Previously in vitro-in vivo extrapolation (IVIVE) with the Simcyp Clearance and Interaction Simulator has been used to predict the clearance of 15 clinically used drugs in humans. The criteria for the selection of the drugs were that they are used as probes for the activity of specific cytochromes P450 (CYPs) or have a single CYP isoform as the major or sole contributor to their metabolism and that they do not exhibit non-linear kinetics in vivo. Where data were available for the clearance of the drugs in at least three animal species, the predictions from IVIVE have now been compared with those based on allometric scaling (AS). Adequate data were available for estimating oral clearance (CLp.o.) in 9 cases (alprazolam, sildenafil, caffeine, clozapine, cyclosporine, dextromethorphan, midazolam, omeprazole and tolbutamide) and intravenous clearance in 6 cases (CLi.v.) (cyclosporine, diclofenac, midazolam, omeprazole, theophylline and tolterodine). AS predictions were based on five different methods: (1) simple allometry (clearance versus body weight); (2) correction for maximum life-span potential (CL x MLP); (3) correction for brain weight (CL x BrW); (4) the use of body surface area; and (5) the rule of exponents. A prediction accuracy was indicated by mean-fold error and the Pearson product moment correlation coefficient. Predictions were considered successful if the mean-fold error was

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT• Several cytochromes P450 (CYPs) have been implicated in the metabolism of methadone, but there is no consensus on their relative contributions to overall disposition and hence variability in response. WHAT THIS STUDY ADDS• Variability in CYP3A4 activity has statistically significant but nonetheless modest influence on the oral clearance of methadone and its enantiomers. • However, CYPs 1A2 and 2D6 appear to have no impact at all. AIMSTo investigate the influence of different cytochrome P450 (CYP) activities and other potential covariates on the disposition of methadone in patients on methadone maintenance therapy (MMT). METHODSEighty-eight patients (58 male; 21-55 years; 84 White) on MMT were studied. CYP2D6 activity [3 h plasma metabolic ratio of dextromethorphan (DEX) to dextrorphan (DOR)] was determined in 44 patients (29 male; 24-55 years), CYP1A2 activity (salivary caffeine elimination half-life) in 44 patients (21 male; 24-55 years) and CYP3A activity (oral clearance of midazolam) in 49 patients (33 male; 23-55 years). Data on all three CYPs were obtained from 32 subjects. Total plasma concentrations of (RS)-methadone and total and unbound plasma concentrations of both enantiomers were measured by LC/MS. Population pharmacokinetics and subsequent multiple regression analysis were used to calculate methadone oral clearance and to identify its covariates. RESULTSBetween 61 and 68% of the overall variation in total plasma trough concentrations of (RS)-, (R)-and (S)-methadone was explained by methadone dose, duration of addiction before starting MMT, CYP3A activity and illicit morphine use. CYP3A activity explained 22, 16, 15 and 23% of the variation in unbound (R)-, unbound (S)-, total (RS)-and total (S)-methadone clearances, respectively. Neither CYP2D6 nor CYP1A2 activity was related to methadone disposition. CONCLUSIONSCYP3A activity has a modest influence on methadone disposition. Inhibitors and inducers of this enzyme should be monitored in patients taking methadone.

The purpose of this study was to create an optimized method for preparation of 5-fluorouracil-loaded magnetic poly lactic-co-glycolic acid nanocapsules and to investigate its potential as multifunctional carriers to deliver therapeutic agents for tumor-targeted therapies. The in vitro release of the newly synthesized 5-fluorouracil-loaded poly lactic-co-glycolic acid magnetic nanocapsules was investigated in phosphate-buffered saline medium using the dialysis method. In vivo release studies of the magnetic nanocapsules were performed in rabbits. Finally, the targeting properties, anti-tumor, and pro-apoptotic effects of this new magnetic nanocapsule on CT26 cells allograft model were studied. The effective diameter of nanocapsules was 67.2 nm. In vivo release investigations showed that 5-fluorouracil has a sustained release profile, prolonged lifetime in the rabbit plasma, and increased tissue appetency when loaded into the magnetic nanocapsule. Magnetic resonance imaging confirmed that the magnetic nanocapsules were successfully targeted to the tumor. Additionally, the anti-tumor studies revealed that the targeted therapy with magnetic nanocapsules containing 5-fluorouracil effectively inhibits the growth of tumors compared with 5-fluorouracil alone (P < 0.01). The present study demonstrates that this new magnetic nanocapsule can be considered a new nanotechnology-based cancer chemotherapy agent in vivo.