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Biomedical subjects

J B Porter

Publications and source records attributed to J B Porter.

At least 37 records · Page 2Linked to original sources
PubMed1999

Determinants of iron status and bilirubin levels in congenital dyserythropoietic anaemia type I.

Seven untransfused patients with congenital dyserythropoietic anaemia type I were investigated to assess the determinants of both iron overload and serum bilirubin levels. The serum ferritin concentration was increased in all patients and non-transferrin-bound iron (NTBI) was increased in all but one patient. None of the patients showed the C282Y mutation in the hereditary haemochromatosis gene, HFE. One patient was homozygous for the H63D mutation in this gene. The data indicated that differences in the extent of iron overload were not mediated by co-inheritance of the C282Y mutation in the HFE gene but could largely be explained by differences in the severity of anaemia and ineffective erythropoiesis, and in the age of the patient. In one patient an unusually high plasma bilirubin level was associated with the variant A[TA]7TAA configuration in the TATA box of the uridine diphosphate glucuronosyltransferase (UGT-1A) gene promoter, the mutation found in most patients with mild Gilbert's syndrome.

Adult↗
PubMed1998

Raised neutrophil phospholipase A2 activity and defective priming of NADPH oxidase and phospholipase A2 in sickle cell disease.

Intermittent painful crises due to vasoocclusion are the major clinical manifestation of sickle cell disease (SCD), but subclinical episodes may also occur. There is sparse evidence for the involvement of neutrophils in the pathophysiology of SCD, but production of cytokines by the damaged endothelium might influence neutrophil function and modulate responses to subsequent cytokine exposure. In addition, the activation of neutrophils in the microcirculation could itself exacerbate vasoocclusion. To test whether neutrophil inflammatory responses were altered in SCD, neutrophil phospholipase A2 and NADPH oxidase activity in response to in vitro priming by granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor-alpha (TNF-alpha) were measured both during and between painful crises. Resting levels of neutrophil phospholipase A2 activity in steady-state SCD (4.0% +/- 0. 5% of total cell radioactivity) were raised relative to control values (2.0% +/- 0.2%, n = 10, P = .008). There was no defect of agonist-stimulated phospholipase A2 or NADPH oxidase activity in steady-state SCD; however, the ability of phospholipase A2 to respond to priming with GM-CSF was attenuated to 63% +/- 17% of control values (n = 10, P = .04). Similarly, neutrophil NADPH oxidase activity after priming with GM-CSF and TNF-alpha was, respectively, 65% +/- 11% (n = 7, P = .03) and 57% +/- 7% of control (n = 10, P = .007) in steady-state disease, and was further reduced during painful vasoocclusive crises to 34% +/- 9% and 25% +/- 3% of control for GM-CSF and TNF-alpha, respectively. These data were not explained by poor splenic function or any racial factor, as normal cytokine responses were seen in splenectomized patients in remission from Hodgkin's disease and in healthy Afro-Caribbean subjects. Abnormal neutrophil cytokine priming responses were not observed in either patients with rheumatoid arthritis or iron-deficiency anemia. Our findings are indicative of an ongoing inflammatory state in SCD between painful crises involving neutrophil activation and an abnormality of cytokine-regulated neutrophil function, which may compromise the host defenses against certain microorganisms.

Adolescent↗
PubMed1998

Endothelial activation in patients with chronic venous disease.

