Evaluation of amoxicillin, metronidazole and gentamicin dosage regimens for use in antibiotic prophylaxis in colorectal surgery

OBJECTIVES

To evaluate amoxicillin, metronidazole and gentamicin dosage regimens for antibiotic prophylaxis in colorectal surgery.

METHODS

The study was conducted in 20 patients undergoing colorectal surgery. Patients received one or two doses of amoxicillin 1000 mg, metronidazole 500 mg and gentamicin 3 mg/kg ideal body weight, banded by height. Antibiotic concentrations were measured up to 7 h post dose. Population pharmacokinetic (PopPK) analysis with NONMEM followed by Monte Carlo simulation of different dosage regimens was used to estimate the PTA for potential organisms associated with surgical site infections (SSIs).

RESULTS

A median of 5 (range 3-6) concentrations were available per patient. CL and V of all antibiotics were related to weight; gentamicin CL was also related to CLCR. The administered doses maintained the desired PTA up to 8 h for the Streptococcus anginosus group but not for enterococci, Bacteroides fragilis group, MSSA, and Escherichia coli. An additional 500 mg amoxicillin every 4 h was sufficient to achieve the PTA for most relevant organisms but 2 hourly dosing was required for patients at risk of infective endocarditis. A metronidazole dose of 1000 mg was required for patients >85 kg. In patients with CLCR >50 mL/min, 5 mg/kg gentamicin (with an additional 2.5 mg/kg in prolonged surgery at 6 h) maintained PTA targets for >10 h.

CONCLUSIONS

PopPK analysis with Monte Carlo simulation identified prophylactic antibiotic regimens that would maintain the PTA for organisms associated with SSIs during short- and long-duration colorectal surgery.

Pharmacokinetic/pharmacodynamic analysis of weight- and height-scaled tobramycin dosage regimens for patients with cystic fibrosis

OBJECTIVES

To determine the outcomes of weight- and height-based tobramycin dosing regimens for patients with cystic fibrosis (CF).

METHODS

A simulated dataset of 5000 patients based on 331 patients with CF was created using NONMEM. Pharmacokinetic (PK) parameters were derived for each patient from a published model using Monte Carlo simulation. The abilities of 10 and 12 mg/kg/day and 3 and 4 mg/cm/day to achieve standard and extended Cmax (20-30 and 20-40 mg/L) and AUC0-24 (80-120 and 80-150 mg·h/L) targets were evaluated. PK/pharmacodynamic (PK/PD) indices were a Cmax/MIC ratio ≥10 and an AUC0-24/MIC ratio ≥110. For these indices and a range of MICs, cumulative fractions of response (CFRs) for Pseudomonas aeruginosa were also determined.

RESULTS

More patients achieved standard Cmax and AUC0-24 targets with 3 mg/cm/day (64% and 62%, respectively) than with 10 mg/kg/day (43% and 48%, respectively). AUC0-24 estimates >120 mg·h/L were more common with weight-based dosing. With higher doses, 72% achieved high target peaks with 4 mg/cm/day and 65% with 12 mg/kg/day. For the Cmax/MIC index, the maximal MIC for the target microorganism was 2 mg/L with lower doses, 2.5 mg/L with higher doses and 0.5 mg/L for AUC0-24/MIC-based regimens. The CFR for all regimens was >90% for Cmax targets and 66% to 79% for AUC0-24 targets.

CONCLUSIONS

A tobramycin dose of 3 mg/cm/day rather than 10 mg/kg/day achieved similar PK/PD outcomes but dose and AUC0-24 ranges were narrower and the incidence of high AUC0-24 values was lower. Height-based doses should therefore be considered for patients with CF.

Pharmacokinetics and dose requirements of disopyramide in neonates and children

The pharmacokinetics and dose requirements of disopyramide were evaluated in 10 infants and children during the course of routine therapeutic drug monitoring. Drug clearance was estimated using a Bayesian parameter estimation program and found to range from 0.09-0.31L/h/kg. Concentrations associated with a satisfactory response were achieved with doses of 15-20mg/kg/day in most patients, but some required higher doses of 25-30mg/kg/day.

Bioavailability study of a concentrated suspension of phenytoin in healthy volunteers

The bioequivalence of a new concentrated suspension of phenytoin was compared with the standard reference solution recommended by the United Kingdom Committee on Review of Medicines in a group of 18 healthy male volunteers. Doses of 5ml (94.6mg) of the new formulation and 50ml (92mg) of the reference solution were administered in a randomised crossover design and blood samples were withdrawn over the next 72 hours. Serum concentrations were analysed by high performance liquid chromatography. There were no significant differences in the areas under the plasma concentration-time curves of the two formulations although the time to peak was shorter and peak concentrations were higher with the reference solution. As the total amount of phenytoin absorbed from each preparation was not significantly different, both formulations have the same relative bioavailability.

This new concentrated phenytoin suspension may have clinical advantages over tablets, capsules and the commercially available low strength suspension, in patients who have difficulty in swallowing but require normal adult doses of phenytoin

Characterisation and evaluation of UK websites on attention deficit hyperactivity disorder

OBJECTIVES

To identify, characterise and evaluate UK websites providing information about attention deficit hyperactivity disorder (ADHD) and its pharmacological management.

DESIGN

Cross-sectional survey of websites identified by entering "ADHD" into five search engines.

DATA SOURCE

48 websites.

MAIN OUTCOME MEASURES

Each website was scored against 26 criteria using a bespoke instrument to evaluate (a) quality of information on the disorder and its drug treatment and (b) physical characteristics of the site.

