Phase I and pharmacokinetic study of tiazofurin (TCAR, NSC 286193) administered by continuous infusion

Anti-Tumor Potential of IMP Dehydrogenase Inhibitors: A Century-Long Story

The purine nucleotides ATP and GTP are essential precursors to DNA and RNA synthesis and fundamental for energy metabolism. Although de novo purine nucleotide biosynthesis is increased in highly proliferating cells, such as malignant tumors, it is not clear if this is merely a secondary manifestation of increased cell proliferation. Suggestive of a direct causative effect includes evidence that, in some cancer types, the rate-limiting enzyme in de novo GTP biosynthesis, inosine monophosphate dehydrogenase (IMPDH), is upregulated and that the IMPDH inhibitor, mycophenolic acid (MPA), possesses anti-tumor activity. However, historically, enthusiasm for employing IMPDH inhibitors in cancer treatment has been mitigated by their adverse effects at high treatment doses and variable response. Recent advances in our understanding of the mechanistic role of IMPDH in tumorigenesis and cancer progression, as well as the development of IMPDH inhibitors with selective actions on GTP synthesis, have prompted a reappraisal of targeting this enzyme for anti-cancer treatment. In this review, we summarize the history of IMPDH inhibitors, the development of new inhibitors as anti-cancer drugs, and future directions and strategies to overcome existing challenges.

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.

Hematological toxicity associated with tiazofurin-influence on erythropoiesis

In this study hematological toxicity was analyzed after the single and repeated applications of tiazofurin (TZF). Cellularity of bone marrow, spleen and peripheral blood was examined, spanning the period of fifty days after the initial application. Analysis of hematological parameters was performed by slightly modified conventional techniques. The fraction of erythroid series was monitored during the experiment. Presented data describe kinetics of damage and recovery of hemopoietic tissue. Our results indicate that the effect of tiazofurin on cellularity of bone marrow and spleen and on erythropoiesis is reversible and dose dependent within tested dose range and therapeutic regimes. Twenty days after the application normal function of hemopoietic tissues was restored. This approach and results can be useful in defining the timing for sequencing and combination therapy with tiazofurin.

Adenocarcinoma of unknown primary: retrospective analysis of chemosensitivity of 313 freshly explanted tumors in a tumor cloning system

Cancer of unknown primary origin is the eighth most common form of malignancy and accounts for up to 10% of all neoplasms diagnosed. It is a set of heterogenous tumors with widely varying sensitivity to systemic chemotherapy. Over the past years progress has been made in identifying subsets of patients that can be effectively treated with chemotherapy and may achieve long-term survival even with metastatic disease. However, the large majority of cancers of unknown origin still is resistant to chemotherapy. In an attempt to identify conventional and investigational new agents with possible activity against cancers of unknown primary, we have retrospectively analyzed the results of chemosensitivity testing in a soft agar cloning system in vitro and have compared these data with published clinical trials. Between 1978 and 1993, a total of 19584 tumor specimens were studied using a variety of investigational or established antitumor agents. Of these, 615 (3.1%) were tumors of unknown origin and confirmed on pathology review. The largest histologic subgroup was adenocarcinoma (332, 54%). Sufficient numbers of cells for in vitro testing were obtained from 313 tumor specimens (94.3%). Of 278 agents tested in adenocarcinoma of unknown origin, borderline activity (< 20% in vitro response) was noted for 5-FU, doxorubicin, bleomycin, mitoxantrone, mitomycin-C, cisplatin, and etoposide. In vitro response rates of > or = 20% were observed for actinomycin-D, BCNU, melphalan, methotrexate, taxol, and vinblastine. In addition, several investigational agents including fludarabine, amira235, bisantrene, Dupont840, echinomycin, tiazofurin, LY104208 (vinzolidine), intoplicine, and topotecan had activity.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Metabolism and action of purine nucleoside analogs

Recent investigations have identified many new purine nucleoside analogs that act as antimetabolites. This article focuses on the metabolism and mechanisms of action of tiazofurin, 3-deazaguanosine, neplanocin A, arabinosyladenine in combination with inhibitors of adenosine deaminase, arabinosyl-2-fluoroadenine, and 2-chloro-2'-deoxyadenosine, drugs that are either currently being evaluated in clinical trials or are close to that stage. The diverse metabolic requirements for activation, unique mechanisms of action, and differential biological activities of these compounds are characterized and evaluated for prospective therapeutic application.

