SC I J. Efficacy of Antimicrobial Preservation

This section provides background information on the purpose and scope of the test for efficacy of antimicrobial preservation. The Annex to this section provides guidance on some practical aspects of testing.

1. The test for efficacy of antimicrobial preservation, included in Appendix XVI C, has been accorded non-mandatory status in the Pharmacopoeia. This status is reflected in the test’s inclusion as a general text in section 5 of the European Pharmacopoeia and in the form of reference to the test within Production sections of relevant general monographs of the European Pharmacopoeia. Non-mandatory status provides a degree of flexibility consistent with the intended purpose of the pharmacopoeial test.

2. The pharmacopoeial test is intended to serve as a model offering a manufacturer guidance concerning this aspect of quality and a foundation on which he can build to meet his own particular needs. The testing procedure is intended to serve as a means whereby, during product development, a manufacturer can assess the efficacy of any antimicrobial preservative included in the product. Within the context of the British Pharmacopoeia, Appendix XVI C serves to define what is meant by the term ‘suitable antimicrobial preservative’ in accordance with the relevant Notice in Part 2 of the General Notices.

3. If during development a fully quantitative, comparative evaluation of different preservative systems would be useful, the procedure described in the Appendix could be extended. For example, the incorporation of additional sample times would allow an estimation of microbial death rates.

4. The test is intended for application to those products that may support the growth or viability of microbial contaminants. It is based on the premise that preservatives are used primarily to protect products from in-use microbial contamination but also recognises their role in reducing the bioburden of ‘non-sterile’ products. In-use contamination is of relevance to multidose products, both sterile and non-sterile. Bioburden is of relevance to non-sterile products, both multidose and single dose.

5. The present European Pharmacopoeia text provides criteria for parenteral, ophthalmic, topical and oral preparations. The criteria for ear preparations are equivalent to those specified for topical preparations thus maintaining parity between these two types of product.

6. An unusual feature of the European Pharmacopoeia text is the inclusion of two sets of criteria (A and B) for parenteral and ophthalmic preparations and for topical preparations. The A criteria express the recommended efficacy to be achieved, that is, they represent generally applicable ‘target’ criteria.

7. It is recognised that for certain products, for example, some antacids and certain biological products, these target criteria are unlikely to be achieved except at the expense of some other property of equal or greater importance. The alternative criteria to be met in these circumstances are a matter for agreement between the manufacturer and the competent authority and should take account of any special considerations relevant to the specific product. The B criteria for parenteral and ophthalmic products and for topical products were adopted by the European Pharmacopoeia Commission in deference to those member states that wanted published guidance on the minimum values below which any alternative criteria should not fall.

8. As noted in paragraph 2 above, the pharmacopoeial test is intended to provide a framework within which to develop a test suitable for the particular product being examined. Detailed descriptions of all aspects of testing are therefore inappropriate within the pharmacopoeial test itself. The following Annex draws attention to some key features of the test and offers additional guidance on certain practical aspects of testing.

Annex

9. Several features have been identified as sources of procedural variation in the test that may contribute to variation in the outcome of the test. A clearly written and sufficiently detailed test protocol is therefore essential. Some of these features are discussed in this Annex.

10. Test organisms; culture maintenance Potential sources of variation to which attention should be given include the source, age and storage of the microbial culture and the number of passages between the freeze-dried strain collection culture (repository culture) and the suspension of organisms used to inoculate the product (the test suspension). It is recommended that the repository culture is cultured and aliquots freeze-dried or stored under liquid nitrogen. It is also recommended that the sub-culture used as the test suspension should be no more than 5 passages from the repository culture.

11. Product inoculation Potential sources of variation to which attention should be given include the culture media and method used to prepare the inoculum, the inoculum:product ratio and the homogeneity of the inoculum–product mixture. Appendix XVI C states that the test suspension shall be used immediately. It is recommended that this is interpreted to mean that, once prepared, the test suspension should not be stored but may be used over a period of 8 hours provided that it is kept at a temperature of 2° to 8°.

12. Test container It is emphasised that, as indicated in the Appendix, wherever possible, the product should be inoculated and incubated in its original container (market pack). Where, however, it is necessary to transfer the product to some other container, for example, in order to achieve homogeneous distribution of the inoculum, careful consideration needs to be given to the choice of test container. Features of such a container that will influence its suitability as a test container include the material from which it is made, its shape and size (volume) and the type of closure. The material from which the container is made should not affect the product, for example, by leaching or by sorption of ingredients. Particular attention should be paid to possible changes in product pH since these can markedly affect preservative activity. A container with smooth surfaces and a wide neck that will allow ease of access and mixing is recommended for creams and other viscous preparations.

13. Test procedure Test conditions such as storage temperatures and sample times are given in Appendix XVI C. Working at a temperature within the stated range of 20° to 25°, it is advisable, wherever possible, to minimise the temperature difference from test to test.

14. Counting survivors The Appendix specifies that the count obtained for the inoculum is used as the baseline for calculating the reduction in viable micro-organisms.

15. Preservative inactivation When residual antimicrobial activity is removed either by the use of an inactivating agent or by dilution, it is necessary to confirm the ability of the system to support the growth of the test organisms by the use of appropriate controls. These controls should simulate test conditions and allow validation of the counts. In designing such controls an appropriate target recovery efficiency, for example 70% recovery, should be set.

16. Interpretation of results Where a test criterion is given as ‘no increase’ this is intended to be interpreted as no increase above the counts obtained at the previous specified sample time. It is expected that, having achieved the required reduction in counts at the shorter time interval specified, the preservative will maintain the microbial population at or below this lower level.