Concentration-dependent Plasma Protein Binding Research Articles

Concentration-dependent plasma protein binding: Expect the unexpected

The unbound fraction of a drug in plasma can profoundly influence both its pharmacokinetics and pharmacodynamics. For most drugs, the unbound fraction is relatively constant across the clinically relevant range of concentrations in a given individual. Nonlinear plasma protein binding involving saturation of binding sites results in increasing unbound fraction as the concentration of the drug increases, a phenomenon that is consistent with the law of mass action and is well recognized. Not widely appreciated is that some drugs undergo atypical concentration-dependent binding to plasma proteins, whereby the unbound fraction decreases with increasing concentration. In this article we review the drugs for which atypical nonlinear plasma protein binding has been reported. For each drug, the evidence for the phenomenon is presented and the proposed mechanism discussed. Also reviewed are the potential implications of atypical nonlinearity in plasma protein binding. Highlighted is the importance of understanding the relationship between unbound fraction and plasma drug concentration during the preclinical and early clinical stages of drug development, and during the routine clinical use of a drug especially if therapeutic drug monitoring is used to assist in optimization of the dosing regimen. The lesson is that the unexpected concentration-dependent behavior that has been observed for a number of drugs should be expected to occur for some other drugs.

Single and multiple dose intravenous and oral pharmacokinetics of the hedgehog pathway inhibitor vismodegib in healthy female subjects

While recent publications have suggested the pharmacokinetics (PK) of vismodegib appear to be non-linear, there has not been a report describing the mechanisms of non-linearity. This study provides evidence that two separate processes, namely, solubility-limited absorption and concentration-dependent plasma protein binding, can explain the non-linear PK of vismodegib. This study provides quantitative results which can account for the lower than expected accumulation of vismodegib with continuous daily dosing. Vismodegib has demonstrated clinical activity in patients with advanced basal cell carcinoma. The pharmacokinetics (PK) of vismodegib are non-linear. The objective of this study was to determine whether vismodegib PK change following repeated dosing by administering a tracer intravenous (i.v.) dose of (14) C-vismodegib with single and multiple oral doses. Healthy post menopausal female subjects (n= 6/group) received either a single or daily 150 mg vismodegib oral dose with a (14) C-labelled 10 µg i.v. bolus dose administered 2 h after the single or last oral dose (day 7). Plasma samples were assayed for vismodegib by LC-MS/MS and for (14) C-vismodegib by accelerator mass spectrometry. Following a single i.v. dose, mean clearance, volume of distribution and absolute bioavailability were 43.4 ml h(-1) , 16.4 l and 31.8%, respectively. Parallel concentration-time profiles following single oral and i.v. administration of vismodegib indicated elimination rate limited PK. Following i.v. administration at steady-state, mean clearance and volume of distribution were 78.5 ml h(-1) and 26.8 l, respectively. Comparison of i.v. PK parameters after single and multiple oral dosing showed similar half-life, increased clearance and volume of distribution (81% and 63% higher, respectively) and decreased bioavailability (77% lower) after repeated dosing. Relative to single dose, the unbound fraction of vismodegib increased 2.4-fold with continuous daily dosing. Vismodegib exhibited a long terminal half-life after oral and i.v. administration, moderate absolute bioavailability and non-linear PK after repeated dosing. Results from this study suggest that the non-linear PK of vismodegib result from two separate, non-linear processes, namely solubility limited absorption and high affinity, saturable plasma protein binding.

Current Issues on Bioavailability and Bioequivalence Determination

Methodologies for determining bioavailability and bioequivalence of endogenous compounds and macromolecules such as polymers are different from that of small exogenous compounds. In many cases, bioavailability and bioequivalence guidelines for exogenous molecules may not be appropriate for endogenous compounds. A common approach of baseline subtraction for endogenous materials is arguable in some cases, since levels of endogenous materials are highly variable during an experimental period. For polymers molecular weight distribution may be different among the materials from different manufactures and/or different manufacturing processes. These issues are addressed along with following issues regarding bioavailability/ bioequivalence determination: use of pharmacokinetic vs. pharmacodynamic parameters, use of a chiral assay vs. total assay for racemic drugs, and total vs. free concentration assays for drugs that exhibit concentration-dependent plasma protein binding, and for liposome/protein encapsulated formulations.

