A multilevel object-oriented modelling methodology for physiologically-based pharmacokinetics (PBPK): Evaluation with a semi-mechanistic pharmacokinetic model.

BACKGROUND AND OBJECTIVE: The aims of this study are (i) to assess the predictive reliability of the physiologically based software PhysPK versus the well-known population approach software NONMEM for the cited semi-mechanistic PK model, (ii) to determine whether these modelling approaches are interchangeable and (iii) to compare acausal with causal modelling approaches in the framework of semi-mechanistic PK models. METHODS: A semi-mechanistic model was proposed, which assumed oral administration of a solid dosage form with a peripheral compartment and two active metabolites. The model incorporates intestinal transit, dissolution limited by solubility, variable efflux transporter expression along the gut and linear and non-linear metabolism in the gut and liver. Four different approximations to the theoretical model were developed in order to validate both the new software and modelling methodology. RESULTS: Plasmatic concentrations correlation plots as well as relative errors in AUC0-48 and Cmax predictions revealed the accuracy of PhysPK in the prediction of these exposition parameters. Physiological and acausal object oriented version systematically under-estimated AUC0-48 and Cmax of the parent drug, whereas metabolites were over-estimated when taking the semi-mechanistic and extraction-based metabolism version as the reference. CONCLUSIONS: PhysPK has been properly validated, where differences are related to numerical precision of integrators and solvers. A systematic bias for the parent drug and active metabolites was predicted when a semi-mechanistic approach including extraction-based metabolism was compared to the physiologic and acausal approach, showing that interchangeability might be possible when intrinsic-clearance metabolism is implemented in the semi-mechanistic approach. The acausal and object-oriented methodology allows for defining the semi-mechanistic model through its local mechanisms and relationships among entities, without the need to build the final set of Ordinary Differential Equations.

Defining level A IVIVC dissolution specifications based on individual in vitro dissolution profiles of a controlled release formulation.

Regulatory guidelines recommend that, when a level A IVIVC is established, dissolution specification should be established using averaged data and the maximum difference between AUC and Cmax between the reference and test formulations cannot be greater than 20%. However, averaging data assumes a loss of information and may reflect a bias in the results. The objective of the current work is to present a new approach to establish dissolution specifications using a new methodology (individual approach) instead of average data (classical approach). Different scenarios were established based on the relationship between in vitro-in vivo dissolution rate coefficient using a level A IVIVC of a controlled release formulation. Then, in order to compare this new approach with the classical one, six additional batches were simulated. For each batch, 1000 simulations of a dissolution assay were run. Cmax ratios between the reference formulation and each batch were calculated showing that the individual approach was more sensitive and able to detect differences between the reference and the batch formulation compared to the classical approach. Additionally, the new methodology displays wider dissolution specification limits than the classical approach, ensuring that any tablet from the new batch would generate in vivo profiles which its AUC or Cmax ratio will be out of the 0.8-1.25 range, taking into account the in vitro and in vivo variability of the new batches developed.

Bioactivity of Ceftazidime and Fluconazole Included in Polymethyl Methacrylate Bone Cement for Use in Arthroplasty.

BACKGROUND: The microorganisms that most frequently cause prosthetic joint infection are methicillin-resistant Staphylococcus aureus and gram-negative aerobic bacillus. Studies have documented the efficacy of mixing antibiotics with polymethyl methacrylate, but that of antifungal drugs has not received much attention. The objective of this in vitro study was to characterize the elution profile and bioactivity of ceftazidime and fluconazole when incorporated into bone cement in proportions intended for prophylaxis and treatment of bone infections. METHODS: Antibiotic-loaded bone cement cylinders in a proportion of 1:40 and 4:40 (ratio of grams of antibiotic to grams of cement) were assayed. Drug delivery was investigated in a flow-through dissolution apparatus (SotaxCE7). To assess bioactivity, antibiotic concentrations were simulated in the joint space of 1000 patients. Antibacterial properties were evaluated by counting colony forming units and the inhibition-halo test. RESULTS: The ratio of released ceftazidime and fluconazole was 453% and 648%, respectively, higher when used for treatment proportions than prophylaxis proportions. A bioactivity simulation exercise showed that the efficacy of ceftazidime/fluconazole determined as the amount of drug is released at the active site in the first 3 days after surgery would depend on the sensitivity of the microorganism and would increase substantially after drain removal. The microbiology study showed that biofilm formation by Pseudomonas aeruginosa could be a problem when ceftazidime was used in treatment or prophylaxis proportions. CONCLUSION: Our in vitro findings suggest that ceftazidime and fluconazole can be added into polymethyl methacrylate for the prevention/treatment of infections associated to joint surgery. Their efficacy depends on the sensitivity of the microorganism causing the infection.

