Showing papers by "Yung-Chi Cheng published in 1992"

Phosphorothioate oligonucleotides are inhibitors of human DNA polymerases and RNase H: implications for antisense technology.

Abstract: Phosphorothioate oligodeoxycytidine (S-dCn) was used as a model compound to examine the impact of the number of phosphorothioate linkages and their position on the inhibition of human DNA polymerases and RNase H in vitro. S-dCn with a chain length longer than 15 could inhibit human DNA polymerases and RNase H activities, in a linkage number-dependent manner. Longer oligomers were more potent inhibitors than shorter ones. Kinetic studies indicated that S-dC28 was a competitive inhibitor of DNA polymerase alpha and beta with respect to the DNA template, whereas it was a noncompetitive inhibitor of polymerases gamma and delta. S-dC28 was also a competitive inhibitor of RNase H1 and H2 with respect to RNA-DNA duplex. Susceptibility of these enzymes to inhibition by S-dC28 was in the order of delta approximately gamma greater than alpha greater than beta and RNase H1 greater than RNase H2. Structural-activity relationships were explored with a group of S-dC28 analogs that have phosphorothioate internucleotide linkages at various positions. The inhibitory effect depended on the total number of thioate linkages, rather than the position of the linkages within the oligomer or the chain length itself. No sequence specificity was found. In the presence of the complementary RNA, antisense phosphorothioates (S-oligos) exerted a biphasic effect on RNase H activity. At low concentrations S-oligos could enhance the cleavage of the RNA portion of S-oligo-RNA duplex, whereas at high concentrations (in excess of the complementary RNA) S-oligos could inhibit RNase H and protect the complementary RNA from degradation. Together, these results suggest that the non-sequence-specific inhibitory effect of S-oligos should be taken into consideration in designing antisense inhibitors. This inhibitory activity could be avoided by decreasing the number of phosphorothioate linkages at the backbone, and S-oligos of 15-20 residues are preferable in antisense molecule design.

Synthesis of enantiomerically pure (2'R,5'S)-(-)-1-(2-hydroxymethyloxathiolan-5-yl)cytosine as a potent antiviral agent against hepatitis B virus (HBV) and human immunodeficiency virus (HIV)

Abstract: The synthesis of the ennatiomerically pure 2'R,5'S-(-) form of BCH-189 from L-gulose has been accomplished, and this isomer has been found to exhibit the most potent anti-HIV and anti-HBV activities among the four possible isomers. This is the first example of an L-like nucleoside which is more potent than its D-like isomer

In vitro selection and molecular characterization of human immunodeficiency virus-1 resistant to non-nucleoside inhibitors of reverse transcriptase.

Abstract: Several newly discovered potent and selective non-nucleoside inhibitors of human immunodeficiency virus-1 reverse transcriptase (RT) are undergoing evaluation in clinical trials. We studied the potential for development of viral resistance to one of the prototype compounds, BI-RG-587, a dipyridodiazepinone derivative. Human immunodeficiency virus-1 resistant to BI-RG-587 emerged after only one cycle of in vitro infection in the presence of the drug. Resistant virus was cross-resistant to the non-nucleoside tetrahydroimidazo[4,5,1-jk][1,4]benzodiazepin-2(1H)-thione derivative R82150 but remained susceptible to 29,39-dideoxynucleosides and phosphonoformate. Both native (virion-associated) and recombinant RT derived from resistant virus were insensitive to BI-RG-587 and R82150. Nucleotide sequence analysis of multiple drug-resistant and -sensitive recombinant RT clones identified a single predicted amino acid change common to all resistant clones (tyrosine-181----cysteine). These studies suggest that the viral resistance to non-nucleoside RT inhibitors may develop in vivo. This possibility should be carefully monitored in clinical trials of these compounds.

