Measurement of Free Plasma Concentrations of Beta-Lactam Antibiotics: An Applicability Study in Intensive Care Unit Patients

BACKGROUND

The antibacterial effect of antibiotics is linked to the free drug concentration. This study investigated the applicability of an ultrafiltration method to determine free plasma concentrations of beta-lactam antibiotics in ICU patients.

METHODS

Eligible patients included adult ICU patients treated with ceftazidime (CAZ), meropenem (MEM), piperacillin (PIP)/tazobactam (TAZ), or flucloxacillin (FXN) by continuous infusion. Up to 2 arterial blood samples were drawn at steady state. Patients could be included more than once if they received another antibiotic. Free drug concentrations were determined by high-performance liquid chromatography with ultraviolet detection after ultrafiltration, using a method that maintained physiological conditions (pH 7.4/37°C). Total drug concentrations were determined to calculate the unbound fraction. In a post-hoc analysis, free concentrations were compared with the target value of 4× the epidemiological cut-off value (ECOFF) for Pseudomonas aeruginosa as a worst-case scenario for empirical therapy with CAZ, MEM or PIP/tazobactam and against methicillin-sensitive Staphylococcus aureus for targeted therapy with FXN.

RESULTS

Fifty different antibiotic treatment periods in 38 patients were evaluated. The concentrations of the antibiotics showed a wide range because of the fixed dosing regimen in a mixed population with variable kidney function. The mean unbound fractions (fu) of CAZ, MEM, and PIP were 102.5%, 98.4%, and 95.7%, with interpatient variability of <6%. The mean fu of FXN was 11.6%, with interpatient variability of 39%. It was observed that 2 of 12 free concentrations of CAZ, 1 of 40 concentrations of MEM, and 11 of 23 concentrations of PIP were below the applied target concentration of 4 × ECOFF for P. aeruginosa. All concentrations of FXN (9 samples from 6 patients) were >8 × ECOFF for methicillin-sensitive Staphylococcus aureus.

CONCLUSIONS

For therapeutic drug monitoring purposes, measuring total or free concentrations of CAZ, MEM, or PIP is seemingly adequate. For highly protein-bound beta-lactams such as FXN, free concentrations should be favored in ICU patients with prevalent hypoalbuminemia.

Microanalysis of amoxicillin, flucloxacillin, and rifampicin in neonatal plasma

Simple and rapid reversed-phase high-performance liquid chromatographic assays with ultraviolet detection have been developed and validated for the determination of amoxicillin, flucloxacillin and rifampicin in neonatal plasma. Plasma samples were either precipitated with perchloric acid (amoxicillin) or methanol (rifampicin) or extracted with methylene chloride (flucloxacillin). Precision coefficients of variation and inaccuracy were less than 15% for all three assays. Only small sample volumes (20-40 microL) were required, making the assays suitable for therapeutic drug monitoring and pharmacokinetic studies in preterm and term neonates. The assays have successfully been applied to analysis of amoxicillin, flucloxacillin and rifampicin in previously published pharmacokinetic studies in neonates.

Protein Binding of Flucloxacillin in Neonates

The isoxazolyl penicillins, including flucloxacillin, have the highest levels of plasma protein binding among the semisynthetic penicillins. Because only the free fraction of the penicillin is pharmacologically active, it would be useful to measure both protein-bound and free flucloxacillin to determine its protein binding. Until now, flucloxacillin protein binding in newborn infants has been investigated in only two studies with relatively small populations. In the present study, flucloxacillin protein binding was investigated in 56 (preterm) infants aged 3 to 87 days (gestational age, 25-41 weeks). Surplus plasma samples from routine gentamicin assays of each infant were collected and combined to obtain a sufficiently large sample for analysis. Free flucloxacillin was separated from protein-bound flucloxacillin using ultrafiltration. Reversed-phase high-performance liquid chromatography with ultraviolet detection was used to measure free flucloxacillin concentrations in ultrafiltrate and total flucloxacillin concentrations in pooled plasma. Flucloxacillin protein binding was 74.5% +/- 13.1% (mean +/- standard deviation) with a high variability among the infants (34.3% to 89.7%). High Pearson correlations were found between protein binding and the covariates-plasma albumin concentration (r = 0.804, P < 0.001, n = 18) and plasma creatinine concentration (r = -0.601, P < 0.001, n = 45). Statistically significant but less striking correlations were found between protein binding and gestational age, postconceptional age, body weight, and triglyceride concentration. Because of the high variability of protein binding among infants, it is difficult to devise a flucloxacillin dosage regimen effective for all infants. Individualized dosing, based on free flucloxacillin concentrations, might help to optimize treatment of late-onset neonatal sepsis, but practical obstacles will probably prevent analysis of free flucloxacillin concentrations in newborn infants on a routine basis.

