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S C Chow

Publications and source records attributed to S C Chow.

At least 73 records · Page 4Linked to original sources
PubMed1994

On statistical characteristics of quality of life assessment.

In recent years, the assessment of drug effects on quality of life (QOL), which is the ability of a person to function normally in society as perceived by the patient, has become a popular issue in clinical trials. Since the response may vary widely from patient to patient and from time to time, the questionnaire (or instrument) for assessing the QOL of patients is required to be sensitive and reliable for detection of meaningful differences. QOL measurements are known to be correlated over time and among QOL indices. In this paper, we study some statistical characteristics of a given QOL instrument, such as precision and power, using a time series model that accounts for the correlation structure of QOL measurements over time. Formulae for sample size determination for clinical trials with time-correlated outcome measurements are presented.

Humans↗
PubMed1993

Two-stage sampling in pharmaceutical applications.

In the pharmaceutical industry, some tests for quality assurance before, during and after the manufacture of a drug product involve a two-stage sampling technique. We propose statistical testing procedures in a two-stage sampling problem with large within-class sample sizes. We also derive a two-stage sampling plan by minimizing the expected squared volume (or the generalized variance) of the confidence region related to the test. We present two examples to illustrate the proposed methods.

Clinical Trials as Topic↗
PubMed1993

Mechanisms of platelet-derived growth factor-induced arachidonic acid release in Swiss 3T3 fibroblasts: the role of a localized increase in free Ca2+ concentration beneath the plasma membrane and the activation of protein kinase C.

Stimulation of Swiss 3T3 fibroblasts with platelet-derived growth factor (PDGF) results in a transient increase in intracellular free Ca2+ concentration ([Ca2+]i) and a phospholipase A2 (PLA2)-dependent release of arachidonic acid (AA) of 500% over control values. In the absence of extracellular Ca2+, both the PDGF-induced transient increase in [Ca2+]i and AA release were markedly reduced. Buffering the increase in [Ca2+]i with EGTA, introduced into the cells in the form of EGTA acetoxymethylester (AM), abolished the PDGF-induced transient increase in [Ca2+]i, but potentiated the AA release by at least 2-fold compared to cells without EGTA. The EGTA potentiated PDGF-induced AA release was sensitive to extracellular Ca2+ and inhibited to various degrees by both receptor-mediated as well as voltage-operated Ca2+ channel blockers, suggesting that the release of AA may be tightly coupled to the influx of Ca2+. Activation of protein kinase C (PKC) by the phorbol ester, phorbol 12-myristate 13-acetate (TPA) had little effect in promoting AA release by itself. Down-regulation of PKC in Swiss 3T3 fibroblasts by chronic stimulation with 300 nM TPA for 24 h, markedly inhibited the PDGF-stimulated AA release in both the EGTA-loaded and control cells. In conditions where PDGF-induced AA release was inhibited or potentiated, the production of inositol phosphates was unaffected. Thus, PDGF-induced PLA2 dependent AA release in Swiss 3T3 fibroblast is regulated by both PKC-dependent and -independent mechanisms, and is activated by high concentrations of free Ca2+ in the microenvironment beneath the plasma membrane during Ca2+ influx via plasma-membrane Ca2+ channels, despite buffering by EGTA of [Ca2+]i in the bulk cytoplasm of the cell.

3T3 Cells↗
PubMed1993

Two independently regulated Ca2+ entry mechanisms coexist in Jurkat T cells during T cell receptor antigen activation.

Receptor-mediated Ca2+ influx was studied in the human leukaemic T cell line, Jurkat. Stimulation of these cells through the T cell antigen-receptor complex with OKT3 (an antibody against the CD3 molecules of the T cell antigen-receptor complex), or inhibition of the endoplasmic reticular Ca(2+)-ATPase with thapsigargin, resulted in Ca2+ mobilization from intracellular stores and the activation of Ca2+ and Mn2+ entry. The rates of thapsigargin-induced Ca2+ and Mn2+ entry in Jurkat cells were 76% and 64% respectively of those observed after treatment of these cells with OKT3. The combined addition of thapsigargin plus OKT3 to Jurkat cells produced an enhanced effect on the sustained increase in the cytosolic free Ca2+ concentration that was greater than that obtained by addition of thapsigargin or OKT3 alone. The rates of Ca2+ and Mn2+ entry were increased to 119% and 112% respectively of the OKT3-induced rates. Taken together, these results suggest that the inositol 1,4,5-trisphosphate-sensitive Ca(2+)-pool-dependent bivalent cation entry only accounts for 57% and 52% respectively of the total OKT3-dependent Ca2+ and Mn2+ entry, and that the rest is mediated by second messenger(s). Thus two separate pathways coexist in regulating Ca2+ entry in Jurkat cells during activation mediated through the T cell receptor.

