Population pharmacokinetics of acyclovir in children and young people with malignancy after administration of intravenous acyclovir or oral valacyclovir.

Acyclovir is effective in the prevention and treatment of herpes simplex virus (HSV) and varicella-zoster virus (VZV) infections. The aim of this study was to characterize the population pharmacokinetics of acyclovir observed following treatment with intravenous acyclovir and oral valacyclovir (valaciclovir) in young people with malignancy. Plasma acyclovir concentration-time data were collected from 43 patients (age range, 9 months to 20 years) who had been given multiple doses of acyclovir (5 mg/kg of body weight) and/or valacyclovir (10 mg/kg). Nonlinear mixed-effect modeling was employed to analyze acyclovir population pharmacokinetics and identify influential covariates. Simulations (n = 1,000) were conducted to explore the ability of the current doses to maintain acyclovir concentrations above the recommended 50% inhibitory concentration for HSV or VZV (0.56 mg/liter or 1.125 mg/liter, respectively) for more than 12 h. A one-compartment pharmacokinetic model with first-order elimination best described the acyclovir concentration-time data. The population mean estimates for clearance (CL), volume of distribution (V), absorption rate (k(a)), and bioavailability (F) were 3.55 liters/h, 7.36 liters, 0.63 h(-1), and 0.60, respectively. Inclusion of body weight and estimated creatinine CL (CL(CR)) in the final model reduced the interindividual variabilities in CL and V from 61% to 24% and from 75% to 36%, respectively. Simulations revealed that with the use of the current doses, maximal efficacy can be achieved in over 45% of patients weighing 25 to 50 kg and with CL(CR) levels of 2.0 to 4.0 liters/h/m(2), but only in a much smaller proportion of patients, with low weights (10 kg) and high CL(CR)s (5.5 liters/h/m(2)), suggesting that higher doses are required for this subgroup. This validated population pharmacokinetic model for acyclovir may be used to develop dosing guidelines for safe and effective antiviral therapy in young people with malignancy.

HPLC-UV assay for monitoring total and unbound mycophenolic acid concentrations in children.

A simple, accurate and sensitive HPLC method was developed for measuring total and unbound mycophenolic acid (MPA) in human plasma. Total MPA was extracted by protein precipitation and ultrafiltration was used to assess unbound MPA concentrations. The supernatant (20 microL) or ultrafiltrate (100 microL) was injected onto a C(18) HPLC column with a mobile phase of 0.05 m sodium phosphate buffer (pH 2.31)-acetonitrile (55:45, v/v for total MPA; 50:50 for unbound MPA) with UV detection at 254 nm. The extraction recovery was over 93% and reproducible. The assay was linear over the concentration range of 0.07-50 mg/L for total MPA and 4-1500 microg/L for unbound MPA. Intra- and inter-day assay reproducibility was less than 10%. Detection limits were 0.04 mg/L and 2 microg/L for total and unbound MPA, respectively. The assay utility was established in samples collected from five paediatric bone marrow transplant recipients who were receiving intravenous doses of mycophenolate mofetil. In these patients MPA concentrations ranged from 0.07 to 7.83 mg/L and unbound drug concentrations ranged from 2.1 to 107.5 microg/L. This method can be effectively applied to MPA pharmacokinetics in paediatric patients.

HPLC-fluorescence assay for acyclovir in children.

A simple, accurate, reliable and sensitive HPLC method was developed and validated for quantitating acyclovir in human plasma. Sample (100 microL) preparation involved addition of guanosine (internal standard) and protein precipitation with 7% perchloric acid and centrifugation. Supernatant (20 microL) was injected onto a C18 HPLC column with a mobile phase of 0.05 m sodium phosphate buffer-acetonitrile (pH 2.35, 992:8, v/v) with 25 microL of 0.4 m tetrabutylammonium hydroxide titrant and fluorescence detection (excitation, 260 nm; emission, 375 nm). Analyte recovery was 101% and the assay response was linear over the acyclovir concentration range of 0.1-20 mg/L. Intra- and inter-day accuracy and precision were less than 7%. The limit of detection and limit of quantitation were 0.033 and 0.1 mg/L, respectively. In five paediatric oncology patients administered intravenous acyclovir, concentrations ranged from 0.24 to 43.65 mg/L. This method can be used to measure acyclovir concentrations in paediatric patients.

