Synthesis of Nucleoside-like Molecules from a Pyrolysis Product of Cellulose and Their Computational Prediction as Potential SARS-CoV-2 RNA-Dependent RNA Polymerase Inhibitors

(1R,5S)-1-Hydroxy-3,6-dioxa-bicyclo[3.2.1]octan-2-one, available by an efficient catalytic pyrolysis of cellulose, has been applied as a chiral building block in the synthesis of seven new nucleoside analogues, with structural modifications on the nucleobase moiety and on the carboxyl- derived unit. The inverted configuration by Mitsunobu reaction used in their synthesis was verified by 2D-NOESY correlations, supported by the optimized structure employing the DFT methods. An in silico screening of these compounds as inhibitors of SARS-CoV-2 RNA-dependent RNA polymerase has been carried out in comparison with both remdesivir, a mono-phosphoramidate prodrug recently approved for COVID-19 treatment, and its ribonucleoside metabolite GS-441524. Drug-likeness prediction and data by docking calculation indicated compound 6 [=(3S,5S)-methyl 5-(hydroxymethyl)-3-(6-(4-methylpiperazin-1-yl)-9H-purin-9-yl)tetrahydrofuran-3-carboxylate] as the best candidate. Furthermore, molecular dynamics simulation showed a stable interaction of structure 6 in RNA-dependent RNA polymerase (RdRp) complex and a lower average atomic fluctuation than GS-441524, suggesting a well accommodation in the RdRp binding pocket.

Discovery of a 2'-fluoro-2'-C-methyl C-nucleotide HCV polymerase inhibitor and a phosphoramidate prodrug with favorable properties

A series of 2'-fluorinated C-nucleosides were prepared and tested for anti-HCV activity. Among them, the triphosphate of 2'-fluoro-2'-C-methyl adenosine C-nucleoside (15) was a potent and selective inhibitor of the NS5B polymerase and maintained activity against the S282T resistance mutant. A number of phosphoramidate prodrugs were then prepared and evaluated leading to the identification of the 1-aminocyclobutane-1-carboxylic acid isopropyl ester variant (53) with favorable pharmacokinetic properties including efficient liver delivery in animals.

Single-Dose and Multiple-Dose Pharmacokinetics and Safety of Telbivudine After Oral Administration in Healthy Chinese Subjects

The pharmacokinetics of telbivudine, an L-nucleoside with potent activity against hepatitis B virus, was assessed in 42 healthy Chinese volunteers. Subjects were assigned to receive a single oral dose of 200, 400, or 800 mg telbivudine or repeat doses of 600 mg/d. Telbivudine was absorbed rapidly and exhibited dose-related plasma exposure. After reaching maximum concentration (C(max)) at a median time of 2.0 to 2.5 hours, plasma disposition of the drug was biphasic with a mean terminal half-life ranging from 39.4 to 49.1 hours. Telbivudine accumulated slightly after repeat doses, and steady state was reached after 5 to 6 consecutive doses of 600 mg/d. The mean steady-state C(max) and area under the plasma concentration-time curve over the dosing interval of telbivudine 600 mg were 3.7 microg/mL and 26.1 microg x h/mL, respectively. Cumulative urinary excretion of telbivudine over 32 hours represented 24.4% of the administered dose, with a mean renal clearance of 6.6 L/h. Telbivudine was well tolerated in the studied dose range in healthy Chinese subjects, with no pattern of dose-related clinical or laboratory adverse events.

Absence of Food Effect on the Pharmacokinetics of Telbivudine Following Oral Administration in Healthy Subjects

The influence of food on the pharmacokinetics of telbivudine, a candidate antiviral agent against hepatitis B virus (HBV), was investigated in healthy adult subjects following a 600-mg oral dose administered with and without a high-fat/high-calorie meal. Telbivudine was well tolerated under fasting and fed conditions. Oral absorption of telbivudine as measured by maximum plasma concentration (Cmax), time to reach Cmax (Tmax), and area under the plasma concentration-time curve (AUC(0-t) and AUC(0-infinity)) was not altered by food intake immediately before oral dosing. Values of Cmax, Tmax, and AUC were comparable when telbivudine was administered under fed and fasting conditions. Results from this study indicated that the absorption of telbivudine was not affected by a high-fat/high-calorie meal; telbivudine can therefore be administered orally with no regard to the timing of meals.

Safety, tolerability, and efficacy of KP-1461 as monotherapy for 124 days in antiretroviral-experienced, HIV type 1-infected subjects

Treatment of HIV infection with conventional antiretroviral therapy (ART) is a lifelong challenge with significant long-term risks of adverse events and treatment failure-induced HIV resistance being major concerns. One potential alternative to standard treatment is the use of viral decay accelerators, antiviral agents that theoretically can drive the rate of viral mutation beyond the compensatory capacity of the virus, thereby inducing viral extinction. One such drug, KP-1461, was tested in a population of HIV-infected persons not receiving ART to assess the safety, tolerability, and efficacy of the strategy in vivo. Of 24 highly treatment-experienced HIV-infected patients who received at least one dose of KP-1461, 13 completed the planned 4 months of monotherapy. The drug was generally well tolerated; it did not significantly affect either HIV viral load or CD4 lymphocyte count over the period of dosing. Pharmacokinetic sampling suggested adequate drug exposure was achieved. There were no new mutations induced by KP-1461 that changed viral susceptibility to standard antiretroviral agents. After the study was completed, analysis of more than 7 million base pairs of HIV samples from study patients and controls demonstrated changes in the pattern of viral mutations that differed significantly from what would be encountered naturally. The identified alterations were consistent with an effect resulting from KP-1461's proposed mechanism of action. These findings suggest that the novel antiretroviral approach illustrated by this study should be further investigated, particularly given the relatively good tolerability and the demonstrated excellent safety in this limited cohort study.

Pharmacokinetics of antifungal agents in children

Invasive fungal infections in immunocompromised children are common and often fatal. The first antifungal agents such as amphotericin B and fluconazole offered effective treatment, but their use was often limited by toxicity and resistance. Numerous new antifungal agents have since been developed and appear to be as effective. Most dosing and safety trials have been done in adults, and extrapolation of this data to children has proven inadequate. We reviewed the literature regarding the pharmacokinetics/pharmacodynamics (PK/PD) and safety of antifungal agents with an emphasis on the newer azoles and echinocandins. From a small but growing number of PK/PD trials, better dosing guidelines have been developed.

R-1626, a specific oral NS5B polymerase inhibitor of hepatitis C virus

Roche Holding AG is developing R-1626, an oral nucleoside inhibitor of HCV RNA polymerase. R-1626 has been demonstrated to be well absorbed and rapidly converted to the active component R-1479. The compound has demonstrated a strong capacity to inhibit HCV replication in vitro and in vivo, without the rapid development of viral resistance. After 4 weeks of treatment with R-1626 in combination with PEG-IFN plus ribavirin in treatment-naïve patients with genotype 1 HCV infection, HCV RNA could no longer be detected in approximately 74% of patients, compared with 5% of patients treated with PEG-IFN plus ribavirin alone, indicating the high potency of R-1626 to induce HCV RNA viral load reductions. R-1626 was generally well tolerated, although severe side effects of neutropenia were observed at high doses. A phase IIb clinical trial was ongoing at the time of publication to test the efficacy of R-1626 in combination with a standard or lower dose of PEG-IFN and ribavirin in HCV genotype 1-infected patients. Given its potent antiviral effect with an apparent high genetic barrier, R-1626 represents an important advancement in improving the outcome of patients with chronic HCV infection.

