Optimisation of the dosage of tranexamic acid in trauma patients with population pharmacokinetic analysis.

Tranexamic acid is used both pre-hospital and in-hospital as an antifibrinolytic drug to treat or prevent hyperfibrinolysis in trauma patients; dosing, however, remains empirical. We aimed to measure plasma levels of tranexamic acid in patients receiving pre-hospital anti-hyperfibrinolytic therapy and to build a population pharmacokinetic model to propose an optimised dosing regimen. Seventy-three trauma patients were enrolled and each received tranexamic acid 1 g intravenously pre-hospital. A blood sample was drawn after arrival in the emergency department, and we measured the plasma tranexamic acid concentration using liquid chromatography-mass spectrometry, and modelled the data using non-linear mixed effect modelling. Tranexamic acid was administered at a median (IQR [range]) time of 43 (30-55 [5-135]) min after trauma. Plasma tranexamic acid levels were determined on arrival at hospital, 57 (43-70 [20-148]) min after pre-hospital administration of the drug. The measured concentration was 28.7 (21.5-38.5 [8.7-89.0]) µg.ml-1 . Our subjects had sustained severe trauma; injury severity score 20 (16-29 [5-75]), including penetrating injury in 2.8% and isolated traumatic brain injury in 19.7%. The pharmacokinetics were ascribed a two-compartment open model with body-weight as the main covariate. As tranexamic acid concentrations may fall below therapeutic levels during initial hospital treatment, we propose additional dosing schemes to maintain a specific target blood concentration for as long as required. This is the first study to investigate plasma level and pharmacokinetics of tranexamic acid after pre-hospital administration in trauma patients. Our proposed dosing regimen could be used in subsequent clinical trials to better study efficacy and tolerance profiles with controlled blood concentrations.

Population pharmacokinetics of tranexamic acid in adults undergoing cardiac surgery with cardiopulmonary bypass.

BACKGROUND: Interest in antifibrinolytic tranexamic acid (TA) has grown since the widespread removal of aprotinin, but its dosing during cardiac surgery is still debated. The objectives of this study were to investigate the population pharmacokinetics (PK) of TA given with either low- or high-dose continuous infusion schemes in adult cardiac surgery patients during cardiopulmonary bypass (CPB). METHODS: Patients were randomized to receive either low-dose (10 mg kg(-1) followed by an infusion of 1 mg kg(-1) h(-1) throughout the operation, and 1 mg kg(-1) into the CPB) or high-dose (30 mg kg(-1), then 16 mg kg(-1) h(-1), and 2 mg kg(-1) into the CPB) TA. Serum TA concentrations were measured in 61 patients and the data were modelled using Monolix. RESULTS: TA concentrations were 28-55 µg ml(-1) in the low-dose group and 114-209 µg ml(-1) in the high-dose group throughout surgery. TA PK was best described by a two-compartment open model. The main covariate effect was bodyweight, whereas the CPB did not influence the PK. Assuming a bodyweight of 70 kg, the population estimates were 4.8 litre h(-1) for clearance, 6.6 litre for the volume of the central compartment, 32.2 litre h(-1) for the diffusional clearance, and the peripheral volume of distribution was 10.8 litre. CONCLUSIONS: The PK of TA was satisfactorily described by an open two-compartmental model, which was used to propose a dosing scheme suitable for obtaining and maintaining the desired plasma concentration in a stable and narrow range in cardiac surgery patients.

Population pharmacokinetics of mycophenolic acid in pediatric renal transplant patients using parametric and nonparametric approaches.