RESULTS

Most sites (n = 22) were hosted by charities and support groups, 12 were by commercial organisations, nine were from government or professional bodies, and five were categorised as miscellaneous. Mean total scores per host category ranged from 18.8 to 21 out of 46, with mean (SD) scores of 5.5 (4.2) out of 28 for content and 14.8 (3.0) out of 18 for physical properties. The government/professional sites scored highest for both content and physical properties. Descriptions of the disorder and its drug treatment were poor and lacking in detail. Although most sites mentioned stimulants, only eight discussed atomoxetine and described how both types of drug worked. Ten sites provided detailed information about side effects. The role of different stimulant brands and formulations was discussed on six sites. Authorship details were generally vague. Physical properties related to navigation and layout performed well. Only four sites used language deemed suitable for consumer-orientated health information.

CONCLUSIONS

Information on UK websites about drug treatment for ADHD is basic and incomplete. Websites by government and professional bodies perform better than those in other categories.

Variability in hydrocortisone plasma and saliva pharmacokinetics following intravenous and oral administration to patients with adrenal insufficiency

OBJECTIVE

The best method for determining hydrocortisone replacement therapy is not well defined. This study aimed to assess interindividual variability in cortisol pharmacokinetics and to investigate whether measurement of salivary cortisol provides a useful alternative to plasma concentration measurements.

DESIGN

Intravenous (IV) and oral crossover.

PATIENTS

Twenty-seven patients with primary or secondary adrenal insufficiency who had been on stable replacement therapy for at least 3 months.

MEASUREMENTS

Plasma and salivary concentrations of cortisol were measured up to 8 h following administration of hydrocortisone.

RESULTS

After IV administration, Cmax ranged from 715 to 8313 nmol/l, area under the curve (AUC) from 1112 to 12 177 nmol h/l and cortisol clearance had a median (range) of 0.267 (0.076-0.540) l/h/kg. After oral administration, Cmax ranged from 422 to 1554 nmol/l, AUC 1081-5471 nmol h/l and oral clearance had a median (range) of 0.267 (0.081-0.363) l/h/kg. There was no clear relationship between paired saliva and plasma cortisol concentrations after IV or oral dosing. Plasma and salivary AUC(2-8 h) after IV administration were highly correlated (r2 = 0.77) but differences between predicted and measured plasma AUCs ranged from 3% to 90%. There was a poor correlation between plasma and saliva AUC(2-6 h) after oral administration (r2 = 0.16).

CONCLUSIONS

The wide interindividual variability in plasma and salivary profiles of cortisol following the administration of IV and oral hydrocortisone to patients with adrenal insufficiency and the poor correlation between salivary and plasma measurements suggest that salivary cortisol measurements cannot be used for individual hydrocortisone dosage adjustment.

'Losing my touch': decline in self-reported confidence in performing practical procedures in consultant oncologists

This study aimed to compare the confidence of oncology consultants and specialist registrars (SpRs) in the performance of practical procedures, to contrast this with confidence in other areas of practice and to determine at what grade they felt most confident. Questionnaires were sent to all 57 oncology consultants and SpRs in the South-West region. Respondents scored confidence on a five-point Likert scale. The response rate was 70%. SpRs were significantly more confident in cardiopulmonary resuscitation (p = 0.003) and central line insertion (p = 0.006). Consultants were significantly more confident in developing management plans (p = 0.001) and performing committee work (p = 0.002). Only 6% of consultants felt most confident performing practical procedures as a consultant, and were less confident about these than other tasks (p = 0.001). Some 86% of SpRs considered they were more confident performing practical procedures as senior house officers (SHOs). In conclusion, self-reported confidence in performing practical procedures declines during career progression in oncology. This raises questions about the teaching and supervision of these procedures. If there is a greater emphasis on a consultant-provided service, their educational needs will need to be recognized and retraining or outsourcing of these procedures to other specialties may be necessary.

Medical management of parapneumonic pleural disease

Considerable heterogeneity exists in the management of parapneumonic pleural disease. A randomized controlled trial (RCT) demonstrated the effectiveness of small-catheter drainage with fibrinolysis, but surgical devotees suggest this may only be applicable to "early" cases. We examined evidence-based medical management in "all-comers." We performed a retrospective database analysis of the management of all children with complex pleural effusion admitted to the John Radcliffe Hospital over the 7-year period 1996-2003. One hundred and ten children were admitted. Ten were excluded as they were part of a multicenter RCT and had received intrapleural saline instead of urokinase. Of the remaining 100, 51 were female and 49 male. Median age on admission was 5.8 years (range, 0.3-16.5). Symptoms preadmission averaged 11 days, with December the most common month for presentation. Ninety-six underwent chest ultrasound, confirming an effusion in all, described as loculated/septated (68) or echogenic (11). In 17 cases, no specific comment was made regarding the nature of the fluid seen on ultrasound. Ninety-five had subsequent chest tube drainage and then received intrapleural fibrinolysis with urokinase. An etiological organism was identified in 21 cases (21%) (Streptococcus pneumoniae in 10, group A Streptococcus in 5, Staphylococcus aureus in 4, Haemophilus influenzae in 1, and coliform in 1). In a further 9 cases (9%), Gram-positive organisms were seen on pleural fluid microscopy, but did not grow on culture. Two (2%) required surgery due to the persistence of symptoms and an inadequate response to medical management. Median duration of admission was 7 days (range, 2-21 days); median duration of stay from intervention was 5 days (range, 2-19 days). At median follow-up of 8 weeks (range, 3-20 weeks), all children were symptom-free, with minimal pleural thickening on chest X-ray. In conclusion, antibiotic therapy with chest drain insertion and intrapleural urokinase is effective in treating complex parapneumonic effusion and is associated with a good long-term outcome.