Biological Transport↗
PubMed1992

Potentiation of transmembrane signaling by cross-linking of antibodies against the beta chain of the T cell antigen receptor of JURKAT T cells.

Three monoclonal antibodies (mAb) 2D1, 3B9, and 3B12 were produced by immunizing BALB/c mice with JURKAT cells. These mAb induce comodulation of the TCR/CD3 complex expressed on JURKAT cells, but do not react with the CD3- JURKAT variant, J.RT3.T3.1. Immunoprecipitation studies with detergent-solubilized JURKAT cell lystes indicate that these mAb react with proteins having characteristics of the TCR molecules. Their low reactivity with peripheral blood mononuclear cells (PBMC) and lack of reactivity with other CD3+ T cell lines suggest that they may be anti-idiotypic mAb. Results from binding inhibition assays, reactivity with PBMC, and generation of transmembrane signals suggest that these three anti-TCR mAb recognized different epitopes on the TCR beta chain of JURKAT cells. Although the three mAb are capable of inducing the production of inositol phosphates and cytosolic free Ca2+ increase in JURKAT cells, their stimulatory capacities vary and are lower than that observed by anti-CD3 antibody (OKT3) stimulation. However, crosslinking these mAb with rabbit antimouse immunoglobulins potentiates the stimulatory response to comparable levels induced by OKT3. These mAb could be useful as tools to study V beta 8+ T cells in relation to antigen-specific activation.

Animals↗
PubMed1992

Tributyltin increases cytosolic free Ca2+ concentration in thymocytes by mobilizing intracellular Ca2+, activating a Ca2+ entry pathway, and inhibiting Ca2+ efflux.

The immunotoxic environmental pollutant tri-n-butyltin (TBT) kills thymocytes by apoptosis through a mechanism that requires an increase in intracellular Ca2+ concentration. The addition of TBT (EC50 = 2 microM) to fura-2-loaded rat thymocytes resulted in a rapid and sustained increase in the cytosolic free Ca2+ concentration ([Ca2+]i) to greater than 1 microM. In nominally Ca(2+)-free medium, TBT slightly but consistently increased thymocyte [Ca2+]i by about 0.11 microM. The subsequent restoration of CaCl2 to the medium resulted in a sustained overshoot in [Ca2+]i; similarly, the addition of MnCl2 produced a rapid decrease in the intracellular fura-2 fluorescence in thymocytes exposed to TBT. The rates of Ca2+ and Mn2+ entry stimulated by TBT were essentially identical to the rates stimulated by 2,5-di-(tert.-butyl)-1,4-benzohydroquinone (tBuBHQ), which has previously been shown to empty the agonist-sensitive endoplasmic reticular Ca2+ store and to stimulate subsequent Ca2+ influx by a capacitative mechanism. The addition of excess [ethylenebis(oxyethylenenitrilo)]tetraacetic acid to thymocytes produced a rapid return to basal [Ca2+]i after tBuBHQ treatment but a similar rapid return to basal [Ca2+]i was not observed after TBT treatment. In addition, TBT produced a marked inhibition of both Ca2+ efflux from the cells and the plasma membrane Ca(2+)-ATPase activity. Also, TBT treatment resulted in a rapid decrease in thymocyte ATP level. Taken together, our results show that TBT increases [Ca2+]i in thymocytes by the combination of intracellular Ca2+ mobilization, stimulation of Ca2+ entry, and inhibition of the Ca2+ efflux process. Furthermore, the ability of TBT to apparently mobilize the tBuBHQ-sensitive intracellular Ca2+ store followed by Ca2+ and Mn2+ entry suggests that the TBT-induced [Ca2+]i increase involves a capacitative type of Ca2+ entry.

Adenosine Triphosphate↗
PubMed1992

On assessment of bioequivalence under a higher-order crossover design.

