Epithelial Transfer of the Tyrosine Kinase Inhibitors Erlotinib, Gefitinib, Afatinib, Crizotinib, Sorafenib, Sunitinib, and Dasatinib: Implications for Clinical Resistance

Simple Summary

Tyrosine kinase inhibitors (TKIs) specifically inhibit phosphorylation of signaling pathways of cancer cells, thereby inhibiting their growth. They are characterized by a poor solubility and high protein binding, leading to a large variability in gut uptake after oral administration and variation in the clinical efficacy. We used the CaCo2 gut epithelial model to characterize the gut absorption of 7 TKIs and observed a large variation in apical/basolateral (mimicking gut/blood) transfer, with 4 TKIs showing a negative and 3 a neutral transfer. A highly negative transfer may lead to pharmacokinetic resistance. Intracellular uptake of TKIs was high for sunitinib and crizotinib, intermediate for gefitinib, dasatinib and sorafenib, low for afatinib and not detectable for erlotinib. These properties may explain a high red blood cell to plasma ratio for most TKIs investigated. Although TKIs are poorly absorbed the latter property may compensate for this.

Abstract

Background: tyrosine kinase inhibitors (TKIs) inhibit phosphorylation of signaling proteins. TKIs often show large variations in the clinic due to poor pharmacology, possibly leading to resistance. We compared gut absorption of inhibitors of epidermal growth factor receptor (erlotinib, gefitinib, and afatinib), ALK-cMET (crizotinib), PDGFR/BCR-Abl (dasatinib), and multikinase inhibitors (sunitinib and sorafenib). In clinical samples, we measured the disposition of each compound within various blood compartments. Methods: we used an optimized CaCo2 gut epithelial model to characterize 20 µM TKI absorption. The apical/basolateral transfer is considered to represent the gut/blood transfer. Drugs were measured using LC-MS/MS. Results: sorafenib and sunitinib showed the highest apical/basolateral transfer (Papp 14.1 and 7.7 × 10⁻⁶ cm/s, respectively), followed by dasatinib (3.4), afatinib (1.5), gefitinib (0.38), erlotinib (0.13), and crizotinib (n.d.). However, the net absorptions for dasatinib, afatinib, crizotinib, and erlotinib were highly negative (efflux ratios >5) or neutral/negative, sorafenib (0.86), gefitinib (1.0), and sunitinib (1.6). A high negative absorption may result in resistance because of a poor exposure of tissues to the drug. Accumulation of the TKIs at the end of the transfer period (A->B) was not detectable for erlotinib, very low for afatinib 0.45 pmol/μg protein), followed by gefitinib (0.79), dasatinib (1.1), sorafenib (1.65), and crizotinib (2.11), being highest for sunitinib (11.9). A similar pattern was found for accumulation of these drugs in other colon cell lines, WiDr and HT29. In clinical samples, drugs accumulated consistently in red blood cells; blood to plasma ratios were all >3 (sorafenib) or over 30 for erlotinib. Conclusions: TKIs are consistently poorly absorbed, but accumulation in red blood cells seems to compensate for this.

Schedule-Dependent Synergy Between the Histone Deacetylase Inhibitor Belinostat and the Dihydrofolate Reductase Inhibitor Pralatrexate in T-and B-cell Lymphoma Cells in vitro

Pralatrexate (Folotyn; PLX) and belinostat (Beleodaq; BLS) are registered for the treatment of patients with peripheral T-cell lymphoma (PTCL) and are being considered for other lymphomas. In this study we investigated whether BLS had the ability to potentiate the cytotoxicity of PLX. A panel of lymphoma cell lines was used for the combination studies: the B-cell SUDHL-4, SUDHL-5, HT, Jeko-1 and T-cell Karpas-299 and Hut-78. Uptake of PLX was mediated by the reduced folate carrier (RFC). PLX showed a 6-fold better RFC substrate affinity compared to methotrexate, and 2-fold better than levoleucovorin (l-LV). Sensitivity expressed as the concentration that resulted in 50% growth inhibition (IC50) after 72 hr exposure to PLX varied from 2.8 to 20 nM and for BLS from 72 to 233 nM, independent of the background of the cell lines. The interaction between BLS and PLX was studied using the median-drug effect analysis. At a fixed molar ratio between the drugs based on the IC50 concentration the average combination index (CI) for all cell lines showed additivity (CI: around 1.0). In three selected cell lines (SUDHL-4, SUDHL-5, and HT) sequential exposure (24 h pretreatment with BLS, followed by 48 h to PLX + BLS), did not improve interaction (CI: 0.9–1.4). As an alternative approach a non-fixed ratio was used by exposing SUDHL-4, SUDHL-5, and HT cells to IC25 concentrations of either BLS or PLX in combination with the other drug. Exposure to IC25 of PLX did not decrease the IC50 for BLS (CI from 0.6–1.2), but exposure to IC25 of BLS markedly increased PLX sensitivity (low CIs from 0.40 to 0.66). Mechanistic studies focused on induction of apoptosis, and showed cleavage of predominantly caspase-9 in HT and SUDHL-4 cells for both drugs at their IC50s, being similar in the combination setting. Moreover, at these concentrations, the drugs were shown to confer an S-phase arrest. In conclusion, the combination of PLX and BLS showed additivity in various lymphoma cell lines, with a schedule-dependent synergism in B-cell lymphoma. Based on these data, proficient inhibition of HDAC activity by BLS holds promise in sensitization of tumor cells to PLX.

RX-3117 (Fluorocyclopentenyl-Cytosine)-Mediated Down-Regulation of DNA Methyltransferase 1 Leads to Protein Expression of Tumor-Suppressor Genes and Increased Functionality of the Proton-Coupled Folate Carrier

(1) Background: RX-3117 (fluorocyclopentenyl-cytosine) is a cytidine analog that inhibits DNA methyltransferase 1 (DNMT1). We investigated the mechanism and potential of RX-3117 as a demethylating agent in several in vitro models. (2) Methods: we used western blotting to measure expression of several proteins known to be down-regulated by DNA methylation: O⁶-methylguanine-DNA methyltransferase (MGMT) and the tumor-suppressor genes, p16 and E-cadherin. Transport of methotrexate (MTX) mediated by the proton-coupled folate transporter (PCFT) was used as a functional assay. (3) Results: RX-3117 treatment decreased total DNA-cytosine-methylation in A549 non-small cell lung cancer (NSCLC) cells, and induced protein expression of MGMT, p16 and E-cadherin in A549 and SW1573 NSCLC cells. Leukemic CCRF-CEM cells and the MTX-resistant variant (CEM/MTX, with a deficient reduced folate carrier) have a very low expression of PCFT due to promoter hypermethylation. In CEM/MTX cells, pre-treatment with RX-3117 increased PCFT-mediated MTX uptake 8-fold, and in CEM cells 4-fold. With the reference hypomethylating agent 5-aza-2′-deoxycytidine similar values were obtained. RX-3117 also increased PCFT gene expression and PCFT protein. (4) Conclusion: RX-3117 down-regulates DNMT1, leading to hypomethylation of DNA. From the increased protein expression of tumor-suppressor genes and functional activation of PCFT, we concluded that RX-3117 might have induced hypomethylation of the promotor.

Crizotinib sensitizes the erlotinib resistant HCC827GR5 cell line by influencing lysosomal function

In non-small cell lung cancer, sensitizing mutations in epidermal growth factor receptor (EGFR) or cMET amplification serve as good biomarkers for targeted therapies against EGFR or cMET, respectively. Here we aimed to determine how this different genetic background would affect the interaction between the EGFR-inhibitor erlotinib and the cMET-inhibitor crizotinib. To unravel the mechanism of synergy we investigated the effect of the drugs on various parameters, including cell cycle arrest, migration, protein phosphorylation, kinase activity, the expression of drug efflux pumps, intracellular drug concentrations, and live-cell microscopy. We observed additive effects in EBC-1, H1975, and HCC827, and a strong synergism in the HCC827GR5 cell line. This cell line is a clone of the HCC827 cells that harbor an EGFR exon 19 deletion and has been made resistant to the EGFR-inhibitor gefitinib, resulting in cMET amplification. Remarkably, the intracellular concentration of crizotinib was significantly higher in HCC827GR5 compared to the parental HCC827 cell line. Furthermore, live-cell microscopy with a pH-sensitive probe showed a differential reaction of the pH in the cytoplasm and the lysosomes after drug treatment in the HCC827GR5 in comparison with the HCC827 cells. This change in pH could influence the process of lysosomal sequestration of drugs. These results led us to the conclusion that lysosomal sequestration is involved in the strong synergistic reaction of the HCC827GR5 cell line to crizotinib-erlotinib combination. This finding warrants future clinical studies to evaluate whether genetic background and lysosomal sequestration could guide tailored therapeutic interventions.

