538 ISSN 1068-1620, Russian Journal of Bioorganic Chemistry, 2018, Vol. 44, No. 5, pp. 538–546. © Pleiades Publishing, Ltd., 2018. Original Russian Text © A.V. Semakov, L.V. Anikina, S.V. Afanasyeva, S.A. Pukhov, S.G. Klochkov, 2018, published in Bioorganicheskaya Khimiya, 2018, Vol. 44, No. 5, pp. 544–553. Synthesis and Antiproliferative Activity of Conjugates of Anthracycline Antibiotics with Sesquiterpene Lactones of the Elecampane A. V. Semakov a, 2 , L. V. Anikina a , S. V. Afanasyeva a , S. A. Pukhov a , and S. G. Klochkov a a Institute of Physiologically Active Compounds, Russian Academy of Sciences, Chernogolovka, 142432 Russia Received December 29, 2017; in final form, February 1, 2018 AbstractThe daunorubicin and doxorubicin anthracycline antibiotics were modified with the Inula hele- nium L. sesquiterpene lactones (alantolactone, isoalantolactone, and alloalantolactone) and with their epoxy derivatives. Antiproliferative properties of these conjugates were studied on tumor and normal cell lines. The daunorubicin conjugates with the sesquiterpene lactones (isoalantolactone, allantolactone, and alloalanto- lactone) and with their epoxy derivatives were found to exhibit the higher activity against human tumor cell lines than the corresponding doxorubicin conjugates. The daunorubicin conjugate with epoxyisoalantolac- tone proved to be the most effective compound, because it was more cytotoxic than daunorubicin towards a number of cell lines, including those daunorubicin-resistant, and did not affect normal human cells. Keywords: daunorubicin, doxorubicin, sesquiterpene lactones, in vitro cytotoxicity, MTT test DOI: 10.1134/S1068162018040167 INTRODUCTION At present, a wide spectrum of antitumor antibiot- ics is still used as an integral part of chemotherapy of cancers [1]. Anthracyclines are a group of antitumor antibiotics with a very high antimitotic activity. Their action is based on an inhibition of the synthesis of nucleic acids due to an intercalation between pairs of nitrogen bases and binding to topoisomerase II as well as on an interaction with lipids of cellular membranes, and, as a result, a disturbance of ionic transport and a number of other functions of the cell [2]. This mode of cytotoxicity results in the high antitumor effect and low selectivity of the action of the anthracycline anti- biotics. Irreversible cumulative dose-dependent car- diotoxicity and immune depressive, embryotoxic, mutagenic, and teratogenic actions are characteristic of these antibiotics [3]. The goal of this study was the modification of the anthracycline antibiotics, in particular daunorubicin (DNR) and doxorubicin (DOX), with the following sesquiterpene lactones: isoalantolactone (I), alanto- lactone (II), alloalantolactone (III), which were iso- lated from the elecampane plant (Inula helenium L.), and their epoxy derivatives (IV)–(VI) (Schemes 1, 2). We also evaluated the antiproliferative properties of the prepared compounds towards the tumor and nor- mal cell lines and investigated the possibility of over- coming the multidrug resistance to the anthracycline antibiotics. The five-member heterocycles of the sesquiterpene lactones are structurally related to the six-member lactone cycles of Camptothecin, Topotecan, Irinote- can, and other known inhibitors of topoisomerase I. In addition, the sesquiterpene lactones exhibit the anti- oxidant effect and chelating and antiradical properties [4]. We have proposed that an attachment of the natu- ral sesquiterpene lactones to the anthracycline antibi- otics will increase the cytotoxicity of the resulting con- jugates towards the tumor cells due to an additional action on new biochemical targets owing to the anti- oxidant properties of the sesquiterpene lactones [5–7]. Isoalantolactone (I) and alantolactone (II) are the well-known and actively studied antitumor agents (see, for example, [5]). They also exhibit other types of activities and can be easily isolated in preparative amounts from the chloroform extract of roots of ele- campane (Inula helenium L.) [8]. A minor component of the extract, alloalantolactone (III), can be prepared in sufficient amounts by isomerization of isoalanto- lactone in an acidic medium. These lactones contain the reactive exomethylene group and easily bind dif- ferent nucleophiles via the Michael reaction. RESULTS AND DISCUSSION Isoalantolactone (I) and alantolactone (II) which were isolated from the elecampane roots served as starting compounds. A number of the sesquiterpene lactones (III)–(VI) with the evdesman skeleton were pre- pared from these lactones by the known procedure [8]. 1 This article is published based on the report presented at the Third Russian Conference on Medicinal Chemistry, September 28– October 3, 2017, Kazan. 2 Corresponding author: phone: +7 (496) 524-25-42; e-mail: l_vok@list.ru. Abbreviations: DNR, daunorubicin, DOX, doxorubicin.