The influence of drug-exposure conditions on the development of resistance to methotrexate or ZD1694 in cultured human leukaemia cells

Acquired immune resistance is associated with interferon signature and modulation of KLF6/c-MYB transcription factors in myeloid leukemia

Resistance to immunity is associated with the selection of cancer cells with superior capacities to survive inflammatory reactions. Here, we tailored an ex vivo immune selection model for acute myeloid leukemia (AML) and isolated the residual subpopulations as "immune-experienced" AML (ieAML) cells. We confirmed that upon surviving the immune reactions, the malignant blasts frequently decelerated proliferation, displayed features of myeloid differentiation and activation, and lost immunogenicity. Transcriptomic analyses revealed a limited number of commonly altered pathways and differentially expressed genes in all ieAML cells derived from distinct parental cell lines. Molecular signatures predominantly associated with interferon and inflammatory cytokine signaling were enriched in the AML cells resisting the T-cell-mediated immune reactions. Moreover, the expression and nuclear localization of the transcription factors c-MYB and KLF6 were noted as the putative markers for immune resistance and identified in subpopulations of AML blasts in the patients' bone marrow aspirates. The immune modulatory capacities of ieAML cells lasted for a restricted period when the immune selection pressure was omitted. In conclusion, myeloid leukemia cells harbor subpopulations that can adapt to the harsh conditions established by immune reactions, and a previous "immune experience" is marked with IFN signature and may pave the way for susceptibility to immune intervention therapies.

The strategy and clinical relevance of in vitro models of MAP resistance in osteosarcoma: a systematic review

Over the last 40 years osteosarcoma (OS) survival has stagnated with patients commonly resistant to neoadjuvant MAP chemotherapy involving high dose methotrexate, adriamycin (doxorubicin) and platinum (cisplatin). Due to the rarity of OS, the generation of relevant cell models as tools for drug discovery is paramount to tackling this issue. Four literature databases were systematically searched using pre-determined search terms to identify MAP resistant OS cell lines and patients. Drug exposure strategies used to develop cell models of resistance and the impact of these on the differential expression of resistance associated genes, proteins and non-coding RNAs are reported. A comparison to clinical studies in relation to chemotherapy response, relapse and metastasis was then made. The search retrieved 1891 papers of which 52 were relevant. Commonly, cell lines were derived from Caucasian patients with epithelial or fibroblastic subtypes. The strategy for model development varied with most opting for continuous over pulsed chemotherapy exposure. A diverse resistance level was observed between models (2.2-338 fold) with 63% of models exceeding clinically reported resistance levels which may affect the expression of chemoresistance factors. In vitro p-glycoprotein overexpression is a key resistance mechanism; however, from the available literature to date this does not translate to innate resistance in patients. The selection of models with a lower fold resistance may better reflect the clinical situation. A comparison of standardised strategies in models and variants should be performed to determine their impact on resistance markers. Clinical studies are required to determine the impact of resistance markers identified in vitro in poor responders to MAP treatment, specifically with respect to innate and acquired resistance. A shift from seeking disputed and undruggable mechanisms to clinically relevant resistance mechanisms may identify key resistance markers that can be targeted for patient benefit after a 40-year wait.

Resistance to paclitaxel induces glycophenotype changes and mesenchymal-to-epithelial transition activation in the human prostate cancer cell line PC-3

The development of the multidrug resistance phenotype is one of the major challenges faced in the treatment of cancer. The multidrug resistance phenotype is characterized by cross-resistance to drugs with different chemical structures and mechanisms of action. In this work, we hypothesized that the acquisition of resistance in cancer is accompanied by activation of the epithelial-to-mesenchymal transition process, where the tumor cell acquires a more mobile and invasive phenotype; a fundamental step in tumor progression and in promoting the invasion of other organs and tissues. In addition, it is known that atypical glycosylations are characteristic of tumor cells, being used as biomarkers. We believe that the acquisition of the multidrug resistance phenotype and the activation of epithelial-to-mesenchymal transition provoke alterations in the cell glycophenotype, which can be used as glycomarkers for chemoresistance and epithelial-to-mesenchymal transition processes. Herein, we induced the multidrug resistance phenotype in the PC-3 human prostate adenocarcinoma line through the continuous treatment with the drug paclitaxel. Our results showed that the induced cell multidrug resistance phenotype (1) acquired a mixed profile between epithelial and mesenchymal phenotypes and (2) modified the glycophenotype, showing an increase in the level of sialylation and in the number of branched glycans. Both mechanisms are described as indicators of poor prognosis.