Method of treating or preventing hepatitis b virus

Abstract: Anti-hepatitis B virus compounds (-)3'-thia-2',3'-dideoxycytidine, (-)5-fluoro-3'-thia-2',3'-dideoxycytidine, (±) β-dioxolane cytosine and (-)-L-β-dioxolane cytosine A method of treating a patient suffering from hepatitis B virus or preventing hepatitis B virus infection comprising administering to the patient an effective amount of an active compound selected from the group consisting of (a) (-)3'-thia-2',3'-dideoxycytidine, (b) (±)3'-thia-2',3'-dideoxycytidine, (c) (-)5-fluoro-3'-thia-2',3'-dideoxycytidine, (d) (±)5-fluoro-3'-thia-2,3-dideoxycytidine, (e) (±) β-dioxolane cytosine and (f) (-)-L-β-dioxolane cytosine, or a salt or an ester thereof, either alone or in admixture within a diluent

Anti-aids agents, 6. Salaspermic acid, an anti-HIV principle from Tripterygium wilfordii, and the structure-activity correlation with its related compounds.

Abstract: Salaspermic acid [1], an inhibitor of HIV reverse transcriptase and HIV replication in H9 lymphocyte cells, was isolated from the roots of Tripterygium wilfordii for the first time. The structure of 1 derived from spectral data was established unequivocally by an X-ray analysis of crystals of the monohydrate. A structure-activity correlation of 1 with ten related compounds indicated that the acetal linkage in ring A and the carboxyl group in ring E of 1 may be required for the anti-HIV activity.

Identification of antisense RNA transcripts from a human DNA topoisomerase I pseudogene.

Abstract: Eukaryotic topoisomerase I (TOP1), a DNA unwinding enzyme, plays an essential role in several cellular functions; however, regulation of TOP1 activity remains unknown. In an effort to identify potential regulators of TOP1 activity, the transcriptional activity of a TOP1 pseudogene in chromosome 1 was studied. By using primers unique to the TOP1 pseudogene, strand-specific polymerase chain reaction analysis of HeLa RNA amplified products from at least two transcripts oriented in the antisense direction of TOP1 mRNA. In one case, polymerase chain reaction and Northern blot analysis found a 0.7-kilobase antisense transcript. Upon estimation of 5′ and 3′ boundaries, a 497 base stretch of homology with the TOP1 mRNA was found. While the function of these TOP1 antisense transcripts remains unknown, recent studies of naturally occurring antisense RNA have demonstrated several potential regulatory roles. The production of antisense transcripts from a TOP1 pseudogene was the first example of a naturally occurring antisense RNA transcript produced from a pseudogene.

Antitumor agents. 124. New 4 beta-substituted aniline derivatives of 6,7-O,O-demethylene-4'-O-demethylpodophyllotoxin and related compounds as potent inhibitors of human DNA topoisomerase II.

Abstract: A series of 6,7-O,O-demethylene-4'-O-demethyl-4 beta-(substituted anilino)-4-desoxypodophyllotoxins (18-23), 6,7-O,O-demethylene-6,7-O,O-dimethyl-4'-O-demethyl-4 beta-(substituted anilino)-4-desoxypodophyllotoxins (28-31), and their corresponding 4'-O-methyl analogues (12-17 and 24-27) have been synthesized and evaluated for their inhibitory activity against the human DNA topoisomerase II as well as for their activity in causing cellular protein-linked DNA breakage. Compounds 18-23 are 2-fold more potent than etoposide and compounds 12, 16, 17, 30, and 31 are as active as etoposide in their inhibition of the human DNA topoisomerase II. Compounds 19 and 20 and 29-31 are as active or more active than etoposide in causing protein-linked DNA breakage. These results indicate that a free C-4' hydroxy group is essential for the DNA breakage activity, and that the hydroxyl groups at C-6 and -7 positions may be involved in an interaction which is responsible for the inhibitory activity of DNA topoisomerase II. The maintenance of an intact methylene dioxy-type ring-A system would contribute to enhanced activity. In addition, the sterically less hindered substitution at C-6 and C-7 positions may be important for optimal interactions with DNA topoisomerase II. There is no correlation between the ability of these compounds to inhibit DNA topoisomerase II and their ability to cause protein-linked DNA breaks in cells. This may relate to the difference in uptake of these compounds. The better correlation was observed between the protein-linked DNA breaks and the cytotoxicity in KB cells of these compounds.

