Synthesis and Cytotoxic Activity of Novel Hybrids of Dehydroabietic Acid With Semi‐, Thiosemicarbazides and 1,3,4‐Oxadiazoles

A series of novel hybrids of dehydroabietic acid (DHA) with semi-, thiosemicarbazides, and amino-1,3,4-oxadiazoles was synthesized starting from respective thioisocyanates or isocyanates. The cyclodehydrosulfurization of thiosemicarbazides with Hg(OAc)₂ was used as a fast and efficient method to obtain regioisomeric 2-alkyl(aralkyl, aryl)amino-5-terpenyl- and 2-terpenylamino-5-alkyl(aralkyl, aryl)-1,3,4-oxadiazoles, even in the case of sterically hindered dehydroabietyl derivatives. Several conformationally flexible acylthiosemicarbazide and 2-amino-1,3,4-oxadiazole hybrids exhibited cytotoxicity against HepG2, MCF-7, and HeLa tumor cell lines that was significantly superior to that of the parent DHA. Among them, 2-(abieta-8,11,13-trien-18-oyl)-N-phenylhydrazincarbothioamide was the most active (GI₅₀ = 12-17 µM), inhibiting the growth of tumor cells with greater selectivity (SI = 4.6-7.2) than doxorubicin, a positive control. The thiosemicarbazide moiety in the molecules of the acylthiosemicarbazide hybrids was important for the inhibition of the tumor cells' growth, while its adjacement mode to the terpenyl fragment was tolerant for retaining cytotoxicity. Replacement of the sulfur-containing thiosemicarbazide moiety with the sulfur-free semicarbazide resulted in the loss of cytotoxic properties. Absorption, distribution, metabolism, and excretion (ADME) parameters of the synthesized hybrids were assessed in silico.