Cytostatic activity of the duplex drug linking 2'-deoxy-5-fluorouridine (5FdU) with 3'-C-ethynylcytidine (ECyd) against gastric adenocarcinoma cell lines.

The cytostatic potential of the new duplex drug 2'-deoxy-5-fluorouridylyl-(5'5')-3'-C-ethynylcytidine (5FdU(5'-5')ECyd) was evaluated in comparison to those of 5-fluorouracil (5FU), 2'-deoxy-5-fluorourindine (5FdU), 3'-C-ethynylycytidine (ECyd), cisplatin, an equimolar mixture of 5FdU + ECyd and a three component-mixture of 0.75 µM epirubicin/0.90 µM cisplatin/3.0 µM 5FU (ECF) by incubation of the two human gastric adenocarcinoma cell lines 23132/87 and MKN-45. The molar composition of ECF was taken from data of a triple combination chemotherapy for human gastric cancer. Time and dose depending inhibition of cell growth was determinated using the CASY technology. A growth decrease of both cell lines from 100% to about 20% was observed by treatment with ECF over a course of 14 days. This result provided basis to estimate the cytostatic potential of all tested drugs and combinations thereof. Corresponding high activities in respect to ECF were achieved by incubation of 23132/87 cells with single drugs 49 µM 5FU, 10 µM cisplatin, 3.4 µM 5FdU, 0.65 µM ECyd, the mixture 0.32 µM 5FdU + 0.32 µM ECyd and 0.32 µM 5FdU(5'-5')ECyd. The less sensitive MKN-45 cells require a 1.5-4 fold higher dose of the standard chemotherapeutics in order to achieve an equivalent cytostatic effect, in respect to the 23132/87 cell line,. However, the effect of the duplex drugs on MKN-45 cells was gained with a 5-fold lower dose than ECF. Due to its high cytostatic potential the duplex drug, which covalently links two active anticancer compounds, could be a new therapeutic alternative for chemotherapy in gastric cancer, currently treated with different combinations.

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

Prodrugs can have the advantage over parent drugs in increased activation and cellular uptake. The multidrug ETC-L-FdUrd and the duplex drug ETC-FdUrd are composed of two different monophosphate-nucleosides, 5-fluoro-2'deoxyuridine (FdUrd) and ethynylcytidine (ETC), coupled via a glycerolipid or phosphodiester, respectively. The aim of the study was to determine cytotoxicity levels and mode of drug cleavage. Moreover, we determined whether a liposomal formulation of ETC-L-FdUrd would improve cytotoxic activity and/or cleavage. Drug effects/cleavage were studied with standard radioactivity assays, HPLC and LC-MS/MS in FM3A/0 mammary cancer cells and their FdUrd resistant variants FM3A/TK(-). ETC-FdUrd was active (IC(50) of 2.2 and 79 nM) in FM3A/0 and TK(-) cells, respectively. ETC-L-FdUrd was less active (IC(50): 7 nM in FM3A/0 vs 4500 nM in FM3A/TK(-)). Although the liposomal formulation was less active than ETC-L-FdUrd in FM3A/0 cells (IC(50):19.3 nM), resistance due to thymidine kinase (TK) deficiency was greatly reduced. The prodrugs inhibited thymidylate synthase (TS) in FM3A/0 cells (80-90%), but to a lower extent in FM3A/TK(-) (10-50%). FdUMP was hardly detected in FM3A/TK(-) cells. Inhibition of the transporters and nucleotidases/phosphatases resulted in a reduction of cytotoxicity of ETC-FdUrd, indicating that this drug was cleaved outside the cells to the monophosphates, which was verified by the presence of FdUrd and ETC in the medium. ETC-L-FdUrd and the liposomal formulation were neither affected by transporter nor nucleotidase/phosphatase inhibition, indicating circumvention of active transporters. In vivo, ETC-FdUrd and ETC-L-FdURd were orally active. ETC nucleotides accumulated in both tumor and liver tissues. These formulations seem to be effective when a lipophilic linker is used combined with a liposomal formulation.

N4-[Alkyl-(hydroxyphosphono)phosphonate]-cytidine-new drugs covalently linking antimetabolites (5-FdU, araU or AZT) with bone-targeting bisphosphonates (alendronate or pamidronate).

