Neil Parkin

Tufts University, Georgia, United States

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Publications (97)586.94 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Hepatitis C virus (HCV) drug development has resulted in treatment regimens composed of interferon-free, all-oral combinations of direct-acting antivirals (DAAs). While the new regimens are potent and highly efficacious, the full clinical impact of HCV drug resistance, its implications for re-treatment, and the potential role of baseline resistance testing, remain critical research and clinical questions. In this report, we discuss the viral proteins targeted by HCV DAAs, and summarize clinically-relevant resistance data for compounds that have been approved or are currently in phase 3 clinical trials. The primary goal of this report is to provide a comprehensive, systematic review of all resistance information available from sponsors' trials as a tool to inform the HCV drug development field. This article is protected by copyright. All rights reserved. © 2015 by the American Association for the Study of Liver Diseases.
    Hepatology 06/2015; DOI:10.1002/hep.27934 · 11.19 Impact Factor
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    ABSTRACT: Background. Sofosbuvir is a chain-terminating nucleotide analogue inhibitor of the hepatitis C virus (HCV) NS5B RNA polymerase that is efficacious in subjects with HCV genotype 1-6 infection. Sofosbuvir resistance is primarily conferred by the S282T substitution in NS5B. Methods. NS5B sequencing and susceptibility testing of HCV from subjects infected with genotypes 1-6 who participated in phase 2 and 3 sofosbuvir clinical trials was performed. Results. No NS5B variants present at baseline among 1645 sofosbuvir-treated subjects were associated with treatment failure; sofosbuvir susceptibility was within 2-fold of reference. Among 282 subjects who did not achieve sustained virologic response, no novel sofosbuvir resistance-associated variants were identified, and the NS5B changes observed did not confer significant reductions in sofosbuvir susceptibility. In 1 subject with S282T observed at relapse 4 weeks after sofosbuvir monotherapy, the resistant variant (13.5-fold reduced sofosbuvir susceptibility, replication capacity <2% of control) became undetectable by deep sequencing 12 weeks after treatment. L159F and V321A were identified as treatment-emergent variants but did not confer resistance to sofosbuvir in the replicon system. Conclusions. These data demonstrate a uniform susceptibility of subject-derived HCV to sofosbuvir, and also show that selection of sofosbuvir-resistant HCV is exceedingly rare and is associated with a significant reduction in viral fitness.
    Clinical Infectious Diseases 09/2014; 59(12). DOI:10.1093/cid/ciu697 · 9.42 Impact Factor
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    ABSTRACT: The nonnucleoside reverse transcriptase (RT) inhibitor rilpivirine (RPV) has been co-formulated with emtricitabine and tenofovir disoproxil fumarate for initial therapy of HIV-1-infected individuals. RPV, formulated as a long-acting nanosuspension, will also be assessed for its ability to prevent HIV-1 infection in resource limited settings. In this study, we determined whether any pre-existing genetic differences occurred among different HIV-1 subtypes at residues in RT associated with decreased virologic response to RPV. We found that the E138A substitution occurs more frequently in subtype C (range: 5.9-7.5%) than B (range: 0-2.3%) sequences from both treatment-naïve and -experienced individuals (p<0.01) in 4 independent genotype databases. In one of the databases (Stanford University), E138K and E138Q were also more common in RTI-experienced subtype C sequences (1.0% and 1.1%, respectively) than in subtype B sequences (0.3% and 0.6%, respectively). E138A/K/Q in subtype C decreased RPV susceptibility 2.9-, 5.8-, and 5.4-fold, respectively. Taken together, these data suggest that E138A could impact treatment or prevention strategies that include RPV in geographic areas where subtype C infection is prevalent.
    Antiviral research 04/2014; 107. DOI:10.1016/j.antiviral.2014.04.001 · 3.94 Impact Factor
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    ABSTRACT: The Abbott RealTime (RT) HCV assay targets the 5'-UTR of the HCV genome. Here we analyzed the sequence variability of the assay target region from 1092 specimens. Thermodynamic modeling of the percentage primer/probe bound at the assay annealing temperature was performed to assess the potential effect of sequence variability. Analysis of this large data set revealed that the primer and probe binding sites of the RealTime HCV viral load assay are highly conserved and that naturally-occurring sequences polymorphisms would not be expected to discernibly impact assay performance.
