Comparison between the Cobas Integra immunoassay and high-performance liquid chromatography for therapeutic monitoring of carbamazepine

OBJECTIVES

Carbamazepine (CBZ) is metabolized by the cytochrome P450 into carbamazepine-10,11-epoxide (CBZepox), a metabolite with similar pharmacological activity to the parent drug. Recently it has been indicated that most current immunoassays for the determination of CBZ are unable to quantitatively measure its active epoxide. An evaluation of the Cobas Integra immunoassay for the determination of CBZ was carried out, and the results are compared with those obtained for CBZ+CBZepox using high-performance liquid chromatography (HPLC) in patients on monotherapy and polytherapy.

DESIGN AND METHODS

Steady-state serum trough concentrations of CBZ were determined in 119 epileptic patients using the Cobas Integra immunoassay and HPLC. In 91 cases CBZ was administered in monotherapy and in 28 cases in polytherapy with other anticonvulsant drugs.

RESULTS

The study of within- and between-run imprecisiom for the Cobas Integra immunoassay led to clinically acceptable coefficients of variation. A high correlation was found between the concentrations of CBZ obtained using the immunoassay and HPLC (r = 0.981, p < 0.001). In both the group of patients on monotherapy and those on polytherapy, the levels of CBZepox were greater than the clinically acceptable error for CBZ; consequently, there is a clinically significant difference between the total of CBZ+CBZepox concentrations (HPLC) and the concentrations of CBZ (immunoassay). In the group of patients under monotherapy, a high correlation coefficient was obtained between the levels of CBZ and CBZ+CBZepox (r = 0.975, p < 0.001) with an standard error of the estimate similar to the clinically acceptable value.

CONCLUSIONS

For the patients on monotherapy, it is possible to make a clinically valid estimation of CBZ+CBZepox from the concentration of CBZ obtained by means the immunoassay. In patients on polytherapy, the analytical determination of CBZepox could be of interest in cases where CBZ+CBZepox would be higher than the critical level of CBZ.

Relationship between serum cystatin C and creatinine in kidney and liver transplant patients

BACKGROUND

The results obtained for serum cystatin C, which has been proposed as a novel marker of glomerular filtration rate (GFR), in kidney and liver transplant are still very limited. In our study, the relationship between serum cystatin C and creatinine in kidney and liver transplant patients was investigated.

METHODS

Serum cystatin C and creatinine concentrations were determined in 182 samples from 52 kidney transplant patients and 71 samples from 28 liver transplant patients at 1-9870 days post-transplantation time. Eighty-seven serum samples from 66 patients with different types of chronic kidney disease were also analysed.

RESULTS

The serum creatinine (r=-0.517, p<0.001) and cystatin C (r=-0.409, p<0.001) concentrations were negatively correlated with the post-transplantation time in the kidney transplant patients. In the liver transplant patients, the correlation between these variables is not statistically significant. The creatinine/cystatin C ratio in the liver transplant group is significantly lower than in the other group of patients (p<0.001). This ratio in the kidney transplant patients groups is significantly lower than in the kidney disease group (p<0.001). In the kidney transplant patients the creatinine/cystatin C ratio and the post-transplantation time were negatively correlated (r=-0.523, p<0.001); however, in the liver transplant patients the correlation between these variables was not significant.

CONCLUSIONS

In the groups of kidney disease and kidney transplant patients, as renal function decreases, there is an increase in the creatinine/cystatin C ratio. This may be due to the fact that, since creatinine is eliminated by glomerular filtration and tubular secretion, as renal function is impaired, its serum concentration increases to a greater extent than that of cystatin C, which is only eliminated by glomerular filtration. In the liver transplant patients, the creatinine/cystatin C ratio is lower than in the other groups. This may be due to better preserved renal function, lower muscular mass and a reduced rate of creatine formation and creatinine production in some of these patients. The serum cystatin C would be a better GFR marker than the widely used creatinine in liver transplant patients.

