isocratic acetonitrile-water mobile phase with 10% ammonium acetate. Detection with an ABI 4000 mass spectrometer utilized electrospray positive mode ionization. The assay was linear from 3 to 3,000ng/mL and proved to be accurate (97.1-103.1%) and precise (<7.4% CV), and met the U.S. FDA guidance for bioanalytical method validation. This LC-MS/MS assay will be an essential tool to further define the pharmacokinetics and oral bioavailability of triapine.

Phase I study of veliparib in combination with gemcitabine

BACKGROUND

Veliparib (ABT-888) is an oral PARP inhibitor expected to increase gemcitabine activity. This phase I determined the maximal tolerable dose (MTD), dose-limiting toxicities (DLT), antitumor activity, pharmacokinetics (PK), and pharmacodynamics (PD) of veliparib combined with gemcitabine.

METHODS

Patients with advanced solid tumors received veliparib (10-40-mg PO BID) on chemotherapy weeks with gemcitabine 500-750-mg/m² IV on days 1, 8, and 15 (28-day cycle), or on days 1 and 8 (21-day cycle). The MTD, DLT, adverse events, PK, and PD were evaluated.

RESULTS

Eleven patients were enrolled on the 28-day schedule. The 28-day schedule was considered intolerable and amended to a 21-day schedule, with 20 patients enrolled. Grade ≥ 3 adverse events were myelosuppression-related. The MTD was determined to be 750-mg/m² gemcitabine IV on days 1 and 8- and 20-mg PO veliparib BID days 1-14 on a 21-day schedule. Of 27 patients evaluable for response, 3 had PR and 15 had SD. There was no evidence of any major drug-drug interaction, and PK parameter values for veliparib, gemcitabine, and dFdU were as expected. Analysis of PBMCs showed evidence of PARP inhibition and DNA damage associated with therapy.

CONCLUSIONS

Gemcitabine at 750-mg/m² IV on days 1 and 8 combined with veliparib at a dose of 20-mg PO BID days 1-14 on a 21-day schedule is relatively well-tolerated, with manageable, expected toxicities. Clinical responses were observed in a pretreated population of patients, suggesting that this combination should be further evaluated in the phase II setting.

Cell-Based High-Throughput Screening Assay Identifies 2',2'-Difluoro-2'-deoxycytidine Gemcitabine as a Potential Antipoliovirus Agent

As we approach the global eradication of circulating wild-type polioviruses (PV), vaccination with oral poliovirus vaccine (OPV) has led to the emergence of circulating vaccine-derived poliovirus (cVDPV) and vaccine-associated paralytic poliomyelitis (VAPP). Complete cessation of all poliovirus infections may require stopping use of OPV and formulating improved vaccines and new antiviral drugs. Currently, no licensed drugs are available to treat chronically infected poliovirus excretors. Here, we created a modified PV expressing Gaussia Luciferase (Sb-Gluc) and developed a cell-based high-throughput screening (HTS) antiviral assay. Using the validated HTS assay, we screened the FDA-approved drug library of compounds and identified candidate agents capable of inhibiting PV replication. We then characterized antipoliovirus activity for the best hit, gemcitabine, a nucleoside analogue used in tumor chemotherapy. We found that gemcitabine inhibited PV Mahoney replication with an IC₅₀ of 0.3 μM. It completely protected HeLa cells from PV-induced cytopathic effects at 25 μM, without detectable toxicity for cell viability. Furthermore, a gemcitabine metabolite directly inhibited the ability of PV RNA polymerase to synthesize or elongate PV RNA. Because PV RNA polymerase is somehow conserved among species in the Picornaviridae family, gemcitabine may be further developed as an attractive broad-spectrum antiviral for PV and others.

