Clinical and molecular impact of inhibition of IMP dehydrogenase activity by tiazofurin

From ribavirin to NAD analogues and back to ribavirin in search for anticancer agents

Ribavirin, a broad-spectrum antiviral agent is used in the clinic alone or in combination with other antivirals and/or interferons. Numerous structural analogues of ribavirin have been developed, among them tiazofurin, which is inactive against viruses but is a potent anticancer drug. Tiazofurin was found to inhibit nicotinamide adenine dinucleotide (NAD)-dependent inosine monophosphate dehydrogenase (IMPDH) after metabolic conversion into tiazofurin adenine dinucleotide (TAD), which binds well but could not serve as IMPDH cofactor. TAD showed high selectivity against human IMPDH vs. other cellular dehydrogenases. Mycophenolic acid (MPA) was even more specific, binding at the cofactor-binding domain of IMPDH. Ribavirin adenine dinucleotide, however, did not show any significant inhibition at the enzymatic level. We synthesized numerous NAD analogues in which natural nicotinamide riboside was replaced by tiazofurin, MPA moiety, or benzamide riboside, and the adenosine moiety as well as the pyrophosphate linker were broadly modified. Some of these compounds were found to be low nanomolar inhibitors of the enzyme and sub-micromolar inhibitors of cancer cell line proliferation. The best were as potent as tyrosine kinase inhibitor gleevec heralded as a ‘magic bullet’ against chronic myelogenous leukemia. In recent years, ribavirin was rediscovered as a potential anticancer agent against number of tumors including leukemia. It was clearly established that its antitumor activity is related to the inhibition of an oncogene, the eukaryotic translation initiation factor (eIF4E).

Antitumor Activity of Tiazofurin in Human Colon Carcinoma HT-29

Tiazofurin is effective in treating end-stage leukemic patients (Tricot et al., Cancer Res 49:3696-3701, 1989). In sensitive tumors, the active metabolite of tiazofurin, TAD, potently inhibits IMP dehydrogenase activity, resulting in reduced guanylate pools. To elucidate tiazofurin activity in human solid tumors, we examined its activity in human colon carcinoma HT-29. Tiazofurin exhibited an LC50 of 35 microM in cultured HT-29 cells. Incubation of HT-29 cells with 100 microM tiazofurin for 2 h resulted in TAD formation (9.3 nmol/g cells) and in a 64% decrease in GTP pools. For biochemical and chemotherapy studies, athymic nude mice were transplanted s.c. with HT-29 cells. Twenty-four days later, mice were injected i.p. with tiazofurin (500 mg/kg); 6 h later, tumors were removed and analyzed. These tumors formed 17 nmol/g of TAD with decreased GTP pools (56%). To study oncolytic activity, transplanted mice were treated 24 h later with tiazofurin (500 mg/kg, once a day for 10 days). To examine the effectiveness of tiazofurin in established tumors, the drug was administered to mice 14 days after tumor implantation (500 mg/kg, once a day for 5 days, course repeated 4 times with a 10-day rest). Both treatment schedules resulted in significant antitumor activity. This study illustrates the potential usefulness of tiazofurin in treating human colon carcinoma.

In vitro sensitivity of hematopoietic progenitors to tiazofurin in refractory acute myeloid leukemia and in the blast crisis of chronic myeloid leukemia

The effect of Tiazofurin (TR) on the in vitro growth of bone marrow (BM) and peripheral blood (PB) leukemic progenitors was investigated in 29 patients. Nineteen of the patients were suffering the blast crisis of chronic myeloid leukemia (bcCML) and ten patients refractory acute myeloid leukemia (AML). PB and BM mononuclear cells were cultured in methylcellulose alone or with concentrations of TR ranging between 10 and 200 microM. TR produced a dose dependent inhibition of colony forming unit (CFU)-blast growth in all the samples tested from BM and PB. The most effective concentrations of TR used were 150 and 200 microM, while concentrations of less than 50 microM TR were not adequate for 50% inhibition of cell growth (IC50). Differences were found in the response of CFU-blasts to TR related to the type of underlying leukemia. Inhibition of CFU-blast growth was more pronounced in bcCML than in AML in both the BM and PB samples. The concentration of TR required to induce IC50 in bcCML was 50 microM, while the same effect in AML required a concentration of 150 microM. Analysis of the control samples also revealed that CFU-blasts from bcCML produced smaller numbers of colonies, though these differences were not statistically significant. It has therefore been demonstrated that TR has strong in vitro anti-leukemic activity, more pronounced in bcCML than in refractory AML. We thus feel this study gives further rationale for the clinical application of TR, and would strongly support this.