IVIVC approach based on carbamazepine bioequivalence studies combination.

The aim of the present study was to explore the feasibility of obtaining an IVIVC by combination of data from two bioequivalence (BE) studies of carbamazepine (CBZ) in order to assess if the previously published dissolution media and conditions could be applicable to any other oral immediate release (IR) CBZ products with conventional excipients. Twenty-four healthy male subjects from two BE study received one IR dose of the test (test 1 or 2) or the reference formulation (Tegretol, 400 mg). Dissolution studies of the IR CBZ tablets were performed in two different laboratories. In order to develop IVIVC, individual or average data analysis were considered. A level C, level B and level A correlation have been successfully developed by combining data from different BE studies of CBZ immediate release drug products. A level A IVIVC was developed with all four datasets with a good R2 for all the combinations of in vivo and in vitro data. A dissolution medium containing 1% SLS has demonstrated its suitability as the universal biopredictive dissolution medium, even if different batches and in vivo/in vitro studies were combined.

Impact of nutritional status on the pharmacokinetics of erlotinib in rats.

Malnourishment is a complex condition in which physiopathological changes take place in multiple systems as a result of energy, protein and nutrient deficiency. The purpose of this study was to evaluate, using an experimental animal model, the impact of nutritional status on the pharmacokinetic profile of erlotinib, a reversible, highly selective, human epidermal growth factor receptor (HER1/EGFR) tyrosine kinase inhibitor. Two groups of rats -WN (well-nourished) and UN (undernourished) - were fed with different diets for 23-26 days. Rats were assigned randomly to one of three erlotinib treatments (n = 42) consisting of a single dose administered intravenously (i.v.), via oral solution or via oral suspension. Blood samples were assayed for erlotinib concentration. A population pharmacokinetic model was developed and pharmacokinetic parameters obtained in UN rats were compared with those in WN rats. Erlotinib clearance suffered a 5% decrease in the mild-undernutrition status. Moreover, when the drug was administered orally as a suspension, the extent and rate of absorption underwent a 20% increase in UN rats. The results of this study might help to explain, at least in part, the variability of erlotinib treatment and could represent the first step towards establishing new dosage guidelines for the treatment of undernourished cancer patients. Copyright © 2015 John Wiley & Sons, Ltd.

Design, characterization and in vitro evaluation of 5-aminosalicylic acid loaded N-succinyl-chitosan microparticles for colon specific delivery.

The objective of this study was to prepare NS-chitosan microparticles for the delivery of 5-aminosalicylic acid (5-ASA) to the colon. Microparticles can spread out over a large area of colon allowing a more effective local efficacy of 5-ASA. N-Succinyl-chitosan was chosen as carrier system because of its excellent pharmaceutical properties in colon drug targeting such as poor solubility in acid environment, biocompatibility, mucoadhesive properties, and low toxicity. It was prepared by introducing succinic group into chitosan N-terminals of the glucosamine units. 5-ASA loaded NS-chitosan microparticles were prepared using spray-drying. As a control, a matrix obtained by freeze-drying technique was also prepared and tested. Fourier transform infrared (FT-IR), differential scanning calorimetry (DSC) and X-ray diffraction studies show the 5-ASA/NS-chitosan electrostatic interactions in both the systems. Mean size of the microparticles was around 5 µm, zeta potential value of both systems was always negative. Scanning electron microscopy (SEM) images show an acceptable spherical non porous structure of microparticles. In vitro swelling and drug release studies were in accordance with the polymer properties, showing the highest swelling ratio and drug release at pH=7.4 (colonic pH) where microparticles were able to deliver more than 90% of 5-ASA during 24h experiments. Rheological studies are in accordance with the swelling and release studies.

