Susan P. Grill

Triapine (3-aminopyridine-2-carboxaldehyde- thiosemicarbazone): A potent inhibitor of ribonucleotide reductase activity with broad spectrum antitumor activity.

Previous studies from our laboratories have shown that (a) Triapine() is a potent inhibitor of ribonucleotide reductase activity and (b) hydroxyurea-resistant L1210 leukemia cells are fully sensitive to Triapine. In an analogous manner, Triapine was similarly active against the wild-type and a hydroxyurea-resistant subline of the human KB nasopharyngeal carcinoma. Triapine was active in vivo against the L1210 leukemia over a broad range of dosages and was curative for some mice. This agent also caused pronounced inhibition of the growth of the murine M109 lung carcinoma and human A2780 ovarian carcinoma xenografts in mice. Optimum anticancer activity required twice daily dosing due to the duration of inhibition of DNA synthesis which lasted about 10 hr in L1210 cells treated with Triapine in vivo. DNA synthesis in normal mouse tissues (i.e. duodenum and bone marrow) uniformly recovered faster than that in L1210 leukemia cells, demonstrating a pharmacological basis for the therapeutic index of this agent. Triapine was more potent than hydroxyurea in inhibiting DNA synthesis in L1210 cells in vivo, and the effects of Triapine were more pronounced. In addition, the duration of the inhibition of DNA synthesis in leukemia cells from mice treated with Triapine was considerably longer than in those from animals treated with hydroxyurea. Combination of Triapine with various classes of agents that damage DNA (e.g. etoposide, cisplatin, doxorubicin, and 1-acetyl-1,2-bis(methylsulfonyl)-2-(2-chloroethyl)hydrazine) resulted in synergistic inhibition of the L1210 leukemia, producing long-term survivors of tumor-bearing mice treated with several dosage levels of the combinations, whereas no enhancement of survival was found when Triapine was combined with gemcitabine or cytosine arabinoside. The findings demonstrate the superiority of Triapine over hydroxyurea as an anticancer agent and further suggest that prevention by Triapine of repair of DNA lesions created by agents that damage DNA may result in efficacious drug combinations for the treatment of cancer.

Novel 4′-Substituted Stavudine Analog with Improved Anti-Human Immunodeficiency Virus Activity and Decreased Cytotoxicity

ABSTRACT The antiviral drug 2′,3′-didehydro-3′-deoxythymidine (D4T; also know as stavudine and Zerit), which is used against human immunodeficiency virus (HIV), causes delayed toxicity (peripheral neuropathy) in long-term use. After examining a series of 2′,3′-didehydro-3′-deoxy-4′-substituted thymidine (4′-substituted D4T) analogs, 4′-ethynyl D4T was found to have a fivefold-better antiviral effect and to cause less cellular and mitochondrial toxicity than D4T. The antiviral activity of this compound can be reversed by dThd but not by dCyd. The compound acted synergistically with β-l-2′,3′-deoxy-3′-thiacytidine (also known as lamivudine) and β-l-2′,3′-dideoxy-2′,3′-didehydro-5-fluorocytidine (also known as elvucitabine) and additively with 2′,3′-dideoxyinosine (also known as didanosine and Videx) and 3′-azido-3′-deoxythymidine (also known as Retovir and zidovudine) against HIV. 4′-Ethynyl D4T is phosphorylated by purified human thymidine kinase 1 (TK-1) from CEM cells with a faster relative Vmax and a lower Km value than D4T. The efficiency of TK-1 in the phosphorylation of 4′-ethynyl D4T is fourfold better than that of D4T. While D4T is broken down by the catabolic enzyme thymidine phosphorylase, the level of breakdown of 4′-ethynyl D4T was below detection. Since 4′-ethynyl D4T has increased anti-HIV activity and decreased toxicity and interacts favorably with other currently used anti-HIV drugs, it should be considered for further development as an anti-HIV drug.