Preclinical pharmacokinetics and metabolism of a potent non-nucleoside inhibitor of the hepatitis C virus NS5B polymerase

The disposition of compound A, a potent inhibitor of the hepatitis C virus (HCV) NS5B polymerase, was characterized in animals in support of its selection for further development. Compound A exhibited marked species differences in pharmacokinetics. Plasma clearance was 44 ml min-1 kg-1 in rats, 9 ml min-1 kg-1 in dogs and 16 ml min-1 kg-1 in rhesus monkeys. Oral bioavailability was low in rats (10%) but significantly higher in dogs (52%) and monkeys (26%). Compound A was eliminated primarily by metabolism in rats, with biliary excretion accounting for 30% of its clearance. Metabolism was mainly mediated by cyclohexyl hydroxylation, with N-deethylation and acyl glucuronide formation constituting minor metabolic pathways. Qualitatively, the same metabolites were identified using in vitro systems from all species studied, including humans. The low oral bioavailability of compound A in rats was mostly due to poor intestinal absorption. This conclusion was borne out by the findings that hepatic extraction in the rat was only 30%, intraperitoneal bioavailability was good, and compound A was poorly absorbed from the rat isolated intestinal loop, with no detectable intestinal metabolism. Compound A was not an inhibitor of major human cytochrome P450 enzymes, indicating minimal potential for clinical drug-drug interactions. The metabolic clearance of compound A in rat, dog and monkey hepatocytes correlated with the systemic clearance observed in these species. Since compound A was very stable in human hepatocytes, the results suggest that it will be a low clearance drug in humans.

Prodrugs of nucleoside analogues for improved oral absorption and tissue targeting

Nucleoside analogues are widely used for the treatment of antiviral infections and anticancer chemotherapy. However, many nucleoside analogues suffer from poor oral bioavailability due to their high polarity and low intestinal permeability. In order to improve oral absorption of these polar drugs, prodrugs have been employed to increase lipophilicity by chemical modification of the parent. Alternatively, prodrugs targeting transporters present in the intestine have been exploited to facilitate the transport of the nucleoside analogues. Valacyclovir and valganciclovir are two successful valine ester prodrugs transported by the PepT1 transporter. Recently, research efforts have focused on design of prodrugs for tissue specific delivery to improve efficacy and safety. This review presents advances of prodrug approaches for improved oral absorption of nucleoside analogues and recent developments in tissue targeting.

Nucleoside analogue delivery systems in cancer therapy

Nucleoside analogues (NAs) are important agents in the treatment of hematological malignancies. They are prodrugs that require activation by phosphorylation. Their rapid catabolism, cell resistance and overdistribution in the body jeopardize nucleoside analogue chemotherapy. Accordingly, therapeutic doses of NAs are particularly high and regularly have to be increased, resulting in severe toxicity and narrow therapeutic index. The major challenge is to concentrate the drug at the tumour site, avoiding its distribution to normal tissues. New drug carriers and biomaterials are being developed to overcome some of these obstacles. This review highlights novel NA delivery systems and discusses new technologies that could improve NA cancer therapy.

Pharmacokinetics of telbivudine in subjects with various degrees of renal impairment

This study evaluates the effect of renal impairment on the pharmacokinetics of telbivudine. Thirty-six subjects were assigned, on the basis of creatinine clearance (CL(CR)), to 1 of 5 renal function groups with 6 to 8 subjects per group: normal renal function; mild, moderate, or severe renal impairment; or end-stage renal disease [ESRD] requiring hemodialysis. Subjects received a single oral dose of telbivudine at 600 mg (normal function and mild impairment), 400 mg (moderate impairment), or 200 mg (severe impairment and ESRD); plasma and/or urine samples were collected over a 48-h period for pharmacokinetic analyses. Telbivudine was well tolerated by all subjects. The pharmacokinetics of 600 mg of telbivudine were comparable for subjects with mild renal impairment and normal renal function. Likewise, for subjects with moderate to severe impairment, including ESRD, reduced doses from 200 to 400 mg produced plasma exposure similar to that for subjects with normal renal function. These results indicate that the pharmacokinetics of telbivudine were dependent on renal function, especially for subjects with moderate to severe renal impairment or ESRD. Apparent total plasma clearance, renal clearance (CL(R)), and urinary excretion of telbivudine decreased as renal function deteriorated. A linear relationship was established between CL(R) and CL(CR). In ESRD subjects, a routine 3.5- to 4-h hemodialysis session removed telbivudine from plasma at an extraction ratio of approximately 45%, representing a approximately 23% reduction in total exposure. These results suggest that while no adjustment of the telbivudine dose appears necessary for subjects with mild renal impairment, dose adjustment is warranted for those with moderate to severe renal impairment or ESRD in order to achieve optimal plasma exposure.

Nucleoside and nucleotide reverse transcriptase inhibitors in children

By the end of 2006, approximately 2.3 million children worldwide were living with HIV infection, representing about 15% of all HIV-infected individuals but only 5-7% of the total population of treated patients worldwide. Despite a general increase in the use of antiretroviral therapy (ART) in resource-limited settings, appropriate care and ART remain inaccessible for most of the world's HIV-infected children. ART of children is challenging because of a general lack of paediatric formulations (including tablets in paediatric strengths), limited options of drugs available for children (some have been approved only for use in adults), different viral and immunological responses, dependency on caregivers for administration of the therapy, and specific issues of toxicity in long-term therapy related to maturation and development. As in adults, nucleoside reverse transcriptase inhibitors (NRTIs) are a key component of any ART schedule in children, being the recommended 'backbone' treatment in US, European and WHO guidelines, and, indeed, NRTIs have been extensively studied in children. NRTIs are the class of antiretroviral drugs that have more drugs licensed for paediatric use and more paediatric formulations.Generally, the dual NRTI backbone treatment of combination with a non-NRTI (NNRTI) or protease inhibitor (PI) should comprise a cytidine analogue (lamivudine, emtricitabine) and a thymidine analogue (stavudine, zidovudine), guanosine analogue (i.e. abacavir), or nucleotide RTI (NtRTI; i.e. tenofovir). European and US guidelines recommend the use of triple NRTI therapy (abacavir/lamivudine/zidovudine) in children with anticipated poor adherence to other treatment regimens because of tablet burden. In conclusion, while use of ART in children needs to be dramatically increased, selecting and administering the best drug combination for children is still limited by a lack of paediatric formulations and knowledge of drug metabolism, safety and efficacy in children. NRTIs are already a key component of paediatric ART, but fixed-dose combinations and specific research in children are needed to optimise their use. In this article we review the available information to facilitate selection of the best NRTI for backbone treatment in combination ART for HIV-infected children.

Evidence and possible consequences of the phosphorylation of nucleoside reverse transcriptase inhibitors in human red blood cells

The intracellular metabolism of nucleoside reverse transcriptase inhibitors (NRTI) in mononuclear cells has been thoroughly studied, but that in red blood cells (RBC) has been disregarded. However, the phosphorylation of other analogous nucleosides (in particular, ribavirin) has been described previously. In this study, we investigated for the first time the phosphorylation of NRTI in human RBC. The presence of intracellular zidovudine (AZT) monophosphate, AZT triphosphate, lamivudine (3TC) triphosphate, and tenofovir (TFV) diphosphate, as well as endogenous dATP, dGTP, and dTTP, in RBC collected from human immunodeficiency virus-infected patients was examined. We observed evidence of a selective phosphorylation of 3TC, TFV, and endogenous purine deoxynucleosides to generate their triphosphate moieties. Conversely, no trace of AZT phosphate metabolites was found, and only faint dTTP signals were visible. A comparison of intracellular TFV diphosphate and 3TC triphosphate levels in RBC and peripheral blood mononuclear cells (PBMC) further highlighted the specificity of NRTI metabolism in each cell type. These findings raise the issue of RBC involvement in drug-drug interaction, drug pharmacokinetics, and drug-induced toxicity. Moreover, the typical preparation of PBMC samples by gradient density centrifugation does not prevent their contamination with RBC. We demonstrated that the presence of RBC within PBMC hampers an accurate determination of intracellular TFV diphosphate and dATP levels in clinical PBMC samples. Thus, we recommend removing RBC during PBMC preparation by using an ammonium chloride solution to enhance both the accuracy and the precision of intracellular drug monitoring.

