Toxicity of methotrexate in rats preexposed to nitrous oxide.

Several chemotherapeutic protocols for the treatment of malignancies include administration of methotrexate (MTX) during or shortly after total anesthesia. Clinical observations in patients treated for breast carcinoma or childhood cancer have shown unexpected myelosuppression and mucosal damage. This phenomenon may be attributed to the synergistic effects of nitrous oxide, which inactivates the cobalamin coenzyme of methionine synthase, and MTX, which inhibits dihydrofolate reductase, on folate metabolism. However, no quantitative data on dose-effect relationships are available regarding the combined toxicity of MTX and N2O. We investigated the effect of exposure to N2O on the toxicity of MTX. Groups of male Wistar rats were exposed to either 50% N2O/50% O2 or air for 12-48 h. Subsequently, a single i.p. injection of 10, 20, 40, or 80 mg MTX/kg body weight was given. Gastrointestinal toxicity resulted in diarrhea and weight loss in all groups for 5 days after MTX administration. Concomitantly, bone marrow depression with leukocytopenia and thrombocytopenia occurred. Exposure to N2O did not alter the plasma clearance of MTX. No substantial liver or kidney toxicity could be detected, but the 50% lethal dose for MTX was reduced from 60 mg/kg to 10 mg/kg if rats had been exposed to N2O for 48 h; the main causes of death were dehydration and bleeding. The administration of 5-formyl-tetrahydrofolate (4 x 10 mg i.p.) but not 5-methyltetrahydrofolate protected completely against the lethal effect of the drug combination. Altogether, cytotoxic effects of MTX on proliferating cells are potentiated by N2O. Therefore, the use of this anesthetic shortly before or during MTX administration should be avoided.

The uptake of R-type cobalamin-binding protein by isolated rat liver cells.

The uptake of R-type cobalamin-binding protein from human granulocytes and plasma by isolated parenchymal rat liver cells has been studied. When [57Co] cyanocobalamin-saturated granulocyte-binding protein or transcobalamin III was incubated with the liver cells in a concentration of 500 pM, more than 80% of the vitamin was taken up in 1 h. Vitamin B-12 bound to plasma transcobalamin I, however, was not taken up unless the protein was desialylated by neuraminidase from Vibrio cholerae. The uptake of iodinated pure granulocyte-binding protein, saturated with cobalamin, reached 100% and was accompanied by increasing intracellular proteolytic degradation of the binding protein. EGTA and asialo-orosomucoid completely inhibited this process of uptake and degradation, whereas partial inhibition was caused by chloroquine and colchicine. These observations provide evidence that these (asialo)-R-type cobalamin-binding proteins are taken up by the cell through the plasma membrane receptor for asialoglycoproteins by means of endocytosis followed by proteolysis of the binding protein in the lysosomes.

Clinical modulation of multidrug resistance in multiple myeloma: effect of cyclosporine on resistant tumor cells.

PURPOSE: In multiple myeloma (MM) refractory to doxorubicin (DXR) and/or vincristine (VCR), myeloma cells frequently express the multidrug resistance (MDR) phenotype, associated with overexpression of P-glycoprotein (Pgp), which acts as a drug efflux pump. Recently, studies have shown that clinical resistance can be modulated by drug resistance modifiers. The present study was performed to investigate if MDR modulation in vivo is caused by a direct effect of cyclosporine (CSA) on resistant myeloma plasma cells (PC). PATIENTS AND METHODS: Eight patients with VAD-refractory MM were treated with DXR, VCR, and dexamethasone (VAD) plus CSA. Pgp expression in PC was determined by flow cytometry/immunocytochemistry before and after clinical treatment. Functional Pgp expression was determined by the effect of CSA on the intracellular accumulation of DXR and VCR. RESULTS: Five of eight patients responded to VAD/CSA. The percentage of Pgp-positive (Pgp+) PC was 30% to 100% (median, 90%) before treatment and 4 to 90% (median, 40%) after treatment. CD56+/- or CD38+/- PC had identical Pgp expression. CSA, as well as SDZ PSC 833, but not dexamethasone, increased pretreatment intracellular accumulation of DXR and VCR in Pgp+ PC in three of four and six of six patients, respectively. After clinical treatment, in vitro drug accumulation in residual Pgp-negative (Pgp-) PC of four of four responding patients was not further modulated by CSA or SDZ PSC 833. At later relapse, PC of two of four patients remained Pgp-. CONCLUSION: These data indicate that Pgp overexpression is functional in refractory myeloma and that clinical modulation of MDR by CSA is mediated through an inhibition of Pgp-associated drug efflux. Pgp-expressing PC can be eliminated by clinical treatment with VAD/CSA.

