It is difficult to deal with the most common skin tumor in horses: the equine sarcoid. Its recurrence rate is very high and no single universally effective treatment without serious side effects is yet available.

Aqueous fermented mistletoe extracts of eight different host trees (pine tree, fir tree, hawthorn, apple tree, poplar tree, lime tree, oak tree, willow tree) were tested for their impact on the proliferation of the equine sarcoid cell line E42/02 in a 2D and 3D design.

All mistletoe preparations inhibit the proliferation of the equine sarcoid cells in vitro. The order of the IC50 concentrations was in accordance with the content of mistletoe lectin of the respective Viscum album preparations. The extract from the pine tree (Pini) showed the lowest potency to reduce the cell growth. The strongest inhibition of cell proliferation was obtained with the mistletoe extract of the willow tree (Salicis) with an IC50 concentration of 11 µg/ml in the 2D Alamar Blue assay and 1.2 µg/ml in the 3D Soft Agar assay. In addition to the high content of mistletoe lectin in the extracts from the mistletoe growing on willow tree, further components need to be taken into consideration to assess the full potential, such as the various acids exhibiting anti-tumoral action as well as the anti-phlogistic salicin.

These results indicate that the treatment with mistletoe preparations presents an effective therapy for horses suffering from equine sarcoids. The preparation from the willow tree mistletoe offers both cytotoxic and anti-inflammatory activity which awaits to be proofen in an in vivo design.

Felenda J, Stintzing F (2018) Mistletoe Preparations as an Option for Treatment of Equine Sarcoids – Results of an In vitro Investigation on Cell Proliferation in 2D And 3D Design. J Vet Med Res 5(11): 1169.

•    Viscum album; Equine sarcoid 
•    Anti-proliferative; Anti-tumoral
•    Mistletoe lectin

IC50: Inhibitory concentration of 50%; BVP: Bovine papillomavirus; BCG: Bacillus Calmette Guérin; DNA: Deoxyribonucleic acid; RNA: Ribonucleic acid; ML: Mistletoe lectin; VT: Viscotoxin

The equine sarcoid is the most prevalent skin tumor and can affect all equidae [1]. Equine sarcoids are caused by infection with BVP types 1 and 2 transmitted through insects [2]. Sarcoids are local aggressive tumors without a tendency to build metastasis and occur at the whole body of horses. Sarcoids are by no means merely a cosmetic problem, but may result in an uselessness or a loss of the horse. According to specific appearance five types of the equine sarcoid can be differentiated: occult, verrucose, nodular, fibroblastic and malignant [3]. The occult type features alopecia, skin thickening and scaling. The verrucose type has a warty or verrucose appearance (Figure 1A). The nodular equine sarcoids are sessile or predunculated, solid, subcutaneous, spherical tumors (Figure 1B). The malignant form is an aggressive locally invasive form. Fibroblatic sarcoids represent the most aggressive type and exhibit a fleshy fibrovascular appearence; either with thin pedicle or a wide flat base that commonly bleed easily. They may have a wet, haemorrhagic surface (Figure1C).

The great variety of treatment methods of sarcoids ranging from chemotherapy to homeopathy shows that an universally effective therapy is often not possible. High tendency to recurrence, for treatment often unfavorable localization and the capacity of sarcoids to develop quickly from a milder form to an aggressive one after even slight damage make a successful treatment difficult. Many veterinarians use a combination of different treatment methods to reduce the relapse risk. Classical surgery is often used but has a high failure rate due to tumor relapse, especially if it used as single therapy [4-6]. In clinical practice further treatment options including cryosurgery, laser surgery, BCG immunotherapy, radiotherapy, intratumoral chemotherapy and topical formulations including zinc chloride, bloodroot extract, imiquimod or aciclovir are used. These therapeutic options often imply undesirable side effects, e. g. necrosis, oedematous swelling or paralysis of nervus facialis after treatments at the head [1,7]. Another treatment option without serious side effects are injections based on mistletoe preparations. In the human tumor therapy the anti-tumoral, anti-angiogenetic, immunomodulating and apoptosis inducing effects of the mistletoe therapy were verified in many clinical and preclinical trials [8-16]. In a study with horses suffering from equine sarcoids complete or partial remission without serious side effects and a low recurrence rate could be achieved by Iscador® P (Iscador AG, Arlesheim, Schweiz), a mistletoe preparation from pine tree (Figure 2A) [17]. The following investigations cover systematic assessment of the impact of mistletoe extracts from different host trees on the growth of equine sarcoid cells. An earlier publication of these investigations was dedicated to therapeutic potential of the mistletoe extracts [18].

