Disclosed herein are pharmaceutical compositions comprising carboxyalkylcellulose esters for sustained delivery of pharmaceutically active substances. The composition comprises: at least one pharmaceutically active agent, and at least one carboxyalkyl cellulose ester comprising an anhydroglucose repeat unit having the structure: wherein: R1-R6 are each independently selected from —OH, —OC(O)(alkyl), and —O(CH2)xC(O)OH, and pharmaceutically acceptable salts thereof, wherein x ranges from 1-3, a degree of substitution per anhydroglucose of —OH ranges from 0.1 to 0.7, a degree of substitution per anhydroglucose of —C(O)(alkyl) ranges from 0.1 to 2.7, and a degree of substitution per anhydroglucose of —O(CH2)xC(O)OH ranges from 0.2 to 0.75, and wherein in pharmaceutically acceptable media, the composition exhibits sustained release of the at least one pharmaceutically active agent. Also disclosed are methods of administering the compositions for sustained delivery, such as delivery following zero order kinetics.