Animal model of undernutrition for the evaluation of drug pharmacokinetics / Modelo de desnutrición animal para la evaluación de estudios de farmacocinética

Background: Protein energy malnutrition is a public health problem affecting a great number of people. Pathophysiological imbalances in malnourished individuals have a profound impact on drug pharmacokinetics. Objective: To develop an animal model of undernutrition using male Wistar rats to be used to assess, in further studies, the impact of nutritional status on the oral bioavailability and pharmacokinetics of drugs. Desing: Animals were randomly assigned to one of two groups and fed different diets for 26 days: WN (well-nourished/regular diet, N = 61) and UN (under-nourished/protein-calorie restricted diet, N = 72). Assessment of the animals' nutritional status was performed taking into account serum albumin, total cholesterol level and total body weight. A kinetic model incorporating population kinetic analysis (NONMEM) was developed to analyze body weight versus time profiles in the adaptation period following administration of the two aforementioned diets. Results: Serum albumin plasma levels were lower than 2.3 g/dL in 80% (60/72) of malnourished animals at the end of the adaptation period. The range of the total serum cholesterol was similar in both groups at the end of the adaptation period. Total body weight in all cases was less than 230 g for malnourished animals and higher than 240 g for well-nourished animals. The kinetic model assayed was confirmed to be an expansion module characterized by linear weight gain and a decline module characterized by exponential weight loss, where the weight loss rate constant is an exponential function of time. The bootstrap resampling method confirmed the stability of the model eventually selected. Conclusions: The animal model developed in this study is reliable and could be of use in evaluating the impact of nutritional state on the pharmacokinetics of drugs. The proposed mathematical model allows the body weight of animals to be predicted at a given time taking into account the diet followed in the experimental period (AU)

Antecedentes: La desnutrición calórico protéica es un problema de salud que afecta a un número de personas muy elevado. Las alteraciones fisiopatológicos originadas por la desnutrición tienen una repercusión elevada en el comportamiento farmacocinético de los fármacos. Objetivo: Desarrollar un modelo de desnutrición en ratas Wistar macho que pueda utilizarse para evaluar el impacto del estado nutricional sobre la biodisponibilidad oral y farmacocinética de los fármacos. Diseño: Los animales fueron asignados de forma aleatoria a uno de los dos grupos y alimentados con diferentes dietas durante 26 días: WN (dieta normo-nutrida/regular, N = 61) y UN (dieta restringida/calórico-proteica N = 72). La clasificación del estado nutricional de los animales se realizó teniendo en cuenta los niveles de albúmina sérica y colesterol total, y el peso corporal total. Se ha desarrollado un modelo cinético poblacional para analizar la evolución del peso corporal de los animales durante el periodo de adaptación. Resultados: Al final del periodo de adaptación los niveles de albúmina plasmática fueron inferiores a 2,3 g/dL en el 80% (60/72) de los animales desnutridos. Sin embargo, los valores de colesterol total en suero fueron similares en ambos grupos. El peso corporal total de los animales desnutridos fue inferior a 230 g y superior a 240 g para los animales alimentados en condiciones de normonutrición. El modelo de cinético de evolución del peso seleccionado se caracteriza por una cinética combinada lineal y exponencial. La fracción lineal representa el aumento de peso, y la fracción exponencial, que a su vez es función del tiempo, caracteriza la pérdida de peso del animal. La estabilidad del modelo seleccionado se ha realizado utilizando la técnica de remuestreo (bootstrap) y la validación mediante el test de predicción visual (visual predictive check, VPC). Conclusiones: El modelo animal desarrollado en este estudio puede ser de utilidad para evaluar el impacto del estado nutricional sobre la farmacocinética de los fármacos. El modelo matemático propuesto permite predecir el peso corporal de los animales, teniendo en cuenta la dieta la dieta administrada durante el período experimental (AU)

