Pharmacokinetics and potential advantages of a new oral solution of levothyroxine vs. other available dosage forms.

To better understand the pharmacokinetics and potential advantages of a levothyroxine oral solution vs. tablets and soft gel capsules.4 randomized, 2-treatment, single-dose (600 mcg levothyroxine), 2-way crossover bioequivalence studies in 84 healthy subjects were analyzed. Samples were collected before dosing and until 48-72 h post-dose to calculate noncompartmental baseline-adjusted pharmacokinetic parameters: maximum concentration, time to maximum concentration, and area-under-the-concentration-time-curve from 0 to 48 h and from 0 to 2 h.Mean pharmacokinetic parameters (±standard deviation) for tablets, capsules and solution, respectively, were: area-under-the-concentration-time-curve from 0 to 2 h (ng*h/mL)=68.4±32.8, 64.4±24.4, 99.1±22.7; area-under-the-concentration-time-curve from 0 to 48 h (ng*h/mL)=1 632±424, 1 752±445, 1 862±439; maximum concentration (ng/mL)=67.6±20.9, 68.0±15.9, 71.4±16.0; time of maximum concentration (hours)=2.25±0.99, 2.38±1.58, 1.96±1.07. Overall rate and extent of exposure were not statistically different between formulations, but a faster onset of absorption for the solution was suggested (greater area-under-the-concentration-time-curve from 0 to 2 h and faster time to maximum concentration by an average of 30 min).Levothyroxine rate and extent of exposure are similar between tested formulations. The solution appears however to reach systemic circulation quicker as dissolution is not needed before absorption starts. The solution's greater early exposure and a faster time to maximal concentration of around 30 min may be of benefit to minimize drug-food interactions and deserves further investigations.

Pharmacodynamic equivalence of two orlistat capsule formulations in healthy volunteers under fed conditions.

OBJECTIVE: Orlistat is a reversible lipase inhibitor for obesity management. Orlistat exerts its pharmacological activity in the lumen of the stomach and small intestine by binding with the active site of gastric and pancreatic lipases, with the consequent inhibition of the systemic absorption of dietary fat. The undigested triglycerides are not absorbed, resulting in caloric deficit and positive effect in weight control. The objective of this study was to assess, using fat excreted in feces, the pharmacodynamic equivalence of orlistat when administered as generic and innovator capsule formulations. MATERIALS AND METHODS: A total of 18 healthy volunteers (12 males and 6 females) followed a 5-day run-in diet period in order to become accustomed to a high fat diet. Subjects were then randomized to receive under fed conditions oral doses of orlistat (120 mg) 3 times daily for 10 consecutive days as the generic (Ranbaxy Laboratories) or innovator (Xenical, Roche Laboratories, Nutley, NJ, USA) capsule formulations. Subjects followed a standardized diet (2,500 kcal/day, 30% as fat) for the entire study. Feces were collected over the last 2 days of the run-in period (baseline) and over the last 5 days of the 2 treatment periods. The amount of fat in meals and feces was assayed with a limit of detection of 0.1 and 0.2%, respectively. Fecal fat excretion over 24 hours (FFE(24), calculated as the percentage of amount of fat excreted in feces relative to the amount of fat ingested) was used as a pharmacodynamic endpoint to assess the therapeutic equivalence between the 2 orlistat formulations. An analysis of variance (ANOVA) was performed on FFE(24) parameters. RESULTS: Mean FFE(24) values at baseline and after repeated oral administrations of the generic and innovator formulations of orlistat were 6.48, 20.0 and 19.6%, respectively. The ratio of least-squares means (LSM) of FFE(24) of the generic to the innovator formulation was 99.1%, with 90% confidence intervals of 83.8 -114.5%. Adverse events for the generic and innovator products were similar in nature and frequency. CONCLUSION: Mean FFE(24) values were used as pharmacodynamic endpoints to assess equivalence between 2 formulations of orlistat. Results from this study suggest that pharmacodynamics of the generic capsule formulation of orlistat were similar to the marketed capsule formulation based on FFE(24) values.