Analysis of Candidate Idarubicin Drug Resistance Genes in MOLT-3 Cells Using Exome Nuclear DNA

Various gene alterations related to acute leukemia are reported to be involved in drug resistance. We investigated idarubicin (IDR) resistance using exome nuclear DNA analyses of the human acute leukemia cell line MOLT-3 and the derived IDR-resistant cell line MOLT-3/IDR. We detected mutations in MOLT-3/IDR and MOLT-3 using both Genome Analysis Toolkit (GATK) and SnpEff program. We found 8839 genes with specific mutations in MOLT-3/IDR and 1162 genes with accompanying amino acid mutations. The 1162 genes were identified by exome analysis of polymerase-related genes using Kyoto Encyclopedia of Genes and Genomes (KEGG) and, among these, we identified genes with amino acid changes. In resistant strains, LIG and helicase plurality genes showed amino-acid-related changes. An amino acid mutation was also confirmed in polymerase-associated genes. Gene ontology (GO) enrichment testing was performed, and lipid-related genes were selected from the results. Fluorescent activated cell sorting (FACS) was used to determine whether IDR permeability was significantly different in MOLT-3/IDR and MOLT-3. The results showed that an IDR concentration of 0.5 μg/mL resulted in slow permeability in MOLT-3/IDR. This slow IDR permeability may be due to the effects of amino acid changes in polymerase- and lipid-associated genes.

Different administration strategies with paclitaxel induce distinct phenotypes of multidrug resistance in breast cancer cells

Both dose-dense and dose-escalation chemotherapy are administered in clinic. By approximately imitating the schedules of dose-dense and dose-escalation administration with paclitaxel, two novel multidrug resistant (MDR) cell lines Bads-200 and Bats-72 were successfully developed from drug-sensitive breast cancer cell line BCap37, respectively. Different from Bads-200, Bats-72 exhibited stable MDR and significantly enhanced migratory and invasive properties, indicating that they represented two different MDR phenotypes. Our results showed that distinct phenotypes of MDR could be induced by altered administration strategies with a same drug. Administrating paclitaxel in conventional dose-escalation schedule might induce recrudescent tumor cells with stable MDR and increased metastatic capacity.

Proteomic approaches for investigation of therapy resistance in cancer

Resistance to anticancer therapy is a major obstacle for successful management of patients in oncology. Although in the past, various biological mechanisms involved in therapy resistance, in particular multidrug resistance, have been identified, cancer patients did not really benefit. The mechanisms include the enhanced activity of drug extrusion pumps, modulation of cellular death pathways, alteration and repair of target molecules and various other mechanisms. Together they build a complex network mediating an individual therapy-resistant phenotype. The improved description of this multifactorial network should be useful for prediction of treatment response and would allow to design an individual-tailored therapy regiment. Proteome analyzing technologies appear as powerful tools for identifying new factors and protein expression profiles associated with anticancer therapy resistance. In the last years, the application of proteomic techniques identified multiple new factors or protein expression signatures in drug-resistant cell models and cancerous tissues. However, the functional role and the clinical impact of these findings are not yet clarified. So far, none of the proteomic data were useful for the development of improved diagnostic tests, for prediction of individual therapy response or for development of updated chemosensitizers. Here, the previous therapy resistance-related proteome data and future perspectives will be discussed.