MDR-1 expression and response to vincristine, doxorubicin, and dexamethasone chemotherapy in multiple myeloma refractory to alkylating agents.

PURPOSE: To assess whether the presence of enhanced multiple drug resistance (MDR)-1 gene expression in multiple myeloma (MM) patients predicts survival, as well as response to vincristine, doxorubicin, and dexamethasone (VAD) chemotherapy. PATIENTS AND METHODS: Sixty-three MM patients refractory to alkylating therapy were studied. The presence of the MDR-1 gene product, a 170-kd glycoprotein (P-170), was analyzed in bone marrow plasma cells by means of the alkaline phosphatase (APAAP) technique using the P-170-specific monoclonal antibody (MoAb) C219. The prognostic value of MDR-1 gene expression, examined before VAD treatment, was compared with other established prognostic factors including beta 2-microglobulin, albumin, lactate dehydrogenase (LDH), and the plasma cell labeling index. RESULTS: Fifty-nine percent of all samples were P-170-positive. No association could be demonstrated between response to VAD and MDR-1 gene expression (chi 2 P = .359), in contrast to high serum beta 2-microglobulin levels, which were positively correlated with response (P = .006). P-170-positive and -negative patients showed a median survival duration of 23 and 22 months, respectively, a difference that was not statistically significant (P = .9). beta 2-microglobulin, LDH, albumin, and the plasma cell labeling index were all significantly correlated with survival. CONCLUSION: These results indicate that other mechanisms of resistance must be involved in MM apart from MDR. The role of MDR status at this stage of disease may be biased by the major contribution of dexamethasone to induction of response by VAD in MM patients.

In vitro effect of GF120918, a novel reversal agent of multidrug resistance, on acute leukemia and multiple myeloma cells.

Resistance to chemotherapy in multiple myeloma (MM) and acute myeloid leukemia (AML) is frequently caused by multiple drug resistance (MDR), characterized by a decreased intracellular drug accumulation. MDR is associated with expression of P-glycoprotein (P-gp). GF120918, an acridine derivative, enhances doxorubicin cell kill in resistant cell lines. In this study, the effect of GF120918 on MDR cell lines and fresh human leukemia and myeloma cells was investigated. The reduced net intracellular rhodamine-123 (Rh-123) accumulation in the MDR cell lines RPMI 8226/Dox1, /Dox4, /Dox6 and /Dox40 as compared with wild-type 8226/S was reversed by GF120918 (0.5-1.0 microM), and complete inhibition of rhodamine efflux was achieved at 1-2 microM. This effect could be maintained in drug-free medium for at least 5 h. GF120918 reversal activity was significantly reduced with a maximum of 70% in cells incubated with up to 100% serum. GF120918 significantly augmented Rh-123 accumulation in vitro in CD34-positive acute leukemia (AML) blasts and CD38-positive myeloma (MM) plasma cells obtained from 11/27 de novo AML and 2/12 refractory MM patients. A significant correlation was observed between a high P-gp expression and GF120918 induced Rh-123 reversal (P=0.0001). Using a MRK16/IgG2a ratio > or = 1.1, samples could be identified with a high probability of GF120918 reversal of Rh-123 accumulation. In conclusion, GF120918 is a promising MDR reversal agent which is active at clinically achievable serum concentrations.

Reversal of multidrug resistance by SDZ PSC 833, combined with VAD (vincristine, doxorubicin, dexamethasone) in refractory multiple myeloma. A phase I study.