Differential response to hypomethylating agents based on sex: a report on behalf of the MDS Clinical Research Consortium (MDS CRC)

First-line therapy for higher-risk myelodysplastic syndromes (MDS) includes decitabine (DAC) or azacitidine (AZA). Variables have not identified differential response rates between these. We assessed the influence of patient sex on outcomes including overall survival (OS) in 642 patients with higher-risk MDS treated with AZA or DAC. DAC-treated patients (35% of females, 31% of males) had marginally better OS than AZA-treated patients (p = .043), (median OS of 18.7 months versus 16.4 months), but the difference varied strongly by sex. Female patients treated with DAC had a longer median OS (21.1 months, 95% CI: 16.0-28.0) than female patients treated with AZA (13.2 months, 95% CI: 11.0-15.9; p = .0014), while for males there was no significant difference between HMAs (median OS 18.3 months with DAC versus 17.9 months for AZA, p = .59). The biological reason for this variability is unclear, but may be a consequence of differences in cytidine deaminase activity between men and women.

Characterization of permeability, stability and anti-HIV-1 activity of decitabine and gemcitabine divalerate prodrugs

BACKGROUND

Over 25 drugs have been approved for the treatment of HIV-1 replication. All but one of these drugs is delivered as an oral medication. Previous studies have demonstrated that two drugs, decitabine and gemcitabine, have potent anti-HIV-1 activities and can work together in synergy to reduce HIV-1 infectivity via lethal mutagenesis. For their current indications, decitabine and gemcitabine are delivered intravenously.

METHODS

As an initial step towards the clinical translation of these drugs for the treatment of HIV-1 infection, we synthesized decitabine and gemcitabine prodrugs in order to increase drug permeability, which has generally been shown to correlate with increased bioavailability in vivo. In the present study we investigated the permeability, stability and anti-HIV-1 activity of decitabine and gemcitabine prodrugs and selected the divalerate esters of each as candidates for further investigation.

RESULTS

Our results provide the first demonstration of divalerate prodrugs of decitabine and gemcitabine that are readily permeable, stable and possess anti-HIV-1 activity.

CONCLUSIONS

These observations predict improved oral availability of decitabine and gemcitabine, and warrant further study of their ability to reduce HIV-1 infectivity in vivo.

A miRNA signature of chemoresistant mesenchymal phenotype identifies novel molecular targets associated with advanced pancreatic cancer

In this study a microRNA (miRNA) signature was identified in a gemcitabine resistant pancreatic ductal adenocarcinoma (PDAC) cell line model (BxPC3-GZR) and this signature was further examined in advanced PDAC tumor specimens from The Cancer Genome Atlas (TCGA) database. BxPC3-GZR showed a mesenchymal phenotype, expressed high levels of CD44 and showed a highly significant deregulation of 17 miRNAs. Based on relevance to cancer, a seven-miRNA signature (miR-100, miR-125b, miR-155, miR-21, miR-205, miR-27b and miR-455-3p) was selected for further studies. A strong correlation was observed for six of the seven miRNAs in 43 advanced tumor specimens compared to normal pancreas tissue. To assess the functional relevance we initially focused on miRNA-125b, which is over-expressed in both the BxPC3-GZR model and advanced PDAC tumor specimens. Knockdown of miRNA-125b in BxPC3-GZR and Panc-1 cells caused a partial reversal of the mesenchymal phenotype and enhanced response to gemcitabine. Moreover, RNA-seq data from each of 40 advanced PDAC tumor specimens from the TCGA data base indicate a negative correlation between expression of miRNA-125b and five of six potential target genes (BAP1, BBC3, NEU1, BCL2, STARD13). Thus far, two of these target genes, BBC3 and NEU1, that are tumor suppressor genes but not yet studied in PDAC, appear to be functional targets of miR-125b since knockdown of miR125b caused their up regulation. These miRNAs and their molecular targets may serve as targets to enhance sensitivity to chemotherapy and reduce metastatic spread.