Amino-bisphosphonates (alendronate, pamidronate) were covalently linked in a three step synthesis, with protected and triazolylated derivatives of therapeutically used nucleoside analogs (5-FdU, araC, AZT) by substitution of their triazolyl residue. From the deprotected and chromatographically purified reaction mixtures N4-[alkyl-(hydroxyphosphono) phosphonate]-cytidine combining two differently cytotoxic functions were obtained. This new family of bisphosphonates (BPs) contains as novelty an alkyl side chain with a cytotoxic nucleoside. The BPs moiety allows for a high binding to hydroxyapatite which is a prerequisite for bone targeting of the drugs. In vitro binding of 5-FdU-alendronate (5-FdU-ale) to hydroxyapatite showed a sixfold increased binding of these BPs as compared to 5-FdU. Exploratory cytotoxic properties of 5-FdU-ale were tested on a panel of human tumor cell lines resulting in growth inhibition ranging between 5% and 38%. The determination of IC50-concentrations of the conjugate in Lewis lung carcinoma and murine macrophages showed an incubation time dependent growth inhibition with higher sensitivity towards the tumor cells. We assume that the antimetabolite-BPs can be cleaved into different active metabolites that may exert cytotoxic and other therapeutic effects. However, the underlying mechanisms of these promising new antimetabolite-BPs conjugates remain to be evaluated in future experiments.

Synthesis and in vitro activities of new anticancer duplex drugs linking 2'-deoxy-5-fluorouridine (5-FdU) with 3'-C-ethynylcytidine (ECyd) via a phosphodiester bonding.

Two isomeric cytostatic duplex drugs 2'-deoxy-5-fluorouridylyl-(3'-->5')-3'-C-ethynylcytidine [5-FdU(3'-->5')ECyd] and 2'-deoxy-5-fluorouridylyl-(5'-->5')-3'-C-ethynylcytidine [5-FdU(5'-->5')ECyd] were designed and synthesized at gram scale according to the hydrogenphosphonate method in an overall yield of about 40%. The in vitro evaluation of the anticancer effects indicated highly varying sensibilities of the panel of 60 tested tumor cell lines against the duplex drugs. 5-FdU(3'-->5')ECyd had a 50% growth inhibition (IC(50) < or = 10(-8) M) in 44/58 cell lines. However, only 25/53 of those cell lines showed corresponding IC(50) values when the isomeric 5-FdU(5'-->5')ECyd was tested. Total growth inhibition was achieved using micromolar concentrations of the duplex drugs. The 5-FdU residue of the duplex drug can cause very different effects like additive, synergistic, antagonistic as well as sequence-depending activities, which drastically changed efficiency as well as specificity of the anticancer activities of the duplex drugs, in comparison to those of the monomeric drugs.

Synthesis and in vitro activities of a new antiviral duplex drug linking Zidovudine (AZT) and Foscarnet (PFA) via an octadecylglycerol residue.

To prepare a new antiviral duplex drug linking Zidovudine (AZT) and Foscarnet (PFA) via a lipophilic octadecylglycerol residue we condensed 1-O-4-monomethoxytrityl-3-O-octadecyl-sn-glycerol-2-hydrogenphosphonate obtained from 3-O-octadecyl-sn-glycerol with AZT by the phosphonate method. The purified condensation product was de-tritylated resulting in 3'-azido-3'-deoxythymidylyl-(5'-->2-O)-3-O-octadecyl-sn-glycerol, followed by treatment with (ethoxycarbonyl)phosphoric dichloride. The resulting 3'-azido-3'-deoxy-thymidylyl-(5'-->2)-3-O-octadecyl-sn-glycerol-1-O-(ethoxycarbonyl)phosphonate was purified by preparative RP-18 column chromatography. The antiviral duplex drug 3'-azido-3'-deoxythymidylyl-(5'-->2-O)-3-O-octadecyl-sn-glycerol-1-O-phosphonoformate trisodium salt (AZT-lipid-PFA) was obtained after alkaline cleavage of the phosphonoformate ethylester residue. The overall yield of the five step synthesis performed at gram scale was about 30%. According to a supposed pathway AZT-lipid-PFA could be cleaved to yield a mixture of different antiviral compounds such as AZT, AZT-5'-monophosphate, octadecylglycerol-AZT, PFA and octadecylglycerol-PFA, possibly producing additive and/or synergistic antiviral effects. In vitro studies showed that the duplex drug exhibits antiviral activities against HIV and especially against drug-resistant strains and clinical isolates of HSV and HCMV. The E(50) values of AZT-lipid-PFA against HIV ranged between 170 and 200 nM. The half-maximal inhibitory doses (IC(50)) against highly acyclovir (ACV)-resistant HSV isolates determined by a plaque reduction assay ranged between 1.87 and 4.59 microM. Using ganciclovir (GCV)-sensitive, GCV resistant and drug cross-resistant HCMV strains the IC(50)-values of AZT-lipid-PFA were between 2.78 and 1.18 microM. With regard to PFA, the IC(50)-value of AZT-lipid-PFA determined on a multi-drug-resistant HCMV strain was about 90-fold lower than that of PFA, demonstrating the superior antiviral effect of the duplex-drug.