RP-HPLC analysis of flucloxacillin in human plasma: validation and application to a bioequivalence study

A RP-HPLC method with rapid sample processing was developed for quantitation of flucloxacillin in human plasma using dicloxacillin as the internal standard. The plasma sample (100 microL) was acidified with glacial acetic acid, and deproteinized by precipitation with acetonitrile. The supernatant was directly injected into the HPLC system. Separation was achieved on an Alltima C18 column (250 mmx4.6 mm I.D., 5 microm), with a mixture of 10 mmol x L(-1) KH2PO4-acetonitrile (64.5:35.5, v/v) as mobile phase. The assay was successfully applied to a randomized, two-period cross-over bioequivalence study in 20 healthy Chinese volunteers following a single oral dose of 250 mg flucloxacillin capsules. A non-compartmental method was used for pharmacokinetic analysis. Compared with data in the literature, flucloxacillin was eliminated more slowly in Chinese than in Caucasians. Cmax, AUC(0-t) and AUC(0-infinity) were tested for bioequivalence after log-transformation of data. No significant difference was found. Tmax was analyzed by Wilcoxon's test and no significant difference was obtained (P > 0.05). Based on these statistical inferences, the two formulations were judged to be bioequivalent and, thus, can be prescribed interchangeably.

Population pharmacokinetics and dosing of flucloxacillin in preterm and term neonates

In total 235 flucloxacillin total (free+protein bound) plasma concentrations were determined in 55 neonates (gestational age 26 to 42 weeks, postnatal age 0 to 44 days) with reversed-phase HPLC in surplus plasma samples from routine gentamicin assays. Population pharmacokinetic parameters were calculated according to an one compartment open model with iterative two-stage Bayesian fitting (MWPHARM 3.50, Mediware, The Netherlands). Mean clearance corrected for weight was 0.18+/-0.10 L kgh and volume of distribution corrected for weight was 0.54+/-0.17 L/kg. Pearson correlations between the individual pharmacokinetic parameters and covariates, like gestational age, plasma creatinine, and gentamicin clearance, were low and therefore not relevant for use in clinical practice. Total plasma concentrations above 200 mg/L were considered toxic and T>MIC (time above minimum inhibitory free plasma concentration) of more than 40% was considered effective. Protein binding was assumed to be 86.3% in all neonates, based on literature. The current dosage regimen, 25 or 50 mg/kg every 8 or 12 hours, did not result in effective plasma concentrations for the treatment of Staphylococcus aureus in 17 (31%) of the 55 neonates. Therefore, the authors suggest an initial dose of 25 mg/kg/4 h for all neonates, irrespective of their age, based on the breakpoint MIC value of flucloxacillin for Staphylococcus aureus (2.0 mg/L). After isolation of the causative agent of infection, flucloxacillin administration ought to be reconsidered based on the expected susceptibility pattern of the isolate. When oxacillin sensitive coagulase negative staphylococci are isolated, the initial dose should be reduced to 10 mg/kg/6 h, based on the breakpoint MIC value of 0.25 mg/L. Simulation with these new dosage regimens indicated that satisfactory plasma concentrations were reached in 52 of the 55 neonates. However, the regimens need prospective verification. Moreover, the exact role of neonatal protein binding needs to be further investigated.

Pharmacokinetics of intravenous flucloxacillin and amoxicillin in neonatal and infant cardiopulmonary bypass surgery

OBJECTIVES

To determine the blood and tissue concentrations of flucloxacillin and amoxicillin during cardiopulmonary bypass (CPB) in infants weighing less than 5 kg.

METHODS

Eleven patients aged between 3 and 60 days and weighing <5 kg. Intravenous flucloxacillin 30 mg kg(-1) and amoxicillin 30 mg kg(-1) were administered at time of anaesthesia. Blood and muscle samples were collected at four stages of the operation: immediately before commencement of CPB; before cross-clamping; after weaning of CPB; and at the time of skin closure. Concentrations, half-lives (t(1/2)), clearance and volume of distribution were calculated for both antibiotics in serum and tissue.

