Evaluation of serum glucose-regulated protein 78 (GRP78) as a biomarker of treatment response to bortezomib-based induction regimen in multiple myeloma: A cross-sectional pilot study.

GRP78 overexpression in myeloma cells has been associated with bortezomib resistance in multiple myeloma (MM). However, serum GRP78 as a maker of bortezomib-based treatment response remains unexplored. The objective of the study was to evaluate serum GRP78 levels in MM patients who underwent a bortezomib-based induction regimen. This cross-sectional study included adult MM patients (n=30) who completed at least four cycles of bortezomib-based induction therapy. Healthy volunteers (n=30) and newly diagnosed MM patients (n=19) were also recruited to identify the disease-associated change in GRP78 levels. Serum GRP78 was estimated by ELISA. Surface and intracellular expression of GRP78 in bone marrow plasma cells was evaluated in ten MM patients by flow cytometry. Among 30 MM patients [median (range): 52 (38-68) years; 20 males] who completed at least four cycles of bortezomib-based induction therapy, 20 were responders and 10 were non-responders. Serum GRP78 levels were not significantly different between responders [median (IQR): 5.2 (3.1, 8.0) μg/ml] and non-responders [median (IQR): 4.3 (0.1, 7.1) μg/ml] (p=0.4). Although non-significant (p=0.3), median serum GRP78 was higher in newly diagnosed patients when compared to healthy volunteers. Bone marrow plasma cells ranged from 0.2 to 57.8% in the analyzed samples. Intracellular GRP78 expression in bone marrow plasma cells was higher (1.6 to 5 times) when compared to surface expression. To conclude, serum GRP78 levels vary widely in different MM patient groups but did not correlate with response to a bortezomib-based induction regimen.