Approximately 900,000 suicide deaths occur worldwide every year, and pesticide poisoning accounts for at least one-third of the total number of cases.^[1] Chlorfenapyr is ued in some countries to kill insects that are resistant to organophosphorus insecticides.^[2]

Chlorfenapyr is a white-tan crystalline solid developed by the American Cyanamid Company.^[3] Its International Union of Pure and Applied Chemistry name is 4-bromo-2-(4-chlorophenyl)-1-ethoxymethyl-5-(trifluoromethyl)pyrrole-3-carbonitrile. Its molecular formula is C₁₅H₁₁BrClF₃N₂O, and its relative molecular weight is 407.61 g/mol. It is a pyrrole class broad-spectrum insecticide (supplementary Figure 1A) currently registered in 19 countries for the control of various insects and mites.^[4] Chlorfenapyr has a more significant insecticidal effect than the traditional insecticide phoxim;^[5] however, it also poses a threat to human health and is classified as a medium risk by the World Health Organization (WHO). Chlorfenapyr has poor water solubility, dissolves in organic solvents, degrades slowly in soil, and is not volatile; this exacerbates the problems caused by insecticide residues in the environment.^[6] Chlorfenapyr is passively degraded into CL303268, CL325195, CL152832, CL152835, CL325157, and other metabolites. Among these compounds, CL303268 (tralopyril) is the most toxic (supplementary Figure 1B),^[7] with a relative molecular weight of 349.53 g/mol and the same solubility as chlorfenapyr.

Chlorfenapyr does not cause any cross-resistance to neurotoxic insecticides.^[5,8] Chlorfenapyr has been reported to have greater toxicity with increasing temperature, which predicts the seasonality of pesticide spraying. Chlorfenapyr has been used to control mites on Lepidoptera, Diptera, Coleoptera, Hemiptera, and other crops and trees.^[3] Its metabolite, tralopyril, was accepted in 2014 as a new-generation alternative antifouling biocide for effectively controlling biological fouling.^[9⇓⇓-12] Tralopyril is often released into the water environment through splashing or soil drainage.^{[13⇓⇓-16]}

Given the high mortality rate associated with chlorfenapyr poisoning, this study aimed to review the mechanisms, clinical presentations, and treatment strategies for chlorfenapyr poisoning.

We conducted a review of the literature using PubMed, Web of Science, and SpringerLink from their beginnings to the end of October 2023. The inclusion criteria were systematic reviews, clinical guidelines, retrospective studies, and case reports on chlorfenapyr poisoning that focused on its mechanisms, clinical presentations, and treatment strategies. The references of the included studies were also examined to identify additional sources.

This article is based on previously conducted studies and does not contain any studies with human participants or animals performed by any of the authors.

In total, 57 articles were identified in an initial search. Chlorfenapyr poisoning can occur through oral ingestion, with a median lethal dose (LD₅₀) of 441 mg/kg for rats^[17] and 45 mg/kg for mice.^[18] Moreover, the LD₅₀ of chlorfenapyr on rabbit skin is >2,000 mg/kg body weight (BW).^[19] The absorption of chlorfenapyr in rats is relatively slow, plasma concentrations peak at 8-12 h after administration, and more than 90% of a dose is gradually transferred from the circulation organs to other organs within 168 h, with the final 75%-85% of the dose excreted in the stool and a small amount via urine. The half-life of plasma clearance is approximately 56 h.^[20] Currently, there are no data on the protein binding rate or apparent volume of distribution (VD). After the rats were fed with carbon-14 labeled chlorfenapyr for 1, 8, 24, or 168 h, the concentrations of the radioisotope carbon 14 in the blood and organs of the rats were measured, and the researchers found that the apparent VD of chlorfenapyr was large.^[21] To date, various pathways of chlorfenapyr poisoning, including oral administration, contact, inhalation, and intraperitoneal injections, have been reported.^[22,23] Following oral administration, chlorfenapyr is absorbed into the blood via the digestive tract. Most poisons are removed from the body through the intestine, while some enter the hepatoenteric circulation and are metabolized by the liver. Figure 1 shows the residues distributed in tissues and organs throughout the body, and Figure 2 shows clinical manifestations of chlorfenapyr poisoning.^[11]