Tiazofurin: biological effects and clinical uses

Inosine 5'-phosphate dehydrogenase (IMPDH) activity is increased in all cancer cells. It is the rate-limiting enzyme of guanosine triphosphate (GTP) biosynthesis, and therefore, a sensitive target of chemotherapy. Tiazofurin selectively blocks IMPDH activity. Tiazofurin was found to have an antiproliferative effect on tumor cells in vitro and in the murine system. Based on these findings, Phase I trials were started elsewhere in patients with solid tumors, but were discontinued because of toxicity. In leukemic patients, we were able to demonstrate a good correlation between biochemical parameters (i.e., decline in IMPDH activity and GTP concentrations in blast cells) and clinical response. The most consistent responses to therapy were seen in patients with myeloid blast crisis of chronic myeloid leukemia. Severe toxicity was seen in the earlier patients in the study. However, better patient selection, limitation of treatment duration and earlier recognition and treatment of complications have now made it possible to administer tiazofurin without undue toxicity.

Clinical toxicity associated with tiazofurin

Tiazofurin, an investigational antimetabolite, is undergoing clinical evaluation in leukemia. We analyzed the data base of 198 patients entered in Phase I trials to characterize the incidence and severity of toxicities associated with tiazofurin according to dose and schedule. Severe myelosuppression occurred infrequently, and was not dose-dependent. A five day bolus schedule had a higher incidence of severe or life-threatening neutropenia than other schedules. Tiazofurin produced lymphopenia which was not dose-dependent in the range of 23-36% decrease from baseline, and the effect on lymphocyte count was generally greater than the decline in neutrophil count. Non-hematologic toxicity of a moderate or worse severity (greater than or equal to grade 2) included nausea and vomiting (18% of all courses), serum transaminase elevations (SGOT, 16%; SGPT, 9%), rash (9%), stomatitis (3%), conjunctivitis (3%), headache (10%), other signs of central nervous system toxicity (8%), and cardiac toxicity, primarily pleuropericarditis (4%). Dose-related cutaneous toxicity, headache, and nausea and vomiting were evident in the five day bolus schedule, and myalgia was more frequently reported at higher doses on the single dose schedule. The five day continuous infusion (CI) schedule had a higher incidence of neurotoxicity, cardiac toxicity, SGPT elevations and ocular toxicity than the daily for five days bolus schedule, but none of these differences attained statistical significance. Although the peak plasma concentrations of tiazofurin achieved with the five day bolus schedule were 3-fold higher than the steady-state plasma levels seen with an equal dose given by CI, the area under the concentration-time curve (AUC) was approximately 1.6-fold higher with CI. These observations suggest that both high peak plasma concentrations (above 400 microM) and prolonged exposure to plasma levels exceeding 50 microM may result in a higher incidence of serious non-hematologic toxicity.

Phase I trial and biochemical evaluation of tiazofurin administered on a weekly schedule

Tiazofurin (2-B-D-Ribofuranosylthiazole-4-Carboxamide: NSC 286193) is a nucleoside antimetabolite that acts as a potent inhibitor of IMP dehydrogenase resulting in a guanine nucleotide deprivation. Recent in vivo biochemical observations in rats bearing hepatoma suggested a correlation between depletion of guanine nucleotides and antitumor effect. The present phase I trial utilized a weekly x 3 bolus infusion schedule, repeated every 5 weeks. Biochemical measurements of GTP and dGTP were performed in patients at each dose level. Twelve patients received 16 courses of the drug in doses ranging from 1100 to 2050 mg/m2 weekly x 3. The dose limiting toxicities were pericarditis and clinical symptoms suggestive of a more generalized serositis (chest and abdominal pain). Other toxicities included reversible elevations in CPK (MM band only) and SGOT, nausea, vomiting, and arthralgias. Neurotoxic effects were generally mild, including headaches, anxiety, and malaise. Only 1 of 6 patients evaluated for tiazofurin's biochemical activity showed a sustained depletion of guanine nucleotide pools. No antitumor activity was observed. The maximally tolerated dose of tiazofurin on this intermittent weekly x 3 schedule was 1650 mg/m2. Toxicity and the overall lack of biochemical and biologic effect at clinically achievable doses may preclude further clinical evaluation of this drug on a weekly schedule. The toxicities observed in our study were similar to those reported for phase I investigations using a considerably higher dose intensity with daily x 5 schedules.

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.