Cost-Effectiveness of Therapeutic Drug Monitoring

There are a number of effective but highly toxic drugs that exhibit a narrow therapeutic index and marked interpatient pharmacokinetic variability. Individualized therapy with such drugs requires therapeutic drug monitoring (TDM) to obtain the desired clinical effects safely. Cost-effectiveness analysis in health care is still at an early stage of development, especially for TDM. A systematic review was carried out to document studies that have addressed the cost-effectiveness of TDM. The Cochrane database and Medline were searched. References identified by this approach were then searched manually for relevant articles. Very few studies have been performed that document the cost-effectiveness of TDM, and TDM has been demonstrated to be cost-effective only for aminoglycosides. For the other classes of drugs that are monitored, the rationale for TDM has been supported, but appropriate cost-effectiveness analyses have not been performed. Because the use of many of these drugs without TDM would increase the risk of under- or overdosing, emphasis should not be placed solely on cost-effectiveness but rather on how such interventions can be applied in the most cost-effective and clinically useful manner.

Reduced gains in fat and fat-free mass, and elevated leptin levels in children and adolescents with cystic fibrosis

AIM

Bodyweight is an important prognostic indicator in children with cystic fibrosis (CF), but the relationships between body composition and clinical outcomes are less clear. We have investigated the role of leptin (a potential satiety factor) and changes in body composition, height and weight with respect to age and clinical outcome.

METHODS

143 children (77 boys) with CF and a median age (range) of 5.99 (2.27-17.98) y were followed with annual measurements of height, weight, skinfolds, forced expiratory volume (FEV1), Shwachman score assessment and fasting blood sample. Our control group comprised 40 children (20 boys, 20 girls) aged 8.6-10.2 y at recruitment who were participating in a longitudinal study of growth and puberty.

RESULTS

SD scores for height, weight and BMI decreased with age; fat and fat-free mass was lower in both sexes compared to controls. Shwachman score decreased with age in both sexes and was related to fat-free mass in girls, and to both fat-free and fat mass in boys. FEV1 decreased with age only in boys and was related to fat-free mass. Leptin levels by age and by fat mass were higher in CF children compared to controls.

CONCLUSION

Despite improvements in management, contemporary children with CF still gain less body fat and fat-free mass and are shorter than controls. The higher leptin levels we observed may be due to stimulatory effects of inflammatory cytokines and we postulate that they may contribute to the anorexia, poor weight gain and growth of these children.

Noninvasive ventilation for the management of pulmonary hypertension associated with congenital heart disease in children

The use of noninvasive ventilation to support children with secondary pulmonary hypertension has not previously been reported. We present four children with secondary pulmonary hypertension in association with complex congenital and acquired cardiorespiratory anomalies who have been successfully managed in hospital and then discharged into the community on noninvasive ventilation, thus placing them in environments more suited for growth and development.

Vancomycin therapeutic drug monitoring: is there a consensus view? The results of a UK National External Quality Assessment Scheme (UK NEQAS) for Antibiotic Assays questionnaire

This study investigated vancomycin therapeutic drug monitoring (TDM) and issues related to patient management. Questionnaires were distributed to 310 participants in the UK National External Quality Assessment Scheme (NEQAS) for Antibiotic Assays. The response rate was 57.4%. The majority (76%) had an 'in-house' assay service based, almost exclusively, in the microbiology department, and a fluorescence polarization immunoassay (FPIA) was used by 97%. Almost half (48.7%) had an assay service available for 24 h/day, 7 days/week and 92.7% expected same-day results. The majority (80%) had issued guidelines for vancomycin use. A 12 hourly initial dosing regimen was used by 89%. Trough assay samples were taken <10 min before the dose by 91.5%. For post-dose assay samples, 44% took a sample at 1 h, 28% at 2 h and the remainder at 'other' times. For trough target ranges, 93% quoted <10 mg/L or 5-10 mg/L. There was no consensus with regard to post-dose assay sample times and 23 ranges were quoted. The majority (74.4%) regarded a trough level of >or=10 mg/L as 'toxic' but 13 concentrations were quoted as toxic post-dose measurements. In conclusion, there was a wide variability and poor consensus with regard to post-dose vancomycin assay sampling times, target ranges and what constituted a toxic level.

Randomised trial of intrapleural urokinase in the treatment of childhood empyema

BACKGROUND

The role of intrapleural fibrinolytic agents in the treatment of childhood empyema has not been established. A randomised double blind placebo controlled trial of intrapleural urokinase was performed in children with parapneumonic empyema.

METHODS

Sixty children (median age 3.3 years) were recruited from 10 centres and randomised to receive either intrapleural urokinase 40 000 units in 40 ml or saline 12 hourly for 3 days. The primary outcome measure was length of hospital stay after entry to the trial.

RESULTS

Treatment with urokinase resulted in a significantly shorter hospital stay (7.4 v 9.5 days; ratio of geometric means 1.28, CI 1.16 to 1.41 p=0.027). A post hoc analysis showed that the use of small percutaneous drains was also associated with shorter hospital stay. Children treated with a combination of urokinase and a small drain had the shortest stay (6.0 days, CI 4.6 to 7.8).

CONCLUSION

Intrapleural urokinase is effective in treating empyema in children and significantly shortens hospital stay.

