Genotoxic Impurities: A Regulatory Toxicology Commentary on Recent Articles in Organic Process Research & Development

The food contaminant acetamide is not an in vivo clastogen, aneugen, or mutagen in rodent hematopoietic tissue

Acetamide (CAS 60-35-5) is classified by IARC as a Group 2B, possible human carcinogen, based on the induction of hepatocellular carcinomas in rats following chronic exposure to high doses. Recently, acetamide was found to be present in a variety of human foods, warranting further investigation. The regulatory body JECFA has previously noted conflicting reports on acetamide's ability to induce micronuclei (MN) in mice in vivo. To better understand the potential in vivo genotoxicity of acetamide, we performed acute MN studies in rats and mice, and a subchronic study in rats, the target species for liver cancer. In the acute exposure, animals were gavaged with water vehicle control, 250, 1000, or 2000 mg/kg acetamide, or the positive control (1 mg/kg mitomycin C). In the subchronic assay, bone marrow of rats gavaged at 1000 mg/kg/day (limit dose) for 28 days was evaluated. Both acute and subchronic exposures showed no change in the ratio of polychromatic to total erythrocytes (P/E) at any dose, nor was there any increase in the incidence of micronucleated polychromatic erythrocytes (MN-PCE). Potential mutagenicity of acetamide was evaluated in male rats gavaged with vehicle control or 1500 mg/kg/day acetamide using the in vivoPig-a gene mutation assay. There was no increase in mutant red blood cells or reticulocytes in acetamide-treated animals. In both acute and sub-chronic studies, elevated blood plasma acetamide in treated animals provided evidence of systemic exposure. We conclude based on this study that acetamide is not clastogenic, aneugenic, or mutagenic in vivo in rodent hematopoietic tissue warranting a formal regulatory re-evaluation.

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In vivo micronucleus tests with acetamide in mice and rats.

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Acetamide blood plasma levels demonstrated evidence of exposure.

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Acetamide does not induce micronuclei in rats and mice.

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Acetamide does not increase mutations in the rat Pig-a gene mutation assay.

Determination of five potential genotoxic impurities in dalfampridine using liquid chromatography

A sensitive and selective HPLC method was developed for identification and quantification of five Potential genotoxic impurities (PGIs) viz. Impurity-I, Impurity-II, Impurity-III, Impurity-IV and Impurity-V in Dalfampridine (Drug substance). The method utilizes Zorbax silica column (250mm×4.6mm, 5.0μm) with UV detector in HILIC (Hydrophilic Interaction Liquid Chromatography) mode for quantitation of five PGIs. It has been validated as per International Council for Harmonisation (ICH) guidelines and is able to quantitate all PGIs at 75ppm with respect to 20mg/mL of sample concentration. It is linear in the range of 22.5-112.5ppm for all PGIs, which matches the range of LOQ-150% of estimated permitted level (75ppm). Its accuracy was established in the range from 88.14 to 107.65% for these PGIs. The correlation coefficient of each impurity was >0.999. It is a good quality control tool for quantitation of PGIs in Dalfampridine at low level.

Mutagenic impurities in pharmaceuticals: a critique of the derivation of the cancer TTC (Threshold of Toxicological Concern) and recommendations for structural-class-based limits

The cancer TTC (Threshold of Toxicological Concern) concept is currently employed as an aid to risk assessment of potentially mutagenic impurities (PMIs) in food, cosmetics and other sectors. Within the pharmaceutical industry the use of one default cancer TTC limit of 1.5 μg/day for PMIs is being increasingly questioned. Its derivation, originally in the context of foodstuffs, can be broken down into five key elements: dataset composition; determination of carcinogenicity/mutagenicity status and carcinogenic potency (based on TD₅₀s) of compounds in the dataset; linear extrapolation of carcinogenic potencies; evaluation of the more potent compounds in each structural category, and presence of representative structural alerts amongst the more potent compounds. A detailed evaluation reveals that the derivation process is distorted by the use of the lowest statistically significant TD₅₀s (which can produce a false-carcinogen phenomenon) and by employing linear extrapolation for non-mutagenic carcinogens. By correcting for these two factors, it is concluded that only around 50% of conventional structural-alert categories were adequately addressed and that limits higher than the default value appear to be justified in many cases. Using similar criteria for PMIs in pharmaceuticals, four distinct potency categories of conventional structural alerts can be distinguished, ranging from alerts with questionable validity to those with high potency, which are considered to provide a range of flexible and pragmatic limits for such impurities.

Regioisomerism in the synthesis of a chiral aminotetralin drug compound: unraveling mechanistic details and diastereomer-specific in-depth NMR investigations

During chemical process development of a novel 2-aminotetralin derivative intended for use as an antidepressant, scrutiny of the byproduct present in the drug molecule revealed a set of regioisomers. Detailed studies showed that this impurity issue originated from an early synthetic step in which a brominated tetralone motif was generated in a ring-closing protocol. It was found that this reaction was accompanied by a migration of the aromatic bromo substituent via different bromonium species along two discrete pathways. This example of the halogen dance reaction resulted in the formation of a series of tetralone impurities with a bromine distributed across all available aromatic positions of the tetralin nucleus. Subsequently, when subjected to reductive amination conditions, each of these tetralones gave rise to pairs of aminotetralins in a diastereomeric relationship. NMR investigations revealed that the alicyclic portion of the compounds thus formed displayed very complex signal patterns, which required further in-depth studies using a variety of sophisticated techniques. As a result, a deep insight into the structural features of the current 2-aminotetralin family was obtained, which is emphasized by the definition of a novel "0.2 ppm rule" allowing the absolute configuration at tetralin C-2 to be determined.