Establishment of in-vitro models of chemotherapy resistance

Chemotherapy resistance is one of the most prevalent obstacles to the treatment of cancer, resulting in increased mortality and prolonged exposure to cytotoxic agents with no treatment benefit. One of the tools utilized in the study of mechanisms of chemotherapy resistance are established cell lines derived from human neoplasms. These cell lines can be challenged in vitro with controlled chemotherapy doses to produce chemotherapy-resistant variants. Analysis of these novel chemotherapy-resistant cell lines may then identify genetic and proteomic changes which are associated with the resistant phenotype. Two very important mediators of chemotherapy resistance (P-glycoprotein and multidrug resistance protein-1) were initially identified in chemotherapy-resistant cell lines. To make these in-vitro studies clinically relevant it is, however, necessary to duplicate as far as possible the treatment conditions used in vivo. Considerations should include clinically relevant drug concentrations, such as those derived from peak plasma values, and the type of treatment schedule to be employed.

Synthesis and biological evaluation of alpha- and gamma-carboxamide derivatives of 10-CF3CO-DDACTHF

Structurally-related, but non-polyglutamylatable, derivatives of 10-CF3CO-DDACTHF (1), which incorporate L-glutamine (2) and L-isoglutamine (3) in place of L-glutamate, were prepared and evaluated as inhibitors of recombinant human (rh) GAR Tfase. While the L-glutamate alpha-carboxamide derivative 3 was much less effective as a rhGAR Tfase inhibitor (K(i) = 4.8 microM) and inactive in cellular functional assays, the gamma-carboxamide derivative 2 was found to be a potent and selective rhGAR Tfase inhibitor (K(i) = 0.056 microM) being only 4-fold less potent than 1 (K(i) = 0.015 microM). Moreover, 2 was effective in cellular functional assays exhibiting purine sensitive cytotoxic activity (IC50 = 300 nM, CCRF-CEM) only 20-fold less potent than 1 (IC50 = 16 nM), consistent with inhibition of de novo purine biosynthesis via selective inhibition of GAR Tfase. Like 1, 2 is transported into the cell by the reduced folate carrier. Unlike 1, the functional activity of 2 is not dependent upon FPGS polyglutamylation.

Effects of impaired renal function on the pharmacokinetics and toxicity of i.v. ZD9331, a novel non-polyglutamated thymidylate synthase inhibitor, in adult patients with solid tumors

ZD9331 is a potent thymidylate synthase inhibitor. Renal and hepatic clearances were found to be important routes of elimination. The objectives of this pharmacologic trial were to investigate the effect of renal impairment on the pharmacokinetics of ZD9331, to study the toxicity profile and to document any antitumor effects of ZD9331 when administered i.v. to patients with different degrees of renal impairment. Patients were treated with ZD9331 130 mg/m2 given as an i.v. infusion on day 1 of a 4-week cycle to allow full pharmacokinetic assessment. Subsequent cycles involved the administration of ZD9331 on days 1 and 8, every 3 weeks. Patients were stratified according to their renal function assessed by the creatinine clearance: normal renal function (creatinine clearance > or =60 ml/min), mildly impaired renal function (creatinine clearance > or =40 to <60 ml/min) and moderately impaired renal function (creatinine clearance >25 to <40 ml/min). For pharmacokinetic analysis plasma sampling was performed during the first course and assayed using a validated liquid chromatographic tandem mass spectrometry assay. Twenty-three patients were entered on the study, of whom 21 received 130 mg/m2 ZD9331 in the first treatment cycle. No relationship was seen between renal impairment and plasma clearance nor with the area under the concentration-time curve of free ZD9331. Increasing renal impairment was associated with a greater incidence of myelosuppression. No predictive relationship between the clearance of free ZD9331 and the degree of renal impairment as determined by creatinine clearance could be assessed. However, data from this trial indicate that increased renal impairment may be associated with greater ZD9331-induced toxicity, particularly myelosuppression, although this cannot be attributed to any alteration in the plasma pharmacokinetics of ZD9331. Therefore, it may be necessary to administer a reduced dose of ZD9331 to patients with impaired renal function.