Use of Freund's Complete Adjuvant (FCA) in inflammatory pain models: consequences on the metabolism and pharmacokinetics of the non-peptidic delta receptor agonist SNC80 in the rat.

This study was initiated to characterize the metabolism and pharmacokinetics of SNC80 in rats and to evaluate the impact of Freund's complete adjuvant (FCA)-induced inflammation on its body disposition. In vitro, the disappearance and intrinsic clearance (CL(int)) of SNC80 were measured following incubations in recombinant rat CYPs and in phenotyped liver microsomes from naive and 24-h FCA-treated rats. The unbound fraction (f(u)) was assessed by ultrafiltration. Based on the Clint values, in vivo blood clearance of 3.35 and 2.48 L/h/kg were predicted in naive and FCA-treated rats. In vivo, SNC80 was administered to naive and 24-h FCA-treated rats at 10 micromol/kg i.v. and 50 micromol/kg p.o. The naive animals showed high plasma clearance (3.1 L/h/kg), low renal clearance (<0.02 L/h/kg) and poor bioavailability (<4%). Following i.v. administration, plasma clearance was lower (22%) in FCA-treated vs. untreated rats. Despite the decreases in f(u) (approximately 30%) and CL(int) (approximately 40%) in vitro, in vivo the apparent bioavailability and oral clearance were not significantly different between FCA-treated and naive rats. Hepatic and possibly intestinal losses contribute to the low bioavailability of SNC80. Non-hepatic mechanisms may compensate for the decrease in plasma clearance found in FCA-treated rats, preventing an increase in the oral bioavailability of SNC80.

Liposomal tobramycin against pulmonary infections of Pseudomonas aeruginosa: a pharmacokinetic and efficacy study following single and multiple intratracheal administrations in rats.

OBJECTIVE: To determine the pharmacokinetics and efficacy of tobramycin against pulmonary infections of Pseudomonas aeruginosa in rats after intratracheal administration of conventional and liposomal formulations. METHODS: Male Sprague-Dawley rats were inoculated with 10(6) cfu of a mucoid variant of P. aeruginosa (MIC of tobramycin for PA 508 = 1 mg/l) and tobramycin (conventional or liposomal formulations) was administered in single (490 microg) and multiple dose (490 microg during 4 days) experiments. Rats were killed at multiple time points to determine the residual cfu of P. aeruginosa and tobramycin amounts in lungs. Pharmacokinetic parameters were calculated using a two-compartment model with NONMEM. RESULTS: Mean (+/-S.D.) elimination half-life (t(1/2beta)) and pulmonary exposure (AUC) of the conventional formulation were 14.0 +/-4.0 h and 663 +/-89 microg x h/lungs, respectively. The pharmacokinetic profile of liposomal tobramycin was markedly different, with a longer t(1/2beta) (34.4 +/-5 h, P < 0.05), resulting in an increased AUC (3890 +/-560 microg x h/lungs, P < 0.05). chi(2) analyses were carried out on cfu data distributed in the following categories: below 10(3), 10(3)-10(5), and above 10(5) cfu. In the single dose experiments, approximately 90% of the observations were above 10(5) cfu for both formulations. Significant differences in cfu distribution were observed after multiple treatments, with approximately 10% of the observations falling below 10(3) cfu of P. aeruginosa for the conventional formulation versus 30% for the liposomal formulation. CONCLUSION: The liposomal formulation of tobramycin promoted drug retention in lungs and improved its efficacy after multiple treatments.

Phase I study of topoisomerase I inhibitor exatecan mesylate (DX-8951f) given as weekly 24-hour infusions three of every four weeks.

