An exonuclease-resistant chain-terminating nucleotide analogue targeting the SARS-CoV-2 replicase complex

Nucleotide analogues (NA) are currently employed for treatment of several viral diseases, including COVID-19. NA prodrugs are intracellularly activated to the 5'-triphosphate form. They are incorporated into the viral RNA by the viral polymerase (SARS-CoV-2 nsp12), terminating or corrupting RNA synthesis. For Coronaviruses, natural resistance to NAs is provided by a viral 3'-to-5' exonuclease heterodimer nsp14/nsp10, which can remove terminal analogues. Here, we show that the replacement of the α-phosphate of Bemnifosbuvir 5'-triphosphate form (AT-9010) by an α-thiophosphate renders it resistant to excision. The resulting α-thiotriphosphate, AT-9052, exists as two epimers (RP/SP). Through co-crystallization and activity assays, we show that the Sp isomer is preferentially used as a substrate by nucleotide diphosphate kinase (NDPK), and by SARS-CoV-2 nsp12, where its incorporation causes immediate chain-termination. The same -Sp isomer, once incorporated by nsp12, is also totally resistant to the excision by nsp10/nsp14 complex. However, unlike AT-9010, AT-9052-RP/SP no longer inhibits the N-terminal nucleotidylation domain of nsp12. We conclude that AT-9052-Sp exhibits a unique mechanism of action against SARS-CoV-2. Moreover, the thio modification provides a general approach to rescue existing NAs whose activity is hampered by coronavirus proofreading capacity.

Bemnifosbuvir (BEM, AT-527), a novel nucleotide analogue inhibitor of the hepatitis C virus NS5B polymerase

INTRODUCTION

Chronic hepatitis C virus (HCV) persists as a public health concern worldwide. Consequently, optimizing HCV therapy remains an important objective. While current therapies are generally highly effective, advanced antiviral agents are needed to maximize cure rates with potentially shorter treatment durations in a broader patient population, particularly those patients with advanced diseases who remain difficult to treat.

AREAS COVERED

This review summarizes the in vitro anti-HCV activity, preclinical pharmacological properties of bemnifosbuvir (BEM, AT-527), a novel prodrug that is metabolically converted to AT-9010, the active guanosine triphosphate analogue that potently and selectively inhibits several viral RNA polymerases, including the HCV NS5B polymerase. Results from clinical proof-of-concept and phase 2 combination studies are also discussed.

EXPERT OPINION

BEM exhibits potent pan-genotype activity against HCV, and has favorable safety, and drug interaction profiles. BEM is approximately 10-fold more potent than sofosbuvir against HCV genotypes (GT) tested in vitro. When combined with a potent NS5A inhibitor, BEM is expected to be a promising once-daily oral antiviral for chronic HCV infection of all genotypes and fibrosis stages with potentially short treatment durations.

AT-752 targets multiple sites and activities on the Dengue virus replication enzyme NS5

AT-752 is a guanosine analogue prodrug active against dengue virus (DENV). In infected cells, it is metabolized into 2'-methyl-2'-fluoro guanosine 5'-triphosphate (AT-9010) which inhibits RNA synthesis in acting as a RNA chain terminator. Here we show that AT-9010 has several modes of action on DENV full-length NS5. AT-9010 does not inhibit the primer pppApG synthesis step significantly. However, AT-9010 targets two NS5-associated enzyme activities, the RNA 2'-O-MTase and the RNA-dependent RNA polymerase (RdRp) at its RNA elongation step. Crystal structure and RNA methyltransferase (MTase) activities of the DENV 2 MTase domain in complex with AT-9010 at 1.97 Å resolution shows the latter bound to the GTP/RNA-cap binding site, accounting for the observed inhibition of 2'-O but not N7-methylation activity. AT-9010 is discriminated ∼10 to 14-fold against GTP at the NS5 active site of all four DENV1-4 NS5 RdRps, arguing for significant inhibition through viral RNA synthesis termination. In Huh-7 cells, DENV1-4 are equally sensitive to AT-281, the free base of AT-752 (EC50 ≈ 0.50 μM), suggesting broad spectrum antiviral properties of AT-752 against flaviviruses.

AT-752, a double prodrug of a guanosine nucleotide analog, inhibits yellow fever virus in a hamster model

Yellow fever virus (YFV) is a zoonotic pathogen re-emerging in parts of the world, causing a viral hemorrhagic fever associated with high mortality rates. While an effective vaccine is available, having an effective antiviral against YFV is critical against unexpected outbreaks, or when vaccination is not recommended. We have previously identified AT-281, the free base of AT-752, an orally available double prodrug of a guanosine nucleotide analog, as a potent inhibitor of YFV in vitro, with a 50% effective concentration (EC₅₀) of 0.31 μM. In hamsters infected with YFV (Jimenez strain), viremia rose about 4 log₁₀-fold and serum alanine aminotransferase (ALT) 2-fold compared to sham-infected animals. Treatment with 1000 mg/kg AT-752 for 7 days, initiated 4 h prior to viral challenge, reduced viremia to below the limit of detection by day 4 post infection (pi) and returned ALT to normal levels by day 6 pi. When treatment with AT-752 was initiated 2 days pi, the virus titer and ALT dropped >2 log₁₀ and 53% by day 4 and 6 pi, respectively. In addition, at 21 days pi, 70–100% of the infected animals in the treatment groups survived compared to 0% of the untreated group (p<0.001). Moreover, in vivo formation of the active triphosphate metabolite AT-9010 was measured in the animal tissues, with the highest concentrations in liver and kidney, organs that are vulnerable to the virus. The demonstrated in vivo activity of AT-752 suggests that it is a promising compound for clinical development in the treatment of YFV infection.

Yellow fever virus (YFV) is transmitted by mosquitoes, and its infection can lead to a lethal viral hemorrhagic fever associated with liver damage. While an effective vaccine is available, in places where the vaccination rate is low, in the event of an unexpected outbreak, or where vaccination is not recommended individually, having an effective antiviral treatment is critical. We previously reported that the nucleotide analog prodrug AT-752 potently inhibited the YFV in cultured cells. Here we showed that in hamsters infected with YFV, oral treatment with 1000 mg/kg AT-752 for 7 days reduced the production of infectious virus particles in the blood, and decreased serum alanine aminotransferase, a marker of liver damage, to levels measured in uninfected animals. In addition, at 21 days after infection, 70–100% of the infected animals in the treatment groups survived compared to 0% in the untreated group. Moreover, the amount of the active metabolite formed from AT-752 was highest in the livers and kidneys of the treated animals, organs that are targeted by the virus. These results suggest that AT-752 is a promising compound to develop for the treatment of YFV infection.

1265. AT-527, an Oral Purine Nucleotide Prodrug Exhibiting Potent In Vitro Antiviral Activity Against Human Coronaviruses, Including SARS-CoV-2

Background

Coronaviruses (CoVs) are the causative pathogens of several human diseases, including seasonal respiratory infections (HCoV-229E and HCoV-OC43), Middle East respiratory syndrome (MERS-CoV), severe acute respiratory syndrome (SARS-CoV-1) and the novel CoV recently identified as the virus responsible for the current COVID-19 pandemic, SARS-CoV-2. AT-527 is currently in Phase 2 clinical trials and has demonstrated potent activity and a well-tolerated safety profile in HCV-infected subjects. Here we report the in vitro activity of AT-511, the free base form of AT-527, against SARS-CoV-2 and other CoVs.

Methods

BHK-21, Huh-7, RD and differentiated normal human bronchial epithelial (dNHBE) cell cultures were exposed to virus and serial dilutions of test compounds. Independent assessments of antiviral activity were obtained by determining effective concentrations of test compounds required to 1) prevent half-maximal (EC₅₀) virus-induced cytopathic effect (CPE) using MTT or neutral red staining and 2) produce virus yield reductions (VYR) by 90% (EC₉₀) using standard endpoint dilution CCID₅₀ assays in Vero 76 cells. Half maximal cytotoxicity of test compounds was determined by dye (MTT or neutral red) staining in the absence of added virus or by microscopic inspection (dNHBE cells only).

Results

Table 1 presents the in vitro activities of AT-511 against several coronaviruses. Also included in these assays are the antiviral activities of potential COVID-19 oral treatments, including chloroquine, hydroxychloroquine and N⁴-hydroxycytidine.

Table 1. In Vitro Activity of AT-511 Against Various Human Coronaviruses

Conclusion

The data demonstrate the potent in vitro activity of AT-511 against several CoVs, with individual EC₉₀ values ranging from 0.34 to 1.2 µM against HCoV-229E, HCoV-OC43, SARS-CoV-1 and SARS-CoV-2 and less activity against MERS-CoV (average EC₉₀ = 36 µM). The potent in vitro antiviral activity of AT-511 against SARS-CoV-2 (EC₉₀ = 0.55 µM), associated with the AT-527 safety profile in treated HCV patients, support the ongoing clinical evaluation of the safety and efficacy of AT-527 in COVID-19 patients.

