The challenges of conducting cleaning validation are documented in the literature. R. Baffi et al., for example, described the diverse analytical challenges arising in validating cleaning procedures for biopharmaceutical products produced by recombinant DNA, in which a broad range of potential residual cellular components and trace levels of detergents must be quantified (1). M.A. Strege et al. described the total organic carbon (TOC) analysis of swab samples for cleaning validation of bioprocess fermentation equipment and discussed accuracy, limits of detection, limit of quantitation, linearity and precision (2). K.M. Jenkins et al. compared the advantages and disadvantages of multiple methods for cleaning validation, including high-performance liquid chromatography (HPLC), thin-layer chromatography (TLC), spectrometry, TOC and conductivity (3). A.J. Holmes et al. described the TOC method for measuring residual aspirin on aluminum, stainless steel, painted carbon steel and Plexiglas (4). The latter two authors describe the swab challenge as noted in the FDA guide to inspections of cleaning surfaces.
For cleaning validation using a TOC analyser, the following types of sampling methods are available:
- rinse sampling
- swab sampling with aqueous extraction
- swab sampling with direct combustion.
These methods were compared using a total organic carbon analyser (TOC-LCPH, Shimadzu) to measure residual pharmaceutical products and their constituent substances.
Preparation of residue samples
Residue samples were prepared by applying various types of pharmaceutical products and their constituents to stainless steel pots. Compounds with varying levels of water solubility (i.e., soluble, insoluble and very insoluble) were evaluated to determine how each method performed. The water-soluble substances were dissolved in water and the water-insoluble substances were dissolved in ethanol or acetone, as shown in Table I. Solution concentrations were adjusted to 2,000 mgC/L (i.e., carbon concentration of 2000 mg/L).
Table I: Substances used for residue measurements.
The carbon contents of tranexamic acid (C8H15NO2), anhydrous caffeine (C8H10N4O2), isopropylantipyrine (C14H18N2O), and nifedipine (C17H18N2O6) were estimated by molecular formula. Carbon contents of Gentashin ointment (aminoglycoside antibiotic) and Rinderon ointment (corticosteroid) were determined with the TOC analyser by adding samples of the ointments directly into a solid-sample combustion unit (SSM-5000A, Shimadzu) since molecular formula for these compounds are unknown.
Each residue sample consisted of a 5-cm2 area on the surface of a pot to which 100 µL of each solution was applied and dried. Thus, there were 200 µg carbon in the sample at each application site.