Exatecan mesylate (DX-8951f) is a topoisomerase I inhibitor that has increased solubility and antitumor activity compared with other topoisomerase I inhibitors. The purpose of this study was to establish a safe dose of DX-8951f given as a weekly 24-h infusion 3 of every 4 weeks. DX-8951f was administered as a 24-h continuous infusion in escalating doses. Twenty-seven patients were treated with 81 courses of the drug. Dose-limiting toxicities included neutropenia, thrombocytopenia, and inability to administer all three doses in the first cycle. In minimally pretreated patients, a dose of 0.8 mg/m(2) was tolerable. In patients who were heavily pretreated, a slightly lower dose, 0.53 mg/m(2), was tolerated without any severe toxicities. Nonhematological toxicities were mild and consisted of mild diarrhea, asthenia, mild nausea, and constipation. Pharmacokinetic parameters could be well described with a one-compartment model in most patients, although the application of the one-compartment model probably resulted in an underestimated elimination half-life. In conclusion, the recommended Phase II dose for DX-8951f administered as a weekly 24-h infusion on a 3-of-4 week schedule is 0.8 mg/m(2) in minimally pretreated patients and 0.53 mg/m(2) in patients who are heavily pretreated.

Phase I and pharmacokinetic study of exatecan mesylate (DX-8951f): a novel camptothecin analog.

PURPOSE: To determine the maximum-tolerated dose (MTD), dose-limiting toxicity (DLT), pharmacokinetic (PK) profile, and recommended phase II dose of Exatecan mesylate (DX-8951f) when administered as a 24-hour continuous infusion every 3 weeks to patients with solid tumors. PATIENTS AND METHODS: Twenty-two patients with advanced solid tumors, all previously treated, and with performance status < or = 2, were entered. The starting dose of DX-8951f was 0.15 mg/m(2); the dose was escalated according to the modified continual reassessment method. The drug was administered until disease progression or until unacceptable toxic effects occurred. RESULTS: Seven dose escalations were completed, and a total of 53 courses were delivered (median, two courses; range, one to eight courses) during the study. At doses 1.2 mg/m(2) and lower, toxicities were mostly grade 1, primarily hematologic. In the initial cohort of three patients treated at 2.4 mg/m(2), grade 2 hematologic toxicity was observed. Of the six additional patients entered at 2.4 mg/m(2), three had grade 3 or 4 granulocytopenia. At doses higher than 2.4 mg/m(2), DLT granulocytopenia was observed. Nonhematologic toxicities, including nausea, vomiting, diarrhea, fatigue, and alopecia, were mild to moderate. Neither complete nor partial responses were observed, but four patients had stable disease. The PK profile of DX-8951f seemed linear at the doses administered. The plasma clearance, total volume of distribution, and terminal elimination half-life were approximately 3 L/h, 40 L, and 14 hours, respectively. CONCLUSION: The DLT of this DX-8951f schedule was granulocytopenia for minimally pretreated patients, and both granulocytopenia and thrombocytopenia for heavily pretreated patients. The MTD for both minimally and heavily pretreated patients was 2.4 mg/m(2). DX-8951f seems to have a linear PK profile on the basis of single-dose administration. The recommended phase II dose with this schedule is 2.4 mg/m(2) for minimally pretreated patients. A lower dose should be used for heavily pretreated patients.

A pharmacokinetic study of amoxycillin in febrile beagle dogs following repeated administrations of endotoxin.

The pharmacokinetics of amoxycillin was studied in nine male beagle dogs under healthy and febrile conditions. In Period 1, dogs received 20 mg/kg of an oral suspension of amoxycillin. Intravenous doses of saline, 2 and 20 microg/kg of endotoxin (LPS from Escherichia coli serotype) were administered to dogs (three per group) prior to administration of 20 mg/kg of amoxycillin in Period 2. Rectal temperature and behavioral changes were recorded and blood samples were collected over 12 h for pharmacokinetic analysis. Amoxycillin was assessed in plasma using liquid chromatography coupled with mass spectrometry. Plasma concentrations were analysed using a one-compartment model with lag-time for absorption using an iterative two-stage method. As compared with control groups, amoxycillin clearance decreased significantly with preliminary treatments of 2 microg/kg endotoxin (0.209 vs. 0.140 L/h kg, P < 0.05) and 20 microg/kg endotoxin (0.214 vs. 0.075 L/h kg, P < 0.05). As a result of this, the area under curve for the 2 and 20 microg/kg endotoxin groups increased significantly 100.4 vs. 149.4 microg h/mL (P < 0.05) and 99.2 vs. 277.7 microg h/mL (P < 0.05), respectively. Other drugs currently used for the treatment of fever and septic shock should be re-evaluated using a febrile animal model to avoid improper dose administration.