Disclosures

Steven S. Good, MS, Atea Pharmaceuticals, Inc. (Employee) Adel Moussa, PhD, Atea Pharmaceuticals, Inc. (Employee) Xiao-Jian Zhou, PhD, Atea Pharmaceuticals, Inc. (Employee) Keith Pietropaolo, B.A., Atea Pharmaceuticals, Inc. (Employee) Jean-Pierre Sommadossi, PhD, Atea Pharmaceuticals, Inc. (Board Member)

169. AT-752, an Oral Guanosine Nucleotide Prodrug, Exhibits Potent in Vitro Activity Against Flaviviruses and Prevents Disease Progression in a Dengue Mouse Model

Background

The increasing global prevalence of human Dengue virus infection and the potential for life-threatening sequelae highlight the significance of this unmet medical need. Here we report the potent in vitro activity of AT-281, the free base form of AT-752, against Dengue virus and other flaviviruses and the in vivo efficacy of AT-752 in a mouse model of Dengue viral disease.

Methods

Antiviral activities of serial dilutions of AT-281 were evaluated in infected Huh-7 cells. Effective concentrations of AT-281 required to inhibit virus yield reduction by 90% (EC₉₀) and to prevent cytopathic effect by 50% (EC₅₀) were determined, respectively, by visual examination and by neutral red staining, as was cytotoxicity. AG129 (α-, β- and γ-interferon knock-out) mice received an oral dose of AT‐752 (1000 mg/kg) 4 h before s.c. inoculation with Dengue virus type 2 (strain D2Y98P, 1x10⁵ virus particles) followed by b.i.d. doses (500 mg/kg) for 7 days starting 1 h post‐inoculation (p.i.). Six groups each (n=5) of treated and control mice were scheduled to be sacrificed on days 4, 6, 7, 8, 10 and 21 p.i. with serum and spleen viral RNA levels determined by plaque assay. AT-281 efficacy was evaluated based on overall health score, survival, weight loss and viral load in serum and spleen.

Results

In vitro EC₉₀ values for AT-281 against Dengue, West Nile and Yellow Fever viruses ranged from 0.26 to 0.64 µM and EC₅₀ values for Zika and Japanese encephalitis were 0.21 and 0.64 µM, respectively (Table 1). No toxicity was observed up to the highest concentrations tested (172 µM). Oral administration of AT-752 to Dengue-infected AG129 mice substantially improved survival, prevented weight loss and lowered viral loads by day 6, with virus being undetectable on day 8 and thereafter (Figure 1). Serum and spleen viral loads in control mice declined between days 4 and 8 but no control mice survived beyond day 8. In contrast, AT-752 treated mice survived up to day 19, eventually succumbing to model-induced CNS sequelae.

Table 1. Antiviral Activity of AT-281 Against Various Flaviviruses in Huh-7 Cell Cultures

Figure 1. Efficacy of AT-752 in the AG129 mouse model of Dengue infection. Panel a: health score: 1, healthy; 2, coat slightly ruffled; 3, coat ruffled/wet; 4, coat very ruffled, eyes slightly closed/inset; 5, coat very ruffled, eyes closed/inset; 6, coat very ruffled, eyes closed/inset, moribund requiring humane euthanasia; 7, found dead. Panel b: Kaplan-Meier survival plot. Panel c: percent weight loss. Panel d: serum viremia. Panel e: spleen viral load.

Conclusion

The potent activity of AT-281 against Dengue virus in vitro and the efficacy of its salt form, AT-752, in the terminal AG129 mouse model warrant further clinical development of the drug. Preclinical safety studies are in progress and clinical trials will be initiated thereafter.

Disclosures

Steven S. Good, MS, Atea Pharmaceuticals, Inc. (Employee) Adel Moussa, PhD, Atea Pharmaceuticals, Inc. (Employee) Xiao-Jian Zhou, PhD, Atea Pharmaceuticals, Inc. (Employee) Jean-Pierre Sommadossi, PhD, Atea Pharmaceuticals, Inc. (Board Member) Keith Pietropaolo, BA, Atea Pharmaceuticals, Inc. (Employee)

Preclinical evaluation of AT-527, a novel guanosine nucleotide prodrug with potent, pan-genotypic activity against hepatitis C virus

Despite the availability of highly effective direct-acting antiviral (DAA) regimens for the treatment of hepatitis C virus (HCV) infections, sustained viral response (SVR) rates remain suboptimal for difficult-to-treat patient populations such as those with HCV genotype 3, cirrhosis or prior treatment experience, warranting development of more potent HCV replication antivirals. AT-527 is the hemi-sulfate salt of AT-511, a novel phosphoramidate prodrug of 2’-fluoro-2’-C-methylguanosine-5'-monophosphate that has potent in vitro activity against HCV. The EC₅₀ of AT-511, determined using HCV laboratory strains and clinical isolates with genotypes 1–5, ranged from 5–28 nM. The active 5'-triphosphate metabolite, AT-9010, specifically inhibited the HCV RNA-dependent RNA polymerase. AT-511 did not inhibit the replication of other selected RNA or DNA viruses in vitro. AT-511 was approximately 10-fold more active than sofosbuvir (SOF) against a panel of laboratory strains and clinical isolates of HCV genotypes 1–5 and remained fully active against S282T resistance-associated variants, with up to 58-fold more potency than SOF. In vitro, AT-511 did not inhibit human DNA polymerases or elicit cytotoxicity or mitochondrial toxicity at concentrations up to 100 μM. Unlike the other potent guanosine analogs PSI-938 and PSI-661, no mutagenic O⁶-alkylguanine bases were formed when incubated with cytochrome P450 (CYP) 3A4, and AT-511 had IC₅₀ values ≥25 μM against a panel of CYP enzymes. In hepatocytes from multiple species, the active triphosphate was the predominant metabolite produced from the prodrug, with a half-life of 10 h in human hepatocytes. When given orally to rats and monkeys, AT-527 preferentially delivered high levels of AT-9010 in the liver in vivo. These favorable preclinical attributes support the ongoing clinical development of AT-527 and suggest that, when used in combination with an HCV DAA from a different class, AT-527 may increase SVR rates, especially for difficult-to-treat patient populations, and could potentially shorten treatment duration for all patients.

2′-C-Methyl Branched Pyrimidine Ribonucleoside Analogues: Potent Inhibitors of RNA Virus Replication

RNA viruses are the agents of numerous widespread and often severe diseases. Their unique RNA-dependent RNA polymerase (RDRP) is essential for replication and, thus, constitutes a valid target for the development of selective chemotherapeutic agents. In this regard, we have investigated sugar-modified ribonucleoside analogues as potential inhibitors of the RDRP. Title compounds retain ‘natural’ pyrimidine bases, but possess a β-methyl substituent at the 2′-position of the D- or L-ribose moiety. Evaluation against a broad range of RNA viruses, either single-stranded positive (ssRNA), single-stranded negative (ssRNA−) or double-stranded (dsRNA), revealed potent activities for D-2′- C-methyl-cytidine and -uridine against ssRNA+, and dsRNA viruses. None of the L-enantiomers were active. Moreover, the 5′-triphosphates of the active D-enantiomers were found to inhibit the bovine virus diarrhoea virus polymerase. Thus, the 2′-methyl branching of natural pyrimidine ribonucleosides transforms physiological molecules into potent, broad-spectrum antiviral agents that merit further development.

Synthesis, Physicochemical and Pharmacokinetic Studies of Potential Prodrugs of β-L-2′-Deoxycytidine, a Selective and Specific Anti-HBV Agent

β-L-2′-Deoxycytidine (β-L-dC) is a potent, selective and specific anti-hepatitis B virus (HBV) agent. To improve its oral bioavailability, several derivatives involving sugar or base acylation, as well as N4-derivatization with an N,N-(dimethyl-amino)methylene function, were synthesized. The physicochemical characteristics (including chemical stabilities, solubilities and distribution coefficient values) and pharmacokinetics of these compounds were determined and compared with those of the parent drug, β-L-dC. Presented in part at the 14th International Conference on Antiviral Research, Seattle, Washington, USA, 8–13 April 2001. Antiviral Reseach 2001; 50:A79.

Synthesis and antiviral evaluation of 4'-C-azidomethyl-beta-D-ribofuranosyl purine and pyrimidine nucleosides

In the search for inhibitors of the replication of RNA viruses, including hepatitis C virus (HCV), the hitherto unknown 4'-C-azidomethyl-beta-D-ribofuranosyl nucleosides of the five naturally occurring nucleic acid bases have been synthesized and their antiviral properties examined. These 4'-C-branched nucleosides were stereospecifically prepared by glycosylation of purine and pyrimidine aglycons with a suitable peracylated 4-C-azidomethyl-D-pentofuranose sugar, followed by removal of the protecting groups. The prepared compounds were tested for their activity against several viruses, but they did not show an antiviral effect.

Synthesis and study of 9-deazaguanosine derivatives as potential inhibitors of RNA virus replication

9-Deazaguanosine and the alpha and beta anomers of its 2'-C-methyl counter part, have been synthesized and evaluated against a broad range of RNA viruses, including hepatitis C virus.

