In Vitro Metabolism of Helenalin Acetate and 11α,13-Dihydrohelenalin Acetate: Natural Sesquiterpene Lactones from Arnica

Study on the metabolism of natural sesquiterpene lactones in human liver microsomes using LC-Q-TOF-MS/MS

Sesquiterpene lactones are naturally occurring, highly active -specialised metabolites, which are biosynthesized by important medicinal plants, fulfilling many functions. The in vitro metabolism of parthenolide (1), grosheimin (2), carbetolide C (3), 8α-O-(3,4-dihydroxy--methylenebutanoyloxy)-dehydromelitensin (4) and arteludovicinolide A (5) was examined using human liver microsomes. Phase I, phase II (glucuronidation), as well as combined phase I + II metabolism were studied. Metabolites were identified via liquid chromatography-high resolution quadrupole time-of-flight mass spectrometry. Monohydroxylated, hydrated, carboxylated, methylated derivatives, together with corresponding monoglucuronides were detected, suggesting that the metabolism of sesquiterpene lactones is changeable due to structural features and scaffold diversity, though the lactone ring is the main site of metabolism.

Evaluation of Pharmacokinetic and Toxicological Parameters of Arnica Tincture after Dermal Application In Vivo

Cutaneous leishmaniasis (CL) is classified as a neglected tropical disease by the World Health Organization. As the standard drugs for the treatment of this disease suffer from severe unwanted effects, new effective and safe therapeutic options are required. In our previous work, Arnica tincture showed promising antileishmanial effects in vitro and in vivo. For the potential treatment of human CL patients with Arnica tincture, data on the pharmacokinetic properties of the bioactive, antileishmanial compounds (the sesquiterpene lactone (STL) helenalin and its derivatives) are needed. Therefore, we studied the in vivo absorption of the bioactive compounds after the dermal application of Arnica tincture in rats. Moreover, we analyzed the blood plasma, urine, and feces of the animals by ultra-high-performance liquid chromatography coupled to high-resolution mass spectrometry (UHPLC-HRMS). Although the majority (84%) of the applied STLs (1.0 mg) were absorbed, the concentrations in the plasma, urine, and feces were below the limit of detection (0.3 ng/mL) in the samples for UHPLC-HRMS analysis. This result may be explained by extensive metabolism and slow permeation accompanied by the accumulation of STLs in the skin, as described in our previous work. Accordingly, the plasma concentration of STLs after the topical application of Arnica tincture was very far from a dose where toxicity could be expected. Additionally, tests for corrosive or irritant activity as well as acute and repeated-dose dermal toxicity did not show any positive results after the administration of the amounts of Arnica tincture that would be needed for the treatment of CL. Consequently, in the treatment of CL patients with Arnica tincture, no toxic effects are expected, other than the known sensitization potential of the STLs.

Glutathione-Mediated Conjugation of Anticancer Drugs: An Overview of Reaction Mechanisms and Biological Significance for Drug Detoxification and Bioactivation

The effectiveness of many anticancer drugs depends on the creation of specific metabolites that may alter their therapeutic or toxic properties. One significant route of biotransformation is a conjugation of electrophilic compounds with reduced glutathione, which can be non-enzymatic and/or catalyzed by glutathione-dependent enzymes. Glutathione usually combines with anticancer drugs and/or their metabolites to form more polar and water-soluble glutathione S-conjugates, readily excreted outside the body. In this regard, glutathione plays a role in detoxification, decreasing the likelihood that a xenobiotic will react with cellular targets. However, some drugs once transformed into thioethers are more active or toxic than the parent compound. Thus, glutathione conjugation may also lead to pharmacological or toxicological effects through bioactivation reactions. My purpose here is to provide a broad overview of the mechanisms of glutathione-mediated conjugation of anticancer drugs. Additionally, I discuss the biological importance of glutathione conjugation to anticancer drug detoxification and bioactivation pathways. I also consider the potential role of glutathione in the metabolism of unsymmetrical bisacridines, a novel prosperous class of anticancer compounds developed in our laboratory. The knowledge on glutathione-mediated conjugation of anticancer drugs presented in this review may be noteworthy for improving cancer therapy and preventing drug resistance in cancers.