Concentration-dependent plasma protein binding of the novel dipeptidyl peptidase 4 inhibitor BI 1356 due to saturable binding to its target in plasma of mice, rats and humans

Abstract Objectives The purpose of this study was to characterise the plasma protein binding of BI 1356. Methods BI 1356 (proposed trade name ONDERO) is a novel dipeptidyl peptidase 4 (DPP-4) inhibitor, which is under clinical development for the treatment of type 2 diabetes. DPP-4 is expressed in various tissues but soluble DPP-4 is also present in plasma. Therefore, binding to soluble DPP-4 may influence the pharmacokinetics of BI 1356. Plasma protein binding of BI 1356 was determined in vitro for wild type mice and rats and the results compared with those for DPP-4 knockout mice and DPP-4 deficient Fischer rats. In addition, protein binding of BI 1356 was examined in plasma from healthy human volunteers and renal excretion of the compound in the DPP-4 knockout mice was compared with that occurring in wild type mice. Key findings The results showed that BI 1356 exhibited a prominent concentration-dependent plasma protein binding due to a saturable high affinity binding to the DPP-4 target in plasma. Differences in renal excretion of BI 1356 between DPP-4 knockout mice and wild type mice suggested that saturable binding of BI 1356 to DPP-4 in the body also influenced elimination. Conclusions High affinity, but readily saturable binding of BI 1356 to its target DPP-4 accounted primarily for the concentration-dependent plasma protein binding at therapeutic plasma concentrations of BI 1356.

Binding to dipeptidyl peptidase‐4 determines the disposition of linagliptin (BI 1356) – investigations in DPP‐4 deficient and wildtype rats

Linagliptin (BI 1356) is a novel dipeptidyl peptidase-4 (DPP-4) inhibitor in clinical development for the treatment of type 2 diabetes. It exhibits non-linear pharmacokinetics and shows concentration-dependent plasma protein binding to its target, DPP-4. The aim of this study was to investigate the impact of saturable binding of linagliptin to plasma and tissue DPP-4 by comparing the pharmacokinetics of linagliptin in wildtype and DPP-4 deficient Fischer rats using non-compartmental and model-based data analysis. The non-compartmental analysis revealed a significantly reduced AUC in DPP-4 deficient rats compared with wildtype rats when single intravenous doses <or=1 mg/kg were administered, but the exposure was similar in both strains at higher doses. The terminal half-lives were significantly shorter in DPP-4 deficient rats compared with wildtype rats. For doses <or=1 mg/kg, DPP-4 deficient rats exhibited linear pharmacokinetics, whereas the pharmacokinetics of wildtype rats was non-linear. In the model-based analysis these differences could be accounted for by assuming concentration-dependent protein binding in the central and one peripheral compartment in wildtype rats. In the model, disposition parameters for unbound linagliptin were assumed to be identical in both rat strains. Simulations with different doses of linagliptin and different concentrations of binding sites further illustrated that the interdependence of linagliptin and DPP-4 in plasma and in the periphery has a major influence on the disposition of linagliptin in wildtype rats. In conclusion, the study showed that the concentration-dependent binding of linagliptin to its target DPP-4 has a major impact on the plasma pharmacokinetics of linagliptin.

Concentration-dependent plasma protein binding of the novel dipeptidyl peptidase 4 inhibitor BI 1356 due to saturable binding to its target in plasma of mice, rats and humans

The purpose of this study was to characterise the plasma protein binding of BI 1356. BI 1356 (proposed trade name ONDERO) is a novel dipeptidyl peptidase 4 (DPP-4) inhibitor, which is under clinical development for the treatment of type 2 diabetes. DPP-4 is expressed in various tissues but soluble DPP-4 is also present in plasma. Therefore, binding to soluble DPP-4 may influence the pharmacokinetics of BI 1356. Plasma protein binding of BI 1356 was determined in vitro for wild type mice and rats and the results compared with those for DPP-4 knockout mice and DPP-4 deficient Fischer rats. In addition, protein binding of BI 1356 was examined in plasma from healthy human volunteers and renal excretion of the compound in the DPP-4 knockout mice was compared with that occurring in wild type mice. The results showed that BI 1356 exhibited a prominent concentration-dependent plasma protein binding due to a saturable high affinity binding to the DPP-4 target in plasma. Differences in renal excretion of BI 1356 between DPP-4 knockout mice and wild type mice suggested that saturable binding of BI 1356 to DPP-4 in the body also influenced elimination. High affinity, but readily saturable binding of BI 1356 to its target DPP-4 accounted primarily for the concentration-dependent plasma protein binding at therapeutic plasma concentrations of BI 1356.

