JPM Vol. 28, No. 1
August 1992:57-60
The Influence of Vancomycin Concentration and the pH of Plasma on Vancomycin Protein Binding Y. Chen, R. L. G. Norris, J. J. Schneider, and P. J. R a v e n s c r o f t Department of Clinical Pharmacology, Princess Alexandra Hospital, Woolloongabba QLD 4102, Australia
A review of numerous studies of the protein binding of vancomycin suggests major discrepancies among their results. The reported percent protein binding of vancomycin varies from 0% to 98%. The influence of pH and concentration on the protein binding of vancomycin was investigated in this study. There was a significant difference (p < 0.001) in percent protein binding in vancomycin-spiked plasma samples across the pH range of 7.0-8.0. There was no significant difference (p > 0.05) in percent protein binding in vancomycinspiked plasma samples across the concentration range of 2-80 mg/L. It is likely that some of the variation reported to date may be due to a lack of control of pH during the measurement of protein binding of vancomycin.
Keywords: pH; Concentration; Vancomycin; Protein binding
Introduction Protein binding is a rapidly reversible interaction between serum protein and a drug. The protein-binding process is influenced by many factors including the concentration and chemical structure of the drug, the concentration and chemical structure of the protein, pH, and competition from other ligands (Craig and Welling, 1977). Clinically, these factors can be influenced by various disease states (Wise, 1958). It is widely accepted that the binding of drugs to plasma protein influences the free concentration of such agents, changing their pharmacokinetic and pharmacodynamic behavior. Published studies report wide variations in the protein binding of vancomycin. The percent bound ranges from 0% to 98% (Ackerman et al., 1988; Cantu et al., 1990; Cutler et al., 1984; Krogstad et al., 1980; Lindholm and Murray, 1966; Tan et al., 1990; Wittendorf et al., 1987; Zokufa et al., 1989). These differences may occur because of variations in assay methodology, sample collection, or differences in the physiological and pathological conditions of the subjects.
Address reprint requests to Dr. Y. Chen, Department of Pharmacology, 1st Floor, Lions Building, Princess Alexandra Hospital, Woolloongabba QLD 4102, Australia. Received January 27, 1992; revised and accepted June 1, 1992. Journal of Pharmacological and Toxicological Methods 28, 57-60 (1992) © 1992 Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas, New York, NY 10010
This study investigates the influence of pH and concentration on the protein binding of vancomycin.
Materials and Methods
Reagents Vancomycin was obtained from Eli Lilly Pty Ltd (Australia), and high-performance liquid chromatography (HPLC) grade methanol from Mallinckrodt Australia Pty Ltd. Potassium dihydrogen orthophosphate, hydrochloric acid, and sodium hydroxide were of analytical reagent grade. The deionized water used was obtained from a Milli-Q Water Purification System (Millipore Corporation, MA, U.S.A.).
HPLC Apparatus The HPLC system included a model 510 pump (Millipore Corporation), a model 480 variable-wavelength ultraviolet (UV) detector, (Millipore Corporation), a model 7125 manual injector, (Rheodyne, CA, U.S.A.), and a Type 3066 Pen Recorder (Yokogawa Electric Corporation, Tokyo, Japan). Expired plasma (two bags, citrate additive, albumin concentration: 34 g/L) from the Blood Bank was used for the study. After thorough mixing, the pooled plasma was subdivided into three groups, and the pH
1056-8719/92/$5.00
58
JPM Vol. 28, No. 1 August 1992:57-60
T a b l e 1. M e a n P e r c e n t P r o t e i n B i n d i n g (SD) of V a n c o m y c i n at V a r y i n g C o n c e n t r a t i o n and p H
Protein Binding (%)
Mean (SD) Concentration (mg/L)
pH
2.0
7.0 7.4 8.0 Mean (SD)
38,8 31.7 27.9 32.8
5.0
(1.5) (1.9) (1.5) (4.9)
37.1 31.7 28.5 32.4
10.0
(1.9) (1.1) (1.6) (4.0)
37.5 34.5 27.7 33.3
(1.0) (0.2) (1.2) (4.4)
80.0 32.4 33.9 29.5 31.9
(1.9) (2.0) (1.7) (2.5)
Mean (SD) 36.5 (2.9) 33.0 (1.9) 28.4 (1.5)
Note: n = 4 at each pH concentration.
was adjusted to 7.0, 7.4, and 8.0, respectively, using 0.1 M hydrochloric acid or 0.1 M sodium hydroxide plus buffer to maintain a constant added volume. All the samples were prepared in glassware or glass tubes with Teflon-lined screw caps. Vancomycin was added to the aliquots of plasma at each pH to produce concentrations of 2.0, 5.0, 10.0, and 80.0 rag/L, respectively. The protein binding at each concentration was measured in quadruplicate.
