Protecting-group-free synthesis of clevudine (L-FMAU), a treatment of the hepatitis B virus.

Unnatural nucleoside analogues are valuable research and clinical tools as antiproliferative, antibacterial or antiviral agents. In this context, clevudine (L-FMAU), a reverse transcriptase inhibitor, is currently used for the treatment of the hepatitis B virus. Herein, we describe a new strategy for the preparation of clevudine. Starting from 2-deoxy-2-fluoro-D-galactopyranose, we developed the shortest and highest yield synthesis of this unnatural L-nucleoside. Key steps involve an iodine-promoted cyclization and oxidative cleavage to access the L-arabinofuranosyl scaffold.

The utility of a differentiated preclinical liver model, HepaRG cells, in investigating delayed toxicity via inhibition of mitochondrial-replication induced by fialuridine.

During its clinical development fialuridine caused liver toxicity and the death of five patients. This case remains relevant due to the continued development of mechanistically-related compounds against a back-drop of simple in vitro models which remain limited for the preclinical detection of such delayed toxicity. Here, proteomic investigation of a differentiated, HepaRG, and proliferating, HepG2 cell model was utilised to confirm the presence of the hENT1 transporter, thymidine kinase-1 and -2 (TK1, TK2) and thymidylate kinase, all essential in order to reproduce the cellular activation and disposition of fialuridine in the clinic. Acute metabolic modification assays could only identify mitochondrial toxicity in HepaRG cells following extended dosing, 2 weeks. Toxic effects were observed around 10 µM, which is within a range of 10-15 X approximate Cmax. HepaRG cell death was accompanied by a significant decrease in mitochondrial DNA content, indicative of inhibition of mitochondrial replication, and a subsequent reduction in mitochondrial respiration and the activity of mitochondrial respiratory complexes, not replicated in HepG2 cells. The structural epimer of fialuridine, included as a pharmacological negative control, was shown to have no cytotoxic effects in HepaRG cells up to 4 weeks. Overall, these comparative studies demonstrate the HepaRG model has translational relevance for fialuridine toxicity and therefore may have potential in investigating the inhibition of mitochondrial replication over prolonged exposure for other toxicants.

Spectrum of activity and mechanisms of resistance of various nucleoside derivatives against gammaherpesviruses.

The susceptibilities of gammaherpesviruses, including Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), and animal rhadinoviruses, to various nucleoside analogs was investigated in this work. Besides examining the antiviral activities and modes of action of antivirals currently marketed for the treatment of alpha- and/or betaherpesvirus infections (including acyclovir, ganciclovir, penciclovir, foscarnet, and brivudin), we also investigated the structure-activity relationship of various 5-substituted uridine and cytidine molecules. The antiviral efficacy of nucleoside derivatives bearing substitutions at the 5 position was decreased if the bromovinyl was replaced by chlorovinyl. 1-ß-D-Arabinofuranosyl-(E)-5-(2-bromovinyl)uracil (BVaraU), a nucleoside with an arabinose configuration of the sugar ring, exhibited no inhibitory effect against rhadinoviruses but was active against EBV. On the other hand, the fluoroarabinose cytidine analog 2'-fluoro-5-iodo-aracytosine (FIAC) showed high selectivity indices against gammaherpesviruses that were comparable to those of brivudin. Additionally, we selected brivudin- and acyclovir-resistant rhadinoviruses in vitro and characterized them by phenotypic and genotypic (i.e., sequencing of the viral thymidine kinase, protein kinase, and DNA polymerase) analysis. Here, we reveal key amino acids in these enzymes that play an important role in substrate recognition. Our data on drug susceptibility profiles of the different animal gammaherpesvirus mutants highlighted cross-resistance patterns and indicated that pyrimidine nucleoside derivatives are phosphorylated by the viral thymidine kinase and purine nucleosides are preferentially activated by the gammaherpesvirus protein kinase.

Noncompetitive inhibition of hepatitis B virus reverse transcriptase protein priming and DNA synthesis by the nucleoside analog clevudine.