Exogenous nucleosides modulate proliferation of rat intestinal epithelial IEC-6 cells

Exogenous nucleotides are considered semiessential nutritional components that play an important role in intestinal development, maintenance, and recovery from tissue damage. Nucleosides (NS) are the best-absorbed chemical form of nucleotides in the intestinal epithelium. The aim of this work was to clarify, at the cellular level, the effects described in vivo. Under conditions of high intracellular availability of NS, we studied the effects of 2 NS mixtures on the NS uptake and intracellular distribution and on the proliferation, morphology, viability, and cell-cycle phase distribution of rat intestinal epithelial cell line 6. Purine and pyrimidine NS showed a similar uptake profile, but the intracellular incorporation of guanosine was greater than that of uridine, without differences in intracellular distribution. Proliferation assays demonstrated that IEC-6 cell proliferation is increased by a mixture containing thymidine but decreased by one containing uridine. In fact, the antiproliferative effect started at 75 micromol/L, which indicated that it may not be correct to consider concentrations of uridine >75 micromol/L as physiological. Interestingly, these effects were not related to increased cell necrosis or apoptosis or to changed cell morphology but rather to a reduced S-phase and increased G0/G1 phase of the cell cycle. In summary, our results suggest that NS molecules are well-absorbed by rat intestinal epithelial cell line 6 cells, whose proliferation can be promoted or inhibited (according to the NS mixtures used) by a mechanism that is not dependent on the toxicity of the mixtures.

[Study on the pharmacokinetic profile of telbivudine]

OBJECTIVE

To evaluate the pharmacokinetic profile of telbivudine in healthy Chinese subjects after oral administration of single and multiple doses.

METHODS

Forty-two healthy adult male and female subjects 18-40 years of age were randomized into four telbivudine dosing groups of 200 mg, 400 mg, 600 mg and 800 mg. Subjects in the 600 mg group received both a single dose and once daily multiple doses for 8 consecutive days. Telbivudine concentrations in plasma and urine samples collected at different time points before and after the drug administration were measured using HPLC-MS/MS. Pharmacokinetic parameters were calculated by using the non-compartmental approach.

RESULTS

After a single dose of 200 mg, 400 mg, 600 mg and 800 mg, tmax (median) were 2.50, 2.00, 2.00 and 2.50 hours respectively; t1/2 were (43.3 +/- 15.2) h, (49.1 +/- 14.4) h, (39.4 +/- 12.1) h and (46.7 +/- 20.8) h respectively; Cmax were (1,753.2 +/- 389.0) ng/ml, (2,586.7 +/- 871.4) ng/ml, (3,703.6 +/- 1,219.0) ng/ml and (3454.6 +/- 953.9) ng/ml respectively; AUC(0-infinity) were (12,843.2 +/- 2,925.6) ng.h(-1).ml(-1), (22,948.9 +/- 5,721.0) ng.h(-1)/ml(-1), (26,440.5 +/- 8,938.1) ng.h(-1).ml(-1) and (28, 820.9 +/- 7 912.9) ng.h(-1).ml(-1) respectively, and CL(R) (600 mg) was (6,545.6 +/- 1 504.4) ml/h. The AUCss from multiple doses was (1,088.5 +/- 299.8) ng/ml; Cmax and AUC accumulation ratio were 1.02 +/- 0.21 and 1.23 +/- 0.26 respectively, which implicated moderated accumulation.

CONCLUSION

Pharmacokinetic parameters of telbivudine in Chinese healthy subjects were determined.

Pharmacokinetics of telbivudine in healthy subjects and absence of drug interaction with lamivudine or adefovir dipivoxil

Two phase I studies were conducted to assess the plasma pharmacokinetics of telbivudine and potential drug-drug interactions between telbivudine (200 or 600 mg/day) and lamivudine (100 mg/day) or adefovir dipivoxil (10 mg/day) in healthy subjects. Study drugs were administered orally. The pharmacokinetics of telbivudine were characterized by rapid absorption with biphasic disposition. The maximum concentrations in plasma (Cmax) were reached at median times ranging from 2.5 to 3.0 h after dosing. Mean single-dose Cmax and area under the plasma concentration-time curve from time zero to infinity (AUC0-infinity) were 1.1 and 2.9 microg/ml and 7.4 and 21.8 microg . h/ml for the 200- and 600-mg telbivudine doses, respectively. Steady state was reached after daily dosing for 5 to 7 days. The mean steady-state Cmax and area under the plasma concentration-time curve over the dosing interval (AUCtau) were 1.2 and 3.4 microg/ml and 8.9 and 27.5 microg . h/ml for the 200- and 600-mg telbivudine repeat doses, respectively. The steady-state AUCtau of telbivudine was 23 to 57% higher than the single-dose values. Concomitant lamivudine or adefovir dipivoxil did not appear to significantly alter the steady-state plasma pharmacokinetics of telbivudine; the geometric mean ratios and associated 90% confidence interval (CI) for the AUCtau of telbivudine alone versus in combination were 106.3% (92.0 to 122.8%) and 98.6% (86.4 to 112.5%) when coadministered with lamivudine and adefovir dipivoxil, respectively. Similarly, the steady-state plasma pharmacokinetics of lamivudine or adefovir were not markedly affected by the coadministration of telbivudine; the geometric mean ratios and associated 90% CI, alone versus in combination with telbivudine, were 99.0% (87.1 to 112.4%) and 92.2% (84.0 to 101.1%), respectively, for the lamivudine and adefovir AUCtau values. Moreover, the combination regimens studied were well tolerated in all subjects. The results from these studies provide pharmacologic support for combination therapy or therapy switching involving telbivudine, lamivudine, and adefovir dipivoxil for the treatment of chronic hepatitis B virus infection.

Expression of human equilibrative nucleoside transporter 1 (hENT1) and its correlation with gemcitabine uptake and cytotoxicity in mantle cell lymphoma

BACKGROUND AND OBJECTIVES

Nucleoside transporters might play a relevant role in the intracellular targeting of many nucleoside analogs used in anticancer therapy. Two gene families (SLC28 and SLC29) encode the two types of human nucleoside transporters, concentrative nucleoside transporter (CNT) and equilibrative nucleoside transporter (ENT) proteins. Chronic lymphocytic leukemia (CLL) cells express both SLC28- and SLC29-related mRNA, although transport function seems to be mostly related to ENT-type transporters. Here we have analyzed the role of nucleoside transporters in nucleoside-derived drug bioavailability and action in mantle cell lymphoma (MCL) cells.

DESIGN AND METHODS

The relative amounts of hENT1 and hENT2-related mRNA and protein were analyzed in five MCL cell lines and 20 primary MCL tumors by real-time quantitative reverse transcriptase polymerase chain reaction and western blots. Cell viability, measured by annexin V-FITC staining, and nucleoside-derived drug transport were also studied.

RESULTS

MCL cells express higher levels of hENT1 protein than do CLL cells, and a good correlation was found between protein and mRNA levels of hENT1, thus indirectly suggesting that hENT1 might be transcriptionally regulated in MCL cells. More importantly, a significant correlation between these two parameters, drug uptake and sensitivity to gemcitabine, was also observed.

INTERPRETATION AND CONCLUSIONS

These results further support the concept that nucleoside transporters are implicated in the therapeutic response to nucleoside analogs, and suggest a particular and novel role for hENT1 in the genotoxic response to selected nucleoside analogs, such as gemcitabine, in MCL cells.

[Clinical pharmacology of nucleoside and nucleotide reverse transcriptase inhibitors]

Options for antiretroviral therapy in patients infected with HIV continue to expand as new drugs are integrated into treatment regimens. Nucleoside/nucleotide reverse transcriptase inhibitors (Nt/NRTIs) remain the backbone of highly active antiretroviral therapy (HAART). Although this is the oldest class of antiretrovirals, pharmacokinetic and pharmacodynamic properties have been less studied as compare to protease inhibotors (PIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs). The aim of this article is to review the current status of clinical pharmacology onf Nt/NRTIs, highlighting the issues with clinical interest. Therefore, implications of intracellular pharmacokinetics on dosing schedule, potential for drug-drug interaction and pharmacodynamics is discussed.

