Association Between Serum Magnesium Levels and Outcomes of Atezolizumab in Patients With Metastatic Urothelial Carcinoma

Background/Aim: Evidence suggests that serum magnesium levels are associated with outcomes of immune checkpoint inhibitors (ICIs). However, this association remains under-explored in patients with metastatic urothelial carcinoma (UC) treated with ICIs. Patients and Methods: This prognostic study used individual participant-level data from 1,281 patients with locally advanced or metastatic UC treated with atezolizumab (N=855) or chemotherapy (N=426) who participated in the IMvigor210 and the IMvigor211 trials. Multivariable Cox proportional hazards regression and Fine-Gray subdistribution hazards regression models were used to examine the association of baseline serum magnesium levels with overall survival (OS), progression-free survival (PFS), and immune-related adverse events (irAEs). Results: No evidence of an association was found between baseline serum magnesium levels and PFS or OS in patients treated with atezolizumab [PFS, hazard ratio (HR)=1.03, 95% confidence interval (CI)=0.78-1.35; OS, HR=1.13, 95%CI=0.84-1.51] or chemotherapy (PFS, HR=0.93, 95%CI=0.62-1.40; OS, HR=0.91, 95%CI=0.59-1.40). We also found no evidence of association with irAEs (subdistribution HR=1.29, 95%CI=0.81-2.07) in patients receiving atezolizumab. Conclusion: This study found no evidence of an association between baseline serum magnesium levels and treatment outcomes or irAEs in patients with metastatic UC receiving atezolizumab. Contrary to previous research suggesting a role for magnesium in cancer therapy, these results indicate that serum magnesium levels may not serve as a biomarker to predict outcomes in these patients.