All currently approved antiviral drugs for the treatment of chronic hepatitis B virus (HBV) infection are nucleos(t)ide reverse transcriptase inhibitors (NRTI), which inhibit the DNA synthesis activity of the HBV polymerase. The polymerase is a unique reverse transcriptase (RT) that has a novel protein priming activity in which HP initiates viral DNA synthesis using itself as a protein primer. We have determined the ability of NRTI-triphosphates (TP) to inhibit HBV protein priming and their mechanisms of action. While entecavir-TP (a dGTP analog) inhibited protein priming initiated specifically with dGTP, clevudine-TP (a TTP analog) was able to inhibit protein priming independently of the deoxynucleoside triphosphate (dNTP) substrate and without being incorporated into DNA. We next investigated the effect of NRTIs on the second stage of protein priming, wherein two dAMP nucleotides are added to the initial deoxyguanosine nucleotide. The obtained results indicated that clevudine-TP as well as tenofovir DF-DP strongly inhibited the second stage of protein priming. Tenofovir DF-DP was incorporated into the viral DNA primer, whereas clevudine-TP inhibited the second stage of priming without being incorporated. Finally, kinetic analyses using the HBV endogenous polymerase assay revealed that clevudine-TP inhibited DNA chain elongation by HP in a noncompetitive manner. Thus, clevudine-TP appears to have the unique ability to inhibit HBV RT via binding to and distorting the HP active site, sharing properties with both NRTIs and nonnucleoside RT inhibitors.

Enhancing the efficacy of Ara-C through conjugation with PAMAM dendrimer and linear PEG: a comparative study.

1ß-d-Arabinofuranosylcytosine (Cytarabine, Ara-C) is a key drug in the treatment of acute myeloid leukemia. Ara-C has a number of limitations such as a rapid deactivation by cytidine deaminase leading to the formation of a biologically inactive metabolite, Ara-U (1ß-d-arabinofuranosyluracil), a low lipophilicity, and fast clearance from the body. To address these problems, we developed a conjugate in which hydroxyl-terminated PAMAM dendrimer, G4-OH ["D"] and PEG were used as carriers for the drug (Ara-C). The conjugates were synthesized using an efficient multistep protection/deprotection method resulting in the formation of a covalent bond between the primary hydroxyl group of Ara-C and dendrimer/PEG. The structure, physicochemical properties, and drug release kinetics were characterized extensively. (1)H NMR and MALDI-TOF mass spectrometry suggested covalent attachment of 10 Ara-C molecules to the dendrimer. The release profile of Ara-C in human plasma and in PBS buffer (pH 7.4) showed that the conjugates released the drug over 14 days in PBS, with the release sped up in plasma. In PBS, while most of the drug is released from PEG-Ara-C, the dendrimer continues to release the drug in a sustained fashion. The results also suggested that the formation of the inactive form of Ara-C (Ara-U) was delayed upon conjugation of Ara-C to the polymers. The inhibition of cancer growth by the dendrimer-Ara-C and PEG-Ara-C conjugates was evaluated in A549 human adenocarcinoma epithelial cells. Both dendrimer- and PEG-Ara-C conjugates were 4-fold more effective in inhibition of A549 cells compared to free Ara-C after 72 h of treatment.

High-dose clevudine impairs mitochondrial function and glucose-stimulated insulin secretion in INS-1E cells.

BACKGROUND: Clevudine is a nucleoside analog reverse transcriptase inhibitor that exhibits potent antiviral activity against hepatitis B virus (HBV) without serious side effects. However, mitochondrial myopathy has been observed in patients with chronic HBV infection taking clevudine. Moreover, the development of diabetes was recently reported in patients receiving long-term treatment with clevudine. In this study, we investigated the effects of clevudine on mitochondrial function and insulin release in a rat clonal ß-cell line, INS-1E. METHODS: The mitochondrial DNA (mtDNA) copy number and the mRNA levels were measured by using quantitative PCR. MTT analysis, ATP/lactate measurements, and insulin assay were performed. RESULTS: Both INS-1E cells and HepG2 cells, which originated from human hepatoma, showed dose-dependent decreases in mtDNA copy number and cytochrome c oxidase-1 (Cox-1) mRNA level following culture with clevudine (10 µM-1 mM) for 4 weeks. INS-1E cells treated with clevudine had reduced total mitochondrial activities, lower cytosolic ATP contents, enhanced lactate production, and more lipid accumulation. Insulin release in response to glucose application was markedly decreased in clevudine-treated INS-1E cells, which might be a consequence of mitochondrial dysfunction. CONCLUSIONS: Our data suggest that high-dose treatment with clevudine induces mitochondrial defects associated with mtDNA depletion and impairs glucose-stimulated insulin secretion in insulin-releasing cells. These findings partly explain the development of diabetes in patients receiving clevudine who might have a high susceptibility to mitochondrial toxicity.