Differentiation induction in non-lymphocytic leukemia cells upon treatment with mycophenolate mofetil

Inosine 5'-monophosphate (IMP) dehydrogenase catalyzes the rate-limiting reaction of guanine nucleotide biosynthesis and has been implicated in the reaction of cell growth and differentiation. We investigated the ability of mycophenolate mofetil, a prodrug of mycophenolic acid, to induce differentiation in HL-60 and U937 leukemic cells as well as in fresh leukemia cells from patients with non-lymphocytic leukemia. Treatment with mycophenolate mofetil reduced the intracellular guanosine 5'-triphosphate (GTP) levels and induced morphologic and functional differentiation in HL-60 and U937 cells dose-dependently. HL-60 and U937 cells developed macrophage-like cytoplasm as well as the expression of CD11b and CD14 antigens and the ability to oxidize nitroblue tetrazorium (NBT). These changes became evident when the intracellular GTP levels decreased to approximately 20-30% of the untreated control level and were abrogated by the addition of guanosine. In the fresh leukemic cells, differentiation induction was shown in the cells derived from seven of 13 patients. The fresh leukemia cells responding to mycophenolate mofetil revealed significant higher positivity to CD11b, CD14, and NBT before treatment and significantly reduced intracellular GTP levels after treatment compared to the non-responding cells. These findings suggest that mycophenolate mofetil induces differentiation in HL-60 and U937 cells and some fresh leukemia cells with moderate tendency to maturation, by causing a decrease in the intracellular GTP levels. Mycophenolate mofetil could be a promising differentiation inducer in vivo.

Pilot trial of ribavirin for the treatment of laryngeal papillomatosis

The antiviral drug ribavirin was used as an adjunct to laser surgery for the treatment of patients with laryngeal papillomatosis (LP). An uncontrolled clinical trial for four patients with ribavirin treatment at a daily dose of 23 mg/kg was performed. Three adults received drug prior to laser surgery and continuing orally for 6 months. One infant was treated for 3 months. Two adults achieved complete remissions for at least 2 consecutive months, and both patients developed only minimal recurrent disease in 4 months of follow-up. The other adult and the child sustained a partial response and an increased interval between the required surgeries. Ribavirin caused only a mild, reversible reduction in hemoglobin and reticulocytosis. This preliminary trial shows that ribavirin may be an effective therapy in combination with surgery for LP in a larger controlled clinical trial.

Synergistic action of tiazofurin and difluorodeoxycytidine on differentiation and cytotoxicity

Tiazofurin (TR), an inhibitor of IMP dehydrogenase, causes remissions and induced differentiation in human leukemia through lowering the concentrations of GTP and dGTP. A deoxycytidine analog, difluorodeoxycytidine (DFDC), is an anti-tumor agent phosphorylated by deoxycytidine kinase, resulting in decreased concentration of dCTP, leading to inhibition of DNA synthesis. In HL-60 cells DFDC induced differentiation and inhibited proliferation in a dose-dependent manner (IC50 = 4 nM); TR provided synergism with DFDC. DFDC inhibited proliferation in OVCAR-5 human ovarian carcinoma cells (IC50 = 25 nM) and colony formation in PANC-1 human pancreatic carcinoma cells (IC50 = 2 nM) and rat hepatoma 3924A cells (IC50 = 22 nM). TR and DFDC are synergistically cytotoxic in hepatoma cells and additive in PANC-1 cells. The two drugs together should be helpful in treating leukemias and solid tumors in humans.