Eriocalyxin B Inhibits Nuclear Factor-κB Activation by Interfering with the Binding of Both p65 and p50 to the Response Element in a Noncompetitive Manner

Nuclear factor-κB (NF-κB) has been recognized to play a critical role in cell survival and inflammatory processes. It has become a target for intense drug development for the treatment of cancer, inflammatory, and autoimmune diseases. Here, we describe a potent NF-κB inhibitor, eriocalyxin B (Eri-B), an ent-kauranoid isolated from Isodon eriocalyx, an anti-inflammatory remedy. The presence of two α,β-unsaturated ketones give this compound the uniqueness among the ent-kauranoids tested. Eri-B inhibited the NF-κB transcriptional activity but not that of cAMP response element-binding protein. It suppressed the transcription of NF-κB downstream gene products including cyclooxygenase-2 and inducible nitric-oxide synthase induced by tumor necrosis factor-α or lipopolysaccharide in macrophages and hepatocarcinoma cells. Chromatin immunoprecipitation assay indicated that Eri-B selectively blocked the binding between NF-κB and the response elements in vivo without affecting the nuclear translocation of the transcription factor. Down-regulation of the endogenous p65 protein sensitized the cells toward the action of the compound. Furthermore, in vitro binding assays suggested that Eri-B reversibly interfered with the binding of p65 and p50 subunits to the DNA in a noncompetitive manner. In summary, this study reveals the novel action of a potent NF-κB inhibitor that could be potentially used for the treatment of a variety of NF-κB-associated diseases. Modification of the structure of this class of compounds becomes the key to the control of the behavior of the compound against different cellular signaling pathways.

Systemic Bcl-2 antisense oligodeoxynucleotide in combination with cisplatin cures EBV+ nasopharyngeal carcinoma xenografts in SCID mice

Nasopharyngeal carcinoma (NPC) is causally linked to Epstein‐Barr virus (EBV), and the EBV oncoprotein, latent membrane protein 1 (LMP‐1), is expressed in the majority of NPCs. LMP‐1 upregulates antiapoptotic genes, including bcl‐2, and Bcl‐2 protein is overexpressed in NPC. Given the antiapoptotic and chemoprotective effects of Bcl‐2, it represents a rational therapeutic target in NPC. We have investigated the antitumor and chemosensitizing effects of the Bcl‐2 antisense oligodeoxynucleotide G3139 (oblimersen, Genasense) in NPC. For these studies, we used the C666‐1 line, a stably infected NPC‐derived line that co‐expresses LMP‐1 and Bcl‐2. We have shown that G3139 treatment of C666‐1 in vitro caused sequence‐dependent suppression of Bcl‐2 protein, inhibition of cell growth and enhanced sensitivity to cisplatin (CDDP), as measured by increased antiproliferative and apoptotic effects. In vivo, G3139 treatment (25 mg/kg every 3 days × 5 doses) delayed engraftment and significantly inhibited growth of established C666‐1 xenografts in SCID mice compared to control oligo‐treated animals. However, G3139 alone did not prevent engraftment or cure established tumors in any animals. In contrast, G3139 treatment (25 mg/kg every 3 days × 5 starting on day 7) in combination with CDDP (8 mg/kg on day 14) completely abrogated tumor engraftment in 80% of animals compared to CDDP (0%) or CDDP + control oligo (0%). When treatment was delayed until tumor was established, G3139 in combination with CDDP significantly inhibited tumor growth compared to CDDP or CDDP + control oligo, and cured 69% animals with established tumors. No animals treated with G3139, CDDP or CDDP + control oligo were cured. Tumor burden and response to treatment correlated with EBV DNA load in serum, measured by real‐time PCR. Western blots of tumor extracts obtained during oligo treatment showed that Bcl‐2 levels were significantly decreased in G3139‐treated animals. Our studies have demonstrated that the Bcl‐2 antisense oligodeoxynucleotide, G3139, has proapoptotic effects in C666‐1, and in combination with CDDP, is curative in C666‐1 NPC xenograft tumors in vivo. The sequence‐dependency of these effects is consistent with an antisense mechanism. These studies suggest that Bcl‐2 may represent a biologically relevant target for the development of novel combinatorial therapies for NPC.

A Novel Class of meso-Tetrakis-Porphyrin Derivatives Exhibits Potent Activities against Hepatitis C Virus Genotype 1b Replicons In Vitro

ABSTRACT Recent years have seen the rapid advancement of new therapeutic agents against hepatitis C virus (HCV) in response to the need for treatment that is unmet by interferon (IFN)-based therapies. Most antiviral drugs discovered to date are small molecules that modulate viral enzyme activities. In the search for highly selective protein-binding molecules capable of disrupting the viral life cycle, we have identified a class of anionic tetraphenylporphyrins as potent and specific inhibitors of the HCV replicons. Based on the structure-activity relationship studies reported herein, meso-tetrakis-(3,5-dicarboxy-4,4′-biphenyl) porphyrin was found to be the most potent inhibitor of HCV genotype 1b (Con1) replicon systems but was less effective against the genotype 2a (JFH-1) replicon. This compound induced a reduction of viral RNA and protein levels when acting in the low nanomolar range. Moreover, the compound could suppress replicon rebound in drug-treated cells and exhibited additive to synergistic effects when combined with protease inhibitor BILN 2061 or with IFN-α-2a. Our results demonstrate the potential use of tetracarboxyphenylporphyrins as potent anti-HCV agents.

