Isolation and characterization of thymitaq (AG337) and 5-fluoro-2-deoxyuridylate-resistant mutants of human thymidylate synthase from ethyl methanesulfonate-exposed human sarcoma HT1080 cells

Inside the biochemical pathways of thymidylate synthase perturbed by anticancer drugs: Novel strategies to overcome cancer chemoresistance

Our current understanding of the mechanisms of action of antitumor agents and the precise mechanisms underlying drug resistance is that these two processes are directly linked. Moreover, it is often possible to delineate chemoresistance mechanisms based on the specific mechanism of action of a given anticancer drug. A more holistic approach to the chemoresistance problem suggests that entire metabolic pathways, rather than single enzyme targets may better explain and educate us about the complexity of the cellular responses upon cytotoxic drug administration. Drugs, which target thymidylate synthase and folate-dependent enzymes, represent an important therapeutic arm in the treatment of various human malignancies. However, prolonged patient treatment often provokes drug resistance phenomena that render the chemotherapeutic treatment highly ineffective. Hence, strategies to overcome drug resistance are primarily designed to achieve either enhanced intracellular drug accumulation, to avoid the upregulation of folate-dependent enzymes, and to circumvent the impairment of DNA repair enzymes which are also responsible for cross-resistance to various anticancer drugs. The current clinical practice based on drug combination therapeutic regimens represents the most effective approach to counteract drug resistance. In the current paper, we review the molecular aspects of the activity of TS-targeting drugs and describe how such mechanisms are related to the emergence of clinical drug resistance. We also discuss the current possibilities to overcome drug resistance by using a molecular mechanistic approach based on medicinal chemistry methods focusing on rational structural modifications of novel antitumor agents. This paper also focuses on the importance of the modulation of metabolic pathways upon drug administration, their analysis and the assessment of their putative roles in the networks involved using a meta-analysis approach. The present review describes the main pathways that are modulated by TS-targeting anticancer drugs starting from the description of the normal functioning of the folate metabolic pathway, through the protein modulation occurring upon drug delivery to cultured tumor cells as well as cancer patients, finally describing how the pathways are modulated by drug resistance development. The data collected are then analyzed using network/netwire connecting methods in order to provide a wider view of the pathways involved and of the importance of such information in identifying additional proteins that could serve as novel druggable targets for efficacious cancer therapy.

Hotspots in an obligate homodimeric anticancer target. Structural and functional effects of interfacial mutations in human thymidylate synthase

Human thymidylate synthase (hTS), a target for antiproliferative drugs, is an obligate homodimer. Single-point mutations to alanine at the monomer-monomer interface may enable the identification of specific residues that delineate sites for drugs aimed at perturbing the protein-protein interactions critical for activity. We computationally identified putative hotspot residues at the interface and designed mutants to perturb the intersubunit interaction. Dimer dissociation constants measured by a FRET-based assay range from 60 nM for wild-type hTS up to about 1 mM for single-point mutants and agree with computational predictions of the effects of these mutations. Mutations that are remote from the active site retain full or partial activity, although the substrate KM values were generally higher and the dimer was less stable. The lower dimer stability of the mutants can facilitate access to the dimer interface by small molecules and thereby aid the design of inhibitors that bind at the dimer interface.

Epigenetics and chemoresistance in colorectal cancer: an opportunity for treatment tailoring and novel therapeutic strategies

Colorectal cancer is the second leading cause of cancer-related deaths in the world. Despite many therapeutic opportunities, prognosis remains dismal for patients with metastatic disease, and a significant portion of early-stage patients develop recurrence after chemotherapy. Epigenetic gene regulation is a major mechanism of cancer initiation and progression, through the inactivation of several tumor suppressor genes. Emerging evidence indicates that epigenetics may also play a key role in the development of chemoresistance. In the present review, we summarize epigenetic mechanisms triggering resistance to three commonly used agents in colorectal cancer: 5-fluorouracil, irinotecan and oxaliplatin. Those epigenetic biomarkers may help stratify colorectal cancer patients and develop a tailored therapeutic approach. In addition, epigenetic modifications are reversible through specific drugs: histone-deacetylase and DNA-methyl-transferase inhibitors. Preclinical studies suggest that these drugs may reverse chemoresistance in colorectal tumors. In conclusion, an epigenetic approach to colorectal cancer chemoresistance may pave the way to personalized treatment and to innovative therapeutic strategies.

