PTGER4 and neoplasm: Several companies are currently conducting clinical trials of EP4 receptor-selective antagonists for cancer therapy (ClinicalTrials ID: NCT03658772, NCT03696212, NCT03152370, NCT03661632) and evaluating EP4 antagonist therapy in anti-PD-1-refractory tumors, microsatellite stable tumors, and in combination with tumor radiotherapy.