The haematopoietic prostaglandin D2 synthase (HPGDS) has proinflammatory effects in various diseases, like Duchenne muscular dystrophy (DMD), where its product prostaglandin D2 (PGD2) has a role in regulating some of the adverse disease characteristics. DMD is typically characterised through progressive muscle degeneration, weakness, and loss of muscle function. Currently, the only pharmacologic agent approved for treatment of DMD are corticosteroids, which cause numerous adverse effects. Here we describe the optimisation of our lead HPGDS inhibitor, PK007. New inhibitor CLS122 exhibited approximately a four-fold increase in potency compared to PK007, whilst CLS134 demonstrated similar potency to PK007. These compounds were also tested for liver microsome stability and cell membrane permeability. Utilising the gold standard DMD mouse mdx model, we evaluated the efficacy of CLS122 and CLS134 in vivo, through measuring changes in grip strength and body weight. Grip strength was significantly improved in the CLS122 and CLS134 treated groups compared to the untreated group and even the PK007 treated group, indicating reduced inflammation and an increase in muscle regeneration. Overall, we successfully optimised the potency and in vivo efficacy of PK007. In all, these findings support the notion that PGD2 plays a vital role in the pathology of DMD, thus validating the use of HPGDS inhibitors as a novel therapeutic for the treatment of DMD.