Rosario Calvo

Effect of P-glycoprotein inhibition on methadone analgesia and brain distribution in the rat.

Methadone is an opiate drug that has been identified as an in‐vitro substrate of the efflux pump P‐glycoprotein (P‐gp), active in the intestinal epithelium and in the blood–brain barrier (BBB), among other sites. The objective of this study was to test in vivo, in the rat model, the role of P‐gp modulation on the analgesic effect and brain uptake of methadone, as well as identify the most relevant site via dual oral and intravenous (i.v.) experiments. The P‐gp specific inhibitor (valspodar or PSC833) was preadministered (10 mg kg−1 i.v.) to test groups. Analgesia was measured using the tailflick test. The ED50 for oral methadone (2, 3, 6 and 8 mg kg−1) decreased three‐fold in valspodar groups compared with controls (2.23 + 0.002 mg kg−1 and 6.07 + 0.07 mg kg−1; P < 0.0001). The overall analgesic effect (% antinociception) was elevated 3.1 times in pretreated compared with control rats (90.65% + 0.22 vs 29.23% + 14.0; P < 0.01) after 6 mg kg−1 oral methadone and 2.8 times after i.v. (0.35 mg kg−1) administration (91.75% + 4.27 vs 32.45% + 9.0; P < 0.01). The brain:plasma distribution ratio was higher in pretreated animals and AUCbrain (overall brain concentration) was 6 times higher after oral methadone and 4 times higher after i.v. compared with controls, disproportionally increased relative to plasma, implying an active process at the BBB. P‐gp, and hence substrate comedication, plays a critical role in the evolution of the methadone analgesic effect and in its brain uptake, independent of the administration route.

Plasma protein carbamylation and decreased acidic drug protein binding in uremia.

The effects of in vitro carbamylation of plasma with potassium cyanate on drug‐protein binding have been investigated. Potassium cyanate added to samples of normal plasma and incubated for 30 to 150 min induced time‐related plasma protein carbamylation. Carbamylation of plasma did not influence quinidine protein binding, but resulted in decreased salicylate binding. The increased free fraction of salicylate in plasma correlated with the degree of carbamylation of plasma proteins (r = 0.99; p < 0.001). Plasma from patients with chronic renal disease showed varying degrees of plasma protein carbamylation, correlating with the values of free plasma salicylate (r = 0.80; p < 0.05). Scatchard plots for sulfadiazine binding in plasma from patients with uremia and in normal plasma carbamylated in vitro with potassium cyanate showed changes in the 2 groups when compared with those in normal individuals. If cyanate is produced in vivo from urea in patients with uremia, plasma protein carbamylation may play a role in the decreased plasma protein binding of some acidic drugs.

Serum protein binding of propofol in patients with renal failure or hepatic cirrhosis

Background: Serum protein binding is a limiting factor in the access of drugs to the central nervous system. Disease‐induced modifications of the degree of binding may influence the effect of anesthetic drugs.

Pharmacokinetic-Pharmacodynamic Modelling of the Antipyretic Effect of Two Oral Formulations of Ibuprofen

ObjectiveTo analyse the population pharmacokinetic-pharmacodynamic relationships of racemic ibuprofen administered in suspension or as effervescent granules with the aim of exploring the effect of formulation on the relevant pharmacodynamic parameters.DesignThe pharmacokinetic model was developed from a randomised, crossover bioequivalence study of the 2 formulations in healthy adults. The pharmacodynamic model was developed from a randomised, multicentre, single dose efficacy and safety study of the 2 formulations in febrile children.Patients and participantsPharmacokinetics were studied in 18 healthy volunteers aged 18 to 45 years, and pharmacodynamics were studied in 103 febrile children aged between 4 and 16 years with bodyweight ≥25kg.MethodsThe pharmacokinetic study consisted of two 1-day study occasions, each separated by a 1-week washout period. On each occasion ibuprofen 400mg was administered orally as suspension or granules. The time course of the antipyretic effect was evaluated in febrile children receiving a single oral dose of 7 mg/kg in suspension or 200 or 400mg as effervescent granules. During the pharmacodynamic analysis, the predicted typical pharmacokinetic profile (based on the pharmacokinetic model previously developed) was used.ResultsThe disposition of ibuprofen was described by a 2-compartment model. No statistical differences (p > 0.05) were found between the 2 formulations in the distribution and elimination parameters. Absorption of ibuprofen from suspension was adequately described by a first-order process; however, a model with 2 parallel first-order input sites was used for the drug given as effervescent granules, leading to time to reach maximum drug concentration (tmax) values of 0.9 and 1.9 hours for suspension and granules, respectively. The time course of the antipyretic effect was best described using an indirect response model. The estimates (with percentage coefficients of variation in parentheses) of Emax (maximum inhibition of the zero-order synthesis rate of the factor causing fever), EC50 (plasma concentration eliciting half of Emax), n (slope parameter) and kout (first order rate constant of degradation) were 0.055 (10), 6.16 (14) mg/L, 2.71 (18) and 1.17 (23) h−1, respectively, where T0 is the estimate of the basal temperature, 38.8 (1) °C. No significant (p > 0.05) covariate effects (including pharmaceutical formulation) were detected in any of the pharmacodynamic parameters.ConclusionsBecause of the indirect nature of the effect exerted by ibuprofen, the implications of differences found in the plasma drug concentration profiles between suspension and effervescent granules are less apparent in the therapeutic response.

