Characterisation of particle properties and compaction behaviour of hydroxypropyl methylcellulose with different degrees of methoxy/hydroxypropyl substitution

The particle characteristics and compaction behaviour of hydroxypropyl methylcellulose (HPMC) powders from two different suppliers were studied regarding effects of methoxy/hydroxypropyl substitution. Samples included Methocel K4M (low substitution ratio), E4M (medium) and F4M (high) and the corresponding substitution ratios from Metolose: 90 SH 4000, 60 SH 4000, and 65 SH 4000. Characterisation of the particle properties and compaction behaviour of the pure polymers suggested that reported differences in drug release behaviour of Methocel E4M compared with the other two powders may be related to the lower powder surface area, differing particle morphology and lower fragmentation propensity during compaction. In addition, compacts of Methocel E4M were weaker when tested in both axial and radial directions and had different porosity and elastic recovery properties. There were no differences between the polymers in degree of disorder, as evaluated by solid-state nuclear magnetic resonance spectroscopy. The different behaviour of Methocel E4M could, however, be related to the overall higher total degree of substitution of this polymer and in particular the high content of methoxy groups compared to the other polymers. The methoxy substituent is hydrophobic and may, when present in sufficiently high concentrations, change the particulate and mechanical properties of the powder, thus potentially affecting the compactability. The high content of methoxy groups might also decrease the development of inter- and intraparticulate hydrogen bonds during compaction, and suppress the actions of the hydrophilic hydroxypropyl groups, both of which could affect drug release.