Potent 2,3-dihydrophthalazine-1,4-dione derivatives as dual inhibitors for mono-ADP-ribosyltransferases PARP10 and PARP15

While human poly-ADP-ribose chain generating poly-ARTs, PARP1 and 2 and TNKS1 and 2, have been widely characterized, less is known on the pathophysiological roles of the mono-ADP-ribosylating mono-ARTs, partly due to the lack of selective inhibitors. In this context, we have focused on the development of inhibitors for the mono-ART PARP10, whose overexpression is known to induce cell death. Starting from OUL35 (1) and its 4- (benzyloxy)benzamidic derivative (2) we herein report the design and synthesis of new analogues from which the cyclobutyl derivative 3c rescued cells most efficiently from PARP10 induced apoptosis. Most importantly, we also identified 2,3-dihydrophthalazine-1,4-dione as a new suitable nicotinamide mimicking PARP10 inhibitor scaffold. When it was functionalized with cycloalkyl (8a-c), o-fluorophenyl (8h), and thiophene (8l) rings, IC50 values in the 130–160 nM range were obtained, making them the most potent PARP10 inhibitors reported to date. These compounds also inhibited PARP15 with low micromolar IC50s, but none of the other tested poly- and mono-ARTs, thus emerging as dual mono-ART inhibitors. Compounds 8a, 8h and 8l were also able to enter cells and rescue cells from apoptosis. Our work sheds more light on inhibitor development against mono-ARTs and identifies chemical probes to study the cellular roles of PARP10 and PARP15. 1. Introduction Human ADP-ribosyltransferases (ARTs) of the Diphtheria toxin like family contain 17 proteins that catalyse ADP-ribosylation, a reversible post-translational modification of proteins, DNA and RNA [1–3]. These enzymes share a catalytic ADP-ribosyltransferase domain able to mediate the covalent attachment of ADP-ribose to substrates using nicotinamide adenine dinucleotide (NAD+) as a co-factor. Four family members, PARP1, PARP2, TNKS1, and TNKS2 are poly-ARTs and ca- talyse the iterative, covalent attachment of multiple ADP-ribose units resulting in poly-ADP-ribosylation (PARylation). On the contrary most of the family members are mono-ARTs able to only mono-ADP-ribosylate substrates (MARylation) [4]. PARylation catalysed by PARP1 and PARP2 is a crucial step in DNA damage recognition and initiation of DNA repair through PAR-mediated recruitment of repair factors [5]. In cancer cells that are deficient in homologous recombination repair, for example due to mutations in the tumour suppressor genes BRCA1 or BRCA2, PARP-mediated DNA repair becomes essential. Such cancer cells are highly sensitive to PARP in- hibitors [6]. This is referred to as synthetic lethality and led to the clinical approval of olaparib as the first PARP inhibitor (PARPi) for the treatment of BRCA-mutated advanced ovarian cancer in 2014 [7]. Additional PARPis including rucaparib, niraparib, and talazoparib were approved for use in various clinical settings more recently. The other two poly-ARTs, TNKS1 and TNKS2, are implicated in controlling telo- mere maintenance, protein and protein complex stability, and signaling.