    Journal of clinical microbiology 01/2014; 52(4). DOI:10.1128/JCM.02661-13 · 4.23 Impact Factor
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    ABSTRACT: The Abbott RealTime (ART) HIV-1 assay targets the integrase region and is designed to tolerate mismatches. Variability in integrase sequences comprising the assay target regions from>1000 clinical specimens submitted for phenotypic and genotypic raltegravir resistance testing were analyzed. In this large collection of sequences from clinical specimens, the number and location of raltegravir resistance associated mutations did not differ from those tested previously and shown not to result in under-estimation of viral loads.
    Journal of virological methods 07/2013; 193(2). DOI:10.1016/j.jviromet.2013.07.028 · 1.88 Impact Factor
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    ABSTRACT: The Abbott RealTime (RT) HBV assay targets the N-terminal region of the S-gene. Here we analyzed the sequence variability of the assay target region from >2100 clinical specimens. Thermodynamic modeling of the percentage primer/probe bound at the assay annealing temperature was performed to assess the potential effect of sequence variability.
    Journal of clinical microbiology 01/2013; 51(4). DOI:10.1128/JCM.03003-12 · 4.23 Impact Factor
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    ABSTRACT: Durable suppression of HIV-1 replication requires the establishment of antiretroviral drug concentrations that exceed the susceptibility of the virus strain(s) infecting the patient. Minimum plasma drug concentrations (Ctrough) are correlated with response, but determination of target Ctrough values is hindered by a paucity of in vivo concentration-response data. In the absence of these data, in vitro susceptibility measurements, adjusted for serum protein binding, can provide estimations of suppressive in vivo drug concentrations. We derived serum protein binding correction factors (PBCF) for protease inhibitors, nonnucleoside reverse transcriptase inhibitors, and an integrase inhibitor by measuring the effect of a range of human serum concentrations on in vitro drug susceptibility measured with the PhenoSense HIV assay. PBCFs corresponding to 100% HS were extrapolated using linear regression and ranged from 1.4 for nevirapine to 77 for nelfinavir. Using the mean 95% inhibitory concentration (IC95) for ≥1,200 drug-susceptible viruses, we calculated protein-bound IC95 (PBIC95) values. PBIC95 values were concordant with the minimum effective Ctrough values that were established in well-designed pharmacodynamic studies (e.g., indinavir, saquinavir, and amprenavir). In other cases, the PBIC95 values were notably lower (e.g., darunavir, efavirenz, and nevirapine) or higher (nelfinavir and etravirine) than existing target recommendations. The establishment of PBIC95 values as described here provides a convenient and standardized approach for estimation of the minimum drug exposure that is required to maintain viral suppression and prevent the emergence of drug-resistant variants, particularly when in vivo concentration-response relationships are lacking.
    Antimicrobial Agents and Chemotherapy 09/2012; 56(11):5938-45. DOI:10.1128/AAC.00691-12 · 4.45 Impact Factor
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    Neil Parkin · James Bremer · Silvia Bertagnolio
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    ABSTRACT: The World Health Organization (WHO) has developed a global laboratory network to support human immunodeficiency virus drug resistance genotyping for public health surveillance in resource-limited countries. Blinded proficiency panels are an essential part of a genotyping quality-assurance program and are used to monitor the reliability of genotyping data in the WHO laboratory network. Laboratories in Europe, North America, Asia, Africa, and the Caribbean have tested panels annually since 2007; 103 of 131 submissions (79%) had >99% nucleotide sequence identity and resistance mutation concordance, compared with consensus. Most errors were associated with mixtures in the test specimen, leading to subjectivity in base-calling or amplification bias. Overall, genotyping assays used by the WHO laboratory network are reliable.
    Clinical Infectious Diseases 05/2012; 54 Suppl 4(Suppl 4):S266-72. DOI:10.1093/cid/cir992 · 9.42 Impact Factor
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    ABSTRACT: In resource-limited settings, there is increased demand for human immunodeficiency virus type 1 drug resistance testing. Because preservation of plasma specimens is often not feasible in resource-limited settings, use of dried blood spots (DBSs) is being adopted. We used 2 panels of DBSs for genotyping assay validation and proficiency testing in selected laboratories in the World Health Organization laboratory network in 14 countries. An amplification sensitivity of 1000 copies/mL was achieved by 2 laboratories. Reproducibility and accuracy of nucleotide sequence determination and resistance-associated mutation identification from DBSs was similar to that previously determined for plasma. International shipping at ambient temperature had no significant effect on amplification success. These studies indicate that DBS-based genotyping is equally reproducible and reliable, although slightly less sensitive, compared with plasma.