SDZ PSC 833, a non-immunosuppressive cyclosporin analogue reverses multidrug resistance (MDR) in vitro by inhibiting P-glycoprotein (P-gp) mediated drug efflux. We performed a dose escalation study of SDZ PSC 833 combined with VAD chemotherapy in refractory multiple myeloma (MM). Twenty-two MM patients who were refractory to doxorubicin/vincristine/dexamethasone (VADr, n=11) or had failed multiple regimens (n=6) or were melphalan-refractory (MELr, n=5), were treated with one to three cycles of VAD combined with oral SDZ PSC 833, which was administered at escalating dosages starting at 5 mg/kg/day to 15 mg/kg/day for 7 days. The median trough and peak blood levels of SDZ PSC 833 ranged from 461/1134 ng/ml at 5 mg/kg/day to 821/2663 ng/ml at 15 mg/kg, respectively. With addition of SDZ PSC 833 (5 mg/kg) the mean plasma AUC 0-->96 h of doxorubicin as compared with control patients treated with VAD increased from 779 to 1510 ng/ml/h (P=0.0071), while the doxorubicin clearance was reduced from 47.6 to 27.8 l/h/m2 (P=0.0002). The clearance of doxorubicinol was reduced accordingly. Because of the increased plasma AUC, the dose of doxorubicin and vincristine had to be reduced in 13 patients to 50% (n=1) or 75% (n=12). A further dose-escalation of SDZ PSC 833 did not lead to a proportional increase of doxorubicin AUC. Toxicity WHO CTC grade 2 or 3 included hypoplasia (18/22), constipation (10/22), hyponatremia (3/22) and infections (6/22). A partial response or stable disease was achieved in eight and six patients, respectively. In 17 evaluable patients the mean percentage of pretreatment bone marrow plasma cells which expressed P-glycoprotein was 40%. The pretreatment in vitro rhodamin retention in CD38++ myeloma cells was reversible by 2 microM SDZ PSC 833 with 15-98% in 7/9 tested patients. In 4/5 responding patients analyzed before and after treatment with VAD + SDZ PSC 833, a reduction of P-gp + plasma cells was observed. It is concluded, that the blood concentrations of SDZ PSC 833 attained in MM patients increase with dose after oral administration. It can be safely combined with VAD chemotherapy. SDZ PSC 833 diminishes the clearance of doxorubicin, leading to an increase of the plasma AUC of doxorubicin. In addition, it is an effective inhibitor of P-gp mediated efflux of doxorubicin in myeloma tumor cells in vitro. Therefore, a proportional dose-reduction of doxorubicin and vincristine is warranted. Phase II/III studies in refractory MM are in progress to evaluate the therapeutic efficacy of SDZ PSC 833 with VAD.

High expression of the multidrug resistance P-glycoprotein in high-risk myelodysplasia is associated with immature phenotype.

The expression of the multidrug resistance (MDR-1) gene product, P-170 glycoprotein (P-170) was investigated in 26 patients with low-risk (n = 9) or high-risk (n = 17) myelodysplastic syndrome (MDS), using a panel of monoclonal antibodies to P-170 (C219, JSB1, C494, MRK16) and quantitative analysis of MDR-1 mRNA. P-170 membrane staining was demonstrated in bone marrow blast cells of 14/17 HR-MDS and in 2/9 LR-MDS patients (p < 0.01). P-170 expression was associated with the presence of blast cells characterized by an immature or early myeloid phenotype as defined by CD34 expression (p = 0.034), CD13 or CD33 expression (p = 0.0006), or CD13/33 plus terminal deoxynucleotidyl transferase (TdT) double expression (p = 0.04). With double fluorescence analysis, P-170 expression was observed in a subset of CD34+ cells, but not in CD34- cells. P-170 expression was present in 13/15 (86%) patient samples with an abnormal karyotype as compared with 3/10 samples (30%) with a normal karyotype (p < 0.05). Nine of these 15 patients had a loss or a deletion of chromosome 7. Thirteen out of 16 (81%) MDR-1 positive patients developed acute leukemia versus two of ten (20%) MDR-1 negative patients (p = 0.025). It is concluded that MDR-1 expression in MDS is present in cells with an immature phenotype and is frequently observed in patients who have an abnormal karyotype and a high risk of leukemic transformation.

MDR1 expression in poor-risk acute myeloid leukemia with partial or complete monosomy 7.