Plasma Ultrafiltration Vancomycin free fraction was measured by ultrafiltration. Of the plasma, 0.5 mL from each combination of pH and concentration was placed into each of four Amicon Centrifree micropartition devices (Amicon Corporation, Dancers, MA, U.S.A.). All the samples were centrifuged in a fixed-angle rotor at 1000 g for 12 rain, at a rotor temperature of 37°C. Preliminary experimentation indicated that adsorption of vancomycin on to these devices was negligible under these conditions.
Determination of Vancomycin Concentrations (HPLC) Vancomycin concentrations in the ultrafiltrate were determined using our modification of a previously reported method (Bauchet et al., 1987).
Data Analysis The percent protein binding of vancomycin was determined from the ratio of the unbound vancomycin concentration to the total vancomycin concentration in the samples. The relationships between vancomycin concentration, pH, and percent protein binding were evaluated by two-way ANOVA analysis, Tukey's Studentized Range (HSD) Test (SAS, 1985), and use of 95% confidence intervals.
Results The data are summarized in Table 1. The results of two-way ANOVA analysis and Tukey's Studentized Range (HSD) Test showed that there was a significant
pH 8.0
7.4
Figure 1. Mean and 95% confidence intervals of percent protein-bound vancomycin at pH 7.0, 7.4, and 8.0 across the concentration of 2-80 mg/L.
7.0
25
~ 30
I 35
Protein binding (%)
I 40
CHEN ET AL. INFLUENCE OF pH ON VANCOMYCINPROTEIN BINDING
59
45 40
7-
Vancomycin Concentration • 2mg/L
Figure 2. Mean and standard deviations of percent protein-bound vancomycin of concentrations of 2, 5, |0, and 80 mg/L at pH 7.0, 7.4, and 8.0, respectively.
[] 5mg/L 10mg/L
•
[] 80mg/L
0
difference (p < 0.001) in percent protein binding in plasma samples across the pH range of 7.0-8.0. The 95% confidence intervals were calculated by using standard deviations (Bolton, 1984). At pH 7.0, 7.4, and 8.0, they were 35.0%-38.0%, 32.0%-34.0%, and 27.6%-29.2%, respectively (Figure 1). There was no significant difference (p > 0.05) in percent protein binding in plasma samples across the concentration range of 2-80 mg/L. The mean and standard deviations of percent protein-bound vancomycin of concentrations of 2.0, 5.0, 10.0, and 80.0 (mg/L) at pH 7.0, 7.4, and 8.0 were respectively shown in Figure 2. An interaction between vancomycin concentration and pH was demonstrated by the analysis (p < 0.01).
Discussion Many studies have been published on vancomycin protein binding since the initial paper on this subject was published by Lindholm and Murray in 1966. The results of the studies so far show major discrepancies with the percentage of protein binding reported ranging from 0% to 98% (Ackerman et al., 1988; Cantu et al., 1990; Cutler et al., 1984; Krogstad et al., 1980; Lindholm and Murray, 1966; Tan et al., 1990; Wittendorf et al., 1987; Zokufa et al., 1989). Vancomycin hydrochloride is an amphoteric substance, and, therefore, may bind to albumin and possibly al-acid glycoprotein, and/or other plasma proteins (Piafsky 1980). However, in one study, there was a strong relationship shown between percent protein-bound vancomycin and albumin concentration, but a poor relationship between percent protein-bound vancomycin and ct1-acid glycoprotein concentration (Zokufa et al., 1989). Cantu et al. (1990) have suggested that vancomycin may bind to IgA. In all these studies, there was no indication of the sample collection methods used, which may affect pro-
7.0
7.4 pH
8.0
tein binding (Cotham and Shand, 1975; Fremstad and Bergerud, 1976; Haughey and Lima, 1982; Sorrells, 1987; Wood et al., 1979), nor information on whether the pH and temperature were controlled during the measurement of protein binding. In our study all above variables have been controlled by using (1) pooled plasma, thereby avoiding variability in the plasma used for all binding experiments; (2) appropriate, standardized collection procedures; and (3) controlled conditions of temperature and pH. We established that change in pH is one of the important factors affecting vancomycin protein binding. By controlling pH, we note that the 95% confidence intervals for vancomycin binding are reduced, thereby making it less likely that real effects on the protein binding would be obscured (Figures 1 and 2). Ackerman et al. (1988) showed that there was no significant relationship (p < 0.05) between vancomycin concentration and percent protein binding in serum samples from 12 infected patients. This finding is in general agreement with our results and suggests vancomycin concentration does not have an important influence on the percent bound. We conclude that the control of pH in studies of vancomycin protein binding is important, and it is l!kely that some of the published variation in the protein binding of vancomycin is due to this hitherto unreported problem. Additional studies are required to establish the extent of influence of pH on the protein binding of vancomycin in vivo.
