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Henk-Jan Guchelaar

Publications and source records attributed to Henk-Jan Guchelaar.

29 records · Page 2Linked to original sources
PubMed2003

CYP1A2 activity is an important determinant of clozapine dosage in schizophrenic patients.

Clozapine is an effective atypical antipsychotic drug applied in the treatment of resistant schizophrenia. The drug is mainly metabolized by cytochrome P-450 (CYP) enzymes especially the isozyme CYP1A2. Remarkably, the effective dosage varies widely among patients, making it necessary to individualize drug therapy with clozapine. The explanation for dosage variation may be differences in drug metabolism, and more specifically of CYP1A2 activity. This study is aimed at determining to what extent variability in clozapine dose can be explained by pharmacokinetic (PK) factors and more specifically by CYP1A2 activity in effectively treated psychiatric patients. In 22 evaluable patients with a schizophrenic disorder chronically using clozapine, the CYP1A2 activity and the clozapine clearance were estimated. For calculation of the pharmacokinetic parameters of clozapine, population PK software based upon Bayesian analysis was used. Caffeine clearance was estimated with the paraxanthine/caffeine ratio and served as estimate of CYP1A2 activity.A significant linear relationship was found between the clozapine dose and clozapine clearance (R: 0.71; P<0.05), whereas no relationship was found between clozapine dosage and clozapine serum trough concentration. Moreover, individual caffeine and clozapine clearances were found to be significantly related (R: 0.62; P<0.05) as were clozapine dose per kg body weight and P/C mol ratio (R: 0.44; P<0.05). We conclude that CYP1A2 activity is an important determinant of the variability of effective clozapine doses in psychiatric patients.

Adult↗
PubMed2003

Implementation of a computerized physician medication order entry system at the Academic Medical Centre in Amsterdam.

In the period 1997-2001 the Academic Medical Centre in Amsterdam implemented the computerized physician medication order entry (CPmOE) system Medicator. This article describes several important aspects of this program: technological architecture, features, implementation project, authentication and training, continuous support, human resource investments, route of prescription, logistics and administration. Furthermore important advantages and disadvantages of the CPmOE system are discussed. Advantages mainly concern patient safety and drug logistics, while disadvantages are related to access to a computer, user friendliness of the software and printer problems.

Academic Medical Centers↗
PubMed2003

The potential role of computerisation and information technology in improving prescribing in hospitals.

For physicians, prescribing of drugs is one of the most common actions in daily practice. In the continuum prescribing, dispensing, administration and use of drugs, failures may occur and can lead to patient harm. The use of computerised physician order entry systems is subject to much discussion regarding medication error reduction. This commentary analyses the issues where such systems can contribute to improved care.

Computers↗
PubMed2003

Risk assessment in clinical pharmacy.

Medication error forms a major proportion of the errors in the medical system. Despite many studies of adverse drug events, there are no systematic ways of ensuring safety, or of assessing how safe a pharmaceutical system is. Risk assessment is required in hazardous industries such as nuclear power or oil and gas. Risk assessments involve identifying the defences and assessing their effectiveness and are relatively uncommon in clinical pharmacy, as opposed to reactive approaches involving incident analyses. Risk factors, that degrade barriers, can be identified and their effect measured. A risk assessment structure for pharmacy processes is proposed that can also be used to support incident investigation and analysis processes and provide a standard for audit.

Humans↗
PubMed2003

Two novel methods for the determination of CYP1A2 activity using the paraxanthine/caffeine ratio.

Cytochrome P450 1A2 (CYP1A2) plays an important role in drug metabolism. Provocation with caffeine is used to estimate CYP1A2 activity, but in most tests a long period of caffeine abstinence has to be taken into account. We compared two novel methods with the currently applied test. The pharmacokinetic (PK) parameters of caffeine and paraxanthine were estimated in eight caffeine-taking healthy volunteers by fitting serum concentration-time data to a two-compartment PK model. Then a three-step approach was followed. Step 1: The caffeine administration regimens of three provocation methods, which differ by their periods of abstinence, together with the PK parameters of each volunteer, were entered in a PK simulation program and the molecular ratio of the paraxanthine/caffeine concentration (P/C molratio) of each method was estimated for the individual volunteers. Step 2: For each method a relationship for the population between the caffeine clearance (Clc) and the corresponding P/C molratio was empirically established. Step 3: The true caffeine clearance (Clc tr) of each volunteer, as found by fitting the individual PK curve, was compared for all three methods with the clearance estimated from the individual P/C molratio using the relationship of step 2. The predictive values for Clc of the three methods did not differ significantly from Clc tr. For the three methods the values for bias were 6.7, 4.3 and 3.1%, respectively and for precision they were 12.3, 20.6 and 17.8%. We conclude that the two novel methods of caffeine provocation show good predictive performance for Clc when compared with the conventional method. Abstaining from caffeine for a long period is not necessary to estimate CYP1A2 activity (using the P/C molratio) accurately.

Adult↗
PubMed2003

An in vitro study on the active conversion of flucytosine to fluorouracil by microorganisms in the human intestinal microflora.

