Stability Of Drugs:Microbiological Stability
From Pharmpedia
Contamination from microorganisms is a big problem for all formulations containing moisture but it can be a bother in solid dosage forms also if some natural polymers are used because many natural polymers are fertile sources of microorganisms. In the type of hygienic manufacture carried out today where “Quality Assurance” is a prerequisite as per the cGMP procedures, there are definite procedures to prevent microbial contamination in all formulations. But way back in the sixties microbial contamination of pharmaceutical formulations was a big problem. One case of outbreak of Salmonellosis in Sweden attributed to contaminated tablets is discussed in “Pharmaceutics, The Science of Dosage Form Design” by Michael E. Aulton. In this case the infection was traced to the original defatted thyroid powder imported from Hungary which was used to make the tablets. The Pharmaceutical Society of Great Britain set up a working party in 1968 to investigate microbial contamination of pharmaceutical preparations in manufacturing establishments and in hospital and retail pharmacies. This investigation shed light on a number of issues including microbial content of some drugs and medicines and suggested many measures to reduce contamination.
Contents |
Sources of Microbial Contamination
| Water | Low demand gram-negative groups: Pseudomonas, Xanthamonas, Flavobacterium, Achromobacter |
| Air | Mould spores: Penicillium, Mucor, Aspergillus Bacterial spores: Bacillus spp. Yeasts |
| Raw Materials | Micrococci |
| Earths | Anaerobic spore formers: Clostridium spp |
| Pigments | Salmonella |
| Starches | Coliforms |
| Gums | Actinomyces |
| Animal products | Salmonella, Coliforms |
| Personnel | Coliforms, Staphylococci, Sterptococci, Coryembacteria |
Table No.1 taken from “Pharmaceutics, The Science of Dosage Form Design” by Michale E. Aulton gives the types of organisms present in different sources
Extremely hygienic manufacture ensures a product that is free of contamination in the case of all non-sterile preparations and a sterile preparation in the case of all
parenterals. There are two strategies followed in the manufacture of microbiologically stable, acceptable pharmaceutical
preparations. The first step is to prevent contamination of the product. The second is to formulate the final product so that it
is hostile to microorganisms and it is usually done by
the addition of preservatives.
For sterile preparations there is either a terminal sterilization process or a closely controlled aseptic manufacturing
procedure. In every case the final product is so made to protect the product during storage and minimize contamination while the
product is in use.
When discussing microbiological stability we have to discuss parenterals as one class and the rest f the formulations as one class.
Parenterals are either terminally sterilized or manufactured by an aseptic manufacturing procedure. To prevent contamination to the formulation during storage and use many steps are taken such as
(1) suitably designing the containers,
(2) usually using single dose containers,
(3) sticking to proper storage conditions and
(4) adding an antimicrobial substance as preservative.
Preservatives used in pharmaceutical preparations
The following Table ---- given in “Pharmaceutics The Science of Dosage Form
Design” by Michael, E. Aulton gives a list of usual preservatives used in pharmaceutical preparations
|
Preparation |
Preservative |
Concentration % w.v |
Special factors |
|
Injections |
Phenol Cresol Chlorocresol Phenylmrcuric nitrate Benzyl alcohol |
0.5 0.3 0.1 0.001 1.0 |
Not for intraocular, intracardiac or intacisternal or over 15 mil single dose. Closures pretreated. |
|
Eye drops |
Phenylmercuric nitrate or acetate Chlorhexidine acetate Benzalkonium chloride |
0.002 0.01 0.01 |
Dropper teat pretreated
Silicone rubber teats |
|
Mixtures |
Chloroform Benzoic acid Methyl paraben Alcohol Sulphur dioxide |
0.25 0.1 0.1 12-20 400 parts/106 |
Adsorptin. Volatile PH (pKa, 4.2) Adsorption Volatile Volatile |
|
Creams |
Parabens Chlorocresol Dichlorobenzyl alcohol Cetyltrimethyl ammonium bromide Phenylmercuric nitrate |
0.1-0.2 0.1 0.05-0.2 0.01-0.1 0.001 |
*Kow R high Kow R high Kow
R high |
|
Tablets |
Methylparaben |
0.1 |
|
| Kow values | Mineral oil | Vegetable oil | |
| Chlorocresol | 1.5 | 117 | |
| Methylparaben | 0.02 | 7.5 | |
| Propylparaben | 0.5 | 80.0 |
R = ratio of total to free preservative in non-ionic surfactant – water system.
Parenterals and ophthalmic preparations have to be totally free from microorganisms i.e. they have to be sterile. This requirement is met by (1) placing a suitable preservative or combination of preservatives wherever required in the products, (2) storing the products properly, (3) stoppering them properly and by following proper aseptic procedures during administration and during any admixture procedures followed prior to administration. In spite of all these precautions if any microbial growth takes place and is observed the product is condemned and the entire batch from which the product has come is recalled. The storage of these products is done under conditions recommended by WHO which prescribe temperature, humidity, cleanliness as well as colour of the walls of the room.
