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1.  Cellular Pharmacology and Potency of HIV-1 Nucleoside Analogs in Primary Human Macrophages 
Understanding the cellular pharmacology of antiretroviral agents in macrophages and subsequent correlation with antiviral potency provides a sentinel foundation for definition of the dynamics between antiretroviral agents and viral reservoirs across multiple cell types, with the goal of eradication of HIV-1 from these cells. Various clinically relevant nucleoside antiviral agents, and the integrase inhibitor raltegravir, were selected for this study. The intracellular concentrations of the active metabolites of the nucleoside analogs were found to be 5- to 140-fold lower in macrophages than in lymphocytes, and their antiviral potency was significantly lower in macrophages constitutively activated with macrophage colony-stimulating factor (M-CSF) during acute infection than in resting macrophages (EC50, 0.4 to 9.42 μM versus 0.03 to 0.4 μM, respectively). Although tenofovir-treated cells displayed significantly lower intracellular drug levels than cells treated with its prodrug, tenofovir disoproxil fumarate, the levels of tenofovir-diphosphate for tenofovir-treated cells were similar in lymphocytes and macrophages. Raltegravir also displayed significantly lower intracellular concentrations in macrophages than in lymphocytes, independent of the activation state, but had similar potencies in resting and activated macrophages. These data underscore the importance of delivering adequate levels of drug to macrophages to reduce and eradicate HIV-1 infection.
doi:10.1128/AAC.02012-12
PMCID: PMC3591918  PMID: 23263005
2.  Evaluation of Single and Combination Therapies with Tenofovir Disoproxil Fumarate and Emtricitabine In Vitro and in a Robust Mouse Model Supporting High Levels of Hepatitis B Virus Replication 
Antimicrobial Agents and Chemotherapy  2012;56(12):6186-6191.
Next-generation therapies for chronic hepatitis B virus (HBV) infection will involve combinations of established and/or experimental drugs. The current study investigated the in vitro and in vivo efficacy of tenofovir disoproxil fumarate (TDF) and/or emtricitabine [(−)-FTC] alone and in combination therapy for HBV infection utilizing the HepAD38 system (human hepatoblastoma cells transfected with HBV). Cellular pharmacology studies demonstrated increased levels of (−)-FTC triphosphate with coincubation of increasing concentrations of TDF, while (−)-FTC had no effect on intracellular tenofovir (TFV) diphosphate levels. Quantification of extracellular HBV by real-time PCR from hepatocytes demonstrated the anti-HBV activity with TDF, (−)-FTC, and their combination. Combination of (−)-FTC with TDF or TFV (ratio, 1:1) had a weighted average combination index of 0.7 for both combination sets, indicating synergistic antiviral effects. No cytotoxic effects were observed with any regimens. Using an in vivo murine model which develops robust HBV viremia in nude mice subcutaneously injected with HepAD38 cells, TDF (33 to 300 mg/kg of body weight/day) suppressed virus replication for up to 10 days posttreatment. At 300 mg/kg/day, (−)-FTC strongly suppressed virus titers to up to 14 days posttreatment. Combination therapy (33 mg/kg/day each drug) sustained suppression of virus titer/ml serum (<1 log10 unit from pretreatment levels) at 14 days posttreatment, while single-drug treatments yielded virus titers 1.5 to 2 log units above the initial virus titers. There was no difference in mean alanine aminotransferase values or mean wet tumor weights for any of the groups, suggesting a lack of drug toxicity. TDF–(−)-FTC combination therapy provides more effective HBV suppression than therapy with each drug alone.
doi:10.1128/AAC.01483-12
PMCID: PMC3497168  PMID: 22985879
3.  Exploiting the Anti-HIV-1 Activity of Acyclovir: Suppression of Primary and Drug-Resistant HIV Isolates and Potentiation of the Activity by Ribavirin 
Multiple clinical trials have demonstrated that herpes simplex virus 2 (HSV-2) suppressive therapy using acyclovir (ACV) or valacyclovir in HIV-1/HSV-2-infected persons increased the patient's survival and decreased the HIV-1 load. It has been shown that the incorporation of ACV-monophosphate into the nascent DNA chain instead of dGMP results in the termination of viral DNA elongation and directly inhibits laboratory strains of HIV-1. We evaluated here the anti-HIV activity of ACV against primary HIV-1 isolates of different clades and coreceptor specificity and against viral isolates resistant to currently used drugs, including zidovudine, lamivudine, nevirapine, a combination of nucleoside reverse transcriptase inhibitors (NRTIs), a fusion inhibitor, and two protease inhibitors. We found that, at clinically relevant concentrations, ACV inhibits the replication of these isolates in human tissues infected ex vivo. Moreover, addition of ribavirin, an antiviral capable of depleting the pool of intracellular dGTP, potentiated the ACV-mediated HIV-1 suppression. These data warrant further clinical investigations of the benefits of using inexpensive and safe ACV alone or in combination with other drugs against HIV-1, especially to complement or delay highly active antiretroviral therapy (HAART) initiation in low-resource settings.
