Isoniazid hydrolysis

The metabolic fate of isoniazid and iproniazid, two isonicotinoylhydra-zides, has been extensively studied, and it has been shown that metabolic hydrolysis represents an important step in their toxification. Isoniazid (4.269) is employed as a first-line tuberculostatic drug, but prolonged therapy is associated in 1 -2% of patients with significant hepatotoxicity. Isoniazid can be metabolized by either of two primary pathways, hydrolysis and direct -acetylation. Isonicotinic acid (4.271), the product of hydrolysis, can be formed either di-... 

The mechanism of toxification of isoniazid was investigated in rats pretreated with inducers or inhibitors of microsomal enzymes or an inhibitor of acylamidases. In animals pretreated with the acylamidase inhibitor bis(4-nitrophenyl) phosphate, isoniazid and acetylisoniazid produced less liver necrosis than in control animals. The treatment had no effect on the necrosis due to acetylhydrazine [173], In animals pretreated with inducers of microsomal cytochrome P450 such as phenobarbital, acetylisoniazid, and acetylhydrazine caused markedly increased necrosis, while pretreatment with cytochrome P450 inhibitors decreased necrosis. In contrast, the toxicity of isoniazid and hydrazine was not modified by phenobarbital pretreatment. From these observations, Trimbell et al. [173] concluded that the hydrolysis of acetylisoniazid is a prerequisite for hepatotoxicity, and that microsomal enzymes transform acetylhydrazine, the product of hydrolysis, to a toxic species. 

The hepatic injury caused by iproniazid could also be due to the formation of the toxic metabolite hydrazine by A-dealkylation followed by hydrolysis. Indeed, A-dealkylation is a main route in the metabolism of iproniazid, with plasma levels of hydrazine in rabbits three- to sixfold higher than after isoniazid [188],... 

Isoniazid, carbidopa, and hydralazine are hydrazine derivatives with therapeutic uses. They form Schiff bases with pyridoxal 5 -phosphate, and rate constants for their formation and hydrolysis have been measured in aqueous solution pH-rate profiles are reported and compared with that of hydrazine itself. 

The major routes of metabolism for isoniazid are acetylation to give acetylisoniazid, followed by hydrolysis to yield isonicotinic acid and acetylhydrazine (Fig. 7.24). The acetylation of isoniazid in human populations is genetically determined and therefore shows a bimodal distribution (see chap. 5). Thus, there are two acetylator phenotypes, termed "rapid and slow acetylators," which may be distinguished by the amount of acetylisoniazid excreted or by the plasma half-life of isoniazid. 

Figure 7.24 Metabolism of isoniazid. The acetylhydrazine released by the hydrolysis of acetylisoniazid is further metabolized to a reactive intermediate thought to be responsible for the hepatotoxicity.

The classical model of drug metabolism by acetylation is the tuberculostatic drug isoniacide (isonicotinic acid hy-drazide). The metabolism of isoniazide has two interesting aspects Firstly, non-enzymatic hydrolysis of the acetyl metabolite releases acetylhydrazine, which in turn is toxic. This, then, is an example of detrimental drug metabolism (Figure 2.30a, b). 

Figure 2.30. Metabolism of isoniazid. a Acetylation, b Hydrolysis of the acetyl conjugate leads to liver toxicity, c Distribution of acetylation rates in the population.

Hydrolysis of isoniazid gives isonicotinic acid which is further metabolised to its glycine conjugate. 

After oral administration, isoniazid reaches a peak plasma concentration of 3-5 micrograms/ml within 1-2 hours. It equihbrates into all body fluids and tissues and 75-95% is excreted in the urine within 24 hours. The most important urinary metabolites are products of acetylation (acetyliso-niazid) and hydrolysis (isonicotinic acid). Isonicotinyl glycine, isonicotinyl hydrazones, and A-methylisoniazid appear in only small amounts. The rate of acetylation of isoniazid significantly alters its plasma concentrations and half-life. The mean half-Ufe of isoniazid in rapid acetyla-tors is about 70 minutes and in slow acetylators 3 hours. 

Acetylation is a very common metabolic reaction, which occurs with amino, hydroxyl or sulfhydryl groups. The acetyl group is transferred from acetyl-Coenzyme A, and the reaction is catalyzed by acetyltransferases. An important aspect of this kind of substitution is the genetic polymorphism of one acetyltrans-ferase in humans, who are divided into fast and slow acetyla-tors. In a few cases, the conjugates are further metabolized to toxic compounds, as is seen with isoniazid. Some evidence exists that acetylation of the antitubercular isoniazid leads to enhanced hepatotoxicity of the drug. " Acetylation followed by hydrolysis and CYP-dependent oxidation yields free acetyl... 

The drug isoniazid (isonicotinic acid hydrazide) is hydrolysed in vivo to the corresponding acid and hydrazine, as shown in figure 4,44. However, in man, in vivo, hydrolysis of the acetylated metabolite acetylisoniazid is quantitatively more important and toxicologically... 

The major routes of metabolism for isoniazid are acetylation to give acetylisoniazid, followed by hydrolysis to yield isonicotinic acid... 

From 75 to 95% of a dose of isoniazid is excreted in the urine within 24 hours, mostly as metabolites. The main excretory products in humans result from acetylation (acetyUsoniazid) and hydrolysis (isonicotinic acid). 

Hydrolysis is a pH-dependent process, which usually proceeds more rapidly in either acidic or alkatine conditions. Therefore, substances that are prone to hydrolysis have a pH optimum. For example, the rate of hydrolysis of indometha-cin is the lowest at pH 4.9 and increases with higher and lower pH values [7—9]. For isoniazid, the stability optimum regarding hydrolysis is pH 6 [10], and also here, hydrolysis increases with higher and lower pH values. A classic example of an investigation of the influence of pH on hydrolysis is that for acetylsalicylic acid [11]. Figure 22.1 shows the pH stability relationship for acetylsalicylic acid that resulted from that investigation. 

An example is the establishment of the shelf life of an Isoniazid injection solution. Isoniazid when dissolved in water, may hydrolyse and oxidise. Upon hydrolysis, isonicotinic acid, isonicotinamide, di-isonicotinoylhydrazine and hydrazine are formed [10, 29b]. The increase in hydrazine does not match a decrease of isoniazid content Since hydrazine is genotoxic, the shelf life of the product is limited to the formation of hydrazine, which is measurable even before the degradation of isoniazid is measureable. 

---