Perfil de toxicidad y adherencia del esquema farmacoterapéutico gemcitabina-carboplatino en cáncer de pulmón no microcítico / Toxicity Profile and Adherence to the Pharmacotherapeutic Regimen of Gemcitabine–carboplatin in Non-small Cell Lung Cancer

Objetivo Analizar la relación entre las dosis administradas de gemcitabina-carboplatino (GEM-CARBO) y la incidencia y grado de toxicidad, hematológica y renal, y la adherencia al tratamiento en pacientes con cáncer de pulmón no microcítico. Métodos Estudio retrospectivo de 37 meses de duración. El conjunto mínimo de datos para realizar el seguimiento de los pacientes se obtuvo con ayuda del programa informático Farmis-Oncofarm® y de las historias clínicas y farmacoterapéuticas. La toxicidad hematológica se evaluó de acuerdo con la Common Toxicity Criteria 3.0. La toxicidad renal se valoró a partir de los datos de concentración sérica de creatinina y el aclaramiento de creatinina. Resultados Se han incluido en el estudio 31 pacientes a los que se les administraron un total de 122 ciclos. La incidencia de anemia y neutropenia grado III fue de un 34,0 y un 30,8%, respectivamente, de trombocitopenia de grado III del 3,8% y de grado IV del 7,7%. No se ha identificado ningún caso de toxicidad renal. El 65,0% de los pacientes recibieron más del 85,0% de la dosis de carboplatino teórica planeada y el 58,0% de los pacientes recibieron más del 85,0% de la dosis de gemcitabina teórica planeada. Se retrasó la administración en el 18,0% de los ciclos prescritos. Conclusiones La indicación y prescripción del esquema GEM-CARBO se ha ajustado con unas evidencias científicas sólidas, pero su toxicidad hematológica ha limitado su uso y ha dificultado la administración de la intensidad de dosis prevista comprometiendo la efectividad del tratamiento (AU)

Objective To analyse the relationship between doses of gemcitabine–carboplatin (GEM-CARBO) administered and incidence and level of haematological and renal toxicity, and the adherence to the treatment in patients with non-small cell lung cancer. Methods Retrospective study, which lasted for 37 months. We were able to obtain the minimum set of data needed to carry out the follow-up with the help of Farmis-Oncofarm® software and the medical and pharmacotherapeutic records. The haematological toxicity was assessed in accordance with the Common Toxicity Criteria 3.0. Renal toxicity was evaluated using serum creatinine levels and creatinine clearance. Results Thirty-one patients were included in the study who were administered a total of 122 cycles. There was a 34.0% and 30.8% incidence of anaemia and grade 3 neutropenia, respectively. There was also a 3.8% and 7.7% incidence of grade 3 and grade 4 thrombocytopenia, respectively. No cases of renal toxicity were found. 65.0% of patients received more than 85.0% of the planned theoretical dosage of carboplatin and 58% of patients received more than 85.0% of the planned theoretical dosage of gemcitabine. Administration was delayed in 18.0% of the cycles prescribed. Conclusions The indication and prescription of the GEM-CARBO regimen was adjusted in accordance with solid scientific evidence, but its haematological toxicity limited its use and made it difficult to maintain the dose intensity foreseen in the study. This compromised the effectiveness of the treatment (AU)

N-Succinyl-chitosan systems for 5-aminosalicylic acid colon delivery: in vivo study with TNBS-induced colitis model in rats.

5-Aminosalicylic acid (5-ASA) loaded N-Succinyl-chitosan (SucCH) microparticle and freeze-dried system were prepared as potential delivery systems to the colon. Physicochemical characterization and in vitro release and swelling studies were previously assessed and showed that the two formulations appeared to be good candidates to deliver the drug to the colon. In this work the effectiveness of these two systems in the treatment of inflammatory bowel disease was evaluated. In vitro mucoadhesive studies showed excellent mucoadhesive properties of both the systems to the inflamed colonic mucosa. Experimental colitis was induced by rectal instillation of 2,4,6-trinitrobenzene sulfonic acid (TNBS) into male Wistar rats. Colon/body weight ratio, clinical activity score system, myeloperoxidase activity and histological evaluation were determined as inflammatory indices. The two formulations were compared with drug suspension and SucCH suspension. The results showed that the loading of 5-ASA into SucCH polymer markedly improved efficacy in the healing of induced colitis in rats.