    Clinical Infectious Diseases 05/2012; 54 Suppl 4(Suppl 4):S273-9. DOI:10.1093/cid/cir982 · 9.42 Impact Factor
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    ABSTRACT: In resource-limited settings, there is increased demand for human immunodeficiency virus type 1 drug resistance testing. Because preservation of plasma specimens is often not feasible in resource-limited settings, use of dried blood spots (DBSs) is being adopted. We used 2 panels of DBSs for genotyping assay validation and proficiency testing in selected laboratories in the World Health Organization laboratory network in 14 countries. An amplification sensitivity of 1000 copies/mL was achieved by 2 laboratories. Reproducibility and accuracy of nucleotide sequence determination and resistance-associated mutation identification from DBSs was similar to that previously determined for plasma. International shipping at ambient temperature had no significant effect on amplification success. These studies indicate that DBS-based genotyping is equally reproducible and reliable, although slightly less sensitive, compared with plasma. Development and transmission of drug-resistant human immunodeficiency virus (HIV) in resource-limited settings is a potential negative consequence of the international effort to provide antiretroviral treat-ment to millions of persons living with HIV infection [1]. HIV drug resistance (HIVDR) is associated with increased risk of therapeutic failures; transmission of drug-resistant virus; and decreased therapeutic options, treatment program effectiveness, and survival. The World Health Organization (WHO) global strategy for prevention and assessment of HIVDR [2] is a coordinated effort to assess HIVDR worldwide. The laboratory testing component of this strategy promotes
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    ABSTRACT: Dried blood spots (DBS) may be a promising alternative specimen type to plasma for measuring the viral load (VL) in HIV-infected individuals in resource-limited settings. However, characterization of assay performance using DBS is incomplete. In this prospective study, the VL was measured in parallel using plasma and DBS specimens collected at the same time from 157 HIV-1-infected individuals. DBS were prepared by dispensing 50 μl of blood onto filter paper cards and were stored desiccated at -20°C. Nucleic acid extraction from plasma and DBS was performed automatically using the Abbott m2000sp instrument, and the VL was measured using the RealTime HIV-1 VL assay, which has a lower limit of detection of 40 HIV RNA copies/ml. The correlation between plasma and DBS results was good (R = 0.91; P < 0.001). The mean difference in the VL (DBS minus plasma) was 0.35 log copies (standard deviation [SD], 0.47 log copies). A total of 40 (26%) paired specimens had a difference of >0.5 log copy, and in 12 (7.8%) it was >1 log copy. the VL from DBS was measurable in 95.7% of specimens with a plasma VL of >2.74 log copies (550 HIV RNA copies/ml). In summary, the VL can reliably be measured using DBS with the Abbott RealTime HIV-1 assay. The estimated lower limit of detection of this automated methodology on DBS is 550 copies/ml, a threshold that may be acceptable for periodic VL monitoring in patients on antiretroviral therapy in resource-limited settings, where early detection of virologic treatment failure is often problematic.
    Journal of clinical microbiology 12/2011; 50(3):569-72. DOI:10.1128/JCM.00418-11 · 4.23 Impact Factor
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    ABSTRACT: Resistance-associated mutations in the HIV-1 protease modify viral fitness through changes in the catalytic activity and altered binding affinity for substrates and inhibitors. In this report, we examine the effects of 31 mutations at 26 amino acid positions in protease to determine their impact on infectivity and protease inhibitor sensitivity. We found that primary resistance mutations individually decrease fitness and generally increase sensitivity to protease inhibitors, indicating that reduced virion-associated protease activity reduces virion infectivity and the reduced level of per virion protease activity is then more easily titrated by a protease inhibitor. Conversely, mutations at more variable positions (compensatory mutations) confer low-level decreases in sensitivity to all protease inhibitors with little effect on infectivity. We found significant differences in the observed effect on infectivity with a pseudotype virus assay that requires the protease to cleave the cytoplasmic tail of the amphotropic murine leukemia virus (MuLV) Env protein. Additionally, we were able to mimic the fitness loss associated with resistance mutations by directly reducing the level of virion-associated protease activity. Virions containing 50% of a D25A mutant protease were 3- to 5-fold more sensitive to protease inhibitors. This level of reduction in protease activity also resulted in a 2-fold increase in sensitivity to nonnucleoside inhibitors of reverse transcriptase and a similar increase in sensitivity to zidovudine (AZT), indicating a pleiotropic effect associated with reduced protease activity. These results highlight the interplay between enzyme activity, viral fitness, and inhibitor mechanism and sensitivity in the closed system of the viral replication complex.