Relationships between serum markers of monocyte/macrophage activation in type 1 Gaucher's disease

We studied 44 patients with type 1 Gaucher's disease (16 non-treated patients and 28 treated with enzyme replacement therapy). We measured serum levels of chitotriosidase (ChT), neopterin, angiotensin-converting enzyme (ACE), adenosine deaminase (ADA) and beta-hexosaminidase (Hex) and its major isoenzymes Hex A and Hex B. In the untreated group of patients, the increase in serum levels was ChT>neopterin>ACE> ADA>Hex, with all decreasing significantly in treated patients (p< 0.001). Highly significant correlations were obtained between the markers of monocyte/macrophage activation which were tested (p<0.001). However, partial correlations between serum Hex B (with Hex A constant) and ChT, ACE, neopterin and ADA did not reach statistical significance. This suggests that hepatocytes are the major cellular source of this isoenzyme. Similarly, partial correlation of ChT with neopterin, with the other variables constant, was not significant, which would suggest a different expression of these two markers in Gaucher's disease.

Assessment of copper status in pregnancy by means of determining the specific oxidase activity of ceruloplasmin

BACKGROUND

Conditions not directly related to copper nutriture, such as pregnancy, infections and inflammation, which increase serum copper concentration even during copper deprivation, may be expected to conceal changes in copper status. It has been suggested that the specific enzymatic activity of ceruloplasmin (activity per unit mass of enzyme protein) may be a sensitive indicator of copper status and is not affected by factors such as hormones or sex. In this study, we investigated the behaviour of specific oxidase activity of ceruloplasmin and the copper/ceruloplasmin ratio in pregnant women.

METHODS

Copper, immunoreactive ceruloplasmin and its oxidase activity were determined in serum from 52 women in the last trimester of normal pregnancy, and in 50 control women of similar age living in the same area and who were not taking oral contraceptives. The results are expressed as mean+/-S.E.M.

RESULTS

In the group of pregnant women, significantly higher serum levels of copper, ceruloplasmin and its oxidase activity were found than in the control group (p < 0.001). In both groups, a high correlation was found between these biochemical variables (r > or =0.905, p < 0.001). However, in the group of pregnant women the specific oxidase activity for ceruloplasmin (364.4+/-3.3 vs. 407.5+/-3.8 U/g) and the copper/ceruloplasmin ratio (2.82+/-0.03 vs. 3.19+/-0.04 microg/mg) were significantly lower than in the control group (p < 0.001).

CONCLUSIONS

Although pregnancy accelerates the rate of ceruloplasmin protein synthesis and release with an increase of serum copper, the decrease in specific oxidase activity of circulating ceruloplasmin would be an indicator of the degree of depletion of the mother's copper deposits in order to deal with the foetus' needs.

Serum cystatin C-immunoglobulin high-molecular-weight complexes in kidney and liver transplant patients

BACKGROUND

It has been suggested recently that the glomerular filtration rate (GFR) in renal transplant patients is underestimated by serum cystatin C due to an impaired filtration of complexed cystatin C with immunoglobulins. Consequently, serum cystatin C may not be a reliable marker of GFR in these patients. Our study was designed to determine whether this supposition is correct.

METHODS

In 87 serum samples from patients with various kidney diseases, 182 samples from renal transplant patients, and 72 samples from liver transplant patients, the concentrations of cystatin C and creatinine were determined, as well as the residual concentration of cystatin C after precipitation of macromolecules with polyethylene glycol (PEG; 6000 molecular weight).

RESULTS

The residual concentration of serum cystatin C after precipitation with PEG in all cases was much higher (70 to 100%) than that expected in the case of the existence of cystatin C-immunoglobulin complexes. In the kidney and liver transplant patients, there was no significant correlation between the residual concentration of cystatin C and the postoperative time (r = -0.098).

CONCLUSIONS

The results suggest that in renal or liver transplant patients there is no formation of high molecular weight serum cystatin C-immunoglobulin complexes, regardless of the post-transplant period.