Busulphan given as four single daily doses of 150 mg/m2 is safe and effective in children of all ages.

We examined the effects of busulphan (BU) dose and patient age on toxicity and outcome in 63 children with acute leukaemia given BUCY prior to allogeneic or autologous BMT. BU was administered as four single daily oral doses, based either on weight (4 x 4 mg/kg) or surface area (4 x 150 mg/m2). BU pharmacokinetic analysis was not used to dose adjust. The average daily (mg/kg) BU dose was 43% higher for the group given 150 mg/m2 compared to the 4 mg/kg dose group. This produced a median BU AUC 61% higher than with the 4 mg/kg dose. Only one child did not achieve full allogeneic donor engraftment. Regimen-related toxicity was low. Although younger children had faster BU clearance, the 4 x 150 mg/m2 dose ensured equivalent systemic exposure to BU, and resulted in a high frequency of engraftment without a significant increase in serious toxicity. BU, given as four single daily doses of 150 mg/m2, is appropriate and safe in all age groups of children. Given the reliable pharmacokinetics, low toxicity and high rate of allogeneic engraftment, there is no need for routine pharmacokinetic monitoring or dose modifications. This dosage regimen may be applicable for use with i.v. BU.

Population pharmacokinetics of amphotericin B in children with malignant diseases.

AIMS: To construct a population pharmacokinetic model for the antifungal agent, amphotericin B (AmB), in children with malignant diseases. METHODS: A two compartment population pharmacokinetic model for AmB was developed using concentration-time data from 57 children aged between 9 months and 16 years who had received 1 mg kg(-1) day(-1) doses in either dextrose (doseform=1) or lipid emulsion (doseform=2). P-Pharm (version 1.5) was used to estimate the basic population parameters, to identify covariates with significant relationships with the pharmacokinetic parameters and to construct a Covariate model. The predictive performance of the Covariate model was assessed in an independent group of 26 children (the validation group). RESULTS: The Covariate model had population mean estimates for clearance (CL), volume of distribution into the central compartment (V) and the distributional rate constants (k12 and k21) of 0.88 l h(-1), 9.97 l, 0.27 h(-1) and 0.16 h(-1), respectively, and the intersubject variability of these parameters was 19%, 49%, 55% and 48%, respectively. The following covariate relationships were identified: CL (l h(-1)) = 0.053 + 0.0456 weight (0.75) (kg) + 0.242 doseform and V (l) = 7.11 + 0.107 weight (kg). Our Covariate model provided unbiased and precise predictions of AmB concentrations in the validation group of children: the mean prediction error was 0.0089 mg l(-1) (95% confidence interval: -0.0075, 0.0252 mg l(-1)) and the root mean square prediction error was 0.1245 mg l(-1) (95% confidence interval: 0.1131, 0.1349 mg l(-1)). CONCLUSIONS: A valid population pharmacokinetic model for AmB has been developed and may now be used in conjunction with AmB toxicity and efficacy data to develop dosing guidelines for safe and effective AmB therapy in children with malignancy.

Pharmacokinetics of cyclosporin in children with stable renal transplants.

Fourteen children, aged between 5 and 17 years, with stable renal graft function and stable cyclosporin A (CSA) trough levels (Cmin) were studied. They had been taking CSA 12-hourly since their transplant 1.5-9 years previously, with the average dose of Neoral being 6.4 (range 4.4-8.4) mg/kg per day. CSA whole blood levels were measured at 0, 20, and 40 min, and at 1, 1.5, 2, 2.5, 3, 4, 6, and 8 h following the morning dose using the Abbott TDx fluorescence polarization immunoassay. The area under the concentration time curve (AUC), clearance adjusted for bioavailability (CL/F), and steady-state volume of distribution adjusted for bioavailability (Vss/F) were determined using model-independent pharmacokinetic analysis. Delay time (Tdel), peak concentration (Cmax), time to peak concentration (Tmax), and Cmin were also determined and correlated with AUC and other parameters. The Tdel in absorption varied from 0.3 to 1.6 (mean 0.73) h, resulting in a similarly variable time to Tmax of 1-2.4 h (mean 1.59). Tmax was related to the age of the patient (Tmax = 0.027 age + 1.41, r2 = 0.56, P < 0.005). The AUC showed good correlation with Cmax (Cmax = 0.25 AUC + 423.32, r2 = 0.96, P < 0.0005). Cmax appears to be a more-suitable measure of exposure to CSA than Cmin. Prediction of Tmax from the age of the child may help to overcome the problem of when to collect blood for peak levels.

