CYP2E1 is an enzyme involved in phase I of isoniazid metabolism [5]. This enzyme leads to the formation of metabolites among which, the more reactive hydrazine, potentiates the direct toxicity of isoniazid.

CYP2E1 polymorphisms have been associated with hepatotoxicities including isoniazid (INH)-induced hepatitis [5]. The c1/c2 genotype of CYP2E1 polymorphisms (CYP2E1 c1/c2) and those of NAT2 were also associated with the development of severe polyneuropathy (PNP) in a 23-year-old man who received INH-containing therapy [15]. Indeed, during his treatment, laboratory results had shown incipient liver and kidney damage [15]. Later, the genetic analyzes carried out revealed the existence of these two polymorphisms in the patient [15].

CYP2E1 has been associated with isoniazid-induced hepatotoxicity due to its ability to activate acetylhydrazine, subsequently forming various hepatotoxins [16]. There is evidence associating CYP2E1 levels with interindividual variation [16]. The CYP2E1*5 allelic variant is common in Japanese (Frequency 0.27) but rare in Europeans (Frequency 0.02). Isoniazid’s toxic metabolites can also cause other toxicities such as peripheral neuropathy and maculopapular rash [4].

Isoniazid hepatotoxicity was significantly associated with the CYP2E1 RsaI polymorphism and the 96 bp deletion-insertion single nucleotide polymorphism of the CYP2E1 gene [1]. Patients with a homozygous or heterozygous mutant genotype for the CYP2E1 RsaI polymorphism are less likely to present with hepatotoxicity compared to homozygous wild-type patients [1]. On the other hand, the CYP2E1 DraI and PstI polymorphisms are not significantly associated with isoniazid-induced hepatotoxicity [1].

In the absence of discontinuation of antituberculosis treatment, hepatotoxicity can lead to death in 6% to 12% of cases [1].

According to Wang et al., the CYP2E1 c1/c1 genotype significantly increases the risk of isoniazid-induced hepatotoxicity [1]. Isoniazid reactive metabolites destroy hepatocytes by directly interfering with cellular homeostasis or by triggering immunological reactions [17].

The hepatic enzyme CYP2E1 is thought to be involved in the metabolism of Rifampicin [18]. Indeed, its polymorphisms have been associated with elevated plasma concentrations of Rifampicin in poor CYP2E1 metabolizers [18]. Some TB patients who were extensive metabolisers had higher hepatic cytolysis (ALT, AST) and biliary stasis (Gamma-GT) indices compared to poor metabolisers before and after hospital treatment [18].

CYP2E1 polymorphism is a genetic marker of susceptibility to hepatotoxicity before and during antituberculosis treatment [18]. However, Xiang et al. found no association between hepatotoxicity and CYP2E1 RsaIc1/c1, CYP2E1 RsaIc1/c2, or c2/c2 polymorphisms [3].

The pathogenetic mechanism of antituberculosis drug-induced hepatotoxicity is still unclear. A better understanding of this mechanism would help guide the medical approach for rapid and optimal patient care. The combination of Isoniazid and Pyrazinamide would increase the risks of hepatotoxicity from Isoniazid. It is therefore possible that there is a drug interaction potentiating the hepatotoxicity of isoniazid.

Hepatotoxins (Hydrazine or acetylhydrazine) generated by CYP2E1 or NAT2 are detoxified after conjugation by Glutathione S-Transferases (GSTM1 and GSTT1) in the liver [3]. Acetylhydrazine can also be detoxified after a second acetylation by NAT2 forming diacetylhydrazine, a non-toxic metabolite of isoniazid. Antituberculosis metabolizing enzymes are polymorphic and their distribution varies according to ethnic groups [3].

The heterozygous CYP2E1 c1/c2 genotype would contribute to a high risk of hepatotoxicity [19]. On the other hand, the absence of the “c2” allele in women would be a protective factor against the onset of hepatotoxicity, while in men, the presence of the “c1/c2” allele would contribute to a high risk of onset. hepatotoxicity [19].

In CYP2E1, two polymorphisms are most commonly studied (RsaI/PstI (rs2031920/rs3813867) and Dral (rs6413432). A recent meta-analysis revealed that the RsaI/PstI c1/c1 polymorphism confers a higher risk of hepatotoxicity induced by Isoniazid compared to c1/c2 or c2/c2 genotypes, but no increased risk of Dral polymorphism [20]. Isoniazid may also cause peripheral neuropathy in some patients during treatment [21].

