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SRO-91 is a non-natural ribofuranosyl-1,2,3-triazole C-nucleoside obtained by a synthetic sequence involving a C-alkynyl glycosylation mediated by metallic indium and a Huisgen cycloaddition for the construction of the triazole. Its structure is close to the one of ribavirin, a drug presenting a broad-spectrum against viral infections. SRO-91 antitumor activities were investigated on 9 strains of tumor cells and IC 50 of the order of 1 μM were obtained on A431 epidermoid carcinoma cells and B16F10 skin melanoma cells. In addition, studies of ovarian tumor cell inhibitions show an interesting activity in regard to the need for new drugs for this pathology. Finally, cytotoxicity and mouse toxicity studies reveal a favorable therapeutic index for SRO-91., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2019 Elsevier Masson SAS. All rights reserved.)

A divergent synthesis of two novel tiazofurin analogues, 2-(3-deoxy-3-fluoro-beta-D-xylofuranosyl)thiazole-4-carboxamide (2) and 2-(3-acetamido-3-deoxy-beta-D-xylofuranosyl)thiazole-4-carboxamide (3), has been achieved starting from D-glucose. Both nucleoside analogues were evaluated for their in vitro cytotoxicity against several human leukaemia and solid tumour cell lines.

Tiazofurine is a nucleoside analog with oncolytic activity being developed by Ribapharm (formerly ICN Pharmaceuticals) as a potential treatment for leukemia. It is metabolized to TAD (thiazole-4-carboxamide adenine dinucleotide), an inhibitor of IMP dehydrogenase. This inhibition results in the reduction of guanylate levels and the halting of neoplastic proliferation. The compound is in phase II/III trials [215553]. It is expected that Ribapharm will file an orphan drug application for tiazofurine, as a treatment for myelogenous leukemia, following the drug's completion of phase III trials by the end of 2002. The company has reported that phase III trials will begin by the end of 2000. Preliminary studies involving 21 patients have been carried out and the results reported by the company. During these studies, seven patients with chronic myelogenous leukemia had a complete hematologic response and two patients had a partial response. Of the patients with a complete response, six had marrow and peripheral responses. Ribapharm, through a Russian subsidiary of ICN, is also planning to conduct phase II studies of tiazofurine involving patients suffering from advanced ovarian cancer or multiple myeloma which is resistant to conventional therapy. The company has reported that the multiple myeloma limited phase II study is still undergoing planning, with an intended start date in late 2000 [381453]. In March 2000, Chase Hambrecht & Quist predicted that first approval could be towards the end of 2001 [384894].

Inosine 5 -monophosphate dehydrogenase (IMPDH) is a rate-limiting enzyme for the synthesis of GTP and dGTP. Two isoforms of IMPDH have been identified. IMPDH Type I is ubiquitous and predominantly present in normal cells, whereas IMPDH Type II is predominant in malignant cells. IMPDH plays an important role in the expression of cellular genes, such as p53, c-myc and Ki-ras. IMPDH activity is transformation and progression linked in cancer cells. IMPDH inhibitors, tiazofurin, selenazofurin, and benzamide riboside share similar mechanism of action and are metabolized to their respective NAD analogues to exert antitumor activity. Tiazofurin exhibits clinical responses in patients with acute myeloid leukemia and chronic myeloid leukemia in blast crisis. These responses relate to the level of the NAD analogue formed in the leukemic cells. Resistance to tiazofurin and related IMPDH inhibitors relate mainly to a decrease in NMN adenylyltransferase activity. IMPDH inhbitors induce apoptosis. IMPDH inhitors are valuable probes for examining biochemical functions of GTP as they selectively reduce guanylate concentration. Incomplete depletion of cellular GTP level seems to down-regulate G-protein function, thereby inhibit cell growth or induce apoptosis. Inosine 5'-monophosphate dehydrogenase (IMPDH, EC 1.1.1.205) catalyzes the dehydrogenation of IMP to XMP utilizing NAD as the proton acceptor. Studies have demonstrated that IMPDH is a rate-limiting step in the de novo synthesis of guanylates, including GTP and dGTP. The importance of IMPDH is central because dGTP is required for the DNA synthesis and GTP plays a major role not only for the cellular activity but also for cellular regulation. Two isoforms of IMPDH have been demonstrated. IMPDH Type I is ubiquitous and predominately present in normal cells, whereas the IMPDH Type II enzyme is predominant in malignant cells. Although guanylates could be salvaged from guanine by the enzyme hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8), the level of circulating guanine is low in dividing cells and this route is probably insufficient to satisfy the needs of guanylates in the cells.

A phase I trial of 2-beta-D-ribofuranosylthiazole-4-carboxamide (NCS 286193, tiazofurin) was conducted using a 5-day i.v. bolus schedule, every 21 days. Thirty one patients with advanced cancer were entered on the trial. A total of 106 cycles were administered with doses ranging from 550 to 2750 mg/m2. Concomitant administration of Allopurinol was necessary to prevent hyperuricemia. Tiazofurin was difficult to evaluate and many side effects were variable and sporadic. The dose limiting toxicities were nonhematologic consisting particularly of myalgias, headaches and general malaise. Other toxicities included nausea, vomiting, stomatitis, lethargy, sleeping difficulty, sinus bradycardia, skin rash, desquamation of the palms and soles, photophobias and burning of the eyes. Hematologic toxicity was mild and not dose related though it led to a neutropenic septic death in one patient at 2750 mg/m2. Anemia was documented in 60% of cycles. Biochemical abnormalities consisted of mild hyperglycemia, hyperuricemia and elevated skeletal creatinine phosphokinase levels which did not correlate with the incidence or degree of myalgias. Though some patients were able to tolerate higher doses, the recommended dose for phase 2 study is 1650 mg/m2. Further studies will be required to achieve a better understanding of this interesting drug.