BACKGROUND: Investigation of the rate of active conversion of flucytosine to fluorouracil by microorganisms in the intestinal microflora. METHODS: Active conversion of flucytosine was investigated using viable and nonviable Escherichia coli at different flucytosine concentrations. Additionally, flucytosine conversion was studied in fecal specimens from 3 neutropenic patients at the start of the antimicrobial/antifungal prophylaxis (C/A regimen) and 1 week later. RESULTS: Flucytosine levels decreased by an average of 72, 71 and 72% flucytosine after incubation for 48 h of 10(10) viable E. coli /ml suspension in broth containing 13, 130 and 1300 mg/l flucytosine, respectively. The decreasing flucytosine levels corresponded approximately to an identical increase in fluorouracil levels. Also, a 44% decrease of flucytosine levels occurred when nonviable E. coli were used, indicating that bacterial viability is not necessary for this conversion. When fecal specimens of 2 patients were investigated prior to the C/A regimen, significant flucytosine conversion occurred, whereas this conversion was not observed in the corresponding fecal specimens after 1 week of C/A regimen. CONCLUSION: These in vitro experiments showed that extensive flucytosine conversion can occur in the human intestinal microflora by E. coli. Consequently, fluorouracil exposure and fluorouracil-related toxicity may occur in the flucytosine-treated patient.

Antifungal Agents↗
PubMed2002

5-fluorocytosine-related bone-marrow depression and conversion to fluorouracil: a pilot study.

The aim of this study is to investigate whether fluorouracil (5-FU) could be responsible for bone-marrow depression occurring in fluorocytosine (5-FC) treated patients. Six 5-FC treated patients were included in this pilot study. Toxicity was monitored by means of thrombocyte and leucocyte counts. 5-FC and 5-FU serum levels were measured using a high-performance liquid chromatography (HPLC) assay that allows simultaneous determination of both compounds. The amounts of 5-FU in the 34 available serum samples remained below the limit of quantitation (< 0.05 mg/L), whereas 5-FC levels could be detected in all samples. Instead, low levels of the 5-FU catabolite alpha-fluoro-beta-alanine (FBAL) were detected in several of the investigated serum samples. In case of three patients thrombocyte counts remained within the normal range during 5-FC treatment, whereas one patient developed thrombocytopenia (50 x 10(9) thrombocytes/L) during therapy. Furthermore, one patient developed leucocytopenia (2.6 x 10(9) leucocytes/L) during 5-FC therapy, whereas the remaining five patients were suffering from leucocytosis prior to 5-FC therapy. In conclusion, we found nondetectable 5-FU serum concentrations (< 0.05 mg/L) in ICU patients treated with intravenous 5-FC, making it unlikely that 5-FC-associated toxicity results from 5-FU exposure in patients receiving intravenous 5-FC therapy. These findings may be explained by the fact that our patients received 5-FC intravenously instead of orally, therefore not allowing active conversion of 5-FC to 5-FU by the human intestinal microflora.

Adult↗
PubMedDate not supplied

Acute effects of high-dose furosemide on residual renal function in CAPD patients.

BACKGROUND: High doses of furosemide can increase urine volume in chronic peritoneal dialysis (CAPD) patients. However, no information is available about effects on urinary solute excretion in relation to residual glomerular filtration rate (GFR), urinary furosemide excretion, and peritoneal solute kinetics. METHODS: Diuretic response and the effect on peritoneal fluid and solute transport parameters were investigated in 7 stable CAPD patients with residual renal function (median urine volume 350 mL/24 hours, range 140- 1900 mL/24 hours). Comparisons were made during two clearance periods of 24 hours: one without (P1) and one during 2 g furosemide (P2). RESULTS: The median increase in urine volume was 400 mL (range 270 - 910 mL, p < 0.02) and the increase in sodium excretion was 54 mmol (range 25 - 118 mmol, p < 0.02). No change in GFR was found between P1 (2.4 mL/ minute, range 0.6 - 5.7 mL/min) and P2 (2.0 mL/min, range 1.0 - 4.8 mL/min). An increase in fractional clearance was found for volume, sodium, potassium, and osmolality (p < 0.02). No change was found in the fractional clearance of urea and electrolyte-free water. Furosemide excretion in urine was 8.7 mg/24 hours (range 2.1 - 38 mg/24 hours) and in dialysate 4.9 mg/24 hours (range 1.9 - 7.8 mg/ 24 hours). Plasma furosemide concentration was 29.5 mg/L (range 6.2 - 43.9 mg/L). A positive correlation was found between residual GFR and total urine furosemide excretion (r = 0.93, p < 0.005). Efficiency, expressed as the increase in fractional sodium clearance (percent) per milligram of furosemide excreted per 24 hours, was 1.2%/mg (range 0.3% - 11.3%/mg). CONCLUSION: High-dose furosemide is effective in CAPD patients in increasing urine volume and electrolyte excretion without affecting urea and creatinine clearance. In CAPD patients, the individual response to an identical high dose of furosemide is dependent on the magnitude of residual GFR.

Adult↗