Aqueous fermented extracts from mistletoe (Viscum album) from eight different host trees (fir tree, hawthorn, apple tree, pine tree, poplar tree, oak, willow and lime tree) produced according to German Homeopathic Pharmacopoeia, version 38 by WALA Heilmittel GmbH for the manufacture of the mistletoe preparations Iscucin® Abietis, Iscucin® Crataegi, Iscucin® Mali, Iscucin® Pini, Iscucin® Populi, Iscucin® Quercus, Iscucin® Salicis and Iscucin® Tiliae (Table 1) were tested for their impact to inhibit cell proliferation of the equine sarcoid cell line E42/02 (fibroblastic type) provided by Friedrich-Loeffler-Institute (Greifswald-Insel Riems, Greifswald).

Cellular proliferation assays conducted and modified by ProQinase GmbH (Freiburg, Germany). They are widely used and accepted in oncology for analysis of the impact of anticancer drugs. The most common cellular phenotypic assay is the 2D Alamar Blue assay [19], which is generally regarded as a pretest to more sophisticated assays such as apoptosis or the 3D Soft Agar assay [20]. Anchorage-independent cell growth measured in the 3D Soft Agar assay is one of the hallmark characteristics of cellular transformation and uncontrolled cell growth, with normal cells typically not capable to proliferate in semisolid matrices. 3D Cell growth is more similar to the in vivo cellular environment. Both, the 2D Alamar Blue and the 3D Soft Agar assays, are based on the quantification of the population of living cells after compound incubation using a fluorescent cell viability dye. Living cells reduce blue resazurin into the pink-colored and highly red fluorescent resorufin [21]. Raw data (fluorescence intensity) were converted into percent cell viability relative to high controls (solvent) and low controls (10-5 M Staurosporine),

Which were set to 100% and 0%, respectively? IC50 calculation was performed using GraphPad Prism 5 software.

At the beginning the conditions and the optimal density of the E 42/02 cells were determined. Both in the 2D Alamar Blue Assay and the 3D Soft Agar assay, the cells showed a clear growth on day 0 to 4. In the Alamar Blue assay the cells did not proliferate after day 4, but in the Soft Agar assay individual cell colonies grew further. The optimal density of the equine sarcoid cells was 10,000 cells per well in the Alamar Blue assay and 20,000 cells in the Soft Agar assay (Figure. 3). A very good correlation between cell number and reduction of resazurin at 5 hours was analysed by fluorescence according to an earlier report [22]. On the basis of these results the incubation time was calculated. The incubation time of the test items was 72 hours in the 2D Alamar Blue assay and 144 hours in the 3D Soft Agar assay. The final assay concentration of the mistletoe extracts started at a dilution of 1:100 with further semi-logarithmic dilution steps in the Alamar Blue assay. In the 3D model the extracts of Viscum album from the coniferous trees were diluted in the same way, but the dilution of the extracts of Viscum album from deciduous trees began at 1:10,000. The antineoplastic agent Actinomycin-D serves as reference substance. It belongs to the group of cytotoxic antibiotics, binds to DNA and can inhibit RNA-synthesis. In Alamar Blue Assay each sample was tested in three independent experiments.

