Effects of benserazide,carbidopa and isoniazid administration on tryptophan-nicotinamide nucleotide metabolism in the rat

Weanling rats were maintained on a diet providing marginally adequate amounts of nicotinamide and vitamin B₆, with a considerable excess of tryptophan to allow endogenous synthesis of nicotinamide nucleotides. Groups of animals received Benserazide or Carbidopa at 0.9 mg/kg body wt/day or isoniazid at 1.3 mg/kg body wt/day. All three drugs led to a reduction in the excretion of N¹-methyl nicotinamide (the principal metabolite of nicotinamide nucleotides in the rat) after 2 and 3 days, compared with control animals receiving the same diet. After 7 days, excretion of this metabolite had returned to the control level. All three drugs are potent inhibitors of tryptophan oxygenase, kynureninase and 3-hydroxyanthranilate oxidase, but the effects of administration in vivo were not related to the apparent K_i, values determined in vitro. All three drugs also caused an increase in the activity of nicotinamide deamidase, which may lead to increased efficiency of utilization of dietary nicotinamide in response to deficiency, and hence explain the recovery in N¹-methyl nicotinamide excretion towards the end of the experiment. Urinary excretion of tryptophan metabolites (xanthurenic and kynurenic acids and kynurenine) was not as expected. Benserazide and Carbidopa had no significant effect, while isoniazid caused an increase in kynurenine excretion and a decrease in xanthurenic acid excretion. It is suggested that this may be due to a reduced activity of kynurenine hydroxylase, although isoniazid had no effect on the activity of this enzyme after administration for 11 days, or when added in vitro. The results are discussed in relation to the mechanisms of isoniazid-induced pellagra and Benserazide and Carbidopa-induced niacin depletion in man.     

The effect of simultaneous administration of 3,4-dihydroxyphenylpyruvic acid and L-dopa on the bioavailability of L-dopa in rat and mouse.

In the rat, administration of 3-(3,4-dihydroxy)-L-phenylalanine (L-dopa) simultaneously with the corresponding alpha-keto acid, 3,4-dihydroxyphenylpyruvic acid (DHPPA), gives significantly higher concentrations of L-dopa in the serum and of dopamine and homovanillic acid in the brain than the same dose of L-dopa alone. Correspondingly, DHPPA potentiates the effect of L-dopa on the locomotor activity in reserpine-treated mice. DHPPA is postulated to increase the absorption of intact L-dopa from the gut by preventing the transamination reaction between L-dopa and the physiological alpha-keto acids.     

Entacapone improves the availability of L-dopa in plasma by decreasing its peripheral metabolism independent of L-dopa/carbidopa dose

AIMS: Entacapone is a peripherally acting catechol-O-methyltransferase (COMT) inhibitor. To improve the benefits of oral L-dopa in the treatment of Parkinson's disease (PD), entacapone is administered as a 200 mg dose with each daily dose of L-dopa. This study evaluated the effects of entacapone 200 mg on the pharmacokinetics and metabolism of L-dopa given as standard release L-dopa/