We will cover three broad issues about TOC (Total Organic Carbon) as an analytical method for cleaning validation samples. The first is the acceptability of TOC for such use. The second is conditions for use if you choose to use TOC. The third is related issues to consider in its implementation.
While there is a myth that regulatory agencies prefer specific analytical methods for cleaning validation protocols, that is somewhat of a myth. Back in 2003 the US FDA issued a lower-level guidance document stating that TOC could be an acceptable analytical technique for cleaning validation (See the August 2003 Cleaning Memo, Why TOC is Acceptable?). That guidance has been updated and reaffirmed as of November 2022. In answer to the question “Can Total Organic Carbon (TOC) be an acceptable method for detecting residues of contaminants in evaluating cleaning effectiveness?”, the FDA’s reply is “Yes. Since the publication of the inspection guide on cleaning validation in 1993, a number of studies have been published to demonstrate the adequacy of TOC in measuring contaminant residues.” Here is the link to this 2022 guidance:
FDA Questions and Answers on CGMP Requirements for Equipment (TOC question)
We should also look at other regulatory guidance documents. Annex 15 of the EU GMPs states in Paragraph 10.6.2 that “If it is not feasible to test for specific product residues, other representative parameters may be selected, e.g. total organic carbon (TOC) and conductivity.” What does “not feasible” mean here? Since TOC is often used even if specific analytical methods might be feasible, how is this statement to be interpreted and applied? Clearly, it is not enough for a company to say, “we only have a TOC analyzer, therefore we may use TOC”. An alternative interpretation might be to argue that TOC is an analytical test for organic carbon, and that we can correlate that organic carbon with the target residue we have set residue limits for. The issue with this EU GMP statement is that, in contrast to the FDA guidance discussion, there is little clarification of how this is applied.
So let’s move on to the needed conditions for the use of TOC in the context of cleaning validation.
As specified by the FDA in its 2022 update, the target residue must contain organic carbon, and that organic carbon must be measured (accurately) under the conditions of the TOC method. This makes common sense, and should be demonstrated in the analytical method validation for that specific residue. Note that this generally means that there should be some documentation for each individual target residue to be measured.
The second condition (as specified by the FDA) is that in the TOC analytical procedure used in a cleaning validation protocol all measured carbon must be attributed to the target residue. This is clearly a worst case assumption. It is possible (and in most cases likely) that there are other sources of organic carbon in the sample analyzed. Those other sources could be organic excipients, impurities, process materials, or materials attributed to the sampling methods. It is not acceptable to assume that if the organic carbon in the drug product is (as an example) 10% due to the API and 90% due to the excipient, that a correction factor of 0.1 (or 10%) may be applied to the actual measured TOC value to account only for the organic carbon due to API. The reason is that 10/90 ratio may apply to the drug product itself, but the ratio that is actually present in the analytical sample may be significantly different.
That said, it is acceptable to adjust the measured TOC in the analytical sample by subtracting out blanks to account for organic carbon in the sampling water used or to account for any contribution from the sampling materials (such as a swab). However, a third FDA comment relates to the need to limit the background or blank carbon in the sampling and measurement. The background/blank should be as low as “possible”, although I would prefer the phrase “as low as practical”. And I would also add that blanks should be as consistent as practical. As a reminder, as part of the analytical method validation to confirm effectiveness of TOC for the specific residue of concern, blanks (as well as controls) should be utilized to avoid inflating precision/accuracy for that method validation.
A fourth issue addressed by the FDA is that sampling recovery studies should be performed, as with any other combination of target residue, analytical method, sampling method, and surface type.
A fifth concern of the FDA is that the “amount of residue detected for comparison to the specification, should correct for the target material’s composition of carbon”. This is addressed in one of two ways. One way is to set an L4 limit of the target residue by conventional calculation, and convert that target residue to an equivalent TOC limit by correcting for the percentage organic carbon in the target residue. That TOC limit is then listed in the acceptance criteria in the protocol. A second way is to utilize the actual target residue L4 limit in the protocol, but when TOC is measured in an analytical sample in the protocol, that measured TOC value is converted to an equivalent amount of the target residue, which is then compared to the calculated L4 limit of the target residue. While either approach produces the same result in terms of a pass/fail determination, I prefer the latter because it reinforces that I am basing results on the target residue, even though TOC is used to determine the measured amount in the sample.
A sixth concern listed by the FDA is that “If TOC samples are being held for long periods of time before analysis, a firm should verify the impact of sample holding time on accuracy and limit of quantitation”. Needless to say, this is not a requirement limited to TOC as analytical method. It should be considered for any analytical method. Although not mentioned by the FDA, one concern for TOC measurements involving swabs is extraction of organic carbon from the swab material over time, which could impact the swab blank in terms of consistency over time.
The first issue here is the mistaken belief that 500 ppb carbon is an acceptable concentration because that is the level “specified” for the quality of certain pharmaceutical waters. First of all, the USP testing of pharmaceutical waters is not directly a limit of 500 ppb carbon, but the requirement is that the test sample have a response in a TOC procedure of less than a sample containing 500 ppb carbon from a sample of sucrose (as corrected by the TOC value of the background water the sucrose is dissolved in). Related to this is that the USP test method for water is not a quantitative measurement over the entire range up to 500 ppb of carbon; it is actually a limit test (the water sample either passes or fails). If the calculated limit of a cleaning validation sample is 200 ppb, the TOC analytical method should be validated to measure accurately at 200 ppb carbon (as well as somewhat below that value). Likewise, if the calculated limit of a cleaning validation sample is 800 ppb, the TOC analytical method should be validated to measure accurately at 800 ppb carbon (as well as somewhat below that value).
A second issue is how TOC may be used by biotechnology firms to measure cleaning effectiveness in upstream or downstream processes. While it may be possible to set limits based on PDE of the intact protein active and convert that calculated protein limit to an equivalent TOC limit based on the percentage of carbon in the intact protein, that is not necessarily the best approach for use of TOC. There are two possible factors why this may not be the best approach. One reason is that the actual residue left after cleaning is not necessarily the intact protein; rather the actual residue is composed of degraded fragments of the protein (degraded by cleaning with hot, alkaline solutions). Based on a smaller molecular weight, those degraded fragments are likely to be less toxic as compared to the intact protein. The second is that, depending on the dose of the intact protein, the equivalent TOC value may be very low, perhaps below the LOD/LOQ of the TOC method. For example, if the analysis involves subtracting the background TOC of the water from the measured cleaning validation sample, it is not likely that TOC could be quantitated with a reasonable LOQ as low as 10 ppb with an expected analytical method RSD of 10%. Note that these figures for LOD/LOQ and RSD are given just as examples; the actual values will depend on the details of the procedure used.
These are not the only issues that might arise in the use of TOC for cleaning validation samples. It should go without saying that there are some cases where it should be acceptable as well as other cases where it should be avoided.
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