Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:2.7.7.7 (DNA polymerase)
17,007 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Hepatitis B virus (HBV) resistance to lamivudine has not been extensively documented in human immunodeficiency virus (HIV)-infected patients. We studied the long-term incidence of HBV resistance to lamivudine in HIV-positive patients. Sixty-six HIV-HBV-coinfected patients were studied while receiving lamivudine (150 mg twice daily) as a part of antiretroviral therapy. All these patients had a detectable serum HBV DNA at the beginning of lamivudine therapy. Serum HBV DNA was quantified by molecular hybridization. Sequence analysis of the HBV polymerase was performed in patients who became resistant to lamivudine. After 2 months of lamivudine, HBV DNA became undetectable in 57 patients (86.4%, 95% CI: 75.7%-93.6%). After 2 years of lamivudine, 47% +/- 18.6% of the patients, had sustained HBV-DNA suppression. All the 22 tested patients with HBV resistance developed mutation at position 550 in the YMDD motif of the DNA polymerase. None of the following variables were associated with an increased risk of lamivudine resistance: age, associated protease inhibitor therapy, Center for Disease Control (CDC) stage C, known HIV-infection duration, serum HBV-DNA level at baseline, CD4 cell count and serum alanine transaminase levels at baseline and at HBV-replication suppression (2 months of lamivudine). Lamivudine (300 mg/d) is effective for the inhibition of HBV replication in HIV-infected patients. However, emergence of lamivudine-resistant HBV may occur in 20% of patients per year.
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PMID:Long-term incidence of hepatitis B virus resistance to lamivudine in human immunodeficiency virus-infected patients. 1077 55

Lamivudine is an effective antiviral agent for the treatment of chronic type B hepatitis. Recent studies have shown the appearance of lamivudine resistant viruses with mutations at the tyrosine-methionine-aspartate-aspartate (YMDD) motif of the viral polymerase in hepatitis B virus (HBV) infected patients who received orthotopic liver transplantation. In order to confirm the appearance of such mutant HBV in immunocompetent patients, the HBV sequences in and around the YMDD motif of HBV DNA polymerase were examined in the sera from 16 lamivudine treated and 10 untreated control patients. Approximately 200 bases including the YMDD motif of HBV DNA polymerase were amplified by polymerase chain reaction (PCR) and sequenced directly by an automated sequencer. Of the 16 patients receiving lamivudine, mutant viruses with mutations in the YMDD motif were found in 3 of 8 patients treated with lamivudine for 52 weeks. However, this mutation was not found in any of the 8 patients treated for 32 weeks or a shorter period. Mutant viruses appeared after 40 weeks of treatment and were undetectable within 12 weeks after the cessation of the treatment. Such mutant viruses were not detected in any of the 10 untreated patients. This study confirms the emergence of YMDD mutant viruses during long-term lamivudine treatment in immunocompetent type B hepatitis patients. The results from this study suggest the need for combination therapies to reduce the levels of such mutant viruses in some patients.
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PMID:Emergence of YMDD motif mutants of hepatitis B virus during lamivudine treatment of immunocompetent type B hepatitis patients. 1056 56

Chronic hepatitis B virus (HBV) infection is a leading cause of cirrhosis and hepatocellular carcinoma worldwide. Its prevalence approaches 10% in hyperendemic areas, such as southeast Asia, China, and Africa. Although chronic HBV infection is seen less frequently in North America and Europe, an estimated 1.25 million persons in the United States are infected. In the past decade, revolutionary strides have been made toward the treatment of chronic HBV infection. Interferon-alpha was once the only available therapy but has recently been joined by the nucleoside analogues, the most extensively studied of which is lamivudine. Interferon therapy continues to have a role in the treatment of a carefully selected group of patients. Lamivudine therapy, which has less stringent selection criteria, suppresses HBV DNA in almost all treated patients: Seventeen percent to 33% experience loss of hepatitis B e antigen, and 53% to 56% have a histologic response. Extended lamivudine treatment leads to the development of a specific lamivudine-resistant virus with base-pair substitutions at the YMDD locus of the DNA polymerase. Newer nucleoside analogues and other immunomodulator therapies are being investigated. In the future, combination therapy with different classes of agents may yield improved response rates and delay the development of resistance.
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PMID:Chronic hepatitis B virus infection: treatment strategies for the next millennium. 1078 66