Plasma beta-N-acetylhexosaminidase isoenzyme composition and temperature conversion factors

Using the chromogenic substrate 3,3'-dichlorophenylsulfonphthaleinyl-N-acetyl-beta-D-glucosaminide for the activity determination of plasma beta-N-acetylhexosaminidase (Hex), the temperature conversion factors (TCF) offer a highly significant positive correlation with the relative proportion of Hex B isoenzyme (P< 0.001). The calculation of TCF 37 degrees/25 degrees C allows the isoenzyme composition of Hex to be determined quickly and cheaply. The results may be superimposed over those obtained in a previously described method based on the calculation of the enzyme's activation energy using four temperatures. However, the use of TCF 37 degrees/30 degrees C does not appear to comply with the required demands.

Specific oxidase activity of cord serum ceruloplasmin in the newborn

In a study of 50 full-term newborns, we found serum levels of ceruloplasmin, its oxidase activity and specific oxidase activity (activity per unit mass of enzyme protein) which were significantly lower than in adult subjects (p<0.001). A significant correlation was obtained between the specific oxidase activity of the neonatal ceruloplasmin and the transferrin/alpha-fetoprotein ratio (p<0.01), which reflects the maturity of the fetal hepatocyte through its ability to synthesise fetal- and non-fetal-associated proteins. The lower specific oxidase activity of the ceruloplasmin in newborns would be due to a higher relative proportion of apoprotein, and/or other molecular forms with a poor copper content in the oxidase sites. This could be due to an inability to transfer copper into a common intracellular pool for holoceruloplasmin synthesis and biliary copper excretion. A high interindividual variability was found for the specific oxidase activity of the neonatal ceruloplasmin, and in the newborns with higher levels of ceruloplasmin this, from a catalytic point of view, is more similar to the adult form.

Relationship between plasma ammonia concentration and beta-N-acetylhexosaminidase isoenzyme activities in liver cirrhosis

Ammonia is known to increase the secretion of beta-N-acetylhexosaminidase (Hex) to the extracellular medium in cultured human fibroblasts and Hep G2 cells. We examined 35 patients with liver cirrhosis and the results revealed a significant increase for the plasma activities of total Hex and its isoenzymes Hex A and Hex B (p < 0.001). The partial correlations, with other biochemical markers of liver injury constant, between plasma ammonia concentration and the activity (r = 0.658) and the proportion in percentage of the Hex B isoenzyme (r = 0.692) were statistically significant (p < 0.001). The increased concentration of ammonia could explain, at least partially, an increased secretion of Hex B isoforms to the plasma in patients with liver disease.

Immunochemical and enzymatic study of ceruloplasmin in rheumatoid arthritis

Forty adult patients (30 women and 10 men) with rheumatoid arthritis (RA), treated with nonsteroidal anti-inflammatory drugs, were studied. Serum levels of immunoreactive ceruloplasmin, oxidase activity of the ceruloplasmin and total copper, as well as the specific oxidase activity (enzyme activity per unit of mass) and the copper/immunoreactive ceruloplasmin relationship were significantly higher in the group of patients than in the healthy control group (p < 0.001). However, no significant difference was found for the concentration of non-ceruloplasmin copper between both groups. A statistically significant negative correlation was obtained for the concentration of serum thiobarbituric acid-reacting substances with the immunoreactive ceruloplasmin and its oxidase activity in the group of patients (p < 0.005). These results suggest that in RA increases of serum copper are produced at the expense of the fraction linked to the ceruloplasmin, diminishing the proportion of apoceruloplasmin and other forms poor in copper. Although the increase in the serum concentration of ceruloplasmin might offer an additional safeguard against oxidative stress. it does not appear to have a beneficial effect upon the activity of the illness as evaluated by means the biological inflammation markers C-reactive protein, erythrocyte sedimentation rate and sialic acid.