RESULTS

After connection to CPB the mean serum concentration of flucloxacillin decreased by 42.5% from 75.5 to 43.4 mg l(-1) (P=0.003) and that of amoxicillin decreased by 36.2% from 73.3 to 46.7 mg l(-1) (P=0.021). Serum concentrations of the two antibiotics decreased by a further 16.5 and 14.5% during the remainder of the surgery, but remained >15-fold above the expected minimum inhibitory concentration (MIC) for target pathogens. Muscle concentrations of both antibiotics reached MIC values by the time of the first sample and there was no decrease associated with connection to CPB. Levels remained >8-fold above MIC for target pathogens throughout the procedure. The t(1/2) for flucloxacillin was 2.64(+/-0.23)h and for amoxicillin was 3.16(+/-0.29)h, both of which are more than double the values in normal adults. There was an equivalent reduction in clearance for both antibiotics.

CONCLUSIONS

Single doses of flucloxacillin and amoxicillin at 30 mg kg(-1) maintain serum and muscle concentrations well above the MIC throughout cardiac surgery. This is partly due to a prolonged t(1/2) and reduced clearance of both antibiotics in infants.

How to calculate clearance of highly protein-bound drugs during continuous venovenous hemofiltration demonstrated with flucloxacillin

BACKGROUND

Flucloxacillin is an important antimicrobial drug in the treatment of infections with Staphylococcus aureus and therefore is often used in staphylococcal infections. Furthermore, flucloxacillin has a high protein binding rate as for example ceftriaxone or teicoplanin--drugs which have formerly been characterized as not being dialyzable.

METHODS

The pharmacokinetic parameters of 4.0 g flucloxacillin every 8 h were examined in 10 intensive care patients during continuous venovenous hemofiltration (CVVH) using a polyamide capillary hemofilter. In addition, the difficulty of calculating the hemofiltration clearance of a highly protein-bound drug is described.

RESULTS

Flucloxacillin serum levels were significantly lowered (56.9 +/- 24.0%) even though only 15% of the drug was detected in the ultrafiltrate. Elimination half-life, total body clearance and sieving coefficient were 4.9 +/- 0.7 h, 117.2 +/- 79.1 ml/min and 0.21 +/- 0.09, respectively. These discrepancies can be explained by the high protein binding of flucloxacillin, the adsorbing property of polyamide and the equation in order to calculate hemofiltration clearance. The unbound fraction of a 4.0 g flucloxacillin dosage facilitates time above the minimum inhibitory concentration (T > MIC) of 60% only for strains up to a minimum inhibitory concentration (MIC) of 0.5 mg/l.

CONCLUSION

Based on the data of this study, we conclude that intensive care patients with staphylococcal infections on CVVH should be treated with 4.0 g flucloxacillin every 8 h which was safe and well tolerated. Moreover, further studies with highly protein-bound drugs are recommended to check the classical 'hemodialysis' equation as the standard equation in calculating the CVVH clearance of highly protein-bound drugs.

The clinical efficacy of continuous-infusion flucloxacillin in serious staphylococcal sepsis

Since the efficacy of beta-lactams against pathogens such as methicillin-susceptible Staphylococcus aureus (MSSA) is related to the time for which serum drug concentrations exceed the MIC for the pathogen, administration of anti-staphylococcal beta-lactams by continuous infusion may provide a more suitable means of drug delivery than intermittent dosing. To assess the clinical efficacy of continuous-infusion therapy, we reviewed the outcomes for 20 consecutive patients with proven serious MSSA sepsis (three with endocarditis, ten osteomyelitis, one endocarditis plus osteomyelitis and six deep abscess) treated with continuous-infusion flucloxacillin (8-12 g/day). Patients initially receiving routine intermittent-dose flucloxacillin therapy were changed to continuous-infusion flucloxacillin (mean duration 29 days; range 4-60 days) for completion of their treatment course. In the majority of cases this was given at home. Serum flucloxacillin concentrations during continuous-infusion flucloxacillin 12 g/day were 11.5->40 mg/L (ten patients) and those during continuous-infusion flucloxacillin 8 g/day were 8->40 mg/L (five patients), these concentrations being well above the expected MIC of flucloxacillin for MSSA. Continuous-infusion flucloxacillin was well tolerated by most patients, and 14/17 patients (82%) who completed their course of continuous-infusion flucloxacillin were judged clinically and microbiologically cured at long-term follow-up (mean 67 weeks; range 4-152 weeks). These preliminary data suggest that, following initial intermittent-dose flucloxacillin therapy, continuous-infusion flucloxacillin is an effective treatment option for serious MSSA sepsis, and forms a feasible and possibly preferable alternative to glycopeptides when considering home-based parenteral therapy for these infections. Further studies are needed to identify whether continuous-infusion flucloxacillin can entirely replace intermittent-dose therapy for such infections.