Identification and characterization of clevudine-resistant mutants of hepatitis B virus isolated from chronic hepatitis B patients.

Clevudine (CLV) is a nucleoside analog with potent antiviral activity against chronic hepatitis B virus (HBV) infection. Viral resistance to CLV in patients receiving CLV therapy has not been reported. The aim of this study was to characterize CLV-resistant HBV in patients with viral breakthrough (BT) during long-term CLV therapy. The gene encoding HBV reverse transcriptase (RT) was analyzed from chronic hepatitis B patients with viral BT during CLV therapy. Sera collected from the patients at baseline and at the time of viral BT were studied. To characterize the mutations of HBV isolated from the patients, we subjected the HBV mutants to in vitro drug susceptibility assays. Several conserved mutations were identified in the RT domain during viral BT, with M204I being the most common. In vitro phenotypic analysis showed that the mutation M204I was predominantly associated with CLV resistance, whereas L229V was a compensatory mutation for the impaired replication of the M204I mutant. A quadruple mutant (L129M, V173L, M204I, and H337N) was identified that conferred greater replicative ability and strong resistance to both CLV and lamivudine. All of the CLV-resistant clones were lamivudine resistant. They were susceptible to adefovir, entecavir, and tenofovir, except for one mutant clone. In conclusion, the mutation M204I in HBV RT plays a major role in CLV resistance and leads to viral BT during long-term CLV treatment. Several conserved mutations may have a compensatory role in replication. Drug susceptibility assays reveal that adefovir and tenofovir are the most effective compounds against CLV-resistant mutants. These data may provide additional therapeutic options for CLV-resistant patients.

Phosphoramidates of 2'-beta-D-arabinouridine (AraU) as phosphate prodrugs; design, synthesis, in vitro activity and metabolism.

2'-Beta-D-arabinouridine (AraU), the uridine analogue of the anticancer agent AraC, was synthesized and evaluated for antiviral activity and cytotoxicity. In addition, a series of AraU monophosphate prodrugs in the form of triester phosphoramidates (ProTides) were also synthesized and tested against a range of viruses, leukaemia and solid tumour cell lines. Unfortunately, neither the parent compound (AraU) nor any of its ProTides showed antiviral activity, nor potent inhibitory activity against any of the cancer cell lines. Therefore, the metabolism of AraU phosphoramidates to release AraU monophosphate was investigated. The results showed carboxypeptidase Y, hog liver esterase and crude CEM tumor cell extracts to hydrolyse the ester motif of phosphoramidates with subsequent loss of the aryl group, while molecular modelling studies suggested that the AraU l-alanine aminoacyl phosphate derivative might not be a good substrate for the phosphoramidase enzyme Hint-1. These findings are in agreement with the observed disappearance of intact prodrug and concomitant appearance of the corresponding phosphoramidate intermediate derivative in CEM cell extracts without measurable formation of araU monophosphate. These findings may explain the poor antiviral/cytostatic potential of the prodrugs.

Epstein-Barr virus: inhibition of replication by three new drugs.

Epstein-Barr virus (EBV) is the cause of infectious mononucleosis and is associated with three human malignancies. Acyclovir [9-(2-hydroxyethoxymethyl)guanine], the first clinically useful drug effective against replication of EBV, is without effect against latent or persistent EBV infection. Three nucleoside analogs, E-5-(2-bromovinyl)-2'-deoxyuridine, 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-iodocytosine, and 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-methyluracil are potent inhibitors of EBV replication in vitro. Moreover, in contrast to the reversibility of viral inhibition by Acyclovir, these three drugs have prolonged effects in suppressing viral replication even after the drugs are removed from persistently infected cell cultures.

New drugs for chronic hepatitis B: a review.