Molecularly evolved thymidylate synthase inhibits 5-fluorodeoxyuridine toxicity in human hematopoietic cells

Thymidylate synthase (TS) inhibitors, such as 5-fluorouracil (5-FU) and 5-fluorodeoxyuridine (5-FUdR), are amongst the most frequently used chemotherapeutic drugs available, although their efficacy is often limited by myelotoxicity. An emerging strategy for overcoming bone marrow toxicity involves ex vivo genetic transfer of drug resistance to autologous hematopoietic progenitor cells, followed by reimplantation of the transfected cells before chemotherapy. Here we establish that expression of mutant TS genes, selected from millions of engineered variants, renders human hematopoietic cells resistant to 5-FUdR, and identify the most efficacious variant for gene therapeutic rescue of drug-induced myelosuppression.

Toward the design of mutation-resistant enzyme inhibitors: further evaluation of the substrate envelope hypothesis

Previous studies have shown the usefulness of the substrate envelope concept in the analysis and prediction of drug resistance profiles for human immunodeficiency virus protease mutants. This study tests its applicability to several other therapeutic targets: Abl kinase, chitinase, thymidylate synthase, dihydrofolate reductase, and neuraminidase. For the targets where many (> or =6) mutation data are available to compute the average mutation sensitivity of inhibitors, the total volume of an inhibitor molecule that projects outside the substrate envelope V(out), is found to correlate with average mutation sensitivity. Analysis of a locally computed volume suggests that the same correlation would hold for the other targets, if more extensive mutation data sets were available. It is concluded that the substrate envelope concept offers a promising and easily implemented computational tool for the design of drugs that will tend to resist mutations. Software implementing these calculations is provided with the 'Supporting Information'.

Design and characterization of a mutation outside the active site of human thymidylate synthase that affects ligand binding

Owing to its central role in DNA synthesis, human thymidylate synthase (hTS) is a well-established target for chemotherapeutic agents, such as fluoropyrimidines. The use of hTS inhibitors in cancer therapy is limited by their toxicity and the development of cellular drug resistance. Here, with the aim of shedding light on the structural role of the A-helix in fluoropyrimidine resistance, we have created a fluoropyrimidine-resistant mutant by making a single point mutation, Glu30Trp. We postulated that residue 30, which is located in the A-helix, close to but outside the enzyme active site, could have a long-range effect on inhibitor binding. The mutant shows 100 times lower specific activity with respect to the wild-type hTS and is resistant to the classical inhibitor, FdUMP, as shown by a 6-fold higher inhibition constant. Circular dichroism experiments show that the mutant is folded. The results of molecular modeling and simulation suggest that the Glu30Trp mutation gives rise to resistance by altering the hydrogen-bond network between residue 30 and the active site.

Identification and characterization of a novel thymidylate synthase from deep-sea thermophilic bacteriophage Geobacillus virus E2

Thymidylate synthase (TS) is essential for de novo synthesis of dTMP and is a key enzyme involved in DNA synthesis and transcriptional regulation of organisms. Due to their biologic importance, TSs have been intensively studied. In this investigation, a thermostable TS was identified from a deep-sea thermophilic bacteriophage Geobacillus virus E2 (GVE2). It was demonstrated that GVE2-TS was highly homologous to known TSs and contained five characteristic conserved domains. The temporal analyses by Northern and Western blots revealed that the GVE2-TS was transcribed and expressed early after Geobacillus virus E2 infection, identifying it as a viral early gene. As shown by gel mobility shift assays, the recombinant GVE2-TS protein had the capacity to bind its own mRNA. Our study presented the first report on thymidylate synthase from deep-sea thermophilic bacteriophage.

Mechanisms of antifolate resistance and methotrexate efficacy in leukemia cells

Antifolates are the first class of antimetabolites introduced to clinic about 6 decades ago. Now, after several years of administration of antifolates against malignancies and particularly leukemia, we are still trying to achieve a full understanding of the mechanisms of action and resistance to these agents. The present article covers different factors able to influence efficacy of antifolates on leukemic cells, the known mechanisms of resistance to methotrexate (MTX) and strategies to overcome these mechanisms. The dominant factors that are contributed to tolerance to cytocidal effects of MTX including pharmacokinetic factors, impaired transmembrane uptake as the most frequent rote of provoking resistance to MTX, augmented drug efflux, impaired intracellular polyglutamation as a determining process of drug efficacy, alterations in expression or activity of target enzymes and increased intracellular folate pools; and finally role of 7-hydroxymethotrexate on response or resistance to MTX will be discussed in more detail. Finally, strategies to overcome resistance to antifolates are discussed.