    Antimicrobial Agents and Chemotherapy 11/2011; 56(2):623-33. DOI:10.1128/AAC.05549-11 · 4.45 Impact Factor
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    ABSTRACT: Access to antiretroviral therapy is increasing globally and drug resistance evolution is anticipated. Currently, protease (PR) and reverse transcriptase (RT) sequence generation is increasing, including the use of in-house sequencing assays, and quality assessment prior to sequence analysis is essential. We created a computational HIV PR/RT Sequence Quality Analysis Tool (SQUAT) that runs in the R statistical environment. Sequence quality thresholds are calculated from a large dataset (46,802 PR and 44,432 RT sequences) from the published literature ( http://hivdb.Stanford.edu ). Nucleic acid sequences are read into SQUAT, identified, aligned, and translated. Nucleic acid sequences are flagged if with >five 1-2-base insertions; >one 3-base insertion; >one deletion; >six PR or >18 RT ambiguous bases; >three consecutive PR or >four RT nucleic acid mutations; >zero stop codons; >three PR or >six RT ambiguous amino acids; >three consecutive PR or >four RT amino acid mutations; >zero unique amino acids; or <0.5% or >15% genetic distance from another submitted sequence. Thresholds are user modifiable. SQUAT output includes a summary report with detailed comments for troubleshooting of flagged sequences, histograms of pairwise genetic distances, neighbor joining phylogenetic trees, and aligned nucleic and amino acid sequences. SQUAT is a stand-alone, free, web-independent tool to ensure use of high-quality HIV PR/RT sequences in interpretation and reporting of drug resistance, while increasing awareness and expertise and facilitating troubleshooting of potentially problematic sequences.
    AIDS research and human retroviruses 09/2011; 28(8):894-901. DOI:10.1089/AID.2011.0120 · 2.46 Impact Factor
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    ABSTRACT: GS-8374 is a novel bis-tetrahydrofuran HIV-1 protease (PR) inhibitor (PI) with a unique diethylphosphonate moiety. It was selected from a series of analogs containing various di(alkyl)phosphonate substitutions connected via a linker to the para position of a P-1 phenyl ring. GS-8374 inhibits HIV-1 PR with high potency (K(i) = 8.1 pM) and with no known effect on host proteases. Kinetic and thermodynamic analysis of GS-8374 binding to PR demonstrated an extremely slow off rate for the inhibitor and favorable contributions of both the enthalpic and entropic components to the total free binding energy. GS-8374 showed potent antiretroviral activity in T-cell lines, primary CD4(+) T cells (50% effective concentration [EC(50)] = 3.4 to 11.5 nM), and macrophages (EC(50) = 25.5 nM) and exhibited low cytotoxicity in multiple human cell types. The antiviral potency of GS-8374 was only moderately affected by human serum protein binding, and its combination with multiple approved antiretrovirals showed synergistic effects. When it was tested in a PhenoSense assay against a panel of 24 patient-derived viruses with high-level PI resistance, GS-8374 showed lower mean EC(50)s and lower fold resistance than any of the clinically approved PIs. Similar to other PIs, in vitro hepatic microsomal metabolism of GS-8374 was efficiently blocked by ritonavir, suggesting a potential for effective pharmacokinetic boosting in vivo. In summary, results from this broad in vitro pharmacological profiling indicate that GS-8374 is a promising candidate to be further assessed as a new antiretroviral agent with potential for clinical efficacy in both treatment-naïve and -experienced patients.