doi:10.1128/AAC.05986-11
PMCID: PMC3346610  PMID: 22314523
4.  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 β-l-2′-deoxyribose of the β-l-2′-deoxynucleoside confers specific antihepadnavirus activity. The unsubstituted nucleosides β-l-2′-deoxycytidine, β-l-thymidine, and β-l-2′-deoxyadenosine had the most potent, selective, and specific antiviral activity against HBV replication. Human DNA polymerases (α, β, and γ) and mitochondrial function were not affected. In the woodchuck model of chronic HBV infection, viral load was reduced by as much as 108 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.
doi:10.1128/AAC.45.1.229-235.2001
PMCID: PMC90266  PMID: 11120971
5.  Mechanism of Action of 1-β-d-2,6-Diaminopurine Dioxolane, a Prodrug of the Human Immunodeficiency Virus Type 1 Inhibitor 1-β-d-Dioxolane Guanosine 
(−)-β-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 (−)-β-d-dioxolane guanine (DXG). By using calf adenosine deaminase a Km value of 15 ± 0.7 μM was determined for DAPD, which was similar to the Km value for adenosine. However, the kcat for DAPD was 540-fold slower than the kcat 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 α and β. Against the large subunit of human DNA polymerase γ a Ki value of 4.3 ± 0.4 μM was determined for DXG-TP. DXG showed little or no cytotoxicity and no mitochondrial toxicity at the concentrations tested.
doi:10.1128/AAC.45.1.158-165.2001
PMCID: PMC90254  PMID: 11120959
6.  Differential Removal of Thymidine Nucleotide Analogues from Blocked DNA Chains by Human Immunodeficiency Virus Reverse Transcriptase in the Presence of Physiological Concentrations of 2′-Deoxynucleoside Triphosphates 
Antimicrobial Agents and Chemotherapy  2000;44(12):3465-3472.
Removal of 2′,3′-didehydro-3′-deoxythymidine-5′-monophosphate (d4TMP) from a blocked DNA chain can occur through transfer of the chain-terminating residue to a nucleotide acceptor by human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT). ATP-dependent removal of either d4TMP or 3′-azido-3′-deoxythymidine-5′-monophosphate (AZTMP) is increased in AZT resistant HIV-1 RT (containing D67N/K70R/T215F/K219Q mutations). Removal of d4TMP is strongly inhibited by the next complementary deoxynucleoside triphosphate (50% inhibitory concentration [IC50] of ∼0.5 μM), whereas removal of AZTMP is much less sensitive to this inhibition (IC50 of >100 μM). This could explain the lack of cross-resistance by AZT-resistant HIV-1 to d4T in phenotypic drug susceptibility assays.
PMCID: PMC90226  PMID: 11083661
7.  In Vitro Selection of Mutations in the Human Immunodeficiency Virus Type 1 Reverse Transcriptase That Decrease Susceptibility to (−)-β-d-Dioxolane-Guanosine and Suppress Resistance to 3′-Azido-3′-Deoxythymidine 
Human immunodeficiency virus type 1 (HIV-1) isolates resistant to (−)-β-d-dioxolane-guanosine (DXG), a potent and selective nucleoside analog HIV-1 reverse transcriptase (RT) inhibitor, were selected by serial passage of HIV-1LAI in increasing drug concentrations (maximum concentration, 30 μM). Two independent selection experiments were performed. Viral isolates for which the DXG median effective concentrations (EC50s) increased 7.3- and 12.2-fold were isolated after 13 and 14 passages, respectively. Cloning and DNA sequencing of the RT region from the first resistant isolate identified a K65R mutation (AAA to AGA) in 10 of 10 clones. The role of this mutation in DXG resistance was confirmed by site-specific mutagenesis of HIV-1LAI. The K65R mutation also conferred greater than threefold cross-resistance to 2′,3′-dideoxycytidine, 2′,3′-dideoxyinosine, 2′,3′-dideoxy-3′-thiacytidine, 9-(2-phosphonylmethoxyethyl)adenine, 2-amino-6-chloropurine dioxolane, dioxolanyl-5-fluorocytosine, and diaminopurine dioxolane but had only marginal effects on 3′-azido-3′-deoxthymidine (AZT) susceptibility. However, when introduced into a genetic background for AZT resistance (D67N, K70R, T215Y, T219Q), the K65R mutation reversed the AZT resistance. DNA sequencing of RT clones derived from the second resistant isolate identified the L74V mutation, previously reported to cause ddI resistance. The L74V mutation also decreased the AZT resistance when the mutation was introduced into a genetic background for AZT resistance (D67N, K70R, T215Y, T219Q) but to a lesser degree than the K65R mutation did. These findings indicate that DXG and certain 2′,3′-dideoxy compounds (e.g., ddI) can select for the same resistance mutations and thus may not be optimal for use in combination. However, the combination of AZT with DXG or its orally bioavailable prodrug (−)-β-d-2,6-diaminopurine-dioxolane should be explored because of the suppressive effects of the K65R and L74V mutations on AZT resistance.