Protection of hematopoietic stem cells from pemetrexed toxicity by retroviral gene transfer with a mutant dihydrofolate reductase-mutant thymidylate synthase fusion gene

Myelosuppression is one of the major side effects of most anticancer drugs. To confer myeloprotection, our laboratory generated drug-resistant mutants of select target human enzymes for gene transfer to the bone marrow. Mutants of two of these enzymes, dihydrofolate reductase (DHFR F/S) and thymidylate synthase (TS G52S), were previously shown to confer resistance to methotrexate and 5-FU, respectively, and recently a fusion cDNA of both mutant enzymes (DHFR F/S-TS G52S) was shown to confer dual resistance to both antimetabolites. In this study, we examined the sensitivity of the DHFR F/S-TS G52S fusion protein to the multitargeted antifolate, pemetrexed (LY231514, Alimta), which targets both DHFR and TS and is currently in phase III trials for the treatment of solid tumors and in combination with cisplatin has been shown to be an advance in the treatment of mesothelioma. The K(i) for the DHFR F/S portion of the purified fusion protein to pemetrexed was increased by greater than 9000-fold when compared to wtDHFR (8000 versus 0.86 nM), while the K(i) for the TS G52S portion of the fusion protein to pemetrexed was similar to that of wtTS (2.8 versus 3.1 nM). When the fusion gene was retrovirally transduced into NIH 3T3 fibroblasts, the IC(50) to pemetrexed was three- to four-fold higher than cells transduced with DHFR F/S or TS G52S alone (163 versus 53 and 45 nM, respectively). Similarly, expression of the DHFR F/S-TS G52S fusion gene in retrovirally transduced mouse marrow cells resulted in an increased survival of CFU-GM colonies when compared to cells transduced with either of the mutants alone. Co-expression of mutant DHFR and TS enzymes has additive effects in conferring resistance to pemetrexed-induced toxicity. This construct may be useful for conferring myeloprotection to patients receiving this drug.

Circulating biomarkers from tumour bulk to tumour machinery: promises and pitfalls

In this paper, we provide a working classification for circulating biomarkers according to their potential clinical application. We broadly divided biomarkers into four groups: (i) biomarkers of cancer risk, (ii) biomarkers of tumour-host interactions, (iii) biomarker of tumour burden, and (iv) function-related biomarkers. We hope this classification will provide a framework to which the results of future studies can be added. We also discuss the promises and pitfalls in the optional use of biomarkers in oncology.

Identification of the thymidylate synthase within the genome of white spot syndrome virus

Thymidylate synthase (TS) (EC 2.1.1.45) is essential for thede novosynthesis of dTMP in prokaryotic and eukaryotic organisms. Within the white spot syndrome virus (WSSV) genome, an open reading frame (WSV067) that encodes a 289 amino acid polypeptide showed significant homology to all known TSs from species including mammals, plants, fungi, protozoa, bacteria and DNA viruses. In this study, WSV067 was expressed inEscherichia coli, and the purified recombinant protein showed TS activity in dUMP−folate-binding assays using ultraviolet difference spectroscopy. RT-PCR and Western blot analyses showed that WSV067 was a genuine and early gene. Phylogenetic analysis revealed that WSSV-TS was more closely related to the TSs of eukaryotes than to those from prokaryotes.

Thymidylate synthase as an oncogene: a novel role for an essential DNA synthesis enzyme

Thymidylate synthase (TS) is an E2F1-regulated enzyme that is essential for DNA synthesis and repair. TS protein and mRNA levels are elevated in many human cancers, and high TS levels have been correlated with poor prognosis in patients with colorectal, breast, cervical, bladder, kidney, and non-small cell lung cancers. In this study, we show that ectopic expression of catalytically active TS is sufficient to induce a transformed phenotype in mammalian cells as manifested by foci formation, anchorage independent growth, and tumor formation in nude mice. In contrast, comparable levels of two TS mutants carrying single point mutations within the catalytic domain had no transforming activity. In addition, we show that overexpression of TS results in apoptotic cell death following serum removal. These data demonstrate that TS exhibits oncogene-like activity and suggest a link between TS-regulated DNA synthesis and the induction of a neoplastic phenotype.

Resistance to antifolates

The antifolates were the first class of antimetabolites to enter the clinics more than 50 years ago. Over the following decades, a full understanding of their mechanisms of action and chemotherapeutic potential evolved along with the mechanisms by which cells develop resistance to these drugs. These principals served as a basis for the subsequent exploration and understanding of the mechanisms of resistance to a variety of diverse antineoplastics with different cellular targets. This section describes the bases for intrinsic and acquired antifolate resistance within the context of the current understanding of the mechanisms of actions and cytotoxic determinants of these agents. This encompasses impaired drug transport into cells, augmented drug export, impaired activation of antifolates through polyglutamylation, augmented hydrolysis of antifolate polyglutamates, increased expression and mutation of target enzymes, and the augmentation of cellular tetrahydrofolate-cofactor pools in cells. This chapter also describes how these insights are being utilized to develop gene therapy approaches to protect normal bone marrow progenitor cells as a strategy to improve the efficacy of bone marrow transplantation. Finally, clinical studies are reviewed that correlate the cellular pharmacology of methotrexate with the clinical outcome in children with neoplastic diseases treated with this antifolate.

