Effect of electron beam irradiation on remaining activity of lyoph-ilized acid phosphatase with water-binding and non-water binding additives

The ability of various additives to protect lyophilized acid phosphatase against the deleterious effects of electron beam irradiation was investigated. At this purpose, ingredients with water-binding (e.g., lactose and sucrose) and non-water-binding (e.g., mannitol and polyvinylpyrrolidone) properties were used. The reconstituted activity of acid phosphatase was found to dramatically decrease when the irradiation dose overcomes a threshold, which is formulation-dependent. That difference has been related to a different affinity with water of the various additives and, thus, with the water-binding property of the formulation. The deactivation of acid phosphatase against irradiation seems to be directly correlated with the residual moisture of the lyophilized product and inversely correlated with the fraction of water strongly confined within the solid. In particular, the highest protection of the enzyme against irradiation was given by lactose-containing formulation, while the polyvinylpyrrolidone-based formulation gave the lowest protection. This finding was also confirmed by the fact that the number of free radicals generated by irradiation directly depends on the amount of water that is not strongly bound to the product surface. Overall, water-binding additives are preferable to non-water-binding ones in order to protect the active ingredients against irradiation. Follow-up studies should focus on the structural properties of confined water and thus on its ability to preserve the active ingredient against the deleterious effects of drying and irradiation.