Antitumor agents. 123. Synthesis and human DNA topoisomerase II inhibitory activity of 2'-chloro derivatives of etoposide and 4 beta-(arylamino)-4'-O-demethylpodophyllotoxins.

Abstract: The 2'-chloro derivatives of etoposide and 4 beta-(arylamino)-4'-O-demethylpodophyllotoxins have been synthesized and evaluated for their inhibitory activity against the human DNA topoisomerase II as well as for their activity in causing cellular protein-linked DNA breakage. The results showed that none of the compounds are active as a result of the C-2' chloro substitution on ring E. This would suggest that the free rotation of ring E is essential for the aforementioned enzyme inhibitory activity. In addition, these 2'-chloro derivatives showed no significant cytotoxicity (KB).

Determination of prodrugs metabolizable by the liver and therapeutic use thereof

Abstract: A method of ascertaining if a prodrug is useful for treating a disease is disclosed. The prodrug is acceptable if it is metabolized in liver cells by aldehyde oxidase to produce an active drug or metabolite. Prodrugs are shown equally effective in treating diseases as the active drug itself with many benefits and without as many associated side effects. Methods for treating cancers with 5-iodo-2-pyrimidinone-deoxyribose and 5-fluoro-2-pyrimidinone are also described.

Antitumor Agents, 130. Novel 4β-Arylamino Derivatives of 3',4'-Didemethoxy-3',4'-dioxo-4-deoxypodophyllotoxin as Potent Inhibitors of Human DNA Topoisomerase II

Abstract: A series of ortho-quinone analogues 1-28 of podophyllotoxin possessing various C-4 beta-aniline moieties have been synthesized and evaluated for their inhibitory activity against human DNA topoisomerase II, their activity in causing cellular protein-linked DNA breakage, and their cytotoxicity against KB cells. Compounds 8-12, 15, 19, and 23-28 are better than or comparable to etoposide in their inhibition of the human DNA topoisomerase II, while compounds 8-10 and 22 are comparable to or more potent than etoposide in causing DNA breakage. There is an apparent lack of correlation between cytotoxicity and the ability of these compounds to cause protein-linked DNA breaks or inhibit DNA topoisomerase II. This suggests that in addition to the mechanism of the topoisomerase II involving DNA breakages, other mechanisms of action, such as the radical mechanism or the direct adduct formation of the ortho-quinone with DNA or protein, may also involve and account for the apparent KB cytotoxicity. Two different modes of DNA topoisomerase II inhibition for 8-28 were proposed.

Cell-protecting effect against herpes simplex virus-1 and cellular metabolism of 9-(2-phosphonylmethoxyethyl)adenine in HeLa S3 cells.

Abstract: 9-(2-Phosphonylmethoxyethyl)adenine (PMEA) is a selective and potent inhibitor of retrovirus and herpesvirus replication in vitro and in vivo. In cell culture studies, pretreatment of HeLa S3 cells with PMEA before infection enhanced its antiviral potency by almost 10-fold, compared with treatment of the cells only after viral infection. To elucidate the basis for this observation, the uptake, metabolism, and retention of PMEA metabolites were examined in uninfected and herpes simplex virus type 1-infected cells, by using [2,8-3H]PMEA. Uptake of the drug into both acid-soluble and acid-insoluble fractions was slow and did not begin to plateau until close to 24 hr. High performance liquid chromatographic analysis of acid-soluble extracts revealed at least four metabolites in addition to PMEA itself, designated as X, Y, DP, and TP. Metabolites X and Y, which were distinct from PMEA and its mono- and diphosphoryl derivatives, represented almost 90% of the radioactivity associated with the cells after 24 hr of incubation. Dephosphorylation of acid-soluble metabolites resulted in accumulation of radioactivity in the peaks associated with PMEA and X. Most of the radioactivity in the acid-insoluble fraction was associated with DNA. Enzymatic digestion of [3H] PMEA-labeled DNA from either infected or uninfected cells yielded both metabolite X and PMEA itself. The role of newly discovered PMEA metabolites in its antiviral activity and cytotoxicity is not clear.

