**1. Introduction**

Natural and synthetic coumarins (2*H*-chromen-2-one) demonstrate diverse biological activities, and are often considered as a privileged scaffold [1–6]. In particular, a large number of coumarin

derivatives with high antitumor activity have been found in recent years [7–18]. Natural derivatives of 7-hydroxycoumarin containing terpene fragments have also attracted the attention of the medicinal chemistry community [19–21]. The best-known compound of this structural type is auraptene (Figure 1), for which a variety of biological activities are known, including antitumor properties [22].

One of the current approaches to increase the efficacy of clinically-established antitumor therapy is the inhibition of DNA repair enzymes that counteract the effect of DNA-damaging chemotherapy agents [23–25]. One of these important enzymes is tyrosyl-DNA phosphodiesterase 1 (Tdp1) [26]. Tdp1 is involved in the repairing of damaged DNA, including the removal of lesions caused by topoisomerase 1 (Top1) inhibitors. Top1 inhibitors such as the camptothecin derivatives (CPTs), topotecan (tpc), and irinotecan are well-established antitumor agents [27] that are widely used [28]. Thus, Tdp1 reduces the impact of Top1 poisons, resulting in diminished DNA damage and reduced efficacy of this class of chemotherapeutic drugs.

Because Tdp1 repairs Top1/DNA cleavage complexes induced by CPTs, inhibitors of Tdp1 can enhance the sensitivity of cancer cells to CPT analogues [29]. Furthermore, increased Tdp1 expression counteracts the cytotoxicity of CPTs [30,31], and is frequently observed in cancers resistant to CPT therapy [31–33]. Convincing evidence exists from preclinical studies that the ratio of Tdp1/Top1 activity influences cellular sensitivity to Top1 inhibitors [34,35], and that the suppression of Tdp1 activity leads to an increase in the sensitivity of tumor cells to CPTs [32,36–39]. It is believed that targeted short-term treatment with a potent Tdp1 inhibitor will not lead to serious poisoning in normal cells. Indeed, it was shown that Tdp1-/- knockout mice were fertile and had a normal life expectancy, with no signs of premature aging [40]. Until now, no inhibitors of the Tdp1 enzyme have reached human clinical testing.

To date, many Tdp1 inhibitors have been identified. A major class of Tdp1 inhibitors comprises those based on natural products including usnic acid derivatives [41–45], coumarins [46], adamantanes [47–49], nucleoside analogs [50], dehydroabietylamine derivatives [51], chromenes [52], bile acids derivatives [53], and fungal products [54–56]. There are also early reports of Tdp1 inhibition based on diamidines [57], antibiotics [58,59], steroids [60], and other compounds [61]. Nevertheless, only a few Tdp1 inhibitors have been tested in cell- or cancer-models. Synergy with tpc has been demonstrated in vivo for just two Tdp1 inhibitors, both of which were derivatives of usnic acid, a natural product [43,44].

Previously, virtual screening of the InterBioScreen natural product library [62] and subsequent testing identified that 3-methoxybenzyl, a derivative of 7-hydroxycoumarin, annelated with the cyclohexane ring **1** (Figure 1) as a new structural type of Tdp1 inhibitors [46]. Further optimization of the inhibitor, including the replacement of the aromatic substituent in the phenolic group with bulky monoterpenoid substituents, allowed us to increase the potency by almost an order of magnitude, thereby reaching nanomolar activity [46]. Most importantly, the use of compound **2** in non-toxic doses significantly increased the cytotoxic activity of CPT in human cancer cells [46].

**Figure 1.** Examples of biologically-active coumarins: auraptene and compounds **1**–**3**.

Based on molecular modeling, it was predicted that the attachment of an aromatic substituent at the fourth position of coumarin would be promising for enhanced binding. Note that 4-arylcoumarins are often considered a separate group of natural products, called neoflavones. Natural and synthetic neoflavones have low toxicities and exhibit a broad spectrum of biological activity, in particular against tumors [7]. The aim of this work was to synthesize neoflavone derivatives of structural type 3 (Figure 1) by varying both aromatic and monoterpene substituents to determine their inhibitory activity against Tdp1, and to study the synergistic effect with tpc, a clinically-important Top1 inhibitor, in in vivo experiments. As a result, it was discovered that arylcoumarins containing monoterpenoid substituents are indeed potent Tdp1 inhibitors and, most importantly, are able to enhance the antitumor activity of tpc in animal models.