Mycophenolic acid (MPA) is an immunosuppressive drug widely used in the prevention of acute rejection in pediatric renal transplant recipients and is characterized by a wide inter-individual variability in its pharmacokinetics. The aim of this study was to compare population pharmacokinetic modeling of MPA in pediatric renal transplant recipients given mycophenolate mofetil, the ester prodrug of MPA, using parametric and nonparametric population methods. The data from 34 pediatric renal transplants (73 full pharmacokinetic profiles obtained on day 21, months 3, 6 and 9 post-transplant) were analyzed using both the nonlinear mixed-effect modeling (NONMEM) and nonparametric adaptive grid (NPAG) approaches, based on a two-compartment model with first order lagged time absorption and first order elimination. The predictive performance of the two models was evaluated in a separate group of 32 patients. Higher mean population parameter values and ranges of individual pharmacokinetic parameters were obtained with NPAG, especially for the elimination constant ke: mean 1.16 h(-1) (0.26-4.33 h(-1)) and 0.78 h(-1) (0.66-1.15 h(-1)) with NPAG and NONMEM, respectively. With NPAG, the skewness and kurtosis values for ke (2.03 and 7.80, respectively) were far from the theoretical values expected for normal distributions. Such a non-normal distribution could explain the high value of shrinkage (35%) obtained for this parameter with the parametric NONMEM method. Bayesian forecasting of mycophenolic acid exposure using the NPAG population pharmacokinetic parameters as priors yielded a better predictive performance, with a significantly smaller bias than with the NONMEM model (-1.68% vs -9.53%, p<0.0001). In conclusion, in the present study, NPAG was found to be the most adequate population pharmacokinetic method to describe the pharmacokinetics of MPA in pediatric renal transplant recipients.

Population pharmacokinetics of cefazolin in critically ill children infected with methicillin-sensitive Staphylococcus aureus.

OBJECTIVES: Cefazolin is one of curative treatments for infections due to methicillin-sensitive Staphylococcus aureus (MSSA). Both growth and critical illness may impact the pharmacokinetic (PK) parameters. We aimed to build a population PK model for cefazolin in critically ill children in order to optimize individual dosing regimens. METHODS: We included all children (age < 18 years, body weight (BW) > 2.5 kg) receiving cefazolin for MSSA infection. Cefazolin total plasma concentrations were quantified by high-performance liquid chromatography. A data modelling process was performed with the software MONOLIX. Monte Carlo simulations were used in order to attain the PK target of 100% fT > 4 ×MIC. RESULTS: Thirty-nine patients with a median (range) age of 7 (0.1-17) years and a BW of 21 (2.8-79) kg were included. The PK was ascribed to a one-compartment model, where typical clearance and volume of distribution estimations were 1.4 L/h and 3.3 L respectively. BW, according to the allometric rules, and estimated glomerular filtration rate (eGFR) on clearance were the two influential covariates. Continuous infusion with a dosing of 100 mg/kg/day to increase to 150 mg/kg/day for children with a BW < 10 kg or eGFR >200 mL/min/1.73m2 were the best schemes to reach the PK target of 100% fT> 4 ×MIC. CONCLUSIONS: In critically ill children infected with MSSA, continuous infusion seems to be the most appropriate scheme to reach the PK target of 100 % fT > 4 ×MIC in children with normal and augmented renal function.

Cyclophosphamide dose adjustment based on body weight and albuminemia in elderly patients treated with R-mini-CHOP.

