Receptor microscopic autoradiography for the study of percutaneous absorption, in vivo skin penetration, and cellular-intercellular deposition

Imaging and quantifying drug delivery in skin - Part 1: Autoradiography and mass spectrometry imaging

In this two-part review we present an up-to-date description of different imaging methods available to map the localization of drugs on skin as a complement of established ex-vivo absorption studies. This first part deals with invasive methods which are grouped in two classes according to their underlying principles: i) methods using radioactivity such as autoradiography and ii) mass spectrometry methods such as MALDI and SIMS. For each method, a description of the principle is given along with example applications of imaging and quantifying drug delivery in human skin. Thanks to these techniques a better assessment of the fate of drugs is obtained: its localization on a particular skin structure, its potential accumulation, etc. A critical comparison in terms of capabilities, sensitivity and practical applicability is included that will help the reader to select the most appropriate technique depending on the particular problem to be solved.

Comparison of reservoir effect of topical corticosteroids in an experimental animal model by histamine-induced wheal suppression test

Introduction:

Topical corticosteroids used in various dermatological diseases are available in different potencies and different formulations. The reservoir effect of different potency corticosteroids in the stratum corneum will help the clinicians to choose an appropriate topical steroid to maximize their efficacy and safety as therapeutic agents.

Aims:

This study was designed to compare the duration of reservoir of different potency topical corticosteroids experimentally in rabbits using histamine-induced wheal suppression test.

Materials and Methods:

The study was carried out in albino rabbits (as their skin is similar to humans) using four different concentrations of topical steroids, namely mometasone furoate ointment (0.1%), fluticasone propionate ointment (0.005%), betamethasone valerate cream (0.1%), and hydrocortisone butyrate cream (0.1%). These were applied on the back of rabbit on one side and the vehicle was applied on the other. One hour later, histamine-induced wheal suppression test was performed on both sides and wheal area was measured at 10 min for 7 days. Statistical analysis was done by ANOVA followed by post hoc test.

Results:

Maximum wheal suppression was seen on day 1 (P<0.001) in all four groups, both at test and at control sites. Interday comparison of mean wheal size by topical steroids showed that the reservoir of mometasone furoate ointment (0.1%) persisted till day 4 in the stratum corneum of the skin. In case of fluticasone propionate ointment (0.005%) and betamethasone valerate cream (0.1%), the reservoir persisted till day 2 and for hydrocortisone butyrate cream (0.1%), the reservoir was present only on day 1.

Conclusions:

It is concluded that the duration of reservoir depends on the potency of topical steroids. Higher the potency more is the duration of reservoir in stratum corneum and vice versa.

Transfollicular delivery of linoleic acid in human scalp skin: permeation study and microautoradiographic analysis

There are a large number of studies on the pharmacological activity of lineolic acid (LA) on the skin; however, very little work has been carried out to determine how LA targets the follicular structure. The aim of this study was to identify the preferred route of penetration of LA and to localize the molecule, using a microautoradiographic technique associated with a compartmental approach. In an in vitro test using topical application of 0.05 g of a formulation containing 1% of LA, 10% of the total recovery was found in the stratum corneum and dermis after 6 h. Microautoradiographic analysis of a 'virtual' slide showed most of the silver grains, i.e. LA, at the hair sheath, and none in the dermal compartment, confirming the diffusion of LA through a preferred transfollicular route. These results show, for the first time on human scalp skin, that the combination of compartmental analysis and microautoradiography points to a preferred transfollicular route of diffusion of topically applied LA. Thanks to its high-resolution, microautoradiography offers the advantage of providing detailed in situ information on the delivery of LA in the skin, including the cellular location of the molecule.