    Antimicrobial Agents and Chemotherapy 03/2011; 55(4):1366-76. DOI:10.1128/AAC.01183-10 · 4.45 Impact Factor
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    ABSTRACT: The Abbott RealTime HIV-1 viral load assay uses primers and probes targeted to integrase, which is also the target of integrase inhibitors such as raltegravir. Viral loads of 42 raltegravir-susceptible and 40 raltegravir-resistant specimens were determined using RealTime HIV-1 and Roche Monitor (v1.5). The differences in viral load measurements between assays were comparable in the two groups, demonstrating that the RealTime HIV-1 assay can tolerate raltegravir-selected mutations.
    Journal of clinical microbiology 02/2011; 49(4):1631-4. DOI:10.1128/JCM.02253-10 · 4.23 Impact Factor
  • Gastroenterology 01/2011; 140(3):755-60. DOI:10.1053/j.gastro.2011.01.029 · 13.93 Impact Factor
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    ABSTRACT: Patterns of HIV-1 protease inhibitor (PI) resistance-associated mutations (RAMs) and effects on PI susceptibility associated with the L76V mutation were studied in a large database. Of 20,501 sequences with ≥1 PI RAM, 3.2% contained L76V; L76V was alone in 0.04%. Common partner mutations included M46I, I54V, V82A, I84V, and L90M. L76V was associated with a 2- to 6-fold decrease in susceptibility to lopinavir, darunavir, amprenavir, and indinavir and a 7- to 8-fold increase in susceptibility to atazanavir and saquinavir.
    Antimicrobial Agents and Chemotherapy 11/2010; 54(11):4903-6. DOI:10.1128/AAC.00906-10 · 4.45 Impact Factor
  • Journal of Hepatology 04/2010; 52. DOI:10.1016/S0168-8278(10)60768-4 · 10.40 Impact Factor
  • Clinical Infectious Diseases 01/2010; · 9.42 Impact Factor
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    ABSTRACT: HIV-1 drug resistance genotyping is an essential component of the World Health Organization global HIV Drug Resistance (HIVDR) prevention and assessment strategy. Plasma is considered to be the most appropriate specimen type for HIV-1 drug resistance genotyping. However, use of plasma may not be feasible in rural, remote areas in resource-limited settings since its preparation and storage requires personnel and laboratory infrastructure that is often lacking. An alternative specimen type for HIVDR genotyping is dried blood spots (DBS). DBS can be made from blood drawn for routine clinical or surveillance purposes without special laboratory processing. The filter paper used is relatively inexpensive, easily obtained and stored, and although procedures for making DBS must be followed precisely, the training required is less intensive than that required for plasma separation. HIV nucleic acids are generally stable over long periods of time and freezing is not required unless storage over two weeks is planned. In addition, DBS are more easily transported than plasma because they can be shipped as non-hazardous materials using regular mail or courier services. Many studies have reported the successful genotyping of HIV-1 from DBS and some have shown a high genotypic concordance with plasma genotypes despite potential DNA interferences. During the past few years DBS have started to be widely used for HIV-1 drug resistance testing, and an increased number of reports from resource-limited areas have indicated DBS as the preferred specimen type for transmitted HIV-1 drug resistance surveillance where plasma collection is not feasible. The World Health Organization has brought together a group of experts (WHO HIVResNet DBS working group) to review current data on DBS preparation, storage, and transport conditions, and provide a reference protocol, which is also summarized in this article.
    AIDS reviews 11/2009; 12(4):195-208. · 4.02 Impact Factor

Publication Stats

4k Citations
586.94 Total Impact Points

Institutions

  • 2014
    • Tufts University
      • Department of Public Health and Community Medicine
      Georgia, United States
  • 2012
    • Emory University
      Atlanta, Georgia, United States
  • 2011–2012
    • World Health Organization WHO
      Genève, Geneva, Switzerland
  • 2006–2009
    • Monogram Biosciences
      San Francisco, California, United States
  • 2006–2007
    • Johns Hopkins University
      • Department of Pathology
      Baltimore, MD, United States
  • 2005
    • CSU Mentor
      Long Beach, California, United States
    • University of North Carolina at Chapel Hill
      • Department of Biochemistry and Biophysics
      Chapel Hill, NC, United States
    • Johns Hopkins Medicine
      • Department of Pathology
      Baltimore, MD, United States
  • 2003
    • Bristol-Myers Squibb
      New York City, New York, United States
  • 2002
    • University of California, Berkeley
      • Department of Chemistry
      Berkeley, California, United States
  • 2000
    • University of California, San Francisco
      San Francisco, California, United States