PMCID: PMC89962  PMID: 10858331
8.  Intracellular Metabolism of β-l-2′,3′-Dideoxyadenosine: Relevance to Its Limited Antiviral Activity 
The intracellular metabolism of the β-l- enantiomer of 2′,3′-dideoxyadenosine (β-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 μM [2′,3′,8-3H]-β-l-ddA resulted in an increased intracellular concentration of β-l-ddA with time, demonstrating that these cells were able to transport β-l-ddA. However, it did not result in the phosphorylation of β-l-ddA to its pharmacologically active 5′-triphosphate (β-l-ddATP). Five other intracellular metabolites were detected and identified as β-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/106 cells at 8, 4, and 2 h in HepG2 cells, PBMC, and hepatocytes, respectively. In addition, a β-glucuronic derivative of β-l-ddA was detected in cultured hepatocytes, accounting for 12.5% of the total metabolite pool. Coincubation of hepatocytes in primary culture with β-l-ddA in the presence of increasing concentrations of 5′-methylthioadenosine resulted in decreased phosphorolysis of β-l-ddA and formation of associated metabolites. These results indicate that the limited antiviral activity of β-l-ddA is the result of its inadequate phosphorylation to the nucleotide level due to phosphorolysis and catabolism of β-l-ddA by methylthioadenosine phosphorylase (EC 2.4.2.28).
PMCID: PMC89782  PMID: 10722481
9.  A New Point Mutation (P157S) in the Reverse Transcriptase of Human Immunodeficiency Virus Type 1 Confers Low-Level Resistance to (−)-β-2′,3′-Dideoxy-3′-Thiacytidine 
A P157S mutation in the reverse transcriptase (RT) of human immunodeficiency virus type 1 conferred fivefold resistance to (−)-β-2′,3′-dideoxy-3′-thiacytidine in cell culture. Interestingly, the P157S mutation resulted in increased sensitivity (two- to threefold) to 3′-azido-3′-deoxythymidine (AZT) and to (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA). A similar increase in susceptibility to AZT and to PMPA was also conferred by the M184V mutation in RT.
PMCID: PMC89420  PMID: 10428942
10.  Pharmacokinetics of β-l-2′,3′-Dideoxy-5-Fluorocytidine in Rhesus Monkeys 
β-l-2′,3′-Dideoxy-5-fluorocytidine (β-l-FddC), a novel cytidine analog with an unnatural β-l sugar configuration, has been demonstrated by our group and others to exhibit highly selective in vitro activity against human immunodeficiency virus types 1 and 2 and hepatitis B virus. This encouraging in vitro antiviral activity prompted us to assess its pharmacokinetics in rhesus monkeys. Three monkeys were administered an intravenous dose of [3H]β-L-FddC at 5 mg/kg of body weight. Following a 3-month washout period, an equivalent oral dose was administered. Plasma and urine samples were collected at various times for up to 24 h after dosing, and drug levels were quantitated by high-pressure liquid chromatography. Pharmacokinetic parameters were obtained on the basis of a two-compartment open model with a first-order elimination from the central compartment. After intravenous administration, the mean peak concentration in plasma (Cmax) was 29.8 ± 10.5 μM. Total clearance, steady-state volume of distribution, terminal-phase plasma half-life (t1/2β), and mean residence time were 0.7 ± 0.1 liters/h/kg, 1.3 ± 0.1 liters/kg, 1.8 ± 0.2 h, and 1.9 ± 0.2 h, respectively. Approximately 47% ± 16% of the intravenously administered radioactivity was recovered in the urine as the unchanged drug with no apparent metabolites. β-l-FddC exhibited a Cmax of 3.2 μM after oral administration, with a time to peak drug concentration of approximately 1.5 h and a t1/2 of 2.2 h. One monkey in the oral administration arm of the study had a significant delay in the absorption of the aqueous administered dose. The absolute bioavailability of orally administered β-l-FddC ranged from 56 to 66%.