New insights into the pharmacokinetics and metabolism of (R,S)-ifosfamide in cancer patients using a population pharmacokinetic-metabolism model.

PURPOSE: To describe the pharmacokinetics of R- and S-Ifosfamide (IFF), and their respective 2 and 3 N-dechloroethylated (DCE) metabolites (R2-, R3-, S2, S3-DCE-IFF) in cancer patients. METHODS: (R,S)-IFF was administered (1.5 g/m2) daily for 5 days in 13 cancer patients. Plasma and urine samples were collected and analyzed using an enantioselective GC-MS method. An average of 97 observations per patient were simultaneously fitted using a pharmacokinetic-metabolism (PK-MB) model. A population PK analysis was performed using an iterative 2-stage method (IT2S). RESULTS: Auto-induction of IFF metabolism was observed over the 5 day period. Increases were seen in IFF clearance (R: 4 vs. 7 L/h; S: 5 vs. 10 L/h), and in the formation of DCE (R: 7 vs. 9%; S: 14 vs. 19%) and active metabolites (4-OHM-IFF; R: 71 vs. 77%; S: 67 vs. 71%). A novel finding of this analysis was that the renal excretion of the DCE metabolites was also induced. CONCLUSIONS: This population PK-MB model for (R,S)-IFF may be useful in the optimization of patient care, and gives new insight into the metabolism of (R,S)-IFF.

A population pharmacokinetic model for vancomycin in pediatric patients and its predictive value in a naive population.

The objectives of this study were to (i) construct a population pharmacokinetic (PK) model able to describe vancomycin (VAN) concentrations in serum in pediatric patients, (ii) determine VAN PK parameters in this population, and (iii) validate the predictive ability of this model in a naive pediatric population. Data used in this study were obtained from 78 pediatric patients (under 18 years old). PK analyses were performed using compartmental methods. The most appropriate model was chosen based on the evaluation of pertinent graphics and calculation of the Akaike information criterion test. The population PK analysis was performed using an iterative two-stage method. A two-compartment PK model using age, sex, weight, and serum creatinine as covariates was determined to be the most appropriate one to describe serum VAN concentrations. The quality of fit was very good, and the distribution of weighted residuals was found to be homoscedastic (Wilcoxon signed rank test). Fitted population PK parameters (mean +/- standard deviation) were as follows: central clearance (0.1 +/- 0.05 liter/h/kg), central volume of distribution (0.27 +/- 0.07 liter/kg), peripheral volume of distribution (0.16 +/- 0.07 liter/kg), and distributional clearance (0.16 +/- 0.07 liter/kg). The predictive ability of the developed model (including the above-mentioned covariates) was evaluated in a naive population of 19 pediatric patients. The predictability was very good. Precision (+/-95% confidence interval [CI]) (peak, 4.1 [+/-1.4], and trough, 2.2 [+/-0.7]) and bias (+/-95% CI) (peak, -0.58 [+/-2.2], and trough, 0.63 [+/-1.1] mg/liter) were significantly (P < 0.05) superior to those obtained using a conventional method (precision [+/-95% CI]: peak, 8.03 [+/-2. 46], and trough, 2.7 [+/-0.74]; bias: peak, -7.1 [+/-2.9], and trough, -1.35 [+/-1.2] mg/liter). We propose the use of this population PK model to optimize VAN clinical therapies in our institution and others with similar patient population characteristics.he object

A population analysis of nebulized (R)-albuterol in dogs using a novel mixed gut-lung absorption PK-PD model.

