Candidate reference method for determining creatinine in serum: method development and interlaboratory validation

We describe a "high-performance" liquid chromatographic (HPLC) method for accurately determining creatinine in serum. After prechromatographic precipitation of protein, we performed isocratic ion-exchange chromatography with ultraviolet detection (234 nm). Analytical results showed linearity up to 1770 mumol/L, a detection limit of 22 mumol/L, an average analytical recovery of 101%, and a CV ranging from 3% to 11%. We used certified human serum (National Institute of Standards and Technology), and additional lyophilized serum pools also assayed by definitive isotope-dilution mass spectrometry, to validate the accuracy of the HPLC method. In addition, the isocratic HPLC results showed close agreement with those obtained with a step-gradient HPLC method. We also compared the isocratic HPLC method with alkaline picrate and enzymatic methods. Our findings with samples from nonuremic, uremic, and diabetic ketoacidotic patients confirmed the positive bias previously reported with the alkaline picrate method. Interlaboratory transferability of the method was demonstrated with various commercial instruments and analytical columns. We evaluated column stability and possible interference from endogenous or exogenous compounds. On the basis of our analytical findings, we recommend the isocratic HPLC method as a candidate Reference Method for determining creatinine in serum.

Assessment of renal function by inulin clearance: comparison with creatinine clearance as determined by enzymatic methods

We compared creatinine clearances determined by enzymatic (Kodak Ektachem 700 single-slide, Boehringer Mannheim creatinine PAP) and nonenzymatic (Jaffé, HPLC) methods with glomerular filtration rate measured by inulin clearance in patients with varying degrees of renal function. The Kodak enzymatic assay gave values for creatinine 2 to 3 mg/L higher than the other methods. This resulted in significantly lower creatinine clearances than inulin clearances and creatinine clearances determined by the other methods. However, correlations between all methods for serum and urinary creatinine values and clearances were good. To avoid between assay (enzymatic vs nonenzymatic) discrepancies, manufacturers should agree to an acceptable standard of calibration under the usual conditions used with patients.

Assessment of renal function by inulin clearance: comparison with creatinine clearance as determined by enzymatic methods

We compared creatinine clearances determined by enzymatic (Kodak Ektachem 700 single-slide, Boehringer Mannheim creatinine PAP) and nonenzymatic (Jaffé, HPLC) methods with glomerular filtration rate measured by inulin clearance in patients with varying degrees of renal function. The Kodak enzymatic assay gave values for creatinine 2 to 3 mg/L higher than the other methods. This resulted in significantly lower creatinine clearances than inulin clearances and creatinine clearances determined by the other methods. However, correlations between all methods for serum and urinary creatinine values and clearances were good. To avoid between assay (enzymatic vs nonenzymatic) discrepancies, manufacturers should agree to an acceptable standard of calibration under the usual conditions used with patients.

Monoclonal antibodies for radioimmunoassay of cyclosporine: a multicenter comparison of their performance with the Sandoz polyclonal radioimmunoassay kit

The performance of a radioimmunoassay kit containing monoclonal specific and nonspecific antibodies to cyclosporine (Sandimmun-Kit; Sandoz Ltd., Basle, Switzerland) was compared with that of the original Sandoz polyclonal radioimmunoassay kit (Ciclosporin RIA-Kit). A total of 1320 blood and plasma samples from patients receiving cyclosporine after kidney, heart, liver, and bone-marrow transplantation were analyzed at six centers. For blood samples the median result on using the specific assay was about 50% of the polyclonal assay result after kidney and bone-marrow transplantation, about 33% after heart and liver transplantation; comparable figures for plasma samples were 70 and 40%. The monoclonal nonspecific-antibody assay produced results 10% to 140% higher than polyclonal-assay results, depending on sample matrix and transplant indication; the largest difference was seen in samples from heart- and liver-transplant recipients. Evidently the specific-antibody assay provides a convenient alternative to high-performance liquid chromatography for specific measurement of the drug, but the role of the new nonspecific antibody, possessing an even broader spectrum of cross-reactivity with cyclosporine metabolites than the original polyclonal antiserum, has yet to be defined.