Gemcitabine diphosphate choline is a major metabolite linked to the Kennedy pathway in pancreatic cancer models in vivo

BACKGROUND

The modest benefits of gemcitabine (dFdC) therapy in patients with pancreatic ductal adenocarcinoma (PDAC) are well documented, with drug delivery and metabolic lability cited as important contributing factors. We have used a mouse model of PDAC: KRAS(G12D); p53(R172H); pdx-Cre (KPC) that recapitulates the human disease to study dFdC intra-tumoural metabolism.

METHODS

LC-MS/MS and NMR were used to measure drug and physiological analytes. Cytotoxicity was assessed by the Sulphorhodamine B assay.

RESULTS

In KPC tumour tissue, we identified a new, Kennedy pathway-linked dFdC metabolite (gemcitabine diphosphate choline (GdPC)) present at equimolar amounts to its precursor, the accepted active metabolite gemcitabine triphosphate (dFdCTP). Utilising additional subcutaneous PDAC tumour models, we demonstrated an inverse correlation between GdPC/dFdCTP ratios and cytidine triphosphate (CTP). In tumour homogenates in vitro, CTP inhibited GdPC formation from dFdCTP, indicating competition between CTP and dFdCTP for CTP:phosphocholine cytidylyltransferase (CCT). As the structure of GdPC precludes entry into cells, potential cytotoxicity was assessed by stimulating CCT activity using linoleate in KPC cells in vitro, leading to increased GdPC concentration and synergistic growth inhibition after dFdC addition.

CONCLUSIONS

GdPC is an important element of the intra-tumoural dFdC metabolic pathway in vivo.

Subchronic oral toxicity study of decitabine in combination with tetrahydrouridine in CD-1 mice

Decitabine (5-aza-2'-deoxycytidine; DAC) in combination with tetrahydrouridine (THU) is a potential oral therapy for sickle cell disease and β-thalassemia. A study was conducted in mice to assess safety of this combination therapy using oral gavage of DAC and THU administered 1 hour prior to DAC on 2 consecutive days/week for up to 9 weeks followed by a 28-day recovery to support its clinical trials up to 9-week duration. Tetrahydrouridine, a competitive inhibitor of cytidine deaminase, was used in the combination to improve oral bioavailability of DAC. Doses were 167 mg/kg THU followed by 0, 0.2, 0.4, or 1.0 mg/kg DAC; THU vehicle followed by 1.0 mg/kg DAC; or vehicle alone. End points evaluated were clinical observations, body weights, food consumption, clinical pathology, gross/histopathology, bone marrow micronuclei, and toxicokinetics. There were no treatment-related effects noticed on body weight, food consumption, serum chemistry, or urinalysis parameters. Dose- and gender-dependent changes in plasma DAC levels were observed with a Cmax within 1 hour. At the 1 mg/kg dose tested, THU increased DAC plasma concentration (∼ 10-fold) as compared to DAC alone. Severe toxicity occurred in females receiving high-dose 1 mg/kg DAC + THU, requiring treatment discontinuation at week 5. Severity and incidence of microscopic findings increased in a dose-dependent fashion; findings included bone marrow hypocellularity (with corresponding hematologic changes and decreases in white blood cells, red blood cells, hemoglobin, hematocrit, reticulocytes, neutrophils, and lymphocytes), thymic/lymphoid depletion, intestinal epithelial apoptosis, and testicular degeneration. Bone marrow micronucleus analysis confirmed bone marrow cytotoxicity, suppression of erythropoiesis, and genotoxicity. Following the recovery period, a complete or trend toward resolution of these effects was observed. In conclusion, the combination therapy resulted in an increased sensitivity to DAC toxicity correlating with DAC plasma levels, and females are more sensitive compared to their male counterparts.