PURPOSE: The objectives of this study were to 1) construct a pharmacokinetic-pharmacodynamic (PK-PD) model, and 2) determine the PKs and PDs of (R)-albuterol when given by nebulization to 8 dogs for 7 consecutive days. METHODS: Four doses were evaluated (0.002, 0.02, 0.1, and 0.4 mg/kg/ day). Blood samples were obtained after drug administration on days 1 and 7. Heart rates (HR) were obtained during treatment days 1, 4 and 7. All (R)-albuterol plasma concentrations were fitted using a mixed gut-lung absorption 2-compartment PK model. Day-1, 4, and 7 HR data were co-modeled using a direct response model with Hilltype equations, including a necessary tolerance phenomenon. The population PK-PD analysis was performed with an iterative 2-stage methodology (IT2S). RESULTS: No chiral inversion was seen, and double absorption peaks on the plasma concentration versus time curves were observed in the majority of dogs. These were hypothesized to be the result of combined gut and lung absorption of (R)-Albuterol. Results indicated that 67% (range: 57-89%) of (R)-albuterol systemic exposure after nebulized administration is due to gut absorption. Mean population PK parameters were KaGI (10+/-5.7 h(-1)), KaLUNG (21+/-9.5 h(-1)), CLc/F (0.6+/-0.2 L/h/kg), CLd/F (1.4+/-0.5 L/h/kg), Vc/F (1.4+/-0.9 L/kg), and Vp/F (4.8+/-2.4 L/kg). (R)-albuterol administration was associated with an increase in the dogs heart rates. A tolerance effect related to the cumulative dose was observed and modeled. CONCLUSIONS: The presented PK-PD model appears to differentiate gut from lung absorption when (R)-albuterol is given by 15-minute nebulization to dogs. These results agree with the accepted hypothesis that most of the systemic exposure of (R)-albuterol after nebulized administration is due to gut absorption.

Enantioselective induction of cyclophosphamide metabolism by phenytoin.

The objective of this study was to investigate the effect of phenytoin (PHE) on cyclophosphamide (CP) disposition. CP was administered to 6 adult patients in a preparative regimen for bone marrow transplantation consisting of busulfan and CP. Three of the patients received PHE and the other 3 "control" patients received diazepam (DZP) as anti-epileptic prophylactic treatment. Plasma samples were collected at intervals up to 24 h after CP administration. The plasma concentrations of (R)- and (S)-CP and their respective N-dechloroethylated metabolites, (R)- and (S)-DCE-CP were simultaneously fitted using an enantiospecific 2-compartment pharmacokinetic (PK) model with Bayesian control estimation. DZP had no significant effect on the metabolism of CP and any of its PK parameters. PHE, however, increased significantly the formation of (S)-DCE-CP while having no effect on the formation of (R)-DCE-CP. These results suggest that different enzymes are responsible for the formation of (S)-DCE-CP from (S)-CP and (R)-DCE-CP from (R)-CP. Additionally, assuming that PHE does not affect the passive renal elimination of (R)- and (S)-CP, this analysis suggests that the clearance of both (R)- and (S)-CP to 4-hydroxy-CP (the activation pathway) is increased by PHE.

Piperacillin and tazobactam exhibit linear pharmacokinetics after multiple standard clinical doses.

A population pharmacokinetic (PK) analysis was conducted to determine if piperacillin and tazobactam exhibited linear or nonlinear PKs and if incremental changes in the daily dosage of piperacillin affected tazobactam PKs. Four dosage groups were evaluated after multiple dosing regimens. Concentrations of drug in plasma and amounts in urine were best fitted by using a linear two-compartment PK model. No significant difference between dosing groups was seen for any piperacillin or tazobactam PK parameters. Both drugs exhibited linear PKs when given at usual clinical doses. Tazobactam PKs did not appear to be affected by the different dosing regimens of piperacillin.

Pharmacokinetics of (R)- and (S)-cyclophosphamide and their dechloroethylated metabolites in cancer patients.