Lamivudine has been shown to be an effective therapy for chronic hepatitis B, but resistance to this nucleoside agent is common after prolonged use. Five patients with chronic hepatitis B virus (HBV) infection developed resistance to lamivudine after 9 to 19 months of treatment. In 4 patients this occurred after liver transplantation and the remaining individual had stable cirrhosis. In each case, resistance was confirmed to be caused by one or more mutations in the HBV-DNA polymerase gene and was associated with active underlying liver disease. The patients were treated with adefovir dipivoxil in a dose of 5 to 30 mg daily. Two to 4 log(10) reductions in HBV-DNA levels were observed in 4 cases, and the fifth patient became negative by quantitative polymerase chain reaction (PCR) after retransplantation in conjunction with hepatitis B immunoglobulin (HBIg). Virologic improvement was associated with stable or declining serum alanine transaminase levels in 4 patients. HBV-DNA suppression has been sustained during a mean treatment period of 13 months (range 11 to 15 months), including 1 patient in whom lamivudine has been discontinued. Mild changes in renal function were observed during treatment in most cases but did not require early discontinuation of the drug. This study provides evidence that adefovir dipivoxil can be an effective treatment for lamivudine-resistant HBV mutants as well as wild-type HBV.
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PMID:Adefovir dipivoxil for the treatment of lamivudine-resistant hepatitis B mutants. 1086

A series of unnatural L-nucleosides such as 3TC, FTC and L-FMAU have been found to be potent antiviral agents. The mode of action of L-nucleosides has been found to be similar to that of D-nucleosides as antiviral agents, despite their unnatural stereochemistry, that is, nucleotide formation by kinases followed by interaction with the reverse transcriptase (RT) of HIV or DNA polymerase. To date, the mode of action of nucleoside inhibitors at the molecular level with respect to the active conformations of the 5'-triphosphates as well as the interaction with the RT is not known. Recently, the X-ray crystal structure of the RT-DNA-dTTP catalytic complex has been reported. Computer modeling has been performed for several pairs of D- and L-nucleoside inhibitors using the HIV-1 RT model and crystal coordinate data from a subset of the protein surrounding the deoxynucleoside triphosphate (dNTP) binding pocket region. Results from our modeling studies of D-/L-zidovudine, D-/L-3TC, D-/L-dideoxycytosine triphosphates, dTTP and dCTP show that their binding energies correlate with the reported 50% effective concentrations. Modeling results are also discussed with respect to favorable conformations of each inhibitor at the dNTP site in the polymerization process. Additionally, the clinically important M184V mutation, which confers resistance against 3TC and FTC, was studied with our modeling system. The binding energy patterns of nucleoside inhibitors at the M184V mutation site correlate with the reported antiviral data.
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PMID:Molecular modeling approach to understanding the mode of action of L-nucleosides as antiviral agents. 1112 Sep 56