Pharmacokinetics of telbivudine in subjects with various degrees of hepatic impairment

This study evaluated the effect of hepatic impairment on the pharmacokinetics of telbivudine, an investigational nucleoside antiviral for the treatment of chronic hepatitis B virus infection. Twenty-four subjects were assigned to four hepatic function groups (normal function and mild, moderate, and severe impairment, with six subjects in each group) on the basis of Child-Pugh scores. The subjects were administered a single oral dose of 600 mg telbivudine, and blood samples were collected over a 48-h interval for pharmacokinetic analyses. Telbivudine was well tolerated by all subjects. Telbivudine plasma concentration-time profiles were similar across the four hepatic function groups. The principal pharmacokinetic parameters of drug exposure, i.e., the maximum plasma concentration and area under the drug concentration-time curve, were comparable between subjects with various degrees of hepatic impairment and those with normal hepatic function. Results from this single-dose pharmacokinetic assessment therefore provide a pharmacologic rationale for further evaluation of the safety and efficacy of telbivudine in hepatitis B virus-infected patients with decompensated liver diseases.

Pharmacokinetics of telbivudine following oral administration of escalating single and multiple doses in patients with chronic hepatitis B virus infection: pharmacodynamic implications

The pharmacokinetics of telbivudine were evaluated in adult patients with chronic hepatitis B virus (HBV) infection following once-daily oral administration at escalating doses of 25, 50, 100, 200, 400, and 800 mg/day for 4 weeks. Telbivudine was rapidly absorbed after oral administration, with the median times T(max) to the maximum plasma concentration (C(max)) ranging from 0.8 to 3.0 h postdosing across cohorts. Single-dose and steady-state maximum C(max)s and the areas under the plasma concentration-time curve from time zero to time t (AUC(0-t)s) increased proportionally with dose. At steady-state, the values of C(max) and AUC(0-t) were higher than those obtained after the administration of a single dose, indicative of a slight accumulation, with the ratios of the steady-state value to the value after the administration of a single dose ranging from 1.14 to 1.49 for C(max) and from 1.40 to 1.70 for AUC(0-t). While the elimination of telbivudine from plasma was apparently monophasic over the 8-h sampling period, the substantial steady-state trough plasma levels observed in the groups receiving doses of 100 to 800 mg were clearly indicative of the presence of a second slower elimination phase, with the mean estimated half-lives ranging from 29.5 to 41.3 h by compartmental modeling analysis. Pharmacokinetic and pharmacodynamic analyses by using maximum-effect modeling established a quantitative relationship between a reduction in serum HBV DNA levels and parameters of drug exposure, in particular, the steady-state C(max) and AUC(0-t). In summary, this study showed that telbivudine exhibits dose-proportional plasma pharmacokinetics with sustained steady-state drug exposure and exposure-related antiviral activity, supporting the need for further clinical studies by use of a once-daily regimen in patients with chronic HBV infection.

Telbivudine: a novel nucleoside analog for chronic hepatitis B

OBJECTIVE

To review available literature on the pharmacology, pharmacokinetics, dosing and administration, efficacy, and safety of the antiviral nucleoside analog telbivudine.

DATA SOURCES

Information was obtained from searching MEDLINE (1966-December 2005), International Pharmaceutical Abstracts (1970-December 2005), and the Cochrane Database of Systematic Reviews (4th quarter 2005) using the search words telbivudine, L-dT, L-deoxythymidine, L-nucleosides, and nucleosides. Abstracts from the Annual Meeting of the American Association for the Study of Liver Diseases and European Association for the Study of the Liver were also searched, including bibliographies from the identified articles.

STUDY SELECTION AND DATA EXTRACTION

Data from double-blind, placebo-controlled clinical trials and unpublished information were extracted.

DATA SYNTHESIS

Telbivudine is a novel, orally administered nucleoside analog under development for use in the treatment of chronic hepatitis B. In contrast to other nucleoside analogs, telbivudine has not been associated with inhibition of mammalian DNA polymerase with mitochondrial toxicity. Telbivudine demonstrated potent activity against hepatitis B with significantly higher rate of response and superior viral suppression compared with lamivudine, the standard treatment. Telbivudine has been generally well tolerated, with low adverse effect profile, and at its effective dose, no dose-limiting toxicity has been observed.

CONCLUSIONS

Telbivudine is a novel oral nucleoside analog effective in the treatment of chronic hepatitis B infection.

Functional and pharmacological mechanisms of nucleoside transport across the basolateral membrane of rabbit tracheal epithelial cells

The role of basolateral membrane nucleoside transport in primary cultured rabbit tracheal epithelial cells (RTEC) was studied. Primary cultured RTEC were grown on permeable support at an air-interface. Transport studies were conducted in the uptake, efflux, and transepithelial transport configurations using (3)H-uridine as a model substrate. Time, temperature and concentration dependency of (3)H-uridine transport were evaluated in parallel to the metabolism of this substrate using scintillation counting and thin layer chromatography. Inhibition of (3)H-uridine uptake from basolateral fluid was estimated in presence of all unlabeled natural nucleosides as well as analogs and nucleobases. Functional modulation pathways of (3)H-uridine uptake were studied after treatment of RTEC with pharmacological levels of A23187, forskolin, tamoxifen, H89 and colchicine. The basolateral aspect has a low-affinity and high-capacity transport system that exhibits characteristics of bi-directionality, temperature/concentration dependency, and broad specificity towards purines and pyrimidines without requiring Na(+). Basolateral equilibrative-sensitive/insensitive (es/ei) type transport machinery manifested as a biphasic dose response to nitro-benzyl-mercapto-purine-ribose (NBMPR) inhibition. In addition, a number of therapeutically relevant nucleoside analogs appeared to compete with the uptake of uridine from basolateral fluid. Short-term pre-incubation of primary cultured RTEC with the calcium ionophore A23187 inhibited basolateral uridine uptake without affecting the J(max) and K(m). The inhibitory effect was not reversible with a protein kinase C (PKC) antagonist, tamoxifen. In contrast, basolateral uridine uptake was increased by adenylyl cyclase activator forskolin (reversible with protein kinase A (PKA) inhibitor H89), resulting in a decreased K(m), but a lower J(max). Uridine exit across the basolateral membrane of primary cultured RTEC occurs via a facilitative diffusion carrier, which can be modulated by intracellular Ca(2+) levels and PKA. Information about these carriers will help improve the transportability of antitumor and antiviral nucleoside analogs in the pulmonary setting.

The ideal nucleoside/nucleotide backbone

The selection of the nucleoside/nucleotide reverse transcriptase inhibitor (NRTI/NtRTI) backbone is an important step in constructing a regimen for the treatment of HIV infection. No single NRTI/NtRTI backbone has attributes that make it ideal for every patient, but each has its advantages. Data from clinical trials provide needed guidance on backbone selection and the choice of a 3rd agent to complete a highly active antiretroviral therapy (HAART) regimen. This article reviews available data published since 2000 that address NRTI/NtRTI backbone performance and safety when included in HAART regimens. Based on these trials, the characteristics of different options are compared with an ideal backbone and put into context with other considerations for successful regimen selection and improved outcomes in treatment-naive patients. Practical strategies are offered for implementing contemporary regimens in settings outside clinical trials.

[High intestinal transport activity for nucleosides in cattle: a synopsis]

In ruminants large amounts of nucleic acids associated with the microbial cell mass leaving the fore-stomach system via the abomasum enter the small intestine. In dairy cows this amounts to 100-200 g microbial nucleic acids per day. These nucleic acids are very efficiently digested in the small intestine whereby nucleosides were found to be the main degradation products. Therefore, this review centers mainly on the mechanisms of intestinal absorption of nucleosides and their role in nucleic acid digestion. Furthermore, metabolism of nucleosides in intestinal epithelial cells and its bearing for nucleoside absorption and salvage of nucleosides for nucleic acid synthesis in various tissues is considered. According to own studies using isolated intestinal brush-border membrane (BBM) vesicles (BBMV) from dairy cows purine and pyimidine nucleosides are transported actively by two separate Na+ co-transport systems (N1 and N2) across the bovine BBM, whereby transport activity in the small intestine decreases from proximal to distal. Guanosine and inosine appeared to be transported exclusively by N1 while thymidine and cytidine appeared to be transported exclusively by N2. Uridine and adenosine had an affinity to both transporters. In comparison to findings in man and rabbit, transport capacity (Vmax) of N1 and N2 in the BBM of cows was more than 10-fold higher. Similar findings were obtained in BBMV isolated from the small intestine of veal calves with rudimentary forestomach development in regard to nucleoside transport. Therefore, the high intestinal transport activity for nucleosides seems to be a genetically fixed property in the bovine, which is not related to a functional fore-stomach system.