Application of ProTide technology to gemcitabine: a successful approach to overcome the key cancer resistance mechanisms leads to a new agent (NUC-1031) in clinical development

Gemcitabine is a nucleoside analogue commonly used in cancer therapy but with limited efficacy due to a high susceptibility to cancer cell resistance. The addition of a phosphoramidate motif to the gemcitabine can protect it against many of the key cancer resistance mechanisms. We have synthesized a series of gemcitabine phosphoramidate prodrugs and screened for cytostatic activity in a range of different tumor cell lines. Among the synthesized compounds, one in particular (NUC-1031, 6f) was shown to be potent in vitro. Importantly, compared with gemcitabine, 6f activation was significantly less dependent on deoxycytidine kinase and on nucleoside transporters, and it was resistant to cytidine deaminase-mediated degradation. Moreover, 6f showed a significant reduction in tumor volumes in vivo in pancreatic cancer xenografts. The ProTide 6f is now in clinical development with encouraging efficacy signals in a Phase I/II study, which strongly supports the ProTide approach to generate promising new anticancer agents.

CTGF antagonism with mAb FG-3019 enhances chemotherapy response without increasing drug delivery in murine ductal pancreas cancer

Pancreatic ductal adenocarcinoma (PDA) is characterized by abundant desmoplasia and poor tissue perfusion. These features are proposed to limit the access of therapies to neoplastic cells and blunt treatment efficacy. Indeed, several agents that target the PDA tumor microenvironment promote concomitant chemotherapy delivery and increased antineoplastic response in murine models of PDA. Prior studies could not determine whether chemotherapy delivery or microenvironment modulation per se were the dominant features in treatment response, and such information could guide the optimal translation of these preclinical findings to patients. To distinguish between these possibilities, we used a chemical inhibitor of cytidine deaminase to stabilize and thereby artificially elevate gemcitabine levels in murine PDA tumors without disrupting the tumor microenvironment. Additionally, we used the FG-3019 monoclonal antibody (mAb) that is directed against the pleiotropic matricellular signaling protein connective tissue growth factor (CTGF/CCN2). Inhibition of cytidine deaminase raised the levels of activated gemcitabine within PDA tumors without stimulating neoplastic cell killing or decreasing the growth of tumors, whereas FG-3019 increased PDA cell killing and led to a dramatic tumor response without altering gemcitabine delivery. The response to FG-3019 correlated with the decreased expression of a previously described promoter of PDA chemotherapy resistance, the X-linked inhibitor of apoptosis protein. Therefore, alterations in survival cues following targeting of tumor microenvironmental factors may play an important role in treatment responses in animal models, and by extension in PDA patients.

High cytidine deaminase expression in the liver provides sanctuary for cancer cells from decitabine treatment effects

We document for the first time that sanctuary in an organ which expresses high levels of the enzyme cytidine deaminase (CDA) is a mechanism of cancer cell resistance to cytidine analogues. This mechanism could explain why historically, cytidine analogues have not been successful chemotherapeutics against hepatotropic cancers, despite efficacy in vitro. Importantly, this mechanism of resistance can be readily reversed, without increasing toxicity to sensitive organs, by combining cytidine analogue with an inhibitor of cytidine deaminase (tetrahydrouridine). Specifically, CDA rapidly metabolizes cytidine analogues into inactive uridine counterparts. Hence, to determine if sheltering/protection of cancer cells in organs which express high levels of CDA (e.g., liver) is a mechanism of resistance, we utilized a murine xenotransplant model of myeloid cancer that is sensitive to epigenetic therapeutic effects of the cytidine analogue decitabine in vitro and hepato-tropic in vivo. Treatment of tumor-bearing mice with decitabine (subcutaneous 0.2mg/kg 2X/week) doubled median survival and significantly decreased extra-hepatic tumor burden, but hepatic tumor burden remained substantial, to which the animals eventually succumbed. Combining a clinically-relevant inhibitor of CDA (tetrahydrouridine) with a lower dose of decitabine (subcutaneous 0.1mg/kg 2X/week) markedly decreased liver tumor burden without blood count or bone marrow evidence of myelotoxicity, and with further improvement in survival. In conclusion, sanctuary in a CDA-rich organ is a mechanism by which otherwise susceptible cancer cells can resist the effects of decitabine epigenetic therapy. This protection can be reversed without increasing myelotoxicity by combining tetrahydrouridine with a lower dose of decitabine.

