Exogenous cytokines cannot overcome the antiproliferative effect of the combination of bortezomib and lonafarnib. MM.1R cells were incubated for 48 hours with exogenous IL-6 (10 ng/mL) or IGF-1 (50 ng/mL) in a final volume of 200 μL. Cell proliferation was then measured using thymidine incorporation in the presence or absence of IL-6 or IGF-1.