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In recent years, human dihydroorotate dehydrogenase inhibitors have been associated with acute myelogenous leukemia as well as studied as potent host targeting antivirals. Starting from MEDS433 (IC 50 1.2 nM), we kept improving the structure-activity relationship of this class of compounds characterized by 2-hydroxypyrazolo[1,5- a ]pyridine scaffold. Using an in silico/crystallography supported design, we identified compound 4 (IC 50 7.2 nM), characterized by the presence of a decorated aryloxyaryl moiety that replaced the biphenyl scaffold, with potent inhibition and pro-differentiating abilities on AML THP1 cells (EC 50 74 nM), superior to those of brequinar (EC 50 249 nM) and boosted when in combination with dipyridamole. Finally, compound 4 has an extremely low cytotoxicity on non-AML cells as well as MEDS433; it has shown a significant antileukemic activity in vivo in a xenograft mouse model of AML.

The connection with acute myelogenous leukemia (AML) of dihydroorotate dehydrogenase ( h DHODH), a key enzyme in pyrimidine biosynthesis, has attracted significant interest from pharma as a possible AML therapeutic target. We recently discovered compound 1 , a potent h DHODH inhibitor (IC 50 = 1.2 nM), able to induce myeloid differentiation in AML cell lines (THP1) in the low nM range (EC 50 = 32.8 nM) superior to brequinar's phase I/II clinical trial (EC 50 = 265 nM). Herein, we investigate the 1 drug-like properties observing good metabolic stability and no toxic profile when administered at doses of 10 and 25 mg/kg every 3 days for 5 weeks (Balb/c mice). Moreover, in order to identify a backup compound, we investigate the SAR of this class of compounds. Inside the series, 17 is characterized by higher potency in inducing myeloid differentiation (EC 50 = 17.3 nM), strong proapoptotic properties (EC 50 = 20.2 nM), and low cytotoxicity toward non-AML cells (EC 30 (Jurkat) > 100 μM).

Human dihydroorotate dehydrogenase ( hDHODH) catalyzes the rate-limiting step in de novo pyrimidine biosynthesis, the conversion of dihydroorotate to orotate. hDHODH has recently been found to be associated with acute myelogenous leukemia, a disease for which the standard of intensive care has not changed over decades. This work presents a novel class of hDHODH inhibitors, which are based on an unusual carboxylic group bioisostere 2-hydroxypyrazolo[1,5- a]pyridine, that has been designed starting from brequinar, one of the most potent hDHODH inhibitors. A combination of structure-based and ligand-based strategies produced compound 4, which shows brequinar-like hDHODH potency in vitro and is superior in terms of cytotoxicity and immunosuppression. Compound 4 also restores myeloid differentiation in leukemia cell lines at concentrations that are one log digit lower than those achieved in experiments with brequinar. This Article reports the design, synthesis, SAR, X-ray crystallography, biological assays, and physicochemical characterization of the new class of hDHODH inhibitors.