OBJECTIVES: Leukocyte trapping due to leukocyte-endothelial activation has been implicated as the cause of lipodermatosclerosis and ulceration in patients with chronic venous disease. We investigated endothelial activity in normal controls and patients subjected to short-term venous hypertension. METHODS: Twenty-five normal volunteers and 30 patients with chronic venous disease divided into two groups: varicose veins with skin changes (LDS, n = 15); and varicose veins without skin changes (VVs, n = 15) were studied. Blood samples were taken from a foot vein before and after experimental venous hypertension. Plasma levels of ELAM-1 (endothelial leukocyte adhesion molecule-1), ICAM-1 (intercellular adhesion molecule-1), VCAM-1 (vascular cell adhesion molecule-1), and von Willebrand factor (vWf) was measured by an ELISA. RESULTS: There was a significant rise in the plasma concentration of ELAM-1, ICAM-1 and VCAM-1 in patients and normal controls in response to venous hypertension. Basal levels of plasma VCAM-1 and vWf were higher in patients with LDS compared to patients with VVs. The magnitude of rise of VCAM-1 was greater in patients with LDS compared to patients with VVs (p = 0.01, Mann-Whitney U-test). There was no difference in the basal levels or in the magnitude of change in plasma ICAM-1 and ELAM-1 between the two patient groups. CONCLUSION: Venous hypertension results in endothelial activation which may aid endothelial-leukocyte adhesion. Patients with LDS exhibit increased VCAM-1, which is a counterligand for receptors expressed by monocytes and lymphocytes signifying that these cells may be more important in the development of skin changes.

Adult↗
PubMed1998

Enhancement of 5-aminolaevulinic acid-induced photodynamic therapy in normal rat colon using hydroxypyridinone iron-chelating agents.

Currently, the clinical use of 5-aminolaevulinic acid (ALA)-induced protoporphyrin IX (PPIX) for photodynamic therapy (PDT) is limited by the maximum tolerated oral ALA dose (60 mg kg(-1)). This study investigates whether hydroxypyridinone iron-chelating agents can be used to enhance the tissue levels of PPIX, without increasing the administered dose of ALA. Quantitative charge-coupled device (CCD) fluorescence microscopy was employed to study PPIX fluorescence pharmacokinetics in the colon of normal Wistar rats. The iron chelator, CP94, when administered with ALA was found to produce double the PPIX fluorescence in the colonic mucosa, compared with the same dose of ALA given alone and to be more effective than the other iron chelator studied, CP20. Microspectrofluorimetric studies demonstrated that PPIX was the predominant porphyrin species present. PDT studies conducted on the colonic mucosa showed that the simultaneous administration of 100 mg kg(-1) CP94 i.v. and 50 mg kg(-1) ALA i.v. produced an area of necrosis three times larger than similar parameters without the iron-chelating agent with the same light dose. It is possible, therefore, to increase the amount of necrosis produced by ALA-induced PDT substantially, without increasing the administered dose of ALA, through the simultaneous administration of the iron-chelating agent, CP94.

Aminolevulinic Acid↗
PubMed1997

Leukocyte activity in the microcirculation of the leg in patients with chronic venous disease.

PURPOSE: It has been suggested that leukocyte trapping and activation in the microcirculation of the leg skin causes lipodermatosclerosis and ulceration in patients with chronic venous disease. Ambulatory venous hypertension is accepted as the physiologic factor that leads to ulceration. We investigated leukocyte endothelial adhesion in patients who were subjected to short-term venous hypertension. METHODS: Two groups of patients with venous disease were studied: group 1, varicose veins with skin changes (n = 15); and group 2, varicose veins without skin changes (n = 15). Blood samples were taken from a foot vein before and after standing for 30 minutes to raise the venous pressure in the lower limb, and after lying supine again for 10 minutes. The samples were analyzed for leukocyte surface CD11b and L-selectin (CD62L) expression using a flow cytometer. Plasma-soluble L-selectin was also measured using an enzyme-linked immunosorbent assay. RESULTS: In patients with skin changes, median neutrophil CD11b levels fell from 4.66 to 3.83 arbitrary units (p = 0.005, Wilcoxon) after 30 minutes of venous hypertension, Median monocyte CD11b levels fell from 7.65 to 5.8 arbitrary units (p = NS, Wilcoxon) after venous hypertension and then fell further to 5.43 arbitrary units (p = 0.02 vs baseline; Wilcoxon) when the venous hypertension was removed. Neutrophil and monocyte L-selectin levels also fell in response to venous hypertension, remaining low even after venous hypertension was removed. A similar pattern was seen in patients with uncomplicated varicose veins. There was a rise in soluble L-selectin in the plasma of both groups of patients after venous hypertension, reflecting leukocyte adhesion to endothelium. In the group of patients with skin changes the level of soluble L-selectin rose from 695 ng/ml to 836 ng/ml (p = 0.02, Wilcoxon), and in the group without skin changes the rise was from 700 ng/ml to 801 ng/ml (p = 0.02, Wilcoxon). CONCLUSION: Venous hypertension results in sequestration of the more activated population of neutrophils and monocytes in the microcirculation of the leg in patients with venous disease. These cells bind to the endothelium, releasing L-selectin, and do not emerge from the limb when venous hypertension is reversed. These findings do not differ between patients with varicose veins and those with skin changes.

