Impurities assay

The host cell impurity assay can be used as a tool for detecting impurities in the various in-process purification steps as well as the final product. When using the assay for the in-process steps, it is important to test the samples in the appropriate buffers or matrix. Any sample recovery problems from buffers need to be determined and accounted for in the analysis. 

Type of Analytical Procedure Characteristics Identification Testing for Impurities Assay - Dissolution (Measurement Only) - Content/ Potency... 

The aim of validation of an analytical procedure is to demonstrate that the method employed in any product testing, such as the identification, control of impurities, assay, dissolution, particle size, water content, or residual solvents, is validated in the most important characteristics. Identification tests, quantitative tests for impurities content, limit tests for control of impurities, and quantitative tests of the active moiety in samples of pharmaceutical product are the most common types of analytical procedures that validation addresses [1]. 

Another common reason for having separate assay and impurity methods is the need to use more concentrated samples with the impurity assay to increase sensitivity for minor impurities. Modern HPLC systems have been shown to adequately detect low-level impurities (i.e., 0.05%) in chromatograms where the parent peak is still on scale (that is, within the linear range of the detector). This level of detection is usually adequate for screening methods therefore, the assay for loss of parent compound and the measurement of the increase in impurities can typically be done using a single HPLC method. 

Figure 1. Key steps in the development of protein impurity assays. The reference impurities may be obtained by a process specific purification of host cell proteins arising from a blank run or a production run. While the production run is a more accurate population of potential impurities, the product removal step involves significant technical difficulty.

Sample Assay % Assay % Impurity %Assay % Impurity Mass... 

Heavy metals Organic volatile impurities Assay (of methoxyl groups)... 

Organic volatile impurities Assay (dried basis) of — +... 

Sorbitan palmitate Sorbitan monostearate Organic volatile impurities Assay for fatty acids Sorbitan monolaurate... 

Type of analytical procedure Identification Testing for impurities Testing for impurities Assay — dissolution (measurement only) — content/potency... 

Special Case - Reviewing Impurity Assay Results... 

The specific case of impurity assay results requires a more detailed discussion because of its impact on the proposed drug substance and drug product specifications during development studies. 

Analytical research department involvement with bulk drug LC methods development commonly begins during early toxicology and safety evaluation, when it is necessary to document dose administered in the vehicle used and stability of the drug in the vehicle over the dosing period. This soon leads to preclinical method validation studies for the bulk drug assay and for the development of an impurities assay, both of which receive full validation at the later time. 

The future development of computer simulation will be shaped in part by the limitations of presently available software. The major shortcoming of present computer programs is their limited ability to carry out peak tracking, which is a prerequisite for the use of computer simulation. Peak tracking becomes more difficult when (a) the number of compounds in the sample exceeds about ten and/or (b) trace components are present This tends to complicate the use of computer simulation for many samples e.g., impurity assays, peptide digests, etc. The increasing use of LC-MS should in time both minimize difficulties in peak tracking and further simplify method development... 

Verification—organic impurities Assay Thin-layer chromatography (TLC)... 

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