Retroviral transduction of a mutant dihydrofolate reductase-thymidylate synthase fusion gene into murine marrow cells confers resistance to both methotrexate and 5-fluorouracil

Gene transfer-based myeloprotection strategies against chemotherapy require the development of effective drug resistance genes or gene combinations. Our laboratory has previously generated drug-resistant mutants of dihydrofolate reductase (DHFR F/S) and thymidylate synthase (TS G52S) for myeloprotection against methotrexate (MTX) and 5-fluorouracil (5-FU), respectively. For the purpose of conferring dual myeloprotection against both MTX and 5-FU, we have generated two retroviral constructs encoding both DHFR F/S and TS G52S as a fusion protein (DHFR F/S-TS G52S) or as individual proteins from a bicistronic gene. The DHFR F/S-TS G52S fusion protein is functional and exhibits kinetic properties similar to that of the individual mutant enzymes. NIH 3T3 cells and mouse bone marrow progenitors retrovirally transduced with the fusion DHFR F/S-TS G52S cDNA provided similar levels of resistance to MTX and 5-FU as cells expressing the individual mutant enzymes and higher levels of resistance to MTX than cells expressing DHFR F/S from the 3' end of a bicistronic gene. As MTX and 5-FU are used in combination therapy for diseases such as breast and colon cancer, this fusion gene may be useful in the clinic to reduce myelosuppressive toxicity associated with this drug combination.

Distribution of mutations in human thymidylate synthase yielding resistance to 5-fluorodeoxyuridine

Thymidylate synthase (TS) catalyzes methylation of dUMP to dTMP and is the target of cancer chemotherapeutic agents (e.g. 5-fluorouracil). Here, we used error-prone PCR to mutagenize the full-length human TS cDNA and then selected mutants resistant to 5-fluorodeoxyuridine in a bacterial complementation system. We found that resistant mutants contained 1-5 amino acid substitutions and that these substitutions were located along the entire length of the polypeptide. Mutations were frequent near the active site Cys(195) and in the catalytically important Arg(50) loop; however, many mutations were also distributed throughout the remainder of the cDNA. Mutants containing a single amino acid replacement identified the following 14 residues as unreported sites of resistance: Glu(23), Thr(51), Thr(53), Val(84), Lys(93), Asp(110), Asp(116), Pro(194), Ser(206), Met(219), His(250), Asp(254), Tyr(258), and Lys(284). Many of these residues are distant from the active site and/or have no documented function in catalysis or resistance. We conclude that mutations distributed throughout the linear sequence and three-dimensional structure of human TS can confer resistance to 5-fluorodeoxyuridine. Our findings imply that long range interactions within proteins affect catalysis at the active site and that mutations at a distance can yield variant proteins with desired properties.

Novel aspects of resistance to drugs targeted to dihydrofolate reductase and thymidylate synthase

Drug resistance is often a limiting factor in successful chemotherapy. Our laboratory has been interested in studying mechanisms of resistance to drugs that are targeted to the thymidylate biosynthesis pathway especially those that target thymidylate synthase (TS) and dihydrofolate reductase (DHFR). We have used leukemia as a model system to study resistance to methotrexate (MTX) and colorectal cancer as the model system to study 5-fluorouracil (5-FU) resistance. In leukemias, we and others have shown that transport, efflux, polyglutamylation and hydrolase activities are major determinants of MTX resistance. We have further reported that some leukemic cells have an increase in DHFR gene copy number possibly contributing to the resistant phenotype. Recently, we have begun to study in detail the molecular mechanisms that govern translational regulation of DHFR in response to MTX as an additional resistance mechanism. Studies thus far involving colorectal tumors obtained from patients have focused predominantly on the predictive value of levels of TS expression and p53 mutations in determining response to 5-FU. Although the predictive value of these two measures appears to be significant, given the variety of resistance to 5-FU observed in cell lines, it is not likely that these are the only measures predictive of response or responsible for acquired resistance to this drug. The enzyme uridine-cytidine monophosphate kinase (UMPK) is an essential and rate-limiting enzyme in 5-FU activation while dihydropyrimidine dehydrogenase (DPD) is a catabolic enzyme that inactivates 5-FU. Alterations in UMPK and DPD may therefore explain failure of 5-FU response in the absence of alterations in TS or p53. Transcription factors that regulate TS may also influence drug sensitivity. We have found that mRNA levels of the E2F family of transcription factors correlates with TS message levels and are higher in lung metastases than in liver metastases of colorectal cancers. Moreover, gene copy number of the E2F-1 gene appears to be increased in a significant number of samples obtained from metastases of colorectal cancer. We have also generated mutants of both DHFR and TS that confer resistance to MTX as well as 5-FU by random as well as site-directed mutagenesis. These mutants used alone or as fusion cDNAs of the mutants have proven to be useful in transplant studies where transfer of these mutant cDNAs to bone marrow cells have been shown to confer drug resistance to recipients. The fusion cDNAs of DHFR such as the DHFR-herpes simplex virus type 1 thymidine kinase (HSVTK) are also useful for regulation of gene expression in vivo using MTX as the small molecule regulator that can be monitored by positron emission tomography (PET) scanning or by optical imaging using a fusion construct such as DHFR-EGFP.