In bioavailability studies of two formulations of a drug, the standard two-sequence, two-period crossover design is usually considered to assess bioequivalence. The standard two-sequence, two-period crossover design, however, may not be useful when differential carryover effects are present. In addition, it does not provide independent estimates of intrasubject variabilities for the two formulations. To overcome these problems, alternatively, a higher-order crossover design may be considered. In this paper, we derive statistical methods based on Schuirmann's two one-sided tests procedure for assessing bioequivalence for some commonly used higher-order crossover designs. Four designs, including Balaam's design, the two-sequence dual design, and two four-period designs (with two and four sequences), are considered. The relative merits of these designs as compared to the standard two-sequence, two-period design are discussed. Two examples concerning bioequivalence are used to illustrate the use of these methods.

Biological Availability↗
PubMed1992

Clostridium difficile toxin A induces multinucleation in the human leukemic T cell line JURKAT.

Clostridium difficile toxin A is a cytotoxic enterotoxin known to be active on all mammalian cell lines tested up to now. It induces a disruption of the cytoskeleton, particularly the microfilament system, leading to inhibition of cell proliferation. Here, we describe some effects of toxin A on the leukemic T cell line JURKAT. Cells exposed to the toxin did not divide, as cell numbers remained constant for 3 days in the presence of 0.5 to 1.0 micrograms/ml of the toxin. However, these cells were found to become multinucleated, a phenomenon which was time- and dose-dependent. After treatment for 72 h with 0.5 micrograms/ml toxin A, 95% of the cells were multinucleated and had a considerably increased cell diameter. These effects in JURKAT cells were partially reversible upon removal of the toxin within 12 h after the beginning of toxin exposure, but irreversible after 24 h of toxin treatment. These results suggest a continuing nuclear division in the absence of cytoplasmic division, i.e., an effect of toxin A on contractile ring formation. The JURKAT cell is the first cell type reported to respond to toxin A with multinucleation.

Bacterial Toxins↗
PubMed1991

Modification of membrane phospholipid fatty acyl composition in a leukemic T cell line: effects on receptor mediated intracellular Ca2+ increase.

The effect of modifying fatty acyl composition of cellular membrane phospholipids on receptor-mediated intracellular free Ca2+ concentration ([Ca2+]i) increase was investigated in a leukemic T cell line (JURKAT). After growing for 72 h in medium supplemented with unsaturated fatty acids (UFAs) and alpha-tocopherol, the fatty acyl composition of membrane phospholipids in JURKAT cells was extensively modified. Each respective fatty acid supplemented in the culture medium was readily incorporated into phosphatidylinositol, phosphatidylserine, phosphatidylethanolamine and phosphatidylcholine in the JURKAT cells. The total n-6 fatty acyl content was markedly reduced in phosphatidylinositol and phosphatidylcholine of cells grown in the presence of n-3 fatty acids (alpha-linolenic acid, eicosapentaenoic acid and docosahexaenoic acid). Conversely, in the presence of n-6 fatty acids (linoleic acid and arachidonic acid), the total n-3 fatty acyl content was reduced in all the phospholipids examined. In n-3 and n-6 polyunsaturated fatty acid (PUFA) modified JURKAT cells, the total n-9 monounsaturated fatty acyl content in the phospholipids were markedly reduced. Changing the fatty acyl composition of membrane phospholipids in the JURKAT cells appears to have no affect on the presentation of the T cell receptor/CD3 complex or the binding of anti-CD3 antibodies (OKT3) to the CD3 complex. However, the peak increase in [Ca2+]i and the prolonged sustained phase elicited by OKT3 activation were suppressed in n-3 and n-6 PUFA but not in n-9 monounsaturated fatty acid modified cells. In Ca2+ free medium, OKT3-induced transient increase in [Ca2+]i representing Ca2+ release from the inositol 1,4,5-trisphosphate-sensitive Ca2+ stores, were similar in control and UFA modified cells. Using Mn2+ entry as an index of plasma membrane Ca2+ permeability, the rate of fura-2 fluorescence quenching as a result of Mn2+ influx stimulated by OKT3 in n-9 monounsaturated fatty acid modified cells was similar to control cells, but the rates in n-3 and n-6 PUFA modified cells were significantly lower. These results suggest that receptor-mediated Ca2+ influx in JURKAT cells is sensitive to changes in the fatty acyl composition of membrane phospholipids and monounsaturated fatty acids appears to be important for the maintenance of a functional Ca2+ influx mechanism.