Subcellular localization of several structurally different tyrosine kinase inhibitors

Protein tyrosine kinases form an important target for a new class of anticancer drugs, the tyrosine kinase inhibitors (TKIs). Recently we demonstrated that sunitinib, an inhibitor of the membrane-associated vascular endothelial growth factor receptor (VEGFR), is trapped in lysosomes which isolates the drug from its intended target. Therefore we investigated whether this also holds for other TKIs, targeted against different protein kinases. For this purpose we used the ProteoExtractR kit, which enables a subcellular extraction separating cellular proteins into four distinct fractions covering the cytosol, membranes and membrane organelles (including lysosomes), nuclear proteins and the cytoskeleton. Since TKIs are 98-100 % protein bound we used this property to study their subcellular distribution and used Caco-2 cells as a model. As expected after 2 hours exposure sunitinib was trapped in cytosol (58 %) and organelles (42 % including lysosomes). Crizotinib, an inhibitor of ALK-EML4, showed a similar distribution. However, erlotinib, an inhibitor of the epidermal growth factor receptor (EGFR) showed a very low cellular accumulation and was limited to the organelle fraction. In contrast, the other EGFR inhibitor, gefitinib was predominantly located in the cytosolic (39 %) and membrane fraction (44 %). Sorafenib, another VEGFR inhibitor was predominantly located in the organelle fraction (85 %) and cytosol (15 %) after 2 hours, while after 24 hours distribution decreased (9.9 fold) with a slight shift. Dasatinib, an inhibitor of BCR-Abl was located only in the cytosol (100 %). In general localization after 24 hours was comparable, albeit several small changes were seen. In conclusion protein fractionation with the ProteoExtractR Subcellular Proteome Extraction kit demonstrated large differences in TKI levels in various cellular organelles, with a pattern in agreement with lysosomal accumulation of sunitinib.

Sensitive liquid chromatography mass spectrometry (LC-MS) assay reveals novel insights on DNA methylation and incorporation of gemcitabine, its metabolite difluorodeoxyuridine, deoxyuridine, and RX-3117 into DNA

Antimetabolites are incorporated into DNA and RNA, affecting their function. Liquid-chromatography-mass-spectrometry (LC-MS-MS) permits the sensitive, selective analysis of normal nucleosides. The method was adapted to measure the incorporation of deoxyuridine, gemcitabine (difluorodeoxycytidine), its metabolite difluorodeoxyuridine (dFdU), and the novel compound fluorocyclopentenylcytosine (RX3117). DNA was degraded to its deoxynucleotides for quantification by LC-MS-MS, gradient chromatography on a Phenomenex prodigy-3-ODS with positive ionization. The range of deoxyuridine DNA-mis-incorporation varied nine-fold in 27 cell lines (leukemia, colon, ovarian, lung cancer). At low-folate conditions a 2.1-fold increase in deoxyuridine was observed. Global methylation (given as % 5-methyl-deoxycytidine) was comparable between the cell lines (4.6-6.5%). Exposure of A2780 cells to 1 μM gemcitabine (4 hours) resulted in 3.6 pmol gemcitabine/μg DNA, but in AG6000 cells (deoxycytidine-kinase-deficient) no incorporation was found. However, when A2780, AG6000, or CCRF-CEM cells were exposed to 100 μM dFdU we found it as gemcitabine, 20.5, 19.6, and 0.51 pmol gemcitabine/μg DNA, respectively. Preincubation of CCRF-CEM cells with cyclopentenyl-cytosine (a CTP-synthetase inhibitor) increased dFdU incorporation four-fold. Apparently dFdU is activated independently of deoxycytidine-kinase and possibly converted in-situ to dFdCMP. RX3117 was incorporated into both DNA and RNA (0.0037 and 0.00515 pmol/μg, respectively). In summary, a sensitive method to quantify the incorporation of gemcitabine, deoxyuridine, and RX-3117 was developed, which revealed that dFdU was incorporated into DNA as the parent compound gemcitabine.

Transport of six tyrosine kinase inhibitors: active or passive?

<p class="ADMETabstracttext">Transport of erlotinib, gefitinib, sorafenib, sunitinib, dasatinib and crizotinib can be active or passive, which was studied by measuring uptake  at low (4 °C; passive) and normal temperature (37 °C; active and passive) and by the use of specific organic cation transporter (OCT) inhibitors. Intracellular accumulation was determined using Caco-2 as monolayers, while for gut permeation we used differentiated Caco-2 as model for intestinal epithelium in the Transwell system. Sorafenib and crizotinib uptake are likely to be dependent on passive transport. Gefitinib, dasatinib and sunitinib uptake seem to be active. Erlotinib’s transport also seems to be active. This study suggests that hOCTs might be involved in the apical to basolateral transport of gefitinib and crizotinib. Overall it can be concluded that the accumulation and transport of these six TKIs are very different, despite the fact that they are all tyrosine kinase inhibitors.</p>

Nitric oxide releasing acridone carboxamide derivatives as reverters of doxorubicin resistance in MCF7/Dx cancer cells

A series of nitric oxide donating acridone derivatives are synthesized and evaluated for in vitro cytotoxic activity against different sensitive and resistant cancer cell lines MCF7/Wt, MCF7/Mr (BCRP overexpression) and MCF7/Dx (P-gp expression). The results showed that NO-donating acridones are potent against both the sensitive and resistant cells. Structure activity relationship indicate that the nitric oxide donating moiety connected through a butyl chain at N(10) position as well as morpholino moiety linkage through an amide bridge on the acridone ring system at C-2 position, are required to exert a good cytotoxic effect. Further, good correlations were observed when cytotoxic properties were compared with in vitro nitric oxide release rate, nitric oxide donating group potentiated the cytotoxic effect of the acridone derivatives. Exogenous release of nitric oxide by NO donating acridones enhanced the accumulation of doxorubicin in MCF7/Dx cell lines when it was coadministered with doxorubicin, which inhibited the efflux process of doxorubicin. In summary, a nitric oxide donating group can potentiate the anti-MDR property of acridones.

Folates provoke cellular efflux and drug resistance of substrates of the multidrug resistance protein 1 (MRP1)

Cellular folate concentration was earlier reported to be a critical factor in the activity and expression of the multidrug resistance protein MRP1 (ABCC1). Since MRP1 mediates resistance to a variety of therapeutic drugs, we investigated whether the cellular folate concentration influences the MRP1-mediated cellular resistance against drugs. As a model system, we used the human ovarian carcinoma cell line 2008wt, and its stably MRP1/ABCC1-transfected subline 2008/MRP1. These cell types have a moderate and high expression of MRP1, respectively. In folate-deprived 2008/MRP1 cells, the MRP1-mediated efflux of its model substrate calcein decreased to ~55 % of the initial efflux rate under folate-rich conditions. In 2008wt cells, only a small decrease in efflux was observed. Folate depletion for 5-10 days markedly increased (~500 %) cellular steady-state accumulation of calcein in 2008/MRP1 cells and moderately in 2008wt cells. A subsequent short (24 h) exposure to 2.3 μM L-leucovorin decreased calcein levels again in MRP1-overexpressing cells. Folate deprivation markedly increased growth inhibitory effects of the established MRP1 substrates daunorubicin (~twofold), doxorubicin (~fivefold), and methotrexate (~83-fold) in MRP1-overexpressing cells, proportional to MRP1 expression. In conclusion, this study demonstrates that increased cellular folate concentrations induce MRP1/ABCC1-related drug efflux and drug resistance. These results have important implications in the understanding of the role of MRP1 and its homologs in clinical drug resistance.

Abstract C82: Role of influx and efflux transporters on gut absorption of selected tyrosine kinase inhibitors in a polarized gut epithelium model system

Tyrosine kinase inhibitors (TKI) are a family of small molecules which inhibit the ATP driven phosphorylation of signaling proteins that normally activate transduction cascades. Aberrations in signal cascades have been linked to the development of tumors and their survival pathways. TKIs tend to be alkali in nature with a correspondingly high pKa (5-8) while absorption properties have been linked to both ABC and Organic Cation Transporters (OCT). Because of their poor solubility, TKI are administered orally but available pharmacokinetic data indicate that in most cases bioavailability is relatively low (∼60% or lower). Despite a molecular weight in a similar range, doses vary significantly: Sunitinib (Sun) - 50 mg/d, Dasatinib (Das) - 150 mg/d, Erlotinib (Erl) - 150 mg/d, Gefitinib (Gef) - 250 mg/d, Crizotinib (Cri) - 250 mg BID and Sorafenib (Sor) - 400 mg BID. Steady state plasma levels vary from 0.13 (Sun), 0.21 (Das), 0.29 (Gef), 0.7 (Cri), 2.54 (Erl) to 12.1 µM (Sor). Variations in intestinal absorption may seriously affect plasma concentrations, tumour exposure and antitumor effect. To investigate the mechanisms behind these differences a well-established model for intestinal transport was used: the human colon cell line, CaCo2, when grown in special coated transwell plates forms a confluent differentiated polarised monolayer resembling gut epithelium. This model was used to determine the permeability of Gef, Erl, Sun, Cri, Sor, and Das, using LC-MS-MS to determine drug concentrations. Absorption from the gut given as the transfer rates from Apical to Basolateral (A/B) sides using 20 µM TKI at the apical side was determined over a 3 hour period. Transfer was linear in this period. Transfer rates varied from about 30 for Cri, 43 for Sun, 209 for Das, 180 for Gef, 223 for Sor, to 479 pmol/hr for Erl. In order to determine the role of ABC pumps, we depleted ATP with azide, which partially reduced transfer of Gef, Sun and Sor, but did not affect Erl. Ko143, a specific ABCG2 inhibitor decreased transfer of Gef and Sor but unexpectedly increased Sun. Remarkably, permeability transfer rates from the basolateral (blood) side to gut (B/A) were much higher (252 for Gef, 621 for Sor, 685 for Sun, 1623 for Erl and 4630 pmol/hr for Das) than A/B transfer. Verapamil, an inhibitor of P-glycoprotein and to some extend OCT1 increased A/B transfer of Sor and Das, but did not affect the transfer of the other compounds. Desipramine, a general OCT inhibitor, did not affect transfer of Erl, increased that of Gef and Cri about 2-fold and that of Sun 1.5-fold, but hardly affected that of Sor and Das. In conclusion, absorption of TKI from the apical side (gut epithelium) is relatively poor, while there was a relatively high negative flow (B/A); this is in line with the low bioavailability of most TKI. Some ABC transporters and OCTs play a role in the absorption, in line with their substrate specificity.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C82.