Phase I and pharmacologic study of oral ZD9331, a novel nonpolyglutamated thymidylate synthase inhibitor, in adult patients with solid tumors

PURPOSE

To assess the toxicity profile and dose-limiting toxicities (DLTs), to determine the maximum-tolerated dose, and to study the pharmacokinetics of ZD9331 when administered orally to patients with advanced solid tumors.

PATIENTS AND METHODS

Patients were treated with oral ZD9331 given once daily (od) or twice daily (bid) for 5, 7, or 10 days; cycles were repeated every 21 days at doses ranging from 2.5 to 40 mg. For pharmacokinetic analysis, plasma sampling was performed during the first course and assayed using a validated liquid chromatographic-tandem mass spectrometry assay. Plasma levels of 2'-deoxyuridine were measured as a surrogate marker for TS inhibition.

RESULTS

Forty-two patients received a total of 166 courses. The DLTs were myelosuppression and skin rash. Dose escalation of oral ZD9331 from 2.5 to 40 mg, as a single daily dose, resulted in a less than proportional increase in the plasma area under the concentration-time curve of ZD9331. The plasma drug exposure per cycle for the schedules 20 mg od for 5 days, 10 mg od for 10 days, and 10 mg bid for 5 days, all resulting in a total dose per cycle of 100 mg, were comparable. One partial response was noted in a patient with gastric cancer.

CONCLUSION

DLTs in this phase I study of oral ZD9331 were myelosuppression and skin toxicity. The recommended dose for phase II studies of oral ZD9331 is 20 mg od for 5 consecutive days, every 3 weeks.

Multiple mechanisms of resistance to methotrexate and novel antifolates in human CCRF-CEM leukemia cells and their implications for folate homeostasis

We determined the mechanisms of resistance of human CCRF-CEM leukemia cells to methotrexate (MTX) vs. those to six novel antifolates: the polyglutamatable thymidylate synthase (TS) inhibitors ZD1694, multitargeted antifolate, pemetrexed, ALIMTA (MTA) and GW1843U89, the non-polyglutamatable inhibitors of TS, ZD9331, and dihydrofolate reductase, PT523, as well as DDATHF, a polyglutamatable glycinamide ribonucleotide transformylase inhibitor. CEM cells were made resistant to these drugs by clinically relevant intermittent 24 hr exposures to 5-10 microM of MTX, ZD1694, GW1843U89, MTA and DDATHF, by intermittent 72 hr exposures to 5 microM of ZD9331 and by continuous exposure to stepwise increasing concentrations of ZD9331, GW1843U89 and PT523. Development of resistance required only 3 cycles of intermittent drug exposure to ZD1694 and MTA, but 5 cycles for MTX, DDATHF and GW1843U89 and 8 cycles for ZD9331. The predominant mechanism of resistance to ZD1694, MTA, MTX and DDATHF was impaired polyglutamylation due to approximately 10-fold decreased folylpolyglutamate synthetase activity. Resistance to intermittent exposures to GW1843U89 and ZD9331 was associated with a 2-fold decreased transport via the reduced folate carrier (RFC). The CEM cell lines resistant to intermittent exposures to MTX, ZD1694, MTA, DDATHF, GW1843U89 and ZD9331 displayed a depletion (up to 4-fold) of total intracellular reduced folate pools. Resistance to continuous exposure to ZD9331 was caused by a 14-fold increase in TS activity. CEM/GW70, selected by continuous exposure to GW1843U89 was 50-fold resistant to GW1843U89, whereas continuous exposure to PT523 generated CEM/PT523 cells that were highly resistant (1550-fold) to PT523. Both CEM/GW70 and CEM/PT523 displayed cross-resistance to several antifolates that depend on the RFC for cellular uptake, including MTX (95- and 530-fold). CEM/GW70 cells were characterized by a 12-fold decreased transport of [3H]MTX. Interestingly, however, CEM/GW70 cells displayed an enhanced transport of folic acid, consistent with the expression of a structurally altered RFC resulting in a 2.6-fold increase of intracellular folate pools. CEM/PT523 cells displayed a markedly impaired (100-fold) transport of [3H]MTX along with 12-fold decreased total folate pools. In conclusion, multifunctional mechanisms of resistance in CEM cells have a differential impact on cellular folate homeostasis: decreased polyglutamylation and transport defects lead to folate depletion, whereas a structurally altered RFC protein can provoke expanded intracellular folate pools.