Intracellular Nucleoside Triphosphate Concentrations in HIV-Infected Patients on Dual Nucleoside Reverse Transcriptase Inhibitor Therapy

Background

Intracellular nucleoside reverse transcriptase inhibitor triphosphate (NRTI-TP) concentrations are crucial in suppressing HIV replication. Little is known about how commonly used dual-NRTI regimens affect the intracellular levels of NRTI-TPs, the active form of these drugs. This study investigates the effect of dual-NRTI therapy in intracellular NRTI-TP levels.

Methods

NRTI and NRTI-TP concentrations were evaluated in HIV-infected patients receiving either lamivudine (3TC) and stavudine (d4T) or lamivudine with zidovudine (ZDV); NRTI and NRTI-TP concentrations were determined using a validated HPLC/MS/MS method. Plasma HIV-1 RNA levels were determined at baseline and monthly to examine the relationship between NRTI-TP concentrations and plasma HIV-1 RNA.

Results

Forty-one subjects completed the study. 3TC-TP significantly increased between day 1 and week 28 from 1.48 to 5.00 pmol/106 peripheral blood mononuclear cells (PBMC; P<0.0001). NRTI-TP concentrations for d4T and ZDV did not significantly increase, with values at week 28 of 0.011 and 0.02 pmol/106 PBMC, respectively. Mean NRTI-TP/plasma ratios were 3%, 0.007% and 0.05% for 3TC, d4T and ZDV, respectively. Linear relationships were observed between ZDV- and 3TC-TP and changes in plasma HIV-1 RNA.

Conclusion

Of the three drugs studied, only 3TC-TP levels increased significantly between day 1 and week 28. ZDV-TP and 3TC-TP levels were unaffected by dual-NRTI therapy relative to monotherapy, regardless of the combination (3TC-ZDV or 3TC-d4T). Intracellular levels of d4T-TP were similar to previous reports for dual-NRTI therapy; however, in the case of d4T, these values appear lower than those achieved with d4T monotherapy.

Intracellular nucleoside triphosphate concentrations in HIV-infected patients on dual nucleoside reverse transcriptase inhibitor therapy

BACKGROUND

Intracellular nucleoside reverse transcriptase inhibitor triphosphate (NRTI-TP) concentrations are crucial in suppressing HIV replication. Little is known about how commonly used dual-NRTI regimens affect the intracellular levels of NRTI-TPs, the active form of these drugs. This study investigates the effect of dual-NRTI therapy in intracellular NRTI-TP levels.

METHODS

NRTI and NRTI-TP concentrations were evaluated in HIV-infected patients receiving either lamivudine (3TC) and stavudine (d4T) or lamivudine with zidovudine (ZDV); NRTI and NRTI-TP concentrations were determined using a validated HPLC/MS/MS method. Plasma HIV-1 RNA levels were determined at baseline and monthly to examine the relationship between NRTI-TP concentrations and plasma HIV-1 RNA.

RESULTS

Forty-one subjects completed the study. 3TC-TP significantly increased between day 1 and week 28 from 1.48 to 5.00 pmol/10(6) peripheral blood mononuclear cells (PBMC; P < 0.0001). NRTI-TP concentrations for d4T and ZDV did not significantly increase, with values at week 28 of 0.011 and 0.02 pmol/10(6) PBMC, respectively. Mean NRTI-TP/plasma ratios were 3%, 0.007% and 0.05% for 3TC, d4T and ZDV, respectively. Linear relationships were observed between ZDV- and 3TC-TP and changes in plasma HIV-1 RNA.

CONCLUSION

Of the three drugs studied, only 3TC-TP levels increased significantly between day 1 and week 28. ZDV-TP and 3TC-TP levels were unaffected by dual-NRTI therapy relative to monotherapy, regardless of the combination (3TC-ZDV or 3TC-d4T). Intracellular levels of d4T-TP were similar to previous reports for dual-NRTI therapy; however, in the case of d4T, these values appear lower than those achieved with d4T monotherapy.

Synthesis and Pharmacokinetics of Valopicitabine (NM283), an Efficient Prodrug of the Potent Anti-HCV Agent 2‘-C-Methylcytidine

In our search for new therapeutic agents against chronic hepatitis C, a ribonucleoside analogue, 2'-C-methylcytidine, was discovered to be a potent and selective inhibitor in cell culture of a number of RNA viruses, including the pestivirus bovine viral diarrhea virus, a surrogate model for hepatitis C virus (HCV), and three flaviviruses, namely, yellow fever virus, West Nile virus, and dengue-2 virus. However, pharmacokinetic studies revealed that 2'-C-methylcytidine suffers from a low oral bioavailability. To overcome this limitation, we have synthesized the 3'-O-l-valinyl ester derivative (dihydrochloride form, valopicitabine, NM283) of 2'-C-methylcytidine. We detail herein for the first time the chemical synthesis and physicochemical characteristics of this anti-HCV prodrug candidate, as well as a comparative study of its pharmacokinetic parameters with those of its parent nucleoside analogue, 2'-C-methylcytidine.

Synthesis of beta-L-2'-deoxythymidine (L-dT)

The "unnatural" l-nucleoside beta-l-2'-deoxythymidine (L-dT) is a potent, specific, and selective inhibitor of the replication of hepatitis B virus (HBV), which is currently in Phase III clinical trials. This unit describes, in detail, a semi-large-scale synthesis of l-dT. This convenient methodology produces l-dT in six steps starting with l-ribose and ending with a satisfactory overall yield of l-dT, and may be applied to other 2'-deoxynucleosides, incorporating different heterocyclic bases.

Broad specificity of human phosphoglycerate kinase for antiviral nucleoside analogs

Nucleoside analogs used in antiviral therapies need to be phosphorylated to their tri-phospho counterparts in order to be active on their cellular target. Human phosphoglycerate kinase (hPGK) was recently reported to participate in the last step of phosphorylation of cytidine L-nucleotide derivatives [Krishnan PGE, Lam W, Dutschman GE, Grill SP, Cheng YC. Novel role of 3-phosphoglycerate kinase, a glycolytic enzyme, in the activation of L-nucleoside analogs, a new class of anticancer and antiviral agents. J Biol Chem 2003;278:36726-32]. In the present work, we extended the enzymatic study of human PGK specificity to purine and pyrimidine nucleotide derivatives in both D- and L-configuration. Human PGK demonstrated catalytic efficiencies in the 10(4)-10(5)M(-1)s(-1) range for purine ribo-, deoxyribo- and dideoxyribonucleotide derivatives, either in D- or L-configuration. In contrast, it was poorly active with natural pyrimidine D-nucleotides (less than 10(3)M(-1)s(-1)). Pyrimidine L-enantiomers, which are promising therapeutic analogs against B hepatitis, were 2-25 times better substrates than their D-counterparts. The broad specificity of substrate of human PGK suggests that this enzyme may be involved in the cellular activation of several antiviral nucleoside analogs including dideoxyinosine, acyclovir, L-2'-deoxycytosine and L-2'-deoxythymidine.

4'-C-methyl-beta-D-ribofuranosyl purine and pyrimidine nucleosides revisited

In order to evaluate their antiviral properties, a series of 4'-C-methyl-beta-D-ribofuranosyl purine and pyrimidine nucleosides has been prepared. Unfortunately, none of these 4'-branched nucleosides showed any antiviral activity or cytotoxcity when tested against HIV, HBV, and Yellow Fever virus.

A randomized trial to study first-line combination therapy with or without a protease inhibitor in HIV-1-infected patients

OBJECTIVE

To compare one protease inhibitor (PI)-based and two PI-sparing antiretroviral therapy regimens.

METHODS

International, open label, randomized study of antiretroviral drug-naive patients, with CD4 lymphocyte counts >/= 200 x 106 cells/l and plasma HIV-1 RNA levels > 500 copies/ml. Treatment assignment to stavudine and didanosine plus indinavir or nevirapine or lamivudine. Primary study endpoint was the percentage of patients with plasma HIV-1 RNA levels < 500 copies/ml after 48 weeks in the intention-to-treat analysis (ITT).

RESULTS

In total, 298 patients were enrolled. After 48 weeks, the percentage of patients in the indinavir, nevirapine and lamivudine arms with HIV-1 RNA < 500 copies/ml was 57.0%, 58.4% and 58.7%, respectively, in an ITT analysis. After 96 weeks of follow-up, these percentages were 50.0%, 59.6% and 45.0%, respectively. The percentage of patients with HIV-1 RNA < 50 copies/ml was significantly less for those allocated to lamivudine in an on-treatment analysis after 48 and 96 weeks of follow-up. Patients in the nevirapine arm experienced a smaller increase in the absolute number of CD4 T lymphocytes. There were no significant differences in the incidence of serious adverse events.

CONCLUSIONS

A comparable virological response can be achieved with first-line PI-base and PI-sparing regimens. The triple nucleoside regimen utilized may be less likely to result in viral suppression to < 50 copies/ml, while the nevirapine-based regimen is associated with a lower increase in CD4 T lymphocytes.