External quality assessment of laboratory performance in analysis of toxicological cases

A mean of 44 members of the United Kingdom national external quality assessment scheme (UKNEQAS) for toxicology reported analytical findings on 10 toxicological cases circulated between December 1995 and February 2000. Material distributed usually consisted of a 5ml blood and a 20ml urine sample simulated by quantitative addition of drugs and their metabolites to material donated by volunteers and patients. The samples were accompanied by a brief outline of the circumstances surrounding the case. Laboratories were requested to report their analytical findings, list methods of analysis, and provide interpretation of their findings. The mean overall success rate for identification of drugs or their pharmacological group was 76%, failure being largely by laboratories providing an immunoassay-based screening service for a fixed range of drug groups. The latter laboratories indicated that cases would be referred to regional toxicology centres for further investigation or confirmation. The coefficient of variation of measurements was <7% for routine analytes, such as ethanol and paracetamol, but 26-44% for tricyclics and opiates. There were 3% false positive reports. The quantity and content of interpretative comment provided by the laboratories was very variable. A number provide nothing in addition to the analytical result.

A survey of extraction techniques for drugs of abuse in urine

Sixty nine participants in the United Kingdom national external quality assessment scheme for drugs of abuse in urine reported details of their sample extraction technique by questionnaire. Laboratories were categorised by differences in technique and their analytical test results compared for samples containing D-amfetamine 0.4 (4) and 0.8 (3) mg/l, morphine 0.4 (4) and 0.8 (4)mg/l, and benzoylecgonine 0.15/0.2 (2) and 0.45/0.5 (4) mg/l. Values in parentheses are numbers of samples. For amfetamine, there was no significant difference in the frequency of true positive results between liquid-liquid or solid phase extraction and the Toxi-Lab A system at 0.8 mg/l. Toxi-Lab A gave significantly fewer positives when operating below its specified threshold at 0.4 mg/l. Paradoxically, laboratories using >5 ml urine volume performed less well. Acidification of the extract before volume reduction gave significantly more true positives. For extraction of morphine, solid phase systems significantly outperformed both liquid-liquid and the Toxi-Lab A system at both 0.8 and 0.4 mg/l. No significant differences between extraction techniques were demonstrated for analysis of benzoylecgonine.

Expiratory airflow patterns in children and adults with cystic fibrosis

STUDY OBJECTIVE

To determine whether tidal expiratory airflow patterns change with increasing airways obstruction in patients with cystic fibrosis.

DESIGN

An observational study.

SETTING

Lung function laboratory.

PATIENTS

Sixty-four children and young adults with cystic fibrosis.

MEASUREMENTS

After measuring FEV(1) and airways resistance using body plethysmography, each subject was seated and asked to mouth breathe through a pneumotachograph for 2 min. The collected data were analyzed, and three expiratory airflow pattern-sensitive indexes were computed. The first index was derived from the ratio of the time to reach peak expiratory flow to the total expiratory time (tPTEF/tE). The second index, Trs, was an estimate of the time constant of the passive portion of expiration. The third index, f1.gif" BORDER="0">, describes the slope of the whole post-peak expiratory flow pattern after scaling.

RESULTS

Compared with FEV(1), the index tPTEF/tE was a poor indicator of airways obstruction (r(2) = 0.15, p = 0.002). Trs showed a strong relationship with the severity of airways obstruction (r(2) = 0.46, p < 0.001). Using f1.gif" BORDER="0">, the postexpiratory profile could be categorized into three shapes, and provided a good indicator of airways obstruction when linear and concave-shaped profiles occurred (r(2) = 0.42, p < 0.001). Convex-shaped flow profiles had to be treated separately and were indicative of normal lung function.

CONCLUSIONS

In a cross-sectional study of patients with cystic fibrosis, increase in airways resistance above normal is reflected by quantifiable changes in the expiratory airflow pattern.

Pharmacokinetics of meropenem in intensive care unit patients receiving continuous veno-venous hemofiltration or hemodiafiltration

OBJECTIVE

To evaluate an intravenous meropenem dosage regimen in adult intensive care patients with acute renal failure treated by continuous renal replacement therapy.

DESIGN

A prospective, clinical study.

SETTING

General intensive care unit of a university hospital.

PATIENTS

Ten critically ill adult patients being treated with meropenem and receiving continuous veno-venous hemofiltration (hemofiltration rates, 1-2 L/hr) (n = 5) or continuous venovenous hemodiafiltration (hemofiltration rates, 1-1.5 L/hr; dialysis rates, 1-1.5 L/hr) (n = 5) via a polyacrylonitrile hollow fiber 0.9-m2 filter.

INTERVENTIONS

Patients received a meropenem dose of 1 g iv every 12 hrs as a 5-min bolus.

MEASUREMENTS AND MAIN RESULTS

Meropenem concentrations were measured by high-performance liquid chromatography in serum taken at timed intervals and in ultrafiltrate/dialysate to determine serum concentration-time profiles, derive pharmacokinetic variable estimates, and determine sieving coefficients and filter clearances. The serum concentrations were examined to see whether they were above the minimum inhibitory concentrations (MICs) for pathogens that may be encountered in intensive care patients. Serum concentrations exceeded 4 mg/L (MIC90 for Pseudomonas aeruginosa) during 67% of the dosage period in all patients. Sub-MIC90 concentrations were obtained in three patients immediately before treatment and in one patient 12 hrs after treatment. Mean (SD) (n = 10) pharmacokinetic variable estimates were as follows: elimination half-life, 5.16 hrs (1.83 hrs); volume of distribution, 0.35 L/kg (0.10 L/kg); and total clearance, 4.30 L/hr (1.38 L/hr). A sieving coefficient of 0.93 (0.06) (n = 9) indicated free flow across the filter. The fraction cleared by the extracorporeal route was 48% (13%) (n = 9), which is clinically important.