Penetration of ertapenem into skeletal muscle and subcutaneous adipose tissue in healthy volunteers measured by in vivo microdialysis

Ertapenem is FDA approved for the treatment of skin and skin-structure infections (SSSI), but its in vivo penetration into the interstitial space of soft tissues is unknown. The present microdialysis study was conducted to measure free, protein-unbound ertapenem concentrations in muscle and subcutaneous tissue. In a single-centre, prospective, open-label study six healthy volunteers (three females, 22-37 years) were treated with 1 g ertapenem given as a single intravenous dose. Microdialysis and plasma samples were collected before and at different time points up to 12 h after medication. Drug concentrations were determined by a validated LC-MS-MS method. No serious or microdialysis-associated adverse events were observed. Ertapenem concentrations in plasma reached a maximum (C(max)) of 103.3 +/- 26.3 mg/L, a terminal elimination half-life (t(1/2)) of 3.8 +/- 0.6 h and an AUC(0-infinity) of 359.7 +/- 66.5 mg.h/L. Mean peak concentrations of free, protein-unbound ertapenem in interstitial space fluid of skeletal muscle and subcutaneous adipose tissue were much lower (C(max) = 6.7 +/- 4.1 and 4.0 +/- 1.6 mg/L, respectively). This degree of tissue distribution is consistent with high concentration-dependent plasma protein binding of ertapenem (84-96%). AUC(0-infinity) values for both muscle and adipose tissue were lower as well (39.7 +/- 24.8 and 18.6 +/- 4.6 mg.h/L). However, unbound interstitial fluid concentrations exceeded MIC(90) values for the important SSSI pathogens for 7 (subcutis) and 10 h (muscle) after dosing. These results support the previously observed clinical efficacy of ertapenem in the treatment of SSSI.

SB-242235, a selective inhibitor of p38 mitogenactivated protein kinase. I: Preclinical pharmacokinetics

1. SB-242235 (1-(4-piperidinyl)-4-(4-fluorophenyl)-5-(2-methoxy-4-pyrimidinyl) imidazole) is a potent and selective p38 MAP kinase inhibitor that may be an effective therapy for cytokine-mediated diseases such as autoimmune or inflammatory diseases. The present studies were conducted to evaluate the pharmacokinetics of SB-242235 in several preclinical species, including rat, dog and monkey. 2. SB-242235 demonstrates generally favourable pharmacokinetic properties in all species examined. Systemic plasma clearance was high in rat, but in the non-rodent species SB-242235 demonstrated low to moderate clearance with plasma half-lives > 4 h. Oral bioavailability in each preclinical species was high. In rat and monkey, SB-242235 demonstrated non-linear elimination kinetics that manifested as a decrease in clearance with increasing dose and apparent oral bioavailability >100% at high oral doses. Furthermore, SB-242235 displayed concentration-dependent plasma protein binding over a concentration range of 1000-10,000 ng ml -1. 3. In conclusion, SB-242235 demonstrates high oral bioavailability across the major preclinical species, and may thus be a useful tool compound for investigation of the role of p38 inhibition in various disease states. However, the observations of non-linear protein binding and disposition also suggest the need for caution in the design of and data interpretation from such studies.

Concentration-dependent plasma protein binding of flurbiprofen in the rat: an in vivo microdialysis study.

