Departments of 1 Clinical Pharmacology and 2 Infectious Diseases, Christchurch Hospital, PO Box 4710, Christchurch, New Zealand
Received 7 December 2001; returned 13 March 2002; revised 15 May 2002; accepted 25 May 2002
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Abstract |
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Introduction |
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Cefazolin has a volume of distribution of 10 L and is 7585% plasma bound.3 The total body clearance is
3.6 L/h in patients with normal renal function with
90% of the dose eliminated unchanged via the kidneys. The reported mean elimination half-life is
2 h (range 1.52.5 h).3 Cefazolin, therefore, needs to be administered at least twice daily by intravenous bolus or by a constant infusion over 24 h to achieve therapeutic concentrations.
A constant infusion may be the most appropriate method of delivering ß-lactam antibiotics, as the major determinant of ß-lactam antibiotic efficacy is widely considered to be the time above the MIC at the site of infection.4 With constant infusions at steady state, and assuming the principles of diffusion, it would be expected that concentrations of free drug (not protein bound) should be similar in the plasma and the interstitial fluid.4 If this can be demonstrated, then free drug concentrations in plasma can easily be measured to check the adequacy of dosing regimens.
The aim of this study was to measure free and total cefazolin concentrations in both plasma and interstitial fluid under steady state conditions using a blister model during continuous infusion.
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Materials and methods |
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Seven consecutive patients were recruited from those who had been enrolled in the home antibiotic programme for the treatment of uncomplicated cellulitis. All patients received cefazolin (Eli Lilly, Auckland, New Zealand) at a fixed dose by continuous intravenous infusion for at least 5 days before recruitment to ensure steady state conditions. The usual starting dose was 3 g per 24 h adjusted at the discretion of the attending physician for body size, severity of infection and renal function. Cefazolin was administered via a Homepump ECLIPSE C-Series (5 mL/h) (I-flow Corporation, Lakefront, CA, USA) through a peripherally inserted catheter (PIC line) (Arrow International Inc., Reading, PA, USA). Patients were excluded if their calculated creatinine clearance by the Cockcroft & Gault equation5 was outside the normal range for their age, weight and gender.
Interstitial fluid and blood/plasma sampling
Interstitial fluid (0.2 mL per patient) was harvested from four skin blisters induced on the forearm of each patient using the technique first described by Schreiner et al.4 Blood (5 mL) was also taken at the beginning and end of the period of blister formation to provide comparative protein free and total drug concentrations in plasma. Samples were stored at 30°C prior to analysis.
Processing of samples
Total and free cefazolin concentrations in plasma and interstitial fluid were analysed by the HPLC method of Kamani et al.,6 with minor modifications. Free drug concentrations and degree of protein binding were determined by ultrafiltration (2600g for 30 min at 37°C) using a Diaflo ultrafiltration membrane, YMT DISCS, 30K NMWL, 14 mm (Amicon Inc., Beverly, MA, USA), for 30 min. The filtrate (50 µL) was injected into the Kontron HPLC system and chromatography performed using an AQUA C18 5 µm 75 x 4.6 mm, ID column (Phenomenex, Torrance, CA, USA). The mobile phase was a mixture of 0.01 M phosphate buffer pH 6.5 and acetonitrile [90:10 (v/v)] at a flow rate of 1 mL/min. This provided a retention time for cefazolin of 8 min.
Statistical analysis
Descriptive statistics and linear regression analysis were undertaken using GraphPad Prism, Version 3.0 (GraphPad Software, San Diego, CA, USA).
Ethics committee approval
The study was reviewed and approved by the regional ethics committee. Patients were informed about all aspects of the study, including possible de-pigmentation of the skin areas used for blister formation, and gave written consent.
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Results |
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Five males and two females were recruited into the study: median age 53 years (range 2574), median weight 90 kg (range 71170), mean creatinine clearance (±S.D.) 1.9 mL/s (±0.9), mean daily dose (±S.D.) 3.5 g (±1.1), and mean dose per kg per day (±S.D.) 36 mg/kg/day (±6.1). The patients received no medications that were known to interfere with the clearance or protein binding of cefazolin.
Assay
Standard curves for total and free cefazolin were linear (r2 > 0.99) over the concentration ranges 1.5200 and 0.0420 mg/L, respectively. The limits of quantification for total and free cefazolin were 1.5 and 0.04 mg/L, respectively. The absolute recoveries of total cefazolin concentrations from plasma and interstitial fluid were >95%, whereas recoveries during ultrafiltration at concentrations of 0.4, 4.0 and 16 mg/L were 90%. Intra- and inter-day coefficients of variation (CV%) of assay precision for total cefazolin were <7 and <5.5%, respectively (concentration range 4.0160 mg/L). For free cefazolin the intra- and inter-day CV% were <1 and <2%, respectively (concentration range 0.416 mg/L). Cefazolin was found to be stable in plasma and interstitial fluid at 30°C for at least 4 weeks.
Cefazolin concentrations
The free and total plasma and interstitial fluid concentrations of cefazolin are presented in Table 1 and Figure 1. The total and free plasma concentrations from samples taken at the beginning and end of blister formation were not significantly different (paired t-test, P = 0.88 and 0.38, respectively). Total concentrations (mean ± S.D.) in plasma were higher than those in interstitial fluid in six of seven patients and lower in one (32 ± 17 versus 17.4 ± 8.3 mg/L). Linear regression analysis of the total concentrations in the plasma and interstitial fluid revealed a squared correlation coefficient (r2) of 0.55 (P = 0.056), with similar results observed for the free concentrations. In five of the seven patients free interstitial concentrations were slightly lower than in plasma, and in the other two patients these were equal (mean ratio = 0.84, 95% CI = 0.6960.998). Linear regression analysis showed a strong correlation between the free concentrations in the plasma and the interstitial fluid [r2 of 0.82 (P = 0.005)].
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Discussion |
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Unlike other cephalosporins, cefazolin demonstrates saturable protein binding in the range of concentrations encountered in clinical practice,10 creating problems in studies where antibiotic concentrations vary over the period of investigation. In contrast, saturable protein binding should not influence the free drug concentrations in either compartment at steady state during continuous infusion, as the only determinants of these are the drug dose administered and free drug clearance.
Our studies have confirmed the results of Waterman et al.,11 who showed similar results using bolus dosing in an animal model. Furthermore, we have confirmed that with a constant infusion, at steady state, free concentrations in plasma reflect interstitial fluid free concentrations, and therefore can be compared with the MICs of infecting organisms. For ß-lactam antibiotics clinical efficacy is improved when concentrations exceed the minimum required to inhibit bacterial proliferation (MIC) for the majority (>50%) of the dosing interval.12 It was, therefore, reassuring that the lowest free concentration in the interstitial fluid was 2 mg/L, as this exceeded the MIC90 for the two most likely organisms, Staphylococcus aureus (1 mg/L) and Streptococcus pyogenes (0.1 mg/L),13 suggesting that the dose regimen used was appropriate.
Further studies need to be undertaken with regard to the time taken to achieve steady state conditions, with or without loading doses, to elucidate the distribution kinetics of cefazolin given by constant infusion. Nevertheless, this study provides an important step in validating home intravenous antibiotic programmes, particularly those based on constant infusion techniques.
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Footnotes |
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References |
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2 . Grayson, M. L., Silvers, J. & Turnidge, J. (1995). Home intravenous antibiotic therapy. A safe and effective alternative to inpatient care. Medical Journal of Australia 162, 24953.[ISI][Medline]
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