Kinetic analysis of uptake and phosphorylation of 5,6-dichlororibofuranosylbenzimidazole (DRB) by salivary gland cells of Chironomus tentans

Synthesis and Biological Evaluation of New 5,6-dichlorobenzimidazole Nucleoside Derivatives

Novel 5,6-dichlorobenzimidazole nucleoside analogues structurally related to the well-known riboside DRB have been synthesized. The 1′,2′- trans nucleosides were prepared by condensation of peracylated sugars with 5,6-dichlorobenzimidazole, whereas the 1′,2′- cis β-D-arabinofuranosyl and β-D-lyxofuranosyl nucleosides were obtained by inversion of configuration on the sugar moiety. Chiral acyclic derivatives were stereospecifically prepared by ring-opening of furano- or pyrano-nucleosides by means of periodate oxidation, followed by borohydride reduction. The in vitro activities against a range of DNA and RNA viruses, as well as the cytotoxicities in human T-lymphocyte MT-4 cells, have been determined for these novel compounds and for DRB. No truly selective activity (i.e. clearly below the cytotoxic concentration) was observed against any of the viruses used. Some of the compounds, including DRB, were cytotoxic to MT-4 cells at CC50 values of less than 10 μg ml−1.

Phosphorylation of beta-D-ribosylbenzimidazoles is not required for activity against human cytomegalovirus

We have previously reported that 2,5,6-trichloro-1-(beta-D-ribofuranosyl)benzimidazole (TCRB) and its 2-bromo analog (2-bromo-5,6-dichloro-1-(beta-D-ribofuranosy)benzimidazole [BDCRB]) are potent and selective inhibitors of human cytomegalovirus (HCMV) replication that block viral DNA maturation via HCMV gene products UL89 and UL56. To determine if phosphorylation is required for antiviral activity, the in vitro metabolism of BDCRB was examined and the antiviral activities of nonphosphorylatable 5'-deoxy analogs were determined. Reverse-phase high-performance liquid chromatography (HPLC) analysis of extracts from uninfected and HCMV-infected cells incubated with [(3)H]BDCRB revealed two major metabolites. Both were less polar than naturally occurring nucleoside monophosphates, but one peak coeluted with a BDCRB-5'-monophosphate (BDCRB-5'-MP) standard. Further analysis revealed, however, that neither metabolite partitioned with BDCRB-5'-MP on anion-exchange HPLC. Their retention patterns were not affected by incubation with alkaline phosphatase, thereby establishing that the compounds were not nucleoside 5'-monophosphates. Both compounds were detected in uninfected and HCMV-infected cells and in mouse live extracts, but neither has been identified. Like TCRB and BDCRB, the nonphosphorylatable 5'-deoxy analogs were potent and selective inhibitors of HCMV replication. The 5'-deoxy analogs maintained inhibition of HCMV replication upon removal of BDCRB, whereas an inhibitor of DNA synthesis did not. Similar to TCRB, its 5'-deoxy analog (5'-dTCRB) did not affect viral DNA synthesis, but 5'-dTCRB did inhibit viral DNA maturation to genome-length units. Additionally, virus isolates resistant to TCRB were also resistant to 5'-dTCRB and the 5'-deoxy analog of BDCRB. Taken together, these results confirm that TCRB, BDCRB, and their 5'-deoxy analogs have common mechanisms of action and establish that these benzimidazole ribonucleosides, unlike other antiviral nucleosides, do not require phosphorylation at the 5' position for antiviral activity.