Therapeutic Efficacy of Arnica in Hamsters with Cutaneous Leishmaniasis Caused by Leishmania braziliensis and L. tropica

Leishmaniasis may occur in three different clinical forms, namely, visceral, mucocutaneous and cutaneous, which are caused by different species of trypanosomatid protozoans of the genus Leishmania. Pentavalent antimonials are the leading treatment for cutaneous leishmaniasis despite the hepatic, renal, and cardiac toxicity. In addition, the response of some Leishmania species to pentavalent antimonials is increasingly poorer, and therefore new and more potent therapeutic alternatives are needed. Arnica montana L., Asteraceae, is a traditional medicinal plant of Europe and preparations of its flowers are commonly used externally to treat disorders of the musculoskeletal system as well as superficial inflammatory conditions. Previous studies have shown that Arnica tincture (AT), an ethanolic extract prepared from the flowerheads of Arnica montana as well as isolated Arnica sesquiterpene lactones (STLs) have antileishmanial activity in vitro against L. donovani and L. infantum, as well as in vivo against L. braziliensis. In this work, we studied the in vitro cytotoxicity and antileishmanial activity of AT and STLs against both L. braziliensis and L. tropica. The in vivo therapeutic effect of AT was studied in hamsters with cutaneous Leishmaniasis (CL) caused by experimental infection with L. braziliensis and L. tropica. Furthermore, various semisolid Arnica preparations were also evaluated against L. braziliensis. The STLs and the AT possess a very high in vitro activity against both Leishmania species with median effective concentrations (EC₅₀) ranging from 1.9 to 5.9 μg/mL. The AT was not cytotoxic for human tissue macrophages, skin fibroblasts, and hepatic cells. The therapeutic response of hamsters infected with L. braziliensis to the topical treatment with AT was 87.5% at a dose of 19.2 μg STL/2× day/60 d, 72.7% at doses of 19.2 μg STL/1× d/60 d and 67% at a dose of 38.4 μg STL/2× d/60 d. In turn, the therapeutic response in hamsters infected with L. tropica was 100% when treated at a dose of 19.2 μg STL/2× day/60 d and 71% at a dose of 38.4 μg STL/2× d/60 d. On the other hand, the effectiveness of treatment with glucantime administered intralesionally at a dose of 200 mg/every three days for 30 days was 62.5% for L. braziliensis and 37.5% for L. tropica infection. These results are promising and encourage the implementation of clinical trials with AT in CL patients as a first step to using AT as a drug against CL.

Dermal Absorption of Sesquiterpene Lactones from Arnica Tincture

Arnica tincture is a traditional herbal medicine used to treat blunt injuries, e.g., bruises and squeezes. In addition, a potential new use in the treatment of cutaneous leishmaniasis is currently under investigation. Therefore, detailed information about the dermal absorption of the tincture and especially its bioactive constituents, sesquiterpene lactones (STLs) of the helenalin- and 11α,13-dihydrohelenalin type, is mandatory. Consequently, this article reports on dermal absorption studies of Arnica tincture using diffusion cells and porcine skin as well as two human skin samples with different permeability. The amounts of STLs on the skin surfaces, in skin extracts and in the receptor fluids were quantified by ultra-high-performance liquid chromatography with high-resolution mass spectrometry (UHPLC-HRMS). It was found that Arnica STLs permeated into the receptor fluid already 4 h after the application, but the amount was rather low. Within 48 h, a maximum of 8.4%, 14.6% and 36.4% of STLs permeated through porcine skin, human skin A (trans-epidermal water loss (TEWL) = 11.518 g·m⁻²·h⁻¹) and the more permeable human skin B (TEWL = 17.271 g·m⁻²·h⁻¹), respectively. The majority of STLs was absorbed (penetrated into the skin; 97.6%, 97.8% and 99.3%) after 48 h but a huge portion could not be extracted from skin and is expected to be irreversibly bound to skin proteins. To better visualize the analytes in different skin layers, a fluorescence-labeled STL, helenalin 3,4-dimethoxycinnamate, was synthesized. Fluorescence microscopic images depict an accumulation of the fluorescent derivative in the epidermis. For the treatment of local, cutaneous complaints, an enrichment of the bioactive substances in the skin may be considered beneficial.