Chlorfenapyr and its metabolites can act on the mitochondria of cells, resulting in decreased mitochondrial membrane potential due to calcium overload. This interferes with the internal and external proton balance of the mitochondrial membrane, causing mitochondrial injury.^[24] Chlorfenapyr reduces energy production by inhibiting the conversion of adenosine diphosphate to adenosine triphosphate (ATP), promoting mitochondria-mediated programmed cell death (PCD)^[25] and contributing to DNA injury.^[26] In human hepatocytes, chlorfenapyr promotes the expression of the cellular oxidative stress factor Bax/Bcl-2, leading to the release of cytochrome c (Cyt c) into the cytoplasm, the activation of Cas9/3, and the cleavage of poly(ADP-ribose) polymerase. The DNA injury and cell cycle arrest were detected in human hepatocytes treated with chlorfenapyr.^[27] Moreover, the serum glutamate aminotransferase, gamma-glutamyl transpeptidase, urea nitrogen, and total protein levels increased in rats poisoned with chlorfenapyr. Hypertrophy and hepatocyte necrosis have also been observed.^[28] These results suggest that chlorfenapyr induces PCD in hepatocytes. Uncoupling protein 1 (UCP1) is specifically and highly expressed in brown fat and is located in the inner mitochondrial membrane. After activation of UCP1, a large amount of heat generated through decoupling increases body temperature.^[29] Sweating and hyperthermia caused by chlorfenapyr are related to the decoupling of mitochondrial oxidative phosphorylation. At 52 weeks following ingestion, the mature central nervous system in rats with chlorfenapyr poisoning exhibited pathological processes of vacuolar myelination and mild myelin swelling, demonstrating white matter degeneration and a high signal intensity on T2-weighted imaging andmagnetic resonance imaging (MRI).^[28] Studies have shown that chlorfenapyr and its metabolites can also affect the muscle system and reproductive system.^[9,30]

Owing to the atypical symptoms of acute chlorfenapyr poisoning and the lack of clinical research on this pesticide, there is no specific antidote available. The diagnosis and treatment of this disease are often delayed. Chlorfenapyr poisoning is characterized by delayed toxicity, and its metabolites can reduce ATP production by inhibiting oxidative phosphorylation in mitochondria, thereby causing severe injury to energy-intensive organs, including the nervous system, muscular system, and cardiovascular system (Figure 2).^[30] Chlorfenapyr poisoning has an incubation period of 1-2 weeks. After absorption, chlorfenapyr is slowly released into the blood, causing delayed toxic reactions.^[31] Chlorfenapyr poisoning is highly lethal; most patients develop high fever and experience mental disturbance within 4-21 d.^[32] Therefore, patients with chlorfenapyr poisoning should be closely monitored, and early symptoms should not be ignored. Almost all patients with severe chlorfenapyr poisoning show progressive and aggravated symptoms of nervous system injury, including fatigue, high fever, and an altered state of consciousness,^[33,35] indicating that the brain is one of the primary target organs.^[36]

Focal neurologic deficits have also been reported in patients with chlorfenapyr poisoning. Kwon et al^[33] reported a case of lower limb weakness with hypoesthesia 2 weeks after oral administration of 10 mL of 10% chlorfenapyr, which progressed to lower limb paralysis 10 d later. Another patient exhibited blurred vision, difficult urinating, and incoherence 2 d after skin exposure to chlorfenapyr.^[37]

High fever and disturbances in consciousness are characteristic clinical manifestations of chlorfenapyr poisoning. The physical and pharmacological cooling effects are usually unsuccessful, which may indicate ATP depletion.^[38] Respiratory and circulatory failures occur quickly in these patients, requiring ventilator-assisted ventilation and high doses of vasoactive drugs.