Adult↗
PubMed1997

The efficacy of an iron chelator (CP94) in increasing cellular protoporphyrin IX following intravesical 5-aminolaevulinic acid administration: an in vivo study.

5-Aminolaevulinic acid (ALA)-induced protoporphyrin IX (PpIX) is proving to be a useful photosensitizer for photodynamic therapy (PDT). Conversion of PpIX to haem requires catalysed chelation with iron, and thus the presence of an iron chelator should, in theory, lead to an increase in cellular PpIX accumulation. This paper assesses the in vivo effect of a new iron chelator, 1,2-diethyl-3-hydroxypyridin-4-one (CP94), on the kinetics of PpIX in different layers of the bladder wall. Wistar rats were given 1% or 10% ALA intravesically with or without intraperitoneal CP94. The biodistribution of ALA-induced PpIX in the bladder was evaluated using fluorescence microscopy. Photodynamic effects on the bladder were compared in rats receiving various drug dosimetries. In CP94-treated rats, 5-7 h after administration of 10% ALA solution, the fluorescence intensity of PpIX in the urothelium was doubled compared with animals given ALA alone, whereas in the muscle layer PpIX remained at a low level similar to that found without the iron chelator. At an ALA concentration of 1%, although the PpIX concentration was not increased with CP94, the urothelial selectivity of PDT compared with the muscle layer was enhanced. In conclusion, by using CP94, a further reduction in skin photosensitization may be possible as similar photodynamic effects can be achieved with a lower dose of ALA. The addition of CP94 seems to be an effective and convenient way to potentiate ALA-induced PpIX tissue selectivity between the urothelium and the underlying layers of the bladder wall.

Administration, Intravesical↗
PubMed1997

Non-transferrin-bound iron induced by myeloablative chemotherapy.

Previous studies have suggested that non-transferrin-bound plasma iron (NTBI) is present in patients undergoing cytotoxic chemotherapy, and that this may exacerbate untoward organ damage and increase the risk of bacterial infections following chemotherapy. However, the source of NTBI during myelosuppressive chemotherapy is controversial. In this study we have examined the kinetics of the appearance and disappearance of NTBI with chemotherapy. NTBI was present in only two out of 24 patients prior to chemotherapy but, following chemotherapy, was present in 19 patients, with peak NTBI levels at 5 d after onset of chemotherapy (mean 3.06 microM). Thereafter levels fell, but were still detectable in seven patients 14 d after the end of chemotherapy. The appearance of NTBI was associated with a sudden rise in transferrin saturation, but NTBI was detected on four occasions in the absence of full transferrin saturation. We have also established that daunorubicin cannot itself liberate NTBI from serum. There was no relationship between induced NTBI levels and serum iron or ferritin levels, previous or current blood transfusions, disease stage, or the type of chemotherapy given. The appearance of NTBI following chemotherapy was inverserely related to the fall in reticulocytes and serum transferrin receptor (TfR) levels, suggesting that NTBI formation is a consequence of suspension of erythropoietic activity.

Adolescent↗
PubMed1997

Relationship between the severity of beta-thalassaemia syndromes and the number of alleviating mutations.

