1 What are genotoxic impurities?
Genotoxic impurities (or genotoxic impurities, GTI) refer to the possibility or tendency that the compound itself directly or indirectly damages cell DNA, produces gene mutation or in vivo mutagenesis, and has carcinogenicity. Potentially genotoxic impurities (PGI) are similar to genotoxic impurities in structure, which is alarming. However, compounds such as aflatoxin, nitrosamine, mesylate, etc., which have not been proved by experiments, are common genotoxic impurities, and many chemotherapy drugs also have certain genotoxicity. Their adverse reactions are caused by the genotoxicity of chemotherapy drugs to normal cells, such as cisplatin, carboplatin and fluorouracil.
2 Why pay attention to genotoxic impurities
Genotoxic substances are characterized by damage to human genetic material at very low concentrations, which leads to gene mutation and may promote tumor occurrence. Because of its strong toxicity, it poses a great threat to the safety of drugs. In recent years, more and more cases of large-scale medical accidents caused by the discovery of trace genotoxic impurities in listed drugs have been forcibly recalled by FDA, which has caused huge economic losses to pharmaceutical companies. For example, Viracept (methanesulfonic acid), an HIV protease inhibitor launched by a well-known international pharmaceutical giant in the European market, suspended all its market activities in Europe in July 2007 because ethyl methanesulfonate was found to be a classic genotoxic impurity in its products, and the enterprise paid a huge price for it. Firstly, the reason for the excessive residue was investigated from the inside, because the ethanol was not completely removed when cleaning the instruments and equipment, and it remained, which was related to methyl. After being asked to solve the pollution problem, he was also asked to do toxicity research to better evaluate the risk to patients. At the same time, as many as 25,000 patients were exposed to this known genotoxicity. It was not until this problem was solved that EMA resumed its market authorization in Europe.
In recent years, regulatory agencies in various countries, such as ICH, FDA, EMA, etc. More and more pharmaceutical companies focus on the control and detection of genotoxic impurities in the process of new drug research and development.
3 which compounds are genotoxic impurities
The structure of impurities is varied, and for most impurities, there is often a lack of sufficient toxicity or carcinogenicity research data, so it is difficult to classify them. In the absence of safety data, these regulations and guidelines use "warning structure" as a sign to distinguish common impurities from genotoxic impurities. For the impurities with warning structure, (Q)SAR prediction, in vivo and in vitro genotoxicity and carcinogenicity should be carried out, or the impurity level should be controlled below the toxicological concern threshold (TTC).
Precursor carcinogens are generally divided into two categories: one is genotoxic carcinogens, which directly destroy genetic materials through chemical bonds to produce carcinogenicity, and most chemical carcinogens are genotoxic; The second category is non-genotoxic carcinogens, which usually do not cooperate with chemical bonds and do not cause direct damage, but cause cancer through indirect mechanisms other than genetic materials (such as promoting excessive cell proliferation).
Summarize the warning structure in several documents (see original PDF). For detailed information about the warning results of genetic impurities, please refer to the warning structure development of the structural warning of micronucleus test in rodents issued by the European Union. Or enter the carcinogen Poticity City database (CPDB), which contains a series of information such as 1547 carcinogen list, structural formula, CAS number, action site, TTC value, etc. It should be noted that the inclusion of warning structure does not mean that the impurity must have genotoxicity, and the substance confirmed to have genotoxicity does not necessarily cause cancer. Physical and chemical properties and other structural characteristics of impurities (such as relative molecular weight, hydrophilicity, molecular symmetry/steric hindrance, reactivity and biological metabolism rate, etc.). ) will inhibit or regulate their toxicity. The importance of warning structure lies in that it indicates possible genotoxicity and carcinogenicity, and points out the direction for further impurity safety evaluation and control strategy selection.
4 regulatory requirements and restrictions of genotoxic impurities
Initially, ICH has successively introduced the Q3A(R2) guiding principles for impurity research of APIs and Q3B(R2) guiding principles for impurity research of preparations. In these guidelines, it is mentioned that "for potential impurities with strong pharmacological activity or toxicity, even if their content is lower than 0. 1%, it is still recommended to conduct structural identification research". In the later revision, it was further clarified that "attention should be paid to the potential genotoxic impurities in APIs" and "a lower limit may need to be set for very toxic impurities", but the research and control of genotoxic impurities were not clearly stated, and no specific research principles, control strategies and limit requirements were put forward.
In EMA (European Organization for Drug Evaluation), the guiding principle of genotoxic impurity limit was introduced, and the concept of acceptable risk intake, namely toxic substance limit or toxicological concern threshold (TTC), was introduced. The limit value TTC( 1.5 μ g/day) is set, which is equivalent to the daily intake of 1.5 μ g of genotoxic impurities, and is considered as the acceptable risk of most drugs (the risk of cancer in a lifetime is less than one in 100,000). According to this threshold, the acceptable impurity level in active drugs can be calculated according to the expected daily intake. It should be pointed out that TTC is a risk management tool, which adopts probability method. If there is a genotoxic impurity, we don't know much about its toxicity.