Clinical pharmacokinetic study of tiazofurin administered as a 1-hour infusion

Tiazofurin, 2-beta-D-ribofuranosylthiazole-4-carboxamide, is cytotoxic to murine and human tumor cells. In earlier Phase-I/-II trials performed in other centers in patients with solid tumors, the drug was given mainly as a 10-min bolus or as a continuous i.v. infusion for 5 days. These protocols were associated with serious side effects, including neurotoxicity, pleuropericarditis, and occasional myelosuppression. In our study, 26 patients with end-stage leukemia were treated with tiazofurin with 1-hr daily i.v. infusions, resulting in lower incidence and less severity of side effects. In this group, 7 attained complete remission and 7 showed hematologic responses. Out of 12 evaluable patients with myeloid blast crisis of chronic granulocytic leukemia, 10 (83%) responded to therapy, with 6 attaining complete response. We present pharmacokinetic parameters of our clinical study and examine some of the reasons for the lower toxicity found in our trials. In leukemic patients during and after infusion at doses of 1,100, 2,200 and 3,300 mg/m2 tiazofurin peak plasma concentrations were 245, 441 and 736 microM, respectively, values one-half of those calculated from other reports with a 10-min bolus administration. In our 1-hr infusion method, biphasic pharmacokinetics were noted with alpha t1/2 and beta t1/2 of 0.5 and 6.2 hr, and tiazofurin was eliminated at a faster rate than in previous trials with continuous infusion. The area under the curve with our 1-hr infusion was 52% of that reported for the same dose given by continuous infusion. Our 1-hr infusion method and prompt and effective treatment of side effects enabled us to administer higher doses and larger total amounts of tiazofurin in longer treatment cycles than in any previous trials elsewhere. Tiazofurin therapy using 1-hr infusion may be feasible for other carefully selected types of malignancies.

Novobiocin-induced anti-proliferative and differentiating effects in melanoma B16

The antibiotic drug novobiocin was evaluated for its anti-tumour properties in B16 melanoma cells. Novobiocin is shown to inhibit melanoma B16 cell proliferation. The anti-proliferative effect was gradually reversible upon removal of novobiocin from the culture medium. Growth inhibition by novobiocin was accompanied by phenotypic alterations, that included morphological changes, lipid accumulation and marked increases in the activities of NADPH cytochrome c reductase and gamma glutamyl transpeptidase. In vivo administration of repeated i.p. doses of novobiocin, to mice implanted with B16 melanoma cells resulted in growth retardation. The combined treatment of the B16 melanoma cells with novobiocin and other chemical inducers of differentiation was examined in a cell growth assay. Novobiocin and sodium butyrate inhibited cell growth in a near additive manner, while combination of novobiocin with the GTP-depleting agents, tiazofurin or mycophenolic acid resulted in a synergistic decrease in cell growth. Our results support the contention further that novobiocin and other differentiating agents might be of potential value in melanoma therapy.

Regulation of GTP biosynthesis

In the regulation of GTP biosynthesis, complex interactions are observed. A major factor is the behavior of the activity of IMPDH, the rate-limiting enzyme of de novo GTP biosynthesis, and the activity of GPRT, the salvage enzyme of guanylate production. The activities of GMP synthase, GMP kinase and nucleoside-diphosphate kinase are also relevant. In neoplastic transformation, the activities and amounts of all these biosynthetic enzymes are elevated as shown by kinetic assays and by immunotitration for IMPDH. In cancer cells, the up-regulation of guanylate biosynthesis is amplified by the concurrent decrease in activities of the catabolic enzymes, nucleotidase, nucleoside phosphorylase, and the rate-limiting purine catabolic enzyme, xanthine oxidase. The up-regulation of the capacity for GTP biosynthesis is also manifested in the stepped-up capacity of the overall pathways of de novo and salvage guanylate production. The linking with neoplasia is also seen in the elevation of the activities of IMPDH and GMP synthase and de novo and salvage pathways as the proliferative program is expressed as cancer cells enter log phase in tissue culture. The activity of GMP reductase showed no linkage with neoplastic or normal cell proliferation; however, in induced differentiation in HL-60 cells the activity increased concurrently with the decline in the activity of IMPDH. This reciprocal regulation of the two enzymes is observed in differentiation induced by retinoic acid, DMSO or TPA in HL-60 cells. In support of enzyme-pattern-targeted chemotherapy, evidence was provided for synergistic chemotherapy with tiazofurin (inhibitor of IMPDH) and hypoxanthine (competitive inhibitor of GPRT and guanine salvage activity) in patients and in tissue culture cell lines. These investigations should contribute to the clarification of the controlling factors of GMP biosynthesis, the role of the various enzymes, the behavior of GMP reductase in mammalian cells and the application of the approaches of enzyme-pattern-targeted chemotherapy in patients.