H-gemcitabine: a new gemcitabine prodrug for treating cancer

In this report, we present a new strategy for targeting chemotherapeutics to tumors, based on targeting extracellular DNA. A gemcitabine prodrug was synthesized, termed H-gemcitabine, which is composed of Hoechst conjugated to gemcitabine. H-gemcitabine has low toxicity because it is membrane-impermeable; however, it still has high tumor efficacy because of its ability to target gemcitabine to E-DNA in tumors. We demonstrate here that H-gemcitabine has a wider therapeutic window than free gemcitabine.

Algorithmic modeling quantifies the complementary contribution of metabolic inhibitions to gemcitabine efficacy

Gemcitabine (2,2-difluorodeoxycytidine, dFdC) is a prodrug widely used for treating various carcinomas. Gemcitabine exerts its clinical effect by depleting the deoxyribonucleotide pools, and incorporating its triphosphate metabolite (dFdC-TP) into DNA, thereby inhibiting DNA synthesis. This process blocks the cell cycle in the early S phase, eventually resulting in apoptosis. The incorporation of gemcitabine into DNA takes place in competition with the natural nucleoside dCTP. The mechanisms of indirect competition between these cascades for common resources are given with the race for DNA incorporation; in clinical studies dedicated to singling out mechanisms of resistance, ribonucleotide reductase (RR) and deoxycytidine kinase (dCK) and human equilibrative nucleoside transporter1 (hENT1) have been associated to efficacy of gemcitabine with respect to their roles in the synthesis cascades of dFdC-TP and dCTP. However, the direct competition, which manifests itself in terms of inhibitions between these cascades, remains to be quantified. We propose an algorithmic model of gemcitabine mechanism of action, verified with respect to independent experimental data. We performed in silico experiments in different virtual conditions, otherwise difficult in vivo, to evaluate the contribution of the inhibitory mechanisms to gemcitabine efficacy. In agreement with the experimental data, our model indicates that the inhibitions due to the association of dCTP with dCK and the association of gemcitabine diphosphate metabolite (dFdC-DP) with RR play a key role in adjusting the efficacy. While the former tunes the catalysis of the rate-limiting first phosphorylation of dFdC, the latter is responsible for depletion of dCTP pools, thereby contributing to gemcitabine efficacy with a dependency on nucleoside transport efficiency. Our simulations predict the existence of a continuum of non-efficacy to high-efficacy regimes, where the levels of dFdC-TP and dCTP are coupled in a complementary manner, which can explain the resistance to this drug in some patients.

In vitro and in vivo studies of pharmacokinetics and antitumor efficacy of D07001-F4, an oral gemcitabine formulation

PURPOSE

The chemotherapy agent gemcitabine is currently administered intravenously because the drug has poor oral bioavailability. In order to assess the pharmacokinetics and antitumor activity of D07001-F4, a new self-microemulsifying oral drug delivery system preparation of gemcitabine, this study was performed to compare the effect of D07001-F4 with administered gemcitabine in vitro and in vivo.

METHODS

D07001-F4 pharmacokinetics was examined by evaluation of in vitro deamination of D07001-F4 and gemcitabine hydrochloride by recombinant human cytidine deaminase (rhCDA) and in vivo evaluation of D07001-F4 pharmacokinetics in mice. Antitumor activity was evaluated by comparing the effect of D07001-F4 and gemcitabine hydrochloride in inhibiting growth in nine cancer cell lines and by examining the effect of D07001-F4 and gemcitabine in two xenograft tumor models in mice.

