Preservation efficacy test

The formulation scientist must be aware of interactions between preservatives and other components of a formulation that could compromise the efficacy of the preservative. For example, proteins can bind thi-merosal, reducing preservative efficacy. Partitioning of preservative into a micellar phase or an oil phase (in an emulsion) can also reduce the effective concentration of preservative available for bactericidal or bacteriostatic action. Preservative efficacy testing should be done on the proposed formulation to assure an effective preservative concentration. 

Where a large pack size is provided, it might be necessary to undertake more extensive in-use testing and/ or to restrict the in-use shelf life once the container has been opened. In addition to preservative efficacy testing the chemical stability of antimicrobial preservatives should also be investigated over the unopened and in-use shelf lives of the product. 

A sample of a standard calcium carbonate slurry was received from a large manufacturer in the USA. This sample was subjected to preservative efficacy testing according to the ASTM E 723-91 test protocol. Preservative treated samples were inoculated with a mixed bacterial inoculum containing organisms with a known tolerance or resistance to BIT (l,2-Benzisothiazolin-3-one). Untreated controls were included for reference purposes. The test procedure is outlined below. 

The main objective of validation of an analytical procedure is to demonstrate that the procedure is suitable for its intended purpose. The procedures presented in this SOP provide basic guidelines for the validation of methods for microbiological assay, estimation of the number of microorganisms, detection of indicators of objectionable microorganisms, validation of preservative efficacy testing, and validation of the sterility testing and endotoxin test (LAL test). 

The objective is to provide standard procedure for performing the preservative efficacy test for nonsterile and sterile dosage forms. 

Product categories under preservative efficacy test (Table 1) Table 1 Product Categories... 

Media perform growth promotion test for all media used in preservative efficacy test... 

Any differences between the release and shelf-life acceptance criteria for antimicrobial preservatives should be supported by the results of preservative efficacy testing. 

Preservative efficacy testing Preparation, review, approval, and revision... 

With drug products containing preservatives, the stability protocol should include preservative efficacy testing. Microbial challenge testing should be conducted at appropriate intervals—at least once a year unless significant losses are observed earlier as a result of assay procedures. 

Microbiological protection of multiple-dose presentations such as liquid inhalations, nasal sprays, oral liquids, creams, and lotions is more complex. Once opened they are susceptible to microbiological contamination. If they are aqueous-based, they are in principle susceptible to proliferation of these new contaminants. To avoid this, they are formulated with antimicrobial agents or preservatives and are expected to be able to comply with preservative efficacy standards specified in the pharmacopeias. Preservative efficacy tests (not harmonized) are described in Section 51 of the USP and Section VIII. 14 of the PhEur (Fig. 3). 

Sulfhydryl compounds cause significant reductions in the activity of bronopol, and cysteine hydrochloride may be used as the deactivating agent in preservative efficacy tests lecithin/ polysorbate combinations are unsuitable for this purpose. Bronopol is incompatible with sodium thiosulfate, with sodium metabisulfite, and with amine oxide or protein hydrolysate surfactants. Owing to an incompatibility with aluminum, the use of aluminum in the packaging of products that contain bronopol should be avoided. 

Hodges NA, Denyer SP, Hanlon GW, Reynolds JP. Preservative efficacy tests on formulated nasal products reproducibility and factors affecting preservative activity. ] Pharm Pharmacol 1996 48 1237-1242. 

It is also useful to conduct a simulated use test in which the product is opened each day and drops administered according to the dosing instructions for 28 days. The remaining formulation should pass the preservative efficacy test at the end of this period. 

The range of testing should cover not only chemical and biological stability but also loss of preservative, physical properties and characteristics, organoleptic properties and, where required, microbiological attributes. Preservative efficacy testing and assays on stored samples should be carried out to determine the content and efficacy of antimicrobial preservatives. 

Limits of acceptance should relate to the release limits (where applicable), to be derived from consideration of all the available stability information. The shelf life specification could allow acceptable and justifiable derivations from the release specification based on the stability evaluation and the changes observed on storage. It will need to include specific upper limits for degradation products, the justification for which should be influenced by the levels observed in material used in pre-clinical studies and clinical trials. The justification for the limits proposed for certain other tests such as particle size and/or dissolution rate will require reference to the results observed for batch(es) used in bioavailability and/or clinical studies. Any differences between the release and shelf life specifications for antimicrobial preservatives should be supported by preservative efficacy testing. 

The stability profile of each product, in its Australian sales pack, should be available. Where it is not, it should be determined according to a written stability program. The profile should be verified at appropriate intervals by supplementary testing of further batches of product. The program should include dissolution, microbiological and preservative efficacy testing where appropriate. 

RM Baird. Preservative efficacy testing in the pharmaceutical industries. In MRW Brown, P Gilbert, eds. Microbiological Quahty Assurance A Guide Towards Relevance and Reproducibility of Inocula. New York CRC Press, 1995, pp. 149-162. 

In pharmaceutical preparations another type of curve is sometimes observed. An initial decrease in the number of colony forming units may occur, followed by an increase. This phenomenon can be observed when analysing data from preservative efficacy testing of inadequately preserved dosage forms. It is the reason why in pharmacopoeial... 

Cosmetics and toiletries represent a highly diversified field with many subsections of science and art. Indeed, even in these days of high technology, art and intuition continue to play an important part in the development of formulations, their evaluation, and the selection of raw materials. There is a move toward more sophisticated scientific methodologies in the fields of preservative efficacy testing, claim substantiation, safety testing, product evaluation, and chemical analyses. 

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