Thalassaemia intermedia, defined as homozygous beta-thalassaemia in which patients are not transfusion-dependent, covers a wide range of clinical severity. It may arise because one or more genetic factors ameliorate the otherwise severe phenotype of thalassaemia major. Exactly which and how many such mutations are necessary to produce a thalassaemia intermedia phenotype is incompletely understood, although such information would be useful both clinically and for prenatal diagnosis. We examined DNA from 28 patients with thalassaemia intermedia resident in London and 28 matched patients with thalassaemia major, for 3 types of genetic modifying factors, namely; mild beta-thalassaemia mutations, the upstream XmnI G-gamma globin gene polymorphism, and alpha-globin gene deletions. The results show that the number of alleviating mutations present has a large influence on the phenotype of patients with homozygous beta-thalassaemias. A single alleviating mutation was present in 56% of thalassaemia intermedia subjects compared with 26% of thalassaemia major subjects. Two alleviating mutations were present in 33% of thalassaemia intermedia subjects compared with 1 thalassaemia major subject. No patients with thalassaemia major had 3 alleviating mutations, in contrast to 11% of those with thalassaemia intermedia. Although the findings did not account for the full range of phenotypic variation, such information is of potential value both in the clinical management and the prenatal diagnosis of homozygous beta-thalassaemia.

Adolescent↗
PubMed1997

A risk-benefit assessment of iron-chelation therapy.

Iron overload caused by lifelong transfusion-dependent anaemias, such as beta-thalassaemia major, usually results in lethal cardiac toxicity in the second decade of life if not treated by iron chelation. There is no physiological mechanism for excreting the excess iron accumulated from blood transfusions and, unlike hereditary haemochromatosis, venesection is not an option. Therefore, chelation therapy is the only way to remove excess iron. This must be removed while not depriving cells of the essential iron needed for normal metabolism. Additionally, the iron chelator must prevent iron from participating in the generation of harmful free radicals. Parenteral chelation therapy with deferoxamine (desferrioxamine) is well established as promoting negative iron balance, reversing cardiac toxicity, and prolonging life expectancy well into the fourth decade of life and, most likely, beyond. Unfortunately, poor compliance with the rigours of parenteral treatment in a minority of patients limits its regular use, resulting in reduced life expectancy in these patients. Use of deferoxamine in excessive dosages may result in growth retardation, sensorineural ototoxicity and ocular toxicity, as well as bone deformities. These effects can be largely avoided if the dosage is adjusted to take account of the degree of iron overload (using the therapeutic index) and if the mean daily dose does not exceed 40 mg/kg. Nevertheless, it is recommended that patients be regularly monitored for such adverse effects. Deferiprone (L1; CP20) is an orally absorbed bidentate hydroxypyridinone iron chelator that can induce urinary iron excretion, promote negative iron balance and reduce hepatic iron levels in some transfusion-dependent patients, particularly in those who are markedly iron overloaded and have not received regular deferoxamine therapy. The long term efficacy and toxicity of deferiprone are the subjects of some controversy, and the published results of randomised controlled trials are awaited. Preliminary results suggest that when currently recommended dosages of deferiprone (75 mg/kg/day) are used, hepatic iron settles at levels that still put most patients at an increased risk from iron overload. A number of adverse effects may occur, and require cessation of therapy in up to 30% of patients. These effects include arthritis, nausea and (most seriously) agranulocytosis in 0.6 to 4% of patients. The risk of the latter complication means that frequent white blood cell counts are mandatory for patients taking this drug. There remains an urgent need to identify an orally active chelator regimen that is as effective as deferoxamine and has an acceptable degree of tolerability.

Administration, Oral↗
PubMed1996

The relationship of intracellular iron chelation to the inhibition and regeneration of human ribonucleotide reductase.

The depletion of cellular iron can lead to the inhibition of ribonucleotide reductase, preventing new DNA synthesis and hence inhibiting cell proliferation. Electron paramagnetic resonance (EPR) spectroscopy has been used to examine simultaneously for the first time the relationship between chelation of intracellular iron and the rate of removal and regeneration of the tyrosyl radical of ribonucleotide reductase within intact human leukemia K562 cells. The different physiochemical characteristics of relatively hydrophobic low molecular weight bidentate hydroxypyridinone chelators and the higher molecular weight hexadentate ferrioxamine have been exploited to elucidate these interactions further. The base-line concentration of EPR-detectable mononuclear nonheme iron complexes was 3.15 =/- 1.05 microM, rising on incubation with chelators more rapidly with hydroxypyridinones than with desferrioxamine. Hydroxypyridinones also removed the tyrosyl radical more rapidly, apparently as a consequence of depletion of the intracellular iron pools necessary to regenerate the active enzyme and compatible with their reportedly greater cell toxicity. The radical decay rate is consistent with previous models, suggesting that iron is spontaneously removed from mammalian ribonucleotide reductase. Upon removal of extracellular chelator the regeneration of the tyrosyl radical was significantly faster for hydroxypyridinones than for desferrioxamine, consistent with their differential effects on cell cycle synchronization.