Pharmacology of beta-L-thymidine and beta-L-2'-deoxycytidine in HepG2 cells and primary human hepatocytes: relevance to chemotherapeutic efficacy against hepatitis B virus

beta-L-Thymidine (L-dT) and beta-L-2'-deoxycytidine (L-dC) are potent and highly specific inhibitors of hepatitis B virus (HBV) replication both in vivo and in vitro (50% effective concentrations, 0.19 to 0.24 microM in 2.2.15 cells). The intracellular metabolisms of L-dT and L-dC were investigated in HepG2 cells and primary cultured human hepatocytes. L-dT and L-dC were extensively phosphorylated in both cell types, with the 5'-triphosphate derivative being the predominant metabolite. In HepG2 cells, the 5'-triphosphate levels were 27.7 +/- 12.1 and 72.4 +/- 1.8 pmol/10(6) cells for L-dT and L-dC, respectively. In primary human hepatocytes, the 5'-triphosphate levels were 16.5 +/- 9.8 and 90.1 +/- 36.4 pmol/10(6) cells for L-dT and L-dC, respectively. Furthermore, a choline derivative of L-dCDP was detected at concentrations of 15.8 +/- 1.8 and 25.6 +/- 0.1 pmol/10(6) cells in human hepatocytes and HepG2 cells, respectively. In HepG2 cells exposed to L-dC, the 5'-monophosphate and 5'-triphosphate derivatives of beta-L-2'-deoxyuridine (L-dUMP and L-dUTP, respectively) were also observed, reaching intracellular concentrations of 6.7 +/- 0.4 and 18.2 +/- 1.0 pmol/10(6) cells, respectively. In human hepatocytes, L-dUMP and L-dUTP were detected at concentrations of 5.7 +/- 2.4 and 43.5 +/- 26.8 pmol/10(6) cells, respectively. It is likely that deamination of L-dCMP by deoxycytidylate deaminase leads to the formation of L-dUMP, as the parent compound, L-dC, was not a substrate for deoxycytidine deaminase. The intracellular half-lives of L-dTTP, L-dCTP, and L-dUTP were at least 15 h, with intracellular concentrations of each metabolite remaining above their respective 50% inhibitory concentrations for the woodchuck hepatitis virus DNA polymerase for as long as 24 h after removal of the drug from cell cultures. Exposure of HepG2 cells to L-dT in combination with L-dC led to concentrations of the activated metabolites similar to those achieved with either agent alone. These results suggest that the potent anti-HBV activities of L-dT and L-dC are associated with their extensive phosphorylation.

Antiviral beta-L-nucleosides specific for hepatitis B virus infection

Three simple, related nucleosides, beta-L-2'-deoxycytidine (LdC), beta-Lthymidine (LdT), and beta-L-2'-deoxyadenosine (LdA), have been discovered to be potent, specific and selective inhibitors of the replication hepatitis B virus (HBV), as well as the closely related duck and woodchuck hepatitis viruses (WHV). Structure-activity relationship analysis indicates that the 3'-OH group of the beta-L-2'-deoxyribose of the beta-L-2'-deoxynucleoside confers specific anti-hepadnavirus activity. The simple nucleosides had no effect on the replication of 15 other RNA and DNA viruses, and did not inhibit human DNA polymerases (alpha, beta and gamma) or compromise mitochondrial function. The nucleosides are efficiently converted intracellularly into active triphosphate metabolites that have a long half-life. Once-daily oral administration of these compounds in the woodchuck efficacy model of chronic HBV infection reduced viral load by as much as 10(8) genome equivalents/ml serum and there was no drug-related toxicity. In addition, a decline in WHV surface antigen (WHsAg) paralleled the decrease in viral load. This class of nucleosides displays an excellent overall safety profile. The first compound, LdT, has already entered clinical trials and LdC, currently being developed as a prodrug, is expected to enter the clinic in the near future. These compounds have the potential for use in combination therapy with the goal of achieving superior viral suppression and diminishing the onset of resistance.

Stable concentrations of zidovudine, stavudine, lamivudine, abacavir, and nevirapine in serum and cerebrospinal fluid during 2 years of therapy

For a number of antiretroviral drugs, prolonged suppression of viral replication is related to drug exposure. Therefore, it is important to maintain stable concentrations during prolonged therapy. While studies suggest that saquinavir concentrations decrease over time, we show that concentrations of zidovudine, stavudine, lamivudine, abacavir, and nevirapine in serum and cerebrospinal fluid are stable during 2 years of therapy.

Symptomatic lactic acidosis in hospitalized antiretroviral-treated patients with human immunodeficiency virus infection: a report of 12 cases

We retrospectively investigated the clinical and histopathologic features of hospitalized patients infected with human immunodeficiency virus who had symptomatic lactic acidosis syndrome at a university teaching hospital during 1995-2000. Twelve patients were identified, 11 during 1998-2000; of these, 5 died with rapid progression to otherwise unexplained multiple-organ failure. All had extensive prior exposure to nucleoside analog reverse-transcriptase inhibitors (NRTIs). At presentation, the most commonly identified NRTI component of antiretroviral regimens was stavudine plus didanosine. Eleven patients presented with abdominal pain, nausea, and/or emesis. Eight patients had prior acute weight loss (mean [+/-SD], 12+/-5.3 kg). Median venous plasma lactate levels were > or =2-fold greater than the upper limit of normal (2.1 mmol/L). Serum transaminase levels were near normal limits at presentation. Histopathologic studies confirmed hepatic macrovesicular and microvesicular steatosis in 6 patients. Concurrent chemical pancreatitis was identified in 6 patients. The increasing number of cases identified during the study period suggests that physicians better recognize symptomatic lactic acidosis and/or that cumulative NRTI exposure may increase the risk for this syndrome.

NDP kinase reactivity towards 3TC nucleotides

Nucleoside diphosphate (NDP) kinase is usually considered as the enzyme responsible for the last step of the cellular phosphorylation pathway leading to the synthesis of biologically active triphospho-derivatives of nucleoside analogs used in antiviral therapies and in particular in the treatment of AIDS. NDP kinase lacks specificity for the nucleobase and can use as substrate both ribo- or 2'-deoxyribonucleotides. However, only nucleoside analogs with a sugar moiety in the D-configuration (e.g. 3'-deoxy-3'-azidothymidine (AZT), 2',3'-didehydro-2',3'-dideoxythymidine (d4T)) have so far been analyzed as substrates of NDP kinase. In contrast, beta-L-2',3'-dideoxy-3'-thiacytidine (3TC), also called lamivudine, is a nucleoside analog that is now widely used in AIDS therapy and has a sugar moiety in the L-configuration. Using protein fluorescence to monitor the phosphotransfer between the enzyme and the nucleotide derivative at the presteady state, we have studied the reactivity of 3TC triphosphate and of other L-dideoxynucleotides with NDP kinase. We found that L-dideoxynucleoside triphosphates have a poor affinity for NDP kinase and that the catalytic efficiency of the phosphorylation of L-dideoxyderivatives is very low as compared with their D-enantiomers. We discuss these results using a computer model of 3TC diphosphate bound to the NDP kinase active site. NDP kinase may not seem to be the major enzyme phosphorylating 3TC-DP, in contrast to current opinion.

Prototype trial design for rapid dose selection of antiretroviral drugs: an example using emtricitabine (Coviracil)

Antiretroviral monotherapy for initial drug characterization risks the selection of resistant virus, yet monotherapy is the only setting where many fundamental properties of a new drug can be reliably determined. Using data on viral replication kinetics and dynamics, we designed an accelerated (14 day) open-label study of single agent emtricitabine (formerly known as FTC)--a nucleoside reverse transcriptase inhibitor--to select a dosing regimen for further therapeutic study. Five regimens (25 mg bd, 100 mg od, 200 mg od, 100 mg bd and 200 mg bd) were evaluated in HIV-1-infected subjects over a 14 day dosing period to determine the optimal dose and pharmacokinetics. Serial blood samples for virological, pharmacokinetic and intracellular FTC-triphosphate measurements were drawn frequently. A dose-response relationship for the antiviral activity of emtricitabine was established, with total daily doses of 200 mg or more producing the greatest median HIV-1 viral load suppression: 1.72-1.92 log10. Based on virological outcomes, dose-response analysis and intracellular triphosphate levels, a once-daily dose of 200 mg was selected for further long-term clinical study. Adverse events possibly related to emtricitabine were unremarkable. The antiviral activity of emtricitabine correlated well with intracellular FTC-triphosphate concentrations. This study design is a safe, useful tool for early dose selection for drugs with potent antiretroviral activity and linear pharmacokinetics.

Intravenous infusion of cereport increases uptake and efficacy of acyclovir in herpes simplex virus-infected rat brains

The outcome of herpes simplex virus (HSV) infections manifesting as encephalitis in healthy or immunocompromised individuals is generally very poor with mortality rates of about 8 to 28% with treatment. The long-term prognosis of survivors is often problematic, posing the need for alternative treatments that may decrease the mortality and morbidity associated with herpes encephalitis. This study addresses one such approach that includes a temporary permeabilization of the blood-brain barrier during treatment with acyclovir (ACV). In these studies we utilized a synthetic bradykinin analog, Cereport (RMP-7), in conjunction with ACV to treat HSV infection of the brain in a rat model. Cereport, infused intravenously via the jugular vein, was shown to increase [(14)C]ACV uptake in both the HSV-1-infected and -uninfected rat brain by approximately two- to threefold, correlating with enhanced efficacy of ACV in various brain compartments. In another series of experiments to determine efficacy, various doses of unlabeled ACV were administered during infusion with RMP-7. The decrease in viral titers in the temporal regions of the brain after 5 days of treatment suggested that this approach enhanced the efficacy of ACV treatment. These data indicated that Cereport infused with ACV enhances both the penetration and efficacy of this drug in the treatment of an experimental HSV-1 infection of the rat brain.

