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Biomedical subjects

R T Walker

Publications and source records attributed to R T Walker.

At least 19 recordsLinked to original sources
PubMed2001

Amputations: no longer the end of the road.

BACKGROUND: The aim of this prospective study was to assess the effect of an intensive in-patient rehabilitation programme upon the mobility of amputees. METHODS: All major lower limb amputations between 1997 and 1999 received a pre-operative mobility assessment and, where appropriate, were referred for a vigorous rehabilitation programme. RESULTS: 92 lower limb amputations were performed in 87 patients (57 below knee, 33 above knee, two hip disarticulations). Overall, 63% of patients were able to ambulate independently following rehabilitation. Univariate analysis revealed that the only predictor of mobility was the level of amputation, below knee gaining better mobility than above knee (p=0.002). CONCLUSION: Lower limb amputees should participate in an aggressive in-patient physiotherapy regimen since reasonable mobility can be achieved in the majority of patients.

Adult↗
PubMed2000

Anti-(herpes simplex virus) activity of 4'-thio-2'-deoxyuridines: a biochemical investigation for viral and cellular target enzymes.

The antiviral activity of several nucleoside analogues is often limited by their rapid degradation by pyrimidine nucleoside phosphorylases. In an attempt to avoid this degradation, several modified nucleosides have been synthesized. A series of 4'-thio-2'-deoxyuridines exhibits an anti-[herpes simplex virus (HSV)] activity significantly higher (20-600 times) than that shown by the corresponding 4'-oxy counterpart. We investigated the mode of action of these compounds and we found that: (i) several 4'-thio-2'-deoxyuridines are phosphorylated to the mono- and di-phosphates by HSV-1 thymidine kinase (TK) more efficiently than their corresponding 4'-oxy counterpart; (ii) both are inhibitors of cellular thymidylate synthase; (iii) 4'-thio-2'-deoxyuridines are resistant to phosphorolysis by human thymidine phosphorylase; (iv) both 4'-oxy- and 4'-thio-2'-deoxyuridines are phosphorylated to deoxyribonucleotide triphosphate in HSV-1-infected cells and are incorporated into viral DNA; (v) 4'-thio-2'-deoxyuridines are better inhibitors than their 4'-oxy counterparts of [(3)H]thymidine incorporation in HSV-1-infected cells; (vi) 4'-thio-2'-deoxyuridines are not recognized by HSV-1 and human uracil-DNA glycosylases. Our data suggest that 4'-thio-2'-deoxyuridines, resistant to pyrimidine phosphorylase, can be preferentially or selectively phosphorylated by viral TK in HSV-infected cells, where they are further converted into triphosphate by cellular nucleotide kinases. Once incorporated into viral DNA, they are better inhibitors of viral DNA synthesis than their corresponding 4'-oxy counterpart, either because they are not recognized, and thus not removed, by viral uracil-DNA glycosylase, or because they preferentially interfere with viral DNA polymerase.

Antiviral Agents↗
PubMed2000

5-Methyl-2'-deoxy-4'-thio-alpha-uridine (R)-S-oxide.

The pyrimidine ring of the title compound, C(10)H(14)N(2)O(5)S, is planar to within 0.024 (1) A and makes an angle of 75.46 (10) degrees with the mean plane of the thiosugar ring. In terms of standard nucleoside nomenclature, this ring has the C3'-endo conformation. The O5'-C5'-C4'-C3' torsion angle is 166.5 (3) degrees and the glycosidic torsion angle S4'-C1'-N1-C2 is -52.1 (2) degrees (syn).

Antiviral Agents↗
PubMed1998

Trichomonas vaginalis thymidine kinase: purification, characterization and search for inhibitors.

We report that a thymidine kinase (TK) activity is present in Trichomonas vaginalis and can be separated from the deoxyribonucleoside phosphotransferase. T. vaginalis TK, purified 11200-fold to apparent homogeneity, has a molecular mass of 31500 Da. It phosphorylates not only thymidine (Km 0.18 microM) but also deoxycytidine (Km 0.88 microM) and deoxyuridine (Km 0.14 microM). In contrast with T. vaginalis deoxyribonucleoside phosphotransferase, the TK activity is strongly inhibited by novel deoxyuridine analogues such as 5-methyl-4'-thio-2'-deoxyuridine (MTdU) (Ki 20 nM) and 5-iodo-4'-thio-2'-deoxyuridine (ITdU) (Ki 24 nM). MTdU and ITdU are phosphorylated by T. vaginalis TK in vitro. In vivo they inhibit [3H]thymidine incorporation in T. vaginalis cultured cells and T. vaginalis growth (IC50 7.5 and 24 microM respectively; minimal lethal dose 100 microM). Thus the TK inhibitors described here demonstrate the key role of T. vaginalis TK for protozoal growth and viability and indicate TK as a new target for the design of antitrichomonal drugs.