Consolidation therapy for childhood acute lymphoblastic leukaemia: clinical and cellular pharmacology of cytosine arabinoside, epipodophyllotoxins and cyclophosphamide

The intensification of post-remission induction therapy has been shown to improve the relapse-free survival for childhood acute lymphoblastic leukaemia (ALL), and is now a standard component of the treatment of childhood acute lymphoblastic leukaemia. For cytosine arabinoside (ara-C), methotrexate, vincristine and corticosteroids, in-vitro studies indicate that the extracellular drug concentration and exposure time are important determinants of cytotoxicity for human leukaemia cell lines. For L-asparaginase, epipodopyllotoxins and cyclophosphamide, there have been few studies of the relationship between cellular pharmacology and cytotoxicity in relation to ALL. The clinical and cellular pharmacology of methotrexate and cytosine arabinoside have been studied in relation to childhood ALL in vivo. For these drugs, there is evidence to suggest that maintenance of plasma concentrations that are biochemically optimal is necessary to maximize anti-leukaemic effects. For cytosine arabinoside in particular, optimal extracellular fluid concentrations are not likely to be achieved or maintained by bolus or short-duration i.v. infusions. A potentially important example of this may be served by the success of antimetabolite-based intrathecal chemotherapy for CNS-directed treatment of childhood ALL. Intrathecal administration of both methotrexate and cytosine arabinoside results in prolonged leukaemic cell exposure to cytotoxic concentrations of the drug. For vincristine, anthracyclines and asparaginase, the actual dose intensity received by children during consolidation therapy may be important, and there is considerable interpatient variation in the pharmacokinetics of cyclophosphamide and teniposide in the therapy of childhood cancers. The importance of this relationship to childhood ALL is not known. The pharmacological and cellular pharmacological studies performed at St Jude Children's Research Hospital (Memphis, TN, USA) have allowed investigation of the relationships between the clinical and cellular pharmacology of methotrexate and prognosis, and have supported the individualization of consolidation therapy with this drug. Cytosine arabinoside has been less well studied in relation to childhood ALL, although evidence exists to suggest that the administration of conventional-dose bolus or infusion schedules may not be optimal in terms of the antileukaemic efficacy of this antimetabolite. For L-asparaginase, ongoing studies may allow the relationship between dose and schedule of administration to be related to pharmacodynamic measures such as asparagine depletion and prognosis. Therefore, through knowledge of clinical and cellular pharmacological properties, it may be possible to optimize the consolidation phase of therapy for childhood ALL, without disrupting the fundamental principles by which the overall treatment is administered. This may be particularly important for children with disease that has inherent or acquired resistance to therapy.

Continuing therapy for childhood acute lymphoblastic leukaemia: clinical and cellular pharmacology of methotrexate, 6-mercaptopurine and 6-thioguanine