In vivo and in vitro activity and mechanism of action of the multidrug cytarabine-L-glycerylyl-fluorodeoxyuridine.

Multidrugs have the potential to bypass resistance. We investigated the in vitro activity and resistance circumvention of the multidrug cytarabine-L-fluorodeoxyuridine (AraC-L-5FdU), linked via a glycerophospholipid linkage. Cytotoxicity was determined using sensitive (A2780, FM3A/0) and resistant (AG6000, AraC resistant, deoxycytidine kinase deficient; FM3A/TK-, 5FdU resistant, thymidine kinase deficient) cell lines. Circumvention of nucleoside transporter and activating enzymes was determined using specific inhibitors, HPLC analysis and standard radioactivity assays. AraC-L-5FdU was active (IC50: 0.03 microM in both A2780 and FM3A/0), had some activity in AG6000 (IC50: 0.28 microM), but no activity in FM3A/TK(-) (IC50: 18.3 microM). AraC-nucleotides were not detected in AG6000. 5FdU-nucleotides were detected in all cell lines. AraC-L-5FdU did not inhibit TS in FM3A/TK(-) (5%). Since phosphatase/nucleotidase-inhibition reduced cytotoxicity 7-70-fold, cleavage seems to be outside the cell, presumably to nucleotides, and then to nucleosides. The multidrug was orally active in the HT-29 colon carcinoma xenografts which are resistant toward the single drugs.

Liposome Formulations of Hydrophobic Drugs.

Here we report methods of preparation for liposome formulations containing lipophilic drugs. In contrast to the encapsulation of water soluble compounds into the entrapped aqueous volume of a liposome, drugs with lipophilic properties are incorporated into the phospholipid bilayer membrane. Water-soluble molecules, for example cytotoxic or antiviral nucleosides can be transformed into lipophilic compounds by attachment of long alkyl chains, allowing their stable incorporation into liposome membranes and taking advantage of the high loading capacity lipid bilayers provide for lipophilic molecules. We created a new class of cytotoxic drugs by chemical transformation of the hydrophilic drugs cytosine-arabinoside (ara-C), 5-fluoro-deoxyuridine (5-FdU), and ethinylcytidine (ETC) into lipophilic compounds and their formulation in liposomes.The concept of chemical modification of water-soluble molecules by attachment of long alkyl chains and their stable incorporation into liposome bilayer membranes represent a very promising method for the development of new drugs not only for the treatment of tumors or infections but also for many other diseases.

Isolation and characterization of a scFv antibody specific for tumor endothelial marker 1 (TEM1), a new reagent for targeted tumor therapy.

Tumor endothelial marker 1 (TEM1) is a protein predominantly expressed on the cell surface of endothelial cells in newly developing blood vessels and on tumor cells. It is therefore ideally suited as a target for anti-angiogenic tumor therapy. Using phage display technology a single chain antibody fragment (scFv-CM6) was isolated that specifically binds to the extracellular part of TEM1. Antibody specificity was determined in ELISA, by Western analysis, fluorescence microscopy and flow cytometry performed with TEM1-expressing cells. ScFv-CM6 was further functionalized and coupled to liposomes. Such immunoliposomes loaded with the cytotoxic drug N4-octadecyl-1-beta-D-arabinofuranosylcytosine-(5'-5')-3'-C-ethinylcytidine showed increased binding affinity and up to 80% higher cytotoxic activity towards TEM1-expressing IMR-32 tumor cells compared with control liposomes.

Lipophilic arabinofuranosyl cytosine derivatives in liposomes.

