Isoniazid

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Isoniazid (Laniazid, Nydrazid), also known as isonicotinylhydrazine (or INH), is an organic compound that is the first-line medication in prevention and treatment of tuberculosis. The compound was first synthesized in the early 20th century, but its activity against tuberculosis was first reported in the early 1950s, and three pharmaceutical companies attempted unsuccessfully to patent the drug simultaneously, the most prominent one being Roche, which launched its version, Rimifon, in 1952. The drug was first tested at Many Farms, a Navajo community, due to the Navajo reservation's dire tuberculosis problem and because the population was naïve with respect to streptomycin, the main tuberculosis treatment at the time. With the introduction of isoniazid, a cure for tuberculosis was first considered reasonable.

Isoniazid is available in tablet, syrup, and injectable forms (given intramuscularly or intravenously). It is available worldwide, is inexpensive, and is generally well tolerated. It is manufactured from isonicotinic acid, which is produced from 4-methylpyridine.

The abbreviation INH is common for referring to isoniazid, but abbreviating drug names is not best practice in medicine.

Preparation
Isoniazid may be prepared by the base hydrolysis of 4-cyanopyridine to give the amide, followed by displacement of ammonia by hydrazine:


 * Preparation of isoniazid.png

Mechanism of action
Isoniazid is a prodrug and must be activated by a bacterial catalase-peroxidase enzyme that in M. tuberculosis is called KatG. KatG couples the isonicotinic acyl with NADH to form isonicotinic acyl-NADH complex. This complex binds tightly to the enoyl-acyl carrier protein reductase known as InhA, thereby blocking the natural enoyl-AcpM substrate and the action of fatty acid synthase. This process inhibits the synthesis of mycolic acid, required for the mycobacterial cell wall. A range of radicals are produced by KatG activation of isoniazid, including nitric oxide, which has also been shown to be important in the action of another antimycobacterial prodrug PA-824.

Isoniazid is bactericidal to rapidly dividing mycobacteria, but is bacteriostatic if the mycobacteria are slow-growing. It inhibits the P450 system.

Metabolism
Isoniazid reaches therapeutic concentrations in serum, cerebrospinal fluid, and within caseous granulomas. It is metabolized in the liver via acetylation. Two forms of the enzyme are responsible for acetylation, so some patients metabolize the drug more quickly than others. Hence, the half-life is bimodal, with peaks at one and three hours in the US population. The metabolites are excreted in the urine. Doses do not usually have to be adjusted in case of renal failure.

Dosing
The standard dose of isoniazid in adults is 5 mg/kg/day (max 300 mg daily) for six months. The CDC and the WHO differ in their recommendation of intermittent dosing. The CDC recommends 15 mg/kg/day twice weekly (900 mg max dose), and the WHO recommends 10 mg/kg/day three times weekly (900 mg max dose) for either six or nine months. When prescribed intermittently (twice or thrice weekly), the dose is 10–15 mg/kg (max 900 mg daily), depending on the regimen chosen. Patients with slow clearance of the drug (via acetylation as described above) may require reduced dosages to avoid toxicity. The American Academy of Pediatrics recommends dosing for children of either 10–15 mg/kg/day for a daily regimen or 20–30 mg/kg/day for a twice-weekly regimen. The recommended duration of treatment is nine months in children.

Side effects
Adverse reactions include rash, abnormal liver function tests, hepatitis, sideroblastic anemia, high anion gap metabolic acidosis, peripheral neuropathy, mild central nervous system (CNS) effects, drug interactions resulting in increased phenytoin (Dilantin) or disulfiram (Antabuse) levels, intractable seizures (status epilepticus) and drug-induced lupus erythematosus.

Peripheral neuropathy and CNS effects are associated with the use of isoniazid and are due to pyridoxine (vitamin B6) depletion, but are uncommon at doses of 5 mg/kg. Persons with conditions in which neuropathy is common (e.g., diabetes, uremia, alcoholism, malnutrition, and HIV infection), as well as pregnant women and persons with a seizure disorder, may be given pyridoxine (10–50 mg/day) with isoniazid.

Isoniazid may cause severe and sometimes fatal liver damage. Hepatotoxicity can be avoided with close clinical monitoring of the patient, to be specific, nausea, vomiting, abdominal pain, and loss of appetite. Isoniazid is metabolized by the liver mainly by acetylation and dehydrazination. The N-acetylhydrazine metabolite is believed to be responsible for the hepatotoxic effects seen in patients treated with isoniazid. The rate of acetylation is genetically determined. About 50% of blacks and Caucasians are slow inactivators; the majority of Inuit and Asians are rapid inactivators. The half-life in fast acetylators is one to two hours, while in slow acetylators, it is two to five hours. Elimination is largely independent of renal function, but the half-life may be prolonged in liver disease. The rate of acetylation has not been shown to significantly alter the effectiveness of isoniazid. However, slow acetylation may lead to higher blood concentrations with chronic administration of the drug, with an increased risk of toxicity. Fast acetylation leads to higher blood levels of the toxic metabolite acetylisoniazid and thus to an increase in toxic reactions - hepatitis which is 250 times more common than in slow acetylators. Isoniazid and its metabolites are excreted in the urine, with 75 to 95% of the dose excreted in 24 hours. Small amounts are also excreted in saliva, sputum, and feces. Isoniazid is removed by hemodialysis and peritoneal dialysis.

Headache, poor concentration, weight gain, poor memory, insomnia, and depression have all been associated with isoniazid use. All patients and healthcare workers should be aware of these serious adverse effects, especially if suicidal thinking or behavior are suspected.

INH is known to reduce cytochrome P450, and in theory promotes the efficacy of contraceptives. Therapy is often combined with rifampin. Rifampin increases the P450 enzyme and also can reduce the efficacy of contraceptives. Alternative means of birth control should be used when taking these medications.

As previously mentioned, isoniazid is associated with pyridoxine deficiency. Pyridoxyl phosphate (derivative of pyridoxine) is required for d-aminolevulinic acid synthase, the enzyme responsible for the rate-limiting step in heme synthesis. As such, isoniazid-induced pyridoxine deficiency leads to insufficient heme formation in early red blood cells, leading to sideroblastic anemia.

Synonyms and abbreviations

 * Isonicotinyl hydrazine
 * Isonicotinic acid hydrazide
 * INH
 * H (for "hydrazide", and also the WHO standard abbreviation)