**3. SHetA2, a Flex-Het**

**2.2. Arotinoids**

70 Anti-cancer Drugs - Nature, Synthesis and Cell

[29].

**2.3. Heteroarotinoids**

reduced toxicities [29, 31].

shown in **Figure 2**.

**2.4. Flexible heteroarotinoids**

Recognizing the potential chemopreventive and chemotherapeutic effects of retinoids, chemists have tried to design and synthesize new retinoid derivatives that express high cytotoxic potential with lower toxicity. The rationale is that by developing compounds that selectively activate only one retinoid receptor subtype, the associated toxicities could be reduced. For example, RXR-selective compounds were thought to have great potential for pharmacological use against tumors and other diseases due to a large number of ligand activated receptors whose activities involve dimerization with RXR receptors [19]. Bexarotene (Targretin®, with a one-carbon linker), is a selective RXR agonist and the first synthetic arotinoid approved for the treatment of all stages of cutaneous T-cell lymphoma [27]. RARγ, selectively expressed at high levels in the skin, has been shown to be the mediator of retinoid activity [28]. Early efforts to increase the therapeutic ratio of retinoids included increasing the structure rigidity by incorporating a double-bond between the two aromatic rings, leading to the development of arotinoids with two atom linker. For example, TTNPB (**Figure 1**, **5**), an arotinoid with significant anti-tumor activity [29] was found to be selective for RAR receptors [30]. Unfortunately, intolerable toxicities observed in animal models limited its clinical utility

Subsequent structural modifications involved the addition of one heteroatom (O, N, and S) in the cyclic ring of the arotinoids to block its oxidation into toxic metabolites. This resulted in the development of a new class of compounds called the heteroarotinoids (Hets, **Figure 1**, **6**– **12**). These Hets exhibited similar biological activities to retinoids, but with significantly

Different linkers were placed between the two aryl groups of the Hets to increase their rigidity, providing a more specific fit into each receptor's binding pocket. Two-atom linkers, such as amide (**Figure 1**, **8**) and esters (**Figure 1**, **9**–**11**) were synthesized, and they showed varying degree of receptor selectivity. Interestingly, Hets with either the three-atom urea (**Figure 1**, **14**, **17**, **18**) or thiourea (**Figure 1**, **15**, **16**, **19**, **20**) linker demonstrated significant anticancer activity without activating any of the retinoid receptors [6]. These compounds showed significant growth inhibitory activities against the ovarian cancer cell lines: Caov-3, OVCAR-3, and SK-OV-3, while exhibiting low activities against normal and benign cells. Moreover, due to the lack of RAR/RXR activation, these compounds did not exhibit the associated toxicities observed with other retinoids. Among these compounds, SHetA2 (**Figure 1**, **16**) demonstrated the greatest potency against the aforementioned ovarian cancer cell lines at concentrations ranging from 0.2–3.7 μM. Since both urea and thiourea linkers are somewhat flexible in nature, these compounds were termed flexible heteroarotinoids (Flex-Hets). The evolution of SHetA2 is Flex-Hets have shown cancer-preventive activity by regulating apoptosis, cell growth, and differentiation in multiple types of cancer cell lines. They display significantly greater growth inhibition activities against ovarian cancer cells than both normal and benign ovarian cells [6]. Among the Flex-Hets, SHetA2 has shown to induce the highest levels of apoptosis in multiple cancer cell types, while retaining the differential resistance in normal cells [5]. SHetA2 functions independently of retinoid receptors [32] and therefore, lacks the toxicities associated with conventional retinoids. More importantly, SHetA2 differentially induces apoptosis in cancer cells while sparing normal cells [6]. All 60 cancer cell lines in NCI human tumor panel were sensitive to the growth inhibition activities of SHetA2 at micromolar concentrations [5] (**Table 1**). Despite this wide range of activity against cancer cells, normal and benign human ovarian, endometrial, and oral cultures were resistant to SHetA2-induced apoptosis and growth inhibition [6, 9]. In a recent study, SHetA2 was tested to be negative across three genetic toxicology assays, namely the *in vitro Salmonella-Escherichia coli* mutagenicity assay, Chinese hamster ovary cell chromosome aberration assay (CHO-CA), and *in vivo* mouse bone marrow micronucleus assay [34]. These assays are recommended by the International Conference on Harmonization (ICH), US Food and Drug Administration (FDA), and other regulatory agencies for the detection of mutagenicity by chemicals [35]. These negative results showed that SHetA2 was neither mutagenic nor genotoxic. In addition, SHetA2 exhibited no evidence of toxicity in animal models including dogs [13], and it did not induce skin irritation or teratogenicity [5, 36]. Encouraged by these promising results, SHetA2 was evaluated by NCI's RAID program for preclinical development as a cancer therapeutic agent (Application 196, Compound NSC 726189), and by RAPID program as a chemopreventive agent. Since SHetA2 showed no toxicity in preclinical studies, it is now in Phase-0 clinical trial.


SHetA2, a New Cancer-Preventive Drug Candidate http://dx.doi.org/10.5772/65365 73


Lower values indicate greater inhibition of cell growth.

**Table 1.** Growth inhibition values (GI50) of NCI's human tumor cell line panel expressed in log scale.