Amphotericin B in children with malignant disease: a comparison of the toxicities and pharmacokinetics of amphotericin B administered in dextrose versus lipid emulsion.

In a prospective, randomized clinical trial, the toxicity of 1 mg of amphotericin B (AmB) per kg of body weight per day infused in 5% dextrose was compared with that of AmB infused in lipid emulsion in children with malignant disease. In an analysis of 82 children who received a full course of 6 days or more of AmB (117 courses), it was shown that there were significant increases in plasma urea and creatinine concentrations and in potassium requirement after 6 days of therapy with both AmB infused in dextrose and AmB infused in lipid emulsion, with there being no difference between the two methods of AmB administration. An intent-to-treat comparison of the numbers of courses affected by acute toxicity (fever, rigors) and chronic toxicity (nephrotoxicity) also indicated that there was no significant difference between AmB infused in dextrose (78 courses) and AmB infused in lipid emulsion (84 courses). The pharmacokinetics of AmB were investigated in 20 children who received AmB in dextrose and 15 children who received AmB in lipid emulsion. Blood samples were collected up to 24 h after administration of the first dose, and the concentration of AmB in plasma was analyzed by a high-performance liquid chromatography assay. The clearance (CL) of AmB in dextrose (0.039 +/- 0.016 liter. h-1. kg-1) was significantly lower (P < 0.005) than the CL of AmB in lipid emulsion (0.062 +/- 0. 024 liter. h-1. kg-1). The steady-state volume of distribution for AmB in dextrose (0.83 +/- 0.33 liter. kg-1) was also significantly lower (P < 0.005) than that for AmB in lipid emulsion (1.47 +/- 0.77 liter. kg-1). Although AmB in lipid emulsion is apparently cleared faster and distributes more widely than AmB in dextrose, this study did not reveal any significant advantage with respect to safety and tolerance in the administration of AmB in lipid emulsion compared to its administration in dextrose in children with malignant disease.

Fetal oxygen saturation monitoring in labour: an analysis of 118 cases.

Fetal oxygen saturation (FSpO2) was recorded during labour to determine the relationship between FSpO2 and indicators of fetal well-being, including umbilical blood gases, xanthine (X), hypoxanthine (Hx) and Apgar scores. This is one of the largest reported series of fetal pulse oximetry, with 118 fetuses monitored for over 329 hours. Mean FSpO2 for all cases was 46.9% (SD = 9.1%). There was no correlation between FSpO2 during the last 10 minutes of monitoring and arterial pH, Hx or X. A mean FSpO2 > or = 30% was associated with a 5 minute Apgar score of > or = 7 in the majority of cases. One fetus had a mean FSpO2 < 30% during the final 10 minutes of monitoring and an umbilical arterial pH < 7.20, while there were 10 fetuses with an umbilical arterial pH < 7.20, and mean FSpO2 > or = 30%. As these numbers are small, a larger series is necessary to further characterize the small number of fetuses who are significantly hypoxic.

Busulfan pharmacokinetics using a single daily high-dose regimen in children with acute leukemia.