Pharmacological modification of the serotonergic transmitter system and beta-N-acetylhexosaminidase activity in rats

The enzyme activity and activation energy of plasma beta-N-acetylhexosaminidase (Hex) was determined in rats whose serotonergic system had been pharmacologically altered. In the group of animals treated with 5-hydroxytryptophan, in the different dissected brain regions (brain stem, cortex and hippocampus) significantly higher levels of serotonin and 5-hydroxyindolacetic acid were found, and significantly lower in the group treated with p-chlorophenylalanine, than in the control group. In the total number of animals studied (n = 21), a statistically significant correlation was found between the plasma concentration of 5-hydroxyindolacetic acid and the levels of this metabolite in the different brain regions (p < 0.001). No significant differences were found for the activity of Hex in the plasma, or for its activation energy, which is a marker of its isoenzyme composition, among the three groups of animals. The results obtained using our experimental model in rats do not confirm the hypothesis of other authors who suggest that the Hex responds secondary to increases or decreases of serotonin turnover, and could be a biological test to monitor the serotonin status in psychiatric patients.

Comparison of predose vs 2-h postdose blood metabolites/cyclosporine ratios in kidney and liver transplant patients

OBJECTIVES

It has recently been suggested that when adjusting doses of cyclosporine (CsA), determining its concentration in blood samples taken 2 h postdose (C(2)) is more clinically beneficial than using the predose concentration (C(0)). We determined C(0) and C(2) concentrations of CsA and their metabolites in samples taken from nine kidney and seven liver transplant patients. Similarly, the so-called metabolic ratios (MR)-metabolites to CsA parent ratios-were calculated to characterise the most suitable moment of blood sampling for obtaining a greater analytical specificity with monoclonal immunoassays.

METHODS

The determination of CsA and CsA + metabolites was made using the enzyme multiplied immunotechnique and the polyclonal fluorescence polarization immunoassay Abbott TDx, respectively.

RESULTS

The poor correlation between C(0) and C(2) of CsA (n = 82, r = 0.387, p < 0.001) is greatly inferior to that obtained between C(0) and C(2) of metabolites (n = 82, r = 0.912, p < 0.001). A highly significant difference (p < 0.001) was found between MR(0) values (mean 2.87 +/- 0.12, median 2.48) and MR(2) values (mean 1.73 +/- 0.09, median 1.46), although there is a good correlation between them (r = 0.878, p < 0.001).

CONCLUSIONS

The extent of the positive bias (deviation) of CsA immunoassays compared with the high-performance liquid chromatography results is related to the MR values. As the MR(2) values are significantly lower than the corresponding MR(0), in practice a greater analytical specificity would be obtained with the different monoclonal immunoassays in the determination of the 2 h postdose CsA concentration than in that of trough concentration.

Thermodynamic characterisation of the mutated isoenzyme A of beta-N-acetylhexosaminidase in GM2-gangliosidosis B1 variant

Here we report the determination of the activation energies of the plasma isoenzymes of beta-N-acetylhexosaminidase (Hex, EC 3.2.1.52), isolated by chromatography in DEAE-cellulose, using the neutral chromogenic substrate 3,3'dichlorophenylsulfonphthaleinyl-N-acetyl-beta-D-glucosaminide. The activation energy of mutated Hex A isoenzyme (Ea approximately 71.5 kJ/mol) from a patient with GM2-gangliosidosis B1 variant, homozygote for the G533-->A (Arg178His) mutation, was significantly higher than that of normal Hex A (Ea approximately 41.8 kJ/mol) and analogous to that of Hex B isoenzyme (Ea approximately 75.1 kJ/mol). The determination of this thermodynamic variable of Hex in different biological specimens could allow for a straightforward biochemical characterisation of the GM2-gangliosidosis B1 variant.