PMCID: PMC89226  PMID: 10103200
11.  A Rapid Non-Culture-Based Assay for Clinical Monitoring of Phenotypic Resistance of Human Immunodeficiency Virus Type 1 to Lamivudine (3TC) 
Monitoring for lamivudine (3TC) resistance is important both for the clinical management of human immunodeficiency virus type 1 (HIV-1)-infected patients treated with 3TC and for surveillance of transmission of 3TC-resistant HIV-1. We developed a novel non-culture-based assay for the rapid analysis of phenotypic resistance to 3TC of HIV-1 in plasma. The assay measures the susceptibility of HIV-1 reverse transcriptase (RT) activity to 3TC triphosphate (3TC-TP) in plasma. RT detection was done by the Amp-RT assay, an ultrasensitive PCR-based RT assay. Under our assay conditions, we found that 5 μM 3TC-TP inhibited RT activity from wild-type (WT), zidovudine-resistant, or nevirapine-resistant HIV-1 but not from HIV-1 carrying either the M184V mutation or multidrug (MD) resistance mutations (77L/116Y/151M or 62V/75I/77L/116Y/151M). Mixing experiments showed a detection threshold of 10% 3TC-resistant virus (M184V) in a background of WT HIV-1. To validate the assay for the detection of phenotypic resistance of HIV-1 to 3TC in plasma samples, HIV-1 RT in 30 plasma specimens collected from 15 patients before and during therapy with 3TC was tested for evidence of phenotypic resistance by the Amp-RT assay. The results were compared with those of genotypic analysis. The RT in 12 samples was found to be 3TC sensitive, while the RT in 18 samples had evidence of phenotypic resistance. All 12 samples with 3TC-sensitive RT had WT genotypes at codon 184 and were retrieved before treatment with 3TC. In contrast, all 18 specimens with 3TC-resistant RT were posttherapy samples. This assay provides a simple, rapid, and reliable method for the detection of phenotypic resistance of HIV-1 to 3TC in plasma.
PMCID: PMC89061  PMID: 9925516
12.  Pharmacokinetics of the Antiviral Agent β-d-2′,3′-Didehydro-2′,3′-Dideoxy-5-Fluorocytidine in Rhesus Monkeys 
The values of the pharmacokinetic parameters of the nucleoside antiretroviral agent β-d-2′,3′-didehydro-2′,3′-dideoxy-5-fluorocytidine (D-D4FC) in rhesus monkeys were determined with a two-compartment model after the administration of a single dose. The average values for the terminal half-life, renal clearance, and total systemic clearance for the intravenous administration route were 3.6 h and 0.31 and 0.43 liter · kg−1 · h−1, respectively. The oral bioavailability of D-D4FC averaged 41%. For the intravenous administration route, 76% of the compound was recovered intact in the urine within 8 h, indicating that D-D4FC was eliminated mainly by renal excretion. D-D4FC was detected in the cerebrospinal fluid (CSF) at similar concentrations after administration by both the intravenous and oral routes. D-D4FC levels in plasma and CSF were higher than the median effective concentration for human immunodeficiency virus type 1 in vitro.
PMCID: PMC89084  PMID: 9925539
14.  Pharmacodynamics of (−)-β-2′,3′-Dideoxy-3′-Thiacytidine in Chronically Virus-Infected Woodchucks Compared to Its Pharmacodynamics in Humans 
Antimicrobial Agents and Chemotherapy  1998;42(11):2804-2809.
The pharmacodynamics of (−)-β-2′,3′-dideoxy-3′-thiacytidine (3TC) was studied in chronically woodchuck hepatitis virus-infected woodchucks and compared to that in previous studies in hepatitis B virus (HBV)-infected humans. Net depletion rates of serum virus DNA in woodchucks receiving 3TC were modeled as a sum of an exponentially declining virus input and a first-order elimination. Preceding shoulders and pseudo-first-order virus half-lives in serum ranged from 1 to 7 days and were dose dependent. Higher plasma 3TC concentrations were needed in woodchucks for virus depletion similar to that attained in humans. Human HBV depletion curves from a previous clinical study with 3TC (≥100 mg per day) were described by a biexponential relationship. The average half-life value in humans, normalized to fraction of area under the serum virus load-time curve, was similar to the average half-life value observed in woodchucks given the highest 3TC dose (2.4 and 2.0 days, respectively). On cessation of therapy, virus load rebounds in woodchucks were dose dependent and resembled posttherapy virus “flares” reported to occur in humans. The estimates of drug exposures that could lead to optimal antiviral effects presented indicate that 3TC should not be underdosed and compliance should be monitored. The study of chronically infected woodchucks may prove useful for optimizing drug regimens for hepadnavirus infections.
PMCID: PMC105947  PMID: 9797207
15.  The Base Component of 3′-Azido-2′,3′-Dideoxynucleosides Influences Resistance Mutations Selected in HIV-1 Reverse Transcriptase▿ 
We recently reported that HIV-1 resistant to 3′-azido-3′-deoxythymidine (AZT) is not cross-resistant to 3′-azido-2′,3′-dideoxypurines. This finding suggested that the nucleoside base is a major determinant of HIV-1 resistance to nucleoside analogs. To further explore this hypothesis, we conducted in vitro selection experiments by serial passage of HIV-1LAI in MT-2 cells in increasing concentrations of 3′-azido-2′,3′-dideoxyguanosine (3′-azido-ddG), 3′-azido-2′,3′-dideoxycytidine (3′-azido-ddC), or 3′-azido-2′,3′-dideoxyadenosine (3′-azido-ddA). 3′-Azido-ddG selected for virus that was 5.3-fold resistant to 3′-azido-ddG compared to wild-type HIV-1LAI passaged in the absence of drug. Population sequencing of the entire reverse transcriptase (RT) gene identified L74V, F77L, and L214F mutations in the polymerase domain and K476N and V518I mutations in the RNase H domain. However, when introduced into HIV-1 by site-directed mutagenesis, these 5 mutations only conferred ∼2.0-fold resistance. Single-genome sequencing analyses of the selected virus revealed a complex population of mutants that all contained L74V and L214F linked to other mutations, including ones not identified during population sequencing. Recombinant HIV-1 clones containing RT derived from single sequences exhibited 3.2- to 4.0-fold 3′-azido-ddG resistance. In contrast to 3′-azido-ddG, 3′-azido-ddC selected for the V75I mutation in HIV-1 RT that conferred 5.9-fold resistance, compared to the wild-type virus. Interestingly, we were unable to select HIV-1 that was resistant to 3′-azido-ddA, even at concentrations of 3′-azido-ddA that yielded high intracellular levels of 3′-azido-ddA-5′-triphosphate. Taken together, these findings show that the nucleoside base is a major determinant of HIV-1 resistance mechanisms that can be exploited in the design of novel nucleoside RT inhibitors.