Toxicity profile and adherence to the pharmacotherapeutic regimen of gemcitabine-carboplatin in non-small cell lung cancer.

OBJECTIVE: To analyse the relationship between doses of gemcitabine-carboplatin (GEM-CARBO) administered and incidence and level of haematological and renal toxicity, and the adherence to the treatment in patients with non-small cell lung cancer. METHODS: Retrospective study which lasted for 37 months. We were able to obtain the minimum set of data needed to carry out the follow-up with the help of Farmis-Oncofarm(®) software and the medical and pharmacotherapeutic records. The haematological toxicity was assessed in accordance with the Common Toxicity Criteria 3.0. Renal toxicity was evaluated using serum creatinine levels and creatinine clearance. RESULTS: Thirty-one patients were included in the study who were administered a total of 122 cycles. There was a 34.0% and 30.8% incidence of anaemia and grade 3 neutropaenia, respectively. There was also a 3.8% and 7.7% incidence of grade 3 and grade 4 thrombocytopaenia, respectively. No cases of renal toxicity were found. 65.0% of patients received more than 85.0% of the planned theoretical dosage of carboplatin and 58% of patients received more than 85.0% of the planned theoretical dosage of gemcitabine. Administration was delayed in 18.0% of the cycles prescribed. CONCLUSIONS: The indication and prescription of the GEM-CARBO regimen was adjusted in accordance with solid scientific evidence, but its haematological toxicity limited its use and made it difficult to maintain the dose intensity foreseen in the study. This compromised the effectiveness of the treatment.

A pharmacokinetic model for evaluating the impact of hepatic and intestinal first-pass loss of saquinavir in the rat.

The aim of this study was to quantify the intestinal and hepatic first-pass loss of saquinavir and to assess the effect of coadministration of ritonavir on this first-pass loss. Single doses of 12, 24, and 48 mg of saquinavir and a dose of 24 mg of saquinavir/6 mg of ritonavir were orally, intravenously, or intraperitoneally administered to 94 rats. Ten groups of animals were studied. A semiphysiological pharmacokinetic model incorporating a population pharmacokinetic analysis [nonlinear mixed-effects model (NONMEM)] was developed to analyze plasma concentration-time profiles after administration via each of the three above-mentioned routes. This model confirmed that saturable metabolism in hepatocytes and enterocytes and dose-dependent precipitation in the peritoneal cavity after intraperitoneal administration characterize the pharmacokinetics of SQV. It also demonstrated that low oral bioavailability of saquinavir is due mainly to intestinal rather than to hepatic first-pass metabolism. In addition, it was shown that ritonavir diminished saquinavir clearance through competitive inhibition. The present report presents a new pharmacokinetic model applied in rats to evaluate the impact of hepatic and intestinal first-pass loss on oral bioavailability.

Animal model of undernutrition for the evaluation of drug pharmacokinetics.

BACKGROUND: Protein energy malnutrition is a public health problem affecting a great number of people. Pathophysiological imbalances in malnourished individuals have a profound impact on drug pharmacokinetics. OBJECTIVE: To develop an animal model of undernutrition using male Wistar rats to be used to assess, in further studies, the impact of nutritional status on the oral bioavailability and pharmacokinetics of drugs. DESIGN: [corrected] Animals were randomly assigned to one of two groups and fed different diets for 26 days: WN (well-nourished/regular diet, N = 61) and UN (under-nourished/protein-calorie restricted diet, N = 72). Assessment of the animals' nutritional status was performed taking into account serum albumin, total cholesterol level and total body weight. A kinetic model incorporating population kinetic analysis (NONMEM) was developed to analyze body weight versus time profiles in the adaptation period following administration of the two aforementioned diets. RESULTS: Serum albumin plasma levels were lower than 2.3 g/dL in 80% (60/72) of malnourished animals at the end of the adaptation period. The range of the total serum cholesterol was similar in both groups at the end of the adaptation period. Total body weight in all cases was less than 230 g for malnourished animals and higher than 240 g for well-nourished animals. The kinetic model assayed was confirmed to be an expansion module characterized by linear weight gain and a decline module characterized by exponential weight loss, where the weight loss rate constant is an exponential function of time. The bootstrap resampling method confirmed the stability of the model eventually selected. CONCLUSIONS: The animal model developed in this study is reliable and could be of use in evaluating the impact of nutritional state on the pharmacokinetics of drugs. The proposed mathematical model allows the body weight of animals to be predicted at a given time taking into account the diet followed in the experimental period.