Monoclonal antibodies for radioimmunoassay of cyclosporine: a multicenter comparison of their performance with the Sandoz polyclonal radioimmunoassay kit

The performance of a radioimmunoassay kit containing monoclonal specific and nonspecific antibodies to cyclosporine (Sandimmun-Kit; Sandoz Ltd., Basle, Switzerland) was compared with that of the original Sandoz polyclonal radioimmunoassay kit (Ciclosporin RIA-Kit). A total of 1320 blood and plasma samples from patients receiving cyclosporine after kidney, heart, liver, and bone-marrow transplantation were analyzed at six centers. For blood samples the median result on using the specific assay was about 50% of the polyclonal assay result after kidney and bone-marrow transplantation, about 33% after heart and liver transplantation; comparable figures for plasma samples were 70 and 40%. The monoclonal nonspecific-antibody assay produced results 10% to 140% higher than polyclonal-assay results, depending on sample matrix and transplant indication; the largest difference was seen in samples from heart- and liver-transplant recipients. Evidently the specific-antibody assay provides a convenient alternative to high-performance liquid chromatography for specific measurement of the drug, but the role of the new nonspecific antibody, possessing an even broader spectrum of cross-reactivity with cyclosporine metabolites than the original polyclonal antiserum, has yet to be defined.

Determination of cyclosporine concentrations with monoclonal antibodies

We measured cyclosporine in whole blood from normal volunteers administered single oral doses of the drug and from two renal-transplant patients on immunosuppressive maintenance therapy, by liquid chromatography (I) and by radioimmunoassay with use of nonspecific polyclonal (II), specific monoclonal (III), and nonspecific monoclonal (IV) antibodies. Concentrations determined by III were equivalent to I, irrespective of cyclosporine dose, concentration, time after dose, or time after transplant. Concentrations determined by II and IV were consistently higher than those by I, owing to cross reactivity with metabolites. Ratios of values by II and IV to those by I increased from less than 1.5 to about 3-4 between 0.5 and 12 h after a single cyclosporine dose, owing to differences in rates of appearance and disappearance of cyclosporine and cross-reacting metabolites, though for the constant 12-h dose intervals in the two renal-transplant patients at steady state these ratios (most within the range 3-4) were relatively stable. Ratios of concentrations measured by IV to those by II (mean of 1.2 for single-dose data, most within the range of 1.2 to 1.5 at steady state) were unaffected by time after dose or time after transplant, suggesting that, despite certain cross-reactivity differences between the two nonspecific antibodies, results are proportional throughout therapy. We therefore propose that III and IV offer alternatives, respectively, to the currently used I and II for cyclosporine monitoring.

Determination of cyclosporine concentrations with monoclonal antibodies

We measured cyclosporine in whole blood from normal volunteers administered single oral doses of the drug and from two renal-transplant patients on immunosuppressive maintenance therapy, by liquid chromatography (I) and by radioimmunoassay with use of nonspecific polyclonal (II), specific monoclonal (III), and nonspecific monoclonal (IV) antibodies. Concentrations determined by III were equivalent to I, irrespective of cyclosporine dose, concentration, time after dose, or time after transplant. Concentrations determined by II and IV were consistently higher than those by I, owing to cross reactivity with metabolites. Ratios of values by II and IV to those by I increased from less than 1.5 to about 3-4 between 0.5 and 12 h after a single cyclosporine dose, owing to differences in rates of appearance and disappearance of cyclosporine and cross-reacting metabolites, though for the constant 12-h dose intervals in the two renal-transplant patients at steady state these ratios (most within the range 3-4) were relatively stable. Ratios of concentrations measured by IV to those by II (mean of 1.2 for single-dose data, most within the range of 1.2 to 1.5 at steady state) were unaffected by time after dose or time after transplant, suggesting that, despite certain cross-reactivity differences between the two nonspecific antibodies, results are proportional throughout therapy. We therefore propose that III and IV offer alternatives, respectively, to the currently used I and II for cyclosporine monitoring.