The charge distribution in the rough endoplasmic reticulum/Golgi complex transitional area investigated by microinjection of charged tracers

The distribution of microinjected ferritin, ranging in charge from anionic to highly cationic, has been used to indicate differences in surface charge on the rough endoplasmic reticulum and the Golgi complex of intact cells. Highly cationic ferritins (HCF)(HCF1, pI 7.9-9.1; HCF2, pI 8.5-9.4; and HCF3.pI 9.5-10.1) were mostly bound and caused swelling of the rough endoplasmic reticulum. Cationic ferritin (CF) (pI 7.0-8.0) and anionic ferritin (AF) (pI 4.0-4.4) caused no changes in morphology. The distribution of these ferritins in the cytoplasmic space varied with their charge. Significantly more CF was bound to surfaces than was found in the free cytoplasmic space. Conversely, there was significantly more AF in the free cytoplasmic space than close to surfaces. Therefore, the intracellular surfaces are negatively charged. Comparison of the structures in the secretory pathway showed no differences in ferritin binding to transition vesicles, rough endoplasmic reticulum, Golgi saccules or secretory vesicles. The Golgi complex beads are not distinguished by their charge. It is therefore unlikely that charge differences play a role in regulating membrane-membrane interactions in this region of the secretory pathway.

Specific inhibition of hnRNA synthesis by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole. Requirement of a free 3'-hydroxyl group, but not 2'- or 5'-hydroxyls

Five structural analogues of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), all with modified sugar moieties, have been examined for their inhibitory activities on RNA transcription in salivary glands of Chironomus tentans. The well-known ability of the parent DRB at 65 microM concentration to selectively inhibit hnRNA/mRNA synthesis by approx. 90% was essentially abolished on methylation of the 3'-OH; but, at an overdose the analogue suppressed labeling of all RNA classes examined (hnRNA/mRNA, rRNA, 4-5 S RNA) by 70-80%. By contrast, the 2'-O-methyl derivative of DRB was almost as effective as DRB itself in blocking transcription of hnRNA/mRNA genes. Blocking of both the 2' and 3' hydroxyls (2',3'-O-isopropylidene-DRB) completely abolished inhibitory activity, irrespective of the concentration employed. The 5'-deoxy-5'-chloro derivative of DRB was only slightly less effective than the parent DRB. An unusual aspect of the activities of 2'-O-methyl-DRB and 5'-deoxy-5'-chloro-DRB was their ability to stimulate synthesis of 4-5 S RNA by 25-45%. Also investigated was the influence of the various analogues on the rate of formation of [3H]UTP from [3H]uridine used as an RNA precursor. The rate of such formation of [3H]UTP was suppressed 2-6-fold by treatment with 2'-O-methyl or 3'-O-methyl-DRB, but was unaffected by 5'-deoxy-5'-chloro-DRB or 5,6-dichloro-1-alpha-D-arabinofuranosylbenzimidazole. The overall data point to the importance of a free 3'-OH in the ribose moiety of DRB for selective inhibitory activity. The alpha-D-arabinofuranosyl analogue, although less selective in inhibition of RNA transcription, still exhibits about 50% of the activity of DRB.

Dichlorobenzimidazole-riboside inhibition of nuclear RNA accumulation initiated with gamma-thio analogues of ATP and GTP

The gamma-thio analogues of ATP and GTP, ATP[S] and GTP[S], have been used as affinity probes to measure RNA synthesis initiated in vitro in L cell nuclei. 5,6-Dichlororibofuranosylbenzimidazole was found to inhibit total RNA synthesis in vitro and the amount bound by mercury-affinity chromatography. Initiation in vitro was also shown by [35S]ATP[S] and [35S]GTP[S] incorporation in RNA molecules with a larger size distribution. Although 66% of RNA molecules labeled with [35S]GTP[S] and 74% of those labeled with [35S]ATP[S] were 3-12 S n size, the remainder of the label was recovered in RNA molecules larger than 12 S and a significant portion in RNA larger tha 18 S. The specificity of the initiation process seems to be indicated by the finding of molecules of the size of pre-tRNA on gel electrophoresis which were labeled with [35S]GTP[S] but not [35S]ATP[S] under our experimental conditions. 5,6-Dichlororibofuranosylbenzimidazole severely inhibited the incorporation of [35S]GTP[S] and [35S]AT[[S] by all size classes, indicating that it can decrease accumulation of RNA initiated in vitro in the L cell nuclear system.