doi:10.1128/AAC.00414-11
PMCID: PMC3147647  PMID: 21646480
16.  Selection and Characterization of HIV-1 with a Novel S68 Deletion in Reverse Transcriptase▿ 
Resistance to human immunodeficiency virus type 1 (HIV-1) represents a significant problem in the design of novel therapeutics and the management of treatment regimens in infected persons. Resistance profiles can be elucidated by defining modifications to the viral genome conferred upon exposure to novel nucleoside reverse transcriptase (RT) inhibitors (NRTI). In vitro testing of HIV-1LAI-infected primary human lymphocytes treated with β-d-2′,3′-dideoxy-2′,3′-didehydro-5-fluorocytidine (DFC; Dexelvucitabine; Reverset) produced a novel deletion of AGT at codon 68 (S68Δ) alone and in combination with K65R that differentially affects drug response. Dual-approach clone techniques utilizing TOPO cloning and pyrosequencing confirmed the novel S68Δ in the HIV-1 genome. The S68Δ HIV-1 RT was phenotyped against various antiviral agents in a heteropolymeric DNA polymerase assay and in human lymphocytes. Drug susceptibility results indicate that the S68Δ displayed a 10- to 30-fold increase in resistance to DFC, lamivudine, emtricitabine, tenofovir, abacavir, and amdoxovir and modest resistance to stavudine, β-d-2′,3′-oxa-5-fluorocytidine, or 9-(β-d-1,3-dioxolan-4-yl)guanine and remained susceptible to 3′-azido-3′-deoxythymidine, 2′,3′-dideoxyinosine (ddI), 1-(β-d-dioxolane)thymine (DOT) and lopinavir. Modeling revealed a central role for S68 in affecting conformation of the β3-β4 finger region and provides a rational for the selective resistance. These data indicate that the novel S68Δ is a previously unrecognized deletion that may represent an important factor in NRTI multidrug resistance treatment strategies.
doi:10.1128/AAC.01700-10
PMCID: PMC3088218  PMID: 21357304
17.  Lack of Pharmacokinetic Interaction between Amdoxovir and Reduced- and Standard-Dose Zidovudine in HIV-1-Infected Individuals▿  
Amdoxovir (AMDX) inhibits HIV-1 containing the M184V/I mutation and is rapidly absorbed and deaminated to its active metabolite, β-d-dioxolane guanosine (DXG). DXG is synergistic with zidovudine (ZDV) in HIV-1-infected primary human lymphocytes. A recent in silico pharmacokinetic (PK)/enzyme kinetic study suggested that ZDV at 200 mg twice a day (b.i.d.) may reduce toxicity without compromising efficacy relative to the standard 300-mg b.i.d. dose. Therefore, an intense PK clinical study was conducted using AMDX/placebo, with or without ZDV, in 24 subjects randomized to receive oral AMDX at 500 mg b.i.d., AMDX at 500 mg plus ZDV at 200 or 300 mg b.i.d., or ZDV at 200 or 300 mg b.i.d. for 10 days. Full plasma PK profiles were collected on days 1 and 10, and complete urine sampling was performed on day 9. Plasma and urine concentrations of AMDX, DXG, ZDV, and ZDV-5′-O-glucuronide (GZDV) were measured using a validated liquid chromatography-tandem mass spectrometry method. Data were analyzed using noncompartmental methods, and multiple comparisons were performed on the log-transformed parameters, at steady state. Coadministration of AMDX with ZDV did not significantly change either of the plasma PK parameters or percent recovery in the urine of AMDX, DXG, or ZDV/GZDV. Larger studies with AMDX/ZDV, with a longer duration, are warranted.