Characterization of the isoenzyme profile of beta-N-acetylhexosaminidase in the urine of newborns

The urinary isoenzymes of beta-N-acetylhexosaminidase (Hex) in newborn infants were characterised by chromatography, electrophoresis, thermodynamic analysis and through substrate specificity. No qualitative difference was found for the major Hex A and Hex B isoenzymes between full-term or premature newborns and adults, although in the latter group the relative proportion of Hex B is much lower (18.5 +/- 2.7% vs. 36.3 +/- 1.0%). An additional minor enzyme form was found in some premature newborns, which eluted from the DEAE-cellulose column at a higher concentration of NaCl than Hex A and, like this isoenzyme, is able to hydrolyse 4-methylumbellipheryl-2-acetamido-2-deoxy-beta-D-glucopyranoside-6 -sulphate, which would suggest that it has alpha subunits in its molecule. These results do not confirm the hypothesis of other authors about the existence of a unique fetal Hex isoenzyme in neonatal urine which eluted before the application of the NaCl gradient, similarly to the Hex B.

Assay of beta-N-acetylhexosaminidase isoenzymes in urine by means of determination of their activation energy without removing endogenous low-molecular-mass components

The determination of the activation energy of beta-N-acetylhexosaminidase (Hex, EC 3.2.1.52), using 3,3'-dichlorophenylsulfonphthaleinyl-N-acetyl-beta-D-glucosaminide as substrate, allows its isoenzyme composition to be evaluated in different biological specimens. However, in the analysis of urine samples, it is necessary first to remove the endogenous low-molecular-mass components, as these provoke an over-estimation of the activation energy of the Hex and, consequently, of the relative proportion of Hex B isoenzyme. The study of this interference has allowed urea to be characterised as the only urinary metabolite that is responsible, and to establish a mathematical expression for the correction, in relation to the endogenous urea concentration, of the activation energy of the Hex obtained experimentally in samples of native urine. The results thus obtained for the isoenzyme composition of urinary Hex are similar to those found using an electrophoretic separation procedure.

Assay of beta-N-acetylhexosaminidase isoenzymes in different biological specimens by means of determination of their activation energies

The activation energy (Ea) of beta-N-acetylhexosaminidase (Hex, EC 3.2.1.52) was determined with 3,3'-dichlorophenylsulfonphthaleinyl-N-acetyl-beta-D-glucosaminide as substrate, with a much higher value being found for the Hex B isoenzyme (Ea = 75.1 kJ/mol) than for the Hex A isoenzyme (Ea = 41.8 kJ/mol). This fact allowed for the development of a fast and reliable thermodynamic method to determine the isoenzyme composition of Hex in different biological specimens (serum/plasma, saliva, cerebrospinal fluid, seminal plasma, urine, and leukocyte lysates). The results in serum given by the proposed method may be superimposed upon those obtained by the heat inactivation assay of O'Brien et al. (N Engl J Med 1970;273:15-20), and the catalytic activity calculated for Hex A offers a good correlation with that obtained by using the specific substrate 4-methylumbelliferyl-N-acetyl-beta-D-glucosaminide-6 sulfate (n = 25, r = 0.953).

Assay of β-N-acetylhexosaminidase isoenzymes in different biological specimens by means of determination of their activation energies

The activation energy (Ea) of β-N-acetylhexosaminidase (Hex, EC 3.2.1.52) was determined with 3,3′-dichlorophenylsulfonphthaleinyl-N-acetyl-β-d-glucosaminide as substrate, with a much higher value being found for the Hex B isoenzyme (Ea = 75.1 kJ/mol) than for the Hex A isoenzyme (Ea = 41.8 kJ/mol). This fact allowed for the development of a fast and reliable thermodynamic method to determine the isoenzyme composition of Hex in different biological specimens (serum/plasma, saliva, cerebrospinal fluid, seminal plasma, urine, and leukocyte lysates). The results in serum given by the proposed method may be superimposed upon those obtained by the heat inactivation assay of O’Brien et al. (N Engl J Med 1970;273:15–20), and the catalytic activity calculated for Hex A offers a good correlation with that obtained by using the specific substrate 4-methylumbelliferyl-N-acetyl-β-d-glucosaminide-6 sulfate (n = 25, r = 0.953).