Three nucleotide/nucleoside analogs are used for chronic hepatitis B (HBV): lamivudine, adefovir dipivoxil, and entecavir. Lamivudine and adefovir are advantageous for oral administration and safety but induce a sustained response after withdrawal of therapy in only a minority of patients. Thus, the treatment should be given in trials in a majority of patients for a long period of time. In addition, the long-term efficacy of lamivudine is limited by the frequent emergence of drug-resistant HBV mutants. Adefovir is associated with a low frequency of resistance, but its antiviral effect is not optimal. Entecavir, a cyclopentyl guanosine analog, is a potent inhibitor of HBV-DNA polymerase and it inhibits both priming and elongation steps of viral DNA replication. In phase II and III clinical trials, entecavir was found to be superior to lamivudine for all primary end points evaluated in both nucleoside-naive and lamivudine-resistant patients, and it was effective in both HBeAg-positive and HBeAg-negative nucleoside-naive patients. Only one trial has shown cases of viral resistance to this drug. The approved dosage in treatment-naive patients is 0.5 mg per day orally, whereas in patients who have failed lamivudine therapy or who are known to harbor lamivudine-resistant mutants, the approved dosage is 1.0 mg per day. Recent preliminary results show that clevudine, telbivudine, and emtricitabine may be potent analogs available for the treatment of HBV. Further studies are being conducted to assess the long-term efficacy and safety of these drugs.

Bioengineering of bacterial pathogens for noninvasive imaging and in vivo evaluation of therapeutics.

Critical bacterial pathogens of public health and biodefense concerns were engineered to constitutively express Escherichia coli enzyme thymidine kinase (TK) that allows for noninvasive nuclear imaging via phosphorylation and entrapment of radiolabeled nucleoside analog 1-(2'deoxy-2'-fluoro-ß-D-arabinofuranosyl)-5-iodouracil (FIAU). Expression of functional TK was established using a nucleoside analog Zidovudine that impeded the growth of tk-engineered bacteria. Significantly, no observable growth differences were detected for FIAU. High resolution mass spectrometry with Pseudomonas aeruginosa PAO1 and its tk variant (PAO1TK) confirmed FIAU phosphorylation and retention only in PAO1TK. In vitro gamma counting with wild-type PAO1, Acinetobacter baumannii and Burkholderia pseudomallei Bp82 and their tk derivatives with [18F]FIAU further confirmed that tk variants selectively incorporated the radiotracer, albeit with varying efficiencies. In vitro [18F]FIAU labeling coupled with in vivo Positron Emission Tomography/Computed Tomography (PET/CT) imaging of PAO1 and PAO1TK confirmed that only PAO1TK can be imaged in mice at sensitivities ≥107 bacteria per infection site. This was further verified by administering [18F]FIAU to animals infected with PAO1 and PAO1TK. Utility of tk-engineered P. aeruginosa in noninvasive PET/CT imaging for bacterial therapeutic evaluation in animals was demonstrated employing antibiotic ciprofloxacin, underscoring the immediate use of PAO1TK and potentially other engineered pathogens for evaluating experimental therapeutics.

Effect of uracil arabinoside on metabolism and cytotoxicity of cytosine arabinoside in L5178Y murine leukemia.

Pretreatment of L5178Y murine leukemia cells with uracil arabinoside (ara-U) enhances the cytotoxicity of cytosine arabinoside (ara-C). This effect is mediated by the cytostatic effect of ara-U, which causes a delay of cell progression through S-phase. Consequently, the specific activity of enzymes that peak during S-phase increases, and deoxycytidine kinase increases 3.6-fold over untreated controls. This allows enhanced anabolism of ara-C to nucleotides, as well as increased incorporation into DNA with ultimate synergistic cytotoxicity. It is postulated that the systemic metabolism of high-dose ara-C to sustained high levels of ara-U in patients with acute leukemia may enhance the activity of subsequent doses of ara-C, and thus contribute to a means for pharmacologic self-potentiation, contributing to the unique therapeutic activity of high-dose ara-C.

A novel mutation pattern emerging during lamivudine treatment shows cross-resistance to adefovir dipivoxil treatment.