Schedule-dependent synergistic action of tiazofurin and dipyridamole on hepatoma 3924A cells

Tiazofurin is an oncolytic nucleoside analog that has shown therapeutic activity in end-stage acute non-lymphocytic leukemia and in chronic granulocytic leukemia in blast crisis. Tiazofurin is anabolized to the active metabolite, TAD, which inhibits IMP dehydrogenase activity, leading to a reduction in guanylate pools and to the cessation of neoplastic cell proliferation. The drug exhibits potent cytostatic and cytotoxic activity against hepatoma 3924A cells in culture. In growth-inhibition and clonogenic assays, the 50% inhibitory concentration of tiazofurin was 3.8 and 4.2 microM, respectively. Dipyridamole, an inhibitor of nucleoside transport, curtails the salvage of nucleosides and bases for nucleotide biosynthesis. Dipyridamole exhibited cytotoxicity against hepatoma 3924A cells, with an LC50 of 24 microM and an IC50 of 29 microM being recorded. A combination of tiazofurin and dipyridamole provided synergistic cytotoxicity in hepatoma 3924A cells in culture. This synergistic activity was dependent on the order of addition of the drugs. Simultaneous addition of the two drugs produced antagonism, whereas preincubation of cells with tiazofurin or dipyridamole followed by addition of the second drug resulted in synergy. TAD concentrations were significantly higher (129% and 135%) in cells that had been pretreated with tiazofurin or dipyridamole before the addition of the second agent as compared with cells that had been treated simultaneously (113%). These studies indicate the importance of the order of the addition of drugs to obtain a synergistic response in combination chemotherapy and suggest the need for a careful selection of drug modulation in clinical trials of tiazofurin and dipyridamole.

Dimethylthiourea inhibition of B16 melanoma growth and induction of phenotypic alterations; relationship to ATP levels

1,3 Dimethylthiourea (DMTU) has previously been shown by us to inhibit the growth of melanoma cells and to induce phenotypic alterations in these cells, including ultrastructural alterations of mitochondria. These findings raised the possibility that impaired mitochondrial function might be involved in mediating the effect of DMTU on cell growth and phenotypic expression. The present study indicates that DMTU as well as another growth inhibitory methylurea derivative, tetramethylurea (TMU) significantly decrease ATP content in the B16 melanoma cell line. 1,3 Dimethylurea (1,3DMU) and 1,1 dimethylurea (1,1DMU) which are poor growth inhibitors, do not reduce ATP content significantly. Altered energy metabolism in the DMTU-treated cells is reflected by inhibition of the activity of cytochrome c oxidase and by increased lactate levels. A cell line selected for resistance to growth inhibition by DMTU was shown to be completely resistant to induction of phenotypic alterations by DMTU. These cells possess high lactate levels, high ATP content and a somewhat decreased Na/K ATPase activity as compared to wild type B16 F10 cells. 1,3 DMTU treatment of the resistant cells leads to a decrease in the activity of the mitochondrial enzyme cytochrome c oxidase, similar to its effect on the wild type B16 F10 cells. DMTU also reduces ATP content moderately in the resistant cells. However, the levels of ATP do not decrease beyond those found in untreated B16 F10 wild type cells. Taken together the results suggest that decreased ATP content might be involved, at least partially, in mediating the effects of DMTU on B16 melanoma cell growth and phenotypic expression.