5-(Trifluoromethyl)-beta-l-2'-deoxyuridine, the L-enantiomer of trifluorothymidine: stereospecific synthesis and antiherpetic evaluations

As a part of our ongoing work on beta-L-nucleoside analogues as potential antiviral drugs, we have synthesized 5-(trifluoromethyl)-beta-L-2'-deoxyuridine (L-TFT), the hitherto unknown L-enantiomer of trifluorothymidine (CF(3)dUrd, TFT). We have also studied the effect of L-TFT on human and herpes simplex virus (HSV) type 1 and 2 thymidine kinases, and human thymidine phosphorylase, as well as its anti-HSV-1 and anti-HSV-2 activities in cell cultures. L-TFT has been found: (i) to inhibit HSV-1 TK with activity comparable to TFT, with no effect on human TK, (ii) to be phosphorylated by the viral enzyme with similar efficiency to TFT, (iii) to be resistant, in contrast to TFT, to hydrolysis by human thymidine phosphorylase. Unfortunately, when evaluated in cell cultures, L-TFT did not show any anti-HSV-1 and anti-HSV-2 activities.

Stereospecific synthesis and biological evaluations of beta-L-pentofuranonucleoside derivatives of 5-fluorouracil and 5-fluorocytosine

In the search for new chemotherapeutic agents, we have focused our work on the synthesis and the study of several unnatural beta-L-nucleoside analogues. In this paper, we report on the synthesis of beta-L-pentofuranonucleosides (and their 2'-deoxy derivatives) of 5-fluorouracil and their inhibitory effects on the proliferation of several murine and human tumor cells. The corresponding 5-fluorocytosine derivatives were also synthesized and their anti-HIV and anti-HBV activities have been evaluated.

Evaluation of the mutagenic and genotoxic activities of anti-hepatitis B analogs of β--adenosine by the Ames test and the Comet assay

beta-L-2'-deoxyadenosine (beta-L-dA), beta-L-2',3'-dideoxyadenosine (beta-L-ddA) and its two bis (S-acyl-2-thioethyl; SATE) phosphotriester derivatives, beta-L-2',3'-dideoxyadenosine-5'-monophosphate-bis(MeSATE) and beta-L-2',3'-dideoxyadenosine-5'-monophosphate-bis(tButylSATE) have been previously shown to exhibit potent and selective anti-hepatitis B activity in vitro. None of the four compounds was mutagenic up to 100 microg in the Ames test (microtechnique) using Salmonella typhimurium strains TA 97a, TA 98, TA 100 and TA 102, with and without metabolic activation. In addition, the genotoxicity of beta-LdA and the three other compounds was evaluated in human lymphocytes using the Comet assay, at doses up to 5 microg with or without the addition of a microsomal S9 fraction. None of the four compounds induced DNA strand breakage with and without metabolic activation. In summary, the data clearly demonstrate that the purine nucleoside beta-L-dA, beta-L-ddA and the two prodrugs, beta-L-ddAMP-bis(MeSATE) and beta-L-ddAMP-bis(tButylSATE) are not mutagenic in the Ames test and do not induce DNA damage in human lymphocytes, as assessed by the Comet assay.

Anti-HBV specific beta-L-2'-deoxynucleosides

A unique series of simple unnatural L-nucleosides that specifically inhibit hepatitis B virus (HBV) replication has been discovered. These molecules have in common a hydroxyl group in the 3'-position (3'-OH) of the beta-L-2'-deoxyribose sugar that confers antiviral activity specifically against hepadnaviruses. Replacement of the 3'-OH broadens activity to other viruses. Substitution in the base decreases antiviral potency and selectivity. Human DNA polymerases and mitochondrial function are not effected. Plasma viremia is reduced up to 8 logs in a woodchuck model of chronic HBV infection. These investigational drugs, used alone or in combination, are expected to offer new therapeutic options for patients with chronic HBV infection.

Antiviral activity and intracellular metabolism of bis(tButylSATE) phosphotriester of beta-L-2',3'dideoxyadenosine, a potent inhibitor of HIV and HBV replication

The beta-L-nucleoside analogue beta-L-2',3'-dideoxy adenosine (beta-L-ddA) has been shown to exhibit limited antiviral activities. This was attributed to its rapid catabolism through cleavage of the glycosidic bond and poor phosphorylation to the nucleotide beta-L-2',3'-dideoxyadenosine-5'-mono phosphate (beta-L-ddAMP) (Placidi et al., 2000). However, the nucleotide beta-L-2',3'-dideoxyadenosine-5'-triphosphate (beta-L-ddATP) inhibited the activity of both HIV-1 reverse transcriptase (RT) and viral DNA polymerase isolated from woodchuck hepatitis virus-infected serum (a model of hepatitis B) with an inhibitory concentration (IC50) of 2.0 microM without inhibiting human DNA polymerases alpha, beta, or gamma up to a concentration of 100 microM. These results suggested that prodrugs of beta-L-ddAMP may bypass the poor metabolic activation of beta-L-ddA and lead to more potent and selective antiviral activity. Therefore, the mononucleoside phosphotriester derivative of beta-L-ddAMP incorporating the S-pivaloyl-2-thioethyl (tButylSATE) groups, beta-L-ddAMP-bis(tButylSATE) was synthesized. Beta-L-ddAMP-bis(tButylSATE) inhibited HIV replication in human peripheral blood mononuclear cells (PBMCs) and HBV replication in 2.2.15 cells with effective concentrations (EC50s) of 2 and 80 nM, respectively. Intracellular metabolism of beta-L-ddAMP-bis(tButylSATE) demonstrated that beta-L-ddATP was the predominant intracellular metabolite in PBMC and liver cells. The intracellular half-life of beta-L-ddATP was 5.4 and 9.2 h in HepG2 and PBMCs, respectively. The intracellular concentrations of beta-L-ddATP were maintained above the EC50 for the inhibition of HIV RT and hepatitis B virus (HBV) for as long as 24 h after removal of the drug.

Indinavir, nevirapine, stavudine, and lamivudine for human immunodeficiency virus-infected, amprenavir-experienced subjects: AIDS Clinical Trials Group protocol 373

This prospective, multicenter, open-label study was designed to determine the antiretroviral activity and safety of a 4-drug regimen: 1000 mg indinavir every 8 h with 200 mg nevirapine, 40 mg stavudine, and 150 mg lamivudine, each given twice daily in amprenavir-experienced subjects. The primary end points of the study were the human immunodeficiency virus (HIV) RNA level and CD4 cell count responses. Fifty-six subjects were enrolled and were changed from amprenavir-containing regimens to the 4-drug regimen. Overall, at week 48, 33 (59%) of 56 subjects had HIV RNA levels <500 copies/mL (intent-to-treat analysis, where missing values equal > or =500 copies/mL) and CD4 cell counts increased by 94 cells/mm(3) from baseline. Subjects who had previously taken amprenavir combination therapy were more likely to experience virologic failure than those who had taken amprenavir monotherapy (odds ratio, 7.7; P=.0012). In this study, most subjects who had taken amprenavir-based regimens and who changed to a 4-drug regimen achieved subsequent durable virologic suppression.

Antiviral L-nucleosides specific for hepatitis B virus infection

A unique series of simple "unnatural" nucleosides has been discovered to inhibit hepatitis B virus (HBV) replication. Through structure-activity analysis it was found that the 3'-OH group of the beta-L-2'-deoxyribose of the beta-L-2'-deoxynucleoside confers specific antihepadnavirus activity. The unsubstituted nucleosides beta-L-2'-deoxycytidine, beta-L-thymidine, and beta-L-2'-deoxyadenosine had the most potent, selective, and specific antiviral activity against HBV replication. Human DNA polymerases (alpha, beta, and gamma) and mitochondrial function were not affected. In the woodchuck model of chronic HBV infection, viral load was reduced by as much as 10(8) genome equivalents/ml of serum and there was no drug-related toxicity. In addition, the decline in woodchuck hepatitis virus surface antigen paralleled the decrease in viral load. These investigational drugs, used alone or in combination, are expected to offer new therapeutic options for patients with chronic HBV infection.

Mechanism of action of 1-beta-D-2,6-diaminopurine dioxolane, a prodrug of the human immunodeficiency virus type 1 inhibitor 1-beta-D-dioxolane guanosine

(-)-beta-D-2,6-Diaminopurine dioxolane (DAPD), is a nucleoside reverse transcriptase (RT) inhibitor with activity against human immunodeficiency virus type 1 (HIV-1). DAPD, which was designed as a water-soluble prodrug, is deaminated by adenosine deaminase to give (-)-beta-D-dioxolane guanine (DXG). By using calf adenosine deaminase a K(m) value of 15 +/- 0.7 microM was determined for DAPD, which was similar to the K(m) value for adenosine. However, the k(cat) for DAPD was 540-fold slower than the k(cat) for adenosine. In CEM cells and peripheral blood mononuclear cells exposed to DAPD or DXG, only the 5'-triphosphate of DXG (DXG-TP) was detected. DXG-TP is a potent alternative substrate inhibitor of HIV-1 RT. Rapid transient kinetic studies show the efficiency of incorporation for DXG-TP to be lower than that measured for the natural substrate, 2'-deoxyguanosine 5'-triphosphate. DXG-TP is a weak inhibitor of human DNA polymerases alpha and beta. Against the large subunit of human DNA polymerase gamma a K(i) value of 4.3 +/- 0.4 microM was determined for DXG-TP. DXG showed little or no cytotoxicity and no mitochondrial toxicity at the concentrations tested.