Bioavailability and pharmacokinetic model for ritonavir in the rat.

The aim of this study is to investigate in vivo the oral bioavailability of ritonavir and to evaluate the pharmacokinetic model that best describes the plasma concentration behavior after oral and intravenous administration. Male Wistar rats were intravenously administered at 3 mg dose of pure ritonavir and oral administered at 4.6 +/- 2.5 mg of diluted Norvir. Blood samples were taken by means of the jugular vein for a 24 h period of time. An analytical high-performance liquid chromatography (HPLC) technique was developed in order to quantify ritonavir plasma concentrations. A nonlinear modeling approach was used to estimate the pharmacokinetic parameters of interest. Results showed that a two-compartmental model with zero-order kinetic in the incorporation process of ritonavir into the body better fitted intravenous and oral data. The estimated oral bioavailability by means of noncompartmental and compartmental approaches resulted in 74% and 76.4%, respectively. These values confirm the ones obtained by other authors in the rat. In conclusion, a zero-order kinetic in the incorporation process at the administered doses suggests the saturation of the possible specialized transport mechanisms involved in the incorporation of ritonavir into the body. These results could justify the use of low doses of ritonavir when improving the bioavailability of other protease inhibitors (PIs) is required.

Estudio de la interaccion entre ritonavir y saquinavir en su absorción intestinal / No disponible

El estudio se ha diseñado con objeto de determinarla interacción entre el ritonavir y el saquinavirdurante su absorción gastrointestinal.Para ello, se han realizado estudios de perfusiónen el intestino delgado completo de ratasWistar, con distintas proporciones de los dosfármacos. Los resultados obtenidos demuestranque la constante aparente de velocidad deabsorción (kap) del ritonavir disminuye, aunqueno significativamente, cuando la concentraciónde saquinavir es al menos un 40% superiora la de ritonavir. Asimismo, la kap saquinavirdisminuye cuando la concentración de ritonavires un 25% superior a la de saquinavir,si bien estas diferencias no son significativas.El mismo comportamiento se observa cuandola concentración de ritonavir es un 40% inferiora la de saquinavir, lo que indicaría que la incorporacióndel saquinavir se modifica por el ritonavir.Se requieren estudios adicionales paradilucidar el mecanismo concreto de interacciónentre los dos fármacos

The study has been designed in order to determinatethe interaction between the ritonavirand the saquinavir during its gastrointestinalabsorption. For it, perfusion studies have beencarried out in the whole intestine of the Wistarrats, with different proportions of the two drugs.The results obtained demonstrate that the apparentabsorption rate constant (kap) of the ritonavirit diminishes, although not significantly,when the saquinavir concentration is at least40% superior to that of ritonavir. Also, the kapsaquinavir diminishes when the ritonavir concentrationis 25% superior to that of saquinavir,although these differences are not significant.The same behavior is observed when the ritonavirconcentration is 40 % smaller to that ofsaquinavir, what would indicate that the incorporationof the saquinavir modifies for the ritonavir.Additional studies are required to elucidatethe concrete mechanism of interactionamong the two drugs

Use of nonlinear mixed effect modeling for the intestinal absorption data: application to ritonavir in the rat.