Citation Format: Richard J. Honeywell, Christien Fatmawati, Marita Boeddha, Sarina Hitzerd, Ietje Kathmann, Elisa Giovannetti, Godefridus J. Peters. Role of influx and efflux transporters on gut absorption of selected tyrosine kinase inhibitors in a polarized gut epithelium model system. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C82.

Folate homeostasis of cancer cells affects sensitivity to not only antifolates but also other non-folate drugs: effect of MRP expression

Sensitivity to antifolates can be decreased by endogenous or exogenous folates. Leucovorin protects cancer patients against toxicity of the dihydrofolate reductase inhibitor methotrexate (MTX), while folic acid is used to protect rheumatoid arthritis patients against MTX. Folates and antifolates can be effluxed from the cell by ABC transporters multidrug resistance protein 1 (MRP1), 2 and 3. We previously demonstrated in 2008 ovarian cancer cells that MRP overexpression reduced cellular folate content by 40%, while folate depletion increased expression of MRP1. As MRPs mediate resistance to several unrelated drugs, we investigated whether folate status would affect sensitivity to doxorubicin, daunorubicin, etoposide and vincristine. Ovarian cancer 2008 cells and its MRP1 transfected variant (2008/MRP1) were adapted from normal folate medium [2.3 μM; high folate (HF) cells] to short-term folate depletion (up to 7 days) (low folate cells); drugs were added after 2 days and sensitivity was tested by the MTT test after 3 additional days. The effect on folate homeostasis was evaluated by measurement of intracellular homocysteine using high-performance liquid chromatography and glutathione using a kit. MRP expression of wild-type (WT) 2008 cells did not increase homocysteine pools in 2008/MRP1 cells. Three day folate depletion increased homocysteine pools 23-fold in 2008 cells and 8.6-fold in the MRP variant. Folate depletion increased glutathione 20%–40% in 2008/WT and 2008/MRP1. In 2008 HF cells MRP1 expression did not affect sensitivity to MTX, but induced 4- to 10-fold resistance to doxorubicin, daunorubicin, etoposide and vincristine. Folate depletion decreased 50% growth inhibition (IC50) for MTX in both 2008 variants 25- to 4-fold, but that to doxorubicin and daunorubicin approximately 2-fold. Sensitivity to etoposide and vincristine was not affected. In conclusion, folate depletion markedly increased homocysteine, but moderately increased glutathione. Folate depletion increased MTX sensitivity, but effects on other drugs were most pronounced in WT cells, probably because MRP expression is already high in transfected variants.

Abstract 4425: Characterisation of cellular transport and accumulation of six clinically approved tyrosine kinase inhibitors (TKIs) in colon cancer cells

TKIs are a loosely characterised family of small molecules which inhibit the ATP driven phosphorylation of signalling proteins that normally activate transduction cascades. Aberrations in signal cascades have been linked to the development of tumours and their survival pathways; TKIs are targeted at these pathways. However, promising preclinical studies on structurally different TKIs often did not translate into clinical success due to cellular resistance, cascade mutations and poor bioavailability. TKIs tend to be alkali in nature with a correspondingly high pKa (5-8) while absorption properties have been linked to both ABC and Organic Cation Transporters (OCT). Previously we demonstrated in a gut epithelium model that absorption of TKIs is relatively poor, while a correspondingly high negative flow was observed. In this study we aimed to characterize tumour cell uptake and accumulation of TKIs using specific transporter inhibitors. We used a sensitive and specific LC-MS-MS method to characterize the role of the transport inhibitors Ko143 (BCRP inhibitor), amantadine (Am;hOCT1&2), cimetidine (Ci;hOCT1&2&3), desipramine (Ds;hOCT1&2&3), β-estradiol (Be; hOCT1&3) and verapamil (Ve; hOCT1&P-gp).

Uptake of TKIs at 4°C and 37°C in CaCo2 and WiDr colon cancer cells showed large differences; uptake of Gefitinib (G - 10μM), Erlotinib (E - 10 μM) & Dasatinib (D - 1μM) was partially active since after 2hr uptake at 4°C was lower compared to 37°C. Uptake of Sunitinib (Su - 10μM), Sorafenib (So - 10 μM) and Crizotinib (Cr - 10 μM) was predominantly passive since values were comparable at 4°C and 37°C. Absolute accumulation over 24hr (37°C) was very different with E and D reaching 20-150 ng/mg protein, while G was 20 fold higher. Su, So and Cr accumulation were even higher reaching 2 to 12 μg/mg protein. This pattern matched accumulation of some TKIs observed in the clinical setting. Accumulation for Su and Cr showed a similar pattern suggesting that Cr accumulated in lysosomes similar to Su. Bafilomycin, a lysosomal inhibitor, decreased total accumulation of Su 14.1-fold, and that of Cr 2.4-fold indicating lysosomal accumulation. The hOCT inhibitor Ds consistently (25-55%) decreased accumulation of G, So Su and Cr; Ci, Be and Am had little effect on cellular accumulation for all TKIs tested. Ve, De and Ci increased accumulation of E, and Ve that of D, suggesting inhibition of efflux via P-gp, MRP or BCRP. Ko143 increased accumulation for G, E, So and Su in WiDr but only for Su in CaCo2 cells indicating a differential role for BCRP depending on TKI and tumour type. In conclusion, several transporters appear to be involved to differing degrees for each molecule, indicating TKIs pharmacokinetics should be investigated on an individual basis.

Citation Format: Richard J. Honeywell, Sarina Hitzerd, Ietje Kathmann, Elisa Giovannetti, Henk Verheul, Frits Peters. Characterisation of cellular transport and accumulation of six clinically approved tyrosine kinase inhibitors (TKIs) in colon cancer cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4425. doi:10.1158/1538-7445.AM2013-4425

Paraparese bei einer Zwergziege: Abklärung mittels Magnetresonanztomographie

A 4-year old pygmy goat with chronic paraparesis of the hindlimbs was referred to the Ruminant Clinic of the University of Berne. The causative lesion was localized to the thoracolumbar spinal cord after a thorough clinical examination. Because a radiographic examination of the spine had not been diagnostic, magnetic resonance imaging (MRI) was performed. A mass compressing the spinal cord in the region of L2-L5 was detected. The goat was euthanized and autopsied, which allowed for the definitive diagnosis of lymphosarcoma. In addition to the changes in the lumbar area, further neoplastic masses were detected in the region of the thoracic vertebrae, near the thoracic aperture, on the lungs and on the pericardium. However, these processes had not yet caused clinical signs. MRI investigation allowed for the ante mortem diagnosis of an infiltrative mass in the spinal canal of this goat.

Abstract 1770: Metabolism and mechanism of action of fluorocyclopentenylcytosine (RX-3117)

Cytidine analogs play an important role in the treatment of various types of cancer, both solid tumors and leukemia. A novel cytidine analog fluorocyclopentenylcytosine (RX-3117) was characterized for its cytotoxic effects, its metabolism and its mechanism of action in a panel of 9 solid tumor and leukemic cell lines, as well as 6 variants resistant to gemcitabine, cytarabine and other pyrimidine analogs. Sensitivity in the parent cell lines after 72 hr exposure varied 75 fold with IC50 values from 0.4 to 30 µM RX-3117. The human equilibrative nucleoside transporter mediates transport of RX-3117, since its inhibition protected cells. Uridine and cytidine also protected cells against RX-3117, indicating that activation of RX-3117 is dependent on phosphorylation catalyzed by uridine-cytidine kinase (UCK), which was abundant in all tested cell lines, including the gemcitabine resistant variants. Deoxycytidine did not protect cells against RX-3117. RX-3117 was a very poor substrate for cytidine deaminase (66,000-fold less than gemcitabine). After its uptake in cells, RX-3117 was rapidly metabolised to its nucleotides with the triphosphate being the most prominent form (90% of all nucleotides), while synthesis of the nucleotides was highest in the most sensitive cell lines (U937 and A2780 cells) and lowest in the least sensitive cells (CCRF CEM cells). No difference in nucleotide formation was observed between the SW1573 and its gemcitabine resistant variant SW1573/G. In the AG6000 cells, the dCK- variant of A2780 and resistant to gemcitabine and RX-3117, a normal monophosphate level was found, but no di-and triphosphates were formed, explaining its resistance. Similarly incorporation of RX-3117 into RNA and DNA was higher in the sensitive A2780 and low in the insensitive SW1573 cells, with no difference between the gemcitabine sensitive and resistant variants. The effect of RX-3117 on synthesis of RNA and DNA was quite different; in the sensitive U937 cells 10 µM RX-3117 inhibited RNA synthesis 90%, while in A2780 and CCRF-CEM cells 100 µM RX-3117 was required for 90% inhibition of RNA synthesis. The effect on DNA synthesis was quite different. 1 µM RX-3117 completely inhibited DNA synthesis in the sensitive U937 cells, 80-90% inhibition was achieved with 10 µM in both CCRF CEM variants and with 100 µM in SW1573/G and AG6000, but in A2780 and SW1573 cells 100 µM only resulted in a partial or no inhibition, respectively. In conclusion, RX-3117 showed a completely different sensitivity profile compared to other cytidine analogs. Its uptake is transporter dependent; it is not activated by dCK, but by UCK. RX-3117 is incorporated into RNA and DNA; RX-3117 hardly affected RNA synthesis at IC50 values, but inhibited DNA synthesis. Its metabolism to nucleotides is related with its sensitivity, possibly because they directly inhibit the target presumably DNA methyltransferase.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1770. doi:1538-7445.AM2012-1770