Deferoxamine↗
PubMed1996

Kinetics of removal and reappearance of non-transferrin-bound plasma iron with deferoxamine therapy.

The rapidity and duration of the response of non-transferrin-bound iron (NTBPI) to chelation therapy are largely unknown and have important implications for the design of optimal chelation regimens. Methodology was developed to measure simultaneously NTBPI, deferoxamine (DFO), and its major metabolite. NTBPI was present in all but 2 of 28 thalassaemia major (TM) patients who had received conventional subcutaneous DFO the previous night, suggesting a short duration of NTBPI clearance by DFO. The detailed kinetics of NTBPI were therefore studied in response to intravenous DFO at 50 mg/kg/27 h for 48 hours and compared in 17 regularly transfused TM and 8 untransfused thalassaemia intermedia (TI) patients to determine the influence of hypertransfusion and iron overload on NTBPI response. Before DFO infusion, NTBPI was present in all patients and was significantly higher in TI (4.52 +/- 0.53 mumol/L) than TM (2.92 +/- 0.03 mumol/L; P = .03). NTBPI values in TM correlated with transferrin saturation (r = .6, P = .03) but not with serum ferritin. Removal of NTBPI by intravenous DFO is in a biphasic manner. The initial rapid rate constant (alpha) was similar in TI (1.5 hour-1) and TM (1.6 hour-1), but the subsequent beta phase was slower (0.04 hour-1) in TI when compared with TM (0.4 hour-1, P = .002). Detectable NTBPI persisted during the beta phase, particularly in TI, despite an excess of plasma DFO also being present (steady state 8 mumol/L). On cessation of DFO infusion, NTBPI reappearance was rapid; the kinetics also being biphasic. The rapid initial rate constant (alpha = 2.5 hour-1) lasted less than 30 minutes and was approximately equal to the summation of the initial rate constant for removal of DFO (1.8 hour-1) and its major metabolite (0.6 hour-1). This was followed by a slower return to pretreatment levels, usually between 6 and 12 hours, which was faster in TI than in TM. This marked NTBPI lability supports the use of continuous rather than intermittent DFO in high risk patients.

Adolescent↗
PubMed1996

The environment of the lipoxygenase iron binding site explored with novel hydroxypyridinone iron chelators.

The mechanisms of lipoxygenase inhibition by iron chelators have been investigated in human neutrophils and in isolated soybean lipoxygenase. Their Fe(III)-containing active sites have been targeted by synthesizing novel bidentate chelators from the hydroxypyridinone family sufficiently small to gain access through the hydrophobic channels of lipoxygenase. In stimulated human neutrophils, release of [3H]arachidonate-labeled eicosanoids is dependent on the lipid solubility of hydroxypyridinones, but larger hexadentate chelators have no effect on this or on total cellular leukotriene B4 production. Lipophilic hydroxypyridinones inhibit 5-lipoxygenase at equivalent concentrations to the established inhibitor, piriprost, and show additional but minor anti-phospholipase A2 activity. Soybean 15-lipoxygenase inhibition is also dependent on the lipid solubility and coordination structure of chelators. Inhibition is associated with the formation of chelate-iron complexes, which are removed by dialysis without restoration of enzyme activity. Only after adding back iron is activity restored. Electron paramagnetic resonance studies show the removal of the iron center signal (g = 6) is concomitant with formation of Fe(III)-chelator complexes, identical in spectral shape and g value to 3:1 hydroxypyridinone Fe(III) complexes. Removal of iron is not the only mechanism by which hydroxypyridinones can inhibit lipoxygenase in intact cells, however, as a lipophilic non-iron-binding hydroxypyridinone, which shows no inhibition of the soybean lipoxygenase activity, partially inhibits 5-lipoxygenase in intact neutrophils without inhibiting neutrophil phospholipase A2.

