3-(1-deoxyribofuranosyl)benzamide and Leukemia--Myeloid

Benzamide riboside, a recently discovered inhibitor of IMP dehydrogenase (IMPDH) exhibits oncolytic activity. IMPDH is the key enzyme of de novo guanylate biosynthesis and was shown to be linked with proliferation. Therefore, IMPDH is a very good target for antitumor therapy. In order to be active, benzamide riboside has to be converted to BAD, an NAD analogue that binds to the NAD site on IMPDH. Inhibition of the enzyme by benzamide riboside selectively inhibits tumor cell growth and induces apoptosis in various human tumor cell lines. In this manuscript we describe the induction of the CD71 transferrin receptor in human promyelocytic leukemia HL-60 cells following treatment with benzamide riboside. The results indicate a possible involvement of the iron metabolism in the action of this new compound. Benzamide riboside might be clinically used in the treatment of leukemia and solid tumors, alone or as part of combination therapy. Since transferrin receptors are overexpressed in certain cancers, such as glioma and colon cancer, a combination therapy that includes benzamide riboside in transferrin-coupled liposomes will not only target cancer cells but also leads to suicidal action because benzamide riboside will upregulate transferrin receptors on cancer cells thereby make it accessible to dose-intensive chemotherapy. We therefore believe that benzamide riboside itself or derivatives of benzamide riboside might become an important addition for the treatment to diseases that are otherwise fatal.

Topics: Antigens, CD; Antigens, Differentiation, B-Lymphocyte; Apoptosis; Cell Division; Drug Screening Assays, Antitumor; HL-60 Cells; Humans; IMP Dehydrogenase; Iron; Leukemia, Myeloid; Nucleosides; Receptors, Transferrin; Ribavirin; Tumor Cells, Cultured

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.

Topics: Antineoplastic Agents; Apoptosis; cdc25 Phosphatases; Clinical Trials as Topic; Clinical Trials, Phase I as Topic; Clinical Trials, Phase II as Topic; Drug Resistance, Neoplasm; Enzyme Inhibitors; Female; Guanosine Triphosphate; HL-60 Cells; Humans; IMP Dehydrogenase; Leukemia, Myeloid; Neoplasms; Nucleosides; Organoselenium Compounds; Ovarian Neoplasms; Protein Tyrosine Phosphatases; Ribavirin; Ribonucleosides; RNA, Messenger; Time Factors; Tumor Cells, Cultured

Benzamide riboside exhibits significant cytotoxicity against a variety of human tumor cells in culture. On the basis of metabolic studies, the primary target of this drug's action appears to be IMP dehydrogenase (IMPDH). Incubation of human myelogenous leukemia K562 cells with an IC50 concentration of benzamide riboside resulted in an expansion of IMP pools (5.9-fold), with a parallel reduction in the concentration of GMP (90%), GDP (63%), GTP (55%) and dGTP (40%). On kinetic grounds, it was deduced that benzamide riboside (whose Ki versus IMPDH is 6.4 mM, while that of its 5'-monophosphate is 3.9 mM) or its 5'-monophosphate were unlikely to be responsible for inhibition of this target enzyme, IMPDH, since only micromolar concentrations of benzamide riboside were needed to exert potent inhibition of tumor-cell growth. Studies on the metabolism of this C-nucleoside have revealed the presence of a new peak eluting in the nucleoside diphosphate area on HPLC. Treatment of this peak with venom phosphodiesterase degraded it and concurrently nullified its inhibitory activity versus IMPDH; alkaline phosphatase, on the other hand, totally failed to digest the anabolite. These results suggest that the metabolite in question is the phosphodiester, benzamide adenine dinucleotide (BAD). Evidence that the inhibitor was an analog of NAD, wherein the nicotinamide moiety has been replaced by benzamide, was provided by both NMR and mass spectrometric analysis and confirmed by enzymatic synthesis. Further insight into the nature of the active principle was obtained from kinetic studies, which established that BAD competitively inhibited NAD utilization by partially purified IMPDH from K562 cells with a Ki of 0.118 microM. In concert, these studies establish that benzamide riboside exhibits potent antiproliferative activity by inhibiting IMPDH through BAD.

Topics: Carcinoma, Non-Small-Cell Lung; Colonic Neoplasms; Drug Screening Assays, Antitumor; Guanosine; Humans; IMP Dehydrogenase; Leukemia, Myeloid; Lung Neoplasms; Magnetic Resonance Spectroscopy; Mass Spectrometry; Nucleosides; Nucleotides; Ribonucleotides; Tumor Cells, Cultured