Distribution of systemically administered ampicillin, benzylpenicillin, and flucloxacillin in excisional wounds in diabetic and normal rats and effects of local topical vasodilator treatment

The present study assessed the suitability of the streptozotocin-treated diabetic rat as a model for the study of diabetes-impaired wound healing. The distribution of three antibiotics, ampicillin, benzylpenicillin, and flucloxacillin, in wound and adjacent tissue sites on the abdomens and legs of normal and diabetic rats was determined 30 min after intravenous administration of a single bolus containing 50 mg of all three antibiotics per kg of body weight. Tissue/plasma ratios showed that antibiotic tissue penetration appeared to be related to protein binding. The treatment of wound sites with vasodilators (1% solution) to increase local blood flow and antibiotic delivery to the site was then determined and appeared to be more effective with endothelium-independent sodium nitroprusside than with endothelium-dependent acetylcholine in diabetic rats. These results suggest that coadministration of topical vasodilators to wound sites in neuropathic diabetic patients undergoing antibiotic therapy for infected ulcers could increase antibiotic delivery to wound tissue sites.

Dicloxacillin and flucloxacillin: pharmacokinetics, protein binding and serum bactericidal titers in healthy subjects after oral administration

The pharmacokinetics of dicloxacillin and flucloxacillin were studied in 12 healthy volunteers after oral administration. The participants received a single dose of either dicloxacillin (0.5 g, 0.75 g or 1.0 g) or flucloxacillin (0.75 g) in a cross-over fashion. Antibiotic concentrations were determined in serum and urine by bioassay and followed for 8 and 24 h, respectively. The three dicloxacillin dosages showed no significant differences for the serum elimination half-lives (t1/2 beta, median: 72 min). Comparing 0.75 g flucloxacillin with the same dose of dicloxacillin, no significant differences between the values of Cmax, t1/2 beta and AUC were found. Protein binding as determined by ultrafiltration in pooled serum was 94.7-96.2% for flucloxacillin and 96.4-97.2% for dicloxacillin. The serum bactericidal titers were similar for the two drugs. In conclusion, dicloxacillin and flucloxacillin showed similar pharmacokinetic behavior after 0.75 g doses in human volunteers.

Pharmacokinetics and bioavailability of flucloxacillin in elderly hospitalized patients

The pharmacokinetics and oral bioavailability of flucloxacillin were studied in five female and two male patients (age 68-87 yr) who had been hospitalized for orthopedic surgeries. A single dose of intravenous or oral flucloxacillin sodium (500 mg) was administered in random order on different occasions separated by at least 2 days. Blood and urine samples were taken up to 24 hours after drug administration and levels of flucloxacillin and 5-hydroxymethylflucloxacillin (5-HMF), a major metabolite, were measured by high-performance liquid chromatography. Flucloxacillin elimination, but not oral absorption, was reduced in the elderly, compared with data from young healthy subjects reported elsewhere. Total clearance, renal clearance, and volume of distribution were 0.083 +/- 0.013 L/kg/hr, 0.038 +/- 0.01 L/kg/hr, and 0.184 +/- 0.034 L/kg, respectively. Regression of flucloxacillin renal clearance (Clr) on estimated creatine clearance (CLcr) gave the relationship: Clr = 0.755 (CLcr) + 10.6 (r = 0.91; P = 0.004). Terminal half-lives for flucloxacillin and 5-HMF were 2.21 +/- 0.51 hr and 3.0 +/- 0.75 hr, respectively after intravenous administration. Flucloxacillin was absorbed rapidly after oral administration with a mean absorption time of 0.95 +/- 0.34 hr, and time to reach peak concentration of 1.20 +/- 0.29 hr. The absolute bioavailability of flucloxacillin from capsules was 54.4 +/- 18.8%.

Rapid column liquid chromatographic analysis of flucloxacillin in plasma on a microparticulate pre-column

A rapid, sensitive, accurate and precise high-performance liquid chromatographic assay is described for flucloxacillin in plasma. Samples (100 microliters) containing dicloxacillin (internal standard) were extracted with ethyl acetate (2 ml). The mobile phase of acetonitrile (18%, v/v) in phosphate buffer (0.01 M, pH 7) was pumped at 1.2 ml/min through a 40 x 3.2 mm I.D. column (3 microns particles). Detection was at 220 nm. Calibration plots were linear (r > 0.9995) from 0.2 to 40 mg/l. Within-day and between-day coefficients of variation were less than 9% (n = 6). The detection limit was 0.05 mg/l and the limit of quantitation was 0.3 mg/l. Of 24 drugs tested, only phenytoin and carbamazepine may interfere in some patients' samples.