Synthesis of prodrug candidates: conjugates of amino acid with nucleoside boranophosphate

[structure: see text] Preparation of antiviral and anticancer prodrug candidates, P-tyrosinyl(P-O)-5'-P-nucleosidyl boranophosphates, is described. One-pot synthesis via a phosphoramidite method resulted in the title compounds with good yields. The P-boranophosphate diastereomers were separated by RP-HPLC, and their structures were confirmed by 1H and 31P NMR spectroscopy and MS analysis.

Differences in natural ligand and fluoropyrimidine binding to human thymidylate synthase identified by transient-state spectroscopic and continuous variation methods

Thymidylate synthase (TS) is a central target for the design of chemotherapeutic agents due to its vital role in DNA synthesis. Structural studies of binary complexes between Escherichia coli TS and various nucleotides suggest the chemotherapeutic agent FdUMP and the natural ligand dUMP bind similarly. We show, however, that FdUMP binding to human TS yields a substantially greater decrease in fluorescence than does dUMP. Because the difference in quenching due to ligand binding was approximately two-fold and this difference was not seen when using ecTS, the intriguing result indicated a significant difference in the mode of FdUMP binding to the human enzyme. We compared the binding affinities of dUMP, FdUMP, and TMP to TS from both species and found no significant differences for the individual ligands. Because binding affinities were not different among the ligands, the method of continuous variation was employed to determine binding stoichiometry. Similar to that found for dUMP binding to human and ecTS, FdUMP displayed single site occupancy with both enzymes. These results show that nucleotide binding differences exist for FdUMP and dUMP binding to the human enzyme. The observed differences are not due to differences in stoichiometry or ligand affinity. Therefore, although the crystal structure of human TS with various nucleotide ligands has not been solved, these results show that the differences observed using fluorescence methods result from as yet unidentified differential interactions between the human enzyme and nucleotide ligands.

Myeloprotection with drug-resistance genes

One of the many applications of gene transfer for cancer gene therapy is the transfer of drug-resistance genes into bone-marrow stem cells for myeloprotection. Protection of the hosts' bone marrow should allow for dose escalation that may be useful for eradicating minimal residual disease in a post-transplant situation. A number of drug resistance genes, whose products include mutant forms of enzymes that confer resistance to chemotherapeutic drugs, are discussed. Advances in hematopoietic stem cell isolation and ex vivo manipulation has kept pace with improvements in retroviral vector technology to make hematopoietic stem cell transduction a distinct reality. Clinical trials, which have established that the approach is safe, are now being designed to address more therapeutically relevant issues.

Multi-targeted antifolates aimed at avoiding drug resistance form covalent closed inhibitory complexes with human and Escherichia coli thymidylate synthases

Crystal structures of four pyrrolo(2,3-d)pyrimidine-based antifolate compounds, developed as inhibitors of thymidylate synthase (TS) in a strategy to circumvent drug-resistance, have been determined in complexes with their in vivo target, human thymidylate synthase, and with the structurally best-characterized Escherichia coli enzyme, to resolutions of 2.2-3.0 A. The 2.9 A crystal structure of a complex of human TS with one of the inhibitors, the multi-targeted antifolate LY231514, demonstrates that this compound induces a "closed" enzyme conformation and leads to formation of a covalent bond between enzyme and substrate. This structure is one of the first liganded human TS structures, and its solution was aided by mutation to facilitate crystallization. Structures of three other pyrrolo(2,3-d)pyrimidine-based antifolates in complex with Escherichia coli TS confirm the orientation of this class of inhibitors in the active site. Specific interactions between the polyglutamyl moiety and a positively charged groove on the enzyme surface explain the marked increase in affinity of the pyrrolo(2,3-d)pyrimidine inhibitors once they are polyglutamylated, as mediated in vivo by the cellular enzyme folyl polyglutamate synthetase.

