Cellular pharmacology of multi- and duplex drugs consisting of ethynylcytidine and 5-fluoro-2'-deoxyuridine

The conjugated antimetabolite 5-FdU-ECyd and its cellular and molecular effects on platinum-sensitive vs. -resistant ovarian cancer cells in vitro

Background

Resistance to platinum-based chemotherapy is a clinical challenge in the treatment of ovarian cancer (OC) and limits survival. Therefore, innovative drugs against platinum-resistance are urgently needed. Our therapeutic concept is based on the conjugation of two chemotherapeutic compounds to a monotherapeutic pro-drug, which is taken up by cancer cells and cleaved into active cytostatic metabolites. We explore the activity of the duplex-prodrug 5-FdU-ECyd, covalently linking 2'-deoxy-5-fluorouridine (5-FdU) and 3'-C-ethynylcytidine (ECyd), on platinum-resistant OC cells.

Methods

In vitro assays and RNA-Sequencing were applied for characterization of 5-FdU-ECyd treated platinum-sensitive A2780 and isogenic platinum-resistant A2780cis and independent platinum-resistant Skov-3-IP OC cells.

Results

Nano molar 5-FdU-ECyd concentrations induced a rapid dose-dependent decline of cell viability in platinum-sensitive and -resistant OC cells. The effect of 5-FdU-ECyd was accompanied by the formation of DNA double strand breaks and apoptosis induction, indicated by a strong increase of pro-apoptotic molecular markers. Moreover, 5-FdU-ECyd efficiently decreased migration of platinum-resistant OC cells and inhibited clonogenic or spheroidal growth. Transcriptome analysis showed early up-regulation of CDKN1A and c-Fos in both, platinum-resistant and -sensitive cells after 5-FdU-ECyd treatment and de-regulation of distinct cellular pathways involved in cell cycle regulation, apoptosis, DNA-damage response and RNA-metabolism. Combined treatment of 5-FdU-ECyd and cisplatin did not show a synergistic cellular response, suggesting the potential use of 5-FdU-ECyd as a monotherapeutic agent.

Conclusion

Our data provide novel mechanistic insight into the anti-tumor effect of 5-FdU-ECyd and we hypothesize that this duplex-prodrug could be a promising therapeutic option for OC patients with resistance to platinum-based chemotherapy.

Improving nucleoside analogs via lipid conjugation: Is fatter any better?

In the past few decades, nucleoside analog drugs have been used to treat a large variety of cancers. These anti-metabolite drugs mimic nucleosides and interfere with chain lengthening upon incorporation into the DNA or RNA of actively replicating cells. However, efficient delivery of these drugs is limited due to their pharmacokinetic properties, and tumors often develop drug resistance. In addition, nucleoside analogs are generally hydrophilic, resulting in poor bioavailability and impaired blood-brain barrier penetration. Conjugating these drugs to lipids modifies their pharmacokinetic properties and may improve in vivo efficacy. This review will cover recent advances in the field of conjugation of phospholipids to nucleoside analogs. This includes conjugation of myristic acid, 12-thioethyldodecanoic acid, 5-elaidic acid esters, phosphoramidate, and self-emulsifying formulations. Relevant in vitro and in vivo data will be discussed for each drug, as well as any available data from clinical trials.

Overexpression of MRP4 (ABCC4) and MRP5 (ABCC5) confer resistance to the nucleoside analogs cytarabine and troxacitabine, but not gemcitabine