Simultaneous determination of tramadol and its major active metabolite O-demethyltramadol by high-performance liquid chromatography with electrochemical detection

A novel, highly sensitive method was developed for simultaneous determination of tramadol and its main active metabolite O-demethyltramadol (ODMT) in rat plasma. The method involves a single-step extraction procedure and a specific determination by high-performance liquid chromatography with electrochemical detection, using an ethoxy analogue of tramadol (L-233) as internal standard. The dual-electrode detector was operated in the oxidation-screening mode. Absolute recoveries of tramadol and ODMT were about 80%. Calibration curves were linear over a concentration range of 10-1000 ng/ml for ODMT and 10-10000 ng/ml for tramadol with intra- and inter-day coefficients of variation not exceeding 10% and 15%, respectively. The limit of quantification for tramadol and ODMT was lower than 15 ng/ml and 10 ng/ml using 100 microl of plasma, respectively. The described method allows an adequate characterization of the plasma vs. time profiles for both compounds.

Plasma protein binding of etomidate in patients with renal failure or hepatic cirrhosis.

SummaryThe influence of renal failure and of hepatic cirrhosis on the plasma protein binding of etomidate, an intravenous anaesthetic agent of basic nature, has been investigated. The percentage of free etomidate in plasma containing 1 μg/ml was markedly increased in patients with renal failure and in patients with hepatic cirrhosis, when compared with a group of healthy volunteers (43.4 ± 2.9% and 44.2 ± 2.1 versus 24.9 ± 1.4%). This decrease in binding correlated inversely with serum albumin levels in both conditions (r = −0.88 and r = −0.72, respectively) but a slight decrease in the amount bound per mole of albumin was also apparent in both types of disease.

Influence of formulation on propofol pharmacokinetics and pharmacodynamics in anesthetized patients.

Background:  In anesthesia with propofol, variability persists besides sophisticated effect targeting. Drug formulation may be another factor. We have analyzed, retrospectively, the pharmacokinetics (PK) and pharmacodynamics (PD) in monitored surgery patients anesthetized with one each of five formulations of propofol.

Serum protein binding of propofol in critically ill patients

Background: Disease‐induced modifications in the level of serum proteins may change the degree of binding of drugs highly bound to serum proteins.

Influence of plasma-protein binding on analgesic effect of methadone in rats with spontaneous withdrawal.

The effect of spontaneous withdrawal on α1‐acid glycoprotein (AAG) levels and methadone protein binding has been studied in the rat.

THE EFFECT OF CHANGES IN GASTRIC pH INDUCED BY OMEPRAZOLE ON THE ABSORPTION AND RESPIRATORY DEPRESSION OF METHADONE

The effect of omeprazole (2 mg kg-1 i.v.) on respiratory depression induced in rats by acute oral methadone administration (5 mg kg-1) was examined and compared with control animals that only received methadone. Quantitative assessments of arterial Pco2,Po2, pH, and respiratory rate were employed as criteria for evaluation. Intragastric pH was measured in each rat immediately before and 2 h after methadone. Plasma concentration of methadone was measured for 3 h. The relationship between drug effect and the systemic bioavailability of methadone, measured as the area under the plasma concentration-time curve (AUC0-180), was also evaluated. The intensity of the methadone-induced respiratory depression was significantly greater in the omeprazole group than in control rats. A significant variation (p < 0.01) in all respiratory parameters was detected from 30 to 120 min after methadone. Omeprazole caused a significant increase in methadone levels (Cmax = 156 +/- 6.5 ng mL-1 against 51 +/- 5.8 ng mL-1 in control; p < 0.05). AUC0-180 was higher (p < 0.05) after omeprazole treatment (18.6 +/- 1.4 micrograms mL-1 min) than in control (6.8 +/- 0.6 microgram mL-1 min). Two hours after treatment with omeprazole, intragastric pH values were significantly elevated (4.7 +/- 0.1 against 2.2 +/- 0.04) and continued increasing, being 6.4 +/- 0.1 at the end of the experiment. Correlation was observed between intragastric pH and the area under the effect- (respiratory depression-) time curve (r = 0.74; p < 0.001). A relationship between plasma methadone levels at 120 min and gastric pH (r = 0.92; p < 0.001) was detected. A significant correlation between the area under the effect-time curve (0-120 min) and AUC0-180 has been also observed (r = 0.90; p < 0.01). These pharmacokinetic and pharmacodynamic changes could be gastric pH dependent because they were mimicked when gastric pH was experimentally modified by bicarbonate whereas opposite results were obtained with acidic pH2 solution.