In Vivo Microdialysis Sampling in the Bile,Blood, and Liver of Rats to Study the Disposition of Phenol

Methods for continuous in vivo sampling in the bile, blood, and liver extracellular fluid are described. These methods are based on microdialysis sampling in anesthetized rats. A new flow-through microdialysis probe is described for sampling bile while maintaining normal bile flow. All three sites are simultaneously and continuously sampled to provide concentration–time profiles at multiple sites in a single experimental animal. This technique is demonstrated by studying the hepatic metabolism and biliary excretion of phenol in rats. Following an i.v. infusion of phenol, the major hepatic metabolite was found to be phenyl-glucuronide. Hydroquinone and 2-glutathionyl–hydroquinone were also detected but at lower concentrations. A similar pattern of metabolites was found in the bile and blood. For all of the metabolites, bile concentrations are higher than liver concentrations, indicating that the metabolites are actively excreted into the bile.     

Development of tissue-targeted metabonomics. Part 1. Analytical considerations

Tissue-targeted metabonomics provides tissue specific metabolic information while still retaining the profiling approach of traditional metabonomics. Microdialysis sampling is used to generate site-specific samples of endogenous metabolites. The dialysate samples are subjected to proton NMR analysis with data analysis by principal components analysis and partial least squares regression. In this study, sample and data pretreatment methods were examined for their impact on the quality of the data analysis. Specifically, the effects of speed vacuuming, sample solubility, sample pH stability, and sample storage stability were examined. Data pretreatment methods examined included the effects of standardization and normalization to internal standards. In addition, the ability of tissue-targeted metabonomics to generate time trend data was explored and more fully characterized using principal components analysis and partial least squares regression.     

Dermal Microdialysis Sampling in Vivo

Microdialysis sampling of the dermis in vivo was accomplished using a linear microdialysis probe. In contrast to previous studies using a commercial cannula-style microdialysis probe, the linear probe had no effect on the flux of drug through the skin in vitro. The extent of tissue damage in vivo due to probe implantation was evaluated by histological examination and microdialysis delivery studies. Tissue damage due to implantation of the linear probe was minimal with no bleeding or edema observed. Infiltration of lymphocytes into the tissue was observed beginning 6 hours after probe implantation with scar tissue beginning to form after approximately 32 hours. The infiltration of lymphocytes had no effect on the behavior of implanted microdialysis probes. Delivery of 5-fluorouracil was between 20 and 25% for six different probes implanted in six different animals demonstrating good probe-to-probe and implantation-to-implantation reproducibility. Constant delivery was maintained for at least 24 hours in all cases indicating that experiments of at least 24 hour duration are feasible. The dermal concentration of topically applied 5-FU cream, Efudex^®, was continuously monitored by an implanted microdialysis probe demonstrating the feasibility of this technique as for monitoring skin drug levels in vivo. The dermal concentration of 5-FU following topical application was approximately 40-fold higher for in vitro excised skin than for in vivo intact skin.     

Comparison of in vitro BBMEC permeability and in vivo CNS uptake by microdialysis sampling

The studies presented in this report were designed to assess the correlation of the bovine brain microvessel endothelial cell (BBMEC) apparent permeability coefficient (P_app) and in vivo BBB penetration using microdialysis sampling. A mathematical model was developed to describe the relationship of brain extracellular fluid (ECF) concentration to free drug in plasma. The compounds studied have a broad range of physico-chemical characteristics and have widely varying in vitro and in vivo permeability across the blood–brain barrier (BBB). BBMEC permeability coefficients vary in magnitude from a low of 0.9×10⁻⁵ cm/s to a high value of 7.5×10⁻⁵ cm/s. Corresponding in vivo measurements of BBB permeability are represented by clearance (CL_in) into the brain ECF and range from a low of 0.023 μl/min/g to a high of 12.9 μl/min/g. While it is apparent that in vitro data from the BBMEC model can be predictive of the in vivo permeability of a compound across the BBB, there are numerous factors both prior to and following entry into the brain which impact the ultimate uptake of a compound. Even in the presence of high BBB permeability, factors such as high plasma protein binding, active efflux across the BBB, and metabolism within the CNS can greatly limit the ultimate concentrations achieved. In addition, concentrations in the intracellular space may not be the same as concentrations in the extracellular space. While these data show that the BBMEC permeability is predictive of the in vivo BBB permeability, the complexity of the living system makes prediction of brain concentrations difficult, based solely on the in vitro measurement.     

