Inhibition of the spontaneous polymorphic transition of pyrazinamide γ form at room temperature by co-spray drying with 1,3-dimethylurea

The present study focuses on the ability of excipients to induce the crystallization of a specific polymorphic form of pyrazinamide (PZA) and more interestingly, to block the irreversible solid–solid transition of the metastable forms of the PZA to the stable form at room temperature. We outline an experimental protocol for the production of a structurally pure γ form of PZA by means of spray drying. Without any particular treatment, phase transition to δ form was detected after 14 days of storage under ambient conditions. In order to prevent this irreversible phase transition, different excipients were co-spray dried with PZA. By co-spray drying 5% in mass of 1,3-dimethylurea (DMU) with PZA, we noticed its ability in preventing phase transitions and thus to maintain PZA under its γ form up to 12 months of storage at room temperature. Raman spectroscopy evidenced how DMU crystals surround particles of γ PZA which suggest that DMU might interact with the surface of PZA particles, thus blocking the phase transition. On the other hand, the co-spray drying of PZA with the polymerpolyvinylpyrrolidone (PVP) resulted in the crystallization of δ form of PZA. The physical mixture was intact over 12 months of storage at room temperature.

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