We aimed to determine whether the multidrug-resistance-proteins MRP4 (ABCC4) and MRP5 (ABCC5) confer resistance to the antimetabolites cytarabine (Ara-C), gemcitabine (GEM), and the L-nucleoside analog troxacitabine. For this purpose we used HEK293 and the transfected HEK/MRP4 (59-fold increased MRP4) or HEK/MRP5i (991-fold increased MRP5) as model systems and tested the cells for drug sensitivity using a proliferation test. Drug accumulation was performed by using radioactive Ara-C, and for GEM and troxacitabine with HPLC with tandem-MS or UV detection. At 4-hr exposure HEK/MRP4 cells were 2-4-fold resistant to troxacitabine, ara-C and 9-(2-phosphonylmethoxyethyl)adenine (PMEA), and HEK/MRP5i to ara-C and PMEA, but none to GEM. The inhibitors probenecid and indomethacin reversed resistance. After 4-hr exposure ara-C-nucleotides were 2-3-fold lower in MRP4/5 cells, in which they decreased more rapidly after washing with drug-free medium (DFM). Trocacitabine accumulation was similar in the 3 cell lines, but after the DFM period troxacitabine decreased 2-4-fold faster in MRP4/5 cells. Troxacitabine-nucleotides were about 25% lower in MRP4/5 cells and decreased rapidly in MRP4, but not in MRP5 cells. Accumulation of GEM-nucleotides was higher in the MRP4/5 cells. In conclusion: MRP4 and MRP5 overexpression confer resistance to troxacitabine and ara-C, but not to GEM, which was associated with a rapid decline of the ara-C and troxacitabine-nucleotides in HEK/MRP4-5 cells.

Induction of apoptosis in cervical cancer cells by the duplex drug 5-FdU-ECyd, coupling 2'-deoxy-5-fluorouridine and 3'-C-ethinylcytidine

OBJECTIVE

Therapeutic options are limited for patients with advanced cervical cancer, and more effective drugs with favorable side-effect profiles are needed. We developed a nucleoside analogue duplex drug (5-FdU-ECyd), in which the DNA synthesis inhibitor 5-fluorodeoxyuridine is coupled to the RNA synthesis inhibitor 3'-C-ethinylcytidine. We therefore aimed to test its efficacy in cervical carcinoma cells in vitro and to establish its mechanism of action.

METHODS

The cytotoxic effects of 5-FdU-ECyd on cervical cancer cells were assessed using the MTT assay, clonality assays, FACScan analysis, and its effect on cancer cell spheroids. Mechanisms of cell death were analyzed by Western blotting for apoptosis and autophagy pathways and mitochondrial membrane potential.

RESULTS

HeLa, CaSki, SiHa, and Me180 cervical cancer cells were highly sensitive to 5-FdU-ECyd in both 2- and 3-dimensional cancer models. The cell death induced by 5-FdU-ECyd was associated with characteristic morphological and biochemical signs of apoptosis, including nuclear chromatin condensation and fragmentation, PARP cleavage, and a breakdown in mitochondrial membrane potential. 5-FdU-ECyd treatment led to an early S-phase arrest and drastically reduced expression of the anti-apoptosis protein Mcl-1 and increased signaling via the JNK and p38 MAPK pathways.

CONCLUSIONS

5-FdU-ECyd is highly cytotoxic in cervical cancer cells and exploits apoptosis pathways that might be specific to cancer, but not normal cells. 5-FdU-ECyd might represent a new chemotherapeutic option for patients with advanced or treatment refractory cervical cancer.

Pemetrexed induced thymidylate synthase inhibition in non-small cell lung cancer patients: a pilot study with 3'-deoxy-3'-[¹⁸F]fluorothymidine positron emission tomography

OBJECTIVES

Pemetrexed is a thymidylate synthase (TS) inhibitor and is effective in non-small cell lung cancer (NSCLC). 3'-deoxy-3'-[¹⁸F]fluorothymidine (¹⁸F-FLT), a proliferation marker, could potentially identify tumor specific TS-inhibition. The aim of this study was to investigate the effect of pemetrexed-induced TS-inhibition on ¹⁸F-FLT uptake 4 hours after pemetrexed administration in metastatic NSCLC patients.

METHODS

Fourteen NSCLC patients underwent dynamic ¹⁸F-FLT positron emission tomography (PET) scans at baseline and 4 hours after the first dose of pemetrexed. Volumes of interest were defined with a 41%, 50% and 70% threshold of the maximum pixel. Kinetic analysis and simplified measures were performed. At one, two, four and six hours after pemetrexed, plasma deoxyuridine was measured as systemic indicator of TS-inhibition. Tumor response measured with response evaluation criteria in solid tumors (RECIST), time to progression (TTP) and overall survival (OS) were determined.

