Pharmacokinetics of mycophenolate mofetil in stable pediatric liver transplant recipients receiving mycophenolate mofetil and cyclosporine

There are few pharmacokinetic data for mycophenolate mofetil (MMF) when used in combination with cyclosporine (CsA) in pediatric liver transplant recipients. The aim of this study was to assess the pharmacokinetics of MMF in stable pediatric liver transplant patients and estimate the dose of MMF required to provide a mycophenolic acid (MPA) exposure similar to that observed in adult liver transplant recipients receiving the recommended dose of MMF (target area under the plasma concentration-time curve from 0 to 12 hours [AUC(0-12)] for MPA of 29 mug.hour/mL in the immediate posttransplantation period and 58 microg x hour/mL after 6 months). A 12-hour pharmacokinetic profile was collected for 8 pediatric patients (mean age 20.9 months) on stable doses of MMF and CsA who had received a liver transplant > or = 6 months prior to entry and who had started on MMF within 2 weeks of transplantation. Mean MMF dosage was 285 mg/m(2) (range, 200-424 mg/m(2)). Of 8 patients, 7 had a MPA AUC(0-12) (range, 11.0-37.2 microg x hour/mL) well below the target. One patient had an AUC(0-12) > or = 58 microg x hour/mL but was considered an outlier and was excluded from analyses. Mean MPA AUC(0-12) and maximum plasma concentration values were 22.7 +/- 10.5 microg x hour/mL and 7.23 +/- 3.27 microg/mL, respectively; values normalized to 600 mg/m(2) (the approved pediatric dose in renal transplantation) were 47.0 +/- 21.8 microg x hour/mL and 14.5 +/- 4.21 microg/mL. In conclusion, assuming that MPA exhibits linear pharmacokinetics, when used in combination with CsA, a MMF dose of 740 mg/m(2) twice daily would be recommended in pediatric liver transplant recipients to achieve MPA exposures similar to those observed in adult liver transplant recipients. This finding should be confirmed by a prospective trial.

Concentrations of gemifloxacin at the target site in healthy volunteers after a single oral dose

Free gemifloxacin concentrations in the interstitial space fluid of skeletal muscle and subcutaneous adipose tissue were measured by means of in vivo microdialysis to characterize the ability of gemifloxacin to penetrate human soft tissues. Twelve healthy volunteers received a single oral dose of 320 mg of gemifloxacin. The mean areas under the concentration-time curves from 0 to 10 h (AUC(0-10)) were significantly higher for soft tissue than for unbound gemifloxacin in plasma (P < 0.05). The ratios of the mean AUC(0-10) for tissue to the AUC(0-10) for free gemifloxacin in plasma were 1.7 +/- 0.7 (mean +/- standard deviation) for skeletal muscle and 2.4 +/- 1.0 for adipose tissue. The AUC(0-24) ratios for free gemifloxacin in tissues to the MIC at which 90% of frequently isolated bacteria are inhibited were close to or higher than 100 h. Therefore, based on pharmacokinetic and pharmacodynamic calculations, we conclude that gemifloxacin might be a useful therapeutic option for the treatment of soft tissue infections.

Human microdialysis

Microdialysis has been used extensively in animal studies for decades and in human pharmacokinetic studies for about 10 years. Microdialysis is based on the passive diffusion of a compound along its concentration gradient from the tissue through the membrane into the dialysate. Microdialysis samples from the interstitial space which is a defined, anatomical compartment; there is no net loss of body fluid; the sample is "purified" and no enzymatic degradation takes place because proteins do not pass through the probe membrane into the dialysate; microdialysis data relate to the intact molecule; time resolution is high compared to biopsy and skin blister techniques; radioabelling or induction of a magnetic response is not needed; microdialysis is also an alternative method to determine protein binding of a compound in vivo; microdialysis can readily be set up in clinical research units without expensive infrastructure. Microdialysis has been used to measure tissue concentrations of endogenous compounds and to investigate the tissue penetration of drugs in a variety of tissues in humans in vivo in both healthy volunteers and patients. Microdialysis data have also been used in PK-PD modelling and to obtain concentration-response relationships locally in tissues in vivo. There are also studies combining microdialysis with imaging techniques, e.g. PET. Microdialysis data may be used in early studies to select the appropriate compound, to optimise dosing regimens and to investigate the kinetic and dynamic consequences in the tissues of drug-drug and drug-disease interactions. Microdialysis can also be used in late phase studies to provide tissue concentration data in support of therapeutic efficacy trials or to create a niche for an already marketed drug. FDA and CPMP documents emphasise the value and importance of human tissue drug concentration data and support the use of microdialysis in humans to obtain such information. Microdialysis can satisfy regulatory requirements by providing data on drug concentrations in a well-defined anatomical tissue compartment at or close to the effect target site. Microdialysis is a versatile technique because of its multifaceted utility, low cost, ease of use, adaptability to different types of compounds and its feasibility for a number of organs and tissues. Equipment and probes for use in various organs have been commercially available for years.

Increased blood-brain barrier permeability of morphine in a patient with severe brain lesions as determined by microdialysis

Intracerebral microdialysis was utilised to obtain information regarding how morphine is transported across the blood-brain barrier (BBB). In a patient with a severe brain injury, we measured simultaneously unbound extracellular fluid (ECF) concentrations of morphine in human brain and in subcutaneous fat tissue, which were compared to morphine levels in arterial blood. This report shows an increase in morphine levels near the trauma site in the brain compared to uninjured brain tissue. The half-life of morphine in uninjured and injured brain tissue of 178 min and 169 min, respectively, were comparable but were longer than in blood (64 min) and adipose tissue (63 min). This indicates that morphine is retained in brain tissue for a longer time than what could be expected from the blood concentration-time profile. These results show the potential of the microdialysis technique in providing new information regarding the pharmacokinetics of drug in the human brain close to the trauma site and in macroscopically intact tissue.

Quantification of effect delay and acute tolerance development to morphine in the rat

In this study the effect delay and the development of acute tolerance to morphine's antinociceptive effect were investigated in the rat. The antinociceptive response induced after three different short infusions of morphine was measured by using the electrical stimulation vocalization method. Three treatment groups received morphine hydrochloride over 10, 60 and 180 min, targeting maximal plasma concentrations of 25 microM at the end of the infusion. The maximal concentrations of morphine obtained for the three groups were 29.4 +/- 3.7, 26.7 +/- 4.7 and 28.9 +/- 7.3 microM, respectively. Both an effect delay and acute tolerance were observed. For each group, the peak effects were 248 +/- 82, 337 +/- 116 and 303 +/- 49% above base line, at 35, 65 and 90 min after the start of the infusions. A pharmacokinetic-pharmacodynamic model with separate effect and tolerance compartments (the ET-model) well described the antinociceptive response over time. The antinociceptive effect was best described by using a linear model, whereas acute tolerance was best described by using an Emax model. The rates of drug equilibration between the plasma and the effect compartment (ke0) and the plasma and the tolerance compartment (kt0), expressed as half-lives, were 34 +/- 2 and 48 +/- 4 min, respectively. High concentrations were required for the acute tolerance to develop (TC50 of 17 microM).