Tuberculosis is an infection caused by M tuberculosis which affects millions of people worldwide. The first reasonable cure for tuberculosis was considered at the discovery of isoniazid 1952 by Roche who trademarked it as Rimifon®. Currently INH is used as the first-line antituberculous medication for prophylaxix and treatment of tubercule infection. However isoniazid is not used alone for treatment of active tuberculosis due to the fact that resistance quickly develops and is thus commonly accompanied by Rifampicin1.
Isoniazid is available world wide due to inexpensive production and good tollereance. The drug is available in tablet, syrup, and injectable forms (delivered IM or IV) and kills 90% of viable bacilli within the first 2 months of treatment, rendering the patient non-infectious and reducing of drug resistance.
The hydrazide of isonicotinic acid is known as isoniazid, a small molecule with a molecular wight of 137.14 and is freely soluble in water1. The IUPAC name for isoniazid is pyridine-4-carbohydrazide and has the chemical formula C6H7N3O as outlined in figure 1.
Mechanism of Action
The site of attack for the majority of currently administered antituberculosis drugs, including isoniazid, is cell wall sysnthesis as shown in figure 2 below.
Figure 2 – Mechanisms of action for current and investigational tuberculosis drugs 2.
This shows targets of current anti-tuberculosis drugs, include those that affect cell wall synthesis such as isoniazid, folate synthesis such as p-aminosalicylate, transcription such as rifampin, translation e.g. aminoglycosides, DNA metabolism e.g. fluoroquinolones and the cell membrane e.g. pyrazinamide.
There are three newly identified compounds for the treatment of tuberculosis which target other bacterial functions. TMC207 inhibits the ATP synthase complex. The pro-drugs OPC-67863 and PA-824 are activated by Rv3547 5.
INH is a pro-drug which is activated by the M tuberculosis catalase KatG 3, 4 a multifunctional catalase-peroxidase enzyme 5. A schematic of M. tuberculosis KatG is shown in figure 3a below, whilst 3b shows Mycobacterium Tuberculosis Enoyl-Acp reductase with bound Inh-NADP. The activated form of INH consists of an isonicotinic-acyl group attached through its carbonyl group to the C4 of the nicotinamide ring, replacing the 4S hydrogen of NADH 6 (the same position involved in the hydride transfer that occurs during reduction of enoyl-ACP substrates). Subsequently, this will then react with a NADH radical or anion to form isonicotinic acyl-NADH complex 7. The complex binds tightly to ketoenoylreductase, known as InhA, and prevents access of the natural enoyl-AcpM substrate. Activation of INH has been proven by spectrometric studies as shown in figure 5 below.
It can, therefore be said that the INH actions rely on inhibiting an NADH-dependent enoyl-acyl carrier protein reductase (InhA) that provides precursors of mycolic acids. Mycolic acids are components of the mycobacterial cell wall, which is why the InhA is the major target of the isoniazid activity 8, 9. Figure 4 below, shows a simplified schematic for the activation of INH by the catalase peroxidase protein
Recent work by Chimote and Banerjee (2008) has shown that isoniazid acts by causing an increase in surface pressure of the mycobacterium, resulting in the insertion of the drug molecules into the phospholipid membrane 11. The destabilized membranes could be linked with drug induced membrane toxicity, which is seen at clinically relevant concentrations. This membrane destabilisation may also explain the broad reach of isoniazid in all the tissues including the nervous system and cerebrospinal fluid. When present in excess, the isoniazid molecules could result in monolayer disorder leading to toxicity in the host.
Due to its activation by KatG, isoniazid is bactericidal to rapidly-dividing mycobacteria, but is bacteriostatic if the mycobacterium is slow-growing.
As with a majority of drugs, isoniazid is metabolized in the liver via acetylation by N-acetyl transferase which produces acetylisoniazid 12 as shown in figure 6 below.
Following this, it reaches therapeutic concentrations in serum, cerebrospinal fluid (CSF), and within caseous granulomas.
The rate of acetylation is genetically determined as there are two forms of the N-acetyl transferase. Thus some patients metabolize the drug quicker than others, which is reflected in the bimodal ½ life with peaks at 1 hour and 3 hours in the population. The metabolites are excreted in the urine along with a small amount of INH. Doses do not normally have to be adjusted in case of renal failure 13. Figure 7 below shows the plasma concentrations of isoniazid in the test subjects 6 hours after an oral dose. The bimodal distribution is caused by polymorphisms in the gene encoding N acetyltransferase-2 (NAT2),
Acetyisoniazid is further hydrolysed by cytochrome P450 enzymes to isonicotinic acid and acetylhydrazine, both of which are excreted in the urine 12, 15 Isonicotinic acid is conjugated with glycine whereas acetylhydrazine is further metabolised to diacetylhydrazine. The diacetylhydrazine may be converted by the hepatic microsomal enzymes to the reactive metabolite, hydrazine, which may be responsible for INH-induced hepatotoxicity 13. Other metabolites found in the urine are acid labile hydrazones of isoniazid. These are formed with a-ketoglutarate and pyruvate. Due to the fact that the hydrazones do not appear in the blood, it may be possible that they are produced in the bladder.
Therapeutic use and Efficacy
As discussed, isoniazid is the first line therapy for tubercle disease. The dosage is given in table 1 below (Obtained from BNF.org).
The standard dose of isoniazid is 3-5mg/kg/day (max 300mg daily). When prescribed intermittently (twice or thrice weekly) the dose is 15mg/kg (max 900mg daily). Patients with slow clearance of the drug (via acetylation) may require reduced dosages to avoid toxicity.