RESULTS

In vitro deamination of D07001-F4 by rhCDA was 3.3-fold slower than deamination of gemcitabine hydrochloride. Growth inhibition by D07001-F4 of 7 of the 8 cancer cell lines was increased compared with that seen with gemcitabine hydrochloride, and D07001-F4 inhibited the growth of pancreatic and colon cancer xenografts. In vivo pharmacokinetics showed the oral bioavailability of D07001-F4 to be 34%.

CONCLUSIONS

D07001-F4 was effective against several cancer types, was metabolized more slowly than gemcitabine hydrochloride, and exhibited enhanced oral bioavailability.

Tetrahydrouridine inhibits cell proliferation through cell cycle regulation regardless of cytidine deaminase expression levels

Tetrahydrouridine (THU) is a well characterized and potent inhibitor of cytidine deaminase (CDA). Highly expressed CDA catalyzes and inactivates cytidine analogues, ultimately contributing to increased gemcitabine resistance. Therefore, a combination therapy of THU and gemcitabine is considered to be a potential and promising treatment for tumors with highly expressed CDA. In this study, we found that THU has an alternative mechanism for inhibiting cell growth which is independent of CDA expression. Three different carcinoma cell lines (MIAPaCa-2, H441, and H1299) exhibited decreased cell proliferation after sole administration of THU, while being unaffected by knocking down CDA. To investigate the mechanism of THU-induced cell growth inhibition, cell cycle analysis using flow cytometry was performed. This analysis revealed that THU caused an increased rate of G1-phase occurrence while S-phase occurrence was diminished. Similarly, Ki-67 staining further supported that THU reduces cell proliferation. We also found that THU regulates cell cycle progression at the G1/S checkpoint by suppressing E2F1. As a result, a combination regimen of THU and gemcitabine might be a more effective therapy than previously believed for pancreatic carcinoma since THU works as a CDA inhibitor, as well as an inhibitor of cell growth in some types of pancreatic carcinoma cells.

nab-Paclitaxel potentiates gemcitabine activity by reducing cytidine deaminase levels in a mouse model of pancreatic cancer

Nanoparticle albumin-bound (nab)-paclitaxel, an albumin-stabilized paclitaxel formulation, demonstrates clinical activity when administered in combination with gemcitabine in patients with metastatic pancreatic ductal adenocarcinoma (PDA). The limited availability of patient tissue and exquisite sensitivity of xenografts to chemotherapeutics have limited our ability to address the mechanistic basis of this treatment regimen. Here, we used a mouse model of PDA to show that the coadministration of nab-paclitaxel and gemcitabine uniquely demonstrates evidence of tumor regression. Combination treatment increases intratumoral gemcitabine levels attributable to a marked decrease in the primary gemcitabine metabolizing enzyme, cytidine deaminase. Correspondingly, paclitaxel reduced the levels of cytidine deaminase protein in cultured cells through reactive oxygen species-mediated degradation, resulting in the increased stabilization of gemcitabine. Our findings support the concept that suboptimal intratumoral concentrations of gemcitabine represent a crucial mechanism of therapeutic resistance in PDA and highlight the advantages of genetically engineered mouse models in preclinical therapeutic trials.

SIGNIFICANCE

This study provides mechanistic insight into the clinical cooperation observed between gemcitabine and nab-paclitaxel in the treatment of pancreatic cancer.