Non-sterile preparations have less stringent requirements regarding exclusion of microbes. They need not be sterile but it has to
be shown that some specifically named organisms are not present in them. Table No.---- taken from Michael, E. Aulton gives the
microbial standards for pharmaceutical preparations.
Microbial standards for pharmaceutical preparations
|
Requirement |
Authority* |
|
Exclusion 1. Complete exclusion – sterility Injections, Ophthalmic preparations 2. Exclusion of named organisms Raw materials: e.g. : Aluminium Hydroxide – Ps. Aeruginosa/1 g, E. coli/1g Cochineal. Gelatin – E. coli/1g Pancreatin, Thyroid – Salmonella/10 g Maize starch Tragacanth E. Coi/1g
Oral dosage forms: Free from E. Coli Free from E. Coli and Salmonella/1 ml. Topical preparations: Free from Ps aeruginosa and S. Aureus Fee from Enterobacteria, S.aureus and Ps. Aeruginosa
Limit upon number of viable organisms Oral dosage froms: Limit upon total aerobic count of non-specified viable organisms, e.g. Milk of Magnesia 100 cells/ml Raw materials: ≯104 aerobic bacteria/ml ≯102 yeasts or moulds/ml ≯102 enterobacteria/ml
Topical preparations: ≯100 organisms/g or ml |
BP, EP, USP
BP BP BP
BP
USP EP
USP EP
USP
EP
EP |
- BP, British Pharmacopeia 1980; BP Addendum 1986; USP, United States Pharmacopeia XXI; EP, European Pharmacopeia (draft proposals).
Vulnerability to Microbial Attacks
So dear students, we now understand that different formulations have differing levels of stringency regarding microbial presence.
You may be thinking “Drug products are made under such extremely hygienic conditions and are stored under very good conditions.
There is no chance for any contamination to enter into them”. But however clever we are microorganisms seem to be cleverer still!
The risk of contamination is more in non sterile than in sterile products and more in multiple use formulations than in unit dose
systems. Most of the raw materials used in the manufacture of medicines support the growth of microbes and so medicines including
dry powders and tablets and capsules are susceptible to microbial spoilage. Microbes can survive in a vast range of habitats
including volcanic regions and icy regions!. They can use glass and concrete as nutritional sources! So a majority of medicines
are a good source of nutrition to the microbes and if the microbe gets access to the medicine it can definitely survive and
proliferate. This growth sometimes gives off offensive odours and colours. Thus we can spot the breakdown of the system due to
microbes. Sometimes the microbes grow without any visible sign and this can be really troublesome because then we will administer
the medicine without realizing its harmful potential.
Understanding the vulnerability of drug systems to microbes is important. When scientists studied the interaction of microorganisms within foods such as milk and meat products they discovered some curious things. One variety of microbes first attack the medicines and degrade complex nutrients and alter pH levels. Then another variety attacks and thrives on the simple nutrients. For example, syrups may be first contaminated by osmophilic yeasts which can thrive at high sugar concentrations. They then utilize the sugars and create conditions suitable for other microbes. When such syrups are examined there may not be any evidence of the yeasts which started the entire spoilage process.
Effects of Microbial instability
Contamination of a product may sometimes cause a lot of damage and sometimes may not be anything at all. For example, spores of the mould Mucor may be present in a dormant form and never produce spoilage and will not harm the patient who takes the medicine. But if Salmonella enters a medicine, it may not cause any visible damage but would cause a serious health hazard to the patient who consumes it.
If contamination happens in parenterals or in ophthalmic preparations it can be very serious in its effects. But contamination in other nonsterile products is usually not so damaging. It results in general spoilage such as discoloration, breakdown of emulsions and the production of gas and other odours. This is good in one way, if prevents the patient form consuming the medicine. In some cases active drugs may be destroyed without any outward signs. Thus, salicylates, phenacetin, paracetamol, atropine, chloramphenicol and hydrocortisone can be degraded to a variety of therapeutically inactive products. Preservatives, especially those that are aromatic in structure can themselves act as a ready source of nutrition to microbes.
Suppose some microorganisms have entered the raw materials used in some parenteral preparation and then are killed in the process
of sterilization – still there is some harm.
Some toxic substances produced by the bacteria may cause harm. Pyrogens which are the metabolic products of bacterial growth are usually lipo-polysaccharides and they represent a particularly hazardous product released by gramnegative bacteria. If administered inadvertently to a patient they may cause chills and fever.
What to do to prevent microbial spoilage
The proper preservative has to be used. By ‘proper’ I mean that the preservative must have the required oil/water partition coefficient, it must be non-toxic, odourless, stable and compatible with other formulation components while exerting its effects.
Containers
Traditional glass containers do not interact with the preservatives. If the closure is airtight there is no problem of
contamination. But plastic containers pose problems such as permeation through the container or interaction with it. Rubber also
reacts with preservatives but it is still used for teats
and closures. These teats and closures are treated with the preservatives they are to be in contact with, in order to minimize
subsequent uptake during storage.
For sterile preparations there is either a terminal sterilization process or a closely controlled aseptic manufacturing procedure. In every case the final product is so made to protect the product during storage and minimize contamination while the product is in use.
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