Metabolism and disposition of the antiviral nucleoside analogue AM365 in the isolated perfused rat liver

The present study was designed to investigate the hepatic disposition of the prodrug AM365 and the generated antiviral guanosine analogue, AM188 in the isolated perfused rat liver (IPL). The livers of rats (n=12) were isolated and perfused with Krebs-Henseleit pH 7.4 buffer to which AM365 was added as a bolus to achieve an initial perfusate concentration of 22.4 micromol/L. During the 120-min period after administration of AM365, bile was collected in 10-min intervals and perfusate was collected at the mid-point of these intervals. Concentrations of AM365 and AM188 in perfusate and bile were quantified by HPLC. Following administration of AM365, its concentration in perfusate declined and the concentration of AM188 increased; the sum of the molar concentrations remained constant. The clearance and hepatic extraction ratio of AM365 were 3.3+/-2.4 mL/min and 0.110+/-0.079, respectively. The cumulative amount of AM365 excreted in bile during the 120-min perfusion period was approximately 0.21% of the bolus dose, and 0.36% of the total amount of AM365 cleared by the liver during the period. The cumulative amount of AM188 excreted in bile was about 0.48% of the total amount of AM188 formed during the perfusion period. In conclusion, AM365 was metabolised to AM188, which appeared to be the only metabolite and was not further biotransformed. The biliary excretion of AM365 and AM188 was negligible.

[Ester derivatives of nucleoside inhibitors of reverse transcriptase: I. Molecular transport systems for 3'-azido-3'-deoxythymidine and 2',3'-didehydro-3'-deoxythymidine]

The methods of synthesis of the derivatives of nucleoside analogues esterified with various aliphatic, aromatic, and heteroaromatic acids and the construction from them of molecular transport systems that involve lipids, carbohydrates, peptides, and amino acids are discussed. The characteristics of the biological activity of a number of such systems are described.

In vitro delivery of novel, highly potent anti-varicella zoster virus nucleoside analogues to their target site in the skin

PURPOSE

To determine the in-vitro dermal delivery of a new class of lipophilic, highly potent and uniquely selective anti-VZV nucleoside analogues in comparison with aciclovir.

METHODS

Three test compounds (Cf1698, Cf1743, Cf1712) and aciclovir were formulated into propylene glycol/aqueous cream BP formulations and finite doses applied to full-thickness pig ear skin for 48 hours in vertical Franz-type diffusion cells. Receptor phase samples were taken at specific intervals to determine permeation, and depth profiles were constructed following tape stripping and membrane separation.

RESULTS

All three test compounds reached the target basal epidermis in concentrations suggesting they would be highly efficacious in reducing viral load. Furthermore, the data showed that each of the test compounds would perform in a far superior manner to aciclovir, the current treatment of choice.

CONCLUSIONS

The dermatomal site of viral replication during secondary infection--the basal epidermis--was successfully targeted. Topical delivery of these compounds is highly promising as a new first line treatment of VZV infections. By attacking the virus at the first sign of reactivation, it is proposed that the extent of damage caused by the virus would be significantly lowered, thereby limiting the extent and severity of post-herpetic neuralgia.

Long term endocrine regulation of nucleoside transporters in rat intestinal epithelial cells

We studied the regulation of nucleoside transporters in intestinal epithelial cells upon exposure to either differentiating or proliferative agents. Rat intestinal epithelial cells (line IEC-6) were incubated in the presence of differentiating (glucocorticoids) or proliferative (EGF and TGF-α) agents. Nucleoside uptake rates and nucleoside transporter protein and mRNA levels were assessed. The signal transduction pathways used by the proliferative stimuli were analyzed. We found that glucocorticoids induce an increase in sodium-dependent, concentrative nucleoside transport rates and in protein and mRNA levels of both rCNT2 and rCNT1, with negligible effects on the equilibrative transporters. EGF and TGF-α induce an increase in the equilibrative transport rate, mostly accounted for by an increase in rENT1 activity and mRNA levels, rENT2 mRNA levels remaining unaltered. This effect is mimicked by another proliferative stimulus that functions as an in vitro model of epithelial wounding. Here, rENT1 activity and mRNA levels are also increased, although the signal transduction pathways used by the two stimuli are different. We concluded that differentiation of rat intestinal epithelial cells is accompanied by increased mature enterocyte features, such as concentrative nucleoside transport (located at the brush border membrane of the enterocyte), thus preparing the cell for its ultimate absorptive function. A proliferative stimulus induces the equilibrative nucleoside activities (mostly through ENT1) known to be located at the basolateral membrane, allowing the uptake of nucleosides from the bloodstream for the increased demands of the proliferating cell.

Regulation of equilibrative nucleoside uptake by protein kinase inhibitors

The uptake of nucleosides and nucleoside analogs into human leukemia K562 cells is facilitated by the equilibrative transporters ENT1 and ENT2. Incubation of K562 cells with a variety of protein kinase inhibitors inhibited the transport of both uridine (ARA-C) and cytidine (CPEC) analogs. These inhibitory effects were observed for a large number of kinase inhibitors including those against p38 MAPK, the EGF receptor kinase, protein kinase C, TOR and others. Thus these results suggest that the nucleoside transporters are unexpected targets for kinase inhibitors and may influence the design and application of combinatorial approaches of nucleoside analogs and kinase inhibitors in clinical applications.

The nucleoside transport proteins, NupC and NupG, from Escherichia coli: specific structural motifs necessary for the binding of ligands

A series of 46 natural nucleosides and analogues (mainly adenosine-based) were tested as inhibitors of [U-(14)C]uridine uptake by the concentrative, H(+)-linked nucleoside transport proteins NupC and NupG from Escherichia coli. The two evolutionarily unrelated transporters showed similar but distinct patterns of inhibition, revealing differing selectivities for the different nucleosides and their analogues. Binding of nucleosides to NupG required the presence of hydroxyl groups at each of the C-3' and C-5' positions of ribose, while binding to NupC required only the C-3' hydroxyl substituent. The greater importance of the ribose moiety for binding to NupG is consistent with the evolutionary relationship between this protein and the oligosaccharide: H(+) symporter (OHS) subfamily of the major facilitator superfamily (MFS) of transporters. For both proteins the natural alpha-configuration at C-3' and the natural beta-configuration at C-1' was mandatory for ligand binding. N-7 in the imidazole ring of adenosine and the amino group at C-6 were found not to be important for binding and both transporters showed flexibility for substitution at C-6/N(6); one or both of N-1 and N-3 were important for adenosine analogue binding to NupC but significantly less so for binding to NupG. From the different effects of 8-bromoadenosine on the two transporters it appears that adenosine selectively binds to NupC in an anti- rather than a syn-conformation, whereas NupG is less prescriptive. The pattern of inhibition of NupC by differing nucleoside analogues confirmed the functional relationship of the bacterial transporter to members of the human concentrative nucleoside transporter (CNT) family and reaffirmed the use of the bacterial protein as an experimental model for these physiologically and clinically important mammalian proteins. The specificity data for NupG have been used to develop a homology model of the protein's binding site, based on the X-ray crystallographic structure of the disaccharide transporter LacY from E. coli. We have also developed an efficient general protocol for the synthesis of adenosine and three of its analogues, which is illustrated by the synthesis of [1'-(13)C]adenosine.