Autoradiography, MALDI-MS, and SIMS-MS imaging in pharmaceutical discovery and development

Whole-body autoradiography ((WBA) or quantitative WBA (QWBA)), microautoradiography (MARG), matrix-assisted laser desorption/ionization mass spectrometric imaging (MALDI-MSI), and secondary ion mass spectrometric imaging (SIMS-MSI) are high-resolution, molecular imaging techniques used to study the tissue distribution of radiolabeled and nonlabeled compounds in ex vivo, in situ biological samples. WBA, which is the imaging of the whole-body of lab animals, and/or their organ systems; and MARG, which provides information on the localization of radioactivity in histological preparations and at the cellular level, are used to support drug discovery and development efforts. These studies enable the conduct of human radiolabeled metabolite studies and have provided pharmaceutical scientists with a high resolution and quantitative method of accessing tissue distribution. MALDI-MSI is a mass spectrometric imaging technique capable of label-free and simultaneous determination of the identity and distribution of xenobiotics and their metabolites as well as endogenous substances in biological samples. This makes it an interesting extension to WBA and MARG, eliminating the need for radiochemistry and providing molecular specific information. SIMS-MSI offers a complementary method to MALDI-MSI for the acquisition of images with higher spatial resolution directly from biological specimens. Although traditionally used for the analysis of surface films and polymers, SIMS has been used successfully for the study of biological tissues and cell types, thus enabling the acquisition of images at submicrometer resolution with a minimum of samples preparation.

The dose makes the medicine

Dose and time considerations in the development and use of a drug are important for assessing actions and side effects, as well as predictions of safety and toxicity. This article deals with epistemological aspects of dose selection by probing into the linguistic and cultural roots for the measure of medicine mediated by the medical doctor. Because toxicity is related to dose, historic and recent views suggest that less can be more. At low, medium and high dose levels, effects can differ not only quantitatively but also qualitatively. Dose-related target activation and recognition of enantiodromic thresholds between beneficial and toxic effects require elucidation of underlying events. Such studies, including hormesis and microdosing, call for extended ADME procedures with high-resolution methods in addition to the current low-resolution approaches. Improved information of drug logistics and target pharmacokinetics enables effective drug selection, dose determination and prediction. It also allows considerations of systems biology [i.e. integral (gestalt) pharmacology] exemplified by the drug homunculus, as in the case of vitamin D, that might lead to new paradigms and drug design.

Drug localization and targeting with receptor microscopic autoradiography

This review is an argument in favor of better drug target identification. It presents the many merits and feasibilities of drug localization and target identification through the use of a suitable technique: receptor microautoradiography. Studies of drug targets and target bioavailability require methods with high resolution and sensitivity to gain information for understanding mechanisms of action, sound modeling, prediction of effects, and toxicity. For in vivo localization of drugs in tissues and cells, receptor microautoradiography was specifically designed to preserve both tissue structure and deposition of noncovalently bound diffusible compounds and to enable microscopic viewing, quantitative analysis, and characterization of target sites. This method and its applications are explained here. Pictorial and quantitative data are provided together with a discussion of identified targets that document the utility of receptor microautoradiography. For example, when applied to quantitative studies of vitamin D compounds, pharmacokinetic data of blood differed from those of target tissues and even among target tissues. Many of the target tissues discovered and characterized with receptor microautoradiography remained unrecognized with common ADME procedures, radioassay-HPLC, and whole-body autoradiography. For a visual overview of the multiple vitamin D targets, a drug homunculus has been composed. Such a drug or target homunculus may be created for any drug, dose, and time to aid in documenting and fingerprinting. Receptor microautoradiography also is a sensitive method. It can be used for the study of low-dose stimulatory actions of toxic substances to show relationships of receptor binding to dose-dependent reversal of effects, known as hormesis. In addition, a combination of autoradiography and immunocytochemistry with radiolabeled drug and antibodies to receptor or other cellular product permits further target characterization. In its own league, receptor microautoradiography provides unique information. Through greater detail and certainty, it can validate and complement less-sensitive approaches, decrease the failure rates of current ADMET predictions, and serve as a diagnostic tool and guide for biochemical, functional, and clinical follow-up in drug research and development.