Expression of the multidrug resistance (MDR1) phenotype, encoded by the MDR1 gene, is an adverse prognostic factor for CR and survival in acute myeloid leukemia (AML). Other prognostic factors, such as specific cytogenetic abnormalities, have been identified in AML. We have investigated the expression of the MDR1 gene in untreated AML patients with monosomy 7 (n = 12), and partial deletions (n = 7) of the long arm of chromosome 7 (respectively -7/7q-), because of the extremely bad prognosis associated with these cytogenetic abnormalities and because of the fact that the MDR1 gene is located on chromosome 7q21.1. The findings were compared with the level of MDR1 expression in a group of 42 other AML patients, matched for age with favourable, neutral or complex cytogenetic abberations. MDR1 mRNA expression, as measured by the RNase protection assay was significantly higher in the -7/7q- group vs other AML patients (median 1.3 vs 0.1 arbitrary units, P = 0.02). Protein expression of MDR1 in the -7/7q- group, as determined with the monoclonal antibody MRK16, was found to be similar to the levels found in the control group. With a functional rhodamine retention assay using the modulator PSC833, increased MDR1 activity was observed in the -7/7q- group as compared to the control group of patients (P = 0.05). Considering the higher MDR1 mRNA expression and equal or slightly elevated level of protein expression of MDR1, we studied the presence of MDR1 genes in this group of -7/7q- patients. Fluorescence in situ hybridization (FISH) studies, using a specific MDR1 probe revealed no loss of an MDR1 allele in any of the deleted q- arms of the seven patients with 7q-, whereas all monosomy 7 patients lacked one MDR1 gene homologue. To determine whether there was selective loss of the MDR1 gene in the -7/7q- patients, the genetic polymorphism of the MDR1 gene was used. Both allelic variants (G and T) were represented in the -7/7q- and in the control group, showing a predominance for GT at position 2677 of the MDR1 gene in the control group. In the 12 monosomy 7 patients loss of the MDR1 allele was random. Methylation studies of the CpG island of the MDR1 gene revealed no hypermethylation in any of the -7/7q- patients. We conclude that MDR1 expression in -7/7q- AML patients is upregulated at transcriptional, but not at translational level, suggesting that mechanisms other than MDR1 are responsible for the poor prognosis in these patients.

Do P-glycoprotein and major vault protein (MVP/LRP) expression correlate with in vitro daunorubicin resistance in acute myeloid leukemia?

Two proteins that have been correlated with the occurrence of multidrug resistance in acute myeloid leukemia (AML) are P-glycoprotein (Pgp) and the major vault protein (Mvp/LRP). With the purpose of further quantifying the potential contributions of Pgp-mediated drug efflux and Mvp/LRP to drug resistance in AML we have investigated whether the transport function of Pgp and the expression of Mvp/LRP correlated with the accumulation of daunorubicin (DNR) and the in vitro resistance to DNR cytotoxicity (LC50 by MTT assay) in AML cells. In de novo adult AML, the steady-state DNR accumulation (in pmol/10(6) cells) correlated with Pgp activity or expression, whereas the LC50 for DNR did not correlate with Pgp activity (measured as the modulation of rhodamine 123 or DNR accumulation by the Pgp inhibitor PSC833) or Pgp expression (measured by flow cytometry with the MRK-16 antibody). The contribution of MRP1 expression to a reduced DNR accumulation seems minor compared to Pgp. In addition, the modulation of the DNR LC50 by PSC833 did not correlate with Pgp protein or activity. The steady-state DNR accumulation showed no correlation with the DNR LC50. The Mvp/LRP expression (immunocytochemical staining) did neither correlate with DNR accumulation nor with the DNR LC50. A significant negative correlation was seen between the Mvp/LRP immunocytochemical staining and Pgp activity, indicating that both markers define (partially) different populations. In conclusion, it is shown that Pgp function, but not Mvp/LRP or MRP1 expression correlate with a low steady-state DNR accumulation in de novo AML. The Pgp activity does, however, not predict the DNR sensitivity in AML measured as in vitro DNR LC50 with an MTT-based assay. The reason for that seems to be that a low DNR accumulation may not be the most important factor in determining the LC50. While the clinical usefulness of these drug resistance tests remains to be proven they do not seem to provide as yet a straightforward explanation for the major cause(s) of clinical chemotherapy failure.

Regulation of interleukin-6 receptor expression by interleukin-6 in human monocytes--a re-examination.

We have studied the expression and regulation of the interleukin-6 receptor (gp80) and its signal transducer gp130 in primary human blood monocytes. Here, we show that freshly isolated human monocytes express mRNAs for gp80 and gp130. In contrast to a previous report [(1989) FEBS Lett. 249, 27-30] we find that neither lipopolysaccharide nor interleukin-6 (IL-6) lead to a down-regulation of IL-6 receptor mRNA in monocytes. Also in the human monocytic cell line Mono Mac 6 no effect of IL-6 on receptor mRNA levels was observed. For signal transducer gp130 mRNA in monocytes a small and transient up-regulation by IL-6 was found.