Interaction of nucleoside analogues with nucleoside transporters in rat brain endothelial cells

A number of nucleoside analogues, consisting of antiviral compounds and agents designed as adenosine A1 receptor agonists, were examined for nucleoside transporter affinity using an in vitro model of the blood-brain barrier (BBB), the rat brain endothelial cell line, RBE4. Structure-activity relationships (SAR) were also performed to identify the key structural requirements for transporter recognition and the suitability of these systems for carrier-mediated strategies to deliver therapeutics across the BBB. Adenosine receptor agonists did not show transport affinity for concentrative nucleoside carriers, but exhibited affinity for equilibrative systems (Ki=10.8-97.9 microM) within the range of Kms for natural substrates. However, none of the antiviral compounds tested in this study showed affinity for either class of nucleoside transporter. SAR studies suggest that the hydroxyl group located at the 3'-position of the ribose moiety is an essential requirement for transporter recognition. This may explain the inability of nucleoside derived anti-viral compounds to use these systems despite the significant structural homology with naturally occurring nucleosides. Sites have also been identified which accommodate structural additions with retention of carrier affinity, suggesting that compounds which fail to penetrate the BBB could be attached to these sites for carrier-mediated delivery using a prodrug strategy.

Nucleoside/nucleotide reverse transcriptase inhibitor drug interactions

Since their advent the nucleoside and nucleotide reverse transcriptase inhibitors have consolidated their position as the 'backbone' of many antiretroviral therapy regimens. The ability of this class of drugs to combine successfully with members of their own as well as other antiretroviral classes has enabled the effective suppression of HIV replication to occur. Many of these therapeutic combinations rely on synergistic interactions to achieve this. There are, however, also many unfavourable pharmacokinetic and pharmacodynamic interactions between the members of nucleoside/nucleotide reverse transcriptase inhibitors, as well as with other antiretroviral classes and non-HIV drugs. This article aims to identify clinically relevant, beneficial and detrimental interactions of this class of antiretroviral agent.

Molecular imaging and treatment of malignant gliomas following adenoviral transfer of the herpes simplex virus-thymidine kinase gene and the somatostatin receptor subtype 2 gene

Patients suffering from malignant glioma have a very poor prognosis. New therapy approaches for gliomas are necessary; these tumors are attractive targets for gene therapy. Our research concentrated on evaluation of the use of the Herpes Simplex Virus-thymidine kinase (tk) enzyme and the somatostatin receptor subtype 2 (sst2). DOTA-Tyr3-octreotate is an analog of somatostatin with high affinity for sst2. It shows rapid internalization in sst2-positive tumor cells in vitro and in vivo. For gene therapy, we used the adenoviral vector Ad5.tk.sstr, which carries the tk gene and the sst2 gene. The aim of our study was to compare uptake of the tk substrate 1-(2-fluoro-2-deoxy-beta-D-ribofuranosyl)-5-[*I]iodouracil (FIRU) labeled with 125I and the somatostatin analog 111In-DOTA-Tyr3-octreotate in several glioma cell lines after infection with Ad5.tk.sstr. Uptake of 125I-FIRU was measured in rat 9L-tk glioma cells without infection with Ad5.tk.sstr. Results showed that the uptake of 125I-FIRU was concentration and time dependent. We also used several rat and human glioma cell lines for infection with Ad5.tk.sstr. Forty-eight hours after infection, uptake studies were performed using 125I-FIRU and 111In-DOTA-Tyr3-octreotate. In all cell lines, the uptake of 125I-FIRU and 111In-DOTA-Tyr3-octreotate increased with increasing multiplicity of infection of virus and showed that the uptake of 111In-DOTA-Tyr3-octreotate was higher than that of 125I-FIRU in all cell lines. We conclude that the sst2 imaging and therapy modality is most promising for glioma gene therapy, either alone or in combination with HSV-tk suicide gene therapy. Therapy can be performed using combinations of DOTA-Tyr3-octreotate radiolabeled with 177Lu or 90Y, 131I-FIRU and/or the prodrug ganciclovir.

Role of human nucleoside transporters in the cellular uptake of two inhibitors of IMP dehydrogenase, tiazofurin and benzamide riboside

Benzamide riboside (BR) and tiazofurin (TR) are converted to analogs of NAD that inhibit IMP dehydrogenase (IMPDH), resulting in cellular depletion of GTP and dGTP and inhibition of proliferation. The current work was undertaken to identify the human nucleoside transporters involved in cellular uptake of BR and TR and to evaluate their role in cytotoxicity. Transportability was examined in Xenopus laevis oocytes and Saccharomyces cerevisiae that produced individual recombinant human concentrative nucleoside transporter (CNT) and equilibrative nucleoside transporter (ENT) types (hENT1, hENT2, hCNT1, hCNT2, or hCNT3). TR was a better permeant than BR with a rank order of transportability in oocytes of hCNT3 >> hENT1 > hENT2 > hCNT2 >> hCNT1. The concentration dependence of inhibition of [(3)H]uridine transport in S. cerevisiae by TR exhibited lower K(i) values than BR: hCNT3 (5.4 versus 226 microM), hENT2 (16 versus 271 microM), hENT1 (57 versus 168 microM), and hCNT1 (221 versus 220 microM). In cytotoxicity experiments, BR was more cytotoxic than TR to cells that were either nucleoside transport-defective or -competent, and transport-competent cells were more sensitive to both drugs. Exposure to nitrobenzylmercaptopurine ribonucleoside conferred resistance to BR and TR cytotoxicity to hENT1-containing CEM cells, thereby demonstrating the importance of transport capacity for manifestation of cytoxicity. A breast cancer cell line with mutant p53 exhibited 9-fold higher sensitivity to BR than the otherwise similar cell line with wild-type p53, suggesting that cells with mutant p53 may be potential targets for IMPDH inhibitors. Further studies are warranted to determine whether this finding can be generalized to other cell types.

A new era of antifungal therapy

Invasive fungal infections pose major management problems for clinicians caring for hematopoietic cell transplant patients. Two major fungal genera, Candida and Aspergillus, account for most fungal infections. Rates of systemic Candida infection range from 15% to 25%, mostly in the pre-engraftment period. Prophylaxis by fluconazole has dramatically reduced the frequency of early Candida infections. Caspofungin has recently been shown to offer an excellent alternative to amphotericin B (with less toxicity) or fluconazole (with a broader spectrum) for therapy of systemic Candida infections. Aspergillus infections occur in 15% to 20% of allogeneic hematopoietic cell transplant patients, most frequently in the post-engraftment period; they are associated with a severe diminution of cell-mediated immune responses by graft-versus-host disease and prolonged corticosteroid use. Voriconazole, a recently introduced broad-spectrum azole, has excellent activity against Aspergillus and is generally well tolerated. Voriconazole currently offers the best prospect for success and tolerance as a first-line treatment for aspergillosis. Second-line therapies include lipid formulations of amphotericin B, caspofungin, or intravenous itraconazole. Unfortunately, early initiation of therapy for aspergillosis is frequently not possible because of inaccurate diagnostics. One new diagnostic, the galactomannan assay, has recently been approved, and others are in development; these offer promise for earlier diagnosis without the need for invasive procedures. It is hoped that these new therapies and new diagnostics will usher in a new era of antifungal therapy.

A dose-finding study of once-daily oral telbivudine in HBeAg-positive patients with chronic hepatitis B virus infection

Current therapy for chronic hepatitis B is suboptimal as a result of limited durable response rates, cumulative viral resistance, and/or poor tolerability. Telbivudine has potent antiviral activity against hepatitis B virus (HBV) in vitro and in the woodchuck model and has a promising preclinical safety profile. In this first clinical study of telbivudine, safety, antiviral activity, and pharmacokinetics were assessed in 43 adults with hepatitis B e antigen-positive chronic hepatitis B. This placebo-controlled dose-escalation trial investigated 6 telbivudine daily dosing levels (25, 50, 100, 200, 400, and 800 mg/d); treatment was given for 4 weeks, with 12 weeks' follow-up. Serum HBV DNA levels were monitored via quantitative polymerase chain reaction. The results indicate that telbivudine was well tolerated at all dosing levels, with no dose-related or treatment-related clinical or laboratory adverse events. telbivudine plasma pharmacokinetics were dose-proportional within the studied dose range. Marked dose-related antiviral activity was evident, with a maximum at telbivudine doses of 400 mg/d or more. In the 800 mg/d cohort, the mean HBV DNA reduction was 3.75 log10 copies/mL at week 4, comprising a 99.98% reduction in serum viral load. Correspondingly, posttreatment return of viral load was slowest in the high-dose groups. Viral dynamic analyses suggested a high degree of efficiency of inhibition of HBV replication by telbivudine and helped refine selection of the optimal dose. In conclusion, these results support expanded clinical studies of this new agent for the treatment of hepatitis B.