Applied molecular evolution of O6-benzylguanine-resistant DNA alkyltransferases in human hematopoietic cells

Applied molecular evolution is a rapidly developing technology that can be used to create and identify novel enzymes that nature has not selected. An important application of this technology is the creation of highly drug-resistant enzymes for cancer gene therapy. Seventeen O(6)-alkylguanine-DNA alkyltransferase (AGT) mutants highly resistant to O(6)-benzylguanine (BG) were identified previously by screening 8 million variants, using genetic complementation in Escherichia coli. To examine the potential of these mutants for use in humans, the sublibrary of AGT clones was introduced to human hematopoietic cells and stringently selected for resistance to killing by the combination of BG and 1,3-bis(2-chloroethyl)-1-nitrosourea. This competitive analysis between the mutants in human cells revealed three AGT mutants that conferred remarkable resistance to the combination of BG and 1,3-bis(2-chloroethyl)-1-nitrosourea. Of these, one was recovered significantly more frequently than the others. Upon further analysis, this mutant displayed a level of BG resistance in human hematopoietic cells greater than that of any previously reported mutant.

Retroviral expression of Escherichia coli thymidylate synthase cDNA confers high-level antifolate resistance to hematopoietic cells

Drug resistance gene therapy has the potential to protect against the myelosuppressive side effects of chemotherapy or to be used as a dominant in vivo selectable marker of genetically modified cells. Steady state kinetic studies have indicated the Escherichia coli thymidylate synthase (ecTS) is intrinsically more resistant to several TS-directed inhibitors as compared with the human enzyme, suggesting that ecTS is suitable for use as a drug-resistant marker. However, we found a disparity between the kinetic properties of ecTS and the degree of resistance conferred to cells transfected with the cDNA encoding this enzyme. It was determined that although ecTS is as stable as human TS (hTS) in transfected mammalian cells, ecTS is produced at only 40% the level of hTS, indicating poor translation of ecTS in eukaryotic cells. To circumvent this problem, the entire cDNA sequence of ecTS was synthesized by using codons optimized for expression in mammalian cells. In transfected Chinese hamster lung cells, expression of ecTS from the optimized construct, termed OPTecTS, is as efficient as hTS. Furthermore, cells transfected with the OPTecTS cDNA are significantly more resistant to the TS inhibitor raltitrexed as compared with transfected cells expressing similar levels of hTS. High-titer retroviral packaging cells were generated with OPTecTS and >80% of transduced mouse hematopoietic progenitor cells are resistant to raltitrexed, Thymitaq, and U89 at concentrations that eliminated colony growth of mock-transduced cells. The transgene was detectable by PCR in transduced bone marrow selected in U89 or raltitrexed, and expression of ecTS from the OPTecTS cDNA in bone marrow exhibited a catalytic rate constant comparable to that of purified recombinant ecTS. These data indicate that OPTecTS is a viable dominant selectable marker that can confer resistance to antifolates when introduced into cells.

Mechanism of resistance to the ABL tyrosine kinase inhibitor STI571 in BCR/ABL–transformed hematopoietic cell lines

The tyrosine kinase activity of the Bcr/Abl oncogene is required for transformation of hematopoietic cells. The tyrosine kinase inhibitor STI571 (formerly called CGP57148B, Novartis Pharmaceuticals) inhibits BCR/ABL, TEL/ABL, and v-ABL kinase activity and inhibits growth and viability of cells transformed by any of these ABL oncogenes. Here we report the generation of 2 BCR/ABL–positive cell lines that have developed partial resistance to STI571. BCR/ABL–transformed Ba/F3 hematopoietic cells and Philadelphia-positive human K562 cells were cultured in gradually increasing concentrations of STI571 over a period of several months to generate resistant lines. Resistant Ba/F3.p210 cells were found to have an increase in Bcr/Abl messenger RNA, amplification of the Bcr/Abl transgene, and a greater than tenfold increase in the level of BCR/ABL protein. In contrast to Ba/F3.p210 cells, drug-resistant K562 cells did not undergo detectable amplification of the BCR/ABL gene, although they displayed a 2-fold to 3-fold increase in p210BCR/ABL protein. The addition of STI571 to both resistant Ba/F3.p210 and K562 cells resulted in a rapid reduction of tyrosine phosphorylation of cellular proteins, similar to that observed for nonresistant cells. However, the inhibition of kinase activity was transient and partial and was not accompanied by apoptosis. The results suggest that resistance to STI571 may be multifactorial. Increased expression of the target protein BCR/ABL was observed in both lines, and resulted from oncogene amplification in one line. However, altered drug metabolism, transport, or other related mechanisms may also contribute to drug resistance.