CYP2E1 could be a good genetic marker of isoniazid -induced hepatotoxicity but larger studies are needed to validate its relevance as a risk factor [16].

NAT2 is one of the phase II enzymes that plays an essential role in the detoxification and metabolism of several environmental toxins and many drugs including Isoniazid [17] [22] [23]. It is the main cause of toxicity of antituberculous drugs in patients carrying mutations [24]. It is mainly found in the liver and intestine [25].

The NAT2 enzyme is encoded by the highly polymorphic NAT2 gene with 108 haplotypes in the general population [26]. NAT2 polymorphisms are responsible for a tri-modal distribution of isoniazid pharmacokinetics [27]. Thus, a distinction is made between slow, intermediate or normal and fast acetylators [27]. Slow acetylators are at risk of hepatotoxicity while fast acetylators face treatment failure [27]. The seven most common NAT2 polymorphisms are located at its exon 2 [26].

About 35 different NAT2 alleles have been reported in the Indian population [28]. The NAT2*4 allele has historically been referred to as “wild type” because it is most common in certain ethnic groups [28]. The NAT2 polymorphisms allowed to distinguish 3 phenotypes of acetylators: the fast acetylators (Homozygotes having two fast alleles), the intermediate acetylators (Heterozygote having a fast allele and a slow allele) and the slow acetylators (Homozygotes, having two slow alleles) [28]. Of these three acetylator phenotypes, the slow acetylator phenotype is clinically the most important [28]. On the other hand, in TB/HIV co-infected patients, there would be a discrepancy between the genotype and the NAT2 acetylator phenotype [28].

The combination of the slow acetylator profile and the c2 variant of the CYP2E1 gene is responsible for an increase in the hepatotoxicity of antituberculosis drugs [29]. A case of isoniazid-induced polyneuropathy has been reported in a 23-year-old patient, carrier of NAT2 and CYP2E1 mutations [15].

Renal failure is also a risk factor for isoniazid-induced hepatotoxicity [14]. In addition, other toxicities linked to the existence of a co-morbidity can also occur during the antituberculous treatment associating isoniazid. Thus, a severe case of encephalopathy was reported in a 71-year-old patient of African origin after being put on anti-tuberculosis treatment [14].

In hemodialysis patients and carriers of the slow isoniazid acetylator phenotype, a reduction in the dosage of isoniazid associated with an increase in the doses supplemented with Vitamin B6 or Pyridoxine (>100 mg/day instead of 10 - 25 mg/day) reduce the occurrence of hepatotoxicity, encephalopathy and neuropathy [14].

Isoniazid hepatotoxicity occurs by a molecular mechanism that depends on the presence of active alleles of the NAT2 gene (NAT2*4 and NAT2*12) [14].

In a study conducted in 2020 by Zahra et al., four new haplotypes NAT2*5TB, NAT2*5AB, NAT2*5ZA and NAT2*6W corresponding to the slow acetylator phenotype were identified [25]. The most frequent single nucleotide polymorphisms and haplotypes in the participants of this study were NAT2 803 A > G (0.510) and NAT2 341T > C (0.427), respectively, then NAT2*6C (25.00%) and NAT2*5A (22. 92%) corresponding to the slow acetylator phenotype [25].

The effectiveness of isoniazid-based chemotherapy depends on the acetylator status or phenotype and the patient’s medical condition [27]. Yet, a standard FDA-recommended dose is administered regardless of acetylator phenotype or immune status resulting in adverse effects occurring in 5% - 33% of patients [27]. Specific dose adjustments according to acetylator phenotype have been suggested to ensure optimal treatment results [27]. Thus, a reduced dose administered to slow acetylators significantly reduces exposure to toxic metabolites and therefore the risk of adverse events, while maintaining sufficient treatment efficacy [27]. In contrast, intermediate and rapid acetylators receive higher doses of isoniazid on a twice-daily schedule [27].

Slow acetylator phenotype, CYP2E1 c2 or A4 variant genotype, and female gender are significantly associated with anti-TB drug-induced hepatotoxicity in some TB patients [29]. Patients with the slow acetylator profile and the c2 variant are more at risk of developing hepatotoxicity [29]. In some patients carrying the GSTT1 and CYP2E1 polymorphisms, there is a synergistic interaction leading to epistaxis and increasing the risk of hepatotoxicity [29].