The aim of this study is to investigate in situ the mechanisms involved in the gastrointestinal absorption of ritonavir in the rat, as an animal model for preclinical studies of anti-HIV agents in vivo. Four ritonavir solutions (40, 27, 13 and 7 microM) in the presence of 1% dimethylsulfoxide (DMSO) were perfused in the small intestine of anaesthetised rats. Effects of DMSO on the intestinal permeability were investigated using solutions containing antipyrine 1.33 mM and ritonavir 7 microM with and without 1% of DMSO. Antipyrine and ritonavir transport was not modified in the presence of 1% of DMSO. The population pharmacokinetic parameters of the ritonavir intestinal transport were obtained by means of nonlinear mixed effect modelling approach according to a nonlinear absorption and nonlinear secretion. The absorption and secretion kinetic parameters for ritonavir were: Vm=47.6 microM/h; Km=8.77 microM; Vms=3.66 microM/h and Kms=0 microM. The interindividual variability found to ritonavir Vm 13.1%, and the residual variability was 8.98%. The Kms value support the saturation of the carrier at the range of concentrations of ritonavir assayed. The interindividual variability value of the Vm could explain, at least in part, the variability in absorption rate constants observed.

Adsorption of methotrexate and calcium leucovorin onto cholestyramine in vitro.

Methotrexate (MTX), an antimetabolite of folic acid, is a drug widely used in the treatment of different types of cancer. When high doses are administered, it is necessary to interrupt its action by administering calcium leucovorin (CaL). The main pathway of MTX and CaL elimination in humans occurs through the kidney, but about 10% is excreted in the faeces via the bile. Drugs, foods and sorbents in intestinal lumen modify MTX and CaL reabsorption. Individual and simultaneous studies on the adsorption of MTX and CaL from aqueous phosphate buffer by cholestyramine were carried out in order to calculate the adsorption process of MTX and CaL to cholestyramine, and to characterize the influence of CaL in the adsorption of MTX to cholestyramine and vice versa. The Langmuir binding isotherms determined in buffer solutions at pH 6 indicated a greater (12.58%) adsorption capacity of cholestyramine (1.43 mmol of drug/g of resin) than at pH 7 (1.25 mmol of drug/g of cholestyramine). The affinity constant of MTX to cholestyramine was a 45.27% higher (6.67 mM(-1)) than the affinity constant of CaL to the resin (3.65 mM(-1)). Results from simultaneous assays indicate that a displacement of the MTX bound to cholestyramine by CaL is not foreseeable. The results suggest that cholestyramine may be a potentially useful adjunctive therapy in the treatment of an overdose of MTX. Consequently, cholestyramine may be of clinical value in patients who develop early renal function impairment whilst undergoing MTX therapy.

Pharmacokinetic models for the saturable absorption of cefuroxime axetil and saturable elimination of cefuroxime.

Since oligopeptidic drugs such as beta-lactam antibiotics share the same carriers in humans and animals, the absorption and elimination kinetics of cefuroxime (C) were investigated in rats. Plasma C concentrations were measured by liquid chromatography. Pharmacokinetics and bioavailability of C in the rat were examined after intravenous (i.v.) administration at three doses (1.78, 8.9 and 17.8mg) of cefuroxime sodium and oral administration at two doses (2.02 and 8.9mg) of cefuroxime axetil (CA). Preliminary fits using data from intravenous administration of C showed that the drug disposition kinetics were clearly nonlinear, with an increase in plasma clearance as the intravenous dose increased. After oral administration of CA, normalized C(max) was higher for smaller dose than for the largest dose. The population pharmacokinetic parameters were obtained by means of nonlinear mixed effect modelling approach according to a nonlinear elimination and nonlinear absorption two-compartment model. The nonlinear elimination could be attributed to a saturable renal tubular reabsorption of the antibiotic and nonlinear intestinal absorption of CA mediated by carrier system. The oral bioavailability of C, calculated by numeric integration of an amount of CA drug absorbed was 22 and 17% for 2.02 and 8.9mg of prodrug administered orally.

Relevance of multidrug resistance proteins on the clinical efficacy of cancer therapy.