Mechanism of resistance to the ABL tyrosine kinase inhibitor STI571 in BCR/ABL–transformed hematopoietic cell lines

The tyrosine kinase activity of the Bcr/Abl oncogene is required for transformation of hematopoietic cells. The tyrosine kinase inhibitor STI571 (formerly called CGP57148B, Novartis Pharmaceuticals) inhibits BCR/ABL, TEL/ABL, and v-ABL kinase activity and inhibits growth and viability of cells transformed by any of these ABL oncogenes. Here we report the generation of 2 BCR/ABL–positive cell lines that have developed partial resistance to STI571. BCR/ABL–transformed Ba/F3 hematopoietic cells and Philadelphia-positive human K562 cells were cultured in gradually increasing concentrations of STI571 over a period of several months to generate resistant lines. Resistant Ba/F3.p210 cells were found to have an increase in Bcr/Abl messenger RNA, amplification of the Bcr/Abl transgene, and a greater than tenfold increase in the level of BCR/ABL protein. In contrast to Ba/F3.p210 cells, drug-resistant K562 cells did not undergo detectable amplification of the BCR/ABL gene, although they displayed a 2-fold to 3-fold increase in p210BCR/ABL protein. The addition of STI571 to both resistant Ba/F3.p210 and K562 cells resulted in a rapid reduction of tyrosine phosphorylation of cellular proteins, similar to that observed for nonresistant cells. However, the inhibition of kinase activity was transient and partial and was not accompanied by apoptosis. The results suggest that resistance to STI571 may be multifactorial. Increased expression of the target protein BCR/ABL was observed in both lines, and resulted from oncogene amplification in one line. However, altered drug metabolism, transport, or other related mechanisms may also contribute to drug resistance.

Isolation and characterization of a thymidylate synthase-deficient human colon tumor cell line

Following mutagenesis of the human colorectal tumor cell line HCT C with ethyl methanesulfonate, clonal sublines were isolated that survived on medium toxic to cells expressing thymidylate synthase (TS). The subline exhibiting the lowest TS activity, designated as C18, was characterized. Extracts from C18 cells were mixed with extracts from parental C cells to determine whether the TS-deficient phenotype is trans-acting. No effect was observed on the activity of TS in parental extracts. The levels of functional TS in C18 cells were analyzed by the binding of the mechanism-based inhibitor 5-fluoro-2'-deoxyuridylate (FdUMP) under conditions that allowed for the detection of 10 fmol of TS. Only a low level of FdUMP-TS complexes was detected in C18 extracts. The level of TS expression in C18 cells was similar to that in parental C cells, as indicated by immunoblot and RNA analyses. DNA sequence analysis of TS cDNA from C18 cells revealed the existence of a point mutation (C-->T) at nucleotide 647 that predicts the replacement of Ser216 by a leucine residue. That the C18 cell line was homozygous for this mutation was indicated by restriction fragment-length polymorphism analysis and by primer extension analysis. To provide additional evidence that substitution of Ser216 by a leucine residue created a defective protein, a TS-deficient bacterial strain was transformed with an expression vector containing the mutated human TS cDNA. The transformed strain exhibited thymidine auxotrophy, indicating that the mutant TS (Leu216) is nonfunctional.

Tolerance of 5-fluorodeoxyuridine resistant human thymidylate synthases to alterations in active site residues

Fluoropyrimidines, such as 5-fluorouracil (5-FU), are used extensively in cancer therapy. In the cell, 5-FU is metabolized to 5-fluorodeoxyuridylate (5-FdUMP), a tight binding covalent inhibitor of thymidylate synthase (TS). In order to create 5-FdUMP resistant enzymes to protect chemosensitive normal cells and further understand mechanisms of 5-FdUMP resistance, we have randomized four residues within the active site of TS. Our previous studies identified alterations in residues which produce active TS with enhanced resistance to 5-fluorouridine (5-FdUR). By remutagenizing a subset of the 13 previously targeted residues (A197, L198, C199 and V204), an unbiased random library can be created allowing for extensive testing of all possible amino acid substitutions at each of the sites. Using genetic complementation and selection in Escherichia coli, we identified the spectrum of substitutions that yield active TS as well as those that resulted in 5-FdUR resistant mutants of TS. The 5-FdUR resistant TS were found to share several structural features including hydrophobic substitutions at residue 197, retention of the wild-type leucine 198, the alteration C199L (present in 64% of the drug-resistant library), and polar alterations of valine 204. The catalytic activity of mutants with these features was approximately equal to that of the wild-type TS.