BACKGROUND: Diffuse large B-cell lymphoma (DLBCL) is the most common lymphoma in elderly patients, and R-CHOP chemotherapy is the standard treatment protocol for DLBCL. Elderly patients (often defined as 75 years of age) are treated with anticancer drugs with precaution; however, the pharmacokinetics and pharmacodynamics (PK and PD) of these agents have not been thoroughly investigated in this population. In this study, we investigated the PK of cyclophosphamide (CP) and doxorubicin (DOXO) in elderly patients in order to verify if there is an influence of age on the PK of these anticancer drugs. MATERIALS AND METHODS: This is a prospective multi-center clinical trial investigating the PK of CP and DOXO in elderly and very elderly patients with DLBCL treated by R-mini-CHOP regimen. Dose levels were 25 mg/m2, 0.7-1.4 mg/m2, 750 mg/m2, and 375 mg/m2 for DOXO, Vincristine (VCR), CP, and Rituximab, respectively. For PK analysis, 7 time point samples were collected over 48 h post-administration on cycle 3. CP and VCR plasma concentrations were measured using UPLC-MS/MS validated method. DOX plasma concentrations were measured using UPLC coupled with fluorescence detection-validated method. PK-POP modeling has been performed with a non-linear mixed-effect model program (Monolix). RESULTS: 31 patients (15 males and 16 females), 75 to 96 years old, were treated with R-miniCHOP protocol. Among them, 19 patients were treated with VCR. A one-compartment (1cpt) open model with linear elimination adequately described CP concentration-time courses. The population PK parameters for CP were: CL = 3.58 L/h, Vmale = 32.2 L, and Vfemale = 28.7 L. Body weight (BW), albuminemia, and gender demonstrated a significant impact on CP PK. A 2-compartment (2cpt) open model with linear elimination best described DOXO concentration-time courses. The population PK parameters for DOXO obtained for the structural model were: CL = 51.1 L/h, Q = 49.6 L/h, V1 = 29.4 L, V2 = 1,130 L (clearances: CL, Q, volumes of distribution: V1, V2). The main covariate effects on DOXO PK were related to gender, BW, and VCR administration. VCR increases DOXO V1 from 29.4 L to 57.5 L (p = 0.02). No hematologic and cardiac grade 3 or 4 toxicity were recorded. CONCLUSIONS: Usually, in the absence of specific data, the majority of the physicians empirically reduce anticancer drug dose in the elderly patients (Tourani in J Geriatr Oncol 3(1): 41-48, 2012), or even does not treat these very-old patients. A better knowledge of the pharmacokinetics in very-old patients should allow a better dose adjustment based on the most significant physiological factors that modify the pharmacokinetic parameters. In this study, no serious toxicity was observed in these very elderly patients (84.1 years). This indicates that dose adjustment of chemotherapies should not only be based on age and creatinine clearance, but also, based upon appropriate physiological and biological data. Our findings indicate that, CP dose adjustment should be done according to serum albumin levels and patients BW and gender.

Variations in schedules of ifosfamide administration: a better understanding of its implications on pharmacokinetics through a randomized cross-over study.

PURPOSE: The metabolism of ifosfamide is a delicate balance between a minor activation pathway (4-hydroxylation) and a mainly toxification pathway (N-dechloroethylation), and there remains uncertainty as to the optimal intravenous schedule. METHODS: This study assesses ifosfamide pharmacokinetics (PK) according to two standard schedules. Using a 1:1 randomized trial design, we prospectively evaluated ifosfamide PK on two consecutive cycles of 3 g/m2/day for 3 days (9 g/m2/cycle) given in one of two schedules either by continuous infusion (CI) or short (3 h) infusion. Highly sensitive analytical methods allowed determination of concentrations of ifosfamide and the key metabolites 4-hydroxy-ifosfamide, 2- and 3-dechloroethyl-ifosfamide. RESULTS: Extensive PK analysis was available in 12 patients and showed equivalence between both schedules (3 h versus CI) based on area under the curves (micromol/l x h) for ifosfamide, 4-hydroxy-ifosfamide, 2- and 3-dechloroethyl-ifosfamide (9,379 +/- 2,638 versus 8,307 +/- 1,995, 152 +/- 59 versus 161 +/- 77, 1,441 +/- 405 versus 1,388 +/- 393, and 2,808 +/- 508 versus 2,634 +/- 508, respectively, all P > 0.2). The classical auto-induction of metabolism over the 3 days of infusion was confirmed for both schedules. CONCLUSION: This study confirms similar PK for both active and toxic metabolites of ifosfamide in adult cancer patients when 9 g/m2 of ifosfamide is administered over 3 days by CI or daily 3-h infusions.

Pre-transplant donor CD4- invariant NKT cell expansion capacity predicts the occurrence of acute graft-versus-host disease.

