As is the case with a number of drugs, most notably azidothymidine (AZT), a nucleoside reverse transcriptase inhibitor (NRTI) used in the treatment of HIV infections, isoniazid was first synthesised many years before the discovery of the antimycobacterial activity which led to its use in the treatment of TB. Isoniazid (isonicotinic acid hydrazide, INH) was originally synthesised in 1912 as part of a purely organic chemistry investigation (Meyer and Mally, 1912) and it was not until 1952 that its potential in the treatment of the dreaded TB (or consumption) was discovered (Wang, 2005).

Isoniazid has excellent bioavailability after oral administration as a single agent, at around 100% absorption, with peak blood concentrations achieved after 0.9–1 hours, peak mean serum concentrations of 3.6 μg/mL, and a half-life of 1.9 hours (Peters, 1960; Ellard and Gammon, 1976). As isoniazid is used as one component of combination therapy for TB, the pharmacokinetic parameters have also been investigated as part of a combination therapy; fortunately, they do not differ significantly from the single-agent parameters (Zwolska et al., 2002).

The absorption of isoniazid is affected by the presence of food (Melander et al., 1976) and by aluminum hydroxide (Hurwitz and Schlozman, 1974), with the result that it should not be taken with food or less than 1 hour before antacid formulations. TB is a complicating factor for many patients with AIDS, some of whom are already receiving didanosine, which incorporates antacid into the formulation to reduce stomach acidity and minimise the acid-catalysed hydrolysis of didanosine; fortunately, the small amount of antacid in the didanosine formulation has little effect upon the absorption of isoniazid, allowing its concurrent administration (Gallicano et al., 1994).

The bioavailability of isoniazid, like that of the sulfonamides, is affected by the rate at which it is metabolised by the human cytochrome enzyme, N-acetyltransferase 2 (NAT2) (Hughes, 1954; Evans et al., 1960). A patient with rapid acetylation of isoniazid metabolises and excretes it at a faster rate than is appropriate for the antibacterial activity, with the result that sub-therapeutic doses are achieved, with associated poor therapeutic outcomes. The situation is little better for those with slow acetylation status, as there is increased opportunity for high serum concentrations to be achieved, due to decreased metabolism and excretion of the isoniazid, and adverse events are more common for these patients; you will discover this in more detail in Subsection 5.4.7. As acetylator status is inherited and can be determined before treatment by genotype analysis, the personalisation of isoniazid treatment according to NAT2 status has been considered, and this would help to avoid adverse events caused by this problem (Evans, 1989; Spielberg, 1996; Butcher et al., 2002). When genotype analysis becomes less expensive and more routine, this may be a development for the future (Kinzig-Schippers et al., 2005).

As mentioned in Section 1, mycobacteria are aerobic organisms, characterised by thicker cell walls than Gram positive and negative organisms. The mycobacterial cell wall is hydrophobic/waxy in nature, being rich in very-long-chain fatty acids, known as mycolic acids, and is resistant to agents which interfere with cell wall synthesis, such as β-lactams. Isoniazid is actually a prodrug and is converted in vivo to the active form by the mycobacterial enzyme catalase-peroxidase (KatG) (Zhou et al., 2006). Catalase-peroxidases protect bacteria from the cellular damage caused by reactive oxygen species and KatG is responsible for the high resistance of M. tuberculosis to hydrogen peroxide (Manca et al., 1999).

As might be expected, M. tuberculosis resistance to isoniazid may be the result of a number of mechanisms, including:

• Mutations/deletions in the katG gene regulatory region, resulting in a decrease in INH-NAD(P) adduct formation. This effect is not solely due to a loss of catalase-peroxidase activity, and in fact the major mutation found in clinical isolates (serine 315 to threonine) (González et al., 1999) leads to a conformational change in the substrate access channel of the enzyme, resulting in decreased isoniazid binding (Cade et al., 2010).
  
• Over-expression of the InhA gene, so that there is insufficient INH-NADP complex for inhibition, results in resistance in M. tuberculosis strains harbouring InhA plasmids (Larsen et al., 2002), while 25% of the mutations in the InhA gene found in clinical isolates are in the promoter or structural regions (with consequences for NADH and INH-NADP binding) (Telenti et al., 1997).
  
• Increased arylamine N-acetyltransferase (NAT) activity, as a result of increased expression of the NAT gene (N-acetylation reduces the activity of INH) (Payton et al., 1999).
  
• For more on the bacterial resistance of isoniazid, see Dye and Espinal (2001).

5.4.6.1 Spectrum of activity

A common adverse effect associated with isoniazid is hepatic toxicity, which is associated with approximately 20% of patients receiving the drug for TB, with severe hepatotoxicity being shown in 1% of exposed patients (Scharer and Smith, 1969; Garibaldi et al., 1972).

Another serious adverse effect associated with the use of isoniazid is systemic lupus erythematosus, which occurs in approximately 1% of patients (Rubin, 2005).

