Novel regioselective hydroxylations of pyridine carboxylic acids at position C2 and pyrazine carboxylic acids at position C3

We have previously described the isolation of the new bacterial species, Ralstonia/Burkholderia sp. strain DSM 6920, which grows with 6-methylnicotinate and regioselectively hydroxylates this substrate in the C2 position by the action of 6-methylnicotinate-2-oxidoreductase to yield 2-hydroxy-6-methylnicotinate (Tinschert et al. 1997). In the present study we show that this enzymatic activity can be used for the preparation of a series of hydroxylated heterocyclic carboxylic acid derivatives. The following products were obtained from the unhydroxylated educts by biotransformation using resting cells: 2-hydroxynicotinic acid, 2-hydroxy-6-methylnicotinic acid, 2-hydroxy-6-chloronicotinic acid, 2-hydroxy-5,6-dichloronicotinic acid, 3-hydroxypyrazine-2-carboxylic acid, 3-hydroxy-5-methylpyrazine-2-carboxylic acid and 3-hydroxy-5-chloropyrazine-2-carboxylic acid. Thus the respective educts were all regioselectively mono-hydroxylated at the carbon atom between the ring-nitrogen and the ring-carbon atom carrying the carboxyl group. In contrast to its relatively broad biotransformation abilities, the strain shows a limited heterocyclic nutritional spectrum. It could grow only with three of the seven transformed educts: 6-methylnicotinate, 2-hydroxy-6-methylnicotinate and 5-methylpyrazine-2-carboxylate. 2-Hydroxynicotinate, 2-hydroxy-6-chloronicotinate, 2-hydroxy-5,6-dichloronicotinate, 3-hydroxypyrazine-2-carboxylate and 3-hydroxy-5-chloropyrazine-2-carboxylate were not degraded by the strain. Therefore, unlike 6-methylnicotinate-2-oxidoreductase, which has a broad substrate spectrum, the second enzyme of the 6-methylnicotinate pathway seems to have a much more limited substrate range. Among 28 aromatic heterocyclic compounds tested as the sole source of carbon and energy, only pyridine-2,5-dicarboxylate was found as a further growth substrate, and this was degraded by a pathway which did not involve 6-methylnicotinate-2-oxidoreductase. To the best of our knowledge the microbial production of 2-hydroxy-6-chloronicotinic acid, 2-hydroxy-5,6-dichloronicotinic acid and 3-hydroxy-5-methylpyrazine-2-carboxylic acid have not been reported before. Strain DSM 6920 is so far the only known strain which allows the microbial production of both these compounds and 3-hydroxypyrazine-2-carboxylic acid and 3-hydroxy-5-chloroypyrazine-2-carboxylic acid.

Characterization of new mutations in pyrazinamide-resistant strains of Mycobacterium tuberculosis and identification of conserved regions important for the catalytic activity of the pyrazinamidase PncA

A new set of mutations, including transposition of the insertion sequence IS6110, was identified in the pncA gene from 19 pyrazinamide-resistant Mycobacterium tuberculosis strains. Alignment of the PncA protein from M. tuberculosis with homologous proteins from different bacterial species revealed three highly conserved regions in PncA which may play an important role in the processing of pyrazinamide.

Mechanisms of pyrazinamide resistance in mycobacteria: importance of lack of uptake in addition to lack of pyrazinamidase activity