Determination of thiazole-4-carboxamide adenine dinucleotide (TAD) levels in mononuclear cells of leukemic patients treated with tiazofurin

Tiazofurin is an oncolytic agent which has shown therapeutic activity in end-stage acute nonlymphocytic leukemia (ANLL) and blast crisis of chronic granulocytic leukemia (CGL-BC). Tiazofurin is anabolized to the active metabolite, thiazole-4-carboxamide adenine dinucleotide (TAD), which inhibits IMP dehydrogenase activity, leading to reduction of guanylate pools and cessation of cancer cell proliferation. The concentration of TAD in neoplastic cells of patients treated with tiazofurin should be a good indicator of sensitivity to the drug and also might herald the emergence of drug-resistant cells. Therefore, the precise quantitation of TAD in cancer cells during tiazofurin treatment is essential. In this paper we report a highly sensitive method for the determination of TAD in biological samples. With this technique, in addition to TAD, thirteen other biologically relevant adenine, guanine, cytosine and uridine nucleotides can be separated and quantitated accurately. TAD standard was separated on a Waters Partisil 10-SAX column in a RCM-10 module using an ammonium phosphate buffer system. TAD eluted at 21 min with a limit of detection of 15 pmol and linearity up to 3 nmol. The coefficient of variation was 0.6 +/- 0.1% for retention time and 2 +/- 0.3% for TAD concentration. Recovery of TAD was 96% with reproducibility of 98%. To examine the applicability of this method to a clinical setting, blood samples were obtained from a patient with CGL-BC and leukocytes were separated on a Ficoll-Hypaq gradient, extracted with trichloroacetic acid, and an aliquot was analyzed on HPLC. The TAD peak was identified by comparing the retention time and spectral analysis of the standard. After the patient was treated with a 2200 mg/m2 (12.7 mM) dose of tiazofurin, the TAD concentrations in the mononuclear cells at 2, 6, and 24 hr were 23.1, 13.6, and 0.8 microM. TAD levels at 2, 6, and 24 hr after a tiazofurin dose of 3300 mg/m2 (21.1 mM) were 42.8, 26.1, and 1.4 microM respectively.

Regulation of de novo and salvage pathways in chemotherapy

An overview was presented of our approach of inhibition of de novo and salvage pathways in pyrimidine and purine metabolism. 1. Combination of acivicin, an inhibitor of de novo biosynthesis, and dipyridamole, a transport inhibitor, provided synergistic cytotoxicity in hepatoma and colon carcinoma cells. 2. AZT, a competitive inhibitor of the salvage enzyme, thymidine kinase, and 5-FU or MTX provided synergistic cytotoxicity in hepatoma 3924A. In human colon carcinoma HT-29 cells AZT and methotrexate yielded synergistic cytotoxicity and thymidine and hypoxanthine together provided protection from the action of these drugs. 3. These observations are significant because in rat hepatoma 3924A and in human cell lines HT-29, HL-60 and K562 thymidine kinase activity was 16- to 67-fold higher than that of dTMP synthase. Therefore, inhibition of dTMP synthase activity alone may provide poor responses because the salvage pathways can circumvent this block. 4. In leukemic patients treated with tiazofurin, an inhibitor of IMP dehydrogenase, the rate-limiting enzyme of GTP biosynthesis, and with allopurinol, which inhibits GPRT activity through raising plasma hypoxanthine levels, synergistic therapeutic results were obtained. The responses in sensitive patients entailed a decrease in IMP dehydrogenase activity and GTP concentration in leukemic cells and down-regulation of the ras and myc oncogenes. The down-regulation of the ras oncogene by tiazofurin through the decrease of GTP concentration has now been shown in K562, HL-60 and hepatoma cells and in patients with chronic granulocytic leukemia in blast crisis. Tiazofurin may be useful in studies on selective depression of the expression of the ras oncogene. 5. In 27 consecutive patients 50% responded positively to tiazofurin treatment. From this group, 10 out of 12 patients (83%) with chronic granulocytic leukemia in blast crisis responded to tiazofurin treatment.