AIMS: This study was conducted to clarify the resistance profile of a novel mutation pattern emerging during lamivudine (3TC) therapy and showing cross-resistance to adefovir dipivoxil (ADV) in a patient with chronic hepatitis B. METHODS AND RESULTS: Successful suppression of hepatitis B virus (HBV) replication by sequential therapy of 9 MU thrice weekly interferon (IFN) and 3TC was followed by genotypical resistance detected at month 28 of therapy (month 19 of lamivudine treatment). ADV was added to 3TC therapy on month 44 of antiviral treatment. Neither alanine aminotransferase normalization nor a stable decrease in HBV viral load was observed, although ADV was used for more than 40 months. The HBV pol region was amplified from serum samples obtained before and after ADV treatment. The complete genome was cloned into a TA vector. PCR products and 7-10 clones from each cloned vector were sequenced. A novel mutation, A181S, in the reverse transcriptase gene leading to a conversion of W172C in the overlapping surface antigen gene was detected along with a M2041 mutation. The complete genome comprising the A181S+M2041 pattern was cloned into an expression vector and its in vitro susceptibility to 3TC, ADV, tenofovir (PMPA), clevudine (L-FMAU) and emtricitabine (FTC) were determined in transiently transfected Huh7 cells. This mutation pattern displayed more than 1000-fold resistance to the nucleoside analogues 3TC and FTC and approximately sixfold resistance to L-FMAU, while it confers 28.23- and 5.57-fold resistance for the nucleotide analogues ADV and PMPA, respectively. CONCLUSION: A new mutation pattern, A181S+M2041, arising under lamivudine treatment confers cross-resistance to ADV both in vivo and in vitro.

Growth inhibition of Mycobacterium bovis, Mycobacterium tuberculosis and Mycobacterium avium in vitro: effect of 1-beta-D-2'-arabinofuranosyl and 1-(2'-deoxy-2'-fluoro-beta-D-2'-ribofuranosyl) pyrimidine nucleoside analogs.

The resurgence of tuberculosis and the emergence of multiple-drug-resistant strains of Mycobacteria necessitate the search for new classes of antimycobacterial agents. We synthesized a series of 1-beta-D-2'-arabinofuranosyl and 1-(2'-deoxy-2'-fluoro-beta-D-ribofuranosyl) pyrimidine nucleosides possessing diverse sets of alkynyl, alkenyl, alkyl, and halo substituents at the C-5 position of the uracil and investigated their effect on activity against M. tuberculosis, M. bovis, and M. avium. Among these molecules, 5-alkynyl-substituted derivatives emerged as potent inhibitors of M. bovis, M. tuberculosis, and M. avium. Nucleosides 1-beta-D-2'-arabinofuranosyl-5-dodecynyluracil (5), 1-(2'-deoxy-2'-fluoro-beta-D-ribofuranosyl)-5-dodecynyluracil (24), and 1-(2'-deoxy-2'-fluoro-beta-D-ribofuranosyl)-5-tetradecynyluracil (25) showed the highest antimycobacterial potency against M. bovis and M. tuberculosis. The MIC90 exhibited by compounds 5, 24, and 25 was similar or close to that of the reference drug rifampicin. The most active compounds 5, 24, and 25 were also found to retain sensitivity against a rifampicin-resistant strain of M. tuberculosis H37Rv at similar concentrations. Some of these analogs also revealed in vitro antimicrobial effect against several other gram-positive pathogens.

Molecular imaging with 123I-FIAU, 18F-FUdR, 18F-FET, and 18F-FDG for monitoring herpes simplex virus type 1 thymidine kinase and ganciclovir prodrug activation gene therapy of cancer.

