John Earl

Population pharmacokinetics of liposomal amphotericin B in pediatric patients with malignant diseases.

ABSTRACT A population pharmacokinetic model of liposomal amphotericin B (L-AmB) in pediatric patients with malignant diseases was developed and evaluated. Blood samples were collected from 39 pediatric oncology patients who received multiple doses of L-AmB with a dose range from 0.8 to 5.9 mg/kg of body weight/day. The patient cohort had an average age of 7 years (range, 0.2 to 17 years) and weighed an average of 28.8 ± 19.8 kg. Population pharmacokinetic analyses were performed with NONMEM software. Pharmacokinetic parameters, interindividual variability (IIV), between-occasion variability (BOV), and intraindividual variability were estimated. The influence of patient characteristics on the pharmacokinetics of L-AmB was explored. The final population pharmacokinetic model was evaluated by using a bootstrap sampling technique. The L-AmB plasma concentration-time data was described by a two-compartment pharmacokinetic model with zero-order input and first-order elimination. The population mean estimates of clearance (CL) and volume of distribution in the central compartment (V1) were 0.44 liters/h and 3.12 liters, respectively, and exhibited IIV (CL, 10%) and significant BOV (CL, 46% and V1, 56%). The covariate models were CL (liters/h) = 0.44 · e0.0152 ·(WT−21) and V1 (liters) = 3.12 · e0.0241 · (WT − 21), where WT is the patients body weight (kg) centered on the study population cohort median of 21 kg. Model evaluation by the bootstrap procedure indicated that the full pharmacokinetic model was robust and parameter estimates were accurate. In conclusion, the pharmacokinetics of L-AmB in pediatric oncology patients were adequately described by the developed population model. The model has been evaluated and can be used in the design of rational dosing strategies for L-AmB antifungal therapy in this special population.

Cerebrospinal fluid neopterin in paediatric neurology: a marker of active central nervous system inflammation.

Aim  Cerebrospinal fluid (CSF) neopterin production is increased by interferon‐gamma stimulation and appears to act as a marker of intrathecal immune activation. We aimed to test the usefulness of elevated CSF neopterin as a biological marker of central nervous system (CNS) inflammation.

Tyrosine hydroxylase deficiency: clinical manifestations of catecholamine insufficiency in infancy.

Inborn errors of catecholamine biosynthesis are rare but of great interest as they are genetic disorders, and in some, treatment may completely reverse severe neurological abnormalities. They also provide insights into the action of the biogenic amines in the developing brain. We describe the clinical course of an infant with tyrosine hydroxylase (TOH) deficiency over a 30‐month period. The parents are consanguineous, and genetic analysis revealed the infant to be homozygous for the common G698A mutation in the TOH gene. TOH deficiency can be seen as a model of pure catecholamine deficiency. Experimental evidence, reports of other disorders of biogenic amines, and our experience with this infant suggest that the symptoms of catecholamine deficiency in infancy can be broadly subdivided. Signs of dopamine deficiency include tremor, hypersensitivity to levadopa (L‐dopa) therapy, oculogyric crises, akinesia, rigidity, and dystonia. Manifestations of norepinephrine deficiency include ptosis, miosis, profuse oropharyngeal secretions, and postural hypotension. Hypersensitivity to L‐dopa was a particular management problem in this infant.

Cerebrospinal fluid concentrations of vincristine after bolus intravenous dosing: a surrogate marker of brain penetration.

Vincristine (VCR) is used widely in oncology practice, and regular dosing is commonly associated with the development of sensorimotor or autonomic neuropathies. However, the incidence of VCR‐related central nervous system (CNS) toxicity is comparatively low, suggesting that the blood‐brain barrier may limit drug penetration into the brain parenchyma. This study determined whether measurable concentrations of VCR could be detected in the cerebrospinal fluid (CSF), as a surrogate marker of brain parenchyma penetration, after bolus intravenous injection in children without primary CNS pathology.

Perinatal hypophosphatasia presenting as neonatal epileptic encephalopathy with abnormal neurotransmitter metabolism secondary to reduced co-factor pyridoxal-5′-phosphate availability

We describe two neonates presenting with perinatal hypophosphatasia and severe epileptic encephalopathy resulting in death. Both had increased levels of urinary vanillactate, indicating functional deficiency of aromatic amino acid decarboxylase, a pyridoxal-5-phosphate (PLP)-dependent enzyme required for dopamine and serotonin biosynthesis. Clinical findings and results of subsequent metabolic investigations were consistent with secondary pyridoxine-deficient encephalopathy. These patients highlight the importance of tissue non-specific alkaline phosphatase in the neuronal PLP-dependent metabolism of neurotransmitters. In addition, the disturbance of PLP metabolism appears to underlie the predominant neurological presentation in our patients. We recommend the measurement of serum alkaline phosphatase (ALP) during the assessment of perinatal seizures.