The increase in plasma concentrations of isoniazid following accumulation by slow acetylators exposes them to toxic effects, in particular hepatotoxicity [4]. It may also benefit the therapeutic efficacy of isoniazid [4]. On the other hand, in fast acetylators, the plasma concentrations of isoniazid are lower with the consequences of the absence of toxicity, the reduction of the efficacy of the drug, therapeutic failure and the appearance of resistant strains of mycobacteria [4]. Fast acetylators are homozygotes of the NAT2*4, NAT2*11A, NAT2*12A, NAT2*12B, NAT2*12C or NAT2*13 alleles. On the other hand, heterozygous subjects or carriers of the wild-type allele are respectively intermediate or slow acetylators [4].

Isoniazid has been associated with a case of severe encephalopathy in a chronic hemodialysis patient [14]. Investigations revealed that he had a slow acetylator phenotype and that Pyridoxal phosphate, the active metabolite of Pyridoxine used for supplementation, would have been removed by dialysis [14]. The risk of isoniazid toxicity is high in hemodialysis patients with NAT2 polymorphisms [14].

The incidence of hepatotoxicity induced by antituberculosis drugs including isoniazid ranges from 1% to 36%, and mortality in such cases is not uncommon [30]. NAT2 exhibits a hereditarily determined polymorphism [30].

There are approximately 36 NAT2 polymorphisms reported in the literature [17]. Fast acetylators metabolize INH five to six times faster than slow acetylators [17].

The slow C 481 A 590 G 857 haplotype and an intermediate acetylator T 481 A 590 G 857 haplotype have been associated with the development of hepatotoxicity [19].

NAT2 genotypes (NAT2*5/*5, NAT2*5/*6, NAT2*6/*6 and NAT2*6/*14) corresponding to the slow acetylator profile are very frequent in certain countries such as Morocco (78%) [31].

The major NAT2*4 haplotype would protect against hepatotoxicity [32]. The increased susceptibility to Isoniazid (INH)-induced hepatotoxicity due to the presence of NAT2 polymorphism has been demonstrated in the West Indian population [32].

The prevalence of isoniazid-induced hepatotoxicity was higher in men than in women, and there was a weak association with NAT2*5 genotypes [3]. In Uyghur tuberculosis patients in Xinjiang, China, liver damage has been associated with the NAT2*5 genetic variant [3].

Multidrug therapy for tuberculosis increases the risk of serious adverse drug reactions such as hepatotoxicity, gastrointestinal disorders, allergic reactions, arthralgias, neurological disorders and other symptoms [3]. Hepatotoxicity results from the production of toxic metabolites associated with the presence of hepatic enzyme polymorphisms [3].

Several candidate single nucleotide polymorphisms have been identified but their clinical utility in predicting isoniazid-induced hepatotoxicity remains uncertain [20]. The NAT2 polymorphisms rs1041983 and rs1495741 have been associated with isoniazid hepatotoxicity in a Singaporean population and have demonstrated clinical utility in the prediction of INH-induced hepatotoxicity [20].

Isoniazid can cause asymptomatic elevation of liver enzymes in 10% - 20% of patients on treatment, clinically significant hepatitis, or even in less than 1% of cases, acute liver failure [21]. Risk factors associated with Isoniazid hepatotoxicity are age, female gender, black race, alcoholism, liver disease, and concurrent use of other drugs such as Rifampin and Pyrazinamide [21]. The molecular mechanisms of isoniazid hepatotoxicity are not yet well understood, but the accumulation of toxic intermediate metabolites is thought to play an important role [21]. From another point of view, pharmacogenetic studies have so far only focused on the main enzymes (NAT2, CYP2E1 and GST) directly involved in the metabolism of isoniazid [21].

Isoniazid hepatotoxicity usually occurs between 2 weeks and 6 months after the start of treatment. The late onset of isoniazid hepatotoxicity indicates the existence of an immunological mechanism that may involve the antioxidant and detoxification pathway genes, the human leukocyte antigen (HLA) system and tumor necrosis factor-α (TNF-α) [21].