Plasma population pharmacokinetics and penetration into cerebrospinal fluid of indinavir in combination with zidovudine and lamivudine in HIV-1-infected patients

OBJECTIVES

To evaluate plasma population pharmacokinetics and penetration into cerebrospinal fluid (CSF) by indinavir (IDV) in HIV-infected individuals receiving IDV, zidovudine and lamivudine.

METHODS

Plasma population pharmacokinetic analysis was performed on 805 IDV plasma values from 171 patients, using a non-linear mixed-effects modeling approach. CSF data from 19 patients were analyzed using an individual approach.

RESULTS

Mean individual Bayesian estimates for oral clearance (CL) and volume of distribution (V) by the final model that incorporated interoccasion variability were 0.75 l/h per kg [coefficient of variation (CV) 54.8%] and 1.74 l/kg (CV 82.7%), respectively. Mean model-predicted plasma IDV level at 8 h, maximal level, area under the plasma level-time curve up to 8 h and plasma half-life were 0.42 micromol/l (CV 57.5%), 9.51 micromol/l (CV 47.3%), 29.56 micromol/l x h (CV 46.9%) and 1.50 h (CV 20.9%), respectively. The mean IDV CSF level was 0.11 micromol/l (CV 49.7%) and the mean CSF:plasma concentration ratio was 0.017.

CONCLUSIONS

Population estimates of pharmacokinetic parameters of IDV and its CSF penetration were in excellent agreement with previously reported data from individual analyses. Intraindividual interoccasion variability of IDV pharmacokinetics was estimated to be of similar order of magnitude to its interindividual variability, which may affect response to long-term antiretroviral therapy involving IDV. CSF levels of IDV exceeded its in vitro 95% inhibitory concentration of HIV replication. Given that CSF is virtually free of protein, viral suppression in the central nervous system should be achievable with an IDV-containing regimen.

Simultaneous quantitation of the 5'-triphosphate metabolites of zidovudine, lamivudine, and stavudine in peripheral mononuclear blood cells of HIV infected patients by high-performance liquid chromatography tandem mass spectrometry

A high-performance liquid chromatography (HPLC) method utilizing triple quadrupole mass spectrometry (MS) detection was developed and validated for the simultaneous measurement of the intracellular nucleoside 5'-triphosphate anabolites of zidovudine (ZDV-TP), lamivudine (3TC-TP), and stavudine (d4T-TP). These compounds were extracted from patient peripheral blood mononuclear cells (PBMCs) which are the sites of HIV replication and drug action. Ion-exchange solid phase extraction (SPE) followed by enzymatic digestion with alkaline phosphatase was utilized to yield the measurable nucleoside forms of the nucleotides. Reversed phase C-18 SPE with addition of a nucleoside internal standard, 3'-azido-2',3'-dideoxyuridine (AzdU) allowed for the indirect measurement of the original 5'-triphosphate concentration by HPLC/MS/MS. Quantitation was performed from calibration curves generated from authentic 5'-triphosphate standards spiked in PBMCs from healthy volunteers. Analytical range for the three 5'-triphosphates was equivalent to 50-45,000 pg. Mean interassay accuracies for 3TC-TP, d4T-TP, and ZDV-TP (n > 90) were 99.4%, 100.1%, and 108.0%, respectively. Mean interassay precisions (%C.V.) for 3TC-TP, d4T-TP, and ZDV-TP (n > 90) were 8.8%, 10.4%, and 8.2%, respectively. Recovery of the extraction method was 79.2%, 83.1%, and 98.3% for 3TC-TP, d4T-TP, and ZDV-TP, respectively. This method can be utilized to measure the intracellular 5'-triphosphate levels in HIV infected patients receiving antiretroviral therapy containing the nucleoside reverse transcriptase inhibitors 3TC, d4T, or ZDV.

Phase I dose escalation pharmacokinetics of O-(chloroacetylcarbamoyl) fumagillol (TNP-470) and its metabolites in AIDS patients with Kaposi's sarcoma

The pharmacokinetics of TNP-470 and its major metabolites were investigated in AIDS patients enrolled in a phase I dose escalation trial for the treatment of Kaposi's sarcoma. The patients received TNP-470 by 1-h intravenous infusion in dose cohorts of 10, 20, 30, 40, 50 and 70 mg/m2. The parent drug and metabolites, MII and MIV, were measured by high-performance liquid chromatography/mass spectrometry (HPLC/MS) in plasma samples collected during and out to 168 h after the beginning of the infusion. Both metabolites were detected in all patients' plasma, while the parent drug was undetectable at time-points as early as 5 min after the end of infusion for some patients. A large interpatient variability of pharmacokinetic parameters among the dosing cohorts was observed for TNP-470, with a mean (+/- SD) plasma elimination half-life (t1/2) of 0.06 +/- 0.04 h, plasma clearance (CL) of 1487 +/- 1216 l/h and an area under the concentration versus time curve (AUC) of 49.9 +/- 35.8 ng/ml x h. Time to maximum plasma concentration (Tmax) typically occurred before the end of the infusion. The predominant plasma metabolite was MII with a t1/2 of 1.21 +/- 0.43 h, AUC of 1226 +/- 2303 l/h and a Tmax occurring between 5 and 15 min after infusion. The reported active metabolite MIV had a t1/2 of 0.24 +/- 0.13 h, AUC of 24.9 +/- 32.6 ng/ml x h and a Tmax occurring between the midpoint of the infusion and 15 min after infusion. The parent drug was undetectable by HPLC/MS/MS in urine samples collected and pooled between 0-6 and 6-24 h from the beginning of drug administration. Metabolite MIV was present in the 0-6-h urine pool of two patients enrolled in the highest dosing cohorts, equivalent to 0.4% of the administered dose. Metabolite MII was present in all 0-6-h samples analyzed and represented 1.12 +/- 0.9% of the administered dose. Renal clearance (CLR) for MII was 140 +/- 70 ml/h.

Effects of beta-L-3'-azido-3'-deoxythymidine 5'-triphosphate on host and viral DNA polymerases

We have previously reported that several beta-L-thymidine analogues including beta-L-3'-azido-3'-deoxythymidine (beta-L-AZT), beta-L-3'-fluoro-2',3'-dideoxythymidine (beta-L-FLT) and beta-L-2', 3'-didehydro-2',3'-dideoxythymidine (beta-L-D4T) did not inhibit HIV replication in human peripheral blood mononuclear (PBM) cells whereas their corresponding beta-D-counterparts are known as potent and selective anti-HIV agents [Faraj et al., 1997. Nucleosides and Nucleotides 16, 1287-1290]. In order to gain insight on the lack of antiviral activities of these beta-L-derivatives, in vitro enzymatic steady state studies were conducted in the present study with beta-L-AZT. beta-L-AZT 5'-triphosphate (L-AZTTP) was chemically synthesized and found to moderately inhibit wild-type HIV reverse transcriptase (HIV-1 RT) with a K(i) value of 2 microM; while lacking any inhibitory effect towards human DNA polymerase alpha, beta or gamma. However, the inhibitory effect of L-AZTTP towards HIV-1 RT was very modest (266-fold less potent) when compared to its isomer beta-D-AZT 5'-triphosphate (D-AZTTP) which exhibits a K(i) value of 0.0075 microM and this finding was further confirmed by DNA chain termination assay. These data suggest that the absence of antiviral activity of the parent beta-L-AZT may in part be explained by the poor inhibition of the targeted viral enzyme by L-AZTTP, the active metabolite. Finally, L-AZTTP was found to lack affinity for the mutant RT at position 184 (M184V) demonstrating that this mutation confers resistance not only to beta-L-2',3'-dideoxycytidine analogs as previously reported by our group [Faraj et al., 1994. Antimicrob. Agents Chemother. 38, 2300-2305] but as well as to beta-L-2',3'-dideoxythymidine analogs.

Continued lamivudine versus delavirdine in combination with indinavir and zidovudine or stavudine in lamivudine-experienced patients: results of Adult AIDS Clinical Trials Group protocol 370

OBJECTIVE

To compare the virologic activity of continued lamivudine (3TC) versus a switch to delavirdine (DLV) when initiating protease inhibitor therapy in nucleoside-experienced patients.

DESIGN

Randomized, open-label, multi-center study.

SETTING

Adult AIDS clinical trials units.

PATIENTS

Protease and non-nucleoside reverse transcriptase inhibitor-naive patients who had received 3TC plus zidovudine (ZDV), stavudine (d4T), or didanosine (ddl) for at least 24 weeks.

