Pharmaceutical Ingredients Susceptible to Microbial Attack

PHARMACEUTICAL INGREDIENTS SUSCEPTIBLE TO MICROBIAL ATTACK

• Therapeutic agents. Through spoilage, active drug constituents may be metabolized to less potent or chemically inactive forms. Under laboratory conditions, it has been shown that a variety of microorganisms can metabolize a wide assortment of drugs, resulting in loss of activity. Materials as diverse as alkaloids (morphine, strychnine, atropine), analgesics (aspirin, paracetamol), thalidomide (still used in the treatment of some forms of cancer), barbiturates, steroid esters and mandelic acid can be metabolized and serve as substrates for growth. Indeed, the use of microorganisms to carry out subtle transformations on steroid molecules forms the basis of the commercial production of potent therapeutic steroidal agents. In practice, reports of drug destruction in medicines are less frequent. There have, however, been some notable exceptions: the metabolism of atropine in eye drops by contaminating fungi; inactivation of penicillin injections by β-lactamase producing bacteria; steroid metabolism in damp tablets and creams by fungi; microbial hydrolysis of aspirin in suspension by esterase-producing bacteria; and chloramphenicol acetylase-producing contaminant.

• Surface-active agents. Anionic surfactants, such as the alkali metal and amine soaps of fatty acids, are generally stable because of the slightly alkaline pH of the formulations, although readily degraded once diluted into sewage. Alkyl and alkylbenzene sulphonates and sulphate esters are metabolized by ω-oxidation of their terminal methyl groups followed by sequential β-oxidation of the alkyl chains and fission of the aromatic rings. The presence of chain branching involves additional α-oxidative processes. Generally, ease of degradation decreases with increasing chain length and complexity of branching of the alkyl chain.

• Non-ionic surfactants, such as alkyl-polyoxy-ethylene alcohol emulsifiers, are readily metabolized by a wide variety of microorganisms. Increasing chain lengths and branching again decrease ease of attack. Alkylphenol polyoxyethylene alcohols are similarly attacked, but are significantly more resistant. Lipolytic cleavage of the fatty acids from sorbitan esters, polysorbates and sucrose esters is often followed by degradation of the cyclic nuclei, producing numerous small molecules readily utilizable for microbial growth. Ampholytic surfactants, based on phosphatides, betaines and alkylamino substituted amino acids, are an increasingly important group of surfactants and are generally reported to be reasonably biodegradable. The cationic surfactants used as antiseptics and preservatives in pharmaceutical applications are usually only slowly degraded at high dilution in sewage. Pseudomonads have been found growing readily in quaternary ammonium antiseptic solutions, largely at the expense of other ingredients such as buffering materials, although some metabolism of the surfactant has also been observed.

• Organic polymers. Many of the thickening and suspending agents used in pharmaceutical formulations are subject to microbial depolymerization by specific classes of extracellular enzymes, yielding nutritive fragments and monomers. Examples of such enzymes, with their substrates in parentheses, are: amylases (starches), pectinases (pectins), cellulases (carboxy methyl celluloses, but not alkylcelluloses), uronidases (polyuronides such as in tragacanth and acacia), dextranases (dextrans) and proteases (proteins). Agar (a complex polysaccharide) is an example of a relatively inert polymer and, as such, is used as a support for solidifying microbiological culture media. The lower molecular weight polyethylene glycols are readily degraded by sequential oxidation of the hydrocarbon chain, but the larger congeners are rather more recalcitrant. Synthetic packaging polymers such as nylon, polystyrene and polyester are extremely resistant to attack, although cellophane (modified cellulose) is susceptible under some humid conditions.

• Humectants. Low molecular weight materials such as glycerol and sorbitol are included in some products to reduce water loss and may be readily metabolized unless present in high concentrations.

• Fats and oils. These hydrophobic materials are usually attacked extensively when dispersed in aqueous formulations such as oil-in-water emulsions, aided by the high solubility of oxygen in many oils. Fungal attack has been reported in condensed moisture films on the surface of oils in bulk, or where water droplets have contaminated the bulk oil phase. Lipolytic rupture of triglycerides liberates glycerol and fatty acids, the latter often then undergoing β-oxidation of the alkyl chains and the production of odiferous ketones. Although the microbial metabolism of pharmaceutical hydrocarbon oils is rarely reported, this is a problem in engineering and fuel technology when water droplets have accumulated in oil storage tanks and subsequent fungal colonization has catalysed serious corrosion.

• Sweetening, flavouring and colouring agents. Many of the sugars and other sweetening agents used in pharmacy are ready substrates for microbial growth. However, some are used in very high concentrations to reduce water activity in aqueous products and inhibit microbial attack (see section 2.3.3). At one time, a variety of colouring agents (such as tartrazine and amaranth) and flavouring agents (such as peppermint water) were kept as stock solutions for extemporaneous dispensing purposes, but they frequently supported the growth of Pseudomonas spp., including Ps. aeruginosa. Such stock solutions should now be preserved, or freshly made as required by dilution of alcoholic solutions which are much less susceptible to microbial attack.

• Preservatives and disinfectants. Many preservatives and disinfectants can be metabolized by a wide variety of Gram-negative bacteria, although most commonly at concentrations below their effective ‘use’ levels. Growth of pseudomonads in stock solutions of quaternary ammonium antiseptics and chlorhexidine has resulted in infection of patients. Pseudomonas spp. have metabolized 4-hydroxybenzoate (parabens) ester preservatives contained in eyedrops and caused serious eye infections, and have also metabolized the preservatives in oral suspensions and solutions. In selecting suitable preservatives for formulation, a detailed knowledge of the properties of such agents, their susceptibility to contamination and limitations clearly provides invaluable information.