As we saw in Subsection 5.4.3, isoniazid is primarily metabolised into acetylisoniazid (acetylated by N-acetyltransferase, NAT2). Individual subjects are either fast or slow acetylators (this is genetically determined); with chronic use, slow acetylation may lead to higher blood levels with increased toxicity. Acetylisoniazid is then further hydrolysed to isonicotinic acid and acetylhydrazine, which can be renally excreted. Isonicotinic acid is conjugated with glycine. Acetylhydrazine may be further metabolised to diacetylhydrazine and it is then suggested that it is converted by CYP 2E1 into hydrazine, leading to hepatotoxicity (Yard and McKennis, 1955; Ellard and Gammon, 1976; Sarich et al., 1996). Other side effects associated with isoniazid are skin rash, sideroblastic anaemia, and peripheral neuropathy, which is exhibited by those patients who are slow acetylators and/or when high doses of isoniazid are used (7.5–9.5 mg/kg) daily (Devadetta et al., 1960). The degree of neuropathy also depends on the overall nutritional state of the patient and, as a result, poorly nourished patients lacking in pyridoxine (vitamin B6, an important component in protein metabolism) are often at a higher risk (Rudman and Williams, 1983). Simple supplementation of pyridoxine in large amounts may contribute to peripheral neuropathy (Schaumberg et al., 1983), so lower doses of pyridoxine (100–200 mg daily) should be used in patients with isoniazid-induced peripheral neuropathy (Nisar et al., 1990).

Sideroblastic anaemia, in which the body has plentiful iron reserves but cannot incorporate them into haemoglobin, has been reported to have been caused by the long-term use of isoniazid (Sharp et al., 1990; Yoshimoto et al., 1992). The reasons for the poor haem incorporation into erythrocytes in sideroblastic anaemia remains elusive but isoniazid does have an effect upon delta-aminolevulinic acid synthase (ALAS), which acts upon proporphyrinogen II and subsequent haem formation (Bottomley and Muller-Eberhard, 1988). Isoniazid-induced sideroblastic anaemia is treated by withdrawing the drug and administering pyridoxine (the precursor of pyridoxal phosphate, which is a co-factor associated with ALAS function).

Isoniazid is metabolised in two phases (described above), in which it acts as both an inhibitor and then an inducer of the isoenzyme CYP 2E1 (Self et al., 1999). In addition, isoniazid is a weak monoamine oxidase inhibitor and has the potential to inhibit histaminase, thus causing many drug–food interactions (Hauser and Baier, 1982).

5.4.8.1 Phenytoin

There are several reports of isoniazid interacting with phenytoin, resulting in an increased serum concentration of phenytoin and, in some cases, patient toxicity (Miller et al., 1979). Isoniazid is seldom used alone and is often combined with rifampicin, which is known to induce the metabolism of phenytoin, and as such the combination of rifampicin with isoniazid in a typical TB treatment regimen will reduce the plasma levels of phenytoin (the induction effect of rifampicin upon the metabolism of phenytoin far outweighs the inhibition by isoniazid), so all patients receiving this combination of drugs need to be carefully monitored (Kay et al., 1985).

5.4.8.2 Carbamazepine

In a similar manner, carbamazepine toxicity may be experienced if given concurrently with isoniazid (Block, 1982; Fleenor et al., 1991).

5.4.8.3 Paracetamol (acetaminophen)

Isoniazid and paracetamol are both suggested to be metabolised via CYP 2E1. Isoniazid inhibits CYP 2E1 and so raises the levels of N-acetyl-para-benzoquinoneimine, which is the toxic metabolite of paracetamol, thus explaining the reports of toxicity experienced when both drugs are administered to some patients (Moulding et al., 1991; Crippin, 1993; Chien et al., 1997). The effect of slow and fast acetylators has also been investigated. Slow acetylators of isoniazid were given both paracetamol and isoniazid, and reduced levels of urinary oxidative metabolites were collected, whereas increased levels were found with rapid acetylators of isoniazid (Chien et al., 1997).

5.4.8.4 Benzodiazepines

Isoniazid treatment causes hepatic impairment of the N-demethylation of diazepam, resulting in a prolonged half-life of this benzodiazepine (Ochs et al., 1981). Isoniazid also increased the half-life of triazolam, but not oxazepam, as the latter is metabolised by glucuronidation only, thus avoiding the interaction with isoniazid (Ochs et al., 1983).

5.4.8.5 Vitamin D

Vitamin D is hydroxylated, in both the liver and the kidneys, by CYP 27B1, and a study has shown that the administration of isoniazid reduced the serum concentration of calcium, 25-hydroxyvitamin D, and 1α,25-dihydroxyvitamin D (Brodie et al., 1981).

5.4.8.6 Foods

As mentioned previously, isoniazid is a known monoamine oxidase inhibitor and so results in the reduced clearance/metabolism of foods high in monoamines, such as tyramine (Hauser and Baier, 1982). High plasma levels of tyramine (found in strong cheeses, etc.) may cause flushing, palpitations, and headache, with sometimes an increase in systolic blood pressure, so these foods should be avoided in patients taking isoniazid.

As isoniazid is also a histaminase inhibitor, foods high in histamine (e.g. tuna, shellfish, cured pork, parmesan cheese) should also be avoided, due to the reduced metabolism of histamine (Baciewicz and Self, 1985).