Prevention of diabetes in BB rats by intermittent administration of cyclosporine

The successful prevention of spontaneous autoimmune diabetes in biobreeding (BB) rats--the closest animal model to human type I diabetes mellitus--by daily administrations of cyclosporine (CsA) has prompted clinical trials of CsA immunosuppression in human diabetes. Although remissions from hyperglycemia have been achieved in human subjects, nephrotoxicity and recurrence of diabetes after discontinuation of CsA have been observed. Therefore we studied the biologic efficacy of intermittent administration of CsA, a theoretically less dangerous immunosuppressive protocol, in the prevention of spontaneous diabetes in the BB rat. Beginning at 30 to 49 or 50 to 55 days of age, treated animals (n = 86) received daily injections of CsA (15 mg/Kg) for 2 weeks (induction phase) and then twice weekly (maintenance phase) until 160 days of age. A third group of animals (n = 31) received daily CsA for 14 days only. Control littermates (n = 121) were not injected. All animals were followed to 275 days of age. Intermittent administration of CsA was determined to be a biologically effective regimen in the prevention of spontaneous diabetes in the BB rat. Blood levels of CsA and the major CsA metabolites were undetectable intermittently during the course of therapy. Major complications associated with CsA immunosuppression (nephrotoxicity, malignancy, and infection) were not associated with the intermittent CsA protocol. We conclude that spontaneous diabetes can be delayed and often permanently prevented by intermittent administration of CsA. This immunosuppressive regimen deserves further consideration as a biologically effective, but theoretically less toxic, therapeutic regimen.

In vitro immunosuppressive properties of cyclosporine metabolites

The in vitro biological activity of cyclosporine (CsA) and four of its metabolites (M1, M8, M17, and M21) was determined. M1, M17, and M21 are primary metabolites, while M8 is a secondary metabolite derived from either M1 or M17. The order of inhibitory activity in production assays was phytohemagglutinin (PHA), concanavalin A (ConA), mixed lymphocyte culture (MLC), and interleukin-2 (IL-2) CsA greater than M17 greater than M1 greater than M21 much greater than M8. In the PHA assay, CsA was significantly more inhibitory than M17, but in Con A and MLC assays, the inhibitory activity of M17 approached that of CsA. More importantly, M17 and M1 inhibited the production of IL-2 in the MLC to the same extent as CsA. M21 was significantly less inhibitory than either M17 or M1, and M8 appeared to be largely devoid of biological activity. These experiments demonstrate that single hydroxylations of amino acids 1 (M17) and 9 (M1) do not significantly affect the ability of the molecule to block IL-2 production, but hydroxylation of both amino acids renders the molecule virtually inactive. In addition, the presence of the N-methyl group on amino acid 4 appears to be very important, since removal of this group (M21) greatly diminishes the immunosuppressive activity.

Immunosuppressive metabolites of cyclosporine in the blood of renal allograft recipients

Cyclosporine levels by radioimmunoassay (RIA) and high-performance liquid chromatography (HPLC) were monitored in serial blood samples (n = 177) from 11 renal allograft recipients. HPLC analysis revealed three primary metabolites of CsA (M17, M1, and M21) in peak and trough blood samples; M17 was the preponderant metabolite. In 4 patients on whom serial metabolite assays were performed, M17 was found in the blood at 86-2004 ng/ml; M1 and M21 were found at up to 100 ng/ml. The immunosuppressive properties of purified metabolites M1, M17, M21, and M8 (which was not detected in the blood) were compared with CsA. M17--and, to a lesser extent, M1 and M21--were found to inhibit the in vitro response of human mononuclear cells in the mixed leukocyte culture and in mitogen (phytohemagglutinin [PHA], concanavalin A [Con A], and pokeweed mitogen [PWM]) assays at 1000 ng/ml. M8 exhibited no in vitro inhibitory activity. M17 was further tested at 10-1000 ng/ml in PHA and mixed lymphocyte culture (MLC) assays. M17 had considerably less inhibitory activity (12-43%) than CsA (18-70%) in the PHA assay. However, in MLC experiments M17 blocked the proliferative response by 39-72% at 100-800 ng/ml, which approached the degree of inhibition exhibited by CsA (63-87%). In 34 of 37 (92%) patient blood samples, the level of metabolite M17 was found to exceed the parent drug level and could not be measured accurately by RIA. The observed in vitro immunosuppressive activity of metabolites (particularly M17) and their presence in the blood of renal allograft recipients suggest a possible role for these metabolites in the immunopharmacology of CsA.

