Duchenne muscular dystrophy (DMD) is a severe X-linked disease resulting in chronic inflammation, muscle degeneration, and muscle fibrosis. Although current therapies like corticosteroids focus on slowing disease progression, they are associated with various adverse effects; corticosteroids downregulate all downstream mediators, including beneficial prostaglandins (PGs) like PGE2, PGI2, and PGF2α. Clearly, there is a vital need for novel therapeutic targets that address underlying inflammation. One promising target is through inhibiting haematopoietic prostaglandin D2 synthase (HPGDS), which is responsible for the generation of prostaglandin D2 (PGD2), an inflammatory mediator known to exacerbate DMD symptoms. An HPGDS inhibitor, TAS-205 (Taiho Pharma) is currently in Phase III clinical trials, however this compound suffers from average potency and pharmacokinetic profiles. Here, we describe the optimisation from our previous tool HPGDS inhibitor, PK007, allowing us to synthesise and characterise our best-in-class HPGDS inhibitor, CLS189. This optimised compound (CLS189: 2.59 ± 0.73 nM; n=3) demonstrated 11-fold higher potency than TAS-205 (28.34 ± 4.71; n=4) in cell-based assays, and 27-fold higher oral exposure. In vitro assays have shown that treatment with CLS189 selectively decreased inflammatory PGD2, whilst maintaining or upregulating beneficial PGs, which promote DMD recovery. Preclinical studies within the acute stage (postnatal day 18-28) of the gold standard DMD mouse model, mdx mice, showed that oral administration of CLS189 (1 mg/kg/day; n=9) displayed similar grip strength efficacy as TAS-205 (10 mg/kg/day; n=6), but at a lower dose. Grip strength was significantly increased compared to vehicle mdx mice (n=7) at the end of the treatment. The body weight of the CLS189 treated mice remained consistent throughout, indicating that the drug was well tolerated. CLS189 mdx mice displayed significant reduction in myonecrosis and fibrotic area of their hindlimb muscles. Our data suggested the successful optimisation from our previous compound, PK007, to develop the best-in-class HPGDS inhibitor, CLS189.