UNLABELLED: The ability to monitor tumor responses during prodrug activation gene therapy and other anticancer gene therapies is critical for their translation into clinical practice. Previously, we demonstrated the feasibility of noninvasive in vivo imaging with 131I-5-iodo-2'-fluoro-1-beta-D-arabinofuranosyluracil (131I-FIAU) for monitoring herpes simplex virus type 1 thymidine kinase (HSV1-tk) cancer gene expression in an experimental animal model. Here we tested the efficacy of SPECT with 123I-FIAU and PET with 5-18F-fluoro-2'-deoxyuridine (18F-FUdR), 2-18F-fluoroethyl-L-tyrosine (18F-FET), and 18F-FDG for monitoring tumor responses during prodrug activation gene therapy with HSV1-tk and ganciclovir (GCV). METHODS: In the flanks of FVB/N female mice, 4 tumors per animal were established by subcutaneous injection of 1 x 10(5) cells of NG4TL4 sarcoma cells, HSV1-tk-transduced NG4TL4-STK cells, or a mixture of these cells in different proportions to model different efficacies of transfection and HSV1-tk gene expression levels in tumors. Ten days later, the animals were treated with GCV (10 mg/kg/d intraperitoneally) for 7 d. Gamma-Imaging with 123I-FIAU and PET with 18F-FUdR, 18F-FET, and 18F-FDG were performed before and after initiation of therapy with GCV in the same animal. RESULTS: Before GCV treatment, no significant difference in weight and size was found in tumors that expressed different HSV1-tk levels, suggesting similar in vivo proliferation rates for NG4TL4 and NG4TL4-STK sarcomas. The accumulation of 123I-FIAU at 24 h after injection was directly proportional to the percentage of NG4TL4-STK cells in the tumors. The 123I-FIAU accumulation at 4 and 7 d of GCV therapy decreased significantly compared with pretreatment levels and was proportional to the percentage of HSV1-tk-positive tumor cells. Tumor uptake of 18F-FUdR in all HSV1-tk-expressing tumors also decreased significantly compared with pretreatment levels and was proportional to the percentage of HSV1-tk-positive tumor cells. The accumulation of 18F-FET decreased minimally (about 1.5-fold) and 18F-FDG decreased only 2-fold after 7 d of GCV therapy, and the degree of reduction was proportional to the percentage of HSV1-tk-positive tumor cells. CONCLUSION: We have shown that gamma-camera imaging with 123I-FIAU was the most reliable method for prediction of tumor response to GCV therapy, which was proportional to the magnitude of HSV1-tk expression in tumor tissue. 123I-FIAU imaging can be used to verify the efficacy of elimination of HSV1-tk-expressing cells by therapy with GCV. PET with 18F-FUdR reliably visualizes proliferating tumor tissue and is most suitable for the assessment of responses in tumors undergoing HSV1-tk plus GCV prodrug activation gene therapy. PET with 18F-FDG or 18F-FET can be used as additional "surrogate" biomarkers of the treatment response, although these radiotracers are less sensitive than 18F-FUdR for monitoring tumor responses to prodrug activation gene therapy with HSV1-tk and GCV in this sarcoma model.

Modulation of the metabolism and pharmacokinetics of 1-beta-D-arabinofuranosylcytosine by 1-beta-D-arabinofuranosyluracil in leukemic mice.

The interaction between high concentrations of 1-beta-D-arabinofuranosyluracil (HiCAU) and 1-beta-D-arabinofuranosylcytosine (ara-C) was investigated in vivo with emphasis on cell kinetics, pharmacokinetics, and drug metabolism. Mice bearing L5178Y leukemia were given a 48-h s.c. infusion of high-dose ara-U (HiDAU) to achieve a plasma level of 0.5 to 1 mM. A total dose of 7.35 g/kg/day for 2 days was nontoxic; the mean survival of control (saline treated) leukemic mice was 12.2 +/- 1.8 days and 11.7 +/- 2.0 days for the HiDAU-treated leukemic mice. Using flow cytometry, cell cycle progression of L5178Y ascites cells was monitored during HiDAU infusion. At 48 h, the proliferative index (PI) percentage of the leukemic cells is significantly different (P less than 0.001) in HiDAU-treated leukemic mice (mean = 50.8) versus control (mean = 45.6). A higher PI percentage is associated with accumulation of cells in S phase. This effect was highly variable in the ara-U-treated mice, and the ara-U "perturbed" group was defined as those mice whose cells had an increase in the PI to greater than or equal to 50%. The higher PI percentage in HiDAU-treated mice correlated with HiCAU in ascites fluid, leukemic cells, and kidney of perturbed mice. HiCAU in the "ara-U-perturbed" group altered the plasma pharmacokinetics of high-dose ara-C (HiDAC, 1 g/kg), increased the cellular metabolism of ara-C to 1-beta-D-arabinofuranosylcytidine triphosphate (ara-CTP) (3-fold), and increased ara-C-DNA synthesis (3-fold). In mice bearing the L5178Y leukemia, a 48-h infusion of ara-U followed by a 24-h s.c. infusion of 40 mg/kg resulted in a 260% increase in life span and seven 90-day survivors among 16 treated mice. In contrast, ara-U or ara-C alone had a negligible therapeutic effect. ara-U-induced alterations in the systemic pharmacokinetics of ara-C are the result of inhibition of cytidine deaminase activity by HiCAU in liver and kidneys. This results in a decrease in ara-C catabolism and prolongs the plasma half-life of ara-C. The dual alteration of the pharmacokinetics of ara-C and cytokinetics of the leukemia cells by HiCAU results in enhanced survival of leukemic mice. These results may help explain the clinical utility of HiDAC treatment programs for patients with acute leukemia.

