Prodrug Approach to Exploit (S) Alanine as Arginine Mimic Moiety in the Development of Protein Arginine Methyltransferase 4 Inhibitors

: Protein arginine methyltransferase (PRMT) 4 (also known as coactivator-associated arginine methyltransferase 1; CARM1) is involved in a variety of biological processes and is considered as an emerging target class in oncology and other diseases. A successful strategy to identify PRMT substrate-competitive inhibitors has been to exploit chemical scaffolds able to mimic the arginine substrate. (S)-Alanine amide moiety is a valuable arginine mimic for the development of potent and selective PRMT4 inhibitors; however, its high hydrophilicity led to derivatives with poor cellular outcomes. Here, we describe the development of PRMT4 inhibitors featuring a central pyrrole core and an alanine amide moiety. Rounds of optimization, aimed to increase lipophilicity and simultaneously preserve the inhibitory activity, produced derivatives that, despite good potency and physicochemical properties, did not achieve on-target effects in cells. On the other hand, masking the amino group with a NAD(P)H:quinone oxidoreductase 1 (NQO1)-responsive trigger group, led to prodrugs able to reduce arginine dimethylation of the PRMT4 substrates BRG1-associated factor 155 (BAF155). These results indicate that prodrug strategies can be successfully applied to alanine-amide containing PRMT4 inhibitors and provide an option to enable such compounds to achieve sufficiently high exposures in vivo.