References Ackerman BH, Taylor EH, Olsen KM, Abdel-Malak W, Pappas AA (1988) Vancomycin serum protein binding determination by ultrafiltration. Drug lntell Clin Pharm 22:300-303. Bauchet J, Pus sard E, Garaud JJ (1987) Determination of vancomycin in serum and tissues by column liquid chromatography using solid-phase extraction. J Chromatogr 414:472-476.
60
Bolton S (1984) Pharmaceutical Statistics. Drugs and Pharmaceutical Sciences, Vol 25. New York: Marcel Dekker, Inc. Cantu TG, Dick JD, Elliott DE, Humphrey RL, Kornhauser DM (1990) Protein binding of vancomycin in a patient with immunoglobulin A m y e l o m a . A n t i m i c r o b A g e n t s C h e m o t h e r 34: 1459-1461. Cotham RH, Shand D (1975) Spuriously low plasma propranolol concentration resulting from blood collection methods. Clin Pharmacol Ther 18:535-538. Craig WA, Welling PG (1977) Protein binding of antimicrobials: Clinical pharmacokinetic and therapeutic implications. Clin Pharmacokinet 2:252-268. Cutler NR, Narang PK, Lesko LJ, Ninos M, Power M (1984) Vancomycin disposition: The importance of age. Clin Pharmacol Ther 36:803-810. Fremstad D, Bergerud K (1976) Plasma protein binding of drugs as influenced by blood collection methods. Acta Pharmacol Toxicol 39:570-572. Haughery DB, Lima JJ (1982) The influence of blood collection technique on serum and plasma protein binding of disopyramide. Eur J Clin Pharmacol 22:185-189. Krogstad DJ, Moellering RC, Greenblatt DJ (1980) Single-dose kinetics of intravenous vancomycin. J Clin Pharmacol 197-201. Lindholm DD, Murray JS (1966) Persistence of vancomycin in the
JPM Vol. 28. N o . 1 August 1992:57-60
blood during renal failure and its treatment by hemodialysis. N Engl J Med 274:1047-105 I. Piafsky KM (1980) Disease-induced changes in the plasma binding of basic drugs. Clin Pharmacokinet 5:246-262. SAS For Personal Computers (1985) Cary, NC, USA, SAS Institute Inc., Version 6 Edition. Sorrells SC (1987) Effect of vacutainer tubes on lidocaine protein binding. Drug Intell Clin Pharm 21:464-465. Tan CC, Lee HS, Ti TY, Lee EJ (1990) Pharmacokinetics of intravenous vancomycin in patients with end-stage renal failure. Ther Drug Monit 12:29-34. Wise R (1985) The relevance of pharmacokinetics to in vitro models: Protein b i n d i n g - - d o e s it matter? J A n t i m i c r o b C h e m o t h e r 15(Suppl A):77-83. Wittendorf RW, Swagzdis JE, Gifford R, Mico BA (1987) Protein binding of glycopeptide antibiotics with diverse physical-chemical properties in mouse, rat, and human serum. J Pharmacokinet Biopharmaceut 15:5-13. Wood M, Shand DG, Wood AJJ (1979) Altered drug binding due to the use of indwelling heparinized cannulas (heparin lock) for sampling. Clin Pharmacol Ther 25:103-107. Zokufa HZ, Solem LD, Rodvold KA, Crossley KB, Fischer JH, Rotschafer J C (1989) The influence of serum albumin and a~-acid glycoprotein on vancomycin protein binding in patients with burn injuries. J Burn Care Rehabilit 10:425-428.