Uptake of radiolabeled 2'-fluoro-2'-deoxy-5-iodo-1-beta-D-arabinofuranosyluracil in cardiac cells after adenoviral transfer of the herpesvirus thymidine kinase gene: the cellular basis for cardiac gene imaging.

BACKGROUND: Gene therapy is a promising approach for the treatment of cardiac diseases. Coexpression of therapeutic genes with a suitable marker gene would allow for the noninvasive imaging of successful gene transfer and expression via radiolabeled marker substrates. In the present study, such an approach was first applied to cardiac tissue. METHODS AND RESULTS: The combination of the herpesvirus thymidine kinase reporter gene (HSV1-tk) and radiolabeled 2'-fluoro-2'-deoxy-5-iodo-1-beta-D-arabinofuranosyluracil (FIAU) was evaluated. H9c2 rat cardiomyoblasts were infected in vitro with a replication-defective HSV1-tk-containing adenovirus and a negative control virus. The intracellular uptake of [(14)C]FIAU increased with increasing multiplicity of infection and with time after infection. Uptake in negative controls remained <15% of positive controls. Additionally, vectors were applied intramyocardially in Wistar rats. The marker substrate [(125)I]FIAU was injected intravenously 3 days later, and animals were killed after 24 hours. Autoradiographically, regional transgene expression was clearly identified in animals receiving the adenovirus containing HSV1-tk (3. 4+/-2.2-fold increase of radioactivity at vector administration site compared with remote myocardium), whereas nonspecific uptake in negative controls was low (<10% of positive controls). CONCLUSIONS: Using an adenoviral vector, HSV1-tk can be successfully expressed in cardiac cells in vitro and in vivo, yielding high uptake of radiolabeled FIAU. The results suggest that imaging transgene expression in the heart is feasible and may be used to monitor gene therapy noninvasively.

Synergy between high-dose cytarabine and asparaginase in the treatment of adults with refractory and relapsed acute myelogenous leukemia--a Cancer and Leukemia Group B Study.

One hundred ninety-five adult patients with refractory or first relapse acute myelogenous leukemia (AML) were randomly assigned to receive high-dose cytarabine (HiDAC), 3 g/m2 as a three-hour intravenous (IV) infusion every 12 hours for four doses, followed by 6,000 IU/m2 asparaginase (ASNase) administered at hour 42, or HiDAC without ASNase. Treatment was repeated on day 8. The median patient age was 52 years. There was an overall superior complete remission (CR) rate for HiDAC/ASNase (40%) v HiDAC (24%), P = .02. Subset analysis according to prior response and age showed the following CR rates: 54% from HiDAC/ASNase treatment of refractory AML in patients less than 60 years, and 31% in patients greater than 60 years; CR from HiDAC in the same refractory groups were 18% (less than 60) and 0% (greater than 60); 37% from HiDAC/ASNase treatment of relapsed AML in patients less than 60 years, and 43% in patients greater than 60 years; CRs from HiDAC in the same relapsed groups were 33% (less than 60) and 21% (greater than 60). Toxicity in the two treatment arms was comparable and consisted primarily of leukopenia, thrombocytopenia, mild hepatic dysfunction, diarrhea, conjunctivitis and serositis, and hyperglycemia. There was only one case of transient cerebellar toxicity and no cutaneous toxicity. Median time to full hematologic recovery was 5 weeks. There was an overall survival benefit for patients treated with HiDAC/ASNase (19.6 weeks) compared with HiDAC (15.9 weeks), P = .046, primarily attributable to effects in refractory patients. Median time to failure for refractory patients who achieved CR was 38.5 weeks with HiDAC/ASNase, and 13.3 weeks for those treated with HiDAC. For relapsed patients in CR from HiDAC/ASNase the median time to failure was 17.7 weeks and 18.3 weeks for HiDAC. The overall 42% CR rate from HiDAC/ASNase v 12% from HiDAC in patients with refractory AML indicates that HiDAC/ASNase is not cross-resistant with standard-dose cytarabine (SDAC) and anthracyclines. We conclude that HiDAC/ASNase has substantial activity in poor-prognosis AML and that this combination warrants further trials in earlier stage disease.