CONCLUSIONS

A meropenem dose of 1g iv every 12 hrs provides adequate serum concentrations in the majority of patients receiving continuous veno-venous hemofiltration or continuous venovenous hemofiltration with a 0.9-m2 polyacrylonitrile filter at combined ultrafiltrate/dialysate flow rates of up to 3 L/hr. A lower dose would not be sufficient for the empirical treatment of potentially life-threatening infections in all patients.

Pharmacokinetics and dose requirements of vancomycin in neonates

AIMS

To design and evaluate dosing guidelines for vancomycin based on data collected during routine use of the drug.

METHODS

Following the observation that 66% of neonatal vancomycin trough concentrations were outside the target range, new dose guidelines were developed using a population pharmacokinetic approach. NONMEM (non-linear mixed effects model) was used to analyse dose histories and 347 concentration measurements collected during routine therapeutic drug monitoring in 59 neonates.

RESULTS

Postconceptual ages in the patient group ranged from 26-45 weeks, weights from 0. 57-4.23 kg, and creatinine concentrations from 18-172 micromol/l. The population estimate of vancomycin clearance (l/h/kg) was 3. 56/creatinine concentration (micromol/l) with an interpatient coefficient of variation (CV) of 22% and volume of distribution 0.67 l/kg with a CV of 18%. Residual error was 4.5 mg/l. When the new recommendations on dosing were used prospectively in a separate group of neonates the proportion of acceptable troughs increased from 33% to 72%.

CONCLUSIONS

The pharmacokinetics of vancomycin in neonates and young infants depend on weight and serum creatinine. Preliminary results from the new guidelines indicate an improvement on previous practice, but also an ongoing need to monitor concentrations.

Population pharmacokinetics in phase I drug development: a phase I study of PK1 in patients with solid tumours

Doxorubicin pharmacokinetics were determined in 33 patients with solid tumours who received intravenous doses of 20-320 mg m(-2) HPMA copolymer bound doxorubicin (PK1) in a phase I study. Since assay constraints limited the data at lower doses, conventional analysis was not feasible and a 'population approach' was used. Bound concentrations were best described by a biexponential model and further analyses revealed a small influence of dose or weight on V1 but no identifiable effects of age, body surface area, renal or hepatic function. The final model was: clearance (Q) 0.194 I h(-1); central compartment volume (V1) 4.48 x (1+0.00074 x dose (mg)) I; peripheral compartment volume (V2) 7.94 I; intercompartmental clearance 0.685 I h(-1). Distribution and elimination half-lives had median estimates of 2.7 h and 49 h respectively. Free doxorubicin was present at most sampling times with concentrations around 1000 times lower than bound doxorubicin values. Data were best described using a biexponential model and the following parameters were estimated: apparent clearance 180 I h-(-1); apparent V1 (I) 1450 x (1+0.0013 x dose (mg)), apparent V2 (I) 21 300 x (1-0.0013 x dose (mg)) x (1+2.95 x height (m)) and apparent Q 6950 I h(-1). Distribution and elimination half-lives were 0.13 h and 85 h respectively.

Population pharmacokinetics of aminoglycoside antibiotics in patients with cystic fibrosis

The population pharmacokinetics of gentamicin and tobramycin were investigated in a group of 51 young adults with cystic fibrosis. Their ages ranged from 14-35 years, weights from 38-82 kg, and 27 of the patients were female. None of the patients had renal impairment, but 3 patients were treated in the intensive therapy unit (ITU) during one of their courses of therapy. Data comprised 219 courses of therapy and 544 concentrations (mean: 11 per patient). Concentration-time data were analyzed using a nonlinear mixed-effects model package (NONMEM) and were best described by a one-compartment model. Factors identified as potentially influencing aminoglycoside pharmacokinetics were added in a stepwise fashion and the best model found that drug clearance and volume of distribution were related to body surface area and admission to ITU. The mean population estimates were a clearance of 2.89 L/hr/m2 and a volume of distribution of 9.21 L/m2 with a 60% increase in patients who were admitted to ITU. Interpatient variability in clearance and volume were 14% and 8%, respectively. The results suggested that a dose of 120 mg/m2 should achieve an average 1 hour postdose peak of 10 mg/L and trough of <1 mg/L and that higher doses might be required in ITU patients.

Phase I clinical and pharmacokinetic study of PK1 [N-(2-hydroxypropyl)methacrylamide copolymer doxorubicin]: first member of a new class of chemotherapeutic agents-drug-polymer conjugates. Cancer Research Campaign Phase I/II Committee

PK1 comprises doxorubicin covalently bound to N-(2-hydroxypropyl)methacrylamide copolymer by a peptidyl linker. Following cellular uptake via pinocytosis, the linker is cleaved by lysosomal enzymes, allowing intratumoral drug release. Radically altered plasma and tumor pharmacokinetics, compared to free doxorubicin, and significant activity in animal tumors have been demonstrated preclinically. We aimed to determine the maximum tolerated dose, toxicity profile, and pharmacokinetics of PK1 as an i.v. infusion every 3 weeks to patients with refractory or resistant cancers. Altogether, 100 cycles were administered (range, 20-320 mg/m2 doxorubicin-equivalent) to 36 patients (20 males and 16 females) with a mean age of 58.3 years (age range, 34-72 years). The maximum tolerated dose was 320 mg/m2, and the dose-limiting toxicities were febrile neutropenia and mucositis. No congestive cardiac failure was seen despite individual cumulative doses up to 1680 mg/m2. Other anthracycline-like toxicities were attenuated. Pharmacokinetically, PK1 has a distribution t(1/2) of 1.8 h and an elimination t(1/2) averaging 93 h. 131I-labeled PK1 imaging suggests PK1 is taken up by some tumors. Responses (two partial and two minor responses) were seen in four patients with NSCLC, colorectal cancer, and anthracycline-resistant breast cancer. PK1 demonstrated antitumor activity in refractory cancers, no polymer-related toxicity, and proof of principle that polymer-drug conjugation decreases doxorubicin dose-limiting toxicities. The recommended Phase II dose is 280 mg/m2 every 3 weeks. Studies are planned in colorectal, NSCLC, and breast cancer patients.

