Microdialysis Sampling of Carbamazepine, Phenytoin and Phenobarbital in Subcutaneous Extracellular Fluid and Subdural Cerebrospinal Fluid in Humans: An in vitro and in vivo Study of Adsorption to the Sampling Device

Abstract

The purpose of the study was to determine if binding of the drugs to the sampling equipment during microdialysis would influence the results for carbamazepine, phenytoin and phenobarbital. In vitro experiments with microdialysis catheters and separate parts of catheters were performed to estimate the degree of drug binding to the dialysis equipment. A mathematical model to calculate drug binding and recovery is proposed. In vivo protein unbound carbamazepine concentrations in subcutaneous extracellular fluid at different flow rates (6 patients), unbound carbamazepine (1 patient) and unbound phenobarbital (1 patient) in subdural cerebrospinal fluid and subcutaneous extracellular fluid were estimated and the in vivo data were compared to the in vitro results and data generated by the mathematical model. Binding to the soft outlet polyurethane tubing was extensive and variable for phenytoin, which precluded in vivo testing, but limited and more predictable for carbamazepine and phenobarbital. None of the three compounds bound to the hard internal tubing. Phenytoin and phenobarbital did not bind to the dialysis membrane, while a small degree of binding may be present for carbamazepine. In vivo estimates of carbamazepine protein unbound subcutaneous extracellular concentrations by microdialysis, adjusted for binding to the plastic tubing, were 81% of protein unbound plasma concentrations. In single case studies, subdural cerebrospinal fluid and subcutaneous extracellular levels of carbamazepine and phenobarbital were similar and when corrected for binding to the plastic tubings they were also close to protein unbound plasma concentrations. Microdialysis can be used for reliable estimations of protein unbound carbamazepine and possibly phenobarbital concentrations when drug binding to the plastic tubing is considered. Reliable estimation of unbound phenytoin is not possible at present.

Approach to reduce the non-specific binding in microdialysis

“…Non-specific binding to the plastic tubing or dialysis membrane has been largely attributed to lipophilicity of the test compound (Lindberger et al, 2002). Additives investigated for reducing non-specific binding include the addition of albumin (Carneheim and Stahle, 1991), beta-cyclodextrin (Khramov and Stenken, 1999a), CHAPS [(3-cholamidopropyl) dimethylammonio-1-propanesulphonate] (Silvester and Zang, 2012) or polysorbate-80 (Loos et al, 2007) in order to saturate the binding sites on the microdialysis tubing and dialysis membrane (Pitt et al, 1988;Traunmuller et al, 2006), thereby reducing the non-specific binding of compounds.…”

Free phenytoin concentration measurement in brain extracellular fluid: a pilot study

“…This proportion is known as the relative recovery and is a consequence of the transit time of the perfusate along the semi-permeable microdialysis membrane being insufficient to allow complete equilibration between ECF and perfusate. Relative recovery of phenytoin has been investigated by several authors using a variety of catheter types, membrane lengths and perfusion rates 10,15,16 . However, it has not previously been reported for the commonly used catheter type and perfusion rate we describe in this study.…”

CNS Penetration of Tyrosine Kinase Inhibitors in Mouse Models

“…One challenge to the use of microdialysis is the low probe recovery of lipophilic compounds. The low recovery of these compounds can be attributed to the tendency of these compounds to bind to the sampling components, their poor solubility in hydrophilic perfusates, their high protein binding affinity (which limits the unbound drug in the sampling space), and their tendency to accumulate in the intracellular compartment, which reduces the sampling pool in tumor ECF (Lindberger et al, 2002;Loos et al, 2007;Wang et al, 2008). This challenge is particularly significant because many of the compounds under development for treatment of brain tumors tend to exhibit lipophilic properties.…”

“…Previous studies have reported irreproducible dialysis of lipophilic drugs (Lindberger et al, 2002;Loos et al, 2007;Wang et al, 2008). In addition, previous studies have only evaluated a single anti-cancer agent, an approach that has limited relevance in light of the prevalent use of combinations of two or more anti-cancer drugs in therapy.…”

“…A major limitation of cerebral microdialysis is associated with the poor recovery of lipophilic compounds due to the tendency of these compounds to stick to the sampling probe and tubing components, poor solubility of these compounds in the hydrophilic perfusate, and their high protein binding affinity which limits the unbound drug in the sampling space (Benfeldt and Groth, 1998;Benveniste and Huttemeier, 1990;Carneheim and Stahle, 1991;de Lange et al, 2000;Lindberger et al, 2002;Oravcova et al, 1996). Several attempts to improve recovery of lipophilic compounds involved replacing the commonly used hydrophilic perfusates with hydrophobic solutions such as 2.25% glycerol in water or 20% soybean oil/ egg phospholipid (Intralipid®) which allow the lipophilic drug to accumulate in the perfusate (Carneheim and Stahle, 1991;Kurosaki et al, 1998).…”