INTERVENTIONS

Patients with plasma HIV-1 RNA levels > 500 copies/ml who previously received d4T + 3TC or ddI + 3TC were randomized to ZDV + 3TC + indinavir (IDV) or ZDV + DLV + IDV.

MAIN OUTCOME MEASURES

Primary endpoints were the proportion of patients with plasma HIV-1 RNA levels < or = 200 copies/ml at 24 weeks, and occurrence of serious adverse events. The proportion of patients with plasma HIV-1 RNA levels < or = 200 copies/ml at week 48 was a secondary endpoint.

RESULTS

At week 24, 58% of subjects in the ZDV + 3TC + IDV arm and 73% in the ZDV + DLV + IDV arm had plasma HIV-1 RNA levels < or = 200 copies/ml (P = 0.29). At week 48, plasma HIV-1 RNA levels were < or = 200 copies/ml in 48% and 83%, respectively (P = 0.007). Rash and hyperbilirubinemia occurred more frequently in the DLV arm than in the 3TC arm. Steady-state plasma IDV levels were higher among patients in the DLV arm as compared with the 3TC arm.

CONCLUSIONS

Substituting DLV for 3TC when adding IDV improved virologic outcome in nucleoside-experienced patients. This result might be explained, in part, by the positive effect of DLV on IDV pharmacokinetics.

In vivo antagonism with zidovudine plus stavudine combination therapy

Human immunodeficiency virus (HIV)-infected subjects receiving zidovudine were randomized either to add stavudine (d4T) or didanosine (ddI) to their current regimen or to switch to ddI or d4T monotherapy. After 16 weeks of therapy, the mean reduction in HIV RNA from baseline was 0.14 log(10) copies/mL in patients receiving d4T or zidovudine plus d4T. In subjects receiving ddI or ddI plus zidovudine, reductions were 0.39 and 0.56 log(10), respectively. CD4 cell counts remained stable or showed modest increases in all arms except the zidovudine plus d4T arm. Patients receiving zidovudine plus d4T showed progressive declines in CD4 cell counts with a median of 22 cells/mm(3) below baseline by 16 weeks. Examination of intracellular levels of d4T-triphosphate in 6 subjects was consistent with previous in vitro studies demonstrating pharmacologic antagonism between zidovudine and d4T. Analysis of these data suggests that zidovudine and d4T should not be prescribed in combination and that ddI provides greater antiviral activity than d4T in zidovudine-treated patients.

Enhanced penetration of indinavir in cerebrospinal fluid and semen after the addition of low-dose ritonavir

OBJECTIVE

Penetration of antiretroviral drugs into anatomical HIV-1 reservoirs such as the male genital tract and the central nervous system is important. Data on indinavir (IDV) concentrations in seminal plasma are lacking and IDV concentrations in cerebrospinal fluid are at best borderline.

DESIGN

Thirteen patients were treated with zidovudine (or stavudine), lamivudine, abacavir, nevirapine and IDV (1000 mg three times daily). When nevirapine led to low IDV concentrations, IDV was changed into the combination IDV/ritonavir (RTV) 800/100 mg twice daily to improve the pharmacokinetic profile of IDV.

METHODS

A serum pharmacokinetic profile, a semen sample and a cerebrospinal fluid sample were collected at weeks 8, 24, 48 and 72.

RESULTS

Addition of RTV increased the median IDV trough concentration in serum from 65 to 336 ng/ml (P = 0.005). Median IDV concentration in seminal plasma increased from 141 to 1634 ng/ml (P = 0.002) (n = 9) and in cerebrospinal fluid from 39 (n = 12) to 104 (n = 7) ng/ml (P < 0.001). In six patients with samples collected both before and after the addition of RTV, the IDV concentration in seminal plasma increased 8.2 times [95% confidence interval (CI) 5.2-11.6], and in cerebrospinal fluid 2.4 times (95% CI 1.8-3.9).

CONCLUSIONS

IDV penetrates well into the male genital tract. The addition of low-dose RTV not only increases IDV concentrations in serum but also in seminal plasma and cerebrospinal fluid, thereby probably improving the potency of the regimen in these anatomical HIV reservoirs. Higher serum trough levels alone can not sufficiently explain the observed increases in seminal plasma and cerebrospinal fluid concentrations. Inhibition of P-glycoprotein-mediated transport by RTV might be an additional mechanism.

In vitro activities of methylenecyclopropane analogues of nucleosides and their phosphoralaninate prodrugs against cytomegalovirus and other herpesvirus infections

Human cytomegalovirus (HCMV) infection does not generally cause problems in the immunocompetent adult but can result in severe clinical disease in the fetus, neonate, and immunocompromised host. Ganciclovir (GCV), the agent currently used to treat most HCMV infections, has resulted in much therapeutic success; however, efficacy remains suboptimal. Therefore, there is still a need to develop new compounds for use against HCMV infections. In the present study, several Z- and E-series methylenecyclopropane analogues and their phosphoroalaninate prodrugs were tested initially for activity against HCMV, strain AD169, and murine cytomegalovirus (MCMV) in vitro. Many were found to exhibit efficacy comparable to that of GCV against HCMV in plaque assays and were active against MCMV as well. The compounds were also tested for efficacy against herpes simplex virus types 1 and 2, varicella-zoster virus, and Epstein-Barr virus, and some had levels of activity that were comparable to that of acyclovir. In addition, the compounds synguanol (QYL-438) and 2-amino-6-cyclopropylamino analogue (QYL-769) were chosen for further evaluation and were found to be effective against additional laboratory and clinical isolates of HCMV and GCV-resistant isolates. QYL-438 and QYL-769 were found to be nontoxic in human and mouse fibroblasts and were considerably less toxic than GCV in granulocyte macrophage CFUs and erythroid burst-forming units. These results provide evidence for the high activity of some of these methylenecyclopropane analogues against various herpesviruses, particularly HCMV, in tissue culture and suggest that further evaluation is warranted to determine their potential for use in future clinical studies.

Intracellular metabolism of beta-L-2',3'-dideoxyadenosine: relevance to its limited antiviral activity

The intracellular metabolism of the beta-L- enantiomer of 2', 3'-dideoxyadenosine (beta-L-ddA) was investigated in HepG2 cells, human peripheral blood mononuclear cells (PBMC), and primary cultured human hepatocytes in an effort to understand the metabolic basis of its limited activity on the replication of human immunodeficiency virus and hepatitis B virus. Incubation of cells with 10 microM [2',3',8-(3)H]-beta-L-ddA resulted in an increased intracellular concentration of beta-L-ddA with time, demonstrating that these cells were able to transport beta-L-ddA. However, it did not result in the phosphorylation of beta-L-ddA to its pharmacologically active 5'-triphosphate (beta-L-ddATP). Five other intracellular metabolites were detected and identified as beta-L-2', 3'-dideoxyribonolactone, hypoxanthine, inosine, ADP, and ATP, with the last being the predominant metabolite, reaching levels as high as 5.14 +/- 0.95, 8.15 +/- 2.64, and 15.60 +/- 1.74 pmol/10(6) cells at 8, 4, and 2 h in HepG2 cells, PBMC, and hepatocytes, respectively. In addition, a beta-glucuronic derivative of beta-L-ddA was detected in cultured hepatocytes, accounting for 12.5% of the total metabolite pool. Coincubation of hepatocytes in primary culture with beta-L-ddA in the presence of increasing concentrations of 5'-methylthioadenosine resulted in decreased phosphorolysis of beta-L-ddA and formation of associated metabolites. These results indicate that the limited antiviral activity of beta-L-ddA is the result of its inadequate phosphorylation to the nucleotide level due to phosphorolysis and catabolism of beta-L-ddA by methylthioadenosine phosphorylase (EC 2.4.2.28).

Pharmacological considerations in antiretroviral therapy

Suboptimum drug exposure arising from pharmacological and pharmacokinetic factors is an important reason that combination antiretroviral therapy fails in patients with human immunodeficiency virus type 1 infection. With all three available drug classes, the major causes are drug-drug interactions and interpatient pharmacokinetic variability. Inadequate concentrations of protease inhibitors and non-nucleoside reverse transcriptase inhibitors occur when concurrently administered drugs interfere with their metabolism by the hepatic cytochrome P450 enzyme system, the major metabolic pathway for these drugs. Combined therapy with certain nucleoside reverse transcriptase inhibitors can reduce systemic drug exposure by interfering with the intracellular conversion of administered drugs to their active metabolites. Important causes of interpatient variability in pharmacokinetic parameters include low oral drug bioavailability and individual differences in drug disposition and metabolism. Careful selection and monitoring of combination drug therapy along with individualized rather than standard dosage regimens may minimize these pharmacological problems and help ensure optimum antiviral activity.