Across the world, therapy with 6-mercaptopurine (6-MP) and methotrexate (MTX) forms the basis of the continuing therapy of childhood acute lymphoblastic leukaemia (ALL). In this review, the pharmacological determinants of the sensitivity of human leukaemia cell lines and lymphoblasts derived from children with ALL will be discussed. In addition, clinical pharmacological studies of 6-MP and MTX in relation to the continuing therapy with childhood ALL will be reviewed. For 6-MP in vitro, prolonged exposure times to relatively high extracellular drug concentrations are necessary for cytotoxicity, and these concentrations are much higher than those achieved during continuing therapy for childhood ALL. For MTX, plasma concentrations are achieved during continuing therapy that would be cytotoxic to human leukaemia cells during prolonged exposures in vitro. For both MTX and 6-MP, wide inter- and intrapatient variation in plasma pharmacokinetic parameters has been described. For 6-MP and MTX, cellular pharmacological studies have been largely restricted to erythrocytes as a surrogate of the possible effects in leukaemic blasts. Although measures of the pharmacology of 6-MP and MTX in erythrocytes has been related to prognosis in many studies, 6-MP systemic exposure and the dose intensity of 6-MP and MTX actually received by children during this phase of therapy seems to be the most important determinant of efficacy. Further studies will be needed to determine the importance of pharmacokinetic variability during continuing therapy as a determinant of outcome for children with ALL. In this respect, minimal residual disease status during this phase of treatment may prove to be a useful pharmacodynamic endpoint.

Cross-resistance in the 2',2'-difluorodeoxycytidine (gemcitabine)-resistant human ovarian cancer cell line AG6000 to standard and investigational drugs

Gemcitabine (2'-2'-difluorodeoxycytidine; dFdC) is a deoxycytidine analogue which is effective against solid tumours, including lung cancer and ovarian cancer. dFdC requires phosphorylation by deoxycytidine kinase (dCK) for activation. In the human ovarian cancer cell line A2780 and its 30,000-fold dFdC-resistant variant AG6000 (P<0.001), we investigated the cross-resistance profile to several drugs. AG6000, which has a complete dCK deficiency, was approximately 1000-10,000-fold resistant to other deoxynucleoside analogues such as 1-beta-D-arabinofuranosyl cytosine, 2-chloro-deoxyadenosine, aza-deoxycytidine and 2', 2'-difluorodeoxyguanosine (dFdG) (P<0.001). dFdG can be activated by dCK and deoxyguanosine kinase (dGK), but the latter enzyme was not altered in AG6000 cells. Thus dFdG resistance was only due to dCK deficiency. AG6000 was 1.6- and 46.7-fold resistant to 5-fluorouracil (5-FU) and ZD1694, respectively (the latter was significant; P<0.01), which may be due to the 1.7-fold higher thymidylate synthase (TS) activity, but AG6000 cells were also 2. 7-fold resistant to the lipophilic TS inhibitor AG337 (P<0.05). Remarkably, AG6000 cells were 2.5-fold more sensitive to methotrexate (MTX) (P<0.01) than A2780 cells, but 1.6-fold more resistant to trimetrexate (TMQ) (P<0.10). However, no differences in reduced folate carrier activity, folylpolyglutamate synthetase (FPGS) activity and polyglutamation of MTX were found between the cell lines. AG6000 cells were approximately 2 to 7.5-fold more resistant to doxorubicin (DOX), daunorubicin (DAU), epirubicin and vincristine (VCR) (the latter was significant; P<0.02) and approximately 4-fold more resistant to the microtubule inhibitors paclitaxel and docetaxel (P<0.001). Fluorescent activated cell sorter (FACS) analysis revealed no P-glycoprotein (Pgp) or multidrug resistance-associated protein (MRP) expression, but less fluorescence of intercalated DAU in AG6000 cells. An approximately 2-fold resistance to the topoisomerase I and II inhibitors etoposide, CPT-11 and SN38 was found in AG6000 cells. Topoisomerase I and IIalpha RNA expression was decreased in AG6000 cells. AG6000 was 2.4, 2.4, 2.3 and 3.7-fold more resistant to EO9 (P<0.02), mitomycin-C (MMC) (P<0.05), cisplatin (CDDP) (P<0.10) and maphosphamide (MAPH), respectively. DT-diaphorase (DTD), which activates EO9, was 2.2-fold lower in AG6000 cells. CDDP resistance might be related to a reduced retention of DNA adducts in AG6000. However, glutathione levels were equal in A2780 and AG6000 cells. A 24 h exposure to DOX, VCR and paclitaxel at equimolar and equitoxic concentrations, resulted in more double-strand breaks (1.5- to 2-fold) in A2780 than in AG6000 cells. MAPH at 1120 nM and 17 nM of EO9 did not cause DNA damage in either cell line. In conclusion, AG6000 is a cell line highly cross-resistant to a wide variety of drugs. This cross-resistance might be related to altered enzyme activities and/or increased DNA repair.