Animals↗
PubMed1998

Sealing capacity in vitro of thermoplasticized gutta-percha with a solid core endodontic filling technique.

This study assessed the sealing capacity of two endodontic gutta-percha filling techniques. Thirty-four single-rooted fully developed teeth were endodontically accessed, instrumented and randomly divided into two experimental groups (n = 12) and two control groups (n = 5). In Group A, root canals were obturated using a solid core thermoplastic technique (Densfil), in Group B and Group C (negative control) canals were obturated with laterally condensed gutta-percha, and in Group D (positive control) canals were left unobturated. AH-26 was used as the sealer. Two days later, the teeth were conventionally prepared for testing apical and coronal leakage, immersed in india ink for 5 days and subsequently cleared. The linear coronal and apical extent of dye penetration was measured under a light dissecting microscope. The mean apical leakage for Group A was 1.39 mm, and for Group B 2.76 mm, whereas the mean coronal leakage for Group A was 2.87 mm, and for Group B 4.03 mm. The differences between the groups were not statistically significant (P > 0.05).

Bismuth↗
PubMed1998

Allosteric inhibitors against HIV-1 reverse transcriptase: design and synthesis of MKC-442 analogues having an omega-functionalized acyclic structure.

Based on X-ray crystallographic analysis of MKC-442/human immunodeficiency virus type 1 reverse transcriptase (HIV-1 RT) complex, analogues in which the N1-substituent is replaced with omega-functionalized alkyl groups were designed to improve the affinity for the enzyme. Synthesis of these compounds was carried out starting from MKC-442 by a sequence of reactions (N3-protection, removal of N1-ethoxymethyl group, alkylation, and N3-deprotection). The compounds were evaluated for anti-HIV activity. Structure-activity relationships are discussed in terms of the possible interaction with the enzyme.

Allosteric Site↗
PubMed1997

DNA containing 4'-thio-2'-deoxycytidine inhibits methylation by HhaI methyltransferase.

4'-Thio-2'-deoxycytidine was synthesized as a 5'- protected phosphoramidite compatible with solid phase DNA synthesis. When incorporated as the target cytosine (C*) in the GC*GC recognition sequence for the DNA methyltransferase M. HhaI, methyl transfer was strongly inhibited. In contrast, these same oligonucleotides were normal substrates for the cognate restriction endonuclease R. HhaI and its isoschizomer R. Hin P1I. M. HhaI was able to bind both 4'-thio-modified DNA and unmodified DNA to equivalent extents under equilibrium conditions. However, the presence of 4'-thio-2'-deoxycytidine decreased the half-life of the complex by >10-fold. The crystal structure of a ternary complex of M. HhaI, AdoMet and DNA containing 4'-thio-2'-deoxycytidine was solved at 2.05 A resolution with a crystallographic R-factor of 0.186 and R-free of 0.231. The structure is not grossly different from previously solved ternary complexes containing M. HhaI, DNA and AdoHcy. The difference electron density suggests partial methylation at C5 of the flipped target 4'-thio-2'-deoxycytidine. The inhibitory effect of the 4'sulfur atom on enzymatic activity may be traced to perturbation of a step in the methylation reaction after DNA binding but prior to methyl transfer. This inhibitory effect can be partially overcome after a considerably long time in the crystal environment where the packing prevents complex dissociation and the target is accurately positioned within the active site.

DNA↗
PubMed1997

Varicella-zoster virus thymidine kinase gene and antiherpetic pyrimidine nucleoside analogues in a combined gene/chemotherapy treatment for cancer.

Ten pyrimidine nucleoside analogues, including (B)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) and closely related analogues, were evaluated for their cytostatic activity against human osteosarcoma cells transfected with the varicella-zoster virus (VZV) thymidine kinase (tk) (ATP:thymidine 5' phosphotransferase, EC 2.7.2.21) gene. (E)-5-(2-bromovinyl)-1-beta-D-arabinofuranosyluracil (BVaraU), (E)-5-(2-iodovinyl)-2'-deoxy-2'-fluoro-1-beta-D-arabinofuranosyluracil (IVFAU) and (E)-5-(2-bromovinyl)-2'-deoxy-4'-thiouridine (S-BVDU) were among the most potent inhibitors of VZVtk gene-transfected cell proliferation. They displayed an inhibitory activity at drug concentrations that were up to four orders of magnitude lower than those required to inhibit the corresponding nontransfected tumor cells. Inhibition of cellular DNA polymerase and/or incorporation of the drugs into cellular DNA may be a likely target for the cytostatic activity of the BVDU derivatives against the VZVtk gene-transfected tumor cells. These compounds were approximately 40- to 80-fold more potent cytostatic agents in VZVtk gene-transfected cells than the anti-VZV compound 6-methoxy-9-beta-D-arabinofuranosylpurine (araM), and at least five- to 50-fold more cytostatic than ganciclovir in HSV-1tk gene-transfected murine mammary carcinoma FM3A cells. In addition, the intrinsic resistance of BVaraU, IVFAU and S-BVDU to glycosidic bond cleavage by mammalian dThd phosphorylases makes them promising candidate compounds for the treatment of VZVtk gene-transfected tumors in vivo.