All tested mistletoe extracts were able to inhibit the proliferation of the equine sarcoid cell line E 42/02 in the analysed concentration range in both test systems. The antiproliferative potential was higher in the 3D model than in the 2D cell layer (Figure. 4). Possible reasons might be that the cells are more sensitive in the 3D system than in the cell layer or / and that the incubation time was longer in the Soft Agar assay. The order of the IC50 concentrations of the different mistletoe extracts was equal in both tests. The Salicis (willow) preparation showed the highest potency in the assays (Alamar Blue assay: IC50: 11 µg/ml; Soft Agar assay: IC50: 1.2 µg/ml), followed by Tiliae (lime tree) (Alamar Blue assay: IC50: 15 µg/ml; Soft Agar assay: IC50: 1.3 µg/ml). Pini (mistletoe preparation from pine tree) had the lowest potency to inhibit the growth of the equine sarcoid cells (Alamar Blue assay: IC50: 273 µg/ml; Soft Agar assay: IC50: 25 µg/ ml) (Figure. 5). The reference substance Actinomycin-D (IC50: 169 µg/ml) was more effective than Pini although Actinomycin-D was not able to inhibit the proliferation totally at the highest concentration tested in the Alamar Blue assay (Figure. 6). Interestingly in the 3D model that most likely compares to the in vivo situation, Actinomycin-D (IC50: 53 µg/ml) was only half of potency than Pini.

The order of the IC50 concentrations of the mistletoe extracts from different host trees followed the order of the mistletoe lectin contents of the respective Viscum album preparations. The calculated correlation coefficient after Pearson (r) amounts for IC50 (Alamar Blue assay) vs. mistletoe lectin -0.81 and for IC50 (Soft Agar assay) vs. mistletoe lectin -0.86, respectively. The Figure showed a good correlation between the mistletoes with higher lectin content. The mistletoes of the conifers (Pini and Abietis) drop a little bit out of the linear correlation (Figure. 7). Compared to other preparations, the one from willow tree showed both a high content of mistletoe lectins and an average level of viscotoxins (Figure. 2B), but also additional features such as high contents of nucleosides, protocatechuic acid, ferulic acid, chlorogenic acid and caffeic acid [23,24]. These substances also are associated with anti-cancer characteristics [24-28]. Moreover, specifically salicin has been found in willow mistletoe extract. Salicin is known for its pain killing as well anti-phlogistic effects [29,30].

The results obtained in the in vitro cell proliferation assay with mistletoe extracts from different host trees using an equine sarcoid cell line are promising. Viscum album preparations (Iscucin®) inhibit the cell proliferation of the equine sarcoid cell line more effectivly than the reference substance Actinomycin-D in 2D and 3D systems. The anti-proliferative effect of mistletoe extracts on the equine sarcoid cell line E 42/02 in vitro pre Figure an effective systemic treatment or local therapy option of the equine sarcoid without severe side effects. In the previous published study the mistletoe injections were well tolerated by the horses. Only mild edema at injection sites occured, which disappeared within 2-3 days without treatment. No systemic signs were reported [17]. The median lethal dose at mice after parenteral administration amounts > 32 mg/kg. Mistletoe extracts showed no genotoxic, immunotoxic or carcinogenic activity [31]. At humans, low side effect, such as local reaction with swelling < 5 cm and erythema, are desirable at the anthroposophic medicine. Other adverse drug reactions (e. g. fever, headache) were mostly mild and self-limiting [13,32-35]. The mistletoe extract from the willow tree (Salicis) containing high mistletoe lectins and an average level of viscotoxins as well other specific ingredients with anti-tumoral actions showed a strong potential to inhibit the growth of fibroblastic equine sarcoid cells. Because of its level of salicin it might an additional role in the treatment of inflammatory processes correlated with equine sarcoid diseases. Continuing investigations in vivo are underway to corroborate these findings and to analyse the practical application as well as differentiation between the host trees.