Effect of hematocrit on cyclosporine (cyclosporin A) in whole blood and plasma of renal-transplant patients

Cyclosporine concentrations in whole blood and plasma were determined in 25 samples from renal-transplant patients. The portion of cyclosporine carried in plasma varied widely, from 9 to 58%. Radioimmunoassay and liquid chromatography both revealed a statistically significant inverse correlation between hematocrit and the plasma fraction of cyclosporine in the circulating blood. Regression analysis indicates that a 10% increase in hematocrit would decrease the portion of cyclosporine in plasma by 12 to 14%. Studies with cyclosporine-supplemented samples further demonstrated an effect of hematocrit on the drug concentration in plasma without a corresponding effect on the concentration in whole blood. The distribution of cyclosporine between plasma and erythrocytes was unaffected by plasma cholesterol and triglycerides for these transplant patients.

Effect of hematocrit on cyclosporine (cyclosporin A) in whole blood and plasma of renal-transplant patients

Cyclosporine concentrations in whole blood and plasma were determined in 25 samples from renal-transplant patients. The portion of cyclosporine carried in plasma varied widely, from 9 to 58%. Radioimmunoassay and liquid chromatography both revealed a statistically significant inverse correlation between hematocrit and the plasma fraction of cyclosporine in the circulating blood. Regression analysis indicates that a 10% increase in hematocrit would decrease the portion of cyclosporine in plasma by 12 to 14%. Studies with cyclosporine-supplemented samples further demonstrated an effect of hematocrit on the drug concentration in plasma without a corresponding effect on the concentration in whole blood. The distribution of cyclosporine between plasma and erythrocytes was unaffected by plasma cholesterol and triglycerides for these transplant patients.

Liquid chromatographic assay of ceftizoxime in sera of normal and uremic patients

The application of high-pressure liquid chromatography assays for cephalosporin serum concentrations is difficult in uremic patients because of interference from nondialyzable substances. We developed a high-pressure liquid chromatography method for determining the serum concentration of ceftizoxime in normal and uremic patients. The method involves protein precipitation with acetonitrile, followed by removal of the acetonitrile with dichloromethane. Separation was accomplished with a reverse-phase (C-18) column and a mobile phase of 13% acetonitrile and 2.8% acetic acid. UV detection at 310 nm was used to monitor the peaks. This assay produced a linear relationship between peak height ratio and ceftizoxime concentration from 1.5 to 100 micrograms/ml. Samples from 30 patients were assayed by this method and by a bioassay, with a good correlation of results (r = 0.9832). The method was applicable equally to normal and uremic serum samples.

Liquid-chromatographic evaluation of age-related changes in the urinary excretion of free catecholamines in pediatric patients

Pediatric patients (to age 18) without neuroblastoma show an age-related decrease in urinary excretion of each of the catecholamines--epinephrine, norepinephrine, and dopamine--in relation to creatinine excretion. From these data, I have developed reference intervals for pediatric age groups. Application of these ranges to seven patients with neuroblastoma and ganglioneuroblastoma indicated a high clinical sensitivity for the urinary dopamine determination but significant false-negative results for epinephrine and norepinephrine.

Liquid-chromatographic evaluation of age-related changes in the urinary excretion of free catecholamines in pediatric patients

Pediatric patients (to age 18) without neuroblastoma show an age-related decrease in urinary excretion of each of the catecholamines--epinephrine, norepinephrine, and dopamine--in relation to creatinine excretion. From these data, I have developed reference intervals for pediatric age groups. Application of these ranges to seven patients with neuroblastoma and ganglioneuroblastoma indicated a high clinical sensitivity for the urinary dopamine determination but significant false-negative results for epinephrine and norepinephrine.