Mariappan and Singh reported that due to its high solubility between pH 1 and 2, the absorption of rifampicin in the stomach is high. On the other hand, isoniazid is poorly absorbed from the stomach, but well absorbed from all sections of the intestine 17. Work by Gohel and Sarvaiya has overcome this drawback due to the identification of a unique solid dosage form of Rifampicin and Isoniazid with improved functionality 18. The work involved encapsulating in a hard gelatine case (size 00), two tablets of rifampicin and 1 capsule (size 4) of isoniazid. The in vitro drug release and in vitro drug degradation studies showed that the degradation of rifampicin to 3-formyl rifampicin SV (3FRSV) was arrested (3.6%-4.8% degradation of rifampicin at 4 hours) as a direct result of minimization the contact between the two drugs.
Adverse Drug Reactions
One of the most common and most serious adverse reactions to anti-tuberculosis medication is Antituberculosis drug-induced hepatotoxicity (ATDH), especially due to the fact that cocktails of drugs are used to treat this type of disease. A hepatotoxic liver is shown in figure 8 below.
ATDH can be fatal when not recognised at an early stage. If identified, therapy should be interrupted in a timely manner. ATDH occurs in around 9% of patients treated for active TB 19. During INH metabolism a small part of INH is directly hydrolysed into isonicotinic acid and HYD. The significance of this is the fact that in slow acetylators, more INH is left for direct hydrolysis into HYD. Additionally accumulated acetylhydrazine can be converted into HYD, thus leading to a higher risk of ATDH in slow acetylators such as HIV-positive patients 20.
Anti-TB drugs are used in combination and as discussed above cytochrome P450 2E1 (CYP2E1) is involved in INH metabolism. Human genetic studies have shown that INH-related hepatotoxicity involves cytochrome P450 2E1 (CYP2E1) 21. Tottsman et al (2008) have shown that pre-treatment with isoniazid or its toxic metabolite hydrazine increases the in vitro toxicity of pyrazinamide. In addition, pre-treatment with isoniazid, hydrazine or rifampicin increases the in vitro toxicity of INH 21. It has been shown that hydrazine is the likely metabolite involved in the cytotoxic mechanism. However hepatoxicity can be avoided with close clinical monitoring of the patient.22, 23.
Another adverse reaction of isoniazid can result in seizures, which may be caused by the deficiency of pyridoxine induced by INH due to the fact that it produces pharmacologic changes in the metabolism of pyridoxine 3. Reduction in pyridoxine and pyridoxal phosphate inhibits the formation of, gamma aminobutyric acid (GABA, an inhibitory neurotransmitter) 24 which results in the seizures experienced by patients with INH poisoning.
Other adverse reactions include rash, abnormal liver function tests, hepatitis 23, sideroblastic anemia, peripheral neuropathy, mild central nervous system (CNS) effects24; can be caused by pyridoxine (vitamin B6) depletion. Conditions in which neuropathy is common such as diabetes, uremia, alcoholism, malnutrition, HIV-infection and pregnancy, the patient could be administered with pyridoxine along with isoniazid. INH therapy will decrease the efficacy of hormonal birth control when combined with Rifampin 19.
Isoniazid is usually administered for 9 to 12 months and is metabolised in the liver, therefore an increased risk for clinically important interactions exists, where the metabolism of other drugs is potently inhibited. A number of studies have shown that isoniazid inhibits the metabolism of drugs such as phenytoin, carbamazepine, anticoagulants, benzodiazepines, and vitamin D 25. There have been reported cases where increased transaminase levels in patients receiving both carbamazepine and isoniazid have been observed.
Isoniazid has been shown to be a potent inhibitor of histaminase26 (both monoamine oxidase and diamine oxidase) which plays an important role in the metabolism of histamine in the body. As the isoniazid inhibits the histaminase, there is accumulation of histamine, thus leading the systematic poisoning.
Due to its metabolic pathway in the liver and its ability to cause monolayers of the cellular membrane 27, a number of important contraindications exist for isoniazid.
Sugars such as glucose, fructose and sucrose should not be used in INH preparations because the absorption of the drug is impaired by the formation of a condensation product29 whereas in infants it can also lead to hypoglycemia 30.
Important contraindications to INH also include hypersensitivity due to the drug and previous hepatitis associated with INH. The risk of hepatotoxicity 20, 21, 22 is high in INH usage and therefore it should not be used in patients with acute liver disease as it may precipitate porphyria.
Patients with epilepsy may also experience increased convulsions due to the inhibition of GABA by INH 24 whilst patients at risk of neuropathy should be administered 10 mg daily pyridoxine to overcome this effect.
Since its discovery in the early 1950’s, isoniazid has remained the cornerstone drug in treating tuberculosis. Although INH is used in conjunction with other anti-tuberculosis drugs, resistance to this drug is reduced due to the fact that it kills 90% of viable bacilli within the first 2 months of treatment, which also renders the patient non-infectious. The widespread use of INH can be attributed to its relatively low toxicity and excellent pharmacokinetics. Because of its high early bactericidal activity, INH is also often the first agent against which M tuberculosis develops resistance. With few contraindications, inexpensive production and suitability for adults and children, this has been used world wide.
INH relies on affecting the mycobacterial cell wall production by inhibiting its sysnthesis inhibiting production of mycolic acids. The fact that INH is immediately present in the CSF and serum; there is a high risk of toxicity in the host. Newer drugs are being developed which target the bacterial cell in alternative locations, however until their development is complete, INH will continue to be used in conjunction with other anti-tuberculosis drugs as the primary line of prophylaxix and treatment.