Nitrous oxide selectively reduces the proliferation of the malignant cells in experimental rat leukemia.

A considerable reduction of hepatosplenomegaly and leucocytosis in leukemic rats of the Brown Norway Myeloid Leukemia (BNML) can be achieved by exposure to 50% nitrous oxide/50% oxygen. In this study the differential antiproliferative effect of nitrous oxide, inactivating vitamin B12, on normal and leukemic hemopoiesis was investigated in this rat model. Rats injected with leukemic cells and exposed to nitrous oxide for 10 days showed 30% reduction of hepatosplenomegaly and 50% reduction of leukocytosis. Similarly treated healthy rats showed no signs of impaired hemopoiesis as measured by peripheral blood parameters. Clonogenic assays of erythroid and myeloid progenitors from both healthy and leukemic rats revealed that exposure to nitrous oxide did not suppress normal bone marrow functioning. On the contrary, the reduction of leukemic proliferation by nitrous oxide retarded the leukemic infiltration of the bone marrow compartment.

The reduction of intracellular polyamines by sequential inhibition of the synthesis of decarboxylated S-adenosylmethionine: effects on rat leukemia.

Decarboxylated S-adenosylmethionine (SAM) is an aminopropyl donor in the synthesis of the polyamines, spermidine and spermine. The decarboxylation of SAM is inhibited by the toxic cytostatic drug methylglyoxalbis-(guanylhydrazone) (MGBG). To achieve more specific and less toxic effects of MGBG, this drug was combined with cycloleucine, which inhibits SAM synthesis, and with nitrous oxide, which inhibits methionine synthetase. This treatment thus aimed at sequential inhibition of the synthesis of decarboxylated SAM, and was studied in a rat leukemia model (BNML). Combined treatment further decreased the level of spermine, but not of spermidine, in leukemic cells, compared to the effects of MGBG alone. The therapeutic effects of this combination were additive or less than additive, however. MGBG was not very effective in reducing leukemic growth and severely toxic, although less with combined treatment. Another inhibitor of SAM decarboxylase, berenil, was also used, and although this drug was about equally active in inhibition of leukemic growth, alterations in intracellular polyamines were not observed. The combination of nitrous oxide and cycloleucine, which effectively reduced leukemic growth at non-toxic dosages, selectively inhibited spermine synthesis, and therefore may be used to interfere with polyamine metabolism. The relevance of this polyamine deprivation to the treatment of leukemia could not be demonstrated.

Effect of nitrous oxide and methotrexate on folate coenzyme pools of blast cells from leukemia patients.

The effects of methotrexate (inhibiting dihydrofolate reductase) and nitrous oxide (inactivating methionine synthase) on intracellular folate coenzyme levels of leukemic cells were studied. Blast cells from 10 cases of acute myeloid leukemia (AML) and 5 cases of acute lymphoid leukemia (ALL) were incubated with 5 x 10(-8) M [3H] 5-formyltetrahydrofolate (5-formylTHF) for 18 h to label intracellular folate pools, which were subsequently quantitated by high performance liquid chromatography (HPLC). In AML, 5-methylTHF made up 53% of the total folate pool followed by 10-formylTHF (26%), 5-formylTHF (10%), THF (9%) and DHF (1%). Cells from ALL differed from AML (p less than 0.05) with respect to 10-formylTHF (17%) and DHF (10%). Exposure to nitrous oxide (8 h) caused an equal decrease of 10-formylTHF and 5-formylTHF in both AML (30%) and ALL (45%), whereas 5-methylTHF increased (130%). Methotrexate (4 h, 10(-6) M) caused an accumulation of DHF and a decrease of 5-methylTHF in both AML (32%) and ALL (12%). A specific reduction of the 10-formylTHF (50%) and 5-formylTHF (25%) pools was noticed in ALL. Exposure to nitrous oxide prior to methotrexate treatment aggravated the reduction of 10-formylTHF and 5-formylTHF presumably by impaired replenishment from the 5-methylTHF pool. In conclusion, this study demonstrates a significant difference in folate coenzyme distribution between cells from AML and ALL. Moreover it is shown that nitrous oxide and methotrexate treatment of leukemic cells cause an accumulation of 5-methylTHF and DHF respectively at the expense of other folate forms. The presence of substantial amounts of DHF in cells from ALL together with the specific reduction of 10-formylTHF (necessary for purine synthesis) during MTX treatment may in part explain the efficacy of methotrexate in the treatment of ALL.