August 1992:57-60
The Influence of Vancomycin Concentration and the pH of Plasma on Vancomycin Protein Binding Y. Chen, R. L. G. Norris, J. J. Schneider, and P. J. R a v e n s c r o f t Department of Clinical Pharmacology, Princess Alexandra Hospital, Woolloongabba QLD 4102, Australia
A review of numerous studies of the protein binding of vancomycin suggests major discrepancies among their results. The reported percent protein binding of vancomycin varies from 0% to 98%. The influence of pH and concentration on the protein binding of vancomycin was investigated in this study. There was a significant difference (p < 0.001) in percent protein binding in vancomycin-spiked plasma samples across the pH range of 7.0-8.0. There was no significant difference (p > 0.05) in percent protein binding in vancomycinspiked plasma samples across the concentration range of 2-80 mg/L. It is likely that some of the variation reported to date may be due to a lack of control of pH during the measurement of protein binding of vancomycin.
Keywords: pH; Concentration; Vancomycin; Protein binding
Introduction Protein binding is a rapidly reversible interaction between serum protein and a drug. The protein-binding process is influenced by many factors including the concentration and chemical structure of the drug, the concentration and chemical structure of the protein, pH, and competition from other ligands (Craig and Welling, 1977). Clinically, these factors can be influenced by various disease states (Wise, 1958). It is widely accepted that the binding of drugs to plasma protein influences the free concentration of such agents, changing their pharmacokinetic and pharmacodynamic behavior. Published studies report wide variations in the protein binding of vancomycin. The percent bound ranges from 0% to 98% (Ackerman et al., 1988; Cantu et al., 1990; Cutler et al., 1984; Krogstad et al., 1980; Lindholm and Murray, 1966; Tan et al., 1990; Wittendorf et al., 1987; Zokufa et al., 1989). These differences may occur because of variations in assay methodology, sample collection, or differences in the physiological and pathological conditions of the subjects.
Address reprint requests to Dr. Y. Chen, Department of Pharmacology, 1st Floor, Lions Building, Princess Alexandra Hospital, Woolloongabba QLD 4102, Australia. Received January 27, 1992; revised and accepted June 1, 1992. Journal of Pharmacological and Toxicological Methods 28, 57-60 (1992) © 1992 Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas, New York, NY 10010
This study investigates the influence of pH and concentration on the protein binding of vancomycin.
Materials and Methods
Reagents Vancomycin was obtained from Eli Lilly Pty Ltd (Australia), and high-performance liquid chromatography (HPLC) grade methanol from Mallinckrodt Australia Pty Ltd. Potassium dihydrogen orthophosphate, hydrochloric acid, and sodium hydroxide were of analytical reagent grade. The deionized water used was obtained from a Milli-Q Water Purification System (Millipore Corporation, MA, U.S.A.).
HPLC Apparatus The HPLC system included a model 510 pump (Millipore Corporation), a model 480 variable-wavelength ultraviolet (UV) detector, (Millipore Corporation), a model 7125 manual injector, (Rheodyne, CA, U.S.A.), and a Type 3066 Pen Recorder (Yokogawa Electric Corporation, Tokyo, Japan). Expired plasma (two bags, citrate additive, albumin concentration: 34 g/L) from the Blood Bank was used for the study. After thorough mixing, the pooled plasma was subdivided into three groups, and the pH
1056-8719/92/$5.00
58
JPM Vol. 28, No. 1 August 1992:57-60
T a b l e 1. M e a n P e r c e n t P r o t e i n B i n d i n g (SD) of V a n c o m y c i n at V a r y i n g C o n c e n t r a t i o n and p H
Protein Binding (%)
Mean (SD) Concentration (mg/L)
pH
2.0
7.0 7.4 8.0 Mean (SD)
38,8 31.7 27.9 32.8
5.0
(1.5) (1.9) (1.5) (4.9)
37.1 31.7 28.5 32.4
10.0
(1.9) (1.1) (1.6) (4.0)
37.5 34.5 27.7 33.3
(1.0) (0.2) (1.2) (4.4)
80.0 32.4 33.9 29.5 31.9
(1.9) (2.0) (1.7) (2.5)
Mean (SD) 36.5 (2.9) 33.0 (1.9) 28.4 (1.5)
Note: n = 4 at each pH concentration.
was adjusted to 7.0, 7.4, and 8.0, respectively, using 0.1 M hydrochloric acid or 0.1 M sodium hydroxide plus buffer to maintain a constant added volume. All the samples were prepared in glassware or glass tubes with Teflon-lined screw caps. Vancomycin was added to the aliquots of plasma at each pH to produce concentrations of 2.0, 5.0, 10.0, and 80.0 rag/L, respectively. The protein binding at each concentration was measured in quadruplicate.
