Replication-dependent irreversible topoisomerase 1 poisoning is responsible for FdUMP[10] anti-leukemic activity

Oligonucleotides Carrying Nucleoside Antimetabolites as Potential Prodrugs

BACKGROUND

Nucleoside and nucleobase antimetabolites are an important class of chemotherapeutic agents for the treatment of cancer as well as other diseases.

INTRODUCTION

In order to avoid undesirable side effects, several prodrug strategies have been developed. In the present review, we describe a relatively unknown strategy that consists of using oligonucleotides modified with nucleoside antimetabolites as prodrugs.

METHODS

The active nucleotides are generated by enzymatic degradation once incorporated into cells. This strategy has attracted large interest and is widely utilized at present due to the continuous developments made in therapeutic oligonucleotides and the recent advances in nanomaterials and nanomedicine.

RESULTS

A large research effort was made mainly in the improvement of the antiproliferative properties of nucleoside homopolymers, but recently, chemically modified aptamers, antisense oligonucleotides and/or siRNA carrying antiproliferative nucleotides have demonstrated a great potential due to the synergetic effect of both therapeutic entities. In addition, DNA nanostructures with interesting properties have been built to combine antimetabolites and enhancers of cellular uptake in the same scaffold. Finally, protein nanoparticles functionalized with receptor-binders and antiproliferative oligomers represent a new avenue for a more effective treatment in cancer therapy.

CONCLUSION

It is expected that oligonucleotides carrying nucleoside antimetabolites will be considered as potential drugs in the near future for biomedical applications.

Evaluation of Floxuridine Oligonucleotide Conjugates Carrying Potential Enhancers of Cellular Uptake

Conjugation of small molecules such as lipids or receptor ligands to anti-cancer drugs has been used to improve their pharmacological properties. In this work, we studied the biological effects of several small-molecule enhancers into a short oligonucleotide made of five floxuridine units. Specifically, we studied adding cholesterol, palmitic acid, polyethyleneglycol (PEG 1000), folic acid and triantennary N-acetylgalactosamine (GalNAc) as potential enhancers of cellular uptake. As expected, all these molecules increased the internalization efficiency with different degrees depending on the cell line. The conjugates showed antiproliferative activity due to their metabolic activation by nuclease degradation generating floxuridine monophosphate. The cytotoxicity and apoptosis assays showed an increase in the anti-cancer activity of the conjugates related to the floxuridine oligomer, but this effect did not correlate with the internalization results. Palmitic and folic acid conjugates provide the highest antiproliferative activity without having the highest internalization results. On the contrary, cholesterol oligomers that were the best-internalized oligomers had poor antiproliferative activity, even worse than the unmodified floxuridine oligomer. Especially relevant is the effect induced by palmitic and folic acid derivatives generating the most active drugs. These results are of special interest for delivering other therapeutic oligonucleotides.

Parallel G-quadruplex Structures Increase Cellular Uptake and Cytotoxicity of 5-Fluoro-2'-deoxyuridine Oligomers in 5-Fluorouracil Resistant Cells

Fluoropyrimidines, such as 5-fluorouracil (5-FU) and related prodrugs have been considered first-line chemotherapy agents for the treatment of colorectal cancer. However, poor specificity and tumor cell resistance remain major limiting bottlenecks. G-quadruplexes, have been suggested as preferred nanostructures for enhancing cellular uptake mediated by G-quadruplex binding proteins which are abundant at the membranes of some tumor cells. In the current study, we propose a new strategy to deliver 5-fluoro-2′-deoxyuridine (5-FdU) monophosphate, the main active drug from 5-FU derivatives that may circumvent the cellular mechanisms of FU-resistant cancer cells. Two G-quadruplexes delivery systems containing four and six G-tetrads ((TG₄T) and (TG₆T)) linked to a FdU oligonucleotide were synthesized. Biophysical studies show that the G-quadruplex parallel structures are not affected by the incorporation of the 5 units of FdU at the 5’-end. Internalization studies confirmed the ability of such G-quadruplex nanostructures to facilitate the transport of the FdU pentamer and increase its cytotoxic effect relative to conventional FU drug in FU-resistant colorectal cancer cells. These results suggest that FdU oligomers linked to G-quadruplex parallel sequences may be a promising strategy to deliver fluoropyrimidines to cancer cells.