Antigens, Differentiation, T-Lymphocyte↗
PubMed1991

On the estimation of total variability in assay validation.

In the pharmaceutical industry, an assay method is considered validated if the accuracy and precision for an assay meet some acceptable limits. This paper discusses the assessment of assay precision in terms of the estimation of total variability of an assay from a one-way random effects model which is often considered in assay validation. We propose a general class of estimators that includes the analysis of variance estimator and the maximum likelihood estimator. We derive the optimal estimator, in terms of smallest mean squared error, within this class and consider an approximate version of this optimal estimator. We report on a Monte Carlo simulation to study its finite sample performance. We also present two examples to illustrate the use of the proposed methodology.

Analysis of Variance↗
PubMed1991

The estimation of parameters from bulked samples.

An estimation procedure has been developed for the estimation of parameters from bulked sample using the parametric bootstrap and density estimation in conjunction with the one-step maximum-likelihood estimator. It is shown that the proposed estimation procedure provides an asymptotically efficient estimator for parameters of interest when the density for the mean of the bulked samples has a certain form. The lognormal density (with sigma 2 assumed known) is an important distribution with the proper form. The finite sample performance for bulked samples based on underlying lognormal observations was examined by Monte Carlo study. The results indicate that the proposed procedure leads to a reduction in mean squared error compared to known procedures.

Biopharmaceutics↗
PubMed1991

Assessment of bioequivalence using a multiplicative model.

In many bioavailability/bioequivalence studies, the distributions of responses such as AUC and Cmax are often positively skewed and exhibit a lack of homogeneity of variances. In this situation, a logarithmic transformation is usually considered for reduction of skewness and achievement of an additive model with relatively homogeneous variance. In this paper, consideration is given to a multiplicative model based on empirical evidence. A method is proposed using individual subject ratios for assessing the bioequivalence of two different formulations. This method is then subsequently extended to several commonly used crossover experiments.

Biological Availability↗
PubMed1991

Characterization of a monoclonal antibody directed against the phosphotyrosine kinase p56lck, in human T cells.

A monoclonal antibody (anti-p56lck) was generated against a fusion protein containing the residues 145-509 of the human p56lck, a lymphocyte-specific membrane-associated protein tyrosine kinase. The involvement of this enzyme in T-cell transmembrane signalling seems to be an early and crucial event during T-cell receptor-mediated activation. We have produced a monoclonal antibody which recognizes p56lck in free form and when associated with CD4. It functions in western blot analysis and is capable of selectively blocking auto-phosphorylation of this kinase. This monoclonal antibody should be useful for investigating the role of p56lck in T-cell activation.

Antibodies, Monoclonal↗
PubMed1991

Changes in Doppler indices of cardiac function during and after percutaneous transluminal coronary angioplasty.

OBJECTIVE: To assess the sensitivities of Doppler indices to changes in global cardiac function during and after controlled myocardial ischaemia induced by coronary angioplasty. DESIGN: Continuous wave Doppler signals of aortic flow were recorded during coronary angioplasty. The following Doppler indices of cardiac function were measured before, during, and after balloon inflation: V (peak velocity), MA (mean acceleration), V2/T (T = time from onset to peak ejection), and MD (minute distance corrected for baseline heart rate). SETTING: A tertiary care cardiological unit in a university hospital. PATIENTS: Sixteen patients undergoing coronary angioplasty of the left anterior descending coronary artery. Eight patients had multivessel disease. MAIN OUTCOME MEASURES: The primary outcome measures were planned before data collection began. RESULTS: 12 patients showed a significant fall of three or more Doppler indices from their baseline values during balloon inflation. This occurred in all patients with multivessel disease. The Doppler indices V2 T, MD, V, and MA fell by 43.7%, 37.7%, 27.4%, and 23% respectively from their baseline values (p less than 0.0001). The relative sensitivities of the Doppler indices to ischaemia were V2/T greater than MD (p less than 0.02), MD greater than (p less than 0.001), and V greater than MA (p less than 0.01). The impairment of global left ventricular function resulting from brief balloon inflation during single vessel angioplasty was reversible in all the patients. CONCLUSIONS: The Doppler indices V2/T, MD, V, and MA are all sensitive, in order of magnitude, to falls in global cardiac function resulting from ischaemia. They may prove useful for assessing cardiac function during ischaemia in the clinical setting.

