**5.4. Three-atom linker with thiochroman ring replaced**

**Compound Cell growth inhibition**

78 Anti-cancer Drugs - Nature, Synthesis and Cell

cancer cell lines (1) SW954 and (2) SW962 from Ref. [43].

cancer cell lines (1) SW954 and (2) SW962 from Ref. [43].

**Table 3.** Structural modifications of Flex-Hets, and their effects on cancer cell growth.

"–" indicates no data available.

**Table 2.** Structural modifications of Hets, and their effects on cancer cell growth.

"–" indicates no data available.

**Renal (%) Ovarian**

**cancer (%)**

Growth inhibition (%) for renal cancer cell lines (1) Caki-1 and (2) 786-0; Normal renal cells (1) HK-2 and (2) RTC91696 [23]. Growth Inhibition (%) for ovarian cancer cell lines (1) CAOV-3, (2) OVCAR-3, and (3) SKOV-3 [6]. EC50 values for 50% growth inhibition for ovarian cancer cell line (4) A2780, and Normal endometrial cells (NE) [6, 41]. Growth inhibition (%) for cervical cancer cell lines (1) SiHa, (2) CC-1, (3) C33a, and (4) HT-3 [5]. Growth inhibition (%) for head and neck squamous cell cancer cell lines (HN) (1) SCC-2 [43], and (2) SCC-38 [32, 42]. Growth inhibition (%) for vulvar

**Cell growth inhibition**

**Renal normal (%)**

**Ovarian cancer (%)**

**Renal cancer (%)**

 O CO2Et H H 69 45 26 40 40 69 47 2.8 4.5 67 59 85 76 42 S CO2Et H H 56 54 35 28 24 58 42 2.9 2.7 58 42 84 65 39 S NO2 H H 84 72 51 37 55 67 45 1.7 3.0 68 58 87 92 81 O NO2 H H 86 79 53 52 – – – 1.0 2.3 – – – – – O CO2Me H H 57 62 26 46 – – – – – – – – – – S CO2Me H H 39 45 44 38 – – – – – – – – – – S H CO2Me H 34 42 48 – – – – – – – – – – – SHetA19 S NO2 H Me – – – – 32 65 51 – – – – – – 68

**X R1 R2 R3 1 2 1 2 1 2 3 4 NE 1 2 3 4 1**

Growth inhibition (%) for renal cancer cell lines (1) Caki-1 and (2) 786-0; Normal renal cells (1) HK-2 and (2) RTC91696 [23]. Growth Inhibition (%) for ovarian cancer cell lines (1) CAOV-3, (2) OVCAR-3, and (3) SKOV-3 [6]. EC50 values for 50% growth inhibition for ovarian cancer cell line (4) A2780, and Normal endometrial cells (NE) [6, 41]. Growth inhibition (%) for cervical cancer cell lines (1) SiHa, (2) CC-1, (3) C33a, and (4) HT-3 [5]. Growth inhibition (%) for head and neck squamous cell cancer cell lines (HN) (1) SCC-2 [43], and (2) SCC-38 [32, 42]. Growth inhibition (%) for vulvar

Substitutions on the phenyl group have also been evaluated [7]. The nitro (NO2) substitution (**Table 3**, **16**, **17**) consistently exhibited greater growth inhibitory and apoptotic activity than

**NE EC50 (µM)**

**Cancer Normal (%)**

– – 11 – 19 – 34 19 – – – – 74 –

**Vulvar cancer (%)** 

**Cervical cancer**

**HN cancer**

**(%)** 

**NE Cervical cancer (%)**

**EC50(µM)**

**HN cancer (%)** In order to expand the potential clinical applications, we designed, synthesized, and evaluated nine *p*-nitrodiarylthiourea analogs in breast (MCF-7, T-47D, MDA-MB-453) and prostate (DU-145, PC-3, LNCaP) cancer cell lines for their anticancer activities. Majority of our compounds were able to inhibit the growth of these six cancer cell lines at low micromolar concentrations. Compound **23** (**Figure 6**) was found to be the most potent anticancer agent in this series with GI50 values of 3.16 μM for MCF-7, 2.53 μM for T-47D, 4.77 μM for MDA-MB-453 breast cancer lines and 3.54 μM for LNCaP prostate cancer cell line. These GI50 values were comparable to the parent compound, SHetA2 [47].

**Figure 6.** Three- and four-atom linkers in retinoid derivatives.

#### **5.5. Four-atom linker**

Another approach to the structure modification of SHetA2 was to keep the thiochroman ring but replace the thiourea linker with a 4-atom acrylamide linker NC(O)C=C and various substitutions on the terminal aryl ring. When evaluated in a cytotoxicity assay of the human A2780 ovarian cancer cell line, results indicate that activity of 4-nitro phenyl analogs are comparable to that of SHetA2 with efficacies slightly reduced compared to SHetA2 [48]. The α,β-unsaturated acrylamide may contribute to its increase in potency and the compound **24** (**Figure 6**) is the example.
