DEVELOPMENT OF A SECOND GENERATION OF ARNT INHIBITORS: ABIETANE-TYPE DITERPENOIDS

Colistin, also known as polymyxin E, is a last-line antibiotic used for the treatment of multidrug resistant Gram-negative bacterial infections.1 However, in recent years, the occurrence of resistance phenomena related to the use of such antibiotics has been documented. The molecular mechanism that gives resistance to colistin in several Gram-negative bacteria, including Pseudomonas aeruginosa, has to be mainly ascribed to a glycosyltransferase enzyme, named ArnT.2 In a previous study, based on microbiological assays coupled with molecular modeling studies performed on a library of natural and semisynthetic small molecules, we have demonstrated that a diterpene scaffold bearing an oxalate-like group at C-18/C-19 or a sugar residue C-19 to resemble L-Ara4N is an essential requirement for more efficient inhibition of bacterial growth in the presence of colistin.3 In order to optimize the hit compound, we set up rational procedures which simplify the ent-beyerane scaffold into drug-like synthetic molecules able to minimize the pharmacophores that proved to be crucial for biological activity. As a result, different naturally occurring diterpene scaffolds have been evaluated as a source of starting material for the synthesis of a second generation of ArnT inhibitors. In particular, the abietane-type diterpenoids (>7000 molecules) have been deeply investigated for further optimization of ArnT-mediated colistin resistance inhibitors with improved activity and, thanks to preliminary in silico studies, we have been able to select the abietic, dehydroabietic and podocarpic acid as some key starting points for the development of a second generation of ArnT inhibitors. Thus, a small focused library of abietane-type diterpenoids derivatives (AB2, DH3, PD3) has been designed and synthesized, combining the abietane scaffold with the functional groups essential for an efficient inhibition of the ArnT activity. The most promising compounds emerging from the experiments will be further optimized through additional cycles of design-synthesis-bioassay and tested in vitro.