doi:10.1128/AAC.01209-09
PMCID: PMC2826005  PMID: 20038617
18.  Anti-Human Immunodeficiency Virus Activity, Cross-Resistance, Cytotoxicity, and Intracellular Pharmacology of the 3′-Azido-2′,3′-Dideoxypurine Nucleosides▿  
Although the approved nucleoside reverse transcriptase (RT) inhibitors (NRTI) are integral components of therapy for human immunodeficiency virus type 1 (HIV-1) infection, they can have significant limitations, including the selection of NRTI-resistant HIV-1 and cellular toxicity. Accordingly, there is a critical need to develop new NRTI that have excellent activity and safety profiles and exhibit little or no cross-resistance with existing drugs. In this study, we report that the 3′-azido-2′,3′-dideoxypurine nucleosides (ADPNs) 3′-azido-2′,3′-dideoxyadenosine (3′-azido-ddA) and 3′-azido-2′,3′-dideoxyguanosine (3′-azido-ddG) exert potent antiviral activity in primary human lymphocytes and HeLa and T-cell lines (50% inhibitory concentrations [IC50s] range from 0.19 to 2.1 μM for 3′-azido-ddG and from 0.36 to 10 μM for 3′-azido-ddA) and that their triphosphate forms are incorporated as efficiently as the natural dGTP or dATP substrates by HIV-1 RT. Importantly, both 3′-azido-ddA and 3′-azido-ddG retain activity against viruses containing K65R, L74V, or M184V (IC50 change of <2.0-fold) and against those containing three or more thymidine analog mutations (IC50 change of <3.5-fold). In addition, 3′-azido-ddG does not exhibit cytotoxicity in primary lymphocytes or epithelial or T-cell lines and does not decrease the mitochondrial DNA content of HepG2 cells. Furthermore, 3′-azido-ddG is efficiently phosphorylated to 3′-azido-ddGTP in human lymphocytes, with an intracellular half-life of the nucleoside triphosphate of 9 h. The present data suggest that additional preclinical studies are warranted to assess the potential of ADPNs for treatment of HIV-1 infection.
doi:10.1128/AAC.00392-09
PMCID: PMC2737880  PMID: 19596885
19.  Development of an Optimized Dose for Coformulation of Zidovudine with Drugs That Select for the K65R Mutation Using a Population Pharmacokinetic and Enzyme Kinetic Simulation Model▿  
Antimicrobial Agents and Chemotherapy  2008;52(12):4241-4250.
In vitro selection studies and data from large genotype databases from clinical studies have demonstrated that tenofovir disoproxil fumarate and abacavir sulfate select for the K65R mutation in the human immunodeficiency virus type 1 polymerase region. Furthermore, other novel non-thymine nucleoside reverse transcriptase (RT) inhibitors also select for this mutation in vitro. Studies performed in vitro and in humans suggest that viruses containing the K65R mutation remained susceptible to zidovudine (ZDV) and other thymine nucleoside antiretroviral agents. Therefore, ZDV could be coformulated with these agents as a “resistance repellent” agent for the K65R mutation. The approved ZDV oral dose is 300 mg twice a day (b.i.d.) and is commonly associated with bone marrow toxicity thought to be secondary to ZDV-5′-monophosphate (ZDV-MP) accumulation. A simulation study was performed in silico to optimize the ZDV dose for b.i.d. administration with K65R-selecting antiretroviral agents in virtual subjects using the population pharmacokinetic and cellular enzyme kinetic parameters of ZDV. These simulations predicted that a reduction in the ZDV dose from 300 to 200 mg b.i.d. should produce similar amounts of ZDV-5′-triphosphate (ZDV-TP) associated with antiviral efficacy (>97% overlap) and reduced plasma ZDV and cellular amounts of ZDV-MP associated with toxicity. The simulations also predicted reduced peak and trough amounts of cellular ZDV-TP after treatment with 600 mg ZDV once a day (q.d.) rather than 300 or 200 mg ZDV b.i.d., indicating that q.d. dosing with ZDV should be avoided. These in silico predictions suggest that 200 mg ZDV b.i.d. is an efficacious and safe dose that could delay the emergence of the K65R mutation.
doi:10.1128/AAC.00054-08
PMCID: PMC2592851  PMID: 18838591
20.  Antiviral Effect of Orally Administered (−)-β-d-2-Aminopurine Dioxolane in Woodchucks with Chronic Woodchuck Hepatitis Virus Infection▿  
(−)-β-d-2-Aminopurine dioxolane (APD) is a nucleoside prodrug that is efficiently converted to 9-(β-d-1,3-dioxolan-4-yl)guanine (DXG). DXG has antiviral activity in vitro against hepatitis B virus (HBV) but limited aqueous solubility, making it difficult to administer orally to HBV-infected individuals. APD is more water soluble than DXG and represents a promising prodrug for the delivery of DXG. A placebo-controlled, dose-ranging efficacy and pharmacokinetic study was conducted with woodchucks that were chronically infected with woodchuck hepatitis virus (WHV). APD was efficiently converted to DXG after oral and intravenous administrations of APD, with serum concentrations of DXG being higher following oral administration than following intravenous administration, suggestive of a considerable first-pass intestinal and/or hepatic metabolism. APD administered orally at 1, 3, 10, and 30 mg/kg of body weight per day for 4 weeks produced a dose-dependent antiviral response. Doses of 3 and 10 mg/kg/day reduced serum WHV viremia by 0.4 and 0.7 log10 copies/ml, respectively. The 30-mg/kg/day dose resulted in a more pronounced, statistically significant decline in serum WHV viremia of 1.9 log10 copies/ml and was associated with a 1.5-fold reduction in hepatic WHV DNA. Individual woodchucks within the highest APD dose group that had declines in serum WHV surface antigen levels, WHV viremia, and hepatic WHV DNA also had reductions in hepatic WHV RNA. There was a prompt recrudescence of WHV viremia following drug withdrawal. Therefore, oral administration of APD for 4 weeks was safe in the woodchuck model of chronic HBV infection, and the effect on serum WHV viremia was dose dependent.