Mycobacteria are known to acquire resistance to the antituberculous drug pyrazinamide (PZA) through mutations in the gene encoding pyrazinamidase (PZase), an enzyme that converts PZA into pyrazinoic acid, the presumed active form of PZA against bacteria. Additional mechanisms of resistance to the drug are known to exist but have not been fully investigated. Among these is the non-uptake of the pro-drug, a possibility investigated in the present study. The uptake mechanism of PZA, a requisite step for the activation of the pro-drug, was studied in Mycobacterium tuberculosis. The incorporation of [14C]PZA by the bacilli was followed in both neutral and acidic environments since PZA activity is known to be optimal at acidic pH. By using a protonophore (carbonyl cyanide m-chlorophenylhydrazone; CCCP), valinomycin, arsenate and low temperature, it was shown that an ATP-dependent transport system is involved in the uptake of PZA. Whilst the structurally analogous compound nicotinamide inhibited the transport system of PZA, other structurally related compounds such as pyrazinoic acid, isoniazid and cytosine did not. Acidic conditions were also without effect. Based on diffusion experiments in liposomes, it was found that PZA diffuses rapidly through membrane bilayers, faster than glycerol, whilst the presence of OmpATb, the porin-like protein of M. tuberculosis, in proteoliposomes slightly increased the diffusion of the drug. This finding may explain why the cell wall mycolate hydrophobic layer does not represent the limiting step in the diffusion of PZA, as judged from comparative experiments using a M. tuberculosis strain and its isogenic mutant elaborating 40% less covalently linked mycolates. PZase activity, and PZA uptake and susceptibility in different mycobacterial species were compared. M. tuberculosis, a naturally PZA-susceptible species, was the only species that exhibited both PZase activity and PZA uptake; no such correlation was observed with the four naturally resistant species examined. Mycobacterium smegmatis possessed a functional PZase but did not take up PZA; the reverse was true for the PZase-negative strain of Mycobacterium avium used, with PZA uptake comparable to that of M. tuberculosis. Mycobacterium bovis BCG and Mycobacterium kansasii exhibited neither a PZase activity nor PZA uptake. These data clearly demonstrate that one of the mechanisms of resistance to PZA resides in the failure of strains to take up the drug, indicating that susceptibility to PZA in mycobacteria requires both the presence of a functional PZase and a PZA transport system. No correlation was observed between the occurrence and cellular location of PZase and of nicotinamidase in the strains examined, suggesting that one or both amides can be hydrolysed by other mycobacterial amidases.

Role of acid pH and deficient efflux of pyrazinoic acid in unique susceptibility of Mycobacterium tuberculosis to pyrazinamide

Pyrazinamide (PZA) is an important antituberculosis drug. Unlike most antibacterial agents, PZA, despite its remarkable in vivo activity, has no activity against Mycobacterium tuberculosis in vitro except at an acidic pH. M. tuberculosis is uniquely susceptible to PZA, but other mycobacteria as well as nonmycobacteria are intrinsically resistant. The role of acidic pH in PZA action and the basis for the unique PZA susceptibility of M. tuberculosis are unknown. We found that in M. tuberculosis, acidic pH enhanced the intracellular accumulation of pyrazinoic acid (POA), the active derivative of PZA, after conversion of PZA by pyrazinamidase. In contrast, at neutral or alkaline pH, POA was mainly found outside M. tuberculosis cells. PZA-resistant M. tuberculosis complex organisms did not convert PZA into POA. Unlike M. tuberculosis, intrinsically PZA-resistant M. smegmatis converted PZA into POA, but it did not accumulate POA even at an acidic pH, due to a very active POA efflux mechanism. We propose that a deficient POA efflux mechanism underlies the unique susceptibility of M. tuberculosis to PZA and that the natural PZA resistance of M. smegmatis is due to a highly active efflux pump. These findings may have implications with regard to the design of new antimycobacterial drugs.

The pncA gene from naturally pyrazinamide-resistant Mycobacterium avium encodes pyrazinamidase and confers pyrazinamide susceptibility to resistant M. tuberculosis complex organisms

The antituberculosis drug pyrazinamide (PZA) needs to be converted into pyrazinoic acid (POA) by the bacterial pyrazinamidase (PZase) in order to show bactericidal activity against Mycobacterium tuberculosis. M. avium is naturally resistant to PZA. To investigate whether this natural resistance to PZA is due to inability of the M. avium PZase to convert PZA to bactericidal POA, the M. avium PZase gene (pncA) was cloned by using the M. tuberculosis pncA gene as a probe. Sequence analysis showed that the M. avium pncA gene is 561 bp long, encoding a protein with a predicted size of about 19.8 kDa; but Western blotting showed that the M. avium PZase migrated as a 24 kDa band when expressed in M. bovis BCG and Escherichia coli. Sequence comparison revealed that M. avium PZase has 67.7% and 32.8% amino acid identity with the corresponding enzymes from M. tuberculosis and E. coli, respectively. Southern blot analysis with the M. avium pncA gene as a probe showed that M. terrae, M. gastri, M. marinum, M. fortuitum, M. xenopi, M. gordonae, M. szulgai, M. celatum and M. kansasii have close pncA homologues, whereas M. chelonae and M. smegmatis did not give significant hybridization signals. Transformation with the M. avium pncA gene conferred PZA susceptibility to PZA-resistant M. tuberculosis complex organisms, indicating that the nonsusceptibility of M. avium to PZA is not due to an ineffective PZase enzyme, but appears to be related to other factors such as transport of POA.