RESULTS

Eleven patients had evaluable ¹⁸F-FLT PET scans at baseline and 4 hours after pemetrexed. Two patients had increased ¹⁸F-FLT uptake of 35% and 31% after pemetrexed, whereas two other patients had decreased uptake of 31%. In the remaining seven patients ¹⁸F-FLT uptake did not change beyond test-retest borders. In all patients deoxyuridine levels raised after administration of pemetrexed, implicating pemetrexed-induced TS-inhibition. ¹⁸F-FLT uptake in bone marrow was significantly increased 4 hours after pemetrexed administration. Six weeks after the start of treatment 5 patients had partial response, 4 stable disease and 2 progressive disease. Median TTP was 4.2 months (range 3.0-7.4 months); median OS was 13.0 months (range 5.1-30.8 months). Changes in ¹⁸F-FLT uptake were not predictive for tumor response, TTP or OS.

CONCLUSIONS

Measuring TS-inhibition in a clinical setting 4 hours after pemetrexed revealed a non-systematic change in ¹⁸F-FLT uptake within the tumor. No significant association with tumor response, TTP or OS was observed.

Development of modified siRNA molecules incorporating 5-fluoro-2'-deoxyuridine residues to enhance cytotoxicity

Therapeutic small interfering RNAs (siRNAs) are composed of chemically modified nucleotides, which enhance RNA stability and increase affinity in Watson–Crick base pairing. However, the precise fate of such modified nucleotides once the siRNA is degraded within the cell is unknown. Previously, we demonstrated that deoxythymidine release from degraded siRNAs reversed the cytotoxicity of thymidylate synthase (TS)-targeted siRNAs and other TS inhibitor compounds. We hypothesized that siRNAs could be designed with specific nucleoside analogues that, once released, would enhance siRNA cytotoxicity. TS-targeted siRNAs were designed that contained 5-fluoro-2′-deoxyuridine (FdU) moieties at various locations within the siRNA. After transfection, these siRNAs suppressed TS protein and messenger RNA expression with different efficiencies depending on the location of the FdU modification. FdU was rapidly released from the siRNA as evidenced by formation of the covalent inhibitory ternary complex formed between TS protein and the FdU metabolite, FdUMP. These modified siRNAs exhibited 10–100-fold greater cytotoxicity and induced multiple DNA damage repair and apoptotic pathways when compared with control siRNAs. The strategy of designing siRNA molecules that incorporate cytotoxic nucleosides represents a potentially novel drug development approach for the treatment of cancer and other human diseases.

Effect of duplex drugs linking 2'-deoxy-5-fluorouridine (5-FdU) with 3'-C-ethynylcytidine (ECyd) on hepatoblastoma cell lines

PURPOSE

Duplex drugs are promising anticancer agents. After in vivo cleavage into active nucleoside analogues, they exert their anti-tumour activity with reduced toxicity and side effects. Here we evaluated the impact of two duplex drugs on the viability of hepatoblastoma (HB) cells lines and their toxicity against human fibroblasts.

METHODS

The duplex drugs 2'-deoxy-5-fluorouridylyl-(3'-5')- 3'-C-ethynylcytidine (5-FdU(3'-5')ECyd) and 3'-C-ethynylcytidinylyl-(5'→1-O)-2-O-octadecyl-sn-glycerylyl-(3'-Ο→5')-2'-deoxy-5-fluorouridine (ECyd-lipid-5-FdU) were analysed in two HB cell lines (HUH6, HepT1) and fibroblasts by MTT assay. The treatment potential was compared to the single substances 2'-deoxy-5-fluorourindine (5-FdU), 3'-C-ethynylycytidine (ECyd) and an equimolar mixture of both. Cell cycle analyses were performed using flow cytometry after 7-AAD staining.

RESULTS

Both duplex drugs achieve a potent cytotoxic effect at low μM concentrations, which was more pronounced than the mixture of ECyd + 5-FdU. Further, both substances exert toxicity on fibroblasts of tumour samples, with less toxicity in foreskin fibroblasts cultures. Cell cycle analyses revealed a shift towards apoptotic cells for both drugs in HB cells.

CONCLUSION

5-FdU(3'-5')ECyd and ECyd-lipid-5-FdU exert a highly potent anti-tumoural effect on HB cells and might therefore be a treatment option in HB. Pharmacological formulations of both duplex drugs have to be evaluated in vivo to reduce possible side effects.

