No abstract

The nonhalogenated double ester of prednisolone, prednicarbate (PC), is the first topical glucocorticoid with an improved benefit/risk ratio verified clinically and in vitro. To evaluate if this is due to unique characteristics of this steroid, a new compound created according to an identical concept, prednisolone 17-ethylcarbonate, 21-phenylacetate (PEP), and the new halogenated monoester desoximetasone 21-cinnamate (DCE) were tested and compared to PC, desoximetasone (DM) and betamethasone 17-valerate (BMV). Isolated foreskin keratinocytes served for in vitro investigations of anti-inflammatory processes in the epidermis, fibroblasts of the same origin were used to investigate the atrophogenic potential. Inflammation was induced by TNFα, resulting in an increased interleukin 1α (Il-1α) synthesis. As quantified by ELISA, all drugs significantly reduced Il-1α production. But PC and BMV appeared particularly potent, followed by DM and the two new congeners, which revealed minor anti-inflammatory activity. Glucocorticoid esters including PEP are rapidly degraded in keratinocytes (85% within 12 h). Hence, a ribonuclease protection assay of Il-1α mRNA was performed allowing short incubation times and thus minimizing biodegradation. This assay confirmed the anti-inflammatory potency of native PC and BMV. In contrary DCE and PEP did not reduce Il-1α mRNA to a significant extent. Therefore PEP acts as a prodrug only. In fibroblasts, Il-1α and Il-6 syntheses indicate proliferation and inflammation, respectively. Whereas PC and PEP inhibited Il-1α and Il-6 production in fibroblasts only to a minor extent, cytokine synthesis was strongly affected by the conventional glucocorticoids BMV and DM, but also by DCE. The minor unwanted effect of PC and PEP on fibroblasts was also reflected by their low influence on cell proliferation as derived from ³H-thymidine incorporation. Again, more pronounced antiproliferative features were seen with the halogenated glucocorticoids. In the following, the correlation between antiphlogistic effects in keratinocytes (suppression of Il-1α) and antiproliferative effects in fibroblasts (suppression of Il-1α and Il-6; ³H-thymidine incorporation) was analyzed. Here, PC is revealed as the only glucocorticoid with an improved benefit/risk ratio. Native PEP is shown to be almost ineffective and DCE presents exactly the opposite features of PC. It is tempting to speculate if this is due to different glucocorticoid receptor subtypes or different signaling pathways in keratinocytes and fibroblasts.

Long term topical glucocorticoid treatment can induce skin atrophy by the inhibition of fibroblasts. We, therefore, looked for the newly developed drug carriers that may contribute to a reduction of this risk by an epidermal targeting. Prednicarbate (PC, 0.25%) was incorporated into solid lipid nanoparticles of various compositions. Conventional PC cream of 0.25% and ointment served for reference. Local tolerability as well as drug penetration and metabolism were studied in excised human skin and reconstructed epidermis. With the latter drug recovery from the acceptor medium was about 2% of the applied amount following PC cream and ointment but 6.65% following nanoparticle dispersion. Most interestingly, PC incorporation into nanoparticles appeared to induce a localizing effect in the epidermal layer which was pronounced at 6 h and declined later. Dilution of the PC-loaded nanoparticle preparation with cream (1:9) did not reduce the targeting effect while adding drug-free nanoparticles to PC cream did not induce PC targeting. Therefore, the targeting effect is closely related to the PC-nanoparticles and not a result of either the specific lipid or PC adsorbance to the surface of the formerly drug free nanoparticles. Lipid nanoparticle-induced epidermal targeting may increase the benefit/risk ratio of topical therapy.

Extrapolating our results to the in-vivo situation, topically applied PC may be metabolized by epidermal cells during skin penetration. A complex mixture of compounds reaches the dermis, whose fibroblasts are barely able to metabolize the steroids. Since skin atrophy is less pronounced with PC as compared to conventional halogenated glucocorticoids, less potent PC metabolites appear to be the dominant species in the dermis.

The inactivation of highly potent, but also cell toxic, 17-monoesters to almost inactive 21-congeners seen with isolated cell monolayers appears less important in the skin. In vitro determination of the dermal 17-monoesters concentrations may allow the prediction of the atrophogenic risk in man. BM17V levels exceeding P17EC concentration about 6-fold may contribute to its lower tolerance when compared to PC.