Relationship between plasma and bone concentrations of cefuroxime and flucloxacillin. Three different parenteral administrations compared in 30 arthroplasties

(i) The objective was to determine the range of bone levels of cefuroxime and flucloxacillin achieved after one intravenous (IV) administration of different dosages of cefuroxime and flucloxacillin. (ii) Six groups of five patients participated in the study. The first three groups (A-C) received respectively 1500 mg, 1000 mg, and 500 mg cefuroxime intravenously and the second three groups (D-F) received 2000 mg, 1500 mg, and 1000 mg flucloxacillin intravenously. (iii) Parenteral administration of cefuroxime and flucloxacillin resulted in measurable bone concentrations in all patients. (iv) Large inter-individual variation in bone concentration was observed. (v) The bone concentrations of IV cefuroxime were higher (1500 mg, p = 0.0057; 1000 mg, p = 0.0260) than those of flucloxacillin. The bone concentrations of cefuroxime and flucloxacillin were dose dependent.

The application of new bioavailability parameters in the bioequivalence testing of antimicrobial agents

Two new bioavailability parameters were recently suggested [Koeleman et al. 1991] to define (i) the time that the concentration in the blood stays above a defined minimum effective concentration, te and (ii) the onset of the effect, to. In addition to conventional bioequivalence parameters, the new bioavailabilty parameters (to and te) were calculated in this study and statistically compared for penicillin, chloroquine, oxytetracycline, amoxycillin and flucloxacillin from available bioequivalence data. For oxytetracycline, flucloxacillin and amoxycillin, the conventional bioavailability parameters indicated partial equivalence whereas using the te and to parameters, more realistic indications of the possible extent of the performance of a drug from dosage forms were obtained than with the conventional bioequivalence parameters. The new parameters gave additional information for a better evaluation of the performance of a drug from a dosage form.

Plasma and bone concentrations of cefuroxime and flucloxacillin. Oral versus parenteral administration in 20 arthroplasties

Our objective was to determine and to compare the range of bone levels of cefuroxime and flucloxacillin achieved after oral and intravenous administration in 20 arthroplasty patients, allocated to 4 groups: 1 x 500 mg or 7 x 500 mg oral cefuroxime was followed by 2000 mg Flucloxacillin i.v.; 1 x 500 mg and 7 x 500 mg oral flucloxacillin was followed by 1500 mg cefuroxime i.v. Bone samples of hip and knee were obtained. Oral administration did not result in a measurable bone concentration of any of the antibiotics. Intravenous administration resulted in measurable bone concentrations of both cefuroxime and flucloxacillin, with large inter-individual variations. The bone concentrations of intravenous cefuroxime were higher than those of flucloxacillin, despite the lower dose. Oral pretreatment had no effect on the bone concentrations after intravenous administration. No accumulation of the drugs in bone was observed.

Efficacy of amoxycillin/clavulanic acid in experimental Staphylococcus aureus endocarditis in the rat

The efficacy of amoxycillin/clavulanic acid was compared with that of flucloxacillin, vancomycin and amoxycillin in an experimental model of Staphylococcus aureus endocarditis. Doses of the antibiotics were selected to produce peak concentrations in rat serum similar to those achievable in man after administration of parenteral therapeutic doses. Amoxycillin clavulanic acid was more effective than amoxycillin alone against endocarditis caused by beta-lactamase producing strains of Staph. aureus, illustrating the beta-lactamase inhibitory activity of clavulanic acid in vivo. Amoxycillin/clavulanic acid was as effective as flucloxacillin in these infections whereas vancomycin was generally less active. These results illustrate the clinical potential of amoxycillin/clavulanic acid in the prophylaxis, or in the therapy of severe staphylococcal infections.

Optimization of high-performance liquid chromatographic analysis for isoxazolyl penicillins using factorial design

A 3 X 3 factorial design has been used to study the effects of pH and acetonitrile concentration of the eluents on the retention and resolution of cloxacillin, flucloxacillin and dicloxacillin on a C18 column. The logarithm of the capacity factors of these solutes have been found to vary linearly with the pH and quadratically with the acetonitrile content. The equations generated have been employed to predict experimental conditions necessary for an optimum separation. The chromatographic condition selected has been applied to the quantitation of flucloxacillin in human plasma using dicloxacillin as the interval standard. Sample preparation consists of protein precipitation and solid-phase extraction. The detection limit of the assay at 220 nm for flucloxacillin is in the region of 0.1 microgram/ml. This assay has been employed in a study of the relative bioavailability of two commercial flucloxacillin sodium capsules in ten healthy volunteers.