Highly lipophilic drugs can be used therapeutically only by the addition of possibly toxic solubilizing agents or by development of complex pharmaceutical formulations. One way of overcoming these disadvantages is the incorporation of such drugs into the bilayer matrix of phospholipid liposomes. To this end, we chose the approach of chemical transformation of water-soluble nucleosides of known cytotoxic properties into lipophilic drugs or prodrugs. Due to their insolubility, we developed formulations that can be used for intravenous applications in which the lipophilic molecules are incorporated into lipid bilayer membranes of small liposomes. We chose 1-beta-d-arabinofuranosylcytosine (ara-C) as a cytotoxic nucleoside, and we demonstrated that N(4)-acyl derivatives of ara-C were active in vivo in various murine tumor models as liposomal formulations. However, the protection against enzymatic deamination was only partially achieved and was insufficient for significant improvement of cytotoxic properties. Thus, we synthesized a new class of N(4)-alkyl-ara-C derivatives. The most effective derivative, N(4)-octadecyl-ara-C (NOAC), is highly lipophilic and extremely resistant toward deamination. NOAC exerts excellent antitumor activity after oral and parenteral therapy. The activity of NOAC against freshly explanted clonogenic cells from human tumors was determined and compared with conventional antitumor agents. NOAC was used in two liposomal preparations, a stable lyophilized and a freshly prepared liquid formulation. Both formulations inhibited tumor colony formation equally in a concentration-dependent fashion. At optimal conditions, liposomal NOAC had significantly better activity compared with the clinically used drugs cisplatin, doxorubicin, 5-fluorouracil, gemcitabine, mitomycin C, and etoposide. Furthermore, in a hematopoietic stem cell assay, NOAC was less toxic than ara-C and doxorubicin by factors ranging from 2.5 to 200, indicating that this drug is well tolerated at high doses.

Cytotoxic effects of novel amphiphilic dimers consisting of 5-fluorodeoxyuridine and arabinofuranosylcytosine in cross-resistant H9 human lymphoma cells.

Various amphiphilic heterodinucleoside phosphates have recently been synthesized in order to overcome drug resistance. These agents contain 5-fluorodeoxyuridine (5-FdUrd) and arabinofuranosylcytosine (Ara-C). We now investigated the action of two of these novel dimers (#2 and #10) in sensitive and 5-FdUrd/Ara-C cross-resistant H9 human lymphoma cells. The dimers were compared with 5-FdUrd and Ara-C for growth inhibition, apoptosis induction, and cell-cycle effects. No significant difference in the cytotoxicity of dimer #2 could be observed between sensitive and 5-FdUrd/Ara-C cross-resistant H9 cells (IC50 values of 220 nM and 200 nM, respectively), indicating that further studies with this compound are warranted.

Synthesis and anticancer activities of amphiphilic 5-fluoro-2'-deoxyuridylic acid prodrugs.

Amphiphilic anticancer prodrugs of 5'-fluoro-2'-deoxyuridine-5'-monophosphate (5-FdUMP) were synthesized according to the hydrogen phosphonate method by coupling lipophilic cytosine derivatives or a phospholipid with 5-fluoro-2'-deoxyuridine (5-FdU). Studies within the in vitro Anticancer Screen Program of the National Cancer Institute have demonstrated high anticancer activities of the heterodinucleoside phosphates: N4-palmitoyl-2'-deoxycytidylyl-(3' --> 5')-3'-O-acetyl-5-fluoro-2'-deoxyuridine (dC(pam)-5-FdU(Ac), N4-palmitoyl-2',3'-dideoxycytidylyl-(5' --> 5')-3'-O-acetyl-5-fluoro-2'-deoxyuridine (ddC(pam)-(5' --> 5')-5-FdU(Ac), 5-fluoro-2'-deoxyuridylyl-(3' --> 5')-5-fluoro-N4-hexadecyl-2'-deoxycytidine (5-FdU-5-FdC(hex)), and of the new liponucleotide 1-O-octadecyl-rac-glycerylyl-(3 --> 5')-5-fluoro-2'-deoxyuridine (Oct1Gro-(3 --> 5')-5-FdU). The anticancer activities of these prodrugs are comparable to those of 5-FdU and the tumor specificities are modulated by their structures. The highest cytotoxic activity being even superior to 5-FdU was expressed by the dimer 5-FdU-5-FdC(hex).