Clinically useful pre-transplant predictive factors of acute graft-versus-host-disease (aGVHD) after allogeneic hematopoietic stem cell transplantation (allo-SCT) are lacking. We prospectively analyzed HSC graft content in CD34+, NK, conventional T, regulatory T and invariant natural killer T (iNKT) cells in 117 adult patients before allo-SCT. Results were correlated with occurrence of aGVHD and relapse. In univariate analysis, iNKT cells were the only graft cell populations associated with occurrence of aGVHD. In multivariate analysis, CD4- iNKT/T cell frequency could predict grade II-IV aGVHD in bone marrow and peripheral blood stem cell (PBSC) grafts, while CD4- iNKT expansion capacity was predictive in PBSC grafts. Receiver operating characteristic analyses determined the CD4- iNKT expansion factor as the best predictive factor of aGVHD. Incidence of grade II-IV aGVHD was reduced in patients receiving a graft with an expansion factor above versus below 6.83 (9.7 vs 80%, P<0.0001), while relapse incidence at two years was similar (P=0.5).The test reached 94% sensitivity and 100% specificity in the subgroup of patients transplanted with human leukocyte antigen 10/10 PBSCs without active disease. Analysis of this CD4- iNKT expansion capacity test may represent the first diagnostic tool allowing selection of the best donor to avoid severe aGVHD with preserved graft-versus-leukemia effect after peripheral blood allo-SCT.

Comparative preclinical toxicology and pharmacology of isophosphoramide mustard, the active metabolite of ifosfamide.

BACKGROUND: Isophosphoramide mustard (IPM) is the cytotoxic alkylating metabolite of Ifosfamide (IFOS). IPM is being readied for a phase I clinical trial. In the present preclinical study, IPM was evaluated for usage in multidose intravenous (IV) infusion protocols. METHODS: Mice and dogs received IV IPM daily for 3 days. Single-day dosing-oral and IV-to mice, rats, and monkeys is also reviewed for comparison. Complete toxicology studies were completed in the mice and dogs. For mice, dogs and monkeys, IV pharmacokinetic studies were conducted and compared. RESULTS: For mice, the LD(10) for the 3-day IV schedule for IPM was calculated to be 119 mg/kg (with 95% confidence limits of 87-134 mg/kg) (combined sexes), and for adult male dogs the maximum tolerated dose (MTD) was 5 mg/kg. Pharmacokinetic studies in mice, dogs and monkeys were compared and projected to human dosing. For dogs that received 10 mg/kg of IPM, T(1/2beta) was 0.99 h, and clearance was constant (1.01 l/h/kg). IPM was detected from 0 h to 1.5 h after the 5 mg/kg dose and from 0 h to 2 h after the 10 mg/kg dose; none was detected after 2 h. The IV MTD in dogs was 5 mg/kg per day for 3 days. Renal tubular necrosis and bone marrow failure were the causes of death. Transient liver, renal and bone marrow toxicity and gastrointestinal dysfunction were seen at low doses (<5 mg/kg) in dogs. In mice (receiving 100 mg/kg IV) plasma concentrations disappeared in less than 1 h (T(1/2alpha) 2 min), with a clearance of 8.44 l/h/kg. For monkeys, the mean T(1/2) was 4.2 h. Median clearance was 1.65 l/h/kg and no IPM was detected 4 h after dosing. No potential IPM metabolites could be detected in any of the studies. In vitro, plasma protein bound 90% of IPM within 5 min of incubation. CONCLUSIONS: Predictions for human pharmacokinetic parameters and dosing are made from allometric analysis using the above three species. Data predicted an acceptable starting dose of 30 mg/m(2) with a clearance of 39.5 l/h, and a T(1/2) of 1 h 45 min for a 70-kg patient.

Role of alpha-1 acid glycoprotein, albumin, and nonesterified fatty acids in serum binding of apazone and warfarin.