Plasma concentration following oral administration of di- and flucloxacillin in infants and children

The bioavailability of commercial liquid preparations of di- and flucloxacillin was compared in infants and children. The plasma concentrations following a dose of 12.5 mg/kg were equal within the two age groups. Infants 0-1 months old, however, demonstrated a better absorption than older children.

Antibiotic prophylaxis in pacemaker surgery: a prospective double blind trial with systemic administration of antibiotic versus placebo at implantation of cardiac pacemakers

In a double blind clinical trial, 106 consecutive patients scheduled for pacemaker implantation were randomly assigned either to a systemic prophylaxis group (SPG) (to be given flucloxacillin) or to a control group who would be given a placebo (CPG). The SPG group received 2 g IV flucloxacillin 1 hour before the operation, then 1 g perorally every 8 hours for the next five days. In the CPG group, placebo infusions and tablets were given at the same schedule. There were a total of 106 patients (SPG 52, CPG 54) who met the criteria of the study. Of these, 102 patients (SPG 50, CPG 52) completed a follow-up of 7-35 months. Infection of the pacemaker system was not diagnosed in any patient in either group. Tissue fluid was drawn 24 hours postoperatively from the pacemaker pocket for culture and for determination of pocket antibiotic concentration. The mean flucloxacillin concentration of pocket fluid from 23 patients in the SPG was 7.5 micrograms/ml. The bacteriological cultures were positive in 9/32 patients in the SPG group and in 10/34 patients in the CPG group. This study suggests that antibiotic prophylaxis need not routinely be given at implantation of permanent pacemaker systems.

Flucloxacillin concentration in serum and wound exudate during open heart surgery

Per-operative serum and wound fluid concentrations achieved by two flucloxacillin dosage regimens were measured in twelve patients undergoing open heart surgery. One 500 mg bolus dose given after the induction of anaesthesia in six patients resulted in adequate serum concentrations (means decreasing from 69 to 9 mg/l) during surgery, but low wound fluid concentrations (mean 4.6 mg/l, range less than 2-7.8 mg/l) at the time of closure. An additional 500 mg bolus dose given after bypass in six further patients gave more satisfactory wound concentrations (mean 16 mg/l, range 8.6-22.5 mg/l). Disc absorption is a useful technique for assaying antibiotics in wound fluid, but the results are difficult to interpret since they represent a sum of concentrations in a complex and changing mixture of interstitial tissue fluid, blood and cardioplegia solution.

Pharmacokinetics and distribution of flucloxacillin in pacemaker patients

The pharmacokinetics of flucloxacillin in plasma and tissue fluid after i.v. infusion of 1 g was analyzed according to an open two-compartment model in 19 patients with bradyarrhythmias (mean age 70.8 years) admitted for implantation or replacement of a permanent pacemaker system. After the first infusion of flucloxacillin (5 min), the distribution phase was rapid (t 1/2 alpha = 0.13 h). The plasma half-life of elimination (t 1/2 beta) was 1.51 h, which is almost twice as long as reported in healthy volunteers. Total plasma clearance (93.1 ml/min) was also lower than is usually found in healthy individuals, due to low renal clearance of flucloxacillin (60.2 ml/min). The total apparent volume of distribution during the beta-phase (Vdarea) was 0.172 l/kg and distribution in the central compartment (Vc) 0.064 1/kg. In each patient plasma protein binding and drug distribution to plasma water, proteins and blood cells in whole blood were determined. Binding in plasma to proteins was 91.0% and distribution to blood cells in whole blood 13.8%. The mean distribution volume of free flucloxacillin during the beta-phase (Vd beta free) was 2.18 1/kg, which exceeds total body water, suggesting possible intracellular distribution and substantial tissue binding. Plasma concentrations of flucloxacillin after the fourth dose (1 g t.i.d.) were very similar to those obtained after the first infusion and those predicted from the single dose kinetics. The concentration of flucloxacillin in fluid from the pacemaker pockets in 5 patients averaged 12.1 micrograms/ml and 9.5 micrograms/ml at 1 and 5 h, respectively, which was more than ten times the MIC-values for Staphylococcus aureus and S. epidermidis.(ABSTRACT TRUNCATED AT 250 WORDS)