AraC-FdUMP[10] Is a Next-Generation Fluoropyrimidine with Potent Antitumor Activity in PDAC and Synergy with PARG Inhibition

AraC-FdUMP[10] (CF10) is a second-generation polymeric fluoropyrimidine that targets both thymidylate synthase (TS), the target of 5-fluorouracil (5-FU), and DNA topoisomerase 1 (Top1), the target of irinotecan, two drugs that are key components of FOLFIRNOX, a standard-of-care regimen for pancreatic ductal adenocarcinoma (PDAC). We demonstrated that F10 and CF10 are potent inhibitors of PDAC cell survival (in multiple cell lines including patient-derived lines) with IC₅₀s in the nanomolar range and are nearly 1,000-fold more potent than 5-FU. The increased potency of CF10 relative to 5-FU correlated with enhanced TS inhibition and strong Top1 cleavage complex formation. Furthermore, CF10 displayed single-agent activity in PDAC murine xenografts without inducing weight loss. Through a focused drug synergy screen, we identified that combining CF10 with targeting the DNA repair enzyme, poly (ADP-ribose) glycohydrolase, induces substantial DNA damage and apoptosis. This work moves CF10 closer to a clinical trial for the treatment of PDAC. IMPLICATIONS: CF10 is a promising polymeric fluoropyrimidine with dual mechanisms of action (i.e., TS and Top1 inhibition) for the treatment of PDAC and synergizes with targeting of DNA repair. VISUAL OVERVIEW: http://mcr.aacrjournals.org/content/molcanres/19/4/565/F1.large.jpg.

Entrapment of DNA topoisomerase-DNA complexes by nucleotide/nucleoside analogs

Topoisomerases are well-validated targets for cancer chemotherapy and DNA topoisomerase 1 (Top1) is the sole target of the camptothecin (CPT) class of anticancer drugs. Over the last 20 years, multiple studies have shown Top1 activity is modulated by non-native DNA structures and this can lead to trapping of Top1 cleavage complexes (Top1cc) and conversion to DNA double strand breaks. Among the perturbations to DNA structure that generate Top1cc are nucleoside analogs that are incorporated into genomic DNA during replication including cytarabine, gemcitabine, and 5-fluoro-2'-deoxyuridine (FdU). We review the literature summarizing the role of Top1cc in mediating the DNA damaging and cytotoxic activities of nucleoside analogs. We also summarize studies demonstrating distinct differences between Top1cc induced by nucleoside analogs and CPTs, particularly with regard to DNA repair. Collectively, these studies demonstrate that, while Top1 is a common target for both Top1 poisons such as CPT and nucleoside analogs such as FdU, these agents are not redundant. In recent years, studies have shown that Top1 poisons and nucleoside analogs together with other anti-cancer drugs such as cisplatin cause replication stress and the DNA repair pathways that modulate the cytotoxic activities of these compounds are being elucidated. We present an overview of this evolving literature, which has implications for how targeting of Top1 with nucleoside analogs can be used more effectively for cancer treatment.

Thymineless Death by the Fluoropyrimidine Polymer F10 Involves Replication Fork Collapse and Is Enhanced by Chk1 Inhibition

We are developing the fluoropyrimidine polymer F10 to overcome limitations of 5-fluorouracil (5-FU) that result from inefficient metabolism to 5-fluoro-2′-deoxyuridine-5′-mono- and tri-phosphate, the deoxyribonucleotide metabolites that are responsible for 5-FU's anticancer activity. F10 is much more cytotoxic than 5-FU to colorectal cancer (CRC) cells; however, the mechanism of enhanced F10 cytotoxicity remains incompletely characterized. Using DNA fiber analysis, we establish that F10 decreases replication fork velocity and causes replication fork collapse, while 1000-fold excess of 5-FU is required to achieve similar endpoints. Treatment of HCT-116 cells with F10 results in Chk1 phosphorylation and activation of intra–S-phase checkpoint. Combining F10 with pharmacological inhibition of Chk1 with either PF-477736 or prexasertib in CRC cells enhanced DNA damage relative to single-agent treatment as assessed by γH2AX intensity and COMET assay. PF-477736 or prexasertib co-treatment also inhibited upregulation of Rad51 levels in response to F10, resulting in reduced homologous repair. siRNA knockdown of Chk1 also increased F10-induced DNA damage assessed and sensitized CRC cells to F10. However, Chk1 knockdown did not inhibit Rad51 upregulation by F10, indicating that the scaffolding activity of Chk1 imparts activity in DNA repair distinct from Chk1 enzymatic activity. Our results indicate that F10 is cytotoxic to CRC cells in part through DNA damage subsequent to replication fork collapse. F10 is ~1000-fold more potent than 5-FU at inducing replication-mediated DNA damage which correlates with the increased overall potency of F10 relative to 5-FU. F10 efficacy can be enhanced by pharmacological inhibition of Chk1.

Improved potency of F10 relative to 5-fluorouracil in colorectal cancer cells with p53 mutations

Aim:

Resistance to fluoropyrimidine drugs (FPs) is a major cause of mortality in colorectal cancer (CRC). We assessed the potency advantage of the polymeric FP F10 relative to 5-fluorouracil (5FU) in four human CRC cell lines that differ only in TP53 mutational status to determine how p53 mutations affect drug response and whether F10 is likely to improve outcomes.