Induction of resistance to the lipophilic cytarabine prodrug elacytarabine (CP-4055) in CEM leukemic cells

The deoxynucleoside analogs cytarabine (Ara-C) and gemcitabine (dFdC) are widely used in the treatment of cancer. Due to their hydrophilic nature they need the equilibrative (hENT) and concentrative (hCNT) nucleoside transporters to enter the cell. To bypass drug resistance due to decreased uptake, lipophilic 5'elaidic acid esters were synthesized, elacytarabine (CP-4055, from ara-C) and CP-4126 (from gemcitabine), which are currently in clinical development for solid and hematological tumors. We investigated whether resistance can be induced in vitro, and treated the CEM leukemic cell line with weekly increasing elacytarabine concentrations, up to 0.28 microM (10 times IC(50)). The IC(50) of the resistant CEM/CP-4055 was 35 microM, about 1,000 times that of the wildtype CEM, and comparable to that of CEM/dCK- (deoxycytidine kinase deficient) (22 microM). CEM/CP-4055 was also cross-resistant to Ara-C, gemcitabine and CP-4126 (28 and 33 microM, respectively). A low level of mRNA dCK was observed, and similar to CEM/dCK-, CEM/CP-4055 did not accumulate Ara-CTP after exposure to Ara-C or elacytarabine, which is consistent with a deficiency in dCK. In conclusion, elacytarabine induced resistance similar to Ara-C. This resistance was caused by downregulation of dCK.

Cellular folate status modulates the expression of BCRP and MRP multidrug transporters in cancer cell lines from different origins

As cellular folate levels seem to have a different effect on cancer cells from different origins, we extended our initial study to a broader panel of cancer cells. BCRP and MRP1-5 expression was determined in KB, OVCAR-3, IGROV-1, ZR75-1/R/MTX, SCC-11B, SCC-22B, and WiDr either grown in standard RPMI 1640 containing 2.3 micromol/L supraphysiologic concentration of folic acid [high folate (HF)] or adapted to more physiologic concentrations [1-5 nmol/L folic acid or leucovorin; low folate (LF)]. Compared with the HF counterparts, KB LF cells displayed 16.1-fold increased MRP3 and OVCAR-3 LF cells showed 4.8-fold increased MRP4 mRNA levels along with increased MRP3 and MRP4 protein expression, respectively. A marked increase on BCRP protein and mRNA expression was observed in WiDr LF cells. These cells acquired approximately 2-fold resistance to mitoxantrone compared with the HF cell line, a phenotype that could be reverted by the BCRP inhibitor Ko143. Of note, WiDr cells expressed BCRP in the intracellular compartment, similarly to what we have described for Caco-2 cells. Our results provide further evidence for an important role of cellular folate status in the modulation of the expression of multidrug resistance transporters in cancer cells. We show that up-regulation of intracellularly localized BCRP in response to adaptation to LF conditions may be a common feature within a panel of colon cancer cell lines. Under these circumstances, folate supplementation might improve the efficacy of chemotherapeutic drugs by decreasing BCRP expression.

Folate deprivation induces BCRP (ABCG2) expression and mitoxantrone resistance in Caco-2 cells

Folates can induce the expression and activity of the breast-cancer-resistance-protein (BCRP) and the multidrug-resistance-protein-1 (MRP1). Our aim was to study the time-dependent effect of folate deprivation/supplementation on (i) BCRP and MRP expression and (ii) on drug resistance mediated by these transporters. Therefore Caco-2 colon cancer cells usually grown in standard RPMI-medium containing supraphysiological folic acid (FA) concentrations (2.3 muM; high-folate, HF) were gradually adapted to more physiological folate concentrations (1 nM leucovorin (LV) or 1 nM FA; low-folate, LF), resulting in the sublines Caco-2-LF/LV and Caco-2-LF/FA. Caco-2-LF/LV and LF/FA cells exhibited a maximal increase of 5.2- and 9.6-fold for BCRP-mRNA and 3.9- and 5.7-fold for BCRP protein expression, respectively, but no major changes on MRP expression. Overexpression of BCRP in the LF-cells resulted in 3.6- to 6.3-fold resistance to mitoxantrone (MR), which was completely reverted by the BCRP inhibitor Ko143. On the other hand, LF-adapted cells were markedly more sensitive to methotrexate than the HF-counterpart, both after 4-hr (9,870- and 23,923-fold for Caco-2-LF/LV and LF/FA, respectively) and 72-hr (11- and 22-fold for Caco-2-LF/LV and LF/FA, respectively) exposure. Immunofluorescent staining observed with a confocal-laser-scan-microscope revealed that in Caco-2 cells (both HF and LF), BCRP is mainly located in the cytoplasm. In conclusion, folate deprivation induces BCRP expression associated with MR resistance in Caco-2 cells. The intracellular localization of BCRP in these cells suggests that this transporter is not primarily extruding its substrates out of the cell, but rather to an intracellular compartment where folates can be kept as storage.

Oxaliplatin activity in selected and unselected human ovarian and colorectal cancer cell lines

Oxaliplatin is used for treatment of colon cancer in combination with 5-fluorouracil or irinotecan. Oxaliplatin has similar, but also different resistant mechanisms as cisplatin. We studied the activity of oxaliplatin in ovarian and colon cancer cells with different resistance patterns to cisplatin. The 40-fold cisplatin-resistant cell line ADDP was only 7.5-fold resistant to oxaliplatin. The gemcitabine-resistant AG6000 cell line, 9-fold resistant to cisplatin, was not cross-resistant. LoVo-175X2, with mutant p53 showed no resistance compared to the empty vector control. However, LoVo-Li, with inactive p53, was 3.6-fold resistant corresponding to decreased accumulation and Pt adducts. Accumulation and DNA adducts formation showed no significant correlation with oxaliplatin sensitivity. Cell cycle distribution after exposure to oxaliplatin showed arrest in G2/M (A2780) or in S-phase (LoVo-92) for wt-p53 cells. ADDP and LoVo-Li showed G1 arrest followed by S-phase arrest and no changes in distribution, respectively. The cell cycle related proteins Cyclins A and B1 and (p)CDC25C were marginally affected by oxaliplatin. Expression of hCTR1 was decreased in ADDP, LoVo-Li and AG6000, OCT1 decreased in ADDP and AG6000 and OCT3 in LoVo-175X2, compared to the parental cell lines. In ADDP and LoVo-175X2 ATP7A and B were decreased but were increased in AG6000. From this study it can be concluded that changes in cell cycle distribution were cell line dependent and not related to changes in expression of Cyclin A or B1. Oxaliplatin accumulation was related to hCTR1 and, at low concentration, ATP7A to DNA adducts formation while the retention was related to hCTR1, OCT2 and ATP7B.

Daily controlled physiotherapy increases survival time in dogs with suspected degenerative myelopathy

The purposes of the study reported here were to evaluate the signalment and clinical presentation in 50 dogs with degenerative myelopathy, to evaluate whether mean survival time was significantly affected by various means of physiotherapy performed in 22 dogs, and to determine whether neurologic status, anatomic localization, or age at onset had an influence on survival time in dogs that received physiotherapy. We found a significant (P < .05) breed predisposition for the German Shepherd Dog, Kuvasz, Hovawart, and Bernese Mountain Dog. Mean age at diagnosis was 9.1 years, and both sexes were affected equally. The anatomic localization of the lesion was spinal cord segment T3-L3 in 56% (n = 28) and L3-S3 in 44% (n = 22) of the dogs. Animals that received intensive (n = 9) physiotherapy had longer (P < .05) survival time (mean 255 days), compared with that for animals with moderate (n = 6; mean 130 days) or no (n = 7; mean 55 days) physiotherapy. In addition, our results indicate that affected dogs which received physiotherapy remained ambulatory longer than did animals that did not receive physical treatment.

[Chronic inflammatory demyelinating polyradiculoneuropathy with hypertrophy of cervico-thoracal nerve roots in a dog]

A case of chronic inflammatory demyelinating polyradiculoneuropathy in a Magyar Vizsla dame, 7 months of age, is described. The neurological deficits such as movement disorders, hyporeflexia and muscle atrophy, were limited to the front legs. The hypertrophied cervico-thoracal nerve roots could be shown by magnetic resonance imaging. The diagnosis was additionally based on clinical findings, the relapsing course, the good response to therapy with prednisolone, the results of electrodiagnostic workup and muscle and nerve biopsy.

Neurodegenerative diseases in domestic animals: A comparative review

Neurodegenerative diseases are characterised by selective damage to specific neurons in the nervous system. Interest in such diseases in humans has resulted in considerable progress in the molecular understanding of these disorders in recent decades. Numerous neurodegenerative diseases have also been described in domestic animals but relatively little molecular work has been reported. In the present review, we have classified neurodegenerative disease according to neuroanatomical criteria. We have established two large groups, based on whether the neuronal cell body or its axon was primarily affected. Conditions such as motor neuron diseases, cerebellar degenerations and neuroaxonal dystrophies are discussed in terms of their clinical and neuropathological features. In the most studied disorders, we also present what is known about underlying pathomechanisms, and compare them with their human counterparts. The purpose of this review is to re-kindle interest in this group of diseases and to encourage veterinary researchers to investigate molecular mechanisms by taking advantage of current diagnostic tools.