Altered concentrations of serum proteins and nonesterified fatty acids (NEFAs) often accompany malignant diseases. Free fractions (fu) of apazone and warfarin were measured by equilibrium dialysis in serum samples obtained from 31 patients with cancer and 18 control subjects. Mean fu values of both drugs were significantly higher in the patient group. Multivariate analysis showed albumin, NEFA, and AAG for apazone and albumin, NEFA, and age for warfarin accounted for 60% and 63%, respectively, of interpatient variation in bound/free drug concentration ratios in the group of patients with cancer. The interactions of apazone and warfarin with AAG were further characterized; the more avid site had association constants of 4.5 X 10(5) and 2.3 X 10(5) L/mol, respectively. Finally, it is strongly suggested that when hypoalbuminemia is present and a drug binds to AAG with an affinity constant comparable to or higher than that to albumin, then fu will become dependent on the concentration of AAG.

Population pharmacokinetics of high-dose thiopental in patients with cerebral injuries.

Thiopental monitoring was performed in 95 critically ill patients hospitalized for neurologic damage, High-dose thiopental was infused during long-term treatment. Total dose of 333 +/- 144 mg/kg (449 +/- 185 mg/kg for females and 302 +/- 113 mg/kg for men) were given in 125 +/- 43 hours. Plasma concentration-time data were analyzed according to a population pharmacokinetic approach with an initial group of 65 patients. Clearance (CL) and central volume of distribution (Vc) were modeled alone and under the influence of demographic covariates, assuming a two-compartment open model with first-order elimination. The final population models were as follows: CL (L/hr) = 11.7.weight (kg).age (yr)/(2136 + age2) and Vc = 1.52.weight (kg) + 44.8. Mean CL and Vc mean population estimates were 8.01 L/hr (133 ml/min or 2.02 ml/min/kg) and 145 L (2.19 L/kg). The predictive performance of the population modeling and parameters was evaluated with a bayesian fitting procedure in an independent validation set of 30 patients with similar physical and clinical characteristics. There was no statistically significant bias or imprecision between measured and predicted thiopental plasma concentrations in this validation group. Moreover, there was a good adequation (r = 0.939) between individual CL values predicted from the population formula and estimated with the bayesian approach.

Glucocorticoids decrease cytochrome c oxidase activity of isolated rat kidney mitochondria.

The importance of mitochondria is rising as a target in pathologic processes such as ischemia. We have investigated the effects of hydrocortisone, prednisolone, dexamethasone and triamcinolone on oxidative phosphorylation, Ca2+ fluxes, swelling and membrane potentials in isolated kidney mitochondria. The measurement of respiration state 3 showed a significant decrease in presence of glucocorticoids whereas the other respiration states were not modified. When mitochondria were uncoupled and either the complexes III and IV or the complex IV were stimulated, the O2 consumption was decreased by glucocorticoids. These results suggest the cytochrome c oxidase is a target of the glucocorticoid effect on the respiratory chain. Indeed, the other mitochondrial functions investigated were unchanged, ruling out a direct effect on Ca2+ fluxes or swelling. A regulation of cytochrome c oxidase activity by glucocorticoids will be of particular interest in pathology involving metabolic insult.

Individual dosing of carboplatin based on drug monitoring in children receiving high-dose chemotherapy.

Individual dosing of carboplatin based on drug monitoring was performed within a multi-centric phase I study based on high AUC-levels in children. Twelve patients (aged 3-17 years old) have been included: 3, 5, and 4 patients at the overall target ultrafilterable carboplatin AUC of 20, 25, or 30 mg/ml x min, respectively. Carboplatin was administered as a daily 60-min infusion, repeated on five consecutive days. The initial daily dose corresponding to the three first days was calculated according to the carboplatin clearance (CL) predicted from patients' characteristics (body weight, serum creatinine and nephrectomy status). Three blood samples were taken per patient. The individual CL were estimated by MAP (maximum a posteriori approach) Bayesian method implemented in the MP-K program. The doses for day 4 and 5 was adjusted in order to obtain the overall target AUC. Drug monitoring led to a change in the carboplatin dose (overall administered dose versus overall dose planned) ranging from -41% to +45%. Pharmacokinetics were performed at day 5 for 7/12 children: mean relative change between day 1 and day 5 was -11% showing a statistically significant, but limited, decrease of CL from day 1 to day 5. The percentage of difference between the observed and target overall AUC ranged between -7% and +14%. Three patients (one at each AUC level) who were previously treated with cisplatin experienced dose-limiting hearing loss. In conclusion, drug monitoring and dose adjustment is needed for the control of carboplatin plasma exposure when administering high doses of carboplatin in children.