Transport of physiological nucleosides and anti-viral and anti-neoplastic nucleoside drugs by recombinant Escherichia coli nucleoside-H(+) cotransporter (NupC) produced in Xenopus laevis oocytes

The recently identified human and rodent plasma membrane proteins CNT1, CNT2 and CNT3 belong to a gene family (CNT) that also includes the bacterial nucleoside transport protein NupC. Heterologous expression in Xenopus oocytes has established that CNT1-3 correspond functionally to the three major concentrative nucleoside transport processes found in human and other mammalian cells (systems cit, cif and cib, respectively) and mediate Na(+) - linked uptake of both physiological nucleosides and anti-viral and anti-neoplastic nucleoside drugs. Here, one describes a complementary Xenopus oocyte transport study of Escherichia coli NupC using the plasmid vector pGEM-HE in which the coding region of NupC was flanked by 5'- and 3'-untranslated sequences from a Xenopus beta-globin gene. Recombinant NupC resembled human (h) and rat (r) CNT1 in nucleoside selectivity, including an ability to transport adenosine and the chemotherapeutic drugs 3'-azido-3'-deoxythymidine (AZT), 2',3'- dideoxycytidine (ddC) and 2'-deoxy-2',2'-difluorocytidine (gemcitabine), but also interacted with inosine and 2',3'- dideoxyinosine (ddl). Apparent affinities were higher than for hCNT1, with apparent K(m) values of 1.5-6.3 microM for adenosine, uridine and gemcitabine, and 112 and 130 microM, respectively, for AZT and ddC. Unlike the relatively low translocation capacity of hCNT1 and rCNT1 for adenosine, NupC exhibited broadly similar apparent V(max) values for adenosine, uridine and nucleoside drugs. NupC did not require Na(+) for activity and was H(+) - dependent. The kinetics of uridine transport measured as a function of external pH were consistent with an ordered transport model in which H(+) binds to the transporter first followed by the nucleoside. These experiments establish the NupC-pGEM-HE/oocyte system as a useful tool for characterization of NupC-mediated transport of physiological nucleosides and clinically relevant nucleoside therapeutic drugs.

Evaluation of microdosing strategies for studies in preclinical drug development: demonstration of linear pharmacokinetics in dogs of a nucleoside analog over a 50-fold dose range

The technique of accelerator mass spectrometry (AMS) was validated successfully and used to study the pharmacokinetics and disposition in dogs of a preclinical drug candidate (7-deaza-2'-C-methyl-adenosine; Compound A), after oral and intravenous administration. The primary objective of this study was to examine whether Compound A displayed linear kinetics across subpharmacological (microdose) and pharmacological dose ranges in an animal model, before initiation of a human microdose study. The AMS-derived disposition properties of Compound A were comparable to data obtained via conventional techniques such as liquid chromatography-tandem mass spectrometry and liquid scintillation counting analyses. Compound A displayed multiphasic kinetics and exhibited low plasma clearance (5.8 ml/min/kg), a long terminal elimination half-life (17.5 h), and high oral bioavailability (103%). Currently, there are no published comparisons of the kinetics of a pharmaceutical compound at pharmacological versus subpharmacological doses using microdosing strategies. The present study thus provides the first description of the full pharmacokinetic profile of a drug candidate assessed under these two dosing regimens. The data demonstrated that the pharmacokinetic properties of Compound A following dosing at 0.02 mg/kg were similar to those at 1 mg/kg, indicating that in the case of Compound A, the pharmacokinetics in the dog appear to be linear across this 50-fold dose range. Moreover, the exceptional sensitivity of AMS provided a pharmacokinetic profile of Compound A, even after a microdose, which revealed aspects of the disposition of this agent that were inaccessible by conventional techniques.

The rise and fall of triple nucleoside reverse transcriptase inhibitor (NRTI) regimens

Triple nucleoside reverse transcriptase inhibitor (NRTI) regimens have attracted much interest due to their potential to (1) simplify dosing, with potential gains in adherence to treatment, and (2) reduce or even reverse dyslipidaemia associated with protease inhibitor (PI) therapy. A variety of triple NRTI combinations have been investigated, in both antiretroviral-naive and antiretroviral-experienced HIV-infected patients. Many of these trials have generated disappointing results, and some have been prematurely discontinued due to poor efficacy. This article reviews the background to the development of triple NRTI regimens, and the mounting evidence that this approach is suboptimal for antiretroviral-naive patients. Indeed, some triple NRTI regimens should never be used in this population. A role for triple NRTI combinations as a simplification strategy in treatment-experienced patients whose HIV is well controlled has been suggested, but emerging evidence indicates that such an approach can, under adequate selection pressure, lead to the emergence of mutations and viral load rebound. This commentary discusses the factors that appear to influence patients' responses to triple NRTI therapy, and their implications for patient selection.

Nucleoside inhibitors of human immunodeficiency virus type 1 reverse transcriptase

The development of novel compounds that can effectively inhibit both wild type and the most consensus resistant strains of human immunodeficiency virus type 1 (HIV-1) is the primary focus in HIV disease management. Combination therapy, comprising at least three classes of drugs, has become the standard of care for acquired immunodeficiency syndrome (AIDS) or HIV-infected individuals. The drug cocktail can comprise all three classes of HIV inhibitors, including nucleoside reverse transcriptase inhibitors (NRTI), non-nucleoside reverse transcriptase inhibitors (NNRTI) and protease inhibitors (PI). Due to their competitive mode of inhibition and requirement for metabolic activation, almost all NRTI drugs lack the virological potency of NNRTI or PI drugs. However, data from clinical trials indicate that sustained viral suppression could not be achieved with NRTI, NNRTI or PIs alone. Therefore, the NRTIs will remain essential components of highly active antiretroviral therapy (HAART) for the foreseeable future, because they enhance the virological potency of the regimen, they do not bind excessively to protein and most regimens are small pills/tablets given once a day. It has become apparent in recent years that the prolonged use of certain NRTIs exhibits adverse events as a class, limiting the length of time for which they can be safely used. Of major clinical concern is their association with the potentially fatal lactic acidaemia and hepatic steatosis. These class events, as well as individual drug effects, such as peripheral neuropathy, are linked to delayed mitochondrial destruction. In addition to toxicity, the development of resistance-conferring mutations against exposure to nucleoside analogs currently in use influences long-term therapeutic benefits. Of critical importance for the evaluation of new NRTIs are recent studies showing that the efficiency of discrimination or excision by pyrophosphorolysis in the presence of nucleotides of a given NRTI is a key determinant in the emergence of one or the other resistance pathway.

Exogenous nucleosides stimulate proliferation of fetal rat hepatocytes

Exogenous nucleotides (NT) have been reported to exert a reparative role in animal models of intestinal and hepatic damage. Thus, the administration of NT in the diet of rats with thioacetamide-induced liver cirrhosis normalized many of the histological and biochemical alterations produced by this hepatotoxin. We are currently studying the mechanism by which NT exert this effect using cell culture models. The aim of this work was to investigate whether exogenous nucleosides (NS) modulate the proliferation of hepatocytes. We used fetal rat hepatocytes, which, unlike adult hepatocytes, are proliferative cells. Fetal rat primary hepatocytes were incubated with mixtures of NS, and cell proliferation was studied. NS added to the medium of fetal hepatocytes were taken up in a selective fashion by the cells. Cell proliferation was enhanced, as demonstrated by the induction of c-myc and h-ras gene expression as well as by the higher percentage of cells in S phase, and exogenous NS increased the expression of alpha-fetoprotein. These results suggest that exogenous NS may in fact stimulate proliferation of hepatic cells and help preserve the undifferentiated state of fetal rat hepatocytes.