Seventy-seven liver transplant candidates were enrolled in a multicenter study in which patients were treated with lamivudine (100 mg daily) without the adjunctive use of hepatitis B immune globulin. Treatment was begun while patients awaited liver transplantation and continued after transplantation. All were hepatitis B surface antigen (HBsAg) positive, and 61% had detectable hepatitis B e antigen (HBeAg) and HBV DNA when treatment was begun. Forty-seven underwent liver transplantation and 30 did not. Median study participation was 38 months (range, 2.7-48.5) in the transplanted patients and 26 months (range, 0.1-37) in the nontransplanted group. Twenty-five of 42 (60%) transplanted patients with 12 or more weeks of posttransplantation follow-up were HBsAg negative at the last study visit. At treatment week 156, 13 of 22 (59%) remained HBsAg negative, and all 9 reinfected patients were HBV-DNA positive before treatment. In the nontransplanted patients, HBeAg was initially detectable in 20 of 27 (74%) but this decreased to 3 of 17 (18%) after 104 weeks of treatment, and significant improvement in biochemical parameters was observed. HBV-DNA polymerase mutants were detected in 15 (21%) and 6 (20%) of the transplanted and nontransplanted patients, respectively. When compared with historical cohorts, lamivudine-treated patients appeared to have improved survival, and transplanted patients had a decrease in the rate of recurrent HBV infection. Lamivudine therapy was partially effective in preventing recurrent HBV infection when given before and after transplantation. Thus, future trials using a combination of HBIg and lamivudine are needed to assess the optimal prophylactic therapy.
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PMID:A multicenter United States-Canadian trial to assess lamivudine monotherapy before and after liver transplantation for chronic hepatitis B. 1117 45

Patients treated with nucleoside analogue reverse transcriptase inhibitors (NRTIs) develop a varying degree of myopathy or neuropathy after long-term therapy. Zidovudine (AZT) causes myopathy; zalcitabine (ddC), didanosine (ddl) and lamuvidine (3TC) cause neuropathy; stavudine (d4T) and fialuridine (FIAU) cause neuropathy or myopathy and lactic acidosis. The tissue distribution of phosphorylases responsible for phosphorylation of NRTIs relates to their selective tissue toxicity. The myopathy is characterized by muscle wasting, myalgia, fatigue, weakness and elevation of CK. The neuropathy is painful, sensory and axonal. In vitro, NRTIs inhibit the gamma-DNA polymerase, responsible for replication of mtDNA, and cause mtDNA dysfunction. In vivo, patients treated with AZT, the best studied NRTI, develop a mitochondrial myopathy with mtDNA depletion, deficiency of COX (complex IV), intracellular fat accumulation, high lactate production and marked phosphocreatine depletion, as determined with in vivo MRS spectroscopy, due to impaired oxidative phosphorylation. Animals or cultured cells treated with NRTIs develop neuropathy, myopathy, or cell destruction with similar changes in the mitochondria. There is evidence that the NRTI-related neuropathy is also due to mitochondrial toxicity. The NRTIs (AZT, ddC, ddl, d4T, 3TC) contain azido groups that compete with natural thymidine triphosphate as substrates of DNA pol-gamma and terminate mtDNA synthesis. In contrast, FIAU that contains 3'-OH groups serves as an alternate substrate for thymidine triphosphate with DNA pol-gamma and is incorporated into the DNA causing permanent mtDNA dysfunction. The NRTI-induced mitochondrial dysfunction has an influence on the clinical application of these agents, especially at high doses and when combined. They have produced in humans a new category of acquired mitochondrial toxins that cause clinical manifestations resembling the genetic mitochondrial disorders.
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PMID:Peripheral neuropathy and antiretroviral drugs. 1129 2

Hepatitis B virus (HBV) reactivation, a well-known complication in immunosuppressed patients, can give rise to acute hepatitis and even fatal fulminant hepatitis. Three Japanese males with non-Hodgkin's lymphoma (NHL) who were carriers of HBV received high-dose chemotherapy followed by autologous peripheral blood stem cell transplantation (PBSCT). To prevent HBV reactivation, all received oral lamivudine (150 mg/day), a nucleoside analogue, at the start of chemotherapy. All were treated at full-dose intensity, including corticosteroids, without modification of treatment regimens. All three patients completed the total course of chemotherapy and PBSCT, with no signs of HBV reactivation. Peripheral blood stem cell (PBSC) harvests and hematological recoveries after transplantation were not affected by lamivudine, which was continued for at least 16 weeks after transplantation. HBV-DNA and DNA polymerase levels remained negative/normal after discontinuation of lamivudine. Lamivudine effectively inhibits HBV replication and has few serious adverse effects, particularly those related to hematopoiesis. Thus, prophylactic use of lamivudine from initiation of chemotherapy deserves consideration in the treatment of HBV carriers who require immunosuppressive chemotherapy, and may prevent HBV reactivation.
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PMID:A possible role for lamivudine as prophylaxis against hepatitis B reactivation in carriers of hepatitis B who undergo chemotherapy and autologous peripheral blood stem cell transplantation for non-Hodgkin's lymphoma. 1131 73