The complete pharmacokinetics (PK) of (R)- and (S)-cyclophosphamide (CP) and their dechloroethylated (DCE) metabolites have not been reported to date. We collected plasma and urine samples from 12 cancer patients and determined concentrations of both enantiomers of CP and DCE-CP using a chiral GC-MS method. All concentrations of (R)-CP, (S)-CP, (R)-DCE-CP, and (S)-DCE-CP were simultaneously modeled using an enantiospecific compartmental PK model. A population PK analysis was performed. Enantiospecific differences between (R)- and (S)-CP were found for the formation clearance of CP to the DCE metabolites (Clf: 0.25 (R) vs. 0.14 (S) L/h). No difference was found between enantiomers for Cl40H, Cld, Cl(m)R, ClT, or T1/2. In contrast to the adolescent and adult group of patients, a child (6 years old) appeared to have a very different PK and metabolic profile (Bayesian control analysis). Proportions of the (R,S)-CP doses transformed to the (R)-DCE- and (S)-DCE-CP were much higher (R: 25 vs. 1.9%, and S: 38 vs. 3.6%), while formation of active metabolites was much lower (R: 42 vs. 74%, and S: 48 vs. 77%). CP appears to be enantioselectively metabolized to the DCE metabolites. This PK model can evaluate the proportion of a CP dose that is transformed to toxic or active metabolites. It may therefore be used to optimize CP treatment, to identify important drug interactions and/or patients with an abnormal metabolic profile.

6',7'-Dihydroxybergamottin in grapefruit juice and Seville orange juice: effects on cyclosporine disposition, enterocyte CYP3A4, and P-glycoprotein.

BACKGROUND: 6',7'-Dihydroxybergamottin is a furanocoumarin that inhibits CYP3A4 and is found in grapefruit juice and Seville orange juice. Grapefruit juice increases the oral bioavailability of many CYP3A4 substrates, including cyclosporine (INN, ciclosporin), but intestinal P-glycoprotein may be a more important determinant of cyclosporine availability. OBJECTIVES: To evaluate the contribution of 6',7'-dihydroxybergamottin to the effects of grapefruit juice on cyclosporine disposition and to assess the role of CYP3A4 versus P-glycoprotein in this interaction. METHODS: The disposition of oral cyclosporine was compared in healthy subjects after ingestion of water, grapefruit juice, and Seville orange juice. Enterocyte concentrations of CYP3A4 were measured in 2 individuals before and after treatment with Seville orange juice. The effect of 6',7'-dihydroxybergamottin on P-glycoprotein was assessed in vitro. RESULTS: Area under the whole blood concentration-time curve and peak concentration of cyclosporine were increased by 55% and 35%, respectively, with grapefruit juice (P < .05). Seville orange juice had no influence on cyclosporine disposition but reduced enterocyte concentrations of CYP3A4 by an average of 40%. 6',7'-Dihydroxybergamottin did not inhibit P-glycoprotein at concentrations up to 50 micromol/L. CONCLUSIONS: 6',7'-Dihydroxybergamottin is not responsible for the effects of grapefruit juice on cyclosporine. Because the interaction did not occur with Seville orange juice despite reduced enterocyte concentrations of CYP3A4, inhibition of P-glycoprotein activity by other compounds in grapefruit juice may be responsible. Reduced enterocyte CYP3A4 by 6',7'-dihydroxybergamottin could be important for other drugs whose bioavailability is less dependent on P-glycoprotein.

Population pharmacokinetics of nitroglycerin and of its two metabolites after a single 24-hour application of a nitroglycerin transdermal matrix delivery system.

The purpose of this study was to assess the ability of our previously constructed pharmacokinetic (PK) model to describe nitroglycerin (GTN), 1,2-dinitroglycerin (1,2-GDN), and 1,3-dinitroglycerin (1,3-GDN) plasma concentrations after a single-dose application of a GTN transdermal matrix delivery system. GTN, 1,2-GDN, and 1,3-GDN plasma concentrations were simultaneously fitted using a first-pass, mixed-order release, one-compartment PK model. Population PK parameter values were derived using an iterative two-stage methodology (IT2S). Some of the mean PK parameters estimates and their interindividual variability (CV%) were the percentage of the delivered GTN dose reaching the systemic circulation released by a first-order process A, 53% (44); the 1,2-GDN and 1,3-GDN formation rate constants, k(f1)9 h(-1) (67) and k(f2) 0.5 h(-1) (38), respectively; the metabolite elimination rate constant, k(m) 1 h(-1) (27); GTN, 1,2-GDN, and 1,3-GDN volumes of distribution (Vc/F 6 L [45]), V2/F 78 L [51]), and V3/F 29 L [40]), respectively). Mean calculated elimination half-lives (t1/2+/-standard deviation [SD]) for GTN and the GDN metabolites were 7+/-4 minutes and 33+/-7 minutes, respectively. The proposed PK model fitted the observed plasma concentrations of GTN, 1,2-GDN, and 1,3-GDN very well. This new transdermal matrix delivery system appears to behave pharmacokinetically in the same manner as a transdermal reservoir delivery system (Transderm-Nitro, Ciba-Geigy, Mississauga, Canada).