Tannins and Related Compounds as Anti-HIV Agents 1

Abstract: Four new tetragalloylquinic acids, 3,5-di-O-galloyl-4-O-digalloylquinic acid (2), 3,4-di-O-galloyl-5-O-digalloylquinic acid (3), 3-O-digalloyl-4,5-di-O-galloylquinic acid (4), and 1,3,4,5-tetra-O-galloylquinic acid (5), were isolated and characterized from a commercial tannic acid as a new class of human immunodeficiency virus (HIV) reverse transcriptase (RT) inhibitor. Compounds 2, 3 and 4 inhibit HIV RT activity 90, 89 and 84% at 100 μM and 73, 70 and 63% at 30 μM, respectively. Compounds 2–5 also inhibit the HIV growth in cells in the range of 61–70% with low cytotoxicity at 25 μM. The HIV cell growth inhibitory effects of these compounds at 25 μM and 6.25 μM (44–57%) are comparable to their effects against the HIV RT at 30 μM and 10 μM, respectively. The inhibitory effect of 3 against DNA polymerases indicates that the selective antiviral action of 3 is determined by more than its action with HIV RT.

Novel pyrazinone n-oxide nucleosides and analogs thereof

Abstract: This invention relates to nucleoside and acyclo analogs containing 5- and 6- substituted 2-pyrazinone-4-N-oxide. These compounds are useful for treating various conditions including viral infections, cancer, fungal infections, bacterial infections, microbial infections and related disease states. This invention also relates to pharmaceutical formulations containing these compounds. In addition, this invention relates to methods of treating the above-described conditions in animals and in particular, humans.

Oxide-trap-induced instability of GIDL of thermally nitrided-oxide N-MOSFET's under stress

Abstract: Some holes created from band-to-band (B-B) tunneling in the deep-depletion region of the drain can be injected into the gate oxide and reduce the vertical field there. As a result, gate-induced drain leakage (GIDL) current decreases. This kind of hot-hole injection depends on the voltage difference between the drain and gate, due to nitridation-induced lowering of the barrier height for hole injection at the SiO/sub 2/-Si interface. The subsequent hot-electron injection can neutralize these trapped holes, and make the GIDL current recover, and even increase beyond its original value. Since the trapped charges also affect the lateral field, the observed change in the ratio of substrate to source currents further confirms the proposed mechanism for the GIDL degradation and recovery behavior. >

Electrical characterization and simulation of substrate current in n-MOSFETs with nitrided/reoxidized-nitrided oxides as gate dielectrics

Abstract: Three kinds of n-MOSFETs were fabricated with different gate dielectrics: (a) OX: as-grown control oxide, (b) RTN: OX oxide with rapid thermal nitridation, and (c) RONO: OX oxide with nitridation and reoxidation by furnace annealing. Both the RTN and RONO devices show a lower substrate current ISUB as compared to the OX devices. Their ISUB can be characterized and simulated reasonably well, by using the ISUB characterization method and the basic ISUB model which are used for the OX devices. The results of characterization and simulation indicate that the nitridation or reoxidation step has little effect on the ionization rate of channel electrons. It is also confirmed that the lower ISUB of the RTN devices mainly resulted from the degradation in inversion-layer effective mobility μEFF due to nitridation, while that for RONO devices is caused by, not only the degraded μEFF, but also an increased gate-oxide thickness due to reoxidation. Therefore, the decrease in ISUB for RTN and RONO devices arises from the nitridation/reoxidation steps. In a word, even if the devices undergo a nitridation step with or without reoxidation, the basic ISUB model has been proved to be still applicable in regardless of the nitridation method.

Impact of nitridation/reoxidation on performance degradation in n-MOSFETs under Fowler-Nordheim injection

Abstract: Performance degradation of n -MOSFETs with conventional oxides, thermally nitrided oxides and reoxidized nitrided oxides as gate dielectrics was studied using Fowler-Nordheim injection. Their respective performance degradation rate and interface-state generation were characterized by combining capacitor capacitance-voltage and MOSFET subthreshold methods. Shift in both threshold voltage and flatband voltage exhibited a turnaround effect for devices with a large amount of hole traps in the oxide, with their values shifted negatively at first and then positively during injection. Light nitridation could induce a large number of hole traps in the oxide which resulted in a large negative threshold voltage shift, while heavy nitridation with sufficient reoxidation could suppress this effect. Nitridation, especially one with reoxidation, could reduce the density of interface states and improve its stability against Fowler-Nordheim injection, leading to a suppression of the positive threshold voltage shift. The stability of maximum transconductance and field-effect electron mobility in the inversion channel against the Fowler-Nordheim stressing can be greatly improved by the nitridation with or without reoxidation.