The in vivo plasma protein binding and pharmacokinetics of flurbiprofen were studied in awake, unrestrained rats using intravenous microdialysis sampling. Flurbiprofen (20 mg/kg) was administered i.v. to 2 groups of 6 rats: in both groups sampling was carried out by microdialysis, but in the second group an additional 10 blood samples were withdrawn via a jugular cannula. In vitro and ex vivo (following i.v. administration of flurbiprofen 20 mg/kg to another group of 13 rats) plasma protein binding of the drug was determined by equilibrium dialysis. The area under the unbound plasma concentration-time profile of flurbiprofen (AUCu), determined by microdialysis sampling was somewhat smaller (-19%, p = 0.666) in the rats undergoing simultaneous serial blood sampling (2.21 +/- 0.36 micrograms.h/ml) as compared to the rats undergoing microdialysis sampling only (2.73 +/- 0.60 micrograms.h/ml). Comparison of total and unbound concentrations of flurbiprofen showed an in vivo plasma binding varying between 99.5% at low and 98.0% at high total flurbiprofen plasma concentrations. Plasma binding of flurbiprofen determined in vitro over the same concentration range was higher (99.5-99.9%) but also concentration-dependent. Plasma binding of flurbiprofen determined ex vivo, on the other hand, corresponded well with the in vivo binding. Monitoring the fraction of drug unbound in blood of an individual rat throughout a pharmacokinetic experiment has now become possible by using simultaneous sampling of blood and intravenous microdialysates.

Pharmacokinetics of ketoprofen in rats: Effect of age and dose

The effects of age and dose on the pharmacokinetics of ketoprofen were evaluated in young adult and senescent male Fischer 344 rats following intravenous administration of 2.5 and 10 mg kg-1. Plasma concentrations were measured by HPLC and free ketoprofen determined by equilibrium dialysis. The glucuronidation of ketoprofen was investigated in a preparation of rat liver microsomes and kinetic analysis of UDP-glucuronyltransferase was carried out by determining the initial rate of metabolic activity as a function of ketoprofen concentration. Mean plasma clearance CLfree and steady-state volume of distribution Vssfree calculated from unbound plasma ketoprofen concentrations were significantly lower in the aged rat, suggesting reduced metabolic activity and decreased ketoprofen binding to tissue components, respectively. Plasma protein binding demonstrated an age-dependent decline due to decreases in both albumin concentration and binding affinity. Thus, plasma clearance CL and steady-state volume of distribution Vss changes were insignificant when total plasma concentrations were examined, due to the greater free fraction of ketoprofen in the plasma of senescent rats. The maximal rate of ketoprofen glucuronidation by hepatic microsomes was reduced whereas the affinity of the metabolic enzymes for the compound was unaffected by age. Dose had a marked effect on the disposition of ketoprofen as well. Saturation of elimination pathways and tissue binding sites contributed to significant declines in CLfree and Vssfree with increasing dose. Likewise, concentration-dependent plasma protein binding occurred, reflecting saturation of albumin binding. Thus, changes in the pharmacokinetic parameters based on total drug concentrations were offset by the increase in the unbound fraction of ketoprofen.

Effect of non‐linear plasma protein binding on unbound and total plasma phenylbutazone concentrations in cows

The influence of plasma protein binding on unbound and total phenylbutazone concentrations in cows was examined employing data from the literature. Protein binding parameters (number of binding sites and affinity constants) were generated by computer analysis to characterize the concentration-dependent plasma protein binding of phenylbutazone. Unbound plasma phenylbutazone concentrations were calculated from total plasma drug concentrations observed after administration of a single dose of phenylbutazone to cows. Pharmacokinetic parameters for unbound phenylbutazone were obtained. Parameters characterizing the plasma protein binding and pharmacokinetics of unbound phenylbutazone derived from single-dose administration were then used to predict unbound and total drug concentrations after multiple-dose administration of phenylbutazone. Total plasma phenylbutazone concentrations predicted from single-dose pharmacokinetic parameters agreed well with observed values following multiple-dose administration of the drug. Thus, the results of this analysis demonstrate that the non-linear pharmacokinetics of phenylbutazone in the cow can be attributed to the concentration-dependent plasma protein binding of the drug.

Nonlinear pharmacokinetics of intravenous sulphadimethoxine and its dosage regimen in pigs.