Mitochondrial toxicity can result from antiviral nucleotide analog therapy used to control human immunodeficiency virus type 1 infection. We evaluated the ability of such analogs to inhibit DNA synthesis by the human mitochondrial DNA polymerase (pol gamma) by comparing the insertion and exonucleolytic removal of six antiviral nucleotide analogs. Apparent steady-state K(m) and k(cat) values for insertion of 2',3'-dideoxy-TTP (ddTTP), 3'-azido-TTP (AZT-TP), 2',3'-dideoxy-CTP (ddCTP), 2',3'-didehydro-TTP (D4T-TP), (-)-2',3'-dideoxy-3'-thiacytidine (3TC-TP), and carbocyclic 2',3'-didehydro-ddGTP (CBV-TP) indicated incorporation of all six analogs, albeit with varying efficiencies. Dideoxynucleotides and D4T-TP were utilized by pol gamma in vitro as efficiently as natural deoxynucleotides, whereas AZT-TP, 3TC-TP, and CBV-TP were only moderate inhibitors of DNA chain elongation. Inefficient excision of dideoxynucleotides, D4T, AZT, and CBV from DNA predicts persistence in vivo following successful incorporation. In contrast, removal of 3'-terminal 3TC residues was 50% as efficient as natural 3' termini. Finally, we observed inhibition of exonuclease activity by concentrations of AZT-monophosphate known to occur in cells. Thus, although their greatest inhibitory effects are through incorporation and chain termination, persistence of these analogs in DNA and inhibition of exonucleolytic proofreading may also contribute to mitochondrial toxicity.
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PMID:Differential incorporation and removal of antiviral deoxynucleotides by human DNA polymerase gamma. 1131 28

Several of the nucleoside analogs used in the treatment of AIDS exhibit a delayed clinical toxicity limiting their usefulness. The toxicity of nucleoside analogs may be related to their effects on the human mitochondrial DNA polymerase (Pol gamma), the polymerase responsible for mitochondrial DNA replication. Among the AIDS drugs approved by the FDA for clinical use, two are modified cytosine analogs, Zalcitabine (2',3'-dideoxycytidine (ddC)) and Lamivudine (beta-d-(+)-2',3'-dideoxy-3'-thiacytidine ((-)3TC])). (-)3TC is the only analog containing an unnatural l(-) nucleoside configuration and is well tolerated by patients even after long term administration. In cell culture (-)3TC is less toxic than its d(+) isomer, (+)3TC, containing the natural nucleoside configuration, and both are considerably less toxic than ddC. We have investigated the mechanistic basis for the differential toxicity of these three cytosine analogs by comparing the effects of dideoxy-CTP), (+)3TC-triphosphate (TP), and (-)3TC-TP on the polymerase and exonuclease activities of recombinant human Pol gamma. This analysis reveals that Pol gamma incorporates (-)3TC-triphosphate 16-fold less efficiently than the corresponding (+)isomer and 1140-fold less efficiently than dideoxy-CTP, showing a good correlation between incorporation rate and toxicity. The rates of excision of the incorporated analogs from the chain-terminated 3'-end of the DNA primer by the 3'-5'-exonuclease activity of Pol gamma were similar (0.01 s(-)1) for both 3TC analogs. In marked contrast, the rate of exonuclease removal of a ddC chain-terminated DNA occurs at least 2 orders of magnitude slower, suggesting that the failure of the exonuclease to remove ddC may play a major role in its greater toxicity. This study demonstrates that direct analysis of the mitochondrial DNA polymerase structure/function relationships may provide valuable insights leading to the design of less toxic inhibitors.
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PMID:Insights into the molecular mechanism of mitochondrial toxicity by AIDS drugs. 1132 13


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