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Drug distribution in wet granulation: foam versus spray

Foam granulation technology is a new wet granulation approach for pharmaceutical formulations. This study evaluates the performance of foam and spray granulation in achieving uniform drug distribution using a model formulation. To observe wetting and nuclei formation, single drop/foam penetration experiments were performed on a static powder bed comprised of varying compositions of hydrophilic/hydrophobic glass ballotini, and hydrophilic lactose/hydrophobic salicylic acid respectively. High shear granulation experiments were performed in a 5L mixer using varying compositions of hydrophilic lactose and hydrophobic salicylic acid. Four percent hydroxylpropyl methylcellulose (HPMC) solution was delivered at 90 g/min as either a foam (92% FQ) or an atomized spray whilst recording impeller power consumption. After drying, the granule size distribution was measured and the granule composition was estimated using gravimetric filtration in methanol. Foam penetration was less dependent on the powder hydrophobicity compared to drop penetration. For glass ballotini powder mixtures, foam induced nucleation created nuclei with relatively uniform structure and size regardless of the powder hydrophobicity. For salicylic acid and lactose mixtures, increasing the proportion of salicylic acid reduced the nuclei granule size for both foam and drop binder addition. The granule drug distribution was not significantly affected by the binder addition method. Processing conditions, including liquid binder amount, impeller speed, wet massing, and the wettability properties of the formulation were the dominant factors for delivering homogeneous granules. The study reveals that foam and spray granulation involve different nucleation mechanisms – spray tends to incur early liquid penetration whereas foam granulation operates well in mechanical dispersion.