Heterodinucleoside phosphates of 5-fluorodeoxyuridine and arabinofuranosylcytosine--new drugs in cancer chemotherapy?

The incidence of cancer is rapidly increasing and malignancies have become the number two cause of deaths in the Western world after cardiovascular diseases. In particular, colon cancer represents one of the most frequent types of malignancy. Chemotherapy is, in addition to surgery and irradiation, still one of the main treatment options against this group of diseases. Here, several chemotherapeutic treatment modalities and anticancer compounds for the treatment of colon cancer are reviewed. In particular, a newer group of heterodinucleoside phosphates (dimers), consisting of two well known antimetabolites (5-FdUrd (5-Fluorodeoxyuridine) and Ara-C (Cytarabine)), are presented. These dimers were evaluated in several studies and might offer an additional option for the treatment of various malignancies, in particular colon carcinomas. The results are summarized in detail, as these dimers might have some significant advantages when compared with conventional regimens; they might be administered orally and might constitute an alternative treatment option for resistant tumors.

In vitro and in vivo antitumor activity of novel amphiphilic dimers consisting of 5-fluorodeoxyuridine and arabinofuranosylcytosine.

Various heterodinucleoside phosphates of 5-fluorodeoxyuridine (5-FdUrd) and arabinofuranosylcytosine (Ara-C) have recently been synthesized as potent chemotherapeutic agents. 5-Fluorodeoxyuridine is being used in patients with colorectal carcinoma, whereas Ara-C is one of the most effective agents in the treatment of hematological malignancies. We now investigated the action of three novel amphiphilic dimers with different structures in various 5-fluorouracil (5-FU) sensitive and resistant human colon tumor cell lines (CCL228, CCL227, 5-FU resistant CCL227 and HT-29) as well as in L1210 murine leukemia cells. Activity of the heterodimers was determined by clonogenic and growth inhibition assays including the induction of programmed cell death. In addition, the in vivo effects were tested in L1210 leukemia bearing mice. We show that these compounds inhibited the number of colonies of 5-FU sensitive and resistant human colon tumor cell lines with IC50 values ranging from 0.65 to 1 nM. The investigated dimers induced dose-dependent apoptosis in HT-29 colon tumor cells as well as in L1210 leukemia cells. No significant difference in the cytotoxicity of these agents could be observed between 5-FU sensitive and resistant cells, indicating that these compounds might be used in the treatment of 5-FU resistant tumors. In L1210 leukemia bearing mice the survival of tumor-bearing animals was significantly increased in comparison with untreated control animals. We therefore conclude that these new heterodinucleoside phosphates of 5-FdUrd and Ara-C might be an additional option for the treatment of sensitive and 5-FU resistant colon cancer and hematological malignancies.

Combination effects of Ara-C and 5-fluorodeoxyuridine against leukemia cells in vitro and in mice.

BACKGROUND: Ara-C (1-beta-D-arabinofuranosylcytosine) is widely used for treatment of human leukemia. However, due to emergence of resistance, new drug combinations need to be developed. MATERIALS AND METHODS: We tested the combination of Ara-C with 5-FdUrd (5-fluorodeoxyuridine) in L1210 and P388D1 mouse leukemia cells in vitro and in vivo by growth inhibition and recovery assay in leukemia-bearing mice. RESULTS: Simultaneous incubation of cells with Ara-C and 5-FdUrd yielded more than additive effects for most combinations in both cell lines; synergy was seen in P388D1 cells. P388D1 cells showed delayed growth after treatment with Ara-C or 5-FdUrd in a growth recovery assay. In animal studies, simultaneous subcutaneous administration of the compounds did not show any significant beneficial effects as compared with monotherapy. Intraperitoneal administration of both compounds, however, significantly prolonged the survival of P388D1 animals. CONCLUSION: Depending on cell type and route of drug administration, the combination of Ara-C and 5-FdUrd might offer a promising additional treatment option for leukemia.

Cytostatic activity of a 5-fluoro-2'-deoxyuridine-alendronate conjugate against gastric adenocarcinoma and non-malignant intestinal and fibroblast cell lines.