Differential effects of antiretroviral nucleoside analogs on mitochondrial function in HepG2 cells

Numerous studies have reported effects of antiviral nucleoside analogs on mitochondrial function, but they have not correlated well with the observed toxic side effects. By comparing the effects of the five Food and Drug Administration-approved anti-human immunodeficiency virus nucleoside analogs, zidovudine (3'-azido-3'-deoxythymidine) (AZT), 2',3'-dideoxycytidine (ddC), 2', 3'-dideoxyinosine (ddI), 2',3'-didehydro-2',3'-deoxythymidine (d4T), and beta-L-2',3'-dideoxy-3'-thiacytidine (3TC), as well as the metabolite of AZT, 3'-amino-3'-deoxythymidine (AMT), on mitochondrial function in a human hepatoma cell line, this issue has been reexamined. Evidence for a number of mitochondrial defects with AZT, ddC, and ddI was found, but only AZT induced a marked rise in lactic acid levels. Only in mitochondria isolated from AZT (50 microM)-treated cells was significant inhibition of cytochrome c oxidase and citrate synthase found. Our investigations also demonstrated that AZT, d4T, and 3TC did not affect the synthesis of the 11 polypeptides encoded by mitochondrial DNA, while ddC caused 70% reduction of total polypeptide content and ddI reduced by 43% the total content of 8 polypeptides (including NADH dehydrogenase subunits 1, 2, 4, and 5, cytochrome c oxidase subunits I to III, and cytochrome b). We hypothesize that in hepatocytes the reserve capacity for mitochondrial respiration is such that inhibition of respiratory enzymes is unlikely to become critical. In contrast, the combined inhibition of the citric acid cycle and electron transport greatly enhances the dependence of the cell on glycolysis and may explain why apparent mitochondrial dysfunction is more prevalent with AZT treatment.

Synthesis and antiviral evaluation of some beta-L-2', 3'-dideoxy-5-chloropyrimidine nucleosides and pronucleotides

The synthesis and in vitro anti human immunodeficiency virus (HIV) and anti-hepatitis B virus (HBV) activities of some unnatural beta-L-nucleoside enantiomers related to the anti-HIV compound 2', 3'-dideoxy-3'-fluoro-5-chlorouridine (beta-D-3'Fdd5ClU) are reported. In contrast to beta-D-3'Fdd5ClU, beta-L-3'Fdd5ClU and the other L-congeners were devoid of significant anti-HIV effects, but beta-L-2',3'-dideoxy-5-chlorocytidine (beta-L-dd5ClC) and beta-L-2', 3'-dideoxy-3'-fluoro-cytidine (beta-L-3'FddC) showed a distinct anti-HBV activity. Three mononucleoside phosphotriester derivatives with S-pivaloyl-2-thioethyl (t-BuSATE) groups as biolabile phosphate protective groups were also synthesized. The bis(t-BuSATE) derivative of beta-D-3'Fdd5ClU retained anti-HIV activity in thymidine kinase deficient (TK(-)) CEM cells.

Drug susceptibility in HIV infection after viral rebound in patients receiving indinavir-containing regimens

CONTEXT

Loss of viral suppression in patients infected with human immunodeficiency virus (HIV), who are receiving potent antiretroviral therapy, has been attributed to outgrowth of drug-resistant virus; however, resistance patterns are not well characterized in patients whose protease inhibitor combination therapy fails afterachieving viral suppression.

OBJECTIVE

To characterize drug susceptibility of virus from HIV-infected patients who are failing to sustain suppression while taking an indinavir-containing antiretroviral regimen.

DESIGN AND SETTING

Substudy of the AIDS Clinical Trials Group 343, a multicenter clinical research trial conducted between February 1997 and October 1998.

PATIENTS

Twenty-six subjects who experienced rebound (HIV RNA level > or =200 copies/mL) during indinavir monotherapy (n = 9) or triple-drug therapy (indinavir, lamivudine, and zidovudine; n = 17) after initially achieving suppression while receiving all 3 drugs, and 10 control subjects who had viral suppression while receiving triple-drug therapy.

MAIN OUTCOME MEASURE

Drug susceptibility, determined by a phenotypic assay and genotypic evidence of resistance assessed by nucleotide sequencing of protease and reverse transcriptase, compared among the 3 patient groups.

RESULTS

Indinavir resistance was not detected in the 9 subjects with viral rebound during indinavir monotherapy or in the 17 subjects with rebound during triple-drug therapy, despite plasma HIV RNA levels ranging from 10(2) to 10(5) copies/mL. In contrast, lamivudine resistance was detected by phenotypic assay in rebound isolates from 14 of 17 subjects receiving triple-drug therapy, and genotypic analyses showed changes at codon 184 of reverse transcriptase in these 14 isolates. Mean random plasma indinavir concentrations in the 2 groups with rebound were similar to those of a control group with sustained viral suppression, although levels below 50 ng/mL were more frequent in the triple-drug group than in the control group (P = .03).

CONCLUSIONS

Loss of viral suppression may be due to suboptimal antiviral potency, and selection of a predominantly indinavir-resistant virus population may be delayed for months even in the presence of ongoing indinavir therapy. The results suggest possible value in assessing strategies using drug components of failing regimens evaluated with resistance testing.

In vitro and in vivo metabolism and pharmacokinetics of bis [(t-butyl)-S-acyl-2-thioethyl]-beta-L-2',3'-dideoxy-5-fluorocytidine monophosphate

Exposure to 10 &M L-FddCMP-bisSATE led to formation of intracellular L-FddCTP levels of 410.1(+/-) +/- 46.2 and 242.1 +/- 13.2 pmol/10(6) cells in unstimulated and PHAstimulated PBM cells, respectively; whereas, exposure of cells to the parent nucleoside, L-FddC, generated 5-10-fold less L-FddCTP. In Hep-G2 cells and EGF/HGF stimulated and unstimulated primary cultured hepatocytes, the active metabolite reached 113 +/- 29, 23.9 +/- 15.6, and 20.6 +/- 10.5 pmol/10(6) cells. Three other metabolites, L-FddCMP-monoSATE, L-FddCMP-SH, and M I, were detected intracellularly and extracellularly in all cell types examined. Intravenous administered dose of 3 mg/kg L-FddCMP-bisSATE to rhesus monkeys resulted in plasma concentration levels of 2.06 +/- 1.00 and 0.39 +/- 0.15 &M of L-FddCMP-monoSATE and L-FddC, respectively, while the prodrug was completely cleared metabolically within 15 min. Following oral administration of an equivalent dose, the absolute oral bioavailability of L-FddC derived from L-FddCMP-bisSATE administration was 65%.

Inhibition of beta-globin gene expression by 3'-azido-3'-deoxythymidine in human erythroid progenitor cells

3'-Azido-3'-deoxythymidine (AZT) treatment in HIV-infected patients is limited by bone marrow suppression including neutropenia and anemia. Previous studies had shown a direct effect of high concentrations of this drug on globin gene expression in K-562 erythroleukemia cells. To better define the mechanism(s) of AZT-induced bone marrow toxicity, the present study evaluates these effects in more relevant human erythroid progenitor liquid cultures, because AZT is 100 times more toxic to human bone marrow cells than K-562 cells. At a clinically relevant concentration of 1 microM, AZT inhibited specifically erythroid cell growth by approximately 58% as compared with untreated cells. The percentage of cells synthesizing hemoglobin was decreased also by 47% in AZT-treated cells with beta-globin mRNA levels accounting for 0.27 pmol in treated cells as compared with 1.44 under control conditions while beta-actin levels remained unchanged. Under the same conditions, AZT inhibited the beta-globin chain synthesis by approximately 60% as compared with the control. Consistent with the data described above was the finding that a concentration as low as 0.1 microM of AZT decreased by almost 40% the binding level of the erythroid-specific transcription factor GATA-1. These findings demonstrate that AZT, at clinical relevant concentrations, specifically inhibits beta-globin gene expression in human erythroid progenitor liquid cell culture.

Determination of zidovudine triphosphate intracellular concentrations in peripheral blood mononuclear cells from human immunodeficiency virus-infected individuals by tandem mass spectrometry

Nucleoside reverse transcriptase inhibitors (NRTIs) used against the human immunodeficiency virus (HIV) need to be activated intracellularly to their triphosphate moiety to inhibit HIV replication. Intracellular concentrations of these NRTI triphosphates, especially zidovudine triphosphate (ZDV-TP), are relatively low (low numbers of femtomoles per 10(6) cells) in HIV-infected patient peripheral blood mononuclear cells. Recently, several methods have used either high-performance liquid chromatography (HPLC) or solid-phase extraction (SPE) coupled with radioimmunoassay to obtain in vivo measurements of ZDV-TP. The limit of detection (LOD) by these methods ranged from 20 to 200 fmol/10(6) cells. In this report, we describe the development of a method to determine intracellular ZDV-TP concentrations in HIV-infected patients using SPE and HPLC with tandem mass spectrometry for analysis. The LOD by this method is 4.0 fmol/10(6) cells with a linear concentration range of at least 4 orders of magnitude from 4. 0 to 10,000 fmol/10(6) cells. In hispanic HIV-infected patients, ZDV-TP was detectable even when the sampling time after drug administration was 15 h. Intracellular ZDV-TP concentrations in these patients ranged from 41 to 193 fmol/10(6) cells. The low LOD obtained with this method will provide the opportunity for further in vivo pharmacokinetic studies of intracellular ZDV-TP in different HIV-infected populations. Furthermore, this methodology could be used to perform simultaneous detection of two or more NRTIs, such as ZDV-TP and lamivudine triphosphate.