Mycobacterium hassiacum sp. nov., a new rapidly growing thermophilic mycobacterium

A new rapidly growing, scotochromogenic mycobacterium was isolated from urine. This strain is thermophilic (it grows at 65 degrees C), tolerates 5% NaCl, and was unable to utilize any of the sugars tested or citrate or to take up iron. The isolate splits benzamide, urea, nicotinamide, and pyrazinamide and is sensitive to streptomycin, ethambutol, cycloserine, ciprofloxacin, and chlarithromycin but resistant to isoniazid, rifampin, and prothionamide. These characteristics clearly place this organism in a new mycobacterial species, which was confirmed by the unique 16S rRNA nucleotide sequence. The high level of similarity between this rapid grower and Mycobacterium xenopi is surprising. For this new rapidly growing scotochromogenic and thermophilic mycobacterium we propose the name Mycobacterium hassiacum sp. nov.

Indirect coupling of urate and p-aminohippurate transport to sodium in human brush-border membrane vesicles

[14C]urate and p-[14C]aminohippurate (PAH) uptake by human brush-border membrane vesicles (BBMV) were measured in the presence of an inwardly oriented sodium gradient. No direct sodium cotransport was observed. Indirect [14C]urate coupling to sodium transport was demonstrated by cis-stimulation of [14C]urate with nicotinate or pyrazinoate (PZA) in the extravesicular medium but not by adding lactate, alpha-ketoglutarate, or beta-hydroxybutyrate. Indirect sodium coupling of [14C]PAH uptake was observed only when alpha-ketoglutarate was added to the extravesicular medium, a mechanism similar to that of basolateral membranes. The ability for PZA (and nicotinate) to cis-stimulate urate uptake was correlated with a high apparent affinity for the urate/anion exchanger. In urate-loaded vesicles, for identical medium concentrations, [14C]PZA uptake via the urateanion exchanger was 10 times higher than [14C]lactate uptake. Such high PZA affinity for the urate exchanger, working in parallel with PZA sodium cotransport can account for the stimulation of urate reabsorption by PZA in vivo.

Effect of BOF-4272 on the oxidation of allopurinol and pyrazinamide in vivo. Is xanthine dehydrogenase or aldehyde oxidase more important in oxidizing both allopurinol and pyrazinamide?

Allopurinol or pyrazinamide was administered to rats treated with BOF-4272 (a potent xanthine oxidase inhibitor) to investigate to what degree xanthine dehydrogenase participates in the oxidation of these agents. BOF-4272 markedly decreased the plasma concentration and the urinary excretion of both oxypurinol and 5-hydroxypyrazinamide. It also decreased the sum of the urinary excretion of allopurinol and oxypurinol and that of pyrazinamide and its metabolites, although it did not affect the sum of the plasma concentrations of allopurinol and oxypurinol at 105 min after administration of allopurinol or the plasma concentration of pyrazinamide during the period after the administration of pyrazinamide. These results suggested that BOF-4272 almost completely inhibited the oxidation of allopurinol and pyrazinamide and had some effect on the excretion and/or the tissue incorporation of these two compounds. Since the in vitro study demonstrated that BOF-4272 did not inhibit the activity of aldehyde oxidase, which oxidized both allopurinol to oxypurinol and pyrazinamide to 5-hydroxypyrazinamide, the results suggested that xanthine dehydrogenase was the more important enzyme in converting allopurinol to oxypurinol and pyrazinamide to 5-hydroxypyrazinamide.