BACKGROUND: 5-Fluoro-2'-deoxyuridine (5-FdU), a drug against gastric cancer, was covalently linked via its nucleobase with the amino-bisphosphonate alendronate (Ale), resulting in a new antimetabolite-bisphosphonate conjugate (5-FdU-Ale), designed for bone-targeting. MATERIALS AND METHODS: The cytostatic effect of 5-FdU-Ale was evaluated in vitro compared to monomers and mixtures using CASY Technologies and the human gastric adenocarcinoma cell lines 23132/87 and MKN-45, in comparison to the intestinal CCL-241 and dermal fibroblast NHDF neonatal cell lines. RESULTS: The adenocarcinoma cell lines demonstrated a slightly higher sensitivity, with respect to the cell lines CCL-241 and NHDF, to incubation with 5-FdU-Ale. In comparison to 5-FdU, 5-FU and an equimolar mixture of Ale+5-FdU and Ale+5-FU, the cytostatic activity of the 5-FdU-Ale was markedly reduced. CONCLUSION: 5-FdU-Ale was only partially or not at all metabolized to a mixture of cytostatic metabolites in vitro. Therefore an in vivo evaluation of the conjugates is indicated.

Liposome formulations of hydrophobic drugs.

Here, we report methods of preparation for liposome formulations containing lipophilic drugs. In contrast to the encapsulation of water-soluble compounds into the entrapped aqueous volume of a liposome, drugs with lipophilic properties are incorporated into the phospholipid bilayer membrane. Water-soluble molecules, for example, cytotoxic or antiviral nucleosides can be transformed into lipophilic compounds by attachment of long alkyl chains, allowing their stable incorporation into liposome membranes and taking advantage of the high loading capacity lipid bilayers provide for lipophilic molecules. We created a new class of cytotoxic drugs by chemical transformation of the hydrophilic drugs cytosine-arabinoside (ara-C), 5-fluoro-deoxyuridine (5-FdU) and ethinylcytidine (ETC) into lipophilic compounds and their formulation in liposomes. The concept of chemical modification of water-soluble molecules by attachment of long alkyl chains and their stable incorporation into liposome bilayer membranes represent a very promising method for the development of new drugs not only for the treatment of tumors or infections, but also for many other diseases.

5-FdUrd-araC heterodinucleoside re-establishes sensitivity in 5-FdUrd- and AraC-resistant MCF-7 breast cancer cells overexpressing ErbB2.

ErbB2 overexpressing breast tumors have a poor prognosis and a high risk to develop chemoresistance to therapeutic treatment. "Chemoresistance" is a response of cells to toxic stress, and, although it is a common phenomenon, it is still poorly defined. However, a detailed understanding is required to target desensitized pathways and mechanisms for successful reactivation as part of a tailored therapy. To gain insight, which malfunctions contribute to chemoresistance, two mechanisms relevant for tissue homeostasis, the regulation of the cell cycle and of apoptosis, were investigated. Maternal MCF-7- and ErbB2-overexpressing MCF-7(erbB2) breast cancer cells were long term pretreated with 2'-deoxy-5-fluorodeoxyuridine (5-FdUrd) or 1-beta-d-arabinofuranosylcytosine (AraC) and the acquisition of drug-insensitivity was analyzed. A phosphate-conjugated heterodinucleoside consisting of one 5-FdUrd- and one AraC-moiety (5-fluoro-2'-desoxyuridylyl-(3'-->5')-Arabinocytidine) was utilized as a tool to assess the type of acquired resistances. ErbB2-overexpression disrupted proper cell cycle regulation and furthermore facilitated the development of an apoptosis-refractory phenotype upon exposure to 5-FdUrd. Experiments with dimer 5-FdUrd-araC in ErbB2-overexpressing MCF-7(erbB2) cells, and also with nucleoside 5-FdUrd in maternal MCF-7 cells, evidenced that the phenotypes of resistance to cell cycle inhibition and to apoptosis induction were differently affected. The expression profile of cyclin D1 (but not that of p53, p21, or p27) correlated with the proliferative phenotypes and nuclear accumulation of apoptosis inducing factor (but not activation of caspase 7) with apoptotic phenotypes. Dimer 5-FdUrd-araC overrode acquired chemoresistances, whereas combined application of 5-FdUrd and AraC exhibited significantly less activity. Dimer 5-FdUrd-araC remained active in MCF-7 clones most likely by circumventing the prerequisite of first-step phosphorylation. The acquisition of chemoresistance encompassed the affection of apoptosis- and cell-cycle regulation to, respectively, different extents. Thus, drug-induced cell cycle arrest and apoptosis induction are independent of each other.