Anti-Epstein-Barr Virus (EBV) Activity of β-l-5-Iododioxolane Uracil Is Dependent on EBV Thymidine Kinase

ABSTRACT β-l-5-Iododioxolane uracil was shown to have potent anti-Epstein-Barr virus (EBV) activity (50% effective concentration = 0.03 μM) with low cytotoxicity (50% cytotoxic concentration = 1,000 μM). It exerts its antiviral activity by suppressing replicative EBV DNA and viral protein synthesis. This compound is phosphorylated in cells where the EBV is replicating but not in cells where the EBV is latent. EBV-specific thymidine kinase could phosphorylate β-l-5-iododioxolane uracil to the monophosphate metabolite. The Km of β-l-5-iododioxolane uracil with EBV thymidine kinase was estimated to be 5.5 μM, which is similar to that obtained with thymidine but about fivefold higher than that obtained with 2′ fluoro-5-methyl-β-l-arabinofuranosyl uracil, the firstl-nucleoside analogue discovered to have anti-EBV activity. The relative Vmax is seven times higher than that of thymidine. The anti-EBV activity of β-l-5-iododioxolane uracil and its intracellular phosphorylation could be inhibited by 5′-ethynylthymidine, a potent EBV thymidine kinase inhibitor. The present study suggests that β-l-5-iododioxolane uracil exerts its action after phosphorylation; therefore, EBV thymidine kinase is critical for the antiviral action of this drug.

Intracellular Metabolism and Persistence of the Anti-Human Immunodeficiency Virus Activity of 2′,3′-Didehydro-3′-Deoxy-4′-Ethynylthymidine, a Novel Thymidine Analog

ABSTRACT The therapeutic benefits of current antiretroviral therapy are limited by the evolution of drug-resistant virus and long-term toxicity. Novel antiretroviral compounds with activity against drug-resistant viruses are needed. 2′,3′-Didehydro-3′-deoxy-4′-ethynylthymidine (4′-Ed4T), a novel thymidine analog, has potent anti-human immunodeficiency virus (HIV) activity, maintains considerable activity against multidrug-resistant HIV strains, and is less inhibitory to mitochondrial DNA synthesis in cell culture than its progenitor stavudine (D4T). We investigated the intracellular metabolism and anti-HIV activity of 4′-Ed4T. The profile of 4′-Ed4T metabolites was qualitatively similar to that for zidovudine (AZT), with the monophosphate metabolite as the major metabolite, in contrast to that for D4T, with relatively poor formation of total metabolites. The first phosphorylation step for 4′-Ed4T in cells was more efficient than that for D4T but less than that for AZT. The amount of 4′-Ed4T triphosphate (4′-Ed4TTP) was higher than that of AZTTP at 24 h in culture. There was a dose-dependent accumulation of 4′-Ed4T diphosphate and 4′-Ed4TTP on up-regulation of thymidylate kinase and 3-phosphoglycerate kinase expression in Tet-On RKO cells, respectively. The anti-HIV activity of 4′-Ed4T in cells persisted even after 48 h of drug removal from culture in comparison with AZT, D4T, and nevirapine (NVP). The order of increasing persistence of anti-HIV activity of these compounds after drug removal was 4′-Ed4T > D4T > AZT > NVP. In conclusion, with the persistence of 4′-Ed4TTP and persistent anti-HIV activity in cells, we anticipate less frequent dosing and fewer patient compliance issues than for D4T. 4′-Ed4T is a promising antiviral candidate for HIV type 1 chemotherapy.