In vitro oxidation of pyrazinamide and allopurinol by rat liver aldehyde oxidase

Aldehyde oxidase was purified about 120-fold from rat liver cytosol by sequential column chromatography using diethylaminoethyl (DEAE) cellulose, Benzamidine-Sepharose 6B and gel filtration. The purified enzyme was shown as a single band with M(r) of 2.7 x 10(5) on polyacrylamide gel electrophoresis (PAGE) and M(r) of 1.35 x 10(5) on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Using this purified enzyme, in vitro conversion of allopurinol, pyrazinamide and pyrazinoic acid was investigated. Allopurinol and pyrazinamide were oxidized to oxypurinol and 5-hydroxy-pyrazinamide, respectively, while pyrazinoic acid, the microsomal deamidation product of pyrazinamide, was not oxidized to 5-hydroxypyrazinoic acid. The apparent Km value of the enzyme for pyrazinamide was 160 microM and that for allopurinol was 1.1 mM. On PAGE, allopurinol- or pyrazinamide-stained band was coincident with Coomassie Brilliant Blue R 250-stained band, respectively. These results suggest that aldehyde oxidase may play a role in the oxidation of allopurinol to oxypurinol and that of pyrazinamide to 5-hydroxypyrazinamide with xanthine dehydrogenase which can oxidize both allopurinol and pyrazinamide in vivo. The aldehyde oxidase may also play a major role in the oxidation of allopurinol and pyrazinamide in the subgroup of xanthinuria patients (xanthine oxidase deficiency) who can oxidize both allopurinol and pyrazinamide.

A case of xanthinuria: a study on the metabolism of pyrazinamide and allopurinol

A 74-year-old female was diagnosed as having xanthinuria by measurement of the uric acid level in plasma, purine bases in urine and activity of xanthine oxidase in the duodenal mucosa. The determination of the urinary excretion of purine bases in her family demonstrated a slightly increased urinary excretion of oxypurines in her younger brother, suggesting that he was a heterozygote. The pyrazinamide-loading test and allopurinol-loading test demonstrated that she could neither metabolize pyrazinoic acid into 5-hydroxypyrazinoic acid nor allopurinol into oxypurinol, although there was a slight metabolizing of prazinamide into 5-hydroxypyrazinamide. This suggested that she belonged to the subgroup which can neither metabolize pyrazinamide into 5-hydroxypyrazinamide, pyrazinoic acid into 5-hydroxypyrazinoic acid nor allopurinol into oxypurinol.

Effect of simultaneous isoniazid administration on pharmacokinetic parameters of pyrazinamide

Pyrazinamide (PZN) was administered to 10 patients of pulmonary tuberculosis (for 7 consecutive days) each day after an overnight fast. On 8th day serum levels and urinary elimination were measured at 2,4,6 and 8 hours. Simultaneous administration of isoniazid to same patients significantly decreased the peak serum concentration (Cmax). Although, time to peak serum concentration (Tmax) remained unaffected, serum half life (t1/2) prolonged, the elimination rate constant (Kel) and area under serum concentration time curve (AUC) decreased and apparent volume of distribution (Vd) and plasma clearance (Clp) of PZN increased significantly. However, the cumulative per cent dose of PZN excreted in urine was not changed significantly. Although, serum levels of PZN were decreased at 2, 4, 6, and 8 hours, PZN levels remained above minimum effective concentration thereby not affecting the therapeutic status of PZN administered in combination with isoniazid, if PZN is administered in moderate doses.

Nitrilase-catalyzed production of pyrazinoic acid, an antimycobacterial agent, from cyanopyrazine by resting cells of Rhodococcus rhodochrous J1

Using resting cells of Rhodococcus rhodochrous J1, in which a large amount of nitrilase is induced, a simple and efficient bioconversion process for the production of pyrazinoic acid, an antimycobacterial agent, through catalysis by a nitrilase was developed. The reaction conditions for production of pyrazinoic acid were optimized. Under optimum conditions, 3.5 M cyanopyrazine was converted to pyrazinoic acid, with a molar conversion yield of 100%. The highest yield achieved corresponded to 434 g of pyrazinoic acid per liter of reaction mixture. The synthesized pyrazinoic acid was isolated and identified physico-chemically.