New diethylsilylacetylenic linker for parallel solid-phase synthesis of libraries of hydroxy acetylenic steroid derivatives with improved metabolic stability

Acetylenic tertiary alcohols are well-known to be compounds that are biologically more stable than their corresponding secondary alcohols. The linkage of an acetylenic compound to a polymer support and further introduction of molecular diversity was found to be an interesting way to generate libraries of hydroxy acetylenic derivatives and thus potentially improve their biological properties. For the first time, we describe the loading of an ethynyl steroid to a polystyrene-diethylsilane resin and its uses for the solid-phase synthesis of a model library of 21 steroid derivatives. Two levels of molecular diversity were introduced by successive addition of amino acids and carboxylic acids, and hydroxy acetylenic steroids were then released by an acidic treatment in high yield and purity without further purification step.

Determination of the phosphorylated metabolites of gemcitabine and of difluorodeoxyuridine by LCMSMS

Gemcitabine is an established chemotherapy agent in several solid tumors. Its mechanism of action has been theoretically established and this is supported with strong experimental evidence. However, certain aspects of the resistance mechanism for this agent remain elusive. We present a method of analysis using tandem liquid chromatography and mass spectrometry that provides a broader, yet more focused view of the action of gemcitabine and its primary metabolite, difluorodeoxyuridine in relation to the (deoxy) nucleoside and (deoxy) nucleotide pools in tumor cell lines. Alcoholic cytosole extracts were incubated with alkaline phosphatase reducing the nucleotide pools to their respective nucleosides. Determination of the nucleoside content by a sensitive LCMSMS method before and after incubation enables the calculation of the total amount of phosphorylation of each (deoxy) nucleoside in the cell. Incubation with clinically relevant levels of gemcitabine (dFdC) or difluorodeoxyuridine (dFdU) for 24 hours enabled the determination of the changes in the (deoxy) nucleotide pools in relation to chemotherapeutic and toxicological effects. Confirmation of the presence of dFdC phosphorylation is presented as well as direct evidence of dFdU phosphorylation after both dFdC and dFdU treatment. Differences in the nucleotide pools are presented after dFdC and dFdU incubation, indicating that dFdU might have more chemotherapeutic properties than previously believed.

Cytotoxicity of new duplex drugs linking 3'-C-ethynylcytidine and 5-fluor-2'-deoxyuridine against human melanoma cells

Melanoma is an increasingly common and potentially fatal malignancy of the skin and some mucous membranes. As no cure exists for metastatic disease, there is an urgent need for novel drugs. 2'-Deoxy-5-fluorouridylyl-(3'-5')-3'-C-ethynylcytidine [5-FdU(3'-5')ECyd] and 3'-C-ethynylcytidinylyl-(5' → 1-O)-2-O-octadecyl-sn-glycerylyl-(3-O → 5')-2'-deoxy-5-fluorouridine [ECyd-lipid-5-FdU] represent cytostatic active duplex drugs, which can be metabolized into various active antimetabolites. We evaluated the cytotoxicity of these heterodinucleoside phosphate analogs, their corresponding monomers ECyd and 5-FdU and combinations thereof on six metastatic melanoma cell lines and six ex vivo patient-derived melanoma cells in comparison to current standard cytostatic agents and the BRAF V600E inhibitor Vemurafenib. In vitro (real-time)-proliferation assays demonstrated that 5-FdU(3'-5')ECyd and ECyd-lipid-5-FdU had a high cytotoxic efficacy causing 75% melanoma cell death at concentrations in the nanomolar and micromolar range. Cytotoxicity was conducted by induction of DNA cleavage indicating apoptotic cells. Chicken embryotoxicity demonstrated that the duplex drugs were less toxic than 5-FdU at 0.01 μM. In vivo the duplex drug 5-FdU(3'-5')ECyd was efficacious in the murine LOX IMVI melanoma xenograph model on administration of 11.2 mg/kg/injection every fourth day. Both duplex drugs are promising novel cytostatic agents for the treatment of malignant melanoma meriting clinical evaluation.