Preservatives and Antioxidants
Methionine, cysteine, tryptophan, tyrosine, and histi-dine are amino acids that are readily oxidized (Table 4). Proteins rich in these amino acids are liable to oxidative degradation. Replacement of oxygen by inert gases in the vials helps to reduce oxidative stress. Moreover, the addition of antiox-idants such as ascorbic acid or acetylcysteine can be considered. Interestingly, destabilizing effects on proteins have been described for antioxidants as well (Vemuri et al., 1993b). Ascorbic acid, for example, can act as an oxidant in the presence of a number of heavy metals.
Certain proteins are formulated in containers designed for multiple injection schemes. After admin-istering the first dose, contamination with microor-ganisms may occur and preservatives are needed to minimize growth. Usually, these preservatives are present in concentrations that are bacteriostatic rather than bactericidal in nature. Antimicrobial agents mentioned in the USP 29 are the mercury-containing phenylmercuric nitrate and thimerosal and p-hydro-xybenzoic acids, phenol, benzyl alcohol and chlor-obutanol (USP 29; Groves, 1988; Pearlman and Bewley, 1993). The use of mercury containing pre-servatives is presently under discussion (FDA, 2006).