Effects of tetrahydrouridine on pharmacokinetics and pharmacodynamics of oral decitabine

The deoxycytidine analog decitabine (DAC) can deplete DNA methyl-transferase 1 (DNMT1) and thereby modify cellular epigenetics, gene expression, and differentiation. However, a barrier to efficacious and accessible DNMT1-targeted therapy is cytidine deaminase, an enzyme highly expressed in the intestine and liver that rapidly metabolizes DAC into inactive uridine counterparts, severely limiting exposure time and oral bioavailability. In the present study, the effects of tetrahydrouridine (THU), a competitive inhibitor of cytidine deaminase, on the pharmacokinetics and pharmacodynamics of oral DAC were evaluated in mice and nonhuman primates. Oral administration of THU before oral DAC extended DAC absorption time and widened the concentration-time profile, increasing the exposure time for S-phase-specific depletion of DNMT1 without the high peak DAC levels that can cause DNA damage and cytotoxicity. THU also decreased interindividual variability in pharmacokinetics seen with DAC alone. One potential clinical application of DNMT1-targeted therapy is to increase fetal hemoglobin and treat hemoglobinopathy. Oral THU-DAC at a dose that would produce peak DAC concentrations of less than 0.2μM administered 2×/wk for 8 weeks to nonhuman primates was not myelotoxic, hypomethylated DNA in the γ-globin gene promoter, and produced large cumulative increases in fetal hemoglobin. Combining oral THU with oral DAC changes DAC pharmacology in a manner that may facilitate accessible noncytotoxic DNMT1-targeted therapy.

p53-Independent, normal stem cell sparing epigenetic differentiation therapy for myeloid and other malignancies

Cytotoxic chemotherapy for acute myeloid leukemia (AML) usually produces only temporary remissions, at the cost of significant toxicity and risk for death. One fundamental reason for treatment failure is that it is designed to activate apoptosis genes (eg, TP53) that may be unavailable because of mutation or deletion. Unlike deletion of apoptosis genes, genes that mediate cell cycle exit by differentiation are present in myelodysplastic syndrome (MDS) and AML cells but are epigenetically repressed: MDS/AML cells express high levels of key lineage-specifying transcription factors. Mutations in these transcription factors (eg, CEBPA) or their cofactors (eg., RUNX1) affect transactivation function and produce epigenetic repression of late-differentiation genes that antagonize MYC. Importantly, this aberrant epigenetic repression can be redressed clinically by depleting DNA methyltransferase 1 (DNMT1, a central component of the epigenetic network that mediates transcription repression) using the deoxycytidine analogue decitabine at non-cytotoxic concentrations. The DNMT1 depletion is sufficient to trigger upregulation of late-differentiation genes and irreversible cell cycle exit by p53-independent differentiation mechanisms. Fortuitously, the same treatment maintains or increases self-renewal of normal hematopoietic stem cells, which do not express high levels of lineage-specifying transcription factors. The biological rationale for this approach to therapy appears to apply to cancers other than MDS/AML also. Decitabine or 5-azacytidine dose and schedule can be rationalized to emphasize this mechanism of action, as an alternative or complement to conventional apoptosis-based oncotherapy.

Deoxyuridine analog nucleotides in deoxycytidine analog treatment: secondary active metabolites?

Deoxycytidine analogs (dCa's) are nucleosides widely used in anticancer and anti (retro) viral therapies. Intracellularly phosphorylated dCa anabolites are considered to be their main active metabolites. This article reviews the literature on the formation and pharmacological activity of deaminated dCa nucleotides. Most dCa's are rapidly deaminated into deoxyuridine analogs (dUa's) which are only slowly phosphorylated and therefore relatively inactive. dUa nucleotides are, however, also formed via deamination of dCa monophosphates by deoxycytidine monophosphate deaminase (dCMPD). dUa-monophosphates can interact with thymidylate synthase (TS), whereas dUa-triphosphates are incorporated into nucleic acids and interfere with polymerases. Administration of dCa's as monophosphate prodrugs or co-administration of the cytidine deaminase inhibitor tetrahydrouridine (THU) does not prevent dUa nucleotide formation which is, on the other hand, influenced by the dose and dCMPD activity. Taken together, these observations show that the formation of dUa nucleotides is a common phenomenon in treatment with dCa's and these compounds may play a role in treatment outcome. We conclude that more attention should be given to these relatively unknown, but potentially important metabolites.