ABCG2 Harboring the Gly482 Mutation Confers High-Level Resistance to Various Hydrophilic Antifolates

ABCG2 is an ATP-binding cassette transporter that confers resistance to various chemotherapeutic agents. Recent studies have established that an Arg (wild-type) to Gly mutation at amino acid 482 in ABCG2 alters substrate specificity. Here, we explored the role of this G482 mutation in antifolate resistance using a clinically relevant 4-hour drug exposure. Stable transfectants overexpressing the mutant G482 transporter displayed 120-, 1,000-, and >6,250-fold resistance to the antifolates methotrexate, GW1843, and Tomudex, respectively, relative to parental human embryonic kidney cells. Moreover, although overexpressing equal transporter levels at the plasma membrane, G482-ABCG2 cells were 6-, 23-, and >521-fold more resistant to methotrexate, GW1843, and Tomudex, respectively, than R482-ABCG2 cells. In contrast, upon a continuous (72-hour) drug exposure, both the G482- and R482-ABCG2 cells lost almost all their antifolate resistance; this result was consistent with the inability of ABCG2 to extrude long-chain antifolate polyglutamates. Ko143, a specific and potent ABCG2 inhibitor reversed methotrexate resistance in both G482- and R482-ABCG2 cells. Consistently, whereas the pool of free methotrexate in parental human embryonic kidney cells was prominent after 4 hours of transport with 1 micromol/L [3H]methotrexate, in R482- and G482-ABCG2 cells, it was minimal. Furthermore, G482-ABCG2 cells contained marked decreases in the di- and triglutamate species of [3H]methotrexate at 4 hours of incubation with methotrexate and in the tetra- and pentaglutamates at 24 hours. These changes were not associated with any significant decrease in folylypoly-gamma-glutamate synthetase activity. These results provide the first evidence that the G482-ABCG2 mutation confers high-level resistance to various hydrophilic antifolates.

The human multidrug resistance protein MRP5 transports folates and can mediate cellular resistance against antifolates

Members of the multidrug resistance protein family, notably MRP1-4/ABCC1-4, and the breast cancer resistance protein BCRP/ABCG2 have been recognized as cellular exporters for the folate antagonist methotrexate (MTX). Here we show that MRP5/ABCC5 is also an antifolate and folate exporter based on the following evidence: (a) Using membrane vesicles from HEK293 cells, we show that MRP5 transports both MTX (KM = 1.3 mmol/L and VMAX = 780 pmol per mg protein per minute) and folic acid (KM = 1.0 mmol/L and VMAX = 875 pmol per mg protein per minute). MRP5 also transports MTX-glu2 (KM = 0.7 mmol/L and VMAX = 450 pmol per mg protein per minute) but not MTX-glu3. (b) Both accumulation of total [3H]MTX and of MTX polyglutamates were significantly reduced in MRP5 overexpressing cells. (c) Cell growth inhibition studies with MRP5 transfected HEK293 cells showed that MRP5 conferred high-level resistance (>160-fold) against the antifolates MTX, GW1843, and ZD1694 (raltitrexed) in short-term (4 hours) incubations with high drug concentrations; this resistance was proportional to the MRP5 level. (d) MRP5-mediated resistance (8.5- and 2.1-fold) was also found in standard long-term incubations (72 hours) at low concentrations of ZD1694 and GW1843. These results show the potential of MRP5 to mediate transport of (anti)folates and contribute to resistance against antifolate drugs.

Disparate mechanisms of antifolate resistance provoked by methotrexate and its metabolite 7-hydroxymethotrexate in leukemia cells: implications for efficacy of methotrexate therapy

Methotrexate (MTX) is one of the leading drugs in the treatment of leukemia, but extensive metabolism to 7-hydroxymethotrexate (7-OHMTX) can limit its therapeutic efficacy. In this study we investigated whether 7-OHMTX itself can provoke anti-folate resistance that may further disrupt MTX efficacy. For this purpose, we developed resistance to 7-OHMTX as well as MTX in 2 human leukemia cell lines (CCRF-CEM and MOLT-4) by stepwise exposure to increasing concentrations of 7-OHMTX and MTX. Consequently, both leukemia cell lines displayed marked levels of resistance to 7-OHMTX (> 10-fold) and MTX (> 75-fold). The underlying mechanism of resistance in the MTX-exposed cells was a marked decrease (> 10-fold) in reduced folate carrier (RFC)-mediated cellular uptake of MTX. This was associated with transcriptional silencing of the RFC gene in MTX-resistant CCRF-CEM cells. In contrast, the molecular basis for the resistance to 7-OHMTX was due solely to a marked decreased (> 95%) in folylpolyglutamate synthetase (FPGS) activity, which conferred more than 100-fold MTX resistance upon a short-term exposure to this drug. This is the first demonstration that 7-OHMTX can provoke distinct modalities of antifolate resistance compared with the parent drug MTX. The implications of this finding for MTX efficacy and strategies to circumvent MTX resistance are discussed.

[Tetanus in cats: 3 case descriptions]

Three cats with spasticity on one leg or on all four limbs were presented between 1996 and 1998 at the Department of clinical veterinary medicine, Section of neurology, Vetsuisse-Faculty of Bern. The presumptive diagnosis was tetanus. A focal form was present in two cases and generalised tetanus in one cat. All cats had a history of injury at the affected legs respectively at the neck. The first clinical signs were seen between two days and three weeks after injury. The bacteriologic examination of serous fluid from the site of injury revealed an infection with Clostridium. EMG in one cat during anaesthesia showed motor united potentials (MUPs) on the spastic leg. All patients received antibiotics (Penicillin, respectively Amoxicillin/Clavulanic acid and Metronidazol). Supportive aid were initially sedation, wound revision and in one cat nutrition through oesophageal sonde. In a second phase physiotherapy was performed. All three animals were significantly better after a couple of weeks, two cats were without symptoms after eight and five weeks respectively.

Folate concentration dependent transport activity of the Multidrug Resistance Protein 1 (ABCC1)

The Multidrug Resistance Protein MRP1 (ABCC1) can confer resistance to a variety of therapeutic drugs. In addition, MRP1/ABCC1 mediates cellular export of natural folates, such as folic acid and l-leucovorin. In this study we determined whether cellular folate status affected the functional activity of MRP1/ABCC1 mediated efflux of an established substrate, the anthracycline daunorubicin (DNR). As a model system we used the human ovarian carcinoma cell line 2008wt, and its MRP1/ABCC1 transfected subline 2008/MRP1. Both types of these moderate- and high-MRP1/ABCC1 expressing cells displayed efflux of DNR when maintained in standard culture media (2.3microM folic acid). The initial total cellular DNR efflux rate in 2008/MRP1 cells was approximately 2-fold higher compared to 2008wt cells. This efflux consisted of MRP1/ABCC1 mediated transport, possibly non-MRP1 mediated transport, as well as passive diffusion. Benzbromarone, a specific MRP1 inhibitor, decreased the initial efflux rate in 2008/MRP1 cells (4-fold) and in 2008wt cells (2-fold). When 2008/MRP1 cells were challenged for 2 days in folate-free medium, total cellular DNR efflux was decreased to 43% of the initial efflux rate under folate-rich conditions. In 2008wt cells DNR efflux was decreased to 84% of the folate-rich conditions. Benzbromarone did not inhibit DNR efflux after the folate-free period in both cell lines. Repletion of folate by a 2-24hr exposure to 2.5microM l-leucovorin or folic acid resulted in a complete restoration of DNR efflux. In contrast, expression of MRP1/ABCC1 protein was not changed significantly during the folate-free period or the repletion-period, nor were cellular ATP or ADP pools. In conclusion, this study demonstrates that the cellular folate status can influence the transport activity of MRP1/ABCC1. These results have potentially important implications in the understanding of the (patho-)physiological roles of MRP1/ABCC1, and possibly other ABC transporter proteins in cellular folate homeostasis and drug resistance.

Acquired resistance of human T cells to sulfasalazine: stability of the resistant phenotype and sensitivity to non-related DMARDs

BACKGROUND

A recent study from our laboratory showed that induction of the multidrug resistance related drug efflux pump ABCG2 contributed to acquired resistance of human T cells to the disease modifying antirheumatic drug (DMARD) sulfasalazine (SSZ).

OBJECTIVES

To investigate the duration of SSZ resistance and ABCG2 expression after withdrawal of SSZ and rechallenging with SSZ, and to assess the impact of SSZ resistance on responsiveness to other DMARDs.

METHODS

Human CEM cells (T cell origin) with acquired resistance to SSZ (CEM/SSZ) were characterised for (a) SSZ sensitivity and ABCG2 expression during withdrawal and rechallenge of SSZ, and (b) antiproliferative efficacy of other DMARDs.

RESULTS

ABCG2 protein expression was stable for at least 4 weeks when CEM/SSZ cells were grown in the absence of SSZ, but gradually declined, along with SSZ resistance levels, to non-detectable levels after withdrawal of SSZ for 6 months. Rechallenging with SSZ led to a rapid (<2.5 weeks) resumption of SSZ resistance and ABCG2 expression as in the original CEM/SSZ cells. CEM/SSZ cells displayed diminished sensitivity to the DMARDs leflunomide (5.1-fold) and methotrexate (1.8-fold), were moderately more sensitive (1.6-2.0 fold) to cyclosporin A and chloroquine, and markedly more sensitive (13-fold) to the glucocorticoid dexamethasone as compared with parental CEM cells.

CONCLUSION

The drug efflux pump ABCG2 has a major role in conferring resistance to SSZ. The collateral sensitivity of SSZ resistant cells for some other (non-related) DMARDs may provide a further rationale for sequential mono- or combination therapies with distinct DMARDs upon decreased efficacy of SSZ.