Plasma Ultrafiltration Vancomycin free fraction was measured by ultrafiltration. Of the plasma, 0.5 mL from each combination of pH and concentration was placed into each of four Amicon Centrifree micropartition devices (Amicon Corporation, Dancers, MA, U.S.A.). All the samples were centrifuged in a fixed-angle rotor at 1000 g for 12 rain, at a rotor temperature of 37°C. Preliminary experimentation indicated that adsorption of vancomycin on to these devices was negligible under these conditions.
Determination of Vancomycin Concentrations (HPLC) Vancomycin concentrations in the ultrafiltrate were determined using our modification of a previously reported method (Bauchet et al., 1987).
Data Analysis The percent protein binding of vancomycin was determined from the ratio of the unbound vancomycin concentration to the total vancomycin concentration in the samples. The relationships between vancomycin concentration, pH, and percent protein binding were evaluated by two-way ANOVA analysis, Tukey's Studentized Range (HSD) Test (SAS, 1985), and use of 95% confidence intervals.
Results The data are summarized in Table 1. The results of two-way ANOVA analysis and Tukey's Studentized Range (HSD) Test showed that there was a significant
pH 8.0
7.4
Figure 1. Mean and 95% confidence intervals of percent protein-bound vancomycin at pH 7.0, 7.4, and 8.0 across the concentration of 2-80 mg/L.
7.0
25
~ 30
I 35
Protein binding (%)
I 40
CHEN ET AL. INFLUENCE OF pH ON VANCOMYCINPROTEIN BINDING
59
45 40
7-
Vancomycin Concentration • 2mg/L
Figure 2. Mean and standard deviations of percent protein-bound vancomycin of concentrations of 2, 5, |0, and 80 mg/L at pH 7.0, 7.4, and 8.0, respectively.
[] 5mg/L 10mg/L
•
[] 80mg/L
0
difference (p < 0.001) in percent protein binding in plasma samples across the pH range of 7.0-8.0. The 95% confidence intervals were calculated by using standard deviations (Bolton, 1984). At pH 7.0, 7.4, and 8.0, they were 35.0%-38.0%, 32.0%-34.0%, and 27.6%-29.2%, respectively (Figure 1). There was no significant difference (p > 0.05) in percent protein binding in plasma samples across the concentration range of 2-80 mg/L. The mean and standard deviations of percent protein-bound vancomycin of concentrations of 2.0, 5.0, 10.0, and 80.0 (mg/L) at pH 7.0, 7.4, and 8.0 were respectively shown in Figure 2. An interaction between vancomycin concentration and pH was demonstrated by the analysis (p < 0.01).
Discussion Many studies have been published on vancomycin protein binding since the initial paper on this subject was published by Lindholm and Murray in 1966. The results of the studies so far show major discrepancies with the percentage of protein binding reported ranging from 0% to 98% (Ackerman et al., 1988; Cantu et al., 1990; Cutler et al., 1984; Krogstad et al., 1980; Lindholm and Murray, 1966; Tan et al., 1990; Wittendorf et al., 1987; Zokufa et al., 1989). Vancomycin hydrochloride is an amphoteric substance, and, therefore, may bind to albumin and possibly al-acid glycoprotein, and/or other plasma proteins (Piafsky 1980). However, in one study, there was a strong relationship shown between percent protein-bound vancomycin and albumin concentration, but a poor relationship between percent protein-bound vancomycin and ct1-acid glycoprotein concentration (Zokufa et al., 1989). Cantu et al. (1990) have suggested that vancomycin may bind to IgA. In all these studies, there was no indication of the sample collection methods used, which may affect pro-
7.0
7.4 pH
8.0
tein binding (Cotham and Shand, 1975; Fremstad and Bergerud, 1976; Haughey and Lima, 1982; Sorrells, 1987; Wood et al., 1979), nor information on whether the pH and temperature were controlled during the measurement of protein binding. In our study all above variables have been controlled by using (1) pooled plasma, thereby avoiding variability in the plasma used for all binding experiments; (2) appropriate, standardized collection procedures; and (3) controlled conditions of temperature and pH. We established that change in pH is one of the important factors affecting vancomycin protein binding. By controlling pH, we note that the 95% confidence intervals for vancomycin binding are reduced, thereby making it less likely that real effects on the protein binding would be obscured (Figures 1 and 2). Ackerman et al. (1988) showed that there was no significant relationship (p < 0.05) between vancomycin concentration and percent protein binding in serum samples from 12 infected patients. This finding is in general agreement with our results and suggests vancomycin concentration does not have an important influence on the percent bound. We conclude that the control of pH in studies of vancomycin protein binding is important, and it is l!kely that some of the published variation in the protein binding of vancomycin is due to this hitherto unreported problem. Additional studies are required to establish the extent of influence of pH on the protein binding of vancomycin in vivo.