Combined therapeutic efficacy of the thymidylate synthase inhibitor ZD1694 (Tomudex) and the immunotoxin B43(anti-CD19)-PAP in a SCID mouse model of human B-lineage acute lymphoblastic leukemia

The quinazoline antifolate N-(5-[N-(3,4-dihydro-2-methyl-4-oxoquinazolin-6-ylmethyl)-N- methylamino]-2-thenoyl)-L-glutamic acid (ZD1694; Tomudex) is a potent inhibitor of thymidylate synthase and causes cell death through disruption of DNA synthesis and repair by blocking the obligatory thymidine nucleotide synthesis. B43(anti-CD19)-PAP immunotoxin is a potent inhibitor of protein synthesis in CD19+ B-lineage acute lymphoblastic leukemia (ALL) cells and causes apoptosis. In this model, 100% of SCID mice challenged with 1 x 10(6) human NALM-6 B-lineage ALL cells develop overt and invariably fatal leukemia. All of the 22 control SCID mice treated with phosphate-buffered saline died of disseminated human leukemia between 31 and 61 days with a median survival of 41.2 days. Treatment with ZD 1694 resulted in improved leukemia-free survival with a median survival of 69.2 days (P < 0.001, log-rank test). B43-PAP treatment was more effective than ZD1694 (P=0.026) and resulted in 51.0% long-term leukemia-free survival with a median survival of 187.5 days (P < 0.0001. log-rank test). The combination of ZD1694 and B43-PAP was more effective than either agent alone and resulted in 100% long-term leukemia-free survival. To our knowledge, this preclinical study is the first to demonstrate the feasibility and therapeutic advantage of combining an anti-leukemia immunotoxin with a thymidylate synthase inhibitor.

Cytotoxicity of trimetrexate against antifolate-resistant human T-cell leukemia cell lines developed in oxidized or reduced folate

Cytotoxicity of trimetrexate (TMQ), a lipophilic dihydrofolate reductase inhibitor, was examined in antifolate-resistant human T-cell leukemia cell lines developed in oxidized or reduced folate. An approximately 60-fold methotrexate (MTX)-resistant subline was developed in oxidized folate (pteroylglutamic acid: PGA) (CCRF-CEM/MTX60-PGA) from human T-cell leukemia cell line CCRF-CEM; this line exhibited impaired membrane transport of the drug. Further enhancement of MTX resistance resulted in selection of an approximately 5000-fold MTX-resistant subline (CCRF-CEM/ MTX5000-PGA), which showed increased dihydrofolate reductase activity due to gene amplification in addition to further impairment of MTX transport. An approximately 140-fold MTX-resistant subline, and then a 1500-fold MTX-resistant subline were developed in reduced folate (10 nM leucovorin) (CCRF-CEM/MTX140-LV and CCRF-CEM/MTX1500-LV); they exhibited increased dihydrofolate reductase due to gene amplification accompanied by increased intracellular drug accumulation of MTX. While CCRF-CEM/MTX140-LV and CCRF-CEM/MTX1500-LV cells showed cross-resistance to TMQ, CCRF-CEM/MTX60-PGA and CCRF-CEM/MTX5000-PGA cells were at least as sensitive to TMQ as the parent cells. TMQ was more potent against approximately 200-fold N10-propargyl-5,8-dideazafolic-acid (CB3717)-resistant human T-cell leukemia MOLT-3 sublines developed in PGA (MOLT-3/CB3717(200)-PGA) or leucovorin (MOLT-3/CB3717(200)-LV), as compared to the parent cells; MOLT-3/CB3717(200)-PGA and MOLT-3/CB3717(200)-LV cells were resistant to CB3717 by virtue of impaired transport, only the former possessing gene amplification of thymidylate synthase. The cytotoxicity of TMQ in both MOLT-3/CB3717(200)-PGA and MOLT-3/CB3717(200)-LV cells was reduced by addition of leucovorin in a dose-dependent manner, suggesting intracellular folate deficiency as a cause of TMQ sensitivity. These results demonstrate that TMQ overcomes transport-impaired antifolate resistance, irrespective of gene amplification of dihydrofolate reductase or thymidylate synthase. Types of folate used during the development of antifolate resistance seem to be important in relation to the mechanism of TMQ responsiveness as well as that of antifolate resistance.