Development of sulfasalazine resistance in human T cells induces expression of the multidrug resistance transporter ABCG2 (BCRP) and augmented production of TNFalpha

OBJECTIVE

To determine whether overexpression of cell membrane associated drug efflux pumps belonging to the family of ATP binding cassette (ABC) proteins contributes to a diminished efficacy of sulfasalazine (SSZ) after prolonged cellular exposure to this disease modifying antirheumatic drug (DMARD).

METHODS

A model system of human T cells (CEM) was used to expose cells in vitro to increasing concentrations of SSZ for a period of six months. Cells were then characterised for the expression of drug efflux pumps: P-glycoprotein (Pgp, ABCB1), multidrug resistance protein 1 (MRP1, ABCC1), and breast cancer resistance protein (BCRP, ABCG2).

RESULTS

Prolonged exposure of CEM cells to SSZ provoked resistance to SSZ as manifested by a 6.4-fold diminished antiproliferative effect of SSZ compared with parental CEM cells. CEM cells resistant to SSZ (CEM/SSZ) showed a marked induction of ABCG2/BCRP, Pgp expression was not detectable, while MRP1 expression was even down regulated. A functional role of ABCG2 in SSZ resistance was demonstrated by 60% reversal of SSZ resistance by the ABCG2 blocker Ko143. Release of the proinflammatory cytokine tumour necrosis factor alpha (TNFalpha) was threefold higher in CEM/SSZ cells than in CEM cells. Moreover, twofold higher concentrations of SSZ were required to inhibit TNFalpha release from CEM/SSZ cells compared with CEM cells.

CONCLUSION

Collectively, ABCG2 induction, augmented TNFalpha release, and less efficient inhibition of TNFalpha production by SSZ may contribute to diminished efficacy after prolonged exposure to SSZ. These results warrant further clinical studies to verify whether drug efflux pumps, originally identified for their roles in cytostatic drug resistance, can also be induced by SSZ or other DMARDs.

[Cerebellar cortical abiotrophy in American Staffordshire terriers: clinical and pathological description of 3 cases]

Three American Staffordshire Terriers were presented with gait abnormalities and loss of balance at the age of 4.5 (female) and 6 years (2 males). The onset varied between 3 and 5 years of age and the clinical signs were slowly progressive. The neurological examination revealed symmetrical generalized cerebellar ataxia with hypermetria, stiffness, and loss of balance with no evidence of paresis. The menace reflex was decreased in one dog and absent in another. A positional nystagmus was found in two dogs. The dogs were euthanized and a histopathological examination of each brain was performed. Pathological changes were confined to the cerebellum. The main finding was loss of Purkinje cells, as well as depletion of granular cell bodies and shrinkage of the granular and molecular cell layer. These findings are consistent with cerebellar cortical abiotrophy. A genetic basis is supposed, but the mode of inheritance is not determined yet. In contrast to some spinocerebellar ataxias in humans, the cause of Purkinje cell degeneration in cerebellar cortical abiotrophy of dogs is not known.

The role of multidrug resistance proteins MRP1, MRP2 and MRP3 in cellular folate homeostasis

Previously, we reported that the multidrug resistance proteins MRP1, MRP2 and MRP3 confer resistance to therapeutic antifolates by mediating their cellular extrusion. We now determined whether MRPs also play a role in controlling cellular homeostasis of natural folates. In MRP1, MRP2 and MRP3-transfected 2008 human ovarian carcinoma cells total cellular folate content was 32-38% lower than in 2008 cells (105+/-14pmolfolate/mgprotein) when grown in medium containing 2.3 microM folic acid (FA). Under these conditions cellular growth rates were not compromised. However, when cells were challenged under folate-depleted conditions with a short exposure (4 hr) to FA or leucovorin, MRP1 and MRP3 overexpressing cells were impaired in their growth. In contrast to wild-type cells, MRP1 transfected cells retained only 60% of the maximum growth when exposed to 500 nM leucovorin or 500 microM FA. For 2008/MRP1 and 2008/MRP3 cells FA growth stimulation capacity was dramatically decreased when, during a 4 hr exposure, metabolism into rapidly polyglutamatable and retainable dihydrofolate was blocked by the dihydrofolate reductase inhibitor trimetrexate. To retain growth under such conditions MRP1 overexpressing cells required much higher concentrations of FA (EC(50) > 500 microM) compared to 2008 cells (EC(50): 12 microM). These results suggest that down- and up-regulation of MRP1 (and MRP3) expression can influence cellular folate homeostasis, in particular when cellular retention by polyglutamylation of folates is attenuated.

Loss of multidrug resistance protein 1 expression and folate efflux activity results in a highly concentrative folate transport in human leukemia cells

We studied the molecular basis of the up to 46-fold increased accumulation of folates and methotrexate (MTX) in human leukemia CEM-7A cells established by gradual deprivation of leucovorin (LCV). CEM-7A cells consequently exhibited 10- and 68-fold decreased LCV and folic acid growth requirements and 23-25-fold hypersensitivity to MTX and edatrexate. Although CEM-7A cells displayed a 74-86-fold increase in the reduced folate carrier (RFC)-mediated influx of LCV and MTX, RFC overexpression per se cannot induce a prominently increased folate/MTX accumulation because RFC functions as a nonconcentrative anion exchanger. We therefore explored the possibility that folate efflux activity mediated by members of the multidrug resistance protein (MRP) family was impaired in CEM-7A cells. Parental CEM cells expressed substantial levels of MRP1, MRP4, poor MRP5 levels, whereas MRP2, MRP3 and breast cancer resistance protein were undetectable. In contrast, CEM-7A cells lost 95% of MRP1 levels while retaining parental expression of MRP4 and MRP5. Consequently, CEM-7A cells displayed a 5-fold decrease in the [(3)H]folic acid efflux rate constant, which was identical to that obtained with parental CEM cells, when their folic acid efflux was blocked (78%) with probenecid. Furthermore, when compared with parental CEM, CEM-7A cells accumulated 2-fold more calcein fluorescence. Treatment of parental cells with the MRP1 efflux inhibitors MK571 and probenecid resulted in a 60-100% increase in calcein fluorescence. In contrast, these inhibitors failed to alter the calcein fluorescence in CEM-7A cells, which markedly lost MRP1 expression. Replenishment of LCV in the growth medium of CEM-7A cells resulted in resumption of normal MRP1 expression. These results establish for the first time that MRP1 is the primary folate efflux route in CEM leukemia cells and that the loss of folate efflux activity is an efficient means of markedly augmenting cellular folate pools. These findings suggest a functional role for MRP1 in the maintenance of cellular folate homeostasis.

Analysis of the 5' region of the canine PAX3 gene and exclusion as a candidate for Dalmatian deafness

The causative mutation in a gene related to hearing loss in Dalmatians has been elusive. Because of its role in melanocyte migration and differentiation as integral component of the inner ear, we hypothesized that the canine PAX3 (paired box homeotic gene 3) gene could be a candidate for Dalmatian deafness. Therefore, we isolated the canine PAX3 gene and searched for causative mutations within the coding region of important regulatory domains of PAX3. However, no mutations were identified when comparing the DNA sequences of healthy and affected dogs. These results were confirmed by a two-point linkage analysis in 203 Dalmatians transmitting deafness. Our data clearly show that the canine PAX3 gene can be excluded as candidate for Dalmatian deafness.

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.

[Rehabilitation methods in small animal neurology]

Rehabilitation is an important part of the treatment of neurological diseases. The primary goal of these methods is an optimal functional restoring of the neuro-muscular system. Massages, thermo-, hydro- and electrotherapy, as well as therapy of movement are different treatment possibilities with their own indication, which are combined in a physiotherapy program. It follows an overview of the different physiotherapeutic methods and their application in some of the most common neurological diseases, as for example intervertebral disc problems or degenerative myelopathy.

Spontaneous lumbar intervertebral disc protrusion in cats: literature review and case presentations

Reports on intervertebral disc disease in cats are rare in the veterinary literature. It has been postulated that intervertebral disc protrusion is a frequent finding during necropsy in cats, without having any clinical relevance (King and Smith 1958, King & Smith 1960a, King & Smith 1960b). However, a total of six cases with disc protrusions and clinically significant neurological deficits have been reported over the past decade. (Heavner 1971, Seim & Nafe 1981, Gilmore 1983, Littlewood et al 1984, Sparkes & Skerry 1990, Bagley et al 1995). As in dogs, there are also two types of intervertebral disc disease in cats: Hansen's type I (extrusion), and type II (herniation). Cervical spinal cord involvement was more commonly recognised in cats than the lumbar or the thoraco lumbar area. Cats over 15 years were mainly affected (King & Smith 1958, King & Smith 1960a, King & Smith 1960b). We describe two cats with lumbar intervertebral disc protrusions. Emphasis is placed on differential diagnoses, treatment and follow-up.

Clinical and genetic investigations of idiopathic epilepsy in the Bernese mountain dog

A study was conducted to investigate the clinical aspects and to define the mode of inheritance of idiopathic epilepsy in the Bernese mountain dog. Pedigree analyses were carried out on an open, non-preselected population of 4005 dogs. Five different subpopulations with 50 epileptic dogs from 13 generations were included. Almost all epileptic patients showed generalised seizures of the grand-mal type with a well-defined prodromal and postictal phase. The majority (62 per cent) of the epileptic dogs had had their first seizures at between one and three years of age and it was found that the age at first seizure was significantly (P < 0.05) lower in dogs from affected parental animals than in dogs from healthy parental animals. A clear predisposition for males was also noted. Additionally, there was no correlation between inbreeding coefficient and age at first seizure or incidence rate of seizures. The increased occurrence of the disease in different subpopulations and different families of the same sires or dams showed that there was a genetic basis for the condition in the Bernese mountain dog. Furthermore, the results of the pedigree analyses and binomial test support the hypothesis that idiopathic epilepsy has a polygenic, recessive mode of inheritance in the breed. Additional objective test-mating programmes would however be necessary to define the exact mode of inheritance.