Antiviral Agents↗
PubMed1996

Investigation of some properties of oligodeoxynucleotides containing 4'-thio-2'-deoxynucleotides: duplex hybridization and nuclease sensitivity.

The thermal stabilities of the duplexes formed between 4'-thio-modified oligodeoxynucleotides and their DNA and RNA complementary strands were determined and compared with those of the corresponding unmodified oligodeoxynucleotides. A 16mer oligodeoxynucleotide containing 10 contiguous 4'-thiothymidylate modifications formed a less stable duplex with the DNA target (deltaTm/modification -1.0 degrees C) than the corresponding unmodified oligodeoxynucleotide. However, when the same oligodeoxynucleotide was bound to the corresponding RNA target, a small increase in Tm was observed (deltaTm/modification +0.16 degrees C) when compared with the unmodified duplex. A study to identify the specificity of an oligodeoxynucleotide containing a 4'-thiothymidylate modification when forming a duplex with DNA or RNA containing a single mismatch opposite the modification found the resulting Tms to be almost identical to the wild-type duplexes, demonstrating that the 4'-thio-modification in oligodeoxynucleotides has no deleterious effect on specificity. The nuclease stability of 4'-thio-modified oligodeoxynucleotides was examined using snake venom phosphodiesterase (SVPD) and nuclease S1. No significant resistance to degradation by the exonuclease SVPD was observed when compared with the corresponding unmodified oligodeoxynucleotide. However, 4'-thio-modified oligodeoxynucleotides were found to be highly resistant to degradation by the endonuclease S1. It was also demonstrated that 4'-thio-modified oligodeoxynucleotides elicit Escherichia coli RNase H hydrolysis of the RNA target only at high enzyme concentration.

DNA↗
PubMed1996

Postgraduate training for specialisation.

With the prospect of changes brought about through the Mouatt Report and the European Union, calls for a more flexible approach to teaching dentistry are being made. This article describes a postgraduate dental education system that involves modules and credits set up at the University of Leeds. The new system moves away from the more traditional methods of educating post-graduate students.

Credentialing↗
PubMed1996

Complexes of HIV-1 reverse transcriptase with inhibitors of the HEPT series reveal conformational changes relevant to the design of potent non-nucleoside inhibitors.

Crystal structures of HIV-1 reverse transcriptase (RT) complexed with a range of chemically diverse non-nucleoside inhibitors (NNIs) have shown a single pocket in which the inhibitors bind and details of the inhibitor-protein interactions. To delineate the structural requirements for an effective inhibitor, we have determined the structures of three closely related NNIs which vary widely in their potencies. Crystal structures of HIV-1 RT complexed with two very potent inhibitors, MKC-442 and TNK-651, at 2.55 angstroms resolution complement our previous analysis of the complex with the less effective inhibitor, HEPT. These structures reveal conformational changes which correlate with changes in potency. We suggest that a major determinant of increased potency in the analogues of HEPT is an improved interaction between residue Tyr181 in the protein and the 6-benzyl ring of the inhibitors which stabilizes the structure of the complex. This arises through a conformational switching of the protein structure triggered by the steric bulk of the 5-substituent of the inhibitor pyrimidine ring.

Antiviral Agents↗
PubMed1996

The crystal structure analysis of d(CGCGAASSCGCG)2, a synthetic DNA dodecamer duplex containing four 4'-thio-2'-deoxythymidine nucleotides.

The crystal structure refinement of the synthetic dodecamer d(CGCGAASSCGCG), where S = 4'-thio-2'-deoxythymidine, has converged at R=0.201 for 2605 reflections with F > 2sigma(F) in the resolution range 8.0-2.4 A for a model consisting of the dodecamer duplex and 66 water molecules. A comparison of its structure with that of the native dodecamer d(CGCGAATTCGCG) has revealed that the major differences between the two structures is a change in the conformation of the sugar-phosphate backbone in the regions at and adjacent to the positions of the modified nucleosides. Examination of the fine structural parameters for each of the structures reveals that the thiosugars adopt a C3'-exo conformation in d(CGCGAASSCGCG), rather than the approximate C1'-exo conformation found for the analogous sugars in the structure of d(CGCGAATTCGCG). The observed differences in structure between the two duplexes may help to explain the enhanced resistance to nuclease digestion of synthetic oligonucleotides containing 4'-thio-2'-deoxynucleotides.

