(1) Background:
multiple myeloma patients have benefited from
bortezomib therapy, though it has often been discontinued owing to
diarrhea. The objective of this study was to verify serum
bortezomib concentration in the emergence of
diarrhea. (2) Methods: this prospective, observational case-control, and monocentric study was performed with an approval by the Ethics Committee of Kumamoto University Hospital in 2015 (No. 1121) from February 2015 to April 2017. (3) Results: twenty-four patients with
bortezomib therapy were recruited; eight patients (33.3%) developed
diarrhea at day 3 as median. Median measured trough
bortezomib concentration at 24 h after first or second dose for patients with or without
diarrhea was 0.87 or 0.48 ng/mL, respectively (p = 0.04, Wilcoxon signed rank test). Receiver operation characteristic (ROC) analysis produced the cut-off concentration of 0.857 ng/mL (area under the ROC curve of 0.797, sensitivity of 0.625, specificity of 0.875). The survival curves between patients with and without
diarrhea were similar (p = 0.667); those between patients with higher and lower concentration than median value (0.61 ng/mL) were also similar (p = 0.940). (4) Conclusions: this study indicated the possible involvement of serum
bortezomib concentration in the emergence of
diarrhea in
bortezomib therapy in patients with
multiple myeloma.