Multiple mechanisms of resistance to polyglutamatable and lipophilic antifolates in mammalian cells: role of increased folylpolyglutamylation, expanded folate pools, and intralysosomal drug sequestration

Chinese hamster ovary PyrR100 cells display more than 1000-fold resistance to pyrimethamine (Pyr), a lipophilic antifolate inhibitor of dihydrofolate reductase. PyrR100 cells had wild-type DHFR activity, lost folate exporter activity, and had a 4-fold increased activity of a low pH folic acid transporter. Here we report on the marked alterations identified in PyrR100 cells compared with parental cells: 1) approximately 100-fold decreased folic acid growth requirement; 2) a 25-fold higher glucose growth requirement in Pyr-containing medium; 3) a 2.5- to 4.1-fold increase in folylpolyglutamate synthetase activity; 4) a 3-fold increase in the accumulation of [3H]folic acid and a 3-fold expansion of the intracellular folate pools; 5) a 4-fold increase in the activity of the lysosomal marker beta-hexoseaminidase, suggesting an increased lysosome number/PyrR100 cell; and 6) a small reduction in the steady-state accumulation of [3H]Pyr and no evidence of catabolism or modification of cellular [3H]Pyr. Consequently, PyrR100 cells were markedly resistant to the lipophilic antifolates trimetrexate (40-fold) and AG377 (30-fold) and to the polyglutamatable antifolates 5,10-Dideaza-5,6,7,8-tetrahydrofolic acid (DDATHF) (26-fold) and AG2034 (14-fold). Resistance to these drugs was reversed in PyrR100 cells transferred into folate-depleted medium. In conclusion, these multiple resistance factors collectively result in a prominent increase in folate accumulation, an expansion of the intracellular folylpolyglutamate pool, and abolishment of the cytotoxic activity of polyglutamatable and lipophilic antifolates. The role of increased lysosome number per cell in sequestration of hydrophobic weak base drugs such as Pyr is also discussed as a novel mechanism of drug resistance.

A structurally altered human reduced folate carrier with increased folic acid transport mediates a novel mechanism of antifolate resistance

CEM/MTX is a subline of human CCRF-CEM leukemia cells which displays >200-fold resistance to methotrexate (MTX) due to defective transport via the reduced folate carrier (RFC). CEM/MTX-low folate (LF) cells, derived by a gradual deprivation of folic acid from 2.3 microM to 2 nM (LF) in the cell culture medium of CEM/MTX cells, resulted in a >20-fold overexpression of a structurally altered RFC featuring; 1) a wild type Km value for MTX transport but a 31-fold and 9-fold lower Km values for folic acid and leucovorin, respectively, relative to wild type RFC; 2) a 10-fold RFC1 gene amplification along with a >20-fold increased expression of the main 3.1-kilobase RFC1 mRNA; 3) a marked stimulation of MTX transport by anions (i.e. chloride); and 4) a G --> A mutation at nucleotide 227 of the RFC cDNA in both CEM/MTX-LF and CEM/MTX, resulting in a lysine for glutamate substitution at amino acid residue 45 predicted to reside within the first transmembrane domain of the human RFC. Upon transfer of CEM/MTX-LF cells to folate-replete medium (2.3 microM folic acid), the more efficient folic acid uptake in CEM/MTX-LF cells resulted in a 7- and 24-fold elevated total folate pool compared with CEM and CEM/MTX cells, respectively (500 versus 69 and 21 pmol/mg of protein, respectively). This markedly elevated intracellular folate pool conferred a novel mechanism of resistance to polyglutamatable (e.g. ZD1694, DDATHF, and AG2034) and lipophilic antifolates (e.g. trimetrexate and pyrimethamine) by abolishing their polyglutamylation and circumventing target enzyme inhibition.

Lipoamino acid conjugates of methotrexate with antitumor activity

The synthesis, characterization, and in vitro antitumor activity against a wild and a transport-resistant CCRF-CEM cell line is described for a series of alpha,gamma-bisamide lipoamino acid and oligomer conjugates of methotrexate. The influence of the lipophilicity of the conjugates on the cytotoxicity and the dihydrofolate reductase inhibition was investigated. All compounds were more active than their fatty acid conjugate analogues. Compound le with a 12-carbon atom aliphatic side chain showed the highest in vitro activity.

Regulation of carrier-mediated transport of folates and antifolates in methotrexate-sensitive and-resistant leukemia cells

Prolonged cell culture of human leukemia cells at folate concentrations in the (sub)physiological range (1-5 nM) rather than at 'standard' supraphysiological concentrations of 2-10 microM folic acid elicited a number of regulatory aspects of the reduced folate carrier (RFC), the membrane transport protein for natural reduced folate cofactors and folate-based chemotherapeutic drugs such as methotrexate (MTX). One subline of human CCRF-CEM leukemia cells grown under folate-restricted conditions (CEM-7A) exhibited a 95-fold increased Vmax for uptake of [3H]-MTX. The increased uptake of MTX in CEM-7A cells is based on at least two factors: (a) a constitutive 10-fold overexpression of the RFC1 gene and RFC1 message; and (b) a 7-9-fold up-regulation of RFC transport activity under low intracellular reduced folate concentrations. This second component appeared to be regulatable by changes in the cellular folate, purine and methylation status as judged from a 7-9 fold down-regulation of RFC transport activity after short term (1-2 hr) incubation of CEM-7A cells with reduced folate cofactors (25 nM LV), purines (100 microM adenosine) or S-adenosylmethionine (100 microM), respectively. Gradual folate restriction in the cell culture medium of CEM/MTX cells, a subline of CCRF-CEM resistant to MTX due to defective transport via the RFC, revealed the up-regulated expression of an altered RFC protein that is characterized by a 35-fold decreased Km for folic acid and a 10-fold decreased Km for the reduced folate cofactor LV compared to the RFC expressed in CCRF-CEM and CEM-7A cells. As a result of the markedly increased efficiency of folic acid uptake in CEM/MTX cells, intracellular folate pools were 7-fold higher than in CCRF-CEM cells when both cell lines were incubated in the presence of 2 microM folic acid. The high intracellular folate pools in CEM/MTX cells appeared to impair the polyglutamylation of antifolates and confer resistance to ZD1694, an antifolate drug that depends on polyglutamylation for its biological activity. Collectively, these studies provide a better insight into the basic regulation of RFC-mediated membrane transport of clinically active antifolates. In addition, these studies may also provide an opportunity to exploit the transport system as a target for biochemical modulation by which it may contribute to an improved efficacy of folate-based chemotherapy in a clinical setting.

Functional aspects of membrane folate receptors in human breast cancer cells with transport-related resistance to methotrexate

Two methotrexate (MTX)-resistant human breast-cancer cell lines with impaired transport via the reduced folate carrier (RFC), one established in vitro (MTX(R)-ZR-75-1) and another inherently resistant (MDA-231), were adapted to grow in medium containing 2 nM folic acid. This induced the expression of previously undetectable membrane folate receptors (MFR) to levels of 8.2 and 2.3 pmol/10(7) cells, respectively. Polymerase chain reaction (PCR) quantitation revealed that MFR messenger-RNA levels of the isoform first described in human nasopharyngeal carcinoma KB cells (MFR-alpha) were increased in low-folate-adapted MTX(R)-ZR-75-1 cells, whereas placental transcripts (MFR-beta) coincided with MFR-alpha expression in low-folate (LF)-adapted MDA-231 cells. These cell lines were used to study the role of MFR in the uptake and growth-inhibitory effects of five different antifolates with varying affinities for MFR: N10-propargyl-5, 8-dideazafolic acid (CB3717) > 5,10-dideazatetra-hydrofolic acid (DDATHF) > N-5-[N-(3,4-dihydro-2-methyl-4-oxoquinazolin-6-methyl) -N-methyl-amino]-2-theonyl}-glutamic acid (ZD1694) >> MTX > edatrexate (EDX). Expression of MFR only slightly decreased the resistant phenotype for MTX, EDX, and ZD1694, suggesting that these drugs are not transported intracellularly to cytotoxic concentrations at these levels of MFR expression. On the other hand, both cell lines became from at least 180- to 400-fold more sensitive to growth inhibition by CB3717 and DDATHF, which may be correlated with their high affinity for MFR. These sensitivity/resistance profiles were largely similar following cell culture in medium containing 1 nM L-leucovorin, a folate with an affinity for MFR 10-fold lower than that of folic acid, the one exception being the increased sensitivity for ZD1694 seen in the LF-adapted cells with the highest level of MFR expression (MTX(R)-ZR-75-1). These results illustrate that the efficacy of MFR in mediating antifolate transport and cytotoxicity depends on their affinity for the folate antagonist, their degree of expression, and the levels of competing folates.