Tomudex (ZD1694): from concept to care, a programme in rational drug discovery

Folate-based anticancer drugs with specificity for thymidylate synthase (TS) have come of age. Ideas nurtured in the early 1970s led to the first-generation of antifolates with TS and dihydrofolate reductase (DHFR) inhibitory activities. Compounds with increased selectivity for TS followed with the highly specific inhibitor, CB3717 being synthesised in 1979 at the Institute of Cancer Research (ICR). CB3717 had significant clinical activity but its development had to be abandoned because its low aqueous solubility led to occasional nephrotoxicity. Collaborative laboratory studies between the ICR and ICI Pharmaceuticals (later to become Zeneca Pharmaceuticals) led to the discovery of ZD1694 (Tomudex), the first antifolate to be licensed for the treatment of cancer (UK 1995) in nearly 40 years and the first new drug for colorectal cancer in about 35 years. Tomudex belongs to a class of compounds that use the reduced-folate carrier (RFC) for uptake into cells and which are excellent substrates for folylpolyglutamate synthetase (FPGS). This paper reviews the underlying philosophies, and the milestones reached during the development of Tomudex.

Biological activity and intracellular metabolism of ZD1694 in human leukemia cell lines with different resistance mechanisms to antifolate drugs

The biological activity and cellular metabolism of ZD1694, a novel folate-based thymidylate synthase (TS) inhibitor, were analyzed in a human leukemia cell line, MOLT-3, and its antifolate-resistant sublines with different mechanisms of resistance to methotrexate (MTX), trimetrexate (TMQ) and N10-propargyl-5,8-dideazafolic acid (CB3717). MOLT-3/CB3717(40), which was selected for CB3717 resistance, demonstrated impaired membrane drug transport via reduced folate carrier (RFC) and lower accumulation of [3H]ZD1694-polyglutamates in the cells with a shift in the polyglutamate distribution profile to shorter chain length polyglutamates, indicating an alteration in polyglutamation capacity in this subline. Impaired RFC and reduced rate of polyglutamation could explain the cross-resistance (12-fold) of this subline to ZD1694. On the other hand, there was little or no cross-resistance to this drug in a subline (MOLT-3/TMQ800) reportedly resistant to TMQ through impaired membrane transport for TMQ and an increase in dihydrofolate reductase (DHFR) activity. Total amount of ZD1694 polyglutamated to a level higher than diglutamate was approximately 1.7-fold higher in the TMQ-resistant cells than that in the parent cells, but a low degree of increase in TS activity in the cells counteracted the supposed increase in sensitivity to ZD1694. MOLT-3/TMQ800-MTX10000 cells, which were established by sequential exposure of the TMQ-resistant cells to MTX and were previously shown to amplify mutated DHFR with low affinity for MTX, showed a decreased accumulation of polyglutamated ZD1694 as compared with the parent line and this was consistent with cross-resistance to ZD1694 in this subline. Overproduction of variant DHFR scarcely influenced the sensitivity to this drug. These results indicate that ZD1694 could overcome antifolate resistance through a mechanism such as amplified DHFR activity, and the biological activity of this drug against the cells paralleled the amount of polyglutamated drug inside the cells. Determination of polyglutamation capacity in tumor cells may allow prediction of sensitivity to this drug.