Genetic complementation and resistance to 5-fluoro-2'-deoxyuridine in thymidine auxotrophs expressing a highly defective mutant of human thymidylate synthase

A mutant human thymidylate synthase (TS) has been created in which a glutamine residue at position 214 has been replaced by glutamate. Glutamine at position 214 is postulated to be involved in maintaining the enzyme in a conformation that facilitates the binding of the substrate dUMP. Although the kcat/Km of the mutant protein for the substrate, dUMP, is 10(3) lower than that of wild-type TS, the mutant TS confers thymidine prototrophy on a TS-deficient bacterial strain when expressed at high levels. In the present investigation, a TS-deficient Chinese hamster lung cell line was transfected with DNA encoding the defective protein. Thymidine prototrophs were isolated that expressed the defective protein at levels that were physiologically relevant. The activities of the enzymes expressed endogenously in representative prototrophs were consistent with the activities observed for the purified proteins. At similar levels of TS expression, thymidine prototrophs expressing Glu214 TS were 8-fold more resistant to 5-fluoro-2'-deoxyuridine (FdUrd) cytotoxicity than are prototrophs expressing Gln214 TS. FdUrd is a prodrug of the tight-binding TS inhibitor, 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP). The resistance to FdUrd was associated with a significant decrease in the binding of FdUMP to the purified mutant enzyme. The data are consistent with the interpretation that TSs that are highly defective are capable of sufficient dTMP production for cell survival and optimal growth, yet may confer resistance to TS-directed inhibitors.

Immunohistochemical quantitation of thymidylate synthase expression in colorectal cancer metastases predicts for clinical outcome to fluorouracil-based chemotherapy

PURPOSE

To determine whether immunohistochemical thymidylate synthase (TS) quantitation predicts for clinical outcome in patients with advanced colorectal cancer treated by fluorouracil (FUra)-based chemotherapy.

PATIENTS AND METHODS

TS levels were measured immunohistochemically on archival specimens of colorectal cancer metastases from 48 patients homogenously treated by bolus FUra plus methotrexate alternating with continuous-infusion FUra plus leucovorin. These measurements were retrospectively correlated with patient characteristics and clinical outcome.

RESULTS

A significant correlation was found between intratumoral TS expression and all the parameters of clinical outcome analyzed. In patients whose tumors had low (n = 27) and high (n = 21) TS levels, the overall response rates were 67% and 24%, respectively (P =.003). The percentage of tumor shrinkage after chemotherapy was linearly related to TS immunoreactivity (r =.56, P =.00004), and its mean values were 65% and 14% with low and high TS levels, respectively (P =.0001). By logistic regression analysis, low TS expression was the single best predictor of response to chemotherapy (relative probability, 5.0). In patients with low and high TS expression, the median time to progression was 9.6 months v 6.2 months (P =.005) and the median survival time 18.4 months v 15.4 months (P =.02), respectively. Two- and 3-year survival rates were 41% v 15% and 19% v 0% (P =.02), respectively.

CONCLUSION

In this cohort of homogenously treated patients, intratumor TS content was a major predictor of clinical outcome. Immunohistochemical TS quantitation provides a convenient, low-cost technique for identifying patients unresponsive to TS inhibitors who may be candidates for alternative chemotherapy regimens.

Improving enzymes for cancer gene therapy

New techniques now make it feasible to tailor enzymes for cancer gene therapy. Novel enzymes with desired properties can be created and selected from vast libraries of mutants containing random substitutions within catalytic domains. In this review, we first consider genes for the ablation of tumors, namely, genes that have been mutated (or potentially can be mutated) to afford enhanced activation of prodrugs and increased sensitization of tumors to specific chemotherapeutic agents. We then consider genes that have been mutated to provide better protection of normal host tissues, such as bone marrow, against the toxicity of specific chemotherapeutic agents. Expression of the mutant enzyme could render sensitive tissues, such as bone marrow, more resistant to specific cytotoxic agents.

Probing the folate-binding site of human thymidylate synthase by site-directed mutagenesis. Generation of mutants that confer resistance to raltitrexed, Thymitaq, and BW1843U89

Human thymidylate synthase (TS) contains three highly conserved residues Ile-108, Leu-221, and Phe-225 that have been suggested to be important for cofactor and antifolate binding. To elucidate the role of these residues and generate drug-resistant human TS mutants, 14 variants with multiple substitutions of these three hydrophobic residues were created by site-directed mutagenesis and transfected into mouse TS-negative cells for complementation assays and cytotoxicity studies, and the mutant proteins expressed and characterized. The I108A mutant confers resistance to raltitrexed and Thymitaq with respective IC50 values 54- and 80-fold greater than wild-type but less resistance to BW1843U89 (6-fold). The F225W mutant displays resistance to BW1843U89 (17-fold increase in IC50 values), but no resistance to raltitrexed and Thymitaq. It also confers 8-fold resistance to fluorodeoxyuridine. Both the kinetic characterization of the altered enzymes and formation of antifolate-resistant colonies in mouse bone marrow cells that express mutant TS are in accord with the IC50 values for cytotoxicity noted above. The human TS mutants (I108A and F225W), by virtue of their desirable properties, including good catalytic function and resistance to antifolate TS inhibitors, confirm the importance of amino acid residues Ile-108 and Phe-225 in the binding of folate and its analogues. These novel mutants may be useful for gene transfer experiments to protect hematopoietic progenitor cells from the toxic effects of these drugs.