Variations in drug uptake and efflux, as well as changes in intracellular drug entrapment and distribution may represent important resistance mechanisms to cancer therapy. A variety of ATP binding cassette transporters (ABC) localised in multiple cell membranes is implied in those phenomena, representing a mechanism of protection of cells against xenobiotics. Many cancer cell lines over express some ABC transporters, especially p-glycoprotein, MRP1 and BCRP. This over expression is related to worse cancer treatment outcome and, in some cases, reduced overall survival of cancer patients. This paper reviews the location and physiological role of the three transporters mentioned and also describes the drugs that are substrates of these proteins. The usefulness of animal and cellular models to evaluate the role of these transporters on the uptake and efflux of anticancer drugs is discussed. Finally, the results of preclinical and clinical studies about the utility of some inhibitors of these pumps, as well as the implications of polymorphism of ABC transporters on the efficacy and safety of anticancer therapeutics are reported.

A Bayesian method for predicting 5-fluorouracil pharmacokinetic parameters following short-term infusion in patients with colorectal cancer.

The objective of this study was to develop a population pharmacokinetic model and validate it using a Bayesian approach for predicting, a priori and a posteriori, the individual volume of distribution (V(d)) and clearance (Cl) of 5-fluorouracil (5-FU) given as short-term intravenous infusion in weekly and multiple doses. Forty-four patients were divided in group A (5-FU weekly doses) including 27 patients with nonmetastatic colorectal adenocarcinoma treated with 450 mg/m(2) of 5-FU, 1 day per week for 48 doses, plus oral levamisol (50 mg/8 h) for 3 days, every 15 days and group B (5-FU multiple doses) including 17 patients with metastatic colorectal adenocarcinoma, receiving 5-FU (425 mg/m(2)) plus intravenous folinic acid (20 mg/m(2)) over 5 consecutive days, every 4 weeks for six cycles. In both groups 5-FU was administered as a 30-60-min infusion. A total of 176 plasma concentrations were analyzed using a NONMEM program according to a linear one-compartment model. In group A, 5-FU population pharmacokinetic parameters were obtained and the covariables studied were age, gender, weight, ideal body weight, height, body surface area, creatinine clearance, and hepatic function tests. A priori and a posteriori validation of this model was carried out with plasma concentrations obtained in day 1 in group B. In group B, population pharmacokinetic parameters of 5-FU following multiple doses were estimated using scale factors to identify differences in 5-FU V(d) and Cl between days 1 and 4, and the interindividual, interoccasion, and residual variabilities studied. V(d) was 0.266 L/kg of ideal body weight and Cl was 1.21 L/h. kg of total weight following weekly doses. The plasma sample obtained at 10 min gave the best accuracy and precision predictions. When 5-FU was administered in multiple doses, the Cl of the drug in day 4 is reduced by 30.14% compared to day 1. The interoccasion variability was lower than interindividual variability for both V(d) and Cl, suggesting that it could be feasible to individualise dosage of 5-FU for subsequent cycles from data obtained in a previous one in an attempt to improve the therapeutic index of colorectal cancer treatment.

Intestinal transport of cefuroxime axetil in rats: absorption and hydrolysis processes.

Studies were performed using three cefuroxime axetil solutions (11.8, 118 and 200 microM) in three selected intestinal segments and one cefuroxime axetil solution (118 microM) in colon of anaesthetized rats. First-order absorption rate pseudoconstants, k(ap) and effective permeability coefficients, P(eff), were calculated in each set. Absorption of cefuroxime axetil can apparently be described as a carrier-mediated transport, which obeys Michaelis-Menten and first order kinetics in the proximal segment of the small intestine and a passive diffusion mechanism in the mean and distal segments. The absorption kinetic parameters for cefuroxime axetil were obtained: Vm=0.613 (0.440) microM min-1; Km=31.49(28.31) microM and ka=0.011(0.003) min-1. Parameters characterizing degradation of the prodrug were obtained in each intestinal segment: proximal segment k(dp)=0.0049(0.0003) min-1, mean segment, k(dm)=0.0131(0.0007) min-1 and distal segment k(dd)=0.019(0.0009) min-1. Therefore, in situ intestinal absorption of cefuroxime axetil in the proximal segment of the rat in the presence of variable concentrations of cefadroxil has been investigated in order to examine the inhibitory effect of cefadroxil on cefuroxime axetil transport. The data suggest that cefadroxil and cefuroxime axetil share the same intestinal carrier.