The time courses of the total (Ct) and unbound plasma (Cf) concentration after the i.v. injection of 20, 50 and 100 mg/kg of sulphadimethoxine (SDM) were examined in pigs. The area under the Ct-time curve per unit dose decreased dose-dependently. Vdarea and total body clearance of Ct increased dose-dependently. The concentration-dependent plasma protein binding of SDM was evident after 50 and 100 mg/kg. The time courses of Cf en Ct after 3 doses were analyzed by a one compartment open model with nonlinear plasma protein binding. The agreement between calculated curves of Cf and Ct and the observed values, and relative constancy of pharmacokinetic parameters were obtained over 3 doses. These results suggested that the nonlinear pharmacokinetics of SDM was caused by saturable plasma protein binding. The multiple i.v. dose of SDM was based on the dosage regimen using the nonlinear pharmacokinetic model (50 mg/kg, 24 hour interval, 4 days). The observed Cf was maintained in the intended range by the dosage regimen. Therefore, the dosage regimen based on the nonlinear pharmacokinetics may allow the unbound concentration after i.v. injection of SDM in pigs to be controlled.

Melphalan concentration dependent plasma protein binding in healthy humans and rats.

The binding of melphalan to plasma proteins from four healthy humans and from rats was measured by centrifugal ultrafiltration. Melphalan concentrations were determined by HPLC and by measuring 14C-melphalan activity. In whole blood, melphalan was distributed preferentially in plasma. However, a constant fraction, 37%, which was independent of the total melphalan concentration in whole blood, was present within the red blood cells. The binding of melphalan to plasma proteins from humans was less than that from rats. In both, however, the fraction bound was constant throughout the concentration range (0.1 to 9.0 microM) that is achieved during standard-dose melphalan therapy. Albumin was the primary binding protein. At concentrations equal to or in excess of 33 microM, which have been achieved during high-dose melphalan therapy, free plasma melphalan concentrations were no longer linearly related to total drug concentrations, and the plasma protein binding of melphalan in the human became concentration dependent. This occurred at concentrations of 70 microM in the rat. Scatchard analysis of the data indicated the presence of 2 groups of binding sites. Class I sites had 0.03 and 0.4 binding sites per albumin molecule in humans and rats, with respective association constants of 4.43 X 10(4) M-1 and 1.92 X 10(4) M-1. Class II sites had 5.18 and 2.60 binding sites per molecule, with respective association constants of 3.82 X 10(2) M-1 and 2.01 X 10(2) M-1.

CEFTRIAXONE (CTX) KINETICS AND CEREBROSPINAL FLUID (CSF) CONCENTRATIONS IN INFANTS AFTER LOADING (LD) AND MAINTENANCE DOSES (MD)

Thirteen infants (0.4-1.4 yrs) with meningitis were studied. CTX therapy consisted of a 75 mg/kg LD, followed by 50 mg/kg MD every 12 hrs IV over 10 minutes. Simultaneous blood and CSF samples were obtained in 11 infants at 11.5-12.8 hr after LD. In 3 of these, multiple blood samples were collected after the LD and after 9-10 days of MD; simultaneous blood and CSF samples were also collected at 1.25-5.75 hr at steady-state. CTX was measured by HPLC. Simultaneous serum and CSF concentration ranged from 10.5-40.3 and 1.8-8.0 μg/ml after the loading dose and 74-139 and 5.8-7.9 μg/ml at steady-state, respectively. After LD and at steady-state, CTX total clearance was 1.06±0.18 and 0.64±0.06 ml/min/kg (P<0.05), distribution volume was 0.33±0.06 and 0.26±0.06 L/kg and elimination half-life was 3.67±0.06 and 4.61±0.28 hr, respectively. A higher CTX clearance at higher dose may be partly due to concentration-dependent plasma protein binding of CTX reported in adults. Although the CSF/serum concentration was low, the CSF bactericidal titers exceeded 1:512 suggesting a great potential for CTX therapy of central nervous system infections due to susceptible organisms.

High-dose alkylating agent therapy: a review of clinical experiences.