doi:10.1128/AAC.00325-07
PMCID: PMC2043196  PMID: 17606676
21.  Pharmacokinetics of the Antiviral Agent β-d-2′-Deoxy-2′-Fluoro-2′-C-Methylcytidine in Rhesus Monkeys▿ †  
β-d-2′-Deoxy-2′-fluoro-2′-C-methylcytidine (PSI-6130) is an effective inhibitor of hepatitis C virus (HCV) replication in vitro. The purpose of this study was to evaluate the single-dose pharmacokinetics of PSΙ-6130 in rhesus monkeys following intravenous (i.v.) and oral administration. Noncompartmental analysis of the serum data obtained following oral and i.v. administration was performed. Pharmacokinetic studies with rhesus monkeys indicated slow and incomplete absorption with a mean absorption time (MAT) of 4.6 h and an oral bioavailability of 24.0% ± 14.3% (mean ± standard deviation), with comparable mean apparent half-lives following i.v. (4.54 ± 3.98 h) and oral (5.64 ± 1.13 h) administrations. The average percentages of the total dose recovered unchanged and in deaminated form in the urine were 32.9% ± 12.6% and 18.9% ± 6.6% (i.v.) and 6.0% ± 3.9% and 3.9% ± 1.0% (oral), respectively. The total bioavailability, taking into account the parent drug and its deaminated metabolite 2′-deoxy-2′-fluoro-2′-C-methyluridine (PSI-6206), was 64% ± 26%. PSI-6130 was present in the cerebrospinal fluid after oral and i.v. dosing. However, no deamination of radiolabeled PSI-6130 was detected after 8 h of incubation in monkey and human whole blood. An N4-modified prodrug of PSI-6130 (PSI-6419) was orally administered to monkeys, but it failed to improve the oral bioavailability of PSI-6130. Further studies are warranted to improve the oral bioavailability and reduce the deamination of PSI-6130 in order to explore the potential of this drug for the treatment of HCV-infected individuals.
doi:10.1128/AAC.00193-07
PMCID: PMC1932527  PMID: 17562805
22.  Pharmacokinetics of the Anti-Human Immunodeficiency Virus Agent 1-(β-d-Dioxolane)Thymine in Rhesus Monkeys▿ † 
β-d-Dioxolane-thymine (D-DOT) has potent and selective in vitro activity against several clinically important resistant human immunodeficiency virus (HIV) mutants and is in advanced preclinical development. Therefore, the single-dose intravenous and oral pharmacokinetics of D-DOT were studied with three rhesus monkeys. The pharmacokinetic profiles of D-DOT in serum and urine were adequately described by a two-compartment open pharmacokinetic model. D-DOT was rapidly and almost completely absorbed (absorption rate constant = 2.7 h−1; fraction of oral dose absorbed = 0.82 to 1.06). The average serum beta half-life was 2.16 h. The average central and steady-state volumes of distributions were 0.52 and 1.02 liter/kg of body weight, respectively, and the average systemic and renal clearance values were 0.36 liter/h/kg and 0.18 liter/h/kg. Four or eight percent of administered D-DOT was eliminated in the urine as glucuronide within 8 h after intravenous or oral administration, respectively. D-DOT reached levels in the cerebrospinal fluid in excess of 10 to 20 times the median effective concentration for wild-type HIV and resistant mutants. The potent antiretroviral activity of D-DOT against a lamivudine- and zidovudine-resistant HIV-1 mutant, together with an excellent pharmacokinetic profile for rhesus monkeys, suggest that further development is warranted.
