Synthesis of Flexible Sulfur-Containing Heteroarotinoids That Induce Apoptosis and Reactive Oxygen Species with Discrimination between Malignant and Benign Cells

Abstract

Regulation of growth, differentiation, and apoptosis by synthetic retinoids can occur through mechanisms that are dependent and independent of their ability to bind and activate nuclear retinoic acid receptors. The objective of this study was to determine if increasing flexibility of the heteroarotinoid structure would affect the specificity of the synthetic retinoids for the receptors and for their regulation of cancerous and nonmalignant cells. Methods were developed to produce the first examples of heteroarotinoids 15a − 15h, which contain urea and/or thiourea linking groups between two aryl rings. Substituents at the para position of the single phenyl ring were either an ester, a nitro group, or a sulfonamide group. Ovarian cancer cell lines Caov-3, OVCAR-3, SK-OV-3, UCI-101, and 222 were utilized, and the inhibitory prowess of the heteroarotinoids was referenced to that of 4-HPR (25). Similar to 4-HPR (25), the heteroarotinoids inhibited growth of all cell lines at micromolar concentrations. Although the heteroarotinoids did not activate retinoic acid receptors, the agents induced potent growth inhibition against the cancer cells with weak activity against normal and benign cells. The growth inhibition was associated with cell loss and induction of reactive oxygen species.

Flex-Hets differentially induce apoptosis in cancer over normal cells by directly targeting mitochondria

“…The mechanism of this activity has been shown to occur through the intrinsic mitochondrial pathway associated with loss of mitochondrial membrane integrity, generation of reactive oxygen species (ROS), release of cytochrome c from mitochondria, and activation of caspase-3 in head and neck cancer cell lines (4). Generation of ROS was confirmed in ovarian cancer cell lines (2).…”

“…Role of ROS in the Flex-Het Mechanism Our previous studies showed that SHetA2 induces ROS in ovarian and in head and neck cancer cell lines (2,4). To test the hypothesis that SHetA2 exerts mitochondrial effects through generation of intracellular ROS, the ability of SHetA2 to induce apoptosis in the presence of the antioxidants BHA or MnTBAP was measured.…”

“…This objective seems to be met by a novel class of synthetic compounds called flexible heteroarotinoids (Flex-Hets). In vitro studies showed that the Flex-Hets, called SHetA2, SHetA3, and SHetA4, induced differentiation and apoptosis in cancer cell lines and primary cultures of cancer cells (1,2). The Flex-Het compound called SHetA2 was chosen as the lead Flex-Het because it induced the highest levels of apoptosis in multiple cancer types.…”

“…Flex-Hets were developed from a class of retinoid drugs, heteroarotinoids, by replacing the conventional two-atom linker with a more flexible urea or thiourea linker (2). Although the retinoids exert their activities through nuclear retinoic acid receptors, the Flex-Hets do not require the retinoid receptors for their action, nor are their activities prevented by retinoid receptor antagonists (2,4).…”

“…Although the retinoids exert their activities through nuclear retinoic acid receptors, the Flex-Hets do not require the retinoid receptors for their action, nor are their activities prevented by retinoid receptor antagonists (2,4). This may explain the lack of conventional retinoid toxicities, such as skin irritation or teratogenicity, observed when Flex-Hets were tested in animal models designed to specifically test for these toxicities (3,5).…”