Base Sequence↗
PubMed1996

Synthesis and anti-herpes virus activity of 2'-deoxy-4'-thiopyrimidine nucleosides.

A series of 5-substituted 2'-deoxy-4'-thiopyrimidine nucleosides was synthesized and evaluated as potential antiviral agents. A number of analogues such as 2'-deoxy-5-propyl-4'-thiouridine (3ii), 2'-deoxy-5-isopropyl-4'-thiouridine (3iii), 5-cyclopropyl-2'-deoxy-4'-thiouridine (3iv), 2'-deoxy-4'-thio-5-vinyluridine (3viii), and 5-(2-chloroethyl)-2'-deoxy-4'-thiouridine (3xx) were found to be highly active against herpes simplex virus type-1 (HSV-1) and varicella zoster virus (VZV) in vitro with no significant cytotoxicity. The compound with the broadest spectrum of activity was 2'-deoxy-5-ethyl-4'-thiouridine (3i) which showed significant activity against HSV-1, HSV-2, and VZV.

Animals↗
PubMed1996

Reasons for apicectomies. A retrospective study.

A retrospective study was carried out to evaluate the clinical factors involved in deciding to perform apicectomies. Five hundred and seventeen teeth from 392 patients (211 women and 181 men) that had undergone apicectomy during the period from September, 1990 to December, 1992 were assessed using the patients' clinical records. The information recorded included the source of referral, the quality of preoperative root canal filling, the size of periradicular lesion, the type of the lesion (for biopsed lesions), the type of coronal and radicular restorations, and the different factors that influenced the decision to perform an apicectomy for each tooth. These factors were classified into technical and biological, and when they occurred together they were classified as combined. The decisions to perform apicectomies most commonly involved combined technical and biological factors. Biological factors alone only amounted to 35% of the total. Technical factors alone amounted to only 3% of the total. When all factors were considered, biological factors constituted 60%, whilst technical factors constituted 40%, of the total. The most common biological factors were persistent symptoms (54%), and continuing presence of a periradicular lesion (44%). The most common technical factors were post crown (60%) and crowned teeth without posts (31%). This study emphasised the need for a high standard of conventional root canal treatment in order to avoid surgical treatment.

Adult↗
PubMed1996

Endodontic disease: development and treatment.

Endodontic disease requires the involvement of micro-organisms. Microbial contamination of the pulp-dentine complex occurs in a number of different ways. The loss of integrity of teeth and the penetration of the pulp-dentine complex by a mixed microbial flora characterises the disease processes which can lead to pulpal and periradicular inflammation and infection. Endodontology is that part of dental science which deals with the biology of the pulp-dentine complex in health and disease, an understanding of which is a prerequisite to the provision of sound conservative care. The principles of endodontic treatment require the control of micro-organisms and potential nutrients by the microbial decontamination of teeth, the denaturing of protein and the sealing of dentine to prevent recolonisation.

Dental Pulp Diseases↗
PubMed1995

Synthesis and antiviral activity of 6-benzyl analogs of 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine (HEPT) as potent and selective anti-HIV-1 agents.

Several 6-benzyl analogs of 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine (1; HEPT) were synthesized and evaluated for their anti-HIV-1 activity. LDA (lithium diisopropylamide) lithiation of 5-ethyluracil derivatives 7 and 8 and subsequent reaction with an aryl aldehyde gave 6-(arylhydroxymethyl)-5-ethyluracil derivatives 9-12. 6-(Arylhydroxymethyl)-5-isopropyluracil derivatives 15-18 were prepared from the 5-isopropyl-2-thiouracil derivatives 13 and 14 by the above procedure following oxidative hydrolysis of the thione. Preparation of the target 5-alkyl-1-(alkoxymethyl)-6-benzyluracil derivatives 27-34 was carried out by acetylation of 9-14 followed by Pd-catalyzed hydrogenolysis. The 1-butyl- (37 and 39) and 1-(2-methoxyl)- (38 and 40) 5-alkyl-6-benzyluracils were synthesized by 1-alkylation of the 3-phenacyl derivatives 35 and 36 with alkyl halides followed by deprotection of the 3-phenacyl group. Compounds synthesized in this study inhibited HIV-1 replication in MT-4 cells in the submicromolar to namomolar concentration range. From this series of compounds, 6-benzyl-1-(ethoxymethyl)-5-isopropyluracil (33) was selected for clinical evaluation.

Animals↗
PubMed1995

4'-Aza-sugar nucleosides.

From readily available starting materials, the preparation and characterization of a variety of pyrimidine 2', 3'-dideoxyribo-, 2'-deoxyribo-, 3'-deoxyribo- and ribo- N-protected 4'-aza nucleosides is reported. None was found to possess any significant antiviral activity.

Aza Compounds↗