On-line deconjugation of glucuronides using an immobilized enzyme reactor based upon beta-glucuronidase.

An immobilized enzyme reactor based upon beta-glucuronidase (BG-IMER) has been developed for the on-line deconjugation of substrates. The activity of the BG-IMER and its applicability to on-line deconjugation was investigated. The BG-IMER was coupled to a reversed-phase column (C8 or C18) and the latter column was used to separate substrates and products eluted from the beta-glucuronidase reactor. The activity of the BG-IMER was followed by measurement of percent deconjugation and the parameters investigated were: substrate concentration, pH (4 to 6), temperature (r.t., 37 degrees C), enzyme-substrate contact time using flow-rates of 0.1 to 1.0 min/min (15-1.5 min). The glucuronides used in the evaluation of the BG-IMER were: 4-methylumbelliferyl-beta-D-glucuronide, p-acetaminophen-beta-D-glucuronide, 3'-azido-3'-deoxythymidine-beta-D-glucuronide, phenyl-beta-D-glucuronide, chloramphenicol-beta-D-glucuronide, estradiol-17-beta-D-glucuronide and morphine-beta-D-glucuronide. The development of on-line HPLC deconjugation of glucuronide substrates using the BG-IMER will facilitate the identification of metabolites and quantification of aglycones in metabolic and pharmacokinetic studies.

Novel pharmacokinetic modelling of transdermal nitroglycerin.

PURPOSE: To construct a pharmacokinetic (PK) model and to determine population PK parameters of nitroglycerin (GTN), 1,2-dinitroglycerin (1,2-GDN), and 1,3-dinitroglycerin (1,3-GDN). METHODS: Data were obtained in thirty healthy volunteers following a single dose of a GTN reservoir transdermal patch. Blood samples were obtained just before and at 0.5, 1, 2, 3, 4, 6, 8, 12, 14, and 24 hours after the patch application and 1 hour after its removal. GTN, 1,2-GDN, and 1,3-GDN concentrations were determined using HPLC and simultaneously best fitted using a first-pass mixed-order release one-compartment PK model. Individual estimates (ADAPT-II) were used as priors for a population PK analysis (IT2S). Fitted parameters included the percentage (A) of the nitroglycerin dose reaching the systemic circulation that was released from the patch by a first-order process (K1); two absorption (ka1 and ka2), two metabolite formation (kf1 and kf2) and one metabolite elimination (k(m)) rate constants; and three volumes of distribution Vc/F, V2/F and V3/F. RESULTS: Nitroglycerin mean population parameter estimates and inter-individual variability (CV%) were: A 35% (65), K1 0.06 h-1(91), ka1 5 h-1(46), ka2 0.47 h-1(39), kf1 11 h-1(42), kf2 0.6 h-1(34), k(m) 1.4 h-1(29), V0/F 6 L(31), V2/F 73 L(34), and V3/F 23 L(29). The average elimination half-lives for GTN and the two metabolites were 5 and 32 minutes, respectively. CONCLUSIONS: The proposed PK model fitted observed concentrations of GTN, 1,2-GDN and 1,3-GDN very well. This model should be useful to predict drug and metabolite concentrations and to assess bioequivalence of two transdermal formulations.

Ability of a first-pass pharmacokinetic model to characterize cyclosporine blood concentrations after administrations of Sandimmune or Neoral formulations.

