Variations in serum concentrations of sunitinib and its metabolites in patients receiving long-term sunitinib treatment.

Purpose: The blood concentrations of some tyrosine kinase inhibitors are known to decrease with long-term administration. We evaluated the variability in the serum concentrations of sunitinib and its metabolites in patients receiving long-term sunitinib treatment.

Methods: This study prospectively recruited patients who received sunitinib for metastatic renal cell carcinoma at the Showa University Hospital between March 2020 and January 2022. Bivariate correlations between the serum concentration/dose (C/D) ratios of sunitinib and its metabolites (i.e., N-desethyl sunitinib and sunitinib N-oxide) and treatment duration were evaluated using Pearson's correlation coefficient.

Results: Seven patients were enrolled, and 79 blood samples were collected. Among six patients who received sunitinib for > 1 year, three showed a decreasing trend in the C/D ratio of sunitinib (Pt1: r = -0.608, p = 0.047; Pt2: r = -0.555, p = 0.077; Pt6: r = -0.590, p = 0.073). In these patients, the median annual decrease in the C/D ratio of sunitinib was 55.8% (26.5-63.2%). Additionally, two of the three patients also showed a decrease in the C/D ratio of N-desethyl sunitinib. The ratio of N-desethyl sunitinib/sunitinib concentration at baseline and the end of follow-up was similar between the C/D-decreased and C/D-non-decreased groups.

Conclusion: This study showed that the C/D ratio of sunitinib decreased by half over time in half of the patients who received long-term sunitinib treatment despite continuing the same dose. Therefore, serum concentrations of sunitinib and its metabolites should be monitored periodically in patients receiving long-term treatment to prevent decrease in serum sunitinib concentrations.