Suicide prodrugs activated by thymidylate synthase: rationale for treatment and noninvasive imaging of tumors with deoxyuridine analogues.

Most tumors are resistant to therapy by thymidylate synthase (TS) inhibitors due to their high levels of TS. Instead of inhibiting TS, we hypothesized that it was possible to use this enzyme to activate suicide prodrugs (deoxyuridine analogues) to more toxic species (thymidine analogues). Tumors with high levels of TS could be particularly sensitive to deoxyuridine analogues because they would be more efficient in producing the toxic methylated species. Furthermore, the accumulation of methylated species within tumors could be visualized externally if a tracer dose of the deoxyuridine analogue was tagged with an isotope, preferably a positron emitter, such as 18F. Higher accumulation of isotope indicates higher activity of TS and lower sensitivity of the tumor to TS inhibitors, but perhaps more sensitivity to therapy with deoxyuridine analogues as suicide prodrugs. 2'-F-ara-deoxyuridine (FAU) was used as a prototype to demonstrate these concepts experimentally. FAU readily entered cells and was phosphorylated, methylated, and subsequently incorporated into cellular DNA. Among different cell lines, FAU produced varying degrees of growth inhibition. Greater DNA incorporation (e.g., for CEM and U-937 cells) was reflected as increased toxicity. FAU produced less DNA incorporation in Raji or L1210 cells, and growth rate was minimally decreased. As the first demonstration that cells with high levels of TS activity can be more vulnerable to therapy than cells with low TS activity, this preliminary work suggests a new therapeutic approach for common human tumors that were previously resistant. Furthermore, it appears that the TS activity of tumors could be noninvasively imaged in situ by tracer doses of [18F]FAU and that this phenotypic information could guide patient therapy.

Functional coexpression of HSV-1 thymidine kinase and green fluorescent protein: implications for noninvasive imaging of transgene expression.

Current gene therapy technology is limited by the paucity of methodology for determining the location and magnitude of therapeutic transgene expression in vivo. We describe and validate a paradigm for monitoring therapeutic transgene expression by noninvasive imaging of the herpes simplex virus type 1 thymidine kinase (HSV-1-tk) marker gene expression. To test proportional coexpression of therapeutic and marker genes, a model fusion gene comprising green fluorescent protein (gfp) and HSV-1-tk genes was generated (tkgfp gene) and assessed for the functional coexpression of the gene product, TKGFP fusion protein, in rat 9L gliosarcoma, RG2 glioma, and W256 carcinoma cells. Analysis of the TKGFP protein demonstrated that it can serve as a therapeutic gene by rendering tkgfp transduced cells sensitive to ganciclovir or as a screening marker useful for identifying transduced cells by fluorescence microscopy or fluorescence-activated cell sorting (FACS). TK and GFP activities in the TKGFP fusion protein were similar to corresponding wild-type proteins and accumulation of the HSV-1-tk-specific radiolabeled substrate, 2'-fluoro-2'-deoxy-1beta-D-arabinofuranosyl-5-iodo-uracil (FIAU), in stability transduced clones correlated with gfp-fluorescence intensity over a wide range of expression levels. The tkgfp fusion gene itself may be useful in developing novel cancer gene therapy approaches. Valuable information about the efficiency of gene transfer and expression could be obtained by non-invasive imaging of tkgfp expression with FIAU and clinical imaging devices (gamma camera, positron-emission tomography [PET], single photon emission computed tomography [SPECT]), and/or direct visualization of gfp expression in situ by fluorescence microscopy or endoscopy.