The pharmacokinetics of busulfan, given as a single daily dose (either 4 mg/kg or 150 mg/m2), was determined in 22 children undergoing bone marrow transplantation for acute leukemia. The single daily dose regimen showed similar pharmacokinetics to previously reported regimens of 4 x 1 mg/kg, except for fourfold higher mean peak plasma levels and negligible trough levels. Daily systemic exposure for single dose regimens based on weight (4 mg/kg) or surface area (150 mg/m2), respectively were very similar to regimens of (4 x 1 mg/kg) or (4 x 37.5 mg/m2). Dose (milligrams per kilogram), peak plasma level, and area under the curve (AUC) were all higher in 12 children treated with 150 mg/m2 busulfan than in 9 children treated with 4 mg/kg. AUC was age dependent for the 4 mg/kg dose but not for the 150 mg/m2 dose. The use of a 150 mg/m2 dose allows escalation of the dose above 4 mg/kg, eliminating the tendency for younger children to receive lower systemic exposure. Little toxicity was observed in this study. Clearance and distribution volume correlated negatively with age, and AUC correlated positively with dose (milligram per kilogram). Administration of busulfan as crushed rather than whole tablets reduced the delay time for appearance of busulfan in plasma but had no effect on absorption or other pharmacokinetic parameters.

Determination of conjugated and unconjugated serum 3 alpha-OH bile acids by high-performance liquid chromatography.

A simple reverse-phase liquid chromatographic method has been developed for the quantitative measurement of 11 3 alpha-OH bile acids in paediatric serum samples. Bile acids are enzymatically reduced to the corresponding 3-keto compound and then derivatized with the fluorophore dansyl hydrazine. Preliminary fractionation of bile acids is not required. The limit of detection is 0.1 mumol/L using a sample size of 200 microL. One hundred and twenty-three serum samples were analysed by the high-performance liquid chromatography (HPLC) method and the results compared with a commercial kit method (Enzabile) presently used in many laboratories for the measurement of total bile acids. There was a good correlation between the two methods (r = 0.96). The method is suitable for use in a clinical laboratory for the further investigation of those patients with abnormally high total bile acid levels where quantification of bile acid fractions is required.

Mystery of the poisoned expedition.

The Burke and Wills expedition through the interior of Australia in the nineteenth century ended in calamity. But the cause of death was more pernicious than anyone at the time had imagined: beriberi due to thiaminase poisoning.

Quality assurance for biogenic amines with authentic patient specimens.

The measurement of biogenic amines and metabolites is essential for diagnosis and follow-up of neural crest tumors. A Quality Assurance programme involving the distribution of urine specimens obtained from patients with neural crest and related tumors was conducted by the Australian Association of Clinical Biochemists' Working Party on Biogenic Amines in 1988. Fifty laboratories participated in the programme and measured a number of analytes with a variety of methods. These included high performance liquid chromatography with electrochemical detection (HPLC-EC), spectrophotometry, fluorescence, fluorescence polarization immunoassay (Abbott TDX) and gas chromatography--mass spectroscopy (GC-MS). The results of this survey indicated that fluorimetric methods for catecholamines are unreliable as they are subject to interference particularly by Labetalol. Twice as many laboratories utilized catecholamines rather than metanephrines for detection of pheochromocytoma. 5-HIAA appears to be the analyte measured with the least accuracy, particularly with spectrophotometric methods of analysis. Several laboratories would have failed to diagnose some neural crest tumors and need to improve their performance.

Successful treatment of dihydropteridine reductase deficiency, with an interesting effect of 5-hydroxytryptophan deficiency on sleep patterns.

We report a child in whom dihydropteridine reductase deficiency was diagnosed prenatally because of an affected sibling and who was treated from birth with apparent good response. This family has been reported before (Firgaira et al., 1980, 1981, 1983; Lipson et al., 1984; Cotton et al., 1986; Dahl et al., 1988). The parents are Lebanese Muslims who are first cousins, and their first and third children, both male, are well. The third child was diagnosed as normal in utero (Firgaira et al., 1983). The second child, the subject of an early report (Lipson et al., 1984), has dihydropteridine reductase deficiency. The dihydropteridine reductase is nonfunctional due to an amino acid insertion (Howells et al., 1990).

Occurrence of platelet-activating factor (PAF) and an endogenous inhibitor of platelet aggregation in diffuse cutaneous mastocytosis.