We thank ProQinase GmbH (Freiburg, Germany) for conducting the in vitro tests and the Friedrich-LoefflerInstitute (Greifswald-Insel Riems, Germany) for providing the equine sarcoid cell line. Also we want thank Christine Brendle (alternative animal health practitioner, Ditzingen, Germany) and Sandra Graf-Schiller (SaluVet GmbH, Bad Waldsee, Germany) for providing the photos of the equine sarcoids.

1. Hainisch EK, Brandt S. Das Sarkoid als häuFigurester Hauttumor bei Pferden und Pferdeartigen. Pferdespiegel. 2014; 2: 75-83.

2. Nasir L, Campo MS. Bovine papillomaviruses: their role in the aetiology of cutaneous tumours of bovids and equids. Vet Dermatol. 2008; 19: 243-254.

3. Knottenbelt DC. A suggested clinical classification for equine sarcoid. Clin Tech Eq Prac. 2005; 4: 278-295.

4. Dietz O, Huskamp B. Handbuch der Pferdepraxis. Stuttgart: Ferdinand Enke Verlag. 1999.

5. Ragland WL, Keown GH, Spencer GR. Equine Sarcoid. Equine Vet J. 1970; 2: 2-11.

6. Tarwid JN, Fretz PB, Clark EG: Equine sarcoids: a study with emphasis on pathological diagnosis. Comp Cont Educ for Pract. 1985; 7: 293- 300.

7. Hamann J, Grabner A. Das Equine Sarkoid –der häuFigureste Hauttumor beim Pferd. Pferdeheilkunde. 2005; 21: 273-279.

8. Büssing A, Schnietzel M. Apoptosis-inducing properties of Viscum album L. extracts from different host trees, correlate with their content of toxic mistletoe lectins. Anticancer Res. 1999; 19: 23-28.

9. Heidecke H, Meyer U: Iscucin®-Wirkung auf die Angiogenese in vitro. Anthromedics. 2005; 58: 302-304.

10. Kienle GS, Kiene H. Complementary cancer therapy: a systematic review of prospective clinical trials on anthroposophic mistletoe extracts. Eur J Med Res. 2007; 12: 103-119.

11. Marvibaigi M, Supriyanto E, Amini N, Abdul Majid FA, Jaganathan SK. Preclinical and clinical effects of mistletoe against breast cancer. Biomed Res Int. 2014; 785479: 1-15.

12. Ostermann T, Raak C, Büssing A. Survival of cancer patients treated with mistletoe extract (Iscador): a systematic literature review. BMC Cancer. 2009; 9: 451.

13. Tröger W, Galun D, Reif M, Schumann A, Stankovic N, Milicevic M. Viscum album [L.] extract therapy in patients with locally advanced or metastatic pancreatic cancer: a randomised clinical trial on overall survival. Eur J Cancer. 2013; 49: 3788-3797.

14. Vivani A, Rist L. Differenzierte Wirkungen von Mistelextrakten auf Krebszellen. Phytotherapie. 2007; 1: 3-7.

15. Werner M, Bock PR, Hanisch J, Stauder G. Supportive therapy with mistletoe extract in tumor patients, results of four controlled pharmacoepidemiological cohort studies as basis for prospective studies (clinical report). Phytomed. 2011; 18: 12-13.

16. Zuzak T, Rist L, Viviani A, Eggenschwiler J, Mol C, Riegert U et al. Das Mistelpräparat Iscucin®–Herstellung, Analytik, Wirkung in vitro. Anthromedics. 2004; 57: 467-473.

17. Christen-Clottu O, Klocke P, Burger D, Straub R, Gerber V. Treatment of clinically diagnosed equine sarcoid with a mistletoe extract (viscum album austriacus). J Vet Intern Med. 2010; 24: 1483-1489.

18. Felenda J, Stintzing F: Die Misteltherapie als Behandlungsmöglichkeit beim equine Sarkoid. Pferdespiegel. 2017; 4: 162-167

19. Page B, Page M, Noel C. A new fluorometic assay for cytotoxicity mesurements in vitro. Int J Oncol. 1993; 3: 473-476.

