Pharmaceutical industry is growing day by day with the aim to develop new drugs extracted from natural products or synthetic chemically produced drug substances, but one thing always remains constant, that is, the product should be as pure as possible. Therefore, purity has always been considered as an essential factor in ensuring drug quality. There is no known drug that is not harmful or even poisonous at high doses.
The gradual change from the use of natural products in their entire state to either purified extracts from those products or to synthetic chemically produced substances，can be said to have been taking place between the time of Paracelsus, who lived in Basel during the first half of the 16th century, to that of Ehrlich, to whom the award of a Nobel prize in 1909 was a fitting reward for his remarkable researches and breakthrough during the first decade of this century. This period has been described as that leading from Quintessence to the Chemical and has been fascinatingly reviewed by Barber. This transition from the Quintessence to the Chemical stimulated a very considerable amount of interest in the analysis as well as determination of purity of natural products, as De Quincey had said 100 years earlier, not the apparent quantities as determined by weighing but the virtual quantities after allowing for the alloy of impurity. Thus, there is no doubt that nearly a century of pharmaceutical research has contributed spectacularly to the improvement in human health and quality of life.
This paper reviews the impurities associated with the active pharmaceutical ingredients (APIs), their identification and characterization using different chromatographic, spectroscopic, and hyphenated techniques.
Drug registration authorities are increasingly interested in pharmaceutical impurities in the range 0.01–0.1%. Pharmaceutical impurities are the unwanted chemicals that remain with APIs or develop during formulation, or upon degradation of both API and formulated APIs to medicines. The presence of these unwanted chemicals even in small amounts might influence the efficacy and safety of the pharmaceutical products. Therefore, for identification and quantification the drug registration authorities have suggested the following steps:
The quality of starting materials, reagents and solvents used during synthesis, chemical reactions involved in the synthesis, reaction conditions, purification steps, and storage of the final drug substance affect the impurity profile of a drug substance. Any minor change in the above conditions may dramatically change the impurity profile.
It is required to detect impurities in drug substance obtained from batches manufactured during the development process, batches from the commercial process and stress conditions.
The structures of impurities should be elucidated when present at level higher than 0.1%or in some cases higher than 0.2%, depending on daily-recommended dosage.
The impurities are synthesized based on the suggested structures.
The synthesized impurities are used as an impurity standard for the development of a selective analyticalmethod for its quantitation in drug substance and/or products.
Impurities associated with APIs are classified into three groups for regulatory purpose as organic, inorganic, and residual solvents.
Organic impurities may arise from starting materials (most often from isomeric impurities), synthetic intermediates (incomplete reaction or excess reagent used), byproducts, degradation products, reagents, ligands, and catalysts. The reagents, ligands, and catalysts are less commonly found in APIs; in some cases, they may pose a problem as impurities.
Inorganic impurities present in pharmaceutical products originate from the equipment used and from reagents, catalysts, heavy metals, drying agents, and filter aids.The chances of having impurities from reagents, ligands, and catalysts are rare: however, in some processes, these could create a problem unless the manufacturers take proper care during production. The main sources of impurity of heavy metals are the water used in the processes and the reactors (if stainless steel reactors are used), where acidification or acid hydrolysis takes place. These impurities of heavy metals can easily be avoided using demineralized water and glass-lined reactors.
Residual solvents and other volatile impurities must be detected and assayed not only because of their potential toxicity and deleterious environmental effects but also because they can impart undesirable organoleptic characteristics to drugs. Since residual solvents arise in excipients and occasionally in the manufacture of drug products. According to ICH guidelines, residual solvents can be grouped into three categories based on the possible risk to human health. Category I incudes solvents such as benzene (2 ppm limit) and carbon tetrachloride (4 ppm limit). The solvents belonging to category II are methylene chloride (600 ppm limit), methanol (3,000 ppm limit), pyridine (200 ppm limit), toluene (890 ppm limit), N,N-dimethyl formamide (880 ppm limit), and acetonitrile (410 ppm limit). The solvents of category II aremost commonly used during themanufacturing process. Acetic acid, acetone, isopropyl alcohol, butanol, ethanol, and ethyl acetate are solvents of category III. These solvents have higher tolerance limits. ICH guidelines have recommended daily exposures of 50 mg or less per day.
The requirement of establishment of stability indicating assay become more clearly mandated with the advent of ICH, US-FDA, WHO, and European Committee for Directorates guidelines. These guidelines explicitly require conduct of forced decomposition studies under a variety of conditions like pH, light, oxidation, dry heat, etc. and separation of drug substance from degradation products. In definition “A stability-indicating assay is a validated quantitative analytical procedure that can detect the changes with time in the pertinent properties of the drug substance and drug product.” Overall, there are two stages in the validation of stability indicating assay. First stage is early in the development cycle when drug substance is subjected to forced decomposition studies and the stability indicating assay is established based on the knowledge of drug degradation behavior. The main focus of validation at this stage is on the establishment of specificity/selectivity, followed by other parameters like accuracy, precision, linearity, range, robustness, etc. This validated method finds application in the analysis of stability samples of bulk drug for determination of its retest or expiry period. In the second stage, when the stability indicating assay so developed is extended to formulations or other matrices, the emphasis gets limited to just prove the pertinence in the presence of excipients or other formulation constituents. Here, only parameters of critical importance like specificity, selectivity, accuracy, and precision are revalidated.
ICH has harmonized the requirements in two guidelines. The first one summarizes and defines the validation characteristics needed for various types of test procedure. The second one extends the previous test to include the experimental data required and some statistical interpretation. These guidelines serve as a basis worldwide both for regulatory authorities and industry and bring the importance of a proper validation to the attention of all those involved in the process submission. In order to fulfill the validation responsibilities properly, the background of the validation parameters and their consequences must be understood. Normally, evaluated validation characteristic and their minimum number of determinations required are five concentration levels, nine determinations over three concentration levels, six determinations at 100% level or nine determinations over three concentration levels for linearity, accuracy, and precision, respectively. Several approaches have been given in the ICH guideline to determine the detection and quantitation limit. Generally, they are based either on the analysis of blanks or on the scattering (variability) of the analytical signals in the low concentration range. Using the blank procedure, the corresponding calculation value ismultiplied by a factor 3.3 and 10 for the detection and quantitation limits, respectively. The calculation value may represent the signal of the blank, the standard deviation of the blank or of the intercept of a calibration line (corresponding to an extrapolated blank). However, these limits are of special importance in the transfer of analytical procedures and for the reporting of impurities.
Naﬁsur Rahman, Syed Najmul Hejaz Azmi, Hui-Fen Wu. Accred Qual Assur (2006) 11: 69–74