The importance of being parasiticidal… an update on drug development for the treatment of alveolar echinococcosis

Abstract

The lethal disease alveolar echinococcosis (AE) is caused by the metacestode stage of the fox tapeworm Echinococcus multilocularis. Current chemotherapeutical treatment of AE relies on albendazole and mebendazole, with the caveat that these compounds are not parasiticidal. Drugs have to be taken for a prolonged period of time, often life-long, which can cause adverse effects and reduces the patients' quality of life. In some individuals, benzimidazoles are inactive or cause toxicity, leading to treatment discontinuation. Alternatives to benzimidazoles are urgently needed. Over the recent years, in vivo and in vitro models for low-to-medium throughput drug discovery against AE have been set in place. In vitro drug tests include the phosphoglucose-isomerase (PGI) assay to measure physical damage induced to metacestodes, and viability assays to assess parasiticidal activity against metacestodes and stem cells. In vitro models are also employed for studies on mechanisms of action. In vivo models are thus far based on rodents, mainly mice, and benefits could be gained in future by comparative approaches in naturally infected dogs or captive monkeys. For the identification of novel drugs against AE, a rare disease with a low expected market return, drug-repurposing is the most promising strategy. A variety of chemically synthesized compounds as well as natural products have been analyzed with respect to in vitro and/or in vivo activities against AE. We here review and discuss the most active of these compounds including anti-infective compounds (benzimidazoles, nitazoxanide, amphotericin B, itraconazole, clarithromycin, DB1127, and buparvaquone), the anti-infective anti-malarials (artemisinin, ozonids, mefloquine, and MMV665807) and anti-cancer drugs (isoflavones, 2-methoxyestradiol, methotrexate, navelbine, vincristine, kinase inhibitors, metallo-organic ruthenium complexes, bortezomib, and taxanes). Taking into account the efficacy as well as the potential availability for patients, the most promising candidates are new formulations of benzimidazoles and mefloquine. Future drug-repurposing approaches should also target the energy metabolism of E. multilocularis, in particular the understudied malate dismutation pathway, as this offers an essential target in the parasite, which is not present in mammals.

Keywords: 2-ME, 2-methoxyestradiol; ABZ, albendazole; AE, alveolar echinococcosis; Albendazole; Chemotherapy; Drug treatment; Echinococcus multilocularis; MAPK, mitogen activated protein kinases; MBZ, mebendazole; MMV, Medicines for Malaria Venture; Malate dismutation; Mefloquine; PGI, phosphoglucose isomerase; SPEMs, small particles of Echinococcus multilocularis.

Fig. 1

Structure of E. multilocularis metacestodes. (A) schematic view of a metacestode vesicle. The main components are color-coded: the laminated layer (LL, red); the syncytial tegument (ST, brown); the germinal layer (GL, green), the vesicle fluid (VF, blue). (B–E) Scanning electron micrographs (SEM) of E. multilocularis metacestodes. (B) View into the interior of a metacestode, showing the germinal layer (GL) and the outer laminated layer (LL). (C) Intact metacestode, with only the LL exposed. (D) Developing brood capsule (BC) still attached to the germinal layer (GL). (E) Higher magnification SEM image of the vesicle wall. (F) Section cut through the vesicle wall, shown by transmission electron microscopy (TEM). Note the outer laminated layer (LL), the syncytial tegument (ST) with microtriches protruding outwards into the LL (arrows), and the complex germinal layer (GL), containing undifferentiated cells (uc), muscle cells (mu), glycogen storing cells (gly), and also connective tissue. Bars in B = 330 μm; C = 1200 μm; D = 360 μm; E = 280 μm; F = 4.1 μm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Different in vivo models for AE in mice. (A) Peroral infection with E. multilocularis eggs resulting in liver lesions. (B) Intraperitoneal infection with metacestode material resulting in peritoneal lesions. (C) Subcutaneous lesions visible from the outside (Küster et al., 2013a). Growing parasites are indicated by arrows.

Fig. 2

In vitro screening cascade of compounds against E. multilocularis. The three first steps of the screening are based on the PGI-assay that detects metacestode damage. Further, parasiticidal potential is assessed in the same model. If a potential therapeutic window can be identified by host cell toxicity assays, tests on isolated germinal layer cells are included to assess parasiticidal activity. The mode of action of a drug is further studied in vitro before studies in the mouse model are performed. Parasite cells are depicted in green, host cells in brown, dead cells in grey, drugs in orange. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)