Contribution of genetic factors other than CFTR to disease severity in cystic fibrosis

BACKGROUND

Disease severity in patients with cystic fibrosis shows marked variability. Attempts to explain this phenotypic heterogeneity on the basis of CFTR genotype have had limited success. A study was undertaken to test the hypothesis that naturally occurring variants of the pro-inflammatory cytokine tumour necrosis factor alpha (TNF-alpha) and the detoxifying enzyme glutathione S-transferase M1 (GSTM1) could influence disease severity in cystic fibrosis.

METHODS

Fifty-three children with cystic fibrosis were studied. To allow for the effect of age, all clinical details were collected during the eighth year of age. The subjects were divided into groups, both according to the presence or absence of the TNF2 TNF-alpha -308 promoter polymorphism (n = 20), and by homozygosity for the null allele of GSTM1 (n = 26).

RESULTS

Percentage predicted forced expiratory volume in one second (FEV1) and weight z scores were significantly lower in the TNF2 group (mean difference (95% confidence intervals) for FEV1 11.6% (1.7 to 21.5) and 0.59 (0.06 to 1.12) for weight z score). The Chrispin-Norman chest radiographic score was significantly higher and the Shwachman score was significantly lower in patients homozygous for the GSTM1 null allele.

CONCLUSIONS

Two independent genetic factors have been identified which appear to influence disease severity in cystic fibrosis. These results support the contention that inflammation in cystic fibrosis contributes to tissue damage. Isolation of further such factors may lead to identification of patients at risk of more severe disease and allow targeted aggressive therapy in this group.

Population pharmacokinetics of gentamicin in patients with cancer

AIMS

The purpose of this study was to describe the population pharmacokinetics of gentamicin in patients with cancer, to identify possible relationships between clinical covariates and population pharmacokinetic parameter estimates and to examine the relevance of existing dosage nomograms in light of the population model developed in these patients.

METHODS

Data were collected prospectively from 210 patients with cancer and were analysed with package NONMEM. Data were split into two sets: a population data set and an evaluation set. Creatinine clearance was estimated using measured creatinine concentrations and using 'low' creatinines set to a minimum of 60 micromol l(-1), 70 micromol l(-1) or 88.4 micromol l(-1)

RESULTS

A two compartment model was fitted to the concentration-time curve. Two best models were obtained, one that related clearance to estimated creatinine clearance (minimum creatinine value 60 micromol l(-1)) and the other that related clearance to age, creatinine concentration and body surface area. Volume of the central compartment was influenced by body surface area and albumin concentration. For both models 90% of measured concentrations lay within the 95% confidence interval of the simulated concentrations and the mean prediction errors were -7.2% and -6.6%, respectively. A final analysis performed in all patients identified the following relationship CL (1 h(-1))=0.88 x (1 + 0.043 x creatinine clearance) and central volume of distribution V1 (1)=8.59 x body surface area x (albumin/34)(-0.39). The mean population estimate of intercompartmental clearance (Q) was 1.301 h(-1) and peripheral volume of distribution (V2) was 9.801. Coefficient of variation was 18.5% on clearance and 28.2% on Q. Residual error expressed as a standard deviation was 0.36 mg l(-1) at 1.0 mg l(-1) and 1.32 mg l(-1) at 8.0 mg l(-1). The mean population estimate of clearance was 4.21 h(-1) and volume of distribution (Vss) was 24.61 (0.381 kg(-1)). The mean population estimates of half-lives were 1.8 h and 8.0 h.

CONCLUSIONS

In the context of published nomograms this analysis indicated that both the traditional approach and the new, 'once daily' approach should achieve satisfactory concentrations in cancer patients although serum concentration monitoring is required to confirm optimal dosing in individual patients.

An audit of therapeutic drug monitoring service provision by laboratories participating in an external quality assessment scheme

Therapeutic drug monitoring services were investigated in a questionnaire sent to all subscribers to the United Kingdom National External Quality Assessment Scheme for Therapeutic Drug Assays. Questions were posed on assay availability and use, target ranges, and reporting procedures for digoxin, lithium, phenytoin, phenobarbitone, carbamazepine, theophylline, and valproic acid. One hundred fifty-seven laboratories replied and, except for lithium, 45% reported in mass units, 34% in molar units, and 22% a mixture of mass and molar units. Target ranges for lithium, digoxin, carbamazepine, and phenobarbitone were highly variable but ranges for phenytoin, theophylline, and valproic acid were more consistent. Immunoassay was the most popular methodology although high-performance liquid chromatography was commonly used for anticonvulsants. Paper copies of results were provided by 93% of laboratories, 40% reported by telephone, 12% by fax, and 28% by computer. Additional data, mainly dose, time of last dose, and duration of therapy were requested by 55% to 67% of laboratories. Grades of staff authorizing results ranged from nurses to senior consultants, and collaboration with pharmacists occurred in 26% of laboratories. Most laboratories provided a daily analytical service and 73% offered a 24-hour emergency service. This audit unexpectedly identified use of a wide range of target concentrations, particularly for digoxin and lithium.