20. Hamburger AW, Salmon SE. Primary bioassay of human tumor stem cells. Science. 1977; 197: 461-463.

21. Präbst K, Engelhardt H, Ringgeler S, Hübner H. Basic Colorimetric Proliferation Assays: MTT, WST, and Resazurin. Methods Mol Biol. 2017; 1601: 1-17.

22. O’Brien J, Wilson I, Orton T, Pognan F. Investigation of the Alamar Blue (resazurin) fluorescent dye for the assessment of mammalian cell cytotoxicity. Eur J Biochem. 2000; 267: 5421-5426.

23. Gärtner T, Link K, Müller MB, Stintzing FC, Kammerer DR. Phenolic profiles of Viscum album L. subspecies from different host trees. In: Scheer R, Alban S, Becker H, Beer AM, Blaschek W, Kreis W, et al., editors. Die Mistel in der Tumortherapie 4. Essen: KVC-Verlag. 2016: 87-96.

24. Wurster M, Albrecht S, Lindequist U, Stintzing F. Comparative investigation of nucleosides in mistletoes (Viscum album L.) from different host trees. In: Scheer R, Alban S, Becker H, Beer AM, Blaschek W, Kreis W et al., editors. Die Mistel in der Tumortherapie 4. Essen: KVC-Verlag. 2016: 77-85.

25. Dziedzic A, Kubina R, Kaba?a-Dzik A, Tanasiewicz M. Induction of cell cycle arrest and apoptotic response of head and neck squamous carcinoma cells (Detroit 562) by caffeic acid and caffeic acid phenethyl ester derivative. Evid Based Complement Alternat Med. 2017: 6793456.

26. Goedings P. Der Mineralstoffhaushalt in Viscum album L. (weißbeerige Mistel). [dissertation]. Universität Witten/Herdecke. 1995.

27. Tsao SM, Hsia TC, Yin MC. Protocatechuic acid inhibits lung cancer cells by modulating FAK, MAPK, and NF-KB pathways. Nutr Cancer. 2014; 66: 1331-1341.

28. Wilkins LR, Brautigan DL, Wu H, Yarmohammadi H, Kubicka E, Serbulea V et al. Cinnamic acid derivates enhance the efficacy of transarterial embolization in a rat model of hepatocellular carcinoma. Cardiovasc Intervent Radiol. 2017; 40: 430-437.

29. Li Y, Wu Q, Deng Y, Lv H, Qiu J, Chi G, et al. D(-)-Salicin inhibits the LPS-induced inflammation in RAW264.7 cells and mouse models. Int Immunopharmacol. 2015; 26: 286-294.

30. Bonaterra GA, Heinrich EU, Kelber O, Weiser D, Metz J, Kinscherf R. Anti-inflammatory effects of willow bark extract STW 33-I (Proaktiv®) in LPS-activated human monocytes and differentiated macrophages. Phytomed. 2010; 17: 1106-1113.

31. HMPC. Assessment report on Viscum album L., herba. London: European Medicines Agency (EMA), Committee on Herbal Medicinal Products (HMPC); 2012: 1-26

32. Eisenbraun J, Scheer R, Kröz M, Schad F, Huber R. Quality of life in breast cancer patients during chemotherapy and concurrent therapy with a mistletoe extract. Phytomed. 2011; 18: 151-157.

33. Huber R, Lüdtke H, Wieber J, Beckmann C. Safety and effects of two mistletoe preparations on production of interleukin-6 and other immune parameters - a placebo controlled clinical trial in healthy subjects. BMC Complement Alt Med. 2011; 11: 116-128.

34. Kienle GS, Grugel R, Kiene H. Safety of higher dosages of Viscum album L. in animals and humans - systematic review of immune changes and safety parameters. Complement Alt Med. 2011; 11: 72-87.

35. Melzer J, Iten F, Hostanska K, Saller R. Efficacy and safety of mistletoe preparations (viscum album) for patients with cancer diseases. A systematic review. Forsch Komplementmed. 2009; 16: 217-226.