Pharmacokinetics of pyrazinamide and its metabolites in patients with hepatic cirrhotic insufficiency

The pharmacokinetics of pyrazinamide (Pirilène) and its metabolites are evaluated in ten subjects with hepatic insufficiency, after an oral dose of 19.3 +/- 0.6 mg.kg-1 and the results are compared to those of a group of nine healthy subjects (control group). The results exhibit a marked reduction of the pyrazinamide total clearance (0.48 vs 0.84 ml.min-1.kg-1) and an increase in half-life from 9.19 h to 15.07 h in the patients group. The area under the curve of pyrazinoic acid (the main metabolite) is increased from 97 to 280 mg.h.l-1 with a half-life twice as much as that of the control group. The hepatic insufficiency entails a marked reduction of the common posology as well as a closer survey of the biologic hepatic parameters and of uric acid the renal elimination of which is inhibited by pyrazinoic acid.

Metabolism of pyrazinamide and allopurinol in hereditary xanthine oxidase deficiency

The metabolism of pyrazinamide and allopurinol was studied in three xanthinuric patients from two families with hereditary xanthinuria to determine whether both substrates were oxidized only by xanthine oxidase or by other oxidases as well. One xanthinuric patient could neither metabolize pyrazinamide into 5-hydroxypyrazinamide nor allopurinol into oxypurinol. Two xanthinuric patients could metabolize both pyrazinamide into 5-hydroxypyrazinamide and allopurinol into oxypurinol but could not oxidize pyrazinoic acid to 5-hydroxypyrazinoic acid. These findings suggest that xanthinuria comprises at least two subgroups.

Haemodialysis of pyrazinamide in uraemic patients

The pharmacokinetics of PZA during haemodialysis were determined in 6 patients with chronic renal impairment after a single oral dose of 25.7 (1.9) mg.kg-1. The dialysis clearance of PZA and of its metabolites were: pyrazinamide 132 ml.min-1; pyrazinoic acid 121 ml.min-1; 5-hydroxy-pyrazinamide 107 ml.min-1; 5-hydroxy-pyrazinoic acid 118 ml.min-1. The average amount extracted during a dialysis session of 4.1 h was 926 mg after an oral dose of 1700 mg. The high dialysability shows that PZA can properly be administered at the end of each dialysis session in the usual dose of 25 to 30 mg.kg-1.

Substrate specificity of nicotinamide methyltransferase isolated from porcine liver

Nicotinamide methyltransferase (EC 2.1.1.1) has been purified over 1300-fold from porcine liver. The enzyme is electrophoretically homogeneous, exhibiting a relative molecular mass of 27,000. In addition to acting on nicotinamide and close structural analogs such as thionicotinamide and 3-acetylpyridine, the enzyme actively accommodates poor analogs such as quinoline, isoquinoline, and 1,2,3,4-tetrahydroisoquinoline as methyl group acceptors. The enzyme may thus have the function of detoxicating numerous alkaloids in vivo. In some cases, the action of the enzyme might paradoxically increase the toxicities of substrates, but the hepatotoxic antibiotic pyrazinamide, which we considered as potentially such an enzyme-activated electrophile, did not function detectably as a substrate for the isolated enzyme.

Metabolic disposition of pyrazinamide in the rat: identification of a novel in vivo metabolite common to both rat and human