Tiazofurin (2-beta-D-ribofuranosylthiazole-4-carboxamide, TCAR) is a synthetic C-nucleoside that demonstrated significant in vivo activity against a variety of animal tumors as well as in vitro activity against human tumor-derived cell lines. Thirteen patients were treated with TCAR administered as a 5-day continuous infusion in this Phase I trial. Seventeen complete cycles were administered in three dose levels ranging from 550 to 1450 mg/M2. Dose-limiting toxicities were myelosuppression and neurotoxicity including severe lethargy. Other toxicities including superficial skin peeling, myalgias, and tearing were seen at all doses. One patient had chest pain on day 4 resulting in stopping the drug, however, there was no evidence of cardiac or pericardial disease. Uric acid levels rose within one day in the absence of allopurinol treatment. There were no treatment related deaths. HPLC measurement of drug levels demonstrated steady-state plasma levels during the infusion, and a half-life following the infusion of 7.7 +/- 0.6 hours. Minor abnormalities in renal function were associated with dramatic changes in pharmacokinetics and toxicity. No clinical responses were observed in this trial.

Tiazofurin is an interesting drug now entering Phase I trials, with marked preclinical antitumor activity against P388 and L1210 leukemias, and the Lewis lung carcinoma. Schedule dependency favoring frequent administration has been noted. The drug has a novel mechanism of action, being metabolized to an inhibitory cofactor of inosine monophosphate dehydrogenase. Tiazofurin is widely distributed after i.v. administration exhibiting a triphasic pattern of plasma decay, with a terminal half-life of 3-16 h in the three species studied. Approximately 90% of the drug was excreted unchanged in the urine within 24 h. A significant potential for the slower release of intracellularly retained drug exists. Anticipated organ toxicities based on the studies described include myelotoxicity, hepatotoxicity and nephrotoxicity. These were mild and reversible at lower doses, and were not seen at levels corresponding to the starting doses in man. A potential for hyperuricemia exists; this should be easily controllable by the use of allopurinol, without compromising the drug's antitumor effect. Phase I trials under the sponsorship of the NCI are underway in a number of institutions.

Tiazofurin, a C-nucleoside, was cytotoxic in hepatoma 3924A cells grown in culture with an LC50 = 7.5 microM. In the culture, a closely linked dose-related response of tumor cell-kill and depletion of GTP pools was observed after tiazofurin treatment. In rats carrying subcutaneously transplanted hepatoma 3924A solid tumors, a single intraperitoneal injection of tiazofurin (200 mg/kg) caused a rapid inhibition of IMP dehydrogenase (EC 1.2.1.14) activity and depleted GDP, GTP, and dGTP pools in the tumor; concurrently, the 5-phosphoribosyl 1-pyrophosphate (PRPP) and IMP pools expanded 8- and 15-fold, respectively. Tiazofurin decreased tumoral IMP dehydrogenase activity and dGTP pools in a dose-dependent manner over a range of 50-200 mg/kg; by contrast, the depletion of GTP and the accumulation of IMP and PRPP pools were near maximum at 50 mg/kg. The increase in PRPP pools may be attributed to an inhibition by IMP of the activity of hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8). The IMP dehydrogenase activity and the pools of ribonucleotides returned to the normal range by 24-48 h after the single injection of tiazofurin. However, the markedly depleted dGTP pools remained low for 72 h. Tiazofurin treatment resulted in significant anti-tumor activity in rats inoculated with hepatoma 3924A. The decrease in GTP levels and particularly the sustained depletion in the dGTP pools may explain, in part at least, the chemo-therapeutic action of tiazofurin on hepatoma 3924A. This is the first report showing that a marked therapeutic response was achieved against rapidly growing hepatoma 3924A by treatment with a single anti-metabolite.

Lewis lung carcinoma (LLC) induces a range of hemopoietic alterations in its murine host including progressive anemia, thrombocytopenia, splenomegaly, neutrophilia, and marrow and splenic myeloid hyperplasia. Concentrations of both pluripotent and committed marrow hemopoietic progenitors is increased and the cycling fraction of granulocyte-macrophage progenitors is accelerated. We have developed a way to study whether these hemopoietic effects become long-term consequences of cancer, using LLC-bearing mice with advanced tumor treated with the antineoplastic agent tiazofurin, 2-beta-D-ribofuranosyl-thiazole-4-carboxyamide, NSC 286193 (TZ). LLC mice were treated with a single dose of TZ either 150, 300, or 600 mg/kg, intraperitoneally on day 6 posttumor implant when lung metastases are present and all hemopoietic effects of the tumor are recognizable. Even a single dose of 150 mg/kg of TZ produced a significant survival advantage, and 600 mg/kg resulted in 30% of the animals remaining disease free during a 5-month follow-up. A 6-week treatment schedule was devised, administering TZ intraperitoneally, 600 mg/kg, weekly beginning on day 6. In this group, median survival was not reached after 9 months of follow-up. The only evidence of myelotoxicity produced by intermittent administration of TZ was a mild anemia which was fully reversible 2 weeks after discontinuance of the drug. No difference in white blood cell count, differential count, or platelet count was detected in tumor bearers and controls treated with TZ. Both pluripotent and committed marrow hemopoietic precursors remained unchanged in TZ-LLC, TZ-controls and untreated controls throughout treatment and 2 weeks thereafter. This study demonstrates that TZ-cured LLC mice are suitable to explore late hemopoietic effects of cancer.