# Design, Synthesis, and Cell Lines Studies of Oleanolic Acid--Hydrogen Sulfide Donor Hybrids
###### Abstract  
In order to develop new oleanolic acid (OA) derivatives endowed with improved antitumor activities, for the first time, a number of new hybrid compounds were reported by combining OA or 3-oxooleanolic acid with appropriate H\({}_{2}\)S-donor moiety, coupled via a suitable linker. The anti-tumor evaluation indicated that they exhibited excellent anti-cancer activities against the tested cancer cell lines. Moreover, **18d** with 5-(4-hydroxyphenyl)-3H-1,2-dithiole-3-thione moiety as H\({}_{2}\)S donor and \(\beta\)-alanine as the linker, showed more potent cytotoxicity against the tested cancer cell lines than OA and 3-oxooleanolic acid, especially for A549 cells. Furthermore, the preferred compound, **18d**, preferentially accumulates in cancer cells (13.6 \(\upmu\)M) over the matched normal cells LO2 (>100 \(\upmu\)M) in vitro. The improved antitumor activity of this hybrid was probably due to its H\({}_{2}\)S-releasing capability. oleanolic acid derivative; hydrogen sulfide; synthesis; anti-tumor activity +
Footnote †: journal: Applied  
18 March 2021  
1  
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Received: 18 March 2021  
Accepted: 5 April 2021  
Published: 8 April 2021  
**Publisher's Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. **  
**Published: 8 April 2021**  
H\({}_{2}\)S donor hybrid compounds have been synthesized and biologically evaluated, which were identified to display increased effectiveness and reduced toxicity when compared with their parent drugs due to synergic effects [22; 23; 24; 25; 26; 27; 28; 29; 30]. However, the linkage between drugs and hydrogen sulfide donors was relatively simple, most of which was connected by ester bonds, not a heterozygous modification [26; 27; 28; 29; 30]. Additionally, compared with the other reported slow-releasing hydrogen sulfide (H\({}_{2}\)S) donors, only a slow-releasing hydrogen sulfide (H\({}_{2}\)S) donor in aqueous media over a period of hours to days reportedly compared with sulfide salts such as sodium hydrosulfide (NaHS) and sodium sulfide (Na\({}_{2}\)S) in its evaluation of biological effects in cells, tissues and animals, but there are few examples of connecting it to other parent anticancer drugs [26; 27; 28; 29; 30]. In recent years, it has been found that the efficiency can be improved by coupling different types of specific or sensitive linkages. Among them, the coupling of amino acids has captured the greatest attention in pro-drug design. The coupling of amino acids can be selectively identified and taken up by peptide transporter 1 (PepT1) to improve the oral absorption of parent drugs with undesirable biopharmaceutical characteristics [4; 9]. PepT1 appears to be an attractive target with its high capacity, broad substrate specificity, and high level of expression in the intestinal epithelium. Many PepT1 targeted prodrugs have been designed and synthesized with the aim of improving oral bioavailability of parent drugs, in which valacyclovir and valgancilovir are two excellent examples (they are \(L\)-Val ester prodrugs of acyclovir and gancilovir, respectively) [8]. Furthermore, the redox sensitiveness disulfide structure is a promising linkage because the redox process is widely present in physiological environments. Especially, there is a significant redox gradient for tumors between the intracellular components and the extracellular environments. Many studies have reported that polymer micelles with disulfide structures have a good ability to transfer antitumor drug to cancer cells and effectively perform intracellular drug release [31; 32]. Regarding the abovementioned studies, it may be interesting to study the hybrid OA compounds containing H\({}_{2}\)S-donating species through different linkers.