Lotte Groth

Feasibility of measuring lipophilic or protein-bound drugs in the dermis by in vivo microdialysis after topical or systemic drug administration.

Our aim was to assess the microdialysis technique for determining in vivo drug levels of a lipophilic and a protein-bound model drug in the dermis. Forearm skin of healthy volunteers received topical 2% fusidic acid or 0.1% betamethasone-17-valerate formulations twice daily as occluded treatment on irritative dermatitis. Microdialysis sampling in the dermis after 48 h was without measurable drug. Hairless rats received maximized treatment with occluded applications of 10% fusidic acid or 4% betamethasone-17-valerate in ethanol for 72 h followed by microdialysis. Mean levels of betamethasone-17-valerate were 11-45 ng/ml; fusidic acid was not measurable. Systemic administration in clinical doses to rats was without measurable drug levels; increasing doses to 312 mg/kg of fusidic acid and 158 mg/kg of betamethasone-17-valerate yielded betamethasone-17-valerate levels of 25-44 ng/ml and fusidic acid levels of 10-90 ng/ml. This study demonstrates the challenges arising when using microdialysis for measuring in vivo-drug levels. For the drugs chosen it was necessary to administer very high systemic doses or apply a high topical drug concentration to obtain measurable drug levels in the dialysates. Drug levels were in the nanomolar range and demonstrated reproducible and dynamic monitoring of in vivo drug levels in the skin. Using microdialysis for sampling highly protein-bound or lipophilic drugs in the skin requires very sensitive analytical methods, and the sensitivity of the analysis is likely to be the limiting factor.

Cutaneous Microdialysis in the Rat: Insertion Trauma and Effect of Anaesthesia Studied by Laser Doppler Perfusion Imaging and Histamine Release

Microdialysis makes it feasible to study compounds in the extracellular space of cutaneous tissue in vivo. The insertion of the microdialysis probe causes a trauma, which may be anticipated to influence the microdialysis process. The effect of anaesthesia on basal skin blood flow and the trauma in the skin after insertion of a microdialysis probe were investigated. Hairless rats were anaesthetized either with halothane or pentobarbital sodium. Basal skin blood flow and the effect of insertion of a microdialysis probe were measured by laser Doppler perfusion imaging. Trauma-induced histamine release was investigated. Rats anaesthetized with pentobarbital sodium showed a stable skin perfusion in contrast to rats anaesthetized with halothane. A significant increase in blood flow was seen after insertion of the microdialysis probe in the dermis of rats anaesthetized with pentobarbital sodium, while a change in skin blood flow was not observed in rats anaesthetized with halothane. Probe insertion caused histamine release in rats. A minimum equilibration period of 30 min between probe insertion and the start of the experiment is recommended in future studies, allowing immediate events of the trauma to subside and stabilize.

In vivo skin penetration of salicylic compounds in hairless rats.

The in vivo skin penetration of four salicylic compounds was investigated using a hairless rat model, which allowed for non-occluded, finite dose application, and free mobility of the rats throughout the test period. The model compounds were applied in equimolal concentrations of 0.4 mmol/g dimethyl isosorbide. At certain times (0.5-24 h) the rats were killed, and the amount of test compound on the skin surface, in the stratum corneum, and in the deeper viable skin layers was determined. Significant different skin concentrations were found with the following ranking: [(14)C]diethylamine salicylate>[(14)C]salicylic acid>[(14)C]salicylamide>[(14)C]butyl salicylate. In addition, the in vivo percutaneous rate of absorption was in the following order: [(14)C]butyl salicylate>[(14)C]salicylic acid> or =[(14)C]salicylamide>[(14)C]diethylamine salicylate. [(14)C]Butyl salicylate was rapidly absorbed and completely depleted from the surface 3 h post application. In comparison with [(14)C]salicylic acid, the ionic [(14)C]diethylamine salicylate had larger surface depots and penetrated the skin at a lower rate. The relatively hydrophilic [(14)C]salicylamide also had larger surface depots but much lower skin levels. For comparison, the in vitro permeation of the formulations was studied through freshly excised hairless rat skin using Franz diffusions cells, and an agreement between the techniques was found.

Cutaneous microdialysis in the rat: insertion trauma studied by ultrasound imaging.

Microdialysis is a method by which compounds can be studied in the extracellular space in skin, in vivo. The microdialysis probe is inserted in the dermis using a guide cannula. It is expected that intradermal oedema associated with insertion trauma as well as the probe depth can influence the results of microdialysis studies. The aim of the present study was to assess the effects of insertion trauma and, additionally, the probe depth by ultrasound. High-frequency (20 MHz) ultrasound is a noninvasive method for measuring the thickness and echostructure of skin. Hairless rats were anaesthetized with either halothane (n = 7) or pentobarbital sodium (n = 6). The insertion of the microdialysis probe resulted in a 38% relative increase in skin thickness. At 120 min the skin thickness had not reached the pre-insertion value. Thus, significant skin thickening, representing traumatic oedema, developed due to insertion of the microdialysis probe in the skin. The microdialysis probe could be inserted reproducibly in the lower dermis.

Use of an intraluminal guide wire in linear microdialysis probes: effect on recovery?

Background/purpose: For microdialysis studies in the skin, laboratory‐made linear probes are often used. Probes can be assembled with or without a stainless‐steel guide wire in the lumen of the dialysis fibre. From a theoretical viewpoint, this will alter the performance of the probe. The aim of this study was to compare the in vitro microdialysis recovery using probes with and without an internal guide wire, employing two different model drugs and a range of perfusate flow rates.

Bioactivation of morphine-3-propionate, a prodrug of morphine, in tissues from different species

Abstract The bioactivation of the morphine prodrug, morphine-3-propionate, has been evaluated by determination of the first-order hydrolysis rate in different tissue homogenates and blood fractions from various mammal species, including man. The hydrolysis rates were determined in whole blood, serum and plasma from rat, rabbit and man, in serum from pig, in liver, kidney, brain, buccal mucosa, muscle and skin homogenate from rat, rabbit and pig and in skin homogenate from man. Within the same species there was no difference in the enzymatic activity in whole blood, serum and plasma. Comparing the enzymatic activity in blood fractions from the various species, the activity was higher in man followed by rabbit, rat and pig, respectively. The enzymatic activity in the tissue homogenates was general highest in liver followed by kidney, brain, buccal mucosa, muscle and skin. The tissue homogenate from rabbit had higher enzymatic activity than those from rat, which again showed higher activity than those from pig. Comparison of the Michaelis-Menten parameters, K m and V max , obtained using pig and rat serum respectively, suggested that morphine-3-propionate has a lower affinity for enzymes present in pig serum than in rat serum and that the enzymes found in pig serum has a lower hydrolytic capacity than those in rat serum. The results obtained in this study indicate that the three animal species investigated, can serve as a model for man in bioactivation studies on morphine-3-propionate and possibly for other short chain morphine-3-esters as well.