Of the many NAT2 polymorphisms, seven alleles (rs1801279, rs1041983, rs1801280, rs1799929, rs1799930, rs1208, and rs1799931) define 34 haplotypes, but most studies consider only a subset to determine the acetylator phenotype of patients [21]. A novel NAT2 polymorphism rs1495741 has recently been associated with isoniazid hepatotoxicity, although it may not be a good marker for determining the acetylator phenotype of patients [21].

Reactive metabolites of isoniazid may cause peripheral neuropathy or maculopapular eruptions [4].

NAT2 polymorphisms have also been associated in some studies with bladder cancer, colorectal cancer, rheumatoid arthritis, and diabetes mellitus [25].

In addition to genetic polymorphisms, environmental factors and concomitant drug intake can modulate the activity of antituberculosis metabolizing enzymes [28]. Thus it has been reported that Isoniazid would cause the decrease in the clearance of many drugs such as Phenytoin, Carbamazepine, Diazepam, Vincristine, and Acetaminophen. [28]. For example, in patients with tuberculous meningitis or tuberculoma having convulsions, the simultaneous use of Isoniazid and Phenytoin leads to an increase in plasma concentrations of the latter and the appearance of its intoxication [28].

It is important to determine the phenotypic status of patients before starting them on an antituberculous treatment regimen including isoniazid [26]. NAT2 genotyping could serve as a marker for the identification of patients predisposed to hepatotoxicity of antituberculosis drugs [32].

GSTs represent a superfamily of enzymes involved in phase II reactions in the metabolism of many drugs, including antituberculosis drugs [33]. They play a protective role in the processes of biological detoxification of drugs by catalysing the conjugation reactions of Glutathione and toxic intermediate metabolites, subsequently facilitating their elimination and then reducing their hepatotoxicity [33]. They exist in two forms (oxidized and reduced) and are involved in maintaining the redox potential of the cytoplasm of hepatic cells [34].

GSTs hydrolyze acetylhydrazine, the hepatotoxic metabolite of isoniazid [29]. These enzymes detoxify a variety of drugs by conjugation with glutathione. They also play a role in the detoxification of the products of oxidative stress [16].

The GSTM1 and GSTT1 isoforms are absent in several individuals because of the large genetic deletions resulting in null genotypes. Carriers of these null genotypes have no functional enzymatic activity and do not express the proteins [34]. It is possible that the GSTM1 and GSTT1 genotypes influence the risk of isoniazid-induced hepatotoxicity. Several independent studies have found an association between GSTM1 null genotypes and hepatotoxicity [35] [36]. The results of these studies had demonstrated that the frequencies of GSTM1 null genotypes were higher among the cases of hepatotoxicity in contrast to the controls [35] [36].

GST deficiency is due to mutations in the Glutathione S-Transferase Mu 1 (GSTM1) and Glutathione S-Transferase Theta 1 (GSTT1) null genes [29]. The genetic polymorphisms frequently associated with the toxicity of antituberculosis drugs are those of the genes Glutathione S-Transferase Mu 1 (GSTM1), Glutathione S-Transferase Theta 1 (GSTT1), and Glutathione S-Transferase Pi 1 (GSTP1) [5] [35] [37] [38]. However, deletions of the GSTM1 and GSTT1 genes are not associated with drug toxicity [39]. The high frequencies of deletions of the GSTM1 and GSTT1 genes in human populations would explain the interindividual variability of the metabolism of xenobiotics [5].

Null genotypes of the GSTM1 and GSTT1 genes are risk factors for hepatocellular carcinoma [40]. Indeed, these genes would be involved in the mechanisms of detoxification of environmental carcinogens, environmental toxins and radicals resulting from oxidative stress [40]. GST genes in the occurrence of hepatotoxicity and schizophrenia in patients on antituberculosis drugs has been reported [41]. In the Iranian population, the distributions of genetic polymorphisms of GSTT1 are more significant than those of other GSTs [42]. GSTM1 and GSTT1 gene deletions (42.9% and 12.4% respectively) in Brazilian populations, these are not associated with the toxicity of antituberculosis drugs [5]. However, this mutation has no influence on drug response [5].