[Interconversion of the results for erythrocyte zinc protoporphyrin expressed in different units]

As stated in a recent document of the National Committee for Clinical Laboratory Standards (NCCLS Document C42-P, April 1995), expression of the results of erythrocyte zinc protoporphyrin (ZPP) in different units is a main source of confusion which may hamper the wide use of this test. The possible inter-conversion of the ZPP values expressed as microgram/g Hb, microgram/dL blood and microgram/dL red cells was assessed. It was found that in all cases, despite a good correlation between these values (r > or = 0.939), the error of the estimation was higher than allowed by the American College of Pathologists criteria. Such findings show that ZPP values expressed in different units cannot be inter-converted as a function of the corresponding regression equation. Similarly, the use of hematofluorometers with an analogic-digital converter to express results as microgram ZPP/ dL blood might pose an additional source of error.

Assay of serum/plasma beta-N-acetylhexosaminidase isoenzymes by heat inactivation using a continuous spectrophotometric method adapted to a centrifugal analyzer

Activity of serum/plasma beta-N-acetylhexosaminidase (EC 3.2.1.52) was determined by means of a continuous spectrophotometric method using 3,3'-dichlorophenylsulphonphthaleinyl-N-acetyl-beta-D-glucosaminid e as substrate, with very satisfactory results. Incubation of an undiluted aliquot (1 ml) of samples at 52 degrees C for 8 hours with an adjusted pH 5.5-6.0 provoked only the inactivation of isoenzyme A, thus allowing the evaluation of beta-N-acetylhexosaminidase isoenzyme composition. In 25 serum samples from control subjects and pregnant women, a good correlation between the percentage of isoenzyme B obtained by this procedure and the fluorimetric assay of O'Brien et al. (New Engl J Med 1970; 273:15-20) was found (r = 0.983, S(yx) = 1.51), with no statistically significant difference between the means (43.2 vs 42.8%). In 84 healthy adult subjects, an average value of 30.3% for the proportion of isoenzyme B was obtained, with an interval of 25.4-35.0%, in agreement with results reported by other authors.

Activation energy of serum 5'-nucleotidase as a marker for enzyme heterogeneity

Using adenosine 5'-monophosphate (Arkesteijn; J. Clin. Chem. Clin. Biochem. 14 (1976) 155) and inosine 5'-monophosphate (Bertrand & Buret; Clin. Chim. Acta 119 (1982) 275) as substrates for the determination of serum 5'-nucleotidase, linear Arrhenius plots were obtained between 25 degrees C and 37 degrees C. With the latter method, in patients with hepatobiliary disease, activation energy is higher than in healthy subjects (67.8 +/- 0.8 kJ.mol-1 vs. 59.2 +/- 1.6 kJ.mol-1, p < 0.001). Neuraminidase treatment resulted in a significant reduction in the activation energy of serum 5'-nucleotidase from patients (63.6 +/- 0.6 kJ.mol-1, p < 0.001). Although the activation energy seen is also influenced by conditions other than sialic acid content, the results obtained are in accordance with the hypothesis that in liver disease serum 5'-nucleotidase becomes more sialylated.

A de-sialylated isoform of serum 5'-nucleotidase: clinical and biological significance in hepatobiliary disease

Using an electrophoretic technique on cellulose acetate, three multiple forms of the 5'-nucleotidase (5'NU) with mobilities alpha 1, alpha 2, and beta appear in the serum of healthy subjects. The difference between the alpha 1 and alpha 2 isoforms lies in their sialylation degree, the alpha 2-5'NU being a de-sialylated form. In 147 patients with different hepatobiliary diseases the alpha 2 isoform was present in only 19% of cases, and there was no significant difference in the activity of total 5'NU, alpha 1-5'NU and beta-5'NU between patients with or without alpha 2-5'NU, alpha 1-5'NU and beta-5'NU (P < 0.001), as with other biochemical indicators of liver damage. It is suggested that in hepatobiliary diseases an increase of the sialylation of serum 5'NU occurs, which would explain the absence of the desialylated alpha 2 isoform in the majority of cases. However, the decrease of hepatic receptors of asialoglycoproteins would lead to an increase of this de-sialylated isoform in the serum of certain patients.