Fluorine Modulates Species Selectivity in the Triazolopyrimidine Class of <i>Plasmodium falciparum</i> Dihydroorotate Dehydrogenase Inhibitors

Malaria is one of the most serious global infectious diseases. The pyrimidine biosynthetic enzyme Plasmodium falciparum dihydroorotate dehydrogenase (<i>Pf</i>DHODH) is an important target for antimalarial chemotherapy. We describe a detailed analysis of protein–ligand interactions between DHODH and a triazolopyrimidine-based inhibitor series to explore the effects of fluorine on affinity and species selectivity. We show that increasing fluorination dramatically increases binding to mammalian DHODHs, leading to a loss of species selectivity. Triazolopyrimidines bind Plasmodium and mammalian DHODHs in overlapping but distinct binding sites. Key hydrogen-bond and stacking interactions underlying strong binding to <i>Pf</i>DHODH are absent in the mammalian enzymes. Increasing fluorine substitution leads to an increase in the entropic contribution to binding, suggesting that strong binding to mammalian DHODH is a consequence of an enhanced hydrophobic effect upon binding to an apolar pocket. We conclude that hydrophobic interactions between fluorine and hydrocarbons provide significant binding energy to protein–ligand interactions. Our studies define the requirements for species-selective binding to <i>Pf</i>DHODH and show that the triazolopyrimidine scaffold can alternatively be tuned to inhibit human DHODH, an important target for autoimmune diseases

Isoxazolopyrimidine-Based Inhibitors of Plasmodium falciparum Dihydroorotate Dehydrogenase with Antimalarial Activity

Malaria kills nearly 0.5 million people yearly and impacts the lives of those living in over 90 countries where it is endemic. The current treatment programs are threatened by increasing drug resistance. Dihydroorotate dehydrogenase (DHODH) is now clinically validated as a target for antimalarial drug discovery as a triazolopyrimidine class inhibitor (DSM265) is currently undergoing clinical development. We discovered a related isoxazolopyrimidine series in a phenotypic screen, later determining that it targeted DHODH. To determine if the isoxazolopyrimidines could yield a drug candidate, we initiated hit-to-lead medicinal chemistry. Several potent analogues were identified, including a compound that showed in vivo antimalarial activity. The isoxazolopyrimidines were more rapidly metabolized than their triazolopyrimidine counterparts, and the pharmacokinetic data were not consistent with the goal of a single-dose treatment for malaria

Tetrahydro-2-naphthyl and 2‑Indanyl Triazolopyrimidines Targeting <i>Plasmodium falciparum</i> Dihydroorotate Dehydrogenase Display Potent and Selective Antimalarial Activity

Malaria persists as one of the most devastating global infectious diseases. The pyrimidine biosynthetic enzyme dihydroorotate dehydrogenase (DHODH) has been identified as a new malaria drug target, and a triazolopyrimidine-based DHODH inhibitor <b>1</b> (DSM265) is in clinical development. We sought to identify compounds with higher potency against <i>Plasmodium</i> DHODH while showing greater selectivity toward animal DHODHs. Herein we describe a series of novel triazolopyrimidines wherein the <i>p</i>-SF₅-aniline was replaced with substituted 1,2,3,4-tetrahydro-2-naphthyl or 2-indanyl amines. These compounds showed strong species selectivity, and several highly potent tetrahydro-2-naphthyl derivatives were identified. Compounds with halogen substitutions displayed sustained plasma levels after oral dosing in rodents leading to efficacy in the <i>P. falciparum</i> SCID mouse malaria model. These data suggest that tetrahydro-2-naphthyl derivatives have the potential to be efficacious for the treatment of malaria, but due to higher metabolic clearance than <b>1</b>, they most likely would need to be part of a multidose regimen