Development of radioiodinated nucleoside analogs for imaging tissue proliferation: comparisons of six 5-iodonucleosides

The aim of this study was to determine the most suitable iodonucleoside analogs for use in tissue proliferation imaging by means of single photon emission tomography (SPECT). In this study, 5-[(125)I]iodo-(2-deoxy-2-fluoro-4-thio-beta-D-arabinofuranosyl)uracil ([(125)I]FITAU, 1E) and 5-[(125)I]iodo-1-methyl-(2-deoxy-2-bromo-beta-D-arabinofuranosyl)uracil ([(125)I]IMBAU, 1F) were synthesized and their biological data were compared with previously published results regarding 4'-thio nucleoside analogs and the reference compound 5-[(125)I]iodo-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)uracil ([(125)I]FIAU, 1D). 5-Iodo-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)uracil (FIAU, 2D), 5-iodo-(2-deoxy-2-fluoro-4-thio-beta-D-arabinofuranosyl)uracil (FITAU, 2E), and 5-iodo-1-methyl-(2-deoxy-2-bromo-beta-D-arabinofuranosyl)uracil (IMBAU, 2F) were successfully labeled with (125)I and their in vitro cytosolic thymidine kinase (TK(1)) phosphorylation, recombinant thymidine phosphorylase enzymatic catabolism, TK(1)-dependent cell uptake, and in vivo biodistribution in normal mice were evaluated. Five compounds (1B, 1C, 1D, 1E, and 1F) were stable against C-N glycoside degradation induced by recombinant thymidine phosphorylase. However, 5-[(125)I]iodo-2'-deoxyuridine ([(125)I]IUdR, 1A) was not shown to be stable against such degradation. The TK(1) assay showed that [(125)I]FIAU (1D) expressed 16% of the phosphorylation potential of [(125)I]IUdR (1A). Furthermore, [(125)I]FITAU (1E) was shown to have reduced phosphorylation potential, in comparison with that of [(125)I]IUdR (1A) (<0.01). [(125)I]IMBAU (1F) did not show any phosphorylation. In vitro cell uptake and in vivo proliferation-selective uptake of each nucleoside was largely dependent on its potential as a TK(1) substrate. Neither [(125)I]FITAU (1E) nor [(125)I]IMBAU (1F) were shown to have distinct TK(1)-dependent cell uptake and retention in the proliferating tissues. From these results, we concluded that [(125)I]FITAU (1E) and [(125)I]IMBAU (1F) are not effective as imaging agents of cell proliferation. The biological data obtained with these nucleosides were compared, and requirements for the design of pharmaceutically useful radioiodinated nucleoside analogs were also considered.

Antiretroviral pharmacokinetics in the paediatric population: a review

Characteristics unique to paediatric pharmacotherapy should be considered when treating children infected with human immunodeficiency virus (HIV). Processes of growth and development in the paediatric patient can significantly affect drug absorption and disposition. Immature renal function, altered hepatic enzyme activity and differences in drug absorption lead to variations in systemic exposure of antiretrovirals among children. Paediatric patients are also subject to unique circumstances that may prevent adherence to antiretroviral regimens. The pharmacokinetics of nucleoside reverse transcriptase inhibitors differ significantly among preterm infants, full-term infants and older children. Decreased hepatic glucuronidation activity in neonates results in pharmacokinetic differences in zidovudine disposition when compared with older children. Didanosine, stavudine and lamivudine are renally eliminated, thus resulting in differences among young children with immature renal function. Pharmacokinetic data for non-nucleoside reverse transcriptase inhibitors in children are limited. Decreased elimination of nevirapine among neonates has been observed, primarily due to decreased enzymatic activity. Pharmacokinetic differences across age groups have been noted for efavirenz, but no formal assessments have been conducted in children weighing less than 10kg. Protease inhibitors are metabolised by the cytochrome P450 enzyme system, which is not fully developed in younger children. Decreased metabolism can result in elevated plasma concentrations, thereby increasing the chance of toxicity. Unfortunately, few studies exist evaluating the pharmacokinetics of antiretrovirals in children. As a result, dosage selection of antiretrovirals in children often occurs without adequate data. As the life expectancy of HIV-infected children increases, use of antiretrovirals to prevent disease progression also increases. If prevention of treatment failure continues to be the goal of antiretroviral therapy, the pharmacokinetics of antiretrovirals in children need to be assessed early in the drug development process.

Functional characterization in yeast of genetic variants in the human equilibrative nucleoside transporter, ENT1

The human equilibrative nucleoside transporter, ENT1, appears to play a critical role in the disposition of nucleosides and nucleoside analogs used clinically as anti-viral and anti-cancer drugs. Recently, we identified variants of ENT1 in an ethnically diverse DNA sample set from 247 individuals, focusing primarily on the coding region. The goal of the present study was to analyse the haplotype structure and functionally characterize the variants of ENT1. We observed that a single haplotype, ENT1*1, accounted for 91.3% of the 494 chromosomes. Functional analysis in Saccharomyces cerevisiae revealed no differences in the kinetics of uptake of nucleosides and nucleoside analogs by the two non-synonymous variant transporters, ENT1-I216T and ENT1-E391K, and the reference ENT1. These results, together with the observation that there are few haplotypes of ENT1, indicate that coding region variants of ENT1 do not contribute to inter-individual differences in response to nucleoside analog drugs.

Characterization of the transport of nucleoside analog drugs by the human multidrug resistance proteins MRP4 and MRP5

The human multidrug resistance proteins MRP4 and MRP5 are organic anion transporters that have the unusual ability to transport cyclic nucleotides and some nucleoside monophosphate analogs. Base and nucleoside analogs used in the chemotherapy of cancer and viral infections are potential substrates. To assess the possible contribution of MRP4 and MRP5 to resistance against these drugs, we have investigated the transport mediated by MRP4 and MRP5. In cytotoxicity assays, MRP4 conferred resistance to the antiviral agent 9-(2-phosphonomethoxyethyl)adenine (PMEA) and high-performance liquid chromatography analysis showed that, like MRP5, MRP4 transported PMEA in an unmodified form. MRP4 also mediated substantial resistance against other acyclic nucleoside phosphonates, whereas MRP5 did not. Apart from low-level MRP4-mediated cladribine resistance, the cytotoxicity of clinically used anticancer nucleosides was not influenced by overexpression of MRP4 or MRP5. In contrast, MRP5 mediated efflux of the pyrimidine-based antiviral 2',3'-dideoxynucleoside 2',3'-didehydro-2',3'-dideoxythymidine 5'-monophosphate (d4TMP) and its phosphoramidate derivative alaninyl-d4TMP from cells loaded with the 2',3'-didehydro-2',3'-dideoxythymidine prodrugs cyclosaligenyl-d4TMP and aryloxyphosphoramidate d4TMP (So324), respectively. Moreover, only inside-out membrane vesicles derived from MRP5-overexpressing cells accumulated alaninyl-d4TMP. Cellular efflux and vesicular uptake studies were carried out to further compare transport mediated by MRP4 and MRP5 and showed that dipyridamole, dilazep, nitrobenzyl mercaptopurine riboside, sildenafil, trequinsin and MK571 inhibited MRP4 more than MRP5, whereas cyclic nucleotides and monophosphorylated nucleoside analogs were equally poor inhibitors of both pumps. These results strongly suggest that the affinity of MRP4 and MRP5 for nucleotide-based substrates is low.

Kinetics of inhibition of ribonuclease P activity by peptidyltransferase inhibitors. Effect of antibiotics on RNase P

A cell-free system derived from Dictyostelium discoideum has been used to study the kinetics of inhibition of RNase P by puromycin, amicetin and blasticidin S. Detailed kinetic analysis showed that the type of inhibition of RNase P activity by puromycin is simple competitive, whereas the type of inhibition by amicetin and blasticidin S is simple non-competitive. On the basis of Ki values amicetin is stronger inhibitor than puromycin and blasticidin S.