Methods:

HCT-116 human CRC cells (p53^+/+) and three isogenic variants (p53^−/−, R248W/+, R248W/−) were assessed for drug response. Resistance factors were derived from cell viability data and used to establish the relative potency advantage for F10. Rescue studies with exogenous uridine/thymidine determined if cytotoxicity resulted from DNA-directed processes.

Results:

Significant resistance to 5-FU resulted from p53-loss or from gain-of-function (GOF) mutation (R248W) and was greatest when GOF mutation was coupled with loss of wild-type p53. F10 is much more potent than 5-FU (137–314-fold depending on TP53 mutational status). F10 and 5-FU induce apoptosis by DNA- and RNA-directed mechanisms, respectively, and only F10 shows a modest enhancement in cytotoxicity upon co-treatment with leucovorin.

Conclusion:

TP53 mutational status affects inherent sensitivity to FPs, with p53 GOF mutations most deleterious. F10 is much more effective than 5-FU regardless of TP53 mutations and has potential to be effective to CRC that is resistant to 5-FU due, in part, to TP53 mutations.6,7

Curcumin stably interacts with DNA hairpin through minor groove binding and demonstrates enhanced cytotoxicity in combination with FdU nucleotides

We report, based on biophysical studies and molecular mechanical calculations that curcumin binds DNA hairpin in the minor groove adjacent to the loop region forming a stable complex. UV-Vis and fluorescence spectroscopy indicated interaction of curcumin with DNA hairpin. In this novel binding motif, two ɣ H of curcumin heptadiene chain are closely positioned to the A₁₆-H8 and A₁₇-H8, while G₁₂-H8 is located in the close proximity of curcumin α H. Molecular dynamics (MD) simulations suggest, the complex is stabilized by noncovalent forces including; π-π stacking, H-bonding and hydrophobic interactions. Nuclear magnetic resonance (NMR) spectroscopy in combination with molecular dynamics simulations indicated curcumin is bound in the minor groove, while circular dichroism (CD) spectra suggested minute enhancement in base stacking and a little change in DNA helicity, without significant conformational change of DNA hairpin structure. The DNA:curcumin complex formed with FdU nucleotides rather than Thymidine, demonstrated enhanced cytotoxicity towards oral cancer cells relative to the only FdU substituted hairpin. Fluorescence co-localization demonstrated stability of the complex in biologically relevant conditions, including its cellular uptake. Acridine orange/EtBr staining further confirmed the enhanced cytotoxic effects of the complex, suggesting apoptosis as mode of cell death. Thus, curcumin can be noncovalently complexed to small DNA hairpin for cellular delivery and the complex showed increased cytotoxicity in combination with FdU nucleotides, demonstrating its potential for advanced cancer therapy.

The applications of the novel polymeric fluoropyrimidine F10 in cancer treatment: current evidence

F10 is a novel polymeric fluoropyrimidine drug candidate with strong anticancer activity in multiple preclinical models. F10 has strong potential for impacting cancer treatment because it displays high cytotoxicity toward proliferating malignant cells with minimal systemic toxicities thus providing an improved therapeutic window relative to traditional fluoropyrimidine drugs, such as 5-fluorouracil. F10 has a unique mechanism that involves dual targeting of thymidylate synthase and Top1. In this review, the authors provide an overview of the studies that revealed the novel aspects of F10's cytotoxic mechanism and summarize results obtained in preclinical models of acute myeloid leukemia, acute lymphocytic leukemia, glioblastoma and prostate cancer that demonstrate the strong potential of F10 to improve treatment outcomes.

The poison oligonucleotide F10 is highly effective against acute lymphoblastic leukemia while sparing normal hematopoietic cells

F10 is an oligonucleotide based on the thymidylate synthase (TS) inhibitory 5-fluorouracil (5-FU) metabolite, 5-fluoro-2'-deoxyuridine-5'-O-monophosphate. We sought to determine the activity of F10 against preclinical models of acute lymphoblastic leukemia (ALL). F10 treatment resulted in robust induction of apoptosis that could not be equaled by 100 fold more 5-FU. F10 was more potent than Ara-C and doxorubicin against a panel of murine and human ALL cells with an average IC₅₀ value of 1.48 nM (range 0.07 to 5.4 nM). F10 was more than 1000 times more potent than 5-FU. In vivo, F10 treatment significantly increased survival in 2 separate syngeneic ALL mouse models and 3 separate xenograft models. F10 also protected mice from leukemia-induced weight loss. In ALL cells made resistant to Ara-C, F10 remained highly active in vitro and in vivo. Using labeled F10, uptake by the ALL cell lines DG75 and SUP-B15 was rapid and profoundly temperature-dependent. Both cell lines demonstrated increased uptake compared to normal murine lineage- depleted marrow cells. Consistent with this decreased uptake, F10 treatment did not alter the ability of human hematopoietic stem cells to engraft in immunodeficient mice.