Antimicrobial practice. Development of guidelines for gentamicin dosing

The performance of dosage guidelines for starting gentamicin therapy was evaluated prospectively in 50 patients with suspected or proven Gram-negative septicaemia and the results were compared with results from similar group of 50 patients for whom the guidelines were not followed. Peak concentrations were significantly higher when the guidelines were followed (7.2 (+/- 1.9) mg/L vs 5.7 (+/- 1.8) mg/L) but there was no difference in trough concentrations. Fifty-eight per cent of patients had both peak and trough concentrations within the target range (peak > 5 mg/L, trough < 2 mg/L) when doses were decided empirically; this increased to 96% when the guidelines were followed. However, use of the guidelines achieved peaks of > 7 mg/L in only 56% of patients. A revised protocol with higher doses given less frequently was therefore developed and a prospective assessment of its performance indicated that satisfactory concentrations were obtained in 96% of patients.

Methotrexate removal during haemodialysis in a patient with advanced laryngeal carcinoma

A 62-year-old patient on long-term haemodialysis who developed an inoperable T2N3Mo squamous-cell carcinoma of the larynx was treated with weekly low-dose methotrexate (MTX) after failing to respond to radiotherapy. The patient was initially given one dose of 10 mg MTX (6 mg/m2) as a 1-h infusion, then he received three further i.v. doses of 20 mg (12 mg/m2). Haemodialysis was performed 15-18 h after each dose and the patient received folinic acid (30 mg i.v.q 6 h) until the MTX concentration was < 0.1 mumol/l. The MTX concentration was measured regularly until it reached < 0.1 mumol/l, and additional samples were withdrawn pre- and post-dialysis. The MTX elimination rate constant and half-life were estimated with the patient on and off dialysis. The patient failed to respond to treatment but did not experience MTX-related toxicity. The elimination half-life ranged from 22 to 42 h when he was off dialysis but fell to a median of 5.5 h during dialysis. Low-dose MTX was given to a patient on regular haemodialysis without evidence of toxicity. The rate of MTX elimination was increased during haemodialysis, although high MTX concentrations persisted for several days and prolonged rescue with folinic acid was required.

Population pharmacokinetics of caffeine in neonates and young infants

The population pharmacokinetics of caffeine were investigated in 60 neonates and young infants using data collected during routine therapeutic drug monitoring. Clearance was influenced by body weight and postnatal age, and increased in the presence of dexamethasone. No clinical factors were identified that influenced volume of distribution. The population pharmacokinetic parameter estimates were then tested prospectively in a further 20 neonates. Although they produced unbiased results, the dexamethasone effect could not be identified. A final analysis using all 80 patients found clearance (L/day) = 0.14 x weight (kg) + 0.0024 x postnatal age (days) (+/- 20%) and volume of distribution = 0.82 L (+/- 24%). Simulations based on these results indicated that the current dosage guidelines of 20 mg/kg loading dose of caffeine citrate followed by a 5 mg/kg/day maintenance dose should achieve concentrations within the traditional target range in > 70% of neonates.

Hirschsprung's disease associated with a deletion of chromosome 10 (q11.2q21.2): a further link with the neurocristopathies?

We report a patient with total colonic aganglionosis in association with a deletion of part of the long arm of chromosome 10: (del(10)(q11.2q21.2)). This deletion includes the ret proto-oncogene, which has recently been implicated in multiple endocrine neoplasia type 2A (MEN 2A). The possible links between Hirschsprung's disease and the neurocristopathies and the aetiological role of abnormalities of neural crest development in these conditions are discussed.

Evaluation of a Bayesian approach to the pharmacokinetic interpretation of cyclosporin concentrations in renal allograft recipients

The utility of a Bayesian parameter estimation program in the interpretation of cyclosporin concentrations was investigated in a group of 32 patients following renal transplantation. The program was evaluated by comparing concentrations predicted from individual estimates of pharmacokinetic parameters with measured concentrations. A one-compartment model incorporating an exponential, time-related change in clearance was used for data collected in the first 4 weeks after transplantation, and predictions of concentrations measured during weeks 5-14 were made using three schemes: a changing clearance model using all data from week 1 onward; a changing clearance model using data from week 4 onward; and a nonchanging clearance model using data from week 4 onward. Results demonstrated that predictions made by the Bayesian program were unreliable during the first 4 weeks of therapy, but that there was a progressive improvement as time after transplantation increased. The changing clearance model was superior to the constant clearance model and its performance was not compromised by including data from the first 4 weeks of therapy. Although the Bayesian approach may help with the interpretation of blood cyclosporin concentrations during maintenance therapy, the large variability in the pharmacokinetics of orally administered cyclosporin limits the usefulness of the approach in the early weeks following transplantation.

Value of theophylline treatment in patients handicapped by chronic obstructive lung disease

BACKGROUND

It is still not certain whether it is worth using theophylline in addition to inhaled bronchodilators and corticosteroids to treat obstructive airways disease. This trial was designed to test whether the addition of prescribed theophylline in doses sufficient for sustained optimal steady state plasma concentrations would produce any detectable additional advantage in spirometric or functional variables in these handicapped patients.