doi:10.1128/AAC.01498-06
PMCID: PMC1913250  PMID: 17485498
23.  Antiviral and Cellular Metabolism Interactions between Dexelvucitabine and Lamivudine▿  
Studies on cellular drug interactions with antiretroviral agents prior to clinical trials are critical to detect possible drug interactions. Herein, we demonstrated that two 2′-deoxycytidine antiretroviral agents, dexelvucitabine (known as β-d-2′,3′-didehydro-2′,3′-dideoxy-5-fluorocytidine, DFC, d-d4FC, or RVT) and lamivudine (3TC), combined in primary human peripheral blood mononuclear (PBM) cells infected with human immunodeficiency virus 1 strain LAI (HIV-1LAI), resulted in additive-to-synergistic effects. The cellular metabolism of DFC and 3TC was studied in human T-cell lymphoma (CEM) and in primary human PBM cells to determine whether this combination caused any reduction in active nucleoside triphosphate (NTP) levels, which could decrease with their antiviral potency. Competition studies were conducted by coincubation of either radiolabeled DFC with different concentrations of 3TC or radiolabeled 3TC with different concentrations of DFC. Coincubation of radiolabeled 3TC with DFC at concentrations up to 33.3 μM did not cause any marked reduction in 3TC-triphosphate (TP) or any 3TC metabolites. However, a reduction in the level of DFC metabolites was noted at high concentrations of 3TC with radiolabeled DFC. DFC-TP levels in CEM and primary human PBM cells decreased by 88% and 94%, respectively, when high concentrations of 3TC (33.3 and 100 μM) were added, which may influence the effectiveness of DFC-5′-TP on the HIV-1 polymerase. The NTP levels remained well above the median (50%) inhibitory concentration for HIV-1 reverse transcriptase. These results suggest that both β-d- and β-l-2′-deoxycytidine analogs, DFC and 3TC, respectively, substrates of 2′-deoxycytidine kinase, could be used in a combined therapeutic modality. However, it may be necessary to decrease the dose of 3TC for this combination to prove effective.
doi:10.1128/AAC.01543-06
PMCID: PMC1891415  PMID: 17403996
24.  Biochemical Studies on the Mechanism of Human Immunodeficiency Virus Type 1 Reverse Transcriptase Resistance to 1-(β-d-Dioxolane)Thymine Triphosphate▿  
A large panel of drug-resistant mutants of human immunodeficiency virus type 1 reverse transcriptase (RT) was used to study the mechanisms of resistance to 1-(β-d-dioxolane)thymine triphosphate (DOT-TP) and other nucleotide analogs. RT containing thymidine analog-associated mutations (TAM) or RT with a T69S-SG insertion in combination with TAM removed 3′-azido-3′-deoxythymidine-5′-monophosphate or tenofovir more efficiently than DOT-monophosphate from chain-terminated DNA primer/template through ATP-mediated pyrophosphorolysis. For non-ATP-dependent discrimination toward DOT-TP, high levels of resistance were found for RT bearing the Q151M mutation with family mutations, while RT bearing only the M184V or the Y115F mutation conferred no resistance to DOT-TP. A lower degree of resistance to DOT-TP than to tenofovir diphosphate or carbovir-TP was found for RT containing the K65R mutation. In the present studies, 1-(β-d-dioxolane)guanine triphosphate, another nucleotide with a dioxolane sugar moiety, showed a resistance profile similar to that of DOT-TP. The results suggest that DOT, compared with other approved nucleoside analogs, is overall more resilient to mutations such as TAM, M184V, and K65R, which are commonly found in viruses derived from subjects failing multinucleoside therapy.
doi:10.1128/AAC.00119-07
PMCID: PMC1891359  PMID: 17403997
25.  Mechanism of Activation of β-d-2′-Deoxy-2′-Fluoro-2′-C-Methylcytidine and Inhibition of Hepatitis C Virus NS5B RNA Polymerase▿  
β-d-2′-Deoxy-2′-fluoro-2′-C-methylcytidine (PSI-6130) is a potent specific inhibitor of hepatitis C virus (HCV) RNA synthesis in Huh-7 replicon cells. To inhibit the HCV NS5B RNA polymerase, PSI-6130 must be phosphorylated to the 5′-triphosphate form. The phosphorylation of PSI-6130 and inhibition of HCV NS5B were investigated. The phosphorylation of PSI-6130 by recombinant human 2′-deoxycytidine kinase (dCK) and uridine-cytidine kinase 1 (UCK-1) was measured by using a coupled spectrophotometric reaction. PSI-6130 was shown to be a substrate for purified dCK, with a Km of 81 μM and a kcat of 0.007 s−1, but was not a substrate for UCK-1. PSI-6130 monophosphate (PSI-6130-MP) was efficiently phosphorylated to the diphosphate and subsequently to the triphosphate by recombinant human UMP-CMP kinase and nucleoside diphosphate kinase, respectively. The inhibition of wild-type and mutated (S282T) HCV NS5B RNA polymerases was studied. The steady-state inhibition constant (Ki) for PSI-6130 triphosphate (PSI-6130-TP) with the wild-type enzyme was 4.3 μM. Similar results were obtained with 2′-C-methyladenosine triphosphate (Ki = 1.5 μM) and 2′-C-methylcytidine triphosphate (Ki = 1.6 μM). NS5B with the S282T mutation, which is known to confer resistance to 2′-C-methyladenosine, was inhibited by PSI-6130-TP as efficiently as the wild type. Incorporation of PSI-6130-MP into RNA catalyzed by purified NS5B RNA polymerase resulted in chain termination.
doi:10.1128/AAC.00400-06
PMCID: PMC1797721  PMID: 17101674

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