Electron Spin Resonance Spectroscopy↗
PubMed1996

Evaluation of new iron chelators for clinical use.

Evaluation of new chelators for clinical use is limited by the availability of models which will predict the therapeutic safety margin of chelators in iron-overloaded humans such as those with thalassaemia major. Animal models show significant differences with respect to the relative toxicity of different chelators compared with human. These differences can be ascribed to several factors: differences in iron metabolism between different species, human metabolism being significantly more conservative than in rodents or nonhuman primates; differences in drug metabolism between different species which are often difficult to predict from first principles, and difficulties in obtaining iron-overloaded models that are truly representative of transfusional iron overload clinically. These differences have been highlighted by clinical studies on hydroxypyridinone iron chelators such as 1,2-dimethyl-3-hydroxypyridin-4-one (L1, CP20, deferiprone) and 1,2-diethyl-3-hydroxypyridin-4-one (CP94). New tissue culture approaches towards understanding the mechanisms of neutropenia, cytostasis and apoptosis induced by chelators as well as the relative rates of inhibition of non-haem-iron-containing enzymes such as ribonucleotide reductase are predicted to identify chelators with a higher therapeutic safety margin.

Animals↗
PubMed1995

Low plasma heparin cofactor II levels in thalassaemia syndromes are corrected by chronic blood transfusion.

Low plasma heparin cofactor II (HCII) levels are associated with a thrombotic tendency, and we have previously shown these to be decreased in a variety of haemolytic conditions. The risk of thrombosis is recognized to be increased in both thalassaemia major (TM) and intermedia (TI), although the exact mechanisms are poorly understood. HCII levels have therefore been compared in 20 untransfused patients with TI and 20 regularly transfused TM patients to determine the influence of transfusion on HCII. Additionally, untransfused TI patients have been commenced on regular red cell transfusion and the effects on correction of low HCII levels investigated. HCII levels were significantly lower in the untransfused TI patients (mean 0.56 +/- 0.06 U/ml) compared to TM patients (mean 0.85 +/- 0.1 U/ml; P < 0.001). Levels in TI were significantly less than in healthy age-matched controls (P < 0.001) and correlated with Hb values (r = 0.8), whereas levels in TM were at the lower end of the normal range. ATIII values were within the normal reference range in both TI and TM, and HCII antigen showed a parallel reduction to HCII activity, indicating that reduction in HCII is not a consequence of increased thrombin consumption. Three patients with TI were commenced prospectively on hypertransfusion programmes which resulted in a slow normalization of their levels taking 2-3 months. These findings support a hypothesis that the low HCII levels are related to increased red cell turnover and can be normalized once this turnover has been suppressed by hypertransfusion. The thrombotic risk to patients with low HCII levels in the presence of haemolysis might in principle be decreased by such transfusion regimes.

Adolescent↗
PubMed1995

The metabolites of nitric oxide in sickle-cell disease.

Plasma NOx concentrations were raised in 22 acute painful crises in SCD. We have measured blood concentrations of nitric oxide metabolites (NOx) in sickle-cell disease (SCD), and shown that they are increased compared with healthy controls (P = 0.002), and haemoglobin E/beta-thalassaemic controls (P = 0.05). Concentrations in steady-state SCD were also higher than in healthy controls (P = 0.04) but not significantly different from the concentrations at the beginning of painful crises (P = 0.34). Importantly, in 12 regularly exchanged sicklers, the mean pre-transfusion NOx concentration did not differ significantly from the control population (P = 0.52), suggesting that the changes in NO metabolism can be reversed. It is unlikely that the increased concentrations of NOx in SCD result from anaemia or haemolysis as the untransfused haemoglobin E/beta-thalassaemics did not show increased levels.

Acute Disease↗