Chlorfenapyr causes mitochondrial dysfunction, which leads to myocardial injury. The heart is an energy-intensive organ with distinct clinical manifestations. Patients may have an increased heart rate and arrhythmias as the disease progresses.^[37,24] Li et al^[39] reported acute myocardial injury caused by chlorfenapyr. Twelve-lead electrocardiography after oral adminstration of chlorfenapyr (unknown dosage) revealed extensive ST-T changes and ventricular arrythmia with troponin elevation, suggesting myocardial injury and involvement of the cardiac conduction system. Notably, cardiac arrest is the direct cause of death in patients with chlorfenapyr poisoning;^[30,32] however, the specific mechanism of cardiac injury has not yet been explored.

It is common for digestive system to be injured following the absorption of chlorfenapyr through the digestive tract. Nausea, vomiting, abdominal pain, abdominal distension, and other manifestations occur 1-2 d after chlorfenapyr ingestion.^[35,40] The severity of injury is dose-related. Patients with acute poisoning often develop liver injury. The main pathological changes associated with liver injury are hepatocyte hypertrophy and necrosis.^[28] Ku et al^[41] reported acute pancreatitis after chlorfenapyr poisoning. There was no biliary obstruction in this patient, and amylase and lipase levels suddenly and progressively increased, suggesting that acute pancreatitis may have been caused by the direct toxicity of chlorfenapyr and the cumulative toxicity of its metabolites.

Chlorfenapyr reduces energy production. Failure of the necessary energy supply leads to multiple organ failure and eventually death. Striated muscles are high-energy and oxygen-consuming organs. Most patients with chlorfenapyr poisoning have symptoms of rhabdomyolysis, including muscle soreness and fatigue.^[40] Chlorfenapyr also causes kidney injury, and some patients may develop renal dysfunction and acute renal failure at later stages.^[30]

The diagnosis of chlorfenapyr poisoning requires knowing the exact amounts of toxicants and their metabolites. These can be achieved by testing vomit, excreta, serum, and residual poisons using gas chromatography-tandem mass spectrometry.^[43] Chung et al^[38] showed that low levels of serum chlorfenapyr were detected 4 h after poisoning but were not detected 4-7 d after poisoning. They also observed a delayed increase in serum CL303268 (tralopyril) levels, suggesting a greater correlation between metabolites and symptoms compared with serum chlorfenapyr.

Timely and effective laboratory and imaging examinations are helpful for determining disease severity and providing guidance for treatment measures. After analyzing the laboratory results of reported poisoning cases, we found that at the early stage of the disease (less than 6 d), the abnormalities were liver and kidney injuries (increased levels of blood urea nitrogen, creatinine, aspartate aminotransferase, and alanine aminotransferase), as well as elevated levels of myoglobin and creatine kinase. Some patients also exhibited increased levels of inflammatory markers, as well as electrolytes and acid-base balance disorders (e.g., hyperkalemia and metabolic acidosis). However, at the later stage (7-14 d), more signs of neurotoxicity and cardiotoxicity were shown, and they were more significant according to the imaging and electrocardiogram findings.^[33,35,42]

Electroencephalography has been performed on patients with chlorfenapyr poisoning and has demonstrated widespread slow delta activity.^[33] White matter lesions may also affect body temperature regulation.^[44] Therefore, in the diagnosis and evaluation of chlorfenapyr poisoning, MRI is particularly important for determining the course of the disease and patient prognosis. Imaging studies have shown that there are extensive and symmetrical signal intensity abnormalities in the white matter of the brain and cerebellum, including the internal capsule, optic nerve, corpus callosum, corticospinal tract, and brainstem; in addition, spinal MRI shows diffuse swelling of the spinal cord.^[34]

Hyperpyrexia-neurological syndrome, including Parkinson’s hyperpyrexia syndrome, neuro-blocker malignant syndrome, and 5-hydroxytryptamine syndrome, often manifests as high fever, fatigue, sweating, and altered mental status, among other symptoms.^{[45⇓⇓-48]} This type of disorder can be identified based on specific medical and medication histories.

