Described herein are liquid pharmaceutical compositions comprising bortezomib, a polyol comprising mannitol, and an aqueous carrier, wherein the compositions are suitable for intravenous or subcutaneous administration. In certain embodiments, the compositions are substantially free of organic solvents and comprise controlled levels of impurities, including not more than 1.0% of each of Impurity E, Impurity L, Impurity J, Impurity F, Impurity I, and Impurity U, and not more than 3.0% total impurities. The compositions are prepared under controlled process conditions, including control of temperature, dissolved oxygen, and headspace oxygen, to reduce impurity formation and improve stability during storage. In certain embodiments, the compositions exhibit extended stability under refrigerated conditions. The disclosure further relates to methods of preparing such compositions and to dosage configurations aligned with clinically administered doses to improve dose matching and reduce residual drug following administration.