Propranolol metabolite and enantiomers affect colorectal cancer development by inhibiting MAPK pathway and downregulating PD-1 expression

Preclinical and clinical studies have demonstrated the tumor-suppressive role of propranolol, a racemic mixture that blocks β₂-adrenergic receptor (β₂-AR). However, the impact of 4-hydroxypropranolol, the primary metabolite of propranolol, as well as the two enantiomers of propranolol, in tumor control remains uncertain. In this study, we investigated the effects of propranolol, 4-hydroxypropranolol, and propranolol enantiomers in controlling colorectal cancer. These compounds demonstrated time- and concentration-dependent inhibition of cancer cell viability in vitro and significantly suppressed tumor growth in vivo. Notably, 4-hydroxypropranolol exhibited the most potent tumor suppressive effect among all tested compounds. Computational analyses revealed that 4-hydroxypropranolol exhibited stronger binding to β₂-AR compared with propranolol (hydrogen-bond distance: 2.8 Å vs 3.1 Å). Additionally, 4-hydroxypropranolol markedly reduced the population of PD-1⁺CD4⁺T cells and PD-1⁺CD8⁺T cells in the spleen of colorectal cancer models. Furthermore, treatment with 4-hydroxypropranolol (8 mg/kg/day) significantly diminished the population of FoxP3⁺CD4⁺CD25⁺ T (Treg) cells in the spleens of animals, compared to the PBS group. These findings suggest that 4-hydroxypropranolol exerts a stronger tumor suppressor effect than propranolol and its enantiomers, likely due to its higher affinity for β₂-AR.