Toxic leukoencephalopathy is a disease characterized by structural changes in the white matter caused by various pathogenic factors, mainly myelin sheath injury. Clinical manifestations include inattention, forgetfulness, and personality changes that can progress to dementia, coma, and even death.^[34] Moreover, imaging studies of chlorfenapyr poisoning should focus on the white matter tracts, which are helpful for differential diagnosis.^[49]

Other poisons, like cyanide, aluminum phosphide, and sodium pentachlorophenol, inhibit mitochondrial oxygen metabolism through different pathways.^[50,51] After chlorfenapyr poisoning, high-energy oxygen-consuming organs are injured, and respiratory circulatory failure occurs in severe cases,^[52] similar to the poisoning of sodium pentachlorophenol, a mitochondrial phosphoric acid decoupling poison.^[53] Patients with pesticide poisoning can be diagnosed and the poison itself can be identified based on the characteristic clinical manifestations.

Currently, there is a lack of effective antidotes for chlorfenapyr poisoning, leading to high mortality rates.

Early emesis, gastric lavage, and catharsis after poisoning are essential first steps. The effects of chlorfenapyr if taken orally are delayed.^[30] Based on experimental data from rats, it is speculated that the apparent VD of chlorfenapyr is large and that sufficient hemoperfusion of tralopyril can effectively reduce the distribution of toxins. Multiple early hemoperfusions are beneficial for the long-term survival of patients with chlorfenapyr poisoning.^[54] Sequential hemoperfusion through hemodialysis can also be used.^[55] Relatively expensive blood transfusions have also been reported to clear chlorfenapyr; however, their efficacy has not yet been determined. In severe cases, hemoperfusion should be considered. For patients with delayed intoxication or hemodynamic instability, the purification method of continuous renal replacement therapy, which can have a better curative effect by extending the filtration time, should be considered.^[36] Owing to the lack of data on the apparent VD, protein binding rate, and clearance rate of chlorfenapyr, clinicians should consider a more efficient blood purification method.

Creatine kinase and myoglobin levels can be used as monitoring indicators after poisoning for >3 weeks.^[31] Early extracorporeal membrane oxygenation is important for reducing the risk of death from cardiac arrest in patients with high-dose intake or severe illness.^[56] The application of excitatory substances to the central nervous system and exogenous ATP supplementation may be helpful in patients with chlorfenapyr poisoning.^[57] Acetylcysteine antioxidants, the protection of mitochondria using coenzyme Q, the promotion of oxidative phosphorylation, vitamin supplementation, and glucocorticoids have all been studied for the treatment of chlorfenapyr poisoning.^[24,30,35]

Chlorfenapyr is a pyrrole insecticide that is often mixed with other pesticides. Although it has been identified as a moderately toxic pesticide by the WHO, the mortality rate of poisoned patients is extremely high. Further research is needed to identify fast and efficient detoxification methods. High fever and consciousness disorders are featured clinical manifestations of chlorfenapyr poisoning. Respiratory failure occurs immediately, ultimately leading to death from cardiac arrest, which is difficult to prevent. To explore treatment options, we should focus on reconstructing intracellular oxidative phosphorylation coupling. Patients with chlorfenapyr poisoning should be closely monitored, and early symptoms should not be ignored. It is important to identify early biomarkers of chlorfenapyr poisoning to determine its prognosis. As there is no specific antidote for chlorfenapyr poisoning, we can consider developing new drugs that block the conversion of chlorfenapyr to tralopyril, reducing its toxic effects. We believe that further research will help clinicians make early diagnoses and treatments and eventually improve patient outcomes.

Funding: This study was supported by the Research Foundation of Ningbo No. 2 Hospital (2023HMKY49) and Ningbo Key Support Medical Discipline (2022-F16).

Ethical approval: Not needed.

Conflicts of interest: The authors do not have a financial interest or relationship to disclose regarding this research project.

Contributors: JC (Ji Cheng) and YLC (Yulu Chen) contributed equally to this work. JC, YLC, and LWD conceived the study concept and design. JC and YLC was involved in the drafting and critical revision of the manuscript. All authors contributed to the design and interpretation of the study and to further drafts.

All the supplementary files in this paper are available at http://wjem.com.cn.