Determinants of Individual Variation in Intracellular Accumulation of Anti-HIV Nucleoside Analog Metabolites

ABSTRACT Individual variation in response to antiretroviral therapy is well-known, but it is not clear if demographic characteristics such as gender, age, and ethnicity are responsible for the variation. To optimize anti-HIV therapy and guide antiretroviral drug discovery, determinants that cause variable responses to therapy need to be evaluated. We investigated the determinants of intracellular concentrations of nucleoside analogs using peripheral blood mononuclear cells from 40 healthy donors. We observed individual differences in the concentrations of the intracellular nucleoside analogs; the mean concentrations of the triphosphate metabolite of ethynylstavudine (4′-Ed4T), zidovudine (AZT), and lamivudine (3TC) were 0.71 pmol/106 cells (minimum and maximum, 0.10 and 3.00 pmol/106 cells, respectively), 0.88 pmol/106 cells (minimum and maximum, 0.10 and 15.18 pmol/106 cells, respectively), and 1.70 pmol/106 cells (minimum and maximum, 0.20 and 7.73 pmol/106 cells, respectively). Gender and ethnicity had no effect on the concentration of 4′-Ed4T and 3TC metabolites. There was a trend for moderation of the concentrations of AZT metabolites by gender (P = 0.17 for gender·metabolite concentration). We observed variability in the activity and expression of cellular kinases. There was no statistically significant correlation between thymidine kinase 1 (TK-1) activity or expression and thymidine analog metabolite concentrations. The correlation between the activity of deoxycytidine kinase (dCK) and the 3TC monophosphate metabolite concentration showed a trend toward significance (P = 0.1). We observed an inverse correlation between the multidrug-resistant protein 2 (MRP2) expression index and the concentrations of AZT monophosphate, AZT triphosphate, and total AZT metabolites. Our findings suggest that the observed variation in clinical response to nucleoside analogs may be due partly to the individual differences in the intracellular concentrations, which in turn may be affected by the cellular kinases involved in the phosphorylation pathway and ATP-binding cassette (ABC) transport proteins.

Impact of Novel Human Immunodeficiency Virus Type 1 Reverse Transcriptase Mutations P119S and T165A on 4′-Ethynylthymidine Analog Resistance Profile

ABSTRACT 2′,3′-Didehydro-3′-deoxy-4′-ethynylthymidine (4′-Ed4T), a derivative of stavudine (d4T), has potent activity against human immunodeficiency virus and is much less inhibitory to mitochondrial DNA synthesis and cell growth than its progenitor, d4T. 4′-Ed4T triphosphate was a better reverse transcriptase (RT) inhibitor than d4T triphosphate, due to the additional binding of the 4′-ethynyl group at a presumed hydrophobic pocket in the RT active site. Previous in vitro selection for 4′-Ed4T-resistant viral strains revealed M184V and P119S/T165A/M184V mutations on days 26 and 81, respectively; M184V and P119S/T165A/M184V conferred 3- and 130-fold resistance to 4′-Ed4T, respectively. We investigated the relative contributions of these mutations, engineered into the strain NL4-3 background, to drug resistance, RT activity, and viral growth. Viral variants with single RT mutations (P119S or T165A) did not show resistance to 4′-Ed4T; however, M184V and P119S/T165A/M184V conferred three- and fivefold resistance, respectively, compared with that of the wild-type virus. The P119S/M184V and T165A/M184V variants showed about fourfold resistance to 4′-Ed4T. The differences in the growth kinetics of the variants were not more than threefold. The purified RT of mutants with the P119S/M184V and T165A/M184V mutations were inhibited by 4′-Ed4TTP with 8- to 13-fold less efficiency than wild-type RT. M184V may be the primary resistance-associated mutation of 4′-Ed4T, and P119S and T165A are secondary mutations. On the basis of our findings and the results of structural modeling, a virus with a high degree of resistance to 4′-Ed4T (e.g., more than 50-fold resistance) will be difficult to develop. The previously observed 130-fold resistance of the virus with P119S/T165A/M184V to 4′-Ed4T may be partly due to mutations both in the RT sequence and outside the RT sequence.

Time-dependent action of 5-propyl deoxyuridine as antiherpes simplex virus type 1 and type 2 agents

Abstract The most effective period of 5-propyl deoxyuridine action was 3–6 hr post-infection in both herpes simplex virus type 1- and type 2-infected HeLa BU cells. This antiviral activity could not be reversed by removing the drug from the culture medium. A correlation exists between inhibition of virus growth and inhibition of induction of virus specific DNase and DNA polymerase. No inhibition of induction of virus specific thymidine kinase was observed when virus replication was inhibited by 5-propyl deoxyuridine.