Concentrations of cloxacillin, dicloxacillin and flucloxacillin in dental alveolar serum and mandibular bone

Deep infections caused by staphylococci are serious complications after oral surgery. Intensive antibiotic treatment is always necessary in these infections. Isoxazolylpenicillins are often used. Levels of three isoxazolylpenicillins, namely, cloxacillin, dicloxacillin and flucloxacillin, were measured in serum, dental alveolar serum and mandibular bone in 60 patients undergoing surgical removal of impacted third molars after a single dose of 500 mg. The systemic serum concentrations were higher than the dental alveolar serum concentrations in most patients. The maximum concentration in the alveolar serum was 3.8-6.4 microgram/ml for cloxacillin, 6.0-15.0 microgram/ml for dicloxacillin and 10.0-15.0 microgram/ml for flucloxacillin. The concentration in mandibular bone was 2.0 +/- 0.4 microgram/g for cloxacillin, 2.0 +/- 0.5 microgram/g for dicloxacillin and 2.0 +/- 0.5 microgram/g for flucloxacillin.

Drug distribution in whole blood of mothers and their newborn infants: studies of cloxacillin and flucloxacillin

The distribution of cloxacillin and flucloxacillin in whole blood from seven newborn infants and their mothers was determined in vitro by equilibrium dialysis at 37 degrees C. Seven healthy, non-pregnant women of reproductive age served as controls. The distribution of the penicillins to erythrocytes was the same in the infants as in the adults. It was significantly lower in the presence of plasma albumin than when plasma was replaced by isotonic phosphate buffer. The plasma protein binding of cloxacillin and flucloxacillin in 22 infants was significantly lower than in the controls, but was slightly higher than in the mothers. A significant correlation between binding of cloxacillin and flucloxacillin in the same individual suggested that the two drugs were bound to similar sites. During the first postnatal week binding in infant plasma decreased. This change was correlated with an increase in the bilirubin levels. In the mothers, the binding increased during the first week after delivery. On the basis of the distribution data, maternal to fetal plasma and whole blood concentration ratios at equilibrium were calculated. These ratios were lower for flucloxacillin (medians 0.770 and 0.821, respectively) than for cloxacillin 0.996 and 1.094). Accordingly, at equilibrium somewhat higher levels of flucloxacillin should appear on the fetal than on the maternal side, whereas the concentrations of cloxacillin would be expected to be approximately the same.

Analysis of isoxazolyl penicillins and their metabolites in body fluids by high-performance liquid chromatography

A high-performance liquid chromatographic assay method to quantitate the isoxazolyl penicillins, their active metabolites, and their penicilloic acids in serum or urine is described. Separation and analysis is performed using reversed-phase chromatography. Urine samples, after the appropriate dilution, can be assayed directly. Serum samples (0.1 ml) are either extracted with methylene chloride or treated with perchloric acid--methanol. Serum levels as low as 0.4 microgram/ml (extraction procedure) can be assayed accurately.

Dicloxacillin and flucloxacillin twice daily with probenecid in staphylococcal infections. A clinical and pharmakokinetic evaluation

The therapeutic efficacy of decloxacillin or flucloxacillin, 1--2 g orally with probenecid twice daily was evaluated in staphylococcal infections. Excellent results were obtained with this as long-term maintenance therapy in 4 patients with relapsing chronic osteomyelitis and in 6 patients with postoperative infections. No side effects were observed in these patients. In 35 patients with furunculosis twice daily administration of dicloxacillin was used as a primary therapeutic regimen. All healed, but 4 recurrencies occurred during a 3-month observation period after therapy. In 5 patients allergic reactions were noted. The unbound fraction of flucloxallin in serum exceeded MIC values of methicillin-susceptible Staphylococcus aureus during 11 of 24 h. Pharmacokinetic data on flucloxallin with and without probenecid are given.