There is a synergistic interaction between the GSTT1 and CYP2E1 genes associated with an increased risk of anti-TB drug-induced hepatotoxicity [29]. Isoniazid inhibits CYP2E1 activity less in individuals carrying the CYP2E1 *1A/*1A genotype compared to other genotypes [4]. Therefore, people carrying the CYP2E1 *1A/*1A genotype would have higher CYP2E1 activity, which leads to the formation of a greater amount of hepatotoxins [4]. Rifampicin and Pyrazinamide have also been associated with hepatotoxicity of antituberculosis drugs [4]. However, their mechanisms are still unknown and unpredictable. OATP1B1 transmembrane transporter polymorphisms have been associated with rifampicin toxicity [4]. Several studies have shown that if GST is not expressed, there will be an accumulation of reactive metabolites, which increases their interactions with cellular macromolecules [17].

East Asian patients carrying a GSTM1 null genotype have a higher risk than Caucasians, while Caucasians carrying a homozygous GSTT1 null genotype are at higher risk of hepatotoxicity unlike Asian patients. GST polymorphisms are highly variable depending on the ethnic groups and the age of the patients [33]. The association between GSTM1/GSTT1 null mutations and an increased risk of anti-TB drug hepatotoxicity has been demonstrated in adults. On the other hand, in children, there would be no significant association.

UDP-Glucuronosyl-Transferases (UGT) are phase II enzymes in the metabolism of certain drugs, including isoniazid. They are responsible for the glucuronidation of endobiotics and xenobiotics [43]. These enzymes are often considered detoxifiers although they also have roles in the formation of reactive intermediates such as acylglucuronides, responsible for hepatotoxicities [16].

UGT1A1 polymorphisms lead to detoxification deficit of free bilirubin and organic anions such as Rifampicin [43]. These genetic mutations are responsible for a double toxicity on the one hand due to the accumulation of free bilirubin and on the other hand, by default of elimination of the metabolites of Rifampicin. The heterozygous variants UGT1A1*27 and UGT1A1*28 would be associated with a high risk of hepatotoxicity in Taiwanese patients [43].

Rifampicin can induce cytochrome P450 3A4 and increase the formation of a by-product, 6-Alpha-hydroxylated bile acid requiring UGT for glucuronidation before renal excretion [43].

Subject to confirmation of this association by studies on a larger number of patients, screening for UGT1A1 polymorphisms before treatment for tuberculosis may reduce the incidence of ATDH and improve compliance with treatment [43].

P-glycoprotein is a transmembrane efflux pump whose synthesis is encoded by the ABCB1 gene, the polymorphisms of which are responsible for resistance to drugs transported by this protein [46]. Among these drugs, resistance to Rifampicin and Ethambutol has not yet been fully elucidated [46]. However, findings from some studies associate ABCB1 polymorphisms and other risk factors with Ethambutol resistance [46]. It should be noted that these resistances are not always of bacterial origin but sometimes linked to the host [46].

MRP2 or multi-drug resistance protein 2 is a transmembrane protein whose synthesis is encoded by the ABCC2 gene. This protein transports bilirubin out of liver cells and into bile.

More than 40 ABCC2 polymorphisms have been associated with Dubin-Johnson syndrome. It is a disease characterized by yellowing of the skin and the whites of the eyes, which usually appears in adolescence or early adulthood. The majority of mutations are single nucleotide polymorphisms that alter a single amino acid in the primary sequence of the MRP2 protein.

The SLCO1B1 gene (Solute Carrier Organic Anion transporter family member 1B1) codes for the synthesis of a transmembrane protein, the organic anion transporter polypeptide 1B1 (OATP1B1). This protein present in the hepatocyte ensures the transport of endogenous substances (bilirubin, hormones, various toxins, etc.) and xenobiotics (statins, antihypertensives, anticancer drugs, antibiotics, etc.) from the blood to the liver for their elimination. SLCO1B1 polymorphisms have been associated with impaired transport functions of the OATP1B1 protein. This is because the mutated protein produced is less efficient in transporting compounds through the liver, leading to elevated levels of compounds in the body. Certain SLCO1B1 polymorphisms are implicated in Rotor syndrome manifesting as elevated levels of bilirubin in the blood. In some cases, the disease is caused by a deletion that removes parts of the SLCO1B1 gene so that no functional OATP1B1 protein is produced. Genetic polymorphisms of this transporter are associated with rifampicin toxicity [4]. However, the scientific evidence for this association is still insufficient.