Distribution of 1,25-dihydroxyvitamin D3[22-oxa] in vivo receptor binding in adult and developing skin

Whole-body and microscopic autoradiography to determine tissue distribution of biopharmaceuticals -- target discoveries with receptor micro-autoradiography engendered new concepts and therapies for vitamin D

Information about the distribution of biopharmaceuticals is basic for understanding their actions. Tissue and cellular localization is a key to function. Autoradiography with radiolabeled compounds has provided valuable information with both low resolution whole-body macro-autoradiography and high resolution microscopic autoradiography (micro-autoradiography). Whole-body macro-autoradiography is a uniform and expedient single method approach, providing convenient dose- and time-related overviews with data similar to those obtained with conventional bioassays - and therefore widely used. However, whole-body macro-autoradiography, like common bioassays, has limitations. High specificity-low capacity sites of binding and deposition frequently remain unrecognized. Lack of cellular resolution can cause false negatives and provide misleading results (e.g., false blood-brain barrier). For micro-autoradiography, different methods are advertised in the literature. Most of them are, however, unsuited for drug localization because of inadequate resolution and frequent artifacts. Most drugs interact with their receptors non-covalently by weak electrostatic forces. Therefore, translocation and loss can occur during tissue preparation. This has complicated the use of micro-autoradiography. Receptor micro-autoradiography has overcome these complications and is a method of choice. It has been validated through several diffusible compounds with known localization, extensively applied. It has contributed numerous discoveries, followed by new concepts and therapies. Pictorial evidence in this review indicates that cellular information is essential, a 'sine qua non' for meaningful drug distribution studies. High resolution cellular microscopic information obtained from autoradiography requires tissue dissection and the necessary precautions for preserving pristine in vivo drug deposition. Receptor micro-autoradiography fulfils these requirements. It reveals crucial information at the subcellular level that cannot currently be obtained with any other type of autoradiography or spectrometric imaging.

Where is the vitamin D receptor?

The vitamin D receptor (VDR) is a member of the nuclear receptor superfamily and plays a central role in the biological actions of vitamin D. VDR regulates the expression of numerous genes involved in calcium/phosphate homeostasis, cellular proliferation and differentiation, and immune response, largely in a ligand-dependent manner. To understand the global function of the vitamin D system in physiopathological processes, great effort has been devoted to the detection of VDR in various tissues and cells, many of which have been identified as vitamin D targets. This review focuses on the tissue- and cell type-specific distribution of VDR throughout the body.

Sweat gland epithelial and myoepithelial cells are vitamin D targets

Nuclear receptor binding of 1,25(OH)(2)-vitamin D(3) (vitamin D) in skin keratinocytes of epidermis, hair sheaths and sebaceous glands was discovered through receptor microscopic autoradiography. Extended experiments with (3)H-1,25(OH)(2)-vitamin D(3) and its analog (3)H-oxacalcitriol (OCT) now demonstrate nuclear receptor binding in sweat gland epithelium of secretory coils and ducts as well as in myoepithelial cells, as studied in paws of nude mice after i.v. injection. The results suggest genomic regulation of cell proliferation and differentiation, as well as of secretory and excretory functions, indicating potential therapies for impaired secretion as in hypohidrosis of aged and diseased skin.

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.

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

INTRODUCTION

Microscopic autoradiography with cellular resolution and preservation of in vivo conditions is potentially the method of choice to gain detailed information about sites of deposition and retention in the epidermis and of penetration to the dermis after topical application of drugs. We tested this using (3)H-Maxacalcitol.

METHODS

Dorsal skin of adult rats was treated in vivo with ointment containing 1 or 40 microg/kg body weight of the vitamin D analogue (3)H-Maxacalcitol for periods of 0.5, 2, 8, 24, 48, or 168 h. Samples of skin exposed to the ointment and control samples remote from the treatment site were excised and freeze-mounted, and 4-microm frozen sections were exposed to nuclear emulsion.

RESULTS

Two penetration routes to the dermis could be distinguished: one via epidermal cell layers and the other via hair follicles. Highest uptake and retention of radiolabeled steroid was observed in stratum corneum and in intercellular spaces of stratum granulosum. By contrast, cell boundaries and intercellular spaces in the stratum spinosum and basale contained low levels of radioactivity. Keratinocytes in these layers showed high concentration in the cytoplasm at early time intervals, when surrounding radioactivity levels were high, but high nuclear and low or no cytoplasmic concentration at late time intervals, when surrounding radioactivity levels were low.

DISCUSSION

The autoradiographic method provides detailed information on time- and dose-related distribution of radiolabeled compound at the cellular level that is not obtainable with common radioassays and biochemical procedures. A sustained concentration and retention of radiolabeled steroid in the stratum corneum and intercellular space of the stratum granulosum indicate a selective deposition in components of secreted-membrane-coating granules and suggest a temporary barrier and depot for slow release. The differential cytoplasmic-nuclear distribution in the stratum Malpighi suggests functional correlation to a toxic-hormetic reversal of action on cell proliferation, from high-dose inhibitory effects associated with high extranuclear concentration as utilized in the treatment of psoriasis, to low-dose stimulatory effects associated with high nuclear and low cytoplasmic concentration as applicable in wound healing.

Modulation of ribonuclease P activity by calcipotriol

The effects of cholesterol, 7-dehydrocholesterol, vitamin D3 and several synthetic vitamin D3 analogs on ribonuclease P (RNase P) were investigated using a cell-free system from the slime mold Dictyostelium discoideum. RNase P is an ubiquitous and essential enzyme that endonucleolytically cleaves all tRNA precursors to produce the mature 5' end. Among the compounds tested, only calcipotriol was capable of affecting RNase P activity, and revealed a bimodal action at the kinetic phase of the reaction. Depending on the concentration of the drug, both activation and inhibition of tRNA maturation were observed, indicating that calcipotriol may have a direct effect on tRNA biogenesis, possibly associated with the presence of a highly reactive small ring on the side chain of its molecule.

In vivo dose-related receptor binding of the vitamin D analogue [3H]-1,25-dihydroxy-22-oxavitamin D3 (OCT) in rat parathyroid, kidney distal and proximal tubules, duodenum, and skin, studied by quantitative receptor autoradiography

1,25-Dihydroxy-22-oxavitamin D3 (OCT) is a new synthetic analogue of 1,25(OH)2D3 with a low calcemic effect. This study utilized quantitative receptor autoradiography to determine the dose-related receptor binding and saturation among the vitamin D target cells: parathyroid chief cells, kidney distal and proximal tubule epithelium, duodenal absorptive epithelium, and epidermal keratinocytes. Rats were injected with 0.25, 0.5, 1.0, 2.0, 4.0, 8.0, or 16.0 microgram/kg bw of [26-3H]-OCT and sacrificed 1 hr afterwards. Then autoradiographs were prepared under identical conditions. In these target cells, nuclear uptake of radioactivity increased with dose and then achieved a plateau. However, their saturation doses showed differences: parathyroid chief cells 1-2 microgram duodenal absorptive epithelium, distal tubule epithelium, and epidermal keratinocytes 4-6 microgram proximal tubule epithelium 8 microgram (per kg bw). In contrast, in nontarget cells, such as liver and duodenal smooth muscle, radioactivity did not concentrate in the nuclei but increased in the cytoplasm with dose, without plateauing. These results provide the first information on the relative saturabilities of various target cell populations with a vitamin D ligand. Parathyroid chief cells required the relatively lowest receptor saturation dose. This suggests a high sensitivity and response to OCT treatment with related therapeutic potential for the regulation of parathyroid function.