Most recent cyclosporine (CsA) pharmacokinetic (PK) studies have focused on noncompartmental analysis. Because CsA undergoes significant first-pass elimination after oral dosing, the most appropriate compartment model may need to take this process into account for the construction of a valid population PK model for Sandimmune (SAN) and Neoral (NEO) formulations. Twenty patients with cardiac transplants were stabilized for at least 4 weeks on a certain dose of SAN, then changed to the same daily dose of NEO. Blood samples were obtained at times 0, 1, 2, 3, 4, 6 and 12 hours after dosing at steady state. Pharmacokinetic modeling was performed using ADAPT II. Quality of fit was assessed by visual graph inspections, R2 values, and Akaike criterion test. Eight pharmacokinetic models were constructed and evaluated. These included one- and two-compartment with and without a first-pass effect and a time-lag. Neoral and SAN data were consistently best fitted using a two-compartment or the two-compartment first-pass model. However, a time-lag process was found to be necessary for SAN. The use of a two-compartment first-pass with (SAN) or without (NEO) a time-lag process appears to fit CsA concentrations at least as well as a two-compartment model. This first-pass model may be very useful for population pharmacokinetics and Bayesian control analysis.

On-line deconjugation of chloramphenicol-beta-D-glucuronide on an immobilized beta-glucuronidase column. Application to the direct analysis of urine samples.

An immobilized HPLC column has been developed for the on-line deconjugation of beta-glucuronides. The enzymatic activity of this column has been previously demonstrated [1]. This study reports on the application of the immobilized beta-glucuronidase column to the analysis of glucuronide metabolites in the urine. The system utilized in this work was composed of an internal-surface reversed-phase (ISRP) column (50 x 4.6 mm) containing a hydrophobic inner phase and a hydrophilic outer phase, a beta-glucuronidase immobilized enzyme reactor (BG-IMER) column (50 x 4.6 mm) and a C8 reversed-phase column (150 x 4.6 mm). The columns were connected with three six-port switching valves. A coupled-column procedure was developed for urine samples containing chloramphenicol-beta-D-glucuronides (0.07-1.1 mM/injection). Urine samples were injected into the ISRP column where the glucuronides were separated from the biological matrix, with matrix contaminants eluting off-line to waste. Eluent from the ISRP column containing the glucuronides was then transferred on-line to the beta-glucuronidase column for deconjugation and passed directly on-line to the C8 column. In this portion of the chromatographic procedure, the mobile phase consisted of 0.01 M ammonium acetate at pH 6.7. The analyte concentrated on the top of the reversed-phase column was then eluted using a gradient mobile phase system of acetonitrile and 0.01 M ammonium acetate (pH 5.0) and detected at UV wavelength of 280 nm.

Phenytoin-induced alteration in the N-dechloroethylation of ifosfamide stereoisomers.

CASE: A suspected alteration in ifosfamide (IFF) metabolism and pharmacokinetics was observed in a pediatric patient receiving phenytoin. METHODS: Sequential plasma samples were obtained and analyzed for the concentrations of the enantiomers of IFF and their N-dechloroethylated metabolites (DCE-IFF) using a validated enantioselective gas chromatographic-mass spectrometric method. RESULTS: In the phenytoin-treated patient, the metabolic formation of IFF enantiomers was increased and the metabolic pattern of the N-dechloroethylation altered from non-phenytoin-treated patients: (R)-3-DCE IFF > > (S)-3-DCE-IFF = (S)-2-DCE-IFF > (R)-2-DCE-IFF (control) vs (S)-3-DCE-IFF = (S)-2-DCE-IFF > (R)-3-DCE-IFF > > (R)-2-DCE-IFF (patient). CONCLUSIONS: Previous studies have attributed the production of the (S)-2-DCE-IFF and (S)-3-DCE-IFF metabolites to the activity of CYP2B6 and (R)-2-DCE-IFF and (R)-3-DCE-IFF to the activity of CYP3A4. The results suggest that phenytoin induced the activity of CYP2B6 to a greater extent than CYP3A4. In addition, the patient, who was at least partially refractory to several other treatments, went into remission after IFF treatment suggesting that phenytoin pretreatment might increase IFF therapeutic efficacy.