Electrochemical investigations of immunologically reactive procainamide metabolites

As a result of the implication of N-oxidized procainamide metabolites in drug-related lupus (DRL), the electrochemical behaviour of these compounds was investigated and a coulometric synthesis of the nitroso derivative developed using a previously described carbon packed bed bulk electrolysis flow cell. The electrochemical characterization of the parent p-substituted aromatic amine and the N-oxidized derivatives was achieved through systematic comparison with previously well described aromatic amine and nitro systems using cyclic voltammetry and liquid chromatography with electrochemical detection (LC-EC). Chromatographically assisted hydrodynamic voltammetry indicated current limiting plateau potentials of 0.45 and -0.2 V versus Ag/AgCl, respectively, for synthetically prepared procainamide hydroxylamine and electrolytically prepared nitrosoprocainamide. Reaction characterization and binding behaviour is described for each of the procainamide metabolites following in vitro incubations with cysteine, glutathione, ascorbic acid and mouse haemoglobin.     

Investigation of microdialysis sampling calibration approaches for lipophilic analytes: Doxorubicin

Microdialysis is an important sampling technique in many pharmacokinetics and pharmacological studies. Applying microdialysis to lipophilic analytes can be difficult as low extraction efficiencies are generally obtained with these types of analytes. In this investigation, the effects of applying microdialysis to the lipophilic compound, doxorubicin are discussed. Using varying concentrations of doxorubicin (DOX) from 1 to 20 μM, in vitro probe calibrations were performed by delivery, recovery and no-net flux. Any changes in the extraction efficiencies calculated were monitored through the addition of an internal standard, antipyrine. DOX was chosen as a representative lipophilic analyte because its red color could be visibly monitored on the probe. At higher concentrations the probe membrane became redder. For delivery experiments, the inlet of the probe was more highly colored than the outlet. The opposite was true for recovery experiments, in which the outlet was more highly colored than the inlet. It was observed that while antipyrine was well-behaved in these experiments, for DOX the extraction efficiency determined by recovery was not the same as the extraction efficiency determined by delivery (p < 0.005, 0.05). It was further observed that for DOX the extraction efficiency determined by a no-net flux experiment was in good agreement with the value determined by delivery and not that determined by recovery. However, the only point in which no DOX was present in the perfusate was not on the no-net flux line. In addition, the transport of DOX across the microdialysis membrane was considerably slower than the transport of antipyrine.     

Focused open necrosectomy in necrotizing pancreatitis

Background

The control of sepsis is the primary goal of surgical intervention in patients with infected necrosis. Simple surgical approaches that are easy to reproduce may improve outcomes when specialists in endoscopy are not available. The aim of the present study was to describe the experience with a focused open necrosectomy (FON) in patients with infected necrosis.

Method

A prospective pilot study conducted to compare a semi-open/closed drainage laparotomy and FON with the assistance of peri-operative ultrasound. The incidence of sepsis, dynamics of C-reactive protein (CRP), intensive care unit (ICU)/hospital stay, complication rate and mortality were compared and analysed.

Results

From a total of 58 patients, 36 patients underwent a conventional open necrosectomy and 22 patients underwent FON. The latter method resulted in a faster resolution of sepsis and a significant decrease in mean CRP on Day 3 after FON, P = 0.001. Post-operative bleeding was in 1 versus 7 patients and the incidence of intestinal and pancreatic fistula was 2 versus 8 patients when comparing FON to the conventional approach. The median ICU stay was 11.6 versus 23 days and the hospital stay was significantly shorter, 57 versus 72 days, P = 0.024 when comparing FON versus the conventional group. One patient died in the FON group and seven patients died in the laparotomy group, P = 0.139.

Discussion

FON can be an alternative method to conventional open necrosectomy in patients with infected necrosis and unresolved sepsis.     

AAPS-FDA Workshop White Paper: Microdialysis Principles, Application and Regulatory Perspectives

Many decisions in drug development and medical practice are based on measuring blood concentrations of endogenous and exogenous molecules. Yet most biochemical and pharmacological events take place in the tissues. Also, most drugs with few notable exceptions exert their effects not within the bloodstream, but in defined target tissues into which drugs have to distribute from the central compartment. Assessing tissue drug chemistry has, thus, for long been viewed as a more rational way to provide clinically meaningful data rather than gaining information from blood samples. More specifically, it is often the extracellular (interstitial) tissue space that is most closely related to the site of action (biophase) of the drug. Currently microdialysis (μD) is the only tool available that explicitly provides data on the extracellular space. Although μD as a preclinical and clinical tool has been available for two decades, there is still uncertainty about the use of μD in drug research and development, both from a methodological and a regulatory point of view. In an attempt to reduce this uncertainty and to provide an overview of the principles and applications of μD in preclinical and clinical settings, an AAPS-FDA workshop took place in November 2005 in Nashville, TN, USA. Stakeholders from academia, industry and regulatory agencies presented their views on μD as a tool in drug research and development. The views expressed in this paper are those of the authors and do not necessarily reflect the opinions of their companies/institutions or the official policy of the FDA. No official support or endorsement by the FDA is intended or should be inferred. The contents of this report were presented by the authors at the Microdialysis Workshop, Nashville, TN, Nov 4–5, 2005 organized by American Association of Pharmaceutical Scientists and Co-Sponsored with the US Food and Drug Administration.