Molecular properties and pharmacokinetic behavior of cetirizine, a zwitterionic H1-receptor antagonist.

The ionization and lipophilicity behavior of the antihistamine (H1-receptor antagonist) cetirizine was investigated, showing the drug to exist almost exclusively as a zwitterion in the pH region 3.5-7.5. In this pH range, its octanol/water lipophilicity is constant and low compared to cationic antihistamines (log D = log PZ = 1.5), whereas its H-bonding capacity is relatively large (delta log PZ > or = 3.1). Conformational, electronic, and lipophilicity potential calculations revealed that zwitterionic cetirizine experiences partial intramolecular charge neutralization in folded conformers of lower polarity. Pharmacokinetic investigations have shown the drug to be highly bound to blood proteins, mainly serum albumin, and to have a low brain uptake, explaining its lack of sedative effects. As such, cetirizine does not differ from "second-generation" antihistamines. In contrast, its very low apparent volume of distribution in humans (0.4 L kg-1, smaller than that of exchangeable water) implies a low affinity for lean tissues such as the myocardium and is compatible with the absence of cardiotoxicity of the drug. The zwitterionic nature and modest lipophilicity of cetirizine may account for this pharmacokinetic behavior. The suggestion is offered that cetirizine and analogous zwitterions, whose physicochemical, pharmacokinetic, and pharmacodynamic properties differ from those of "first-" and "second-generation" drugs in this class, could be considered as "third-generation" antihistamines.

Drug binding in plasma. A summary of recent trends in the study of drug and hormone binding.

The ligands are generally bound in plasma to a significant extent by several transport proteins (both high and low affinity), irrespective of their endogenous or exogenous origin. The protein binding of endogenous compounds (such as hormones) exhibits higher affinity and specificity than those of exogenous compounds (such as drugs). For plasma proteins that bind the same ligand(s), structural similarities or a common genetic origin may be found, although this is not a general rule. Alterations in ligand binding may be due to modifications of either the structure or the level of the binding protein. These modifications may result from genetic make up, physiology or pathology. In some situations, plasma binding may impair the distribution of drugs to tissues, with drug distribution then mainly restricted to the distribution compartment of the drug-binding protein. In other instances, the plasma drug-binding is permissive, and does not limit drug distribution to tissues. A given drug-transport protein system may have either a permissive or a restrictive effect on the drug distribution, depending on the tissue. The physiological significance of the high-affinity transport proteins is not completely understood. These proteins may increase the plasma concentration of poorly hydrosoluble ligands, ensure a more uniform tissue distribution and increase the life of the ligands. The life of the protein may also be increased by ligand binding. High-affinity transport proteins are also involved in some specific carrier mediated transfer mechanisms. It is possible to demonstrate structure-binding relationships or binding selectivity for the plasma transport proteins, but these are quite independent of relationships observed at the receptor level.

Nimesulide binding to components within blood.

