Coronaviruses have caused three large outbreaks in the past century. The most recent one, also still ongoing, is represented by the SARS-CoV-2/Covid-19 pandemic. Efforts have been taken to develop efficient vaccines and antivirals and one of the major virus-based targets in drug development is represented by the main protease of these viruses. Main proteases are proteins (cysteine hydrolases) with high level of conservation among different coronaviruses and have an important role in the virus life cycle. Due to the need of developing broad-spectrum antivirals against Coronaviruses, this study aimed to set up a CPE-based assay for testing compounds against the main protease of human coronavirus 229E. An optimized TCID50 protocol was established by using MRC-5 cells, at a density of 1x104 cells/ml with a 3h incubation prior infection with a concentration of 10-1 of HCoV-229E. The cell viability was assessed through MTT assay. Using reference compounds, with previously demonstrated antiviral potency against the main protease of different coronaviruses (GC-376, Nirmatrelvir), the efficiency of the conceived assay was validated (GC-376 EC50 = 1.24 μM; Nirmatrelvir Ec50= 0.72 μM). Compound 19 was proved to also be active against the main protease of HCoV-229E (EC50 = 0.22 μM), and together with previous findings, it was concluded that this compound has a broad-spectrum activity. Newly developed compounds MP17 and MP19 were also demonstrated to be efficient against HCoV-229E. As a future perspective, further investigations of these compounds should take place for the identification of EC50 values.