Tiazofurin action in leukemia: evidence for down-regulation of oncogenes and synergism with retinoic acid

New light was thrown on the action of tiazofurin in the treatment of end-stage leukemic patients and in leukemic cells in tissue culture. 1. In a population of 21 consecutive patients 50% responded to tiazofurin treatment, confirming the usefulness of this therapy in end-stage leukemia. 2. In leukemic patients treated with tiazofurin and allopurinol reciprocal action was manifested in the increase in hypoxanthine and the decrease in uric acid concentrations in the plasma. On discontinuation of allopurinol, hypoxanthine levels steeply declined but uric acid concentration increased slowly, taking days to reach pretreatment level. 3. With a new and sensitive method the concentration of the active metabolite of tiazofurin, TAD, was measured in the mononuclear cells of tiazofurin-treated patients. Approximately 5 to 13% of the plasma tiazofurin level was observed as TAD in the mononuclear cells. This TAD concentration was sufficient to account for the inhibition of IMP DH in these cells. 4. Tiazofurin or retinoic acid caused differentiation of HL-60 leukemic cells and inhibition of cell proliferation. 5. By treating leukemic cells incubated with tiazofurin or retinoic acid also with guanosine it was elucidated that the mechanism of the two drugs differed since only the tiazofurin effects were counteracted by guanosine. 6. Tiazofurin and retinoic acid together in HL-60 cells provided synergistic impact on differentiation and cytotoxicity. 7. Tiazofurin resulted in down-regulation of the expression of ras and myc oncogenes in three systems: K562 human erythroleukemic cells, rat hepatoma 3924A cells and human HL-60 leukemia cells. 8. Because both tiazofurin and retinoic acid are licensed drugs, their potential use in combination chemotherapy may have clinical relevance in the treatment of end-stage leukemia where our earlier studies have demonstrated the usefulness of tiazofurin.

Critical issues in chemotherapy with tiazofurin

Some of the current critical issues in the tiazofurin treatment of end-stage leukemia were presented and discussed. 1. Tiazofurin infusions (daily X 10 to 15) provided remissions in 50% of end-stage leukemic patients. The remissions, of 1 to 10 months' duration, varied from antileukemic effect or hematologic improvement to complete response and complete remission. The total survival of the responding patients was from about 1 to 15 months. 2. Our administration of tiazofurin in a 60-min infusion by pump decreased the incidence and severity of toxicity. 3. It was shown that tiazofurin dose does not need to be escalated at each relapse. Depending on the biochemical and hematological response in this novel protocol, 2,200 to 4,400 mg/m2 tiazofurin appeared to be sufficient to provide remissions. 4. A new role was identified for allopurinol, originally given to decrease uric acid in the plasma. Allopurinol markedly increased plasma hypoxanthine concentrations which competitively inhibited the activity of the salvage enzyme, guanine phosphoribosyltransferase, in the blast cells. Thus, the elevated hypoxanthine plasma levels inhibited guanine salvage. To maintain high hypoxanthine levels allopurinol (100 mg) was given every 4 to 6 hr. This provided combination chemotherapy with tiazofurin which inhibited IMP dehydrogenase activity and blocked the de novo biosynthesis of guanylates in the blast cells. 5. Preliminary evidence was obtained in the patients that tiazofurin induced differentiation of the bone marrow. Recent studies also showed that tiazofurin down-regulated the expression of the c-Ki-ras oncogene in K562 erythroleukemic cells. Therefore, tiazofurin treatment provides an impact by chemotherapy, induced differentiation, and, if applicable, through down-regulation of the ras oncogene. 6. Novel aspects of tiazofurin treatment include rational targeting and a continuously monitored trial by measurement of the activity of IMP dehydrogenase and of GTP and TAD concentrations in blast cells and of tiazofurin and hypoxanthine in plasma. 7. Since tiazofurin has not yet achieved lasting remissions in patients nor terminal differentiation of leukemic cells it probably will be advantageous to combine tiazofurin with other drugs to provide synergism. In preclinical tissue culture studies in HL-60 cells synergy was observed with retinoic acid. This may be of interest because retinoic acid also caused differentiation and down-regulation of the myc oncogene.