We have identified PAF in the blister fluid from a patient with bullous mastocytosis, a rare form of mast-cell disease. We have found a novel endogenous inhibitor of platelet aggregation which obscured the presence of the PAF in unprocessed blister fluid and in ethanol or lipid extracts. The PAF was characterized by the demonstration of chromatographic, mass spectral and biological properties identical to those of authentic PAF. Thus this is the first demonstration of PAF in biological fluid from a patient with mastocytosis. High levels of immunoreactive prostaglandin D2 (PGD2) and histamine were also present in the blister fluid. The interaction between PAF and the inhibitor of platelet aggregation in patients with systemic mastocytosis may provide an explanation for some of the manifestations of the disease, in particular the episodic hypotension, cutaneous flushing and pallor.

Distribution of plasma vitamin E levels in supplemented very low birthweight infants.

The distribution of plasma Vitamin E (VE) was determined in 25 very low birthweight (VLBW) infants who were supplemented with 100 mg/kg per day of alpha-tocopherol acetate, given intragastrically. Their mean birthweight was 917 g and mean gestational age was 28 weeks. Mean plasma VE levels after 1 and 6 weeks' supplementation were 2.7 mg/dL (s.e.m. = 1.0) and 6.4 mg/dL (s.e.m. = 1.4), respectively (the difference was not significant). There was wide variability in plasma VE levels in these infants despite being on an identical dose of tocopherol. Plasma VE was less than 0.5 mg/dL in 12% of samples, 0.5-3.0 mg/dL in 32%, 3.1-5.0 mg/dL in 18%, and 5.1-20 mg/dL in 38%. Fifteen of the 25 infants had at least one level in the range which has been associated with an increased incidence of septicaemia and necrotizing enterocolitis (greater than 5.0 mg/dL). These data suggest that if a policy of VE supplementation for VLBW infants is chosen, monitoring of plasma VE levels appears necessary so that the dosage can be adjusted in order to maintain plasma VE within the optimal range. This study's dosage regimen of supplementing infants with 100 mg/kg per day of VE was associated with a high incidence of elevated plasma VE levels and it is concluded that it is not advisable to use such large doses of VE in the premature newborn.

Liver disease and common-bile-duct stenosis in cystic fibrosis.

To determine the incidence of common-bile-duct lesions and their relation to liver disease in cystic fibrosis, we performed hepatobiliary scanning in 50 of 61 patients with cystic fibrosis who had hepatomegaly, abnormal liver function, or both and in 31 of 92 patients with cystic fibrosis who did not have hepatomegaly or abnormal liver function. Ninety-six percent of the patients with liver disease had evidence of biliary tract obstruction, which was defined cholangiographically as a stricture of the distal common bile duct in the majority of cases. All the patients without liver disease had normal intrahepatic and common-duct excretion of tracer. Abdominal pain was significantly more common in patients with common-duct obstruction (P less than 0.001), and enlarged gallbladders occurred only in such patients. Since fasting levels of serum bile acids were elevated in nearly half these patients, irrespective of the severity of their liver disease, serum bile acids may be markers of the severity of the common-duct lesion. We conclude that strictures of the distal common bile duct are common in patients with cystic fibrosis and liver disease. This association requires further study, since surgical relief of common-duct obstruction may prevent or ameliorate the hepatic complications of cystic fibrosis.

Determination of delta-aminobutyric acid and other amino acids in cerebrospinal fluid of pediatric patients by reversed-phase liquid chromatography.

The reversed-phase liquid-chromatographic system described here is capable of resolving the neurotransmitter amino acids aspartic acid, glutamic acid, and gamma-aminobutyric acid (GABA) plus 21 other amino acids in cerebrospinal fluid (CSF) in a single analysis. The amino acids, derivatized with o-phthalaldehyde, are separated in 65 min. Concentrations of glutamine less than or equal to 600 mumol/L can be measured at the same time as GABA greater than or equal to 10 nmol/L. Using this method, we have determined reference intervals for amino acids, including GABA, in CSF in a group of pediatric patients who underwent lumbar puncture before myelography, and who were subsequently shown to have normal myelograms. These intervals are generally lower than those previously reported for childhood, but we believe this results from a more rigid selection of the control group. In addition, artifactual increases in concentrations of free neurotransmitters, caused by breakdown of amino acid conjugates, are minimized by (a) immediate freezing of the CSF samples to prevent enzyme-mediated changes, (b) omission of a deproteinization step, and (c) precolumn derivatization to reduce on-column breakdown of amide and peptide forms.