Cancer Drug DeliveryVol. 1, No. 3 High-Dose Alkylating Agent Therapy: A Review of Clinical ExperiencesM.A. CORNBLEET, R.C.F. LEONARD, and J.F. SMYTHM.A. CORNBLEETSearch for more papers by this author, R.C.F. LEONARDSearch for more papers by this author, and J.F. SMYTHSearch for more papers by this authorPublished Online:2 Apr 2009https://doi.org/10.1089/cdd.1984.1.227AboutSectionsPDF/EPUB ToolsPermissionsDownload CitationsTrack CitationsAdd to favorites Back To Publication ShareShare onFacebookTwitterLinked InRedditEmail FiguresReferencesRelatedDetailsCited byAlkylating Agents3 November 2010Application of hematopoietic progenitor assays for the estimation of hematotoxicity in ratsJournal of Pharmacological and Toxicological Methods, Vol. 31, No. 2A phase I trial of continuous-infusion cyclophosphamide in refractory cancer patientsCancer Chemotherapy and Pharmacology, Vol. 29, No. 1Delivery of vital drugs to the brain for the treatment of brain tumorsJournal of Controlled Release, Vol. 11, No. 1-3Physicochemical and pharmacokinetic parameters of seven lipophilic chlorambucil esters designed for brain penetrationCancer Chemotherapy and Pharmacology, Vol. 25, No. 5Brain and plasma pharmacokinetics and anticancer activities of cyclophosphamide and phosphoramide mustard in the ratCancer Chemotherapy and Pharmacology, Vol. 27, No. 1Drug Delivery to the Brain by Blood-Brain Barrier Circumvention and Drug ModificationOptimizing drug delivery to brain tumorsCancer Treatment Reviews, Vol. 14, No. 1Melphalan concentration dependent plasma protein binding in healthy humans and ratsEuropean Journal of Clinical Pharmacology, Vol. 32, No. 2 Volume 1Issue 3Jan 1984 To cite this article:M.A. CORNBLEET, R.C.F. LEONARD, and J.F. SMYTH.High-Dose Alkylating Agent Therapy: A Review of Clinical Experiences.Cancer Drug Delivery.Jan 1984.227-238.http://doi.org/10.1089/cdd.1984.1.227Published in Volume: 1 Issue 3: April 2, 2009PDF download

Experimental Evidence for Concentration-Dependent Plasma Protein Binding Effects on the Apparent Half-Lives of Restrictively Cleared Drugs

Experimental Evidence for Concentration-Dependent Plasma Protein Binding Effects on the Apparent Half-Lives of Restrictively Cleared Drugs

Volume of distribution terms for a drug (ceftriaxone) exhibiting concentration-dependent protein binding. II. Physiological significance.

Guidelines presented previously for the analysis of plasma concentration versus time data for a drug exhibiting concentration-dependent plasma protein binding were successfully applied to the distributional parameters of a new cephalosporin, ceftriaxone. This approach provided several striking observations when the pharmacokinetics of ceftriaxone in a healthy and uremic population were re-examined. First, the parameter -fp converted the apparent dose-dependent distributional terms of ceftriaxone into a function of the concentration-dependent plasma protein binding. Second, a strong correlation between the term VUSS and the reciprocal of -fp was established within each of the two populations. While this -fp term accounted for the variability within the respective populations due to ceftriaxone-albumin binding differences, it did not account for all of the distributional differences between the two populations. The present analysis revealed that the altered physiologic state of uremia (larger plasma volumes and interstitial to intravascular albumin ratios), in addition to differences in plasma protein binding, dictated the distribution of ceftriaxone in healthy and uremic subjects. Furthermore, the binding-disposition model which accounts for the presence of plasma proteins outside the vascular space, was established to be appropriate in describing the distribution of ceftriaxone.

Pharmacokinetic characteristics of intravenous ceftriaxone in normal adults.