Carrier- and receptor-mediated transport of folate antagonists targeting folate-dependent enzymes: correlates of molecular-structure and biological activity

The transport properties and growth-inhibitory potential of 37 classic and novel antifolate compounds have been tested in vitro against human and murine cell lines expressing different levels of the reduced folate carrier (RFC), the membrane-associated folate binding protein (mFBP), or both. The intracellular targets of these drugs were dihydrofolate reductase (DHFR), glycinamide ribonucleotide transformylase (GARTF), folylpolyglutamate synthetase (FPGS), and thymidylate synthase (TS). Parameters that were investigated included the affinity of both folate-transport systems for the antifolate drugs, their growth-inhibitory potential as a function of cellular RFC/mFBP expression, and the protective effect of either FA or leucovorin against growth inhibition. Methotrexate, aminopterin, N10-propargyl-5,8-dideazafolic acid (CB3717), ZD1694, 5,8-dideazaisofolic acid (IAHQ), 5,10-dideazatetrahydrofolic acid (DDATHF), and 5-deazafolic acid (efficient substrate for FPGS) were used as the basic structures in the present study, from which modifications were introduced in the pteridine/quinazoline ring, the C9-N10 bridge, the benzoyl ring, and the glutamate side chain. It was observed that RFC exhibited an efficient substrate affinity for all analogues except CB3717, 2-NH2-ZD1694, and glutamate side-chain-modified FPGS inhibitors. Substitutions at the 2-position (e.g., 2-CH3) improved the RFC substrate affinity for methotrexate and aminopterin. Other good substrates included PT523 (N alpha-(4-amino-4-deoxypteroyl)-N delta-hemiphthaloyl-L-ornithine), 10-ethyl-10-deazaaminopterin, and DDATHF. With respect to mFBP, modifications at the N-3 and 4-oxo positions resulted in a substantial loss of binding affinity. Modifications at other sites of the molecule were well tolerated. Growth-inhibition studies identified a series of drugs that were preferentially transported via RFC (2,4-diamino structures) or mFBP (CB3717, 2-NH-ZD1694, or 5,8-dideazaisofolic acid), whereas other drugs were efficiently transported via both transport pathways (e.g., DDATHF, ZD1694, BW1843U89, or LY231514). Given the fact that for an increasing number of normal and neoplastic cells and tissue, different expression levels of RFC and mFBP are being recognized, this folate antagonist structure-activity relationship can be of value for predicting drug sensitivity and resistance of tumor cells or drug-related toxicity to normal cells and for the rational design and development of novel antifolates.

Membrane transport of natural folates and antifolate compounds in murine L1210 leukemia cells: role of carrier- and receptor-mediated transport systems

L1210-B73 cells, variants of L1210 cells grown in medium containing nanomolar concentrations of folates, express a membrane associated folate binding protein (mFBP) in addition to the classical reduced folate/methotrexate carrier (RF/MTX-carrier) present in L1210 cells grown in standard high folate medium (G. Jansen et al., Cancer Res., 49: 1959-1963, 1989). In this study we used L1210-B73 and L1210 cells as a model system to study the affinity of the RF/MTX-carrier and the mFBP for the natural folate compounds folic acid and 5-formyltetrahydrofolate (5-CHO-THF), as well as a number of antifolate compounds. Furthermore we studied the contribution of the RF/MTX-carrier and the mFBP in membrane transport of these (anti)folates, and finally we analyzed the role of the mFBP and RF/MTX-carrier in the cytotoxic effects of the antifolates. The antifolates used were either inhibitors of dihydrofolate reductase, including methotrexate (MTX) and 10-ethyl-10-deazaaminopterin (10-EdAM), or two folate-based inhibitors of thymidylate synthase, N10-propargyl-5,8-dideazafolic acid (CB3717) and 2-deamino-2-methyl-N10-propargyl-5,8-dideazafolic acid (ICI-198,583). The affinity of the RF/MTX-carrier for natural and antifolate compounds declined in the order 10-EdAM greater than or equal to ICI-198,583 greater than or equal to 5-CHO-THF greater than MTX much greater than CB3717 much greater than folic acid. The mFBP exhibited a high binding affinity for CB3717 and ICI-198,583 but a poor binding affinity for MTX and 10-EdAM. Binding affinities of the mFBP decreased in the order CB3717 greater than or equal to folic acid = ICI-198,583 greater than or equal to 5-CHO-THF much greater than MTX = 10-EdAM. Over 24 h, at 25 nM, [3H]folic acid uptake in L1210-B73 cells was found to proceed for more than 98% via the mFBP. Uptake of [3H]-5-CHO-THF, at 50 nM extracellular concentration, occurred via both the mFBP (81%) and the RF/MTX-carrier (19%). With respect to antifolates, the mFBP in L1210-B73 cells contributed for less than 30% in the uptake of [3H]MTX but was the predominant route (92%) in the uptake of [3H]ICI-198,583. Results from affinity and membrane transport observations were consistent with growth inhibition studies on L1210-B73 cells demonstrating that the mFBP played only a minor role in the cytotoxic effects of MTX or 10-EdAM. On the other hand, L1210-B73 cells were significantly more sensitive to CB3717 (220-fold) and ICI-198,583 (10-fold) than parental L1210 cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanisms of acquired resistance to methotrexate in a human squamous carcinoma cell line of the head and neck, exposed to different treatment schedules

Mechanisms of acquired resistance to methotrexate (MTX) were evaluated in HNSCC-11B cells which were made resistant to methotrexate either by continuous (11B-MTX-C) or by pulse exposure (11B-MTX-P) to the drug. 11B-MTX-C cells were 91-fold resistant to methotrexate and 30-fold or 49-fold crossresistant to trimetrexate and 10-EdAM, respectively. Dihydrofolate reductase (DHFR) activity was increased 63-fold in 11B-MTX-C cells together with a decrease in [3H]-methotrexate transport and folylpolyglutamate synthase (FPGS) activity (2.5-fold and 3.8-fold, respectively). Against two novel antifolates targetting enzymes other than DHFR, minor crossresistance was observed for ICI-198, 583, but full sensitivity was retained for DDATHF. 11B-MTX-P cells were 46-fold resistant to methotrexate and 47-fold crossresistant to ICI-198,583 in short-term drug exposure, but showed only minor changes in methotrexate sensitivity following prolonged drug exposure. The resistant phenotype in 11B-MTX-P cells were characterised by a 5.6-fold decrease in FPGS activity. These results suggest that different mechanisms of methotrexate resistance in HNSCC cells in vitro can be obtained dependent on the schedule of exposure to the drug.

Growth-inhibitory effects of 5,10-dideazatetrahydrofolic acid on variant murine L1210 and human CCRF-CEM leukemia cells with different membrane-transport characteristics for (anti)folate compounds

5,10-Dideazatetrahydrofolic acid (DDATHF) is a potent inhibitor of glycinamide ribonucleotide transformylase, one of the folate-dependent key enzymes in de novo purine biosynthesis. The present report demonstrates that multiple membrane-transport routes may be involved in the cellular uptake of DDATHF. These routes include the classic reduced folate carrier and a membrane-associated folate-binding protein (mFBP). The role of an mFBP in the uptake of DDATHF was suggested from observations that (a) the mFBP showed a very high binding affinity for DDATHF, (b) murine and human leukemia cells expressing an mFBP were highly sensitive to growth inhibition by DDATHF, and (c) protection against this growth inhibition could be achieved using folic acid rather than reduced folate compounds.

Methotrexate transport in variant human CCRF-CEM leukemia cells with elevated levels of the reduced folate carrier. Selective effect on carrier-mediated transport of physiological concentrations of reduced folates

This study reports the isolation and characterization of a variant of the human CCRF-CEM leukemia cell line that overproduces the carrier protein responsible for the uptake of reduced folates and the folate analogue methotrexate. The variant was obtained by adapting CCRF-CEM cells for prolonged times to stepwise decreasing concentrations of 5-formyltetrahydrofolate as the sole folate source in the cell culture medium. From cells that were grown on less than 1 nM 5-formyl-tetrahydrofolate, a variant (CEM-7A) was isolated exhibiting a 95-fold increased Vmax for [3H]methotrexate influx compared to parental CCRF-CEM cells. The values for influx Km, efflux t0.5, and Ki for inhibition by other folate (analogue) compounds were unchanged. Affinity labeling of the carrier with an N-hydroxysuccinimide ester of [3H]methotrexate demonstrate an approximately 30-fold increased incorporation of [3H] methotrexate in CEM-7A cells. This suggests that the up-regulation of [3H]methotrexate influx is not only due to an increased amount of carrier protein, but also to an increased rate of carrier translocation or an improved cooperativity between carrier protein molecules. Incubation for 1 h at 37 degrees C of CEM-7A cells with a concentration of 5-formyltetrahydrofolate or 5-methyltetrahydrofolate in the physiological range (25 nM) resulted in a 7-fold decline in [3H]methotrexate influx. This down-regulation during incubations with 5-formyltetrahydrofolate or 5-methyltetrahydrofolate could be prevented by either the addition of 10-25 nM of the lipophilic antifolate trimetrexate or by preincubating CEM-7A cells with 25 nM methotrexate. The down-regulatory effect was specifically induced by reduced folates since incubation of CEM-7A cells with 25 nM of either methotrexate, 10-ethyl-10-deazaaminopterin, aminopterin, or folic acid, or a mixture of purines and thymidine, had no effect on [3H]methotrexate influx. Similarly, these down-regulatory effects on [3H]methotrexate transport by 5-formyltetrahydrofolate, and its reversal by trimetrexate or methotrexate, were also observed, though to a lower extent, for parental CCRF-CEM cells grown in folate-depleted medium rather than in standard medium containing high folate concentrations. These results indicate that mediation of reduced folate/methotrexate transport can occur at reduced folate concentrations in the physiological range, and suggest that the intracellular folate content may be a critical determinant in the regulation of methotrexate transport.