Investigations on the flucloxacillin levels in human serum, lung tissue, pericardial fluid and heart tissue

Following intramuscular administration of 500 mg flucloxacillin, the levels of this antibiotic were measured in the serum, lung, and heart tissue as well as pericardial fluid obtained during 24 lung and 19 heart operations. Between 55 and 160 min following administration, the intact, inflamed, and tumorous lung tissue pieces contained 3.3-4.0, 2.0-2.7, and 1.5-2.4 mcg/g flucloxacillin, respectively, representing 16-36, 13-16, and 10-18% of the actual serum levels. Pericardial fluid contained practically no flucloxacillin. The cardiac auricle contained 2.3-3.5 mcg/g between 35 and 130 min after administration; the mitral valve showed 1.1-2.5 mcg/g between 90 and 170 min afterward, whereas in the aortic valve 1.6 mcg/g was found between 105-220 min later. These concentrations represented 16-20, 7-22, and 12% of the actual serum levels. Both the serum and tissue levels proved to be higher than those of oxacillin at the same dosage. The flucloxacillin levels of the serum and tissues studied were much higher than the minimum inhibiting concentrations for the sensitive bacteria. Therefore, flucloxacillin is considered an excellent antibiotic for the treatment of respiratory infections and endocarditis, whereas its levels in the pericardial fluid show it insufficient for treatment of pericarditis.

The effect of semisynthetic penicillins on the binding of bilirubin by neonatal serum

1. The effect of ampicillin, cloxacillin, flucloxacillin and sulphafurazole on bilirubin on bilirubin binding by pooled human umbilical cord serum and bovine serum albumin was studied in vitro using Sephadex gel filtration. 2. Sulphafurazole displaced bilirubin from binding; both cloxacillins displaced bilirubin from pooled cord serum but not bovine serum albumin. 4. No displacement of bilirubin by the cloxacillins from pooled cord serum could be detected at therapeutic plasma concentrations of these drugs. 5. Scatchard analysis of the interactions showed that displacement of bilirubin by these drugs occurred principally at the primary, high affinity, low capacity binding site.

Flucloxacillin in bone

Ten patients undergoing total hip replacement for osteoarthritis were each given intramuscular flucloxacillin about two hours preoperatively; bone and serum were sampled simultaneously at operation. Trabecular and compact bone were separated, partly dried, reduced to powders, and then extracted with buffer. The concentration of flucloxacillin in bone washings and serum was determined by well-diffusion assay. The mean concentration of flucloxacillin in serum was 8.9 mg/l, in trabecular bone washings, 1.3 mg/l, and in compact bone washings 0.9 mg/l. The amount of blood contaminating the bone washings was measured, and was calculated to account for at most 26% of the flucloxacillin present. The significance of these findings is discussed in relation to the prophylactic use of flucloxacillin in hip replacement surgery.

[Pharmacokinetic and clinical research on a new antibiotic combination (amoxicillin and flucloxacillin in equivalent-weight dose)]

A controlled double-blind biometric and an open clinical trial were conducted to determine the therapeutic effectiveness of a new equal-dose w/w association of amoxicillin and flucloxacillin. The following conclusions were drawn. Both antibiotics were present in high serum levels; those of flucloxacillin were higher and more persistent. Analysis of variance on 89 patients pointed to the superiority of the association by comparison with amoxicillin alone. The clinical study made it clear that the broad and complementary spectrum of the association, its synergy, absence of toxicity and good gastric tolerance make it a valuable and effective therapeutic aid, also in presence of germs that produce beta-lactase.

Pharmacokinetics of flucloxacillin and cloxacillin in healthy subjects and patients on chronic intermittent haemodialysis

1 A pharmacokinetic study on flucloxacillin and cloxacillin was performed to investigate the factors contributing to the higher serum concentrations reported for the former after oral administration. 2 The results obtained in a study performed in a group of volunteers with flucloxacillin administered orally and by continuous infusion, were compared with the results of a similar investigation with cloxacillin. Patients on chronic intermittent haemodialysis received flucloxacillin orally and as a single i.v. injection. The results of this part of the study were compared with those of an earlier study on cloxacillin in haemodialysis patients. Serum and urine concentrations of flucloxacillin and cloxacillin were determined by bio-assay, and a one-compartment model was used for the calculations. 3 Higher serum concentrations reached after oral administration of flucloxacillin as compared with cloxacillin were based not only on better oral absorption (53.7% and 32.9%, respectively) but also on slower (renal and extra-renal) elimination (T1/2 : 46 and 32 min, respectively). A significant difference between the apparent volumes of distribution of flucloxacillin and cloxacillin, which could contribute to higher serum concentrations, could not be demonstrated. Considerable individual variation occurs in the rate and amount of oral absorption, especially in patients. The elimination rate of flucloxacillin in haemodialysis patients (T1/2 : 2h 53 min) corresponds with the extra-renal elimination rate in healthy subjects. No influence of haemodialysis on the elimination rate constant of flucloxacillin was found; total plasma clearance was, however, slightly but significantly higher during dialysis.