The cytotoxic and pro-apoptotic activities of the novel fluoropyrimidine F10 towards prostate cancer cells are enhanced by Zn(2+) -chelation and inhibiting the serine protease Omi/HtrA2

BACKGROUND

Intracellular Zn(2+) levels decrease during prostate cancer progression and agents that modulate intracellular Zn(2+) are cytotoxic to prostate cancer cells by an incompletely described mechanism. F10 is a new polymeric fluoropyrimidine drug-candidate that displays strong activity with minimal systemic toxicity in pre-clinical models of prostate cancer and other malignancies. The effects of exogenous Zn(2+) or Zn(2+) chelation for enhancing F10 cytotoxicity are investigated as is the role of Omi/HtrA2, a serine protease that promotes apoptosis in response to cellular stress.

METHODS

To test the hypothesis that the pro-apoptotic effects of F10 could be enhanced by modulating intracellular Zn(2+) we investigated cell-permeable and cell-impermeable Zn(2+) chelators and exogenous Zn(2+) and evaluated cell viability and apoptosis in cellular models of castration-resistant prostate cancer (CRPC; PC3, C4-2). The role of Omi/HtrA2 for modulating apoptosis was evaluated by pharmacological inhibition and Western blotting.

RESULTS

Exogenous Zn(2+) initially reduced prostate cancer cell viability but these effects were transitory and were ineffective at enhancing F10 cytotoxicity. The cell-permeable Zn(2+) -chelator tetrakis-(2-pyridylmethl) ethylenediamine (TPEN) induced apoptosis in prostate cancer cells and enhanced the pro-apoptotic effects of F10. The pro-apoptotic effects of Zn(2+) -chelation in combination with F10 treatment were enhanced by inhibiting Omi/HtrA2 implicating this serine protease as a novel target for prostate cancer treatment.

CONCLUSIONS

Zn(2+) -chelation enhances the pro-apoptotic effects of F10 and may be useful for enhancing the effectiveness of F10 for treatment of advanced prostate cancer. The serine protease Omi/HtrA2 modulates Zn(2+) -dependent apoptosis in prostate cancer cells and represents a new target for treatment of CRPC. Prostate 75:360-369, 2015. © 2014 Wiley Periodicals, Inc.

Thymineless death in F10-treated AML cells occurs via lipid raft depletion and Fas/FasL co-localization in the plasma membrane with activation of the extrinsic apoptotic pathway

The polymeric fluoropyrimidine F10 displays excellent anti-leukemia activity in pre-clinical models of acute myelogenous leukemia (AML) through dual targeting of thymidylate synthase and DNA topoisomerase 1. Here we report that F10 activates the extrinsic apoptotic pathway in AML cells by enhancing localization of Fas and Fas ligand (FasL) at the plasma membrane and while reducing overall lipid raft levels promotes Fas/FasL co-localization in remaining lipid rafts. The HMG-CoA synthase inhibitor simvastatin was synergistic with F10 and induced cell death via similar apoptotic processes. Our results are consistent with diverse processes activating a common apoptotic pathway characterized by reduced overall levels of lipid rafts and Fas/FasL co-localization in the plasma membrane, including in remaining lipid rafts which may play a role in both cell-survival and cell death signaling.

F10 Inhibits Growth of PC3 Xenografts and Enhances the Effects of Radiation Therapy

Chemotherapy remains of limited use for the treatment of prostate cancer with only one drug, docetaxel, demonstrating a modest survival advantage for treatment of late-stage disease. Data from the NCI 60 cell line screen indicated that the castration-resistant prostate cancer cell lines PC3 and DU145 were more sensitive than average to the novel polymeric fluoropyrimidine (FP), F10, despite displaying less than average sensitivity to the widely-used FP, 5FU. Here, we show that F10 treatment of PC3 xenografts results in a significant survival advantage (treatment to control ratio (T/C) days = 18; p < 0.001; n = 16) relative to control mice treated with saline. F10 (40 mg/kg/dose) was administered via jugular vein catheterization 3-times per week for five weeks. This aggressive dosing regimen was completed with no drug-induced weight loss and with no evidence of toxicity. F10 was also shown to sensitize PC3 cells to radiation and F10 was also shown to be a potent radiosensitizer of PC3 xenografts in vivo with F10 in combination with radiation resulting in significantly greater regression of PC3 xenografts than radiation alone. The results indicate that F10 in this pre-clinical setting is an effective chemotherapeutic agent and possesses significant radiosensitizing properties.