References Ackerman BH, Taylor EH, Olsen KM, Abdel-Malak W, Pappas AA (1988) Vancomycin serum protein binding determination by ultrafiltration. Drug lntell Clin Pharm 22:300-303. Bauchet J, Pus sard E, Garaud JJ (1987) Determination of vancomycin in serum and tissues by column liquid chromatography using solid-phase extraction. J Chromatogr 414:472-476.
60
Bolton S (1984) Pharmaceutical Statistics. Drugs and Pharmaceutical Sciences, Vol 25. New York: Marcel Dekker, Inc. Cantu TG, Dick JD, Elliott DE, Humphrey RL, Kornhauser DM (1990) Protein binding of vancomycin in a patient with immunoglobulin A m y e l o m a . A n t i m i c r o b A g e n t s C h e m o t h e r 34: 1459-1461. Cotham RH, Shand D (1975) Spuriously low plasma propranolol concentration resulting from blood collection methods. Clin Pharmacol Ther 18:535-538. Craig WA, Welling PG (1977) Protein binding of antimicrobials: Clinical pharmacokinetic and therapeutic implications. Clin Pharmacokinet 2:252-268. Cutler NR, Narang PK, Lesko LJ, Ninos M, Power M (1984) Vancomycin disposition: The importance of age. Clin Pharmacol Ther 36:803-810. Fremstad D, Bergerud K (1976) Plasma protein binding of drugs as influenced by blood collection methods. Acta Pharmacol Toxicol 39:570-572. Haughery DB, Lima JJ (1982) The influence of blood collection technique on serum and plasma protein binding of disopyramide. Eur J Clin Pharmacol 22:185-189. Krogstad DJ, Moellering RC, Greenblatt DJ (1980) Single-dose kinetics of intravenous vancomycin. J Clin Pharmacol 197-201. Lindholm DD, Murray JS (1966) Persistence of vancomycin in the
JPM Vol. 28. N o . 1 August 1992:57-60
blood during renal failure and its treatment by hemodialysis. N Engl J Med 274:1047-105 I. Piafsky KM (1980) Disease-induced changes in the plasma binding of basic drugs. Clin Pharmacokinet 5:246-262. SAS For Personal Computers (1985) Cary, NC, USA, SAS Institute Inc., Version 6 Edition. Sorrells SC (1987) Effect of vacutainer tubes on lidocaine protein binding. Drug Intell Clin Pharm 21:464-465. Tan CC, Lee HS, Ti TY, Lee EJ (1990) Pharmacokinetics of intravenous vancomycin in patients with end-stage renal failure. Ther Drug Monit 12:29-34. Wise R (1985) The relevance of pharmacokinetics to in vitro models: Protein b i n d i n g - - d o e s it matter? J A n t i m i c r o b C h e m o t h e r 15(Suppl A):77-83. Wittendorf RW, Swagzdis JE, Gifford R, Mico BA (1987) Protein binding of glycopeptide antibiotics with diverse physical-chemical properties in mouse, rat, and human serum. J Pharmacokinet Biopharmaceut 15:5-13. Wood M, Shand DG, Wood AJJ (1979) Altered drug binding due to the use of indwelling heparinized cannulas (heparin lock) for sampling. Clin Pharmacol Ther 25:103-107. Zokufa HZ, Solem LD, Rodvold KA, Crossley KB, Fischer JH, Rotschafer J C (1989) The influence of serum albumin and a~-acid glycoprotein on vancomycin protein binding in patients with burn injuries. J Burn Care Rehabilit 10:425-428.