Improvement of punchthrough‐induced gate‐oxide breakdown in n‐channel metal‐oxide‐semiconductor field‐effect transistors using rapid thermal nitridation

Abstract: Gate‐dielectric breakdown in small n‐channel metal‐oxide‐semiconductor field‐effect transistors has been investigated. It is found that such breakdown is accelerated by large channel current when the devices operate in punchthrough conditions. The situation can be greatly improved by rapid thermal nitridation (RTN) of the gate oxide in the transistors. In addition, the time‐dependent dielectric breakdown of the devices operating beyond punchthrough is also considerably improved by RTN. The results indicate that the charge to breakdown Qbd is increased by three orders of magnitude as compared to conventional‐oxide devices. All these improvements can be attributed to the nitridation‐induced hardening of SiO2/Si interface.

Synthesis and Anticancer and Antiviral Activities of Various 2′‐ and 3′ ‐Methylidene‐Substituted Nucleoside Analogues and Crystal Structure of 2′‐Deoxy‐2′‐methylidenecytidine Hydrochloride.

Abstract: Various 2'- and 3'-methylidene-substituted nucleoside analogues have been synthesized and evaluated as potential anticancer and/or antiviral agents. Among these compounds, 2'-deoxy-2'-methylidene-5-fluorocytidine (22) and 2'-deoxy-2'-methylidenecytidine (23) not only demonstrated potent anticancer activity in culture against murine L1210 and P388 leukemias, Sarcoma 180, and human CCRF-CEM lymphoblastic leukemia, producing ED50 values of 1.2 and 0.3 microM, 0.6 and 0.4 microM, 1.5 and 1.5 microM, and 0.05 and 0.03 microM, respectively, but also were active in mice against murine L1210 leukemia. Of all the tested drug dosage levels (25, 50, and 75 mg/kg, respectively) compound 23 had no toxic deaths and compound 22 yielded only one toxic death at the highest dosage level. On the contrary, in the same study, 1-beta-D-arabinofuranosylcytosine (ara-C) resulted in 2/5, 5/5, and 5/5 toxic deaths, respectively. Both compounds 22 and 23 have shown better anticancer activity than ara-C, yielding higher T/C x 100 values and some long-term survivors (greater than 60 days). In addition, compounds 22 and 23 were found to have, respectively, approximately 130 and 40 times lower binding affinity for cytidine/deoxycytidine deaminase derived from human KB cells compared to ara-C, suggesting that the two 2'-methylidene-substituted analogues may be more resistant to deamination. Cytoplasmic deoxycytidine kinase (dCK) was required for compounds 22 and 23 action. Furthermore, compounds 14, 22, 23, and 24 also have antiherpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) activity in cell culture. In addition, the crystal structure of 2'-deoxy-2'-methylidenecytidine hydrochloride (23-HCl) was determined by X-ray crystallography.

Hot-carrier-induced degradation in reoxidized-nitrided-oxide n-MOSFETs under mixed AC-DC stressing

Abstract: Reduced degradation rate can be observed for reoxidized-nitrided-oxide (RNO) n-MOSFETs under dynamic stressing versus the corresponding static stressing. A new degradation mechanism is proposed in which trapped holes in gate oxide are neutralized by the hot-electron injection, with no significant generation of interface states because of the hardening on the Si-SiO/sub 2/ interface by nitridation/reoxidation steps. The RNO device degradation during AC stressing arises mainly from the charge trapping in the gate oxide rather than the generation of interface states. Moreover, the AC-stressed RNO devices are significantly inferior to the fresh RNO devices in terms of DC stressing, possibly due to lots of neutral electron traps in the gate oxide resulting from the AC stressing. >