Effect of cobalamin inactivation on folate metabolism of leukemic cells.

Exposure to nitrous oxide inactivates the cobalamin coenzyme of methionine synthetase, an essential enzyme in folate metabolism. Hemopoietic cells are especially dependent on the function of cobalamin for the folate-dependent synthesis of thymidylate (dTMP). Inhibition of methionine synthetase may therefore be of potential value in the treatment of hematological malignancies. In the present study we investigated the effect of nitrous oxide induced cobalamin inactivation on folate metabolism of fresh leukemic cells and the human myelomonocytic cell line U937. Cells were exposed to nitrous oxide for 20 h. Subsequently they were subjected to the deoxyuridine suppression test (dU test), which measures the disturbance of folate-dependent dTMP synthesis. In all bone marrow samples, cobalamin inactivation resulted in a 200% increase of the dU test value, implicating a decreased de-novo synthesis of dTMP. Incubation of leukemic cells with methotrexate, 5-fluorouracil or cycloleucine induced similar increases of the dU test values which could be further raised to 400% with the addition of N2O exposure. Prolonged experiments with U937 cells revealed that the disturbance of folate metabolism aggravated up to 48 h of nitrous oxide exposure. It can be concluded that cobalamin inactivation in human leukemic cells results in disturbed folate-dependent dTMP synthesis. Moreover, effects of several drugs interfering with folate metabolism can be enhanced.

Reversal of typical multidrug resistance by cyclosporin and its non-immunosuppressive analogue SDZ PSC 833 in Chinese hamster ovary cells expressing the mdr1 phenotype.

The new non-immunosuppressive cyclosporin derivative SDZ PSC 833 (PSC) is a potent agent used to overcome typical multidrug resistance (MDR) associated with overexpression of the mdr1 gene encoding for a P-170 glycoprotein. In the present study, the efficacy of PSC as compared with cyclosporin was determined in Chinese hamster ovary cell lines exhibiting different levels of resistance to colchicine (0, 0.1, 0.2 and 10 micrograms/ml, respectively). Low concentrations of PSC (8.2 nM) increased the cytotoxicity of colchicine in cell lines expressing low levels of drug resistance. The concentration resulting in 50% cell survival (LC50 value) found for colchicine alone or in combination with PSC in the CHO-A3 cell line that was resistant to 100 ng colchicine/ml decreased from greater than 500 to 200 ng/ml at 8.2 nM PSC and to less than 100 ng/ml at 82 and 820 nM PSC. In the CHO-A3 cell line that was resistant to 200 ng colchicine/ml, the LC50 values decreased from greater than 500 ng/ml for colchicine alone to 500 ng/ml for colchicine used in combination with 8.2 nM PSC and to less than 100 ng/ml for colchicine combined with 82 or 820 nM PSC. At a concentration of 82 nM PSC, the maximal effect in MDR reversal was observed in the cell lines exhibiting moderate resistance. In the highly resistant cell line, PSC (820 nM) also reversed colchicine resistance. In drug-accumulation experiments, we obtained a 4-fold increase in intracellular doxorubicin accumulation using 820 nM PSC. A comparison of PSC with cyclosporin revealed that a cyclosporin concentration 20-fold that of PSC was required to obtain the same sensitising effect. On the basis of these data, it may be concluded that PSC is a most promising chemosensitiser.

Enhanced therapeutic effect of methotrexate in experimental rat leukemia after inactivation of cobalamin (vitamin B12) by nitrous oxide.