Only limited studies have been reported on the disposition and pharmacokinetics of pyrazinamide (PZA) in both animals and humans. The metabolism of PZA has never been completely elucidated, consequently the metabolites of PZA, pyrazinoic acid (PA), 5-hydroxypyrazinoic acid (5-HOPA), and 5-hydroxypyrazinamide (5-HOPZA) were characterized and the disposition of PZA was examined following administration of 150 mg kg-1 of 14C-PZA to male Wistar rats. Comparable t1/2 for total radiolabel 14C (1.45 +/- 0.06 h) and PZA (1.39 +/- 0.04 h) in the blood compartment were observed. Cumulative 48 h excretion in urine and faeces accounted for 82.6 +/- 3.2 per cent and 11.0 +/- 1.3 per cent, respectively, of the dose administered. In the 0-6 h urine collections PA, 5-HOPA, 5-HOPZA, and PZA, respectively, accounted for 25.4 +/- 1.7, 17.7 +/- 1.2, 11.6 +/- 0.8, and 2.7 +/- 0.2 per cent of the administered dose. In the 6-12 h urine samples the proportions of PA and 5-HOPA increased statistically over the 0-6 h excretion whereas 5-HOPZA decreased. Administration of PZA to humans indicated 5-HOPZA was a major urinary metabolite in human. These data suggested that direct hydroxylation of PZA was an alternative pathway in the oxidation of PZA of importance to both human and rat.

Study of the metabolism of pyrazinamide using a high-performance liquid chromatographic analysis of urine samples

A reversed-phase high-performance liquid chromatographic method was developed for the simultaneous determination of pyrazinamide and its metabolites in urine. Study of the metabolism of pyrazinamide by this method demonstrated that 5-hydroxypyrazinamide excretion was compatible with pyrazinoic acid excretion and allopurinol decreased in vivo conversion of pyrazinamide to 5-hydroxypyrazinamide and blocked that of pyrazinoic acid to 5-hydroxypyrazinoic acid.

Pyrazinoate transport in the isolated perfused rabbit proximal tubule

The bidirectional tubular transport of pyrazinoate (PZA) was studied in the isolated perfused proximal S2 segment of rabbit kidney. PZA reabsorption was a mechanism of large capacity, temperature-dependent and requiring a normal Na+/K+-ATPase activity. PZA reabsorption was reversibly decreased when lactate was added to the perfusate, indicating that it might occur through the sodium-lactate cotransport. The addition of PAH to the bath had a slight stimulatory effect on PZA reabsorption, suggesting a component of anion exchange in the overall PZA reabsorption. However, SITS added to either the perfusate or the bathing medium induced a non-significant decrease in PZA reabsorption, confirming the minor part of an anion exchange mechanism in this reabsorptive process. PZA reabsorption was not affected by the establishment of a bath-to-lumen H+ gradient, and was only moderately decreased after carbonic anhydrase inhibition by ethoxyzolamide, in opposition to what is known for the reabsorbed anion salicylate. The secretory transport of PZA was saturable and also dependent on a normal Na+/K+-ATPase activity. It is concluded that PZA is bidirectionally transported by facilitated mechanisms in the rabbit proximal S2 segment, one major reabsorptive mechanism appearing to be a sodium-anion cotransport, which might be the sodium-lactate reabsorbing mechanism.

The bioavailability of isoniazid, rifampin, and pyrazinamide in two commercially available combined formulations designed for use in the short-course treatment of tuberculosis

The bioavailability of isoniazid, rifampin, and pyrazinamide in 2 combined formulations of the 3 drugs (Rifater) for use primarily in the short-course chemotherapy of tuberculosis has been studied in Chinese patients in Singapore and Hong Kong. One formulation, containing 50 mg isoniazid, 120 mg rifampin, and 300 mg pyrazinamide per tablet is suitable for daily use, whereas the other, containing higher proportions of isoniazid and pyrazinamide, is designed for intermittent treatment, each tablet containing 125 mg isoniazid, 100 mg rifampin, and 375 mg pyrazinamide. Appropriate dosages for the Chinese patients, whose average weight was approximately 50 kg, were 5 and 6 tablets, respectively. Plasma concentrations of the 3 drugs after giving such dosages of the 2 combined formulations were compared in 16 patients, 8 in Singapore and 8 in Hong Kong, by means of a crossover study, with the concentrations obtained when identical doses of the 3 drugs were given using standard separate drug formulations. The concomitant urinary excretions of the drugs and their major metabolites were also estimated. Very similar results were obtained whether the drugs were given as the combined preparations or in their standard separate formulations, demonstrating the excellent bioavailability of all 3 drugs in each of the 2 combined formulations.