The TACITO trial was a double-blind, placebo-controlled phase 2 trial of patients with metastatic renal cell carcinoma treated with a combination of an anti-PD-1 inhibitor (pembrolizumab) and a VEGFR tyrosine kinase inhibitor (axitinib). The research team tested whether faecal microbiota transplantation (FMT) from donors with a complete response to primary immunotherapy could improve treatment outcomes. The study's primary endpoint—an improvement in progression-free survival (PFS) by 2 months—was not achieved with FMT. Nevertheless, the study demonstrated that FMT significantly improved median PFS compared with placebo. The results suggest that the composition of the faecal microbiota may influence the effectiveness of immunotherapy in metastatic renal cell carcinoma, although further research is needed to confirm these findings and optimize the treatment approach.