Adult↗
PubMed1991

Estimating drug shelf-life with random batches.

The problem of assessing the shelf-lives of drug products in the market is considered. We focus on the situation where the drug characteristic of interest has a linear relationship over time and batch-to-batch variation is present. Two methods for assessing shelf-lives are proposed based on the weighted least squares method under a regression model with random coefficients. An application to some stability data from the pharmaceutical industry is presented.

Drug Stability↗
PubMed1990

Receptor-operated calcium influx in rat hepatocytes. Identification and characterization using manganese.

Agonist-stimulated divalent cation entry was studied in fura-2-loaded hepatocytes. In the presence of extracellular Mn2+, the Ca2(+)-mobilizing hormone vasopressin produced a severalfold stimulation of the basal rate of fura-2 fluorescence quenching as a result of Mn2+ influx; this effect was blocked by the presence of Ni2+ in the incubation medium. Half-maximum and maximum stimulation of Mn2+ influx was observed with 0.1 and 0.8 nM vasopressin, respectively. Agonist-stimulated Mn2+ influx was also seen with angiotensin II, ATP, phenylephrine, and the combination of AlCl3 and NaF. The stimulation of Mn2+ influx did not occur immediately after addition of Ca2(+)-mobilizing agents, but was characterized by a latency period of 20-30 s. In contrast to vasopressin, glucagon did not stimulate Mn2+ influx into hepatocytes, but produced both a 3-fold enhancement of the rate of vasopressin-stimulated Mn2+ entry and the abolishment of the latency period. The effects of glucagon were mimicked by forskolin and dibutyryl cAMP. Pretreatment of hepatocytes with pertussis toxin or depolarization of the cells altered neither the basal rate of Mn2+ entry nor the ability of vasopressin to stimulate this rate. Emptying of the inositol 1,4,5-trisphosphate-sensitive Ca2+ store by treatment with 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBuBHQ) did not enhance Mn2+ entry into hepatocytes; however, exposure of the cells to tBuBHQ for 2 min markedly enhanced the ability of vasopressin, alone or in combination with glucagon, to increase the rate of Mn2+ influx. Furthermore, pretreatment with tBuBHQ for 2 min abolished the latency of vasopressin-stimulated Mn2+ influx. It is concluded that Ca2(+)-mobilizing hormones stimulate Ca2+ influx in hepatocytes, possibly through receptor-operated Ca2+ channels. The stimulation of divalent cation entry is transduced by a G protein, and the rate of influx appears to be controlled both by the intracellular level of cAMP and the empty state of an intracellular Ca2+ pool that may be inositol 1,4,5-trisphosphate-insensitive.

Angiotensin II↗
PubMed1990

Inhibition of receptor-mediated calcium influx in T cells by unsaturated non-esterified fatty acids.

The effect of omega-3, omega-6 and omega-9 unsaturated fatty acids (UFAs) on receptor-mediated Ca2+ entry was investigated in a T-cell line (JURKAT) by using anti-CD3 antibodies (OKT3) to induce intracellular Ca2+ [( Ca2+]i) increase and Ca2+ influx. All the UFAs, as well as Ni2+ ions and 12-O-tetradecanoylphorbol 13-acetate, decreased the OKT3-induced sustained [Ca2+]i increase to basal levels. Although non-esterified fatty acids activate protein kinase C (PKC) [McPhail, Clayton & Snyderman (1984) Science 224, 622-624; Murakami, Chan & Routtenberg (1986) J. Biol. Chem. 261, 15424-15429], studies using H-7 and analysis of the PKC-dependent phosphorylation of 19 and 80 kDa marker substrates ruled out the involvement of PKC in UFA-induced inhibition of Ca2+ entry. Flow-cytometry analysis showed that UFAs do not interfere with antibody-receptor binding. BSA (0.2%, w/v) reversed the effect of UFAs after these fatty acids have decreased the OKT3-induced [Ca2+]i increase to basal levels. The relevance of these findings and possible mechanisms for inhibition by UFAs of receptor-mediated Ca2+ influx were discussed.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