METHODS

A randomised, double blind, placebo controlled, crossover study of added theophylline treatment was aimed at steady state plasma concentrations of 10 and 17 mg/l, the dose being calculated individually by Bayesian parameter estimation and maintained for six weeks along with the patient's previously prescribed bronchodilators and steroids. Of 20 patients sequentially recruited, 15 provided data that could be analysed. All had chronic obstructive lung disease with a mean forced expiratory volume in the first second (FEV1) up to about 30% of the predicted value and gave no history of being treated with theophylline. The protocol included spirometry, whole body plethysmography, and treadmill exercise. Measurements also included steady state plasma theophylline concentrations and trapped gas volume. Quality of life was assessed by an established questionnaire method covering breathlessness in everyday activities, fatigue, emotional function, and control over the disease.

RESULTS

Both target plasma concentrations were achieved. Improvements in peak flow (PEF; mean 20%), trapped gas volumes (38%), two stage vital capacity (15%), distances walked (48%), breathlessness in everyday activities (32%), and fatigue (18%) were found at the higher plasma concentration only. FEV1, forced vital capacity (FVC), emotional function, and control did not change.

CONCLUSION

Theophylline treatment with sustained steady state concentrations about 17 mg/l provides worthwhile objective and subjective further benefits for patients handicapped by chronic obstructive lung disease when it is added to bronchodilators and corticosteroids.

Bayesian estimation of streptomycin pharmacokinetics

Streptomycin dose requirements were determined in an 83-year-old man with renal impairment who was being treated for miliary tuberculosis. Concentration measurements were interpreted using a Bayesian parameter estimation program. Estimated creatinine clearance (1.1 L/h) was used as a starting value for streptomycin clearance, and volume was initially assumed to be 0.3 L/kg. Bayesian estimates of clearance were close to the starting value and declined from 1.4 L/h to 0.9 L/h during the course of therapy. Volume was higher than the initial estimate (0.4-0.5 L/kg), possibly due to the patient having a low albumin and being underweight. Satisfactory concentrations were maintained for several weeks with doses of 500 mg every 36-48 h.

Pharmacokinetic optimisation of anticonvulsant therapy

Changing attitudes towards the use of antiepileptic drugs have led to an emphasis on monotherapy with serum concentration measurement coupled with standard, weight-adjusted starting and maintenance regimens to guide initial therapy and subsequent dosage titration. Currently, the established anticonvulsants are carbamazepine, valproic acid (sodium valproate) and phenytoin. Phenobarbital is now less commonly prescribed due to its propensity to produce sedation and impair cognitive function. The value of pharmacokinetic optimisation with valproic acid is limited by its wide therapeutic index, large fluctuations in the concentration-time profile and concentration-dependent protein binding. Thus, although serum concentrations are often measured, they are rarely subjected to pharmacokinetic interpretation. Carbamazepine has a flatter concentration-time profile than valproic acid. Its target range is more clearly defined and it undergoes autoinduction of metabolism and interacts with other drugs. Pharmacokinetic principles can, therefore, be more readily applied to carbamazepine, although, in general, a simple clinical approach to its use is usually satisfactory. Phenytoin has required the greatest pharmacokinetic input due to its nonlinear pharmacokinetics and narrow target range. Many different graphical methods, equations and computer programs have been reported, some of which demand 2 steady-state, dose-concentration pairs; others function satisfactorily with only 1. Recent attempts have been made to interpret non-steady-state data. In addition, a number of workers have demonstrated the value of altering the population parameter estimates to account for ethnic differences. A pharmacokinetic approach can also be used to tailor the use of phenytoin in the treatment of status epilepticus. Dosage alterations may be needed for specific patient groups. In particular, children generally require higher dosages on a weight-for-weight basis than adults, while equivalently lower dosages should be given to neonates. Most anticonvulsants are principally cleared by hepatic mechanisms, so dosage adjustment is not usually required in renal disease, although care must be taken in interpreting serum concentrations because of changes in protein binding. Close monitoring is required in the elderly and patients with hepatic impairment, while increased dosages may be needed in critically ill patients and during pregnancy. Pharmacokinetic principles can be used in the treatment of treat self-poisoning with anticonvulsants. There are few data available on the pharmacokinetics of vigabatrin, lamotrigine, oxcarbazepine and gabapentin in patients. Due to its mode of action in binding irreversibly to its target enzyme, serum concentration monitoring of vigabatrin plays no role in optimising therapy. The value of applying pharmacokinetic principles with the other 3 drugs remains to be investigated.(ABSTRACT TRUNCATED AT 400 WORDS)

Bayesian parameter estimation and population pharmacokinetics

The widespread application of Bayesian parameter estimation in the area of therapeutic drug monitoring (TDM) has prompted the need for well conducted population studies to obtain relevant prior pharmacokinetic parameter estimates. In many cases the population has consisted of a relatively small number of subjects. This may be unavoidable for drugs used in cancer chemotherapy or in small, specific populations of patients. In contrast, information about drugs which are used extensively, such as the aminoglycosides, can be obtained by population studies which involve a large number of individuals. Indeed, this technique has proved particularly useful for determining parameter estimates which can be employed in neonatal TDM. Bayesian parameter estimation has been most frequently used for drugs with narrow therapeutic ranges such as the aminoglycosides, cyclosporin, digoxin, anticonvulsants (especially phenytoin), lithium and theophylline. However, the technique has now been extended to cytotoxic drugs, Factor VIII and warfarin. Bayesian methods have also been used to limit the number of samples required in more conventional pharmacokinetic studies with new drugs. Further advances in the use of these methods are likely to include measures of drug response and toxicity requiring population studies which also include relevant pharmacodynamic information.