The multiple-dose pharmacokinetics and tolerance of intravenous ceftriaxone were investigated in 44 adults with normal renal function. Doses of 0.5, 1.0, and 2.0 g every 12 h and 2 g every 24 h were administered intravenously at a constant rate over 30 min. Plasma and urine samples were collected after the first (day 1) and last (day 4) dose and assayed for ceftriaxone by high-pressure liquid chromatography. Considering all four doses, mean peak plasma concentrations ranged from 79 to 255 micrograms/ml on day 1 and from 101 to 280 micrograms/ml on day 4. Trough concentrations at 12 h on day 1 were 15 to 45 micrograms/ml and 20 to 59 micrograms/ml on day 4. After a dose regimen of 2 g every 24 h, trough levels were still in the clinically therapeutic range (13 to 15 microgram/ml). The mean beta-phase t1/2 was markedly long (6.3 to 6.9 h) and was independent of dose. The fraction of dose excreted unchanged in the urine (0.33 to 0.44) indicated a substantial nonrenal mechanism of elimination. The plasma clearance ranged between 1,002 and 1,449 ml/h, and renal clearance ranged from 353 to 529 ml/h. The apparent volume of distribution varied from 9.2 to 13.5 liters. The dose-related increases in calculated Vd and Clp could be attributed to concentration-dependent plasma protein binding because of a larger free fraction of drug at higher concentrations. The drug was well tolerated, and no significant clinical or laboratory abnormalities were noted.

Pharmacokinetics of ceftriaxone in humans.

Pharmacokinetics of the investigational cephalosporin ceftriaxone were studied after 30-min intravenous infusions of three ascending single doses of 0.5, 1, and 2 g crossed over in 12 normal subjects. Serially collected plasma and urine samples were analyzed for ceftriaxone by high-performance liquid chromatography. Plasma concentration-time profiles were characterized by a linear two-compartment open model with the following respective mean (+/- standard deviation) parameters at 0.5-, 1-, and 2-g dose levels: elimination half-life, 6.5 +/- 0.7, 6.2 +/- 0.8, and 5.9 +/- 0.7 h; apparent volume of distribution, 8.5 +/- 1.1, 9.0 +/- 1.1, and 10.1 +/- 1.0 liters; and plasma clearance, 929 +/- 150, 1,007 +/- 130, and 1,190 +/- 150 ml/h. The respective renal excretion parameters were as follows: renal clearance, 373 +/- 60, 399 +/- 50, and 533 +/- 128 ml/h; and percentage of dose excreted unchanged in the 48-h urine samples, 41 +/- 8, 39 +/- 5, and 43 +/- 10. The 6-h elimination half-life of ceftriaxone was 2- to 10-fold longer than those reported for marketed and other known investigational cephalosporins. The small dose-related increases in the apparent volume of distribution and clearance parameters can be explainhe 48-h urine samples, 41 +/- 8, 39 +/- 5, and 43 +/- 10. The 6-h elimination half-life of ceftriaxone was 2- to 10-fold longer than those reported for marketed and other known investigational cephalosporins. The small dose-related increases in the apparent volume of distribution and clearance parameters can be explainhe 48-h urine samples, 41 +/- 8, 39 +/- 5, and 43 +/- 10. The 6-h elimination half-life of ceftriaxone was 2- to 10-fold longer than those reported for marketed and other known investigational cephalosporins. The small dose-related increases in the apparent volume of distribution and clearance parameters can be explained by the concentration-dependent plasma protein binding of ceftriaxone in humans. The impact of the small dose-dependent changes in the pharmacokinetics of ceftriaxone is anticipated to be of negligible clinical significance.

Effects of concentration-dependent plasma protein binding on ceftriaxone kinetics.

The kinetics of ceftriaxone, a cephalosporin, was studied in six healthy subjects who received bolus injections of 150, 500, and 1,500 mg intravenously in a random crossover fashion. Although total drug concentration time profiles after all doses could be described by biexponential equation, simple compartment analysis was inappropriate because a disproportional increase in the area under the total drug concentration time curves occurred with dose. This resulted in a dose-dependent increase in total systemic clearance (ClTS) from 9.7 ml/min at the 150-mg dose to 13 ml/min at the 1500-mg dose. The dose-dependent changes in ClTS could be explained in terms of the concentration-dependent plasma protein binding of ceftriaxone (fplasma ranging from 0.04 to 0.167), because the area under the free drug concentration time curves (AUCFO-infinity) increased proportionately to dose. Mean total clearance with reference to free (unbound) ceftriaxone (ClFS) was constant at 255 ml/min. Calculated mean renal clearance with reference to free ceftriaxone (ClFR) was 173 ml/min, or slightly more than the average glomerular filtration rate in humans. Mean plasma ceftriaxone t1/2 was not influenced by dose and averaged 8 hr. This biological t1/2 is by far the longest ever for a cephalosporin in healthy subjects.