The binding of nimesulide within human serum to isolated proteins and to erythrocytes was studied by equilibrium dialysis. Within the range of therapeutic concentrations, nimesulide was 99% bound to serum involving a nonsaturated process (NKA = 91). This binding was almost identical to binding of nimesulide to serum albumin (NKA = 95). Physiological concentrations of free fatty acids did not affect binding of nimesulide to serum albumin. The retention of nimesulide by erythrocytes suspended in buffer was moderate (67%), although in whole blood no erythrocyte binding was observed because of the greater affinity of this drug for serum. Over the range of therapeutic concentrations (2.5 to 63 mumol/L), the free fraction of nimesulide in serum remains constant. Serum binding was decreased in samples obtained from patients with renal failure or hepatic cirrhosis associated with hypoalbuminaemia and hyperbilirubinaemia, respectively. At therapeutic concentrations, the binding of nimesulide was unaffected by warfarin, cefoperazone, furosemide (frusemide), glibenclamide, tamoxifen or digitoxin. However, valproic acid and fenofibrate (80 mumol/L) may displace nimesulide. 4-Hydroxy-nimesulide, the principal metabolite, significantly increased the free fraction of nimesulide. Although methotrexate had no effect on the free fraction of nimesulide, the free fraction of methotrexate was significantly increased in the presence of nimesulide. The present study also demonstrated 2 distinct nimesulide binding sites (site I and site II) on serum albumin (10 mumol/L) with different affinities: site II KA = 3.57 x 10(5) L/mol and site I KA = 1.24 x 10(5) L/mol. Interaction studies using markers that bind specifically to site I (warfarin and azapropazone) and site II (diazepam and ibuprofen) indicated that nimesulide binds to site II with higher affinity and to a lesser extent to site I.

The binding of aryl carboxylic acid derivatives to human serum albumin--a structure-activity study.

The binding interactions of some aryl carboxylic acid derivatives have been examined by circular dichroism and fluorescence spectroscopy. With specific probes, we have shown that the seven ligands under study bind primarily to the benzodiazepine site on HSA. Their association constants are in the range of 10(5)-10(6) M-1 as found by spectropolarimetric titration, and appear to be closely related to some chemical features. It is concluded that the binding is enhanced by the lengthening of the carbon chain substituent with a terminating carboxyl moiety and by halogen substitution in the aromatic rings. It is further shown that hydrophilic substitutions such as hydroxyl or ketone groups in the carbon chain substituent will decrease the binding.

Comparative binding of etretinate and acitretin to plasma proteins and erythrocytes.

The interactions of etretinate and its main metabolite acitretin with human plasma proteins have been investigated in vitro by an erythrocyte partitioning technique that allows a quantitative estimation of the plasma and erythrocyte binding. Etretinate was extensively lipoprotein-bound (75% of plasma etretinate), with a binding constant for its main low density lipoprotein carrier of 40 x 10(6) M-1, accounting for 48% of the total plasma-bound drug. Acitretin was mainly albumin-bound (91% of plasma acitretin), with a binding constant of 0.7 x 10(6) M-1. The total plasma binding of both drugs was > 99% and, in blood, the fractions associated with erythrocytes were 14.5 and 8.1% of the total amount for etretinate and acitretin, respectively.

Analysis of free fatty acid effect on methotrexate binding to albumin.

The binding of methotrexate to human serum albumin and the inhibitory effect of serum free fatty acids (FFA) have been studied by equilibrium dialysis with radiolabeled methotrexate. Methotrexate was bound to albumin via a single site (1.03 +/- 0.02) with a low affinity (1350 +/- 60 M-1). The effect of FFA on binding by albumin of methotrexate was analysed according to the classical inhibition models with computation of the free inhibitor concentration and was ascribed to an uncompetitive type of inhibition. These results were in agreement with the observed serum binding of methotrexate (45-50%) and allowed the simulation of the effect of various concentrations of FFA on methotrexate albumin binding in human serum.

Evidence for binding of certain acidic drugs to alpha 1-acid glycoprotein.

The binding of some acidic drugs to alpha 1-AGP was studied by equilibrium dialysis at 37 degrees, pH 7.4. Certain acidic drugs bound to alpha 1-AGP at one binding site with a high affinity. Though the alpha 1-AGP plasma concentration is far lower than the HSA concentration, the association constants of some acidic drugs with alpha 1-AGP are high enough to suggest that binding to alpha 1-AGP will contribute significantly to the total plasma binding of these drugs.