Exposure to nitrous oxide interferes selectively with the coenzyme function of vitamin B12 and causes inactivation of methionine synthetase, with subsequent impairment of folate metabolism and reduction of cellular proliferation. In a rat leukemia model (BNML) we investigated the combined administration of nitrous oxide, inactivating vitamin B12, and methotrexate (MTX), a folate antagonist inhibiting the enzyme dihydrofolate reductase. Through different mechanisms, both agents decrease the availability of tetrahydrofolate, and subsequently of other reduced folates, with increased impairment of folate-dependent synthesis of thymidylate. Effects on leukemic growth and on hematological values in rats demonstrated enhancement of the therapeutic effect of MTX by exposure to nitrous oxide. With several treatment schedules, the results of combined treatment were seen to be better than additive when compared with the effects of single agents. In particular, pretreatment of leukemic rats with nitrous oxide for 3 days before administration of MTX appeared effective. With higher doses of MTX, concomitant exposure to nitrous oxide even resulted in toxic effects. These findings were in accordance with the results of some metabolic studies performed in leukemic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Application of a simple immunoadsorption assay for the measurement of saturated and unsaturated transcobalamin II and R-binders.

In this paper an immunoadsorption method for the selective measurement of the concentrations of saturated and unsaturated transcobalamin II and R-binders in human plasma is presented. With this assay the concentrations of saturated transcobalamin II found in the plasma of 70 normal individuals ranged from 20-220 pmol per litre, with a mean of 70 pmol/l. The concentration of R-binders, carrying cobalamin, ranged from 87-491 pmol/l (mean 195 pmol/l). The 33-203 pmol/l (mean 111 pmol/l) of cobalamin analogues were found to be almost exclusively bound to the R-binder fraction. The level of saturated binding proteins is not, or only marginally, influenced by the absorption of ingested cobalamin, even with an oral dose of 15 micrograms. The concentration of transcobalamin II-bound cobalamin is apparently not determined by the availability of unsaturated binding protein. On the contrary, a positive correlation was found between the R-binder-cobalamin concentration and total R-binder level. These observations suggest that the concentration of transcobalamin II-bound cobalamin is primarily determined by the concentration of cobalamin in the tissues.

Effect of nitrous oxide on folate coenzyme distribution and de novo synthesis of thymidylate in human bone marrow cells.

The effect of nitrous oxide on intracellular folate metabolism of the human bone marrow was studied in vitro. Bone marrow cells, obtained from healthy volunteers, were incubated with 5 x 10(-8)m-[(3)H]5-formyltetrahydrofolate (5-formylTHF) for 18 hr to label intracellular folate pools. Subsequently the cells were exposed to nitrous oxide for up to 10 hr, and the intracellular folate coenzyme levels were quantitated by HPLC. The dU suppression test was carried out on part of the bone marrow samples in order to measure folate-dependent synthesis of the DNA precursor thymidylate (dTMP). After 5 hr exposure to nitrous oxide the de novo dTMP synthesis of the bone marrow cells was significantly decreased (P < 0.05), and this reduced synthesis persisted at 10 hr. After both 5 and 10 hr of exposure to nitrous oxide the amount of 10-formylTHF was reduced (P < 0.05) while that of 5-methylTHF was increased (P < 0.05). At 10 hr the level of THF was also decreased (P < 0.05). This study shows that nitrous oxide exposure of human bone marrow cells causes a redistribution of the various folate coenzymes which supports the idea of 'functional cobalamin deficiency'. Moreover it seems probable that following prolonged exposure to nitrous oxide, not only folate-dependent dTMP synthesis but also de novo purine synthesis is reduced.

In vitro Ig-synthesis and proliferative activity in multiple myeloma are stimulated by different growth factors.

Plasma cells isolated from bone marrow (BM) aspirates of 15 patients with active multiple myeloma (MM) were cultured and analysed for in vitro proliferative response and Ig-synthesis upon stimulation with interleukin-3 (IL-3), interleukin-4 (IL-4) and interleukin-6 (IL-6). The proliferative response, determined as Ki-67 positivity in concentrated plasma cells, was increased by IL-6 (Stimulation Index, SI = 1.77 +/- 0.21 (M +/- SEM] but not by IL-3 or IL-4. This proliferation could be blocked by anti-IL-6. In vitro Ig-synthesis was stimulated by IL-4 (SI = 1.62 +/- 0.12 (M +/- SEM) P less than 0.05) but not by IL-6 or IL-3. This effect was not antagonized by anti-IL-6. An inverse correlation was found in this group of patients between the IL-6 induced stimulation of plasma cell proliferative activity and the IL-4 induced increase of Ig-synthesis (P = 0.027). These data indicate in MM that Ig-synthesis and the in vitro proliferative activity may be stimulated by different haematopoietic growth factors, which may reflect the involvement of different responding cells.