Nitrosamine formation in Lidocaine active pharmaceutical ingredients and drug products

Abstract

N–nitrosamine compounds have long been known for their carcinogenic properties, but they came into the focus of the pharmaceutical industry in 2018 when N-nitrosodimethylamine was detected in the active ingredient Valsartan. Since then, there has been a growing literature on the formation of N-nitrosamines in pharmaceutical products. In our study Lidocaine semi-solid and liquid pharmaceutical formulations were tested with a limit of 26.5-53.0 ppb N-nitrosodiethylamine (NDEA) and 100-200 ppb N-nitroso-desethyl lidocaine. It was found that the lidocaine active ingredient degrades to the secondary amine precursors of these nitrosamines at elevated temperatures and that the formation of nitrosamines is feasible in the drug products where the excipients contain trace amounts of nitrite. Nitrite was detected at ppb level in the excipients of the formulations, while diethylamine and desethyl lidocaine were detected at ppm level during the degradation of the active substance. The nitrosamine formation was investigated at their respective production temperatures (ointment: 70°C/3 hours; injection: 125°C/15 minutes), and the NDEA and N-nitroso-desethyl lidocaine contents were measured by GC-MS and HPLC-MS. The formation of nitrosamines in the drug formulations was found to be not only time, temperature, and nitrite dependent but was also different in the base and salt form of the API and in the studied semi-solid and liquid pharmaceutical formulations. These experiments have proven to be a useful tool for predicting nitrosamine formation in lidocaine pharmaceutical formulations and can be used as a basis for making recommendations to reduce nitrosamine concentrations.

Introduction

Nitrosamines are unpleasant, potentially dangerous by-products of the pharmaceutical industry that require strict control. To ensure the safety of medicines, work is ongoing to develop appropriate manufacturing and testing protocols to avoid the presence of nitrosamines in the final products. Their carcinogenic properties were reported as early as 1956 by Barnes and Magee, who observed the growth of malignant primary liver tumors in animal species exposed to N-nitrosodimethylamine (NDMA)¹. Subsequently, other low molecular weight nitrosamines have been reported, followed by the International Agency for Research on Cancer (IARC, 1978)² review, concluding that, among others, NDEA (Figure 1) was carcinogenic in all animal species tested and that there was sufficient evidence of carcinogenicity to classify NDEA as a probable carcinogen (Group 2A) in humans. Due to its high carcinogenicity, the acceptable intake (AI) of NDEA is very low, 26.5 ng/day^3,4. Thus, developing analytically sensitive methods was recommended for the pharmaceutical industry5, 6, 7, 8. The main challenge is the control of nitrosamine drug substance-related impurities (NDSRIs) because they are unique to each active pharmaceutical ingredient (API), and for many, there is insufficient empirical data to determine AI limits. The Carcinogenic Potency Categorization Approach (CPCA) provides a new method for determining recommended AI limits for previously unknown nitrosamines and NDSRIs based solely on their chemical structure⁹. It is a rapid structure-activity relationship (SAR) approach that classifies nitrosamines into five categories based on predicted carcinogenicity, each with a corresponding AI limit. In November 2023, the CPCA classification of N-nitroso-desethyl lidocaine (Figure 1) was published as Category 2, with an AI limit of 100 ng/day. This limit was derived using the SAR approach, taking the TD₅₀ of N-methyl-N-(4-oxo-4-pyridin-3-ylbutyl)nitrous amide as structurally similar to N-nitroso-desethyl lidocaine⁴.

In general, N-nitrosamines are formed from secondary or tertiary amine precursors in the presence of a nitrosating agent under favorable reaction conditions, which include acidic pH, elevated temperature, metal ion catalysts, and humidity¹⁰. These precursors may originate from several sources in the pharmaceutical formulation; for example, they may be the API or the degradation products thereof, impurities, or trace amounts of reagents from the synthesis. Nitrite may be present in the highest amounts in excipients used in the manufacturing process of the pharmaceutical formulation, and materials from packaging operations and degradation¹¹. Sources in the literature mostly investigate nitrosamine formation in solid preparations (tablets, capsules)12, 13, 14, 15, 16, 17, there are very few publications evaluating semi-solid¹⁸ and liquid formulations^19,20.

Lidocaine, formerly known as lignocaine, is a local anaesthetic. First synthesized between 1943 and 1946 by Nils Löfgren and Bengt Lundquist²¹, it is a tertiary amine derived from xylidine, and its use spread rapidly because of its improved safety profile compared to older local anaesthetics. It is often mixed with small amounts of adrenaline to prolong its local effect and reduce bleeding. It is used in a wide variety of preparations, such as an injection for surgery, as a cream for minor burns, scratches, and insect bites, and as a spray for dental and oral surgery, ear, nose, and throat procedures.

In this paper, we have investigated the possible routes of nitrosamine formation in ointment, and injection formulations of lidocaine API in hydrochloride monohydrate and in base form. Temperature stress experiments were carried out on these diethylamino functional group-containing API forms (Figure 1) at temperatures used during the formulation process. The extent to which the cleavage of the diethylamino or ethyl group takes place and the conversion of the resulting precursors to nitrosamine impurities in semi-solid and liquid formulations were investigated. The experimental results were supported by theoretical chemical calculations.

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Andrea Kalauz, Sarolta Boglárka Szabó, Adrienn Tóth-Malik, Zsolt Kelemen, Viola Horváth, Imre Kapui, Nitrosamine formation in Lidocaine active pharmaceutical ingredients and drug products, Journal of Pharmaceutical Sciences, 2025, 103921, ISSN 0022-3549, https://doi.org/10.1016/j.xphs.2025.103921.

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