Variability in the pharmacokinetics of intravenous busulphan given as a single daily dose to paediatric blood or marrow transplant recipients

AIM To examine inter- and intrapatient variability in the pharmacokinetics of intravenous (i.v.) busulphan given as a single daily dose to children with malignant (n = 19) and nonmalignant (n = 21) disease. METHODS Busulphan (120 mg m(-2), 130 mg m(-2) or 3.2 mg kg(-1)) was administered over median 2.1 h. Blood samples (4-10) were collected after the first dose, busulphan concentrations were measured and pharmacokinetic parameters, including clearance (CL) and area under the concentration-time curve (AUC), were determined using the Kinetica software (Innaphase). Interpatient variability was assessed as percent coefficient of variation (% CV). Intrapatient variability was assessed by calculating percent differences between observed full dose AUC and AUC predicted from an initial 65 mg m(-2) dose in 13 children who had busulphan pharmacokinetic monitoring. RESULTS Clearance of i.v. busulphan in 40 children was 4.78 +/- 2.93 l h(-1) (% CV 61%), 0.23 +/- 0.08 l h(-1) kg(-1) (% CV 35%) and 5.79 +/- 1.59 l h(-1) m(-2) (% CV 27%). Age correlated significantly (p < 0.001) with CL (l h(-1)) and CL (l h(-1) kg(-1)), but not with CL (l h(-1) m(-2)). AUC normalized to the 130 mg m(-2) dose ranged from 14.1 to 56.3 mg l(-1) x h (% CV 37%) and also did not correlate with age. Interpatient variability in CL (l h(-1) m(-2)) was highest in six children with immune deficiencies (60%) and lowest in seven children with solid tumours (14%). Intrapatient variability was <13% for nine (of 13) children, but between 20 and 44% for four children. CONCLUSIONS There is considerable inter- and intrapatient variability in i.v. busulphan CL (l h(-1) m(-2)) and exposure that is unrelated to age, especially in children with immune deficiencies. These results suggest that monitoring of i.v. busulphan pharmacokinetics is required.

HeLa cells cocultured with peripheral blood lymphocytes acquire an immuno‐inhibitory phenotype through up‐regulation of indoleamine 2,3‐dioxygenase activity

The mechanisms by which tumour cells escape recognition by the immune system or subvert antitumour effector responses remain poorly understood. In the course of investigating the potential of costimulatory signals in anticancer immunotherapy strategies, we have observed that HeLa cells (a human cervical carcinoma cell line) cocultured with peripheral blood lymphocytes (PBL) acquire the capacity to inhibit PBL proliferation in response to interleukin‐2 (IL‐2). This immuno‐inhibitory phenotype was further shown to result from induction of the tryptophan‐catabolizing enzyme, indoleamine 2,3‐dioxygenase (IDO), by interferon‐γ (IFN‐γ) secreted from cocultured allo‐reactive PBL. This enzyme has recently been shown to be a critically important modulator of immunological responses, most notably through the capacity to protect allogeneic concepti from alloreactive maternal lymphocytes. While the cytostatic consequences of IDO activity in tumour cells has received attention, the data presented in this report support the hypothesis that IDO activity may also act to impair antitumour immune responses.

Population pharmacokinetics of melphalan in patients with multiple myeloma undergoing high dose therapy

AIMS To i) investigate the pharmacokinetics of total and unbound plasma melphalan using a population approach, ii) identify clinical factors that affect melphalan disposition and iii) evaluate the role of melphalan exposure in melphalan-related toxicity and disease response. METHODS Population pharmacokinetic modelling (using NONMEM) was performed with total and unbound concentration-time data from 100 patients (36-73 years) who had received a median 192 mg m(-2) melphalan dose. Model derived estimates of total and unbound melphalan exposure (AUC) in patients with serious melphalan toxicity and those who had a good disease response (>or=90% decrease in paraprotein concentrations) were compared using the Mann-Whitney test. RESULTS A two compartment model generated population mean estimates for total and unbound melphalan clearance (CL) of 27.8 and 128 l h(-1), respectively. Estimated creatinine clearance, fat free mass and haematocrit were important determinants of total and unbound CL, reducing the inter-individual variability in total CL from 34% to 27% and in unbound CL from 42% to 30%. Total AUC (range 4.9-24.4 mg l(-1) h) and unbound AUC (range 1.0-6.5 mg l(-1) h) were significantly higher in patients who had oral mucositis (>or=grade 3) and long hospital admissions (P < 0.01). Patients who responded well had significantly higher unbound AUC (median 3.2 vs. 2.8 mg l(-1) h, P < 0.05) when assessed from diagnosis to post-melphalan and higher total AUC (median 21.3 vs. 13.4 mg l(-1) h, P= 0.06), when assessed from pre- to post-melphalan. CONCLUSIONS Creatinine clearance, fat free mass and haematocrit influence total and unbound melphalan plasma clearance. Melphalan exposure is related to melphalan toxicity while the association with efficacy shows promising trends that will be studied further.

Increasing the Dosage of Vincristine A Clinical and Pharmacokinetic Study of Continuous-Infusion Vincristine in Children with Central Nervous System Tumors

Vincristine (VCR) is widely used to treat patients with malignant disease; among the patients treated with VCR are children with brain tumors. In vitro studies have demonstrated that the cytotoxic activity of VCR is related to both extracellular concentration and duration of exposure. The attainment of higher plasma concentrations by injecting larger bolus doses of VCR has been limited by concerns about neurotoxicity. One possible alternative strategy for enhancing the antitumor efficacy of VCR involves prolonging the duration of in vivo exposure. Therefore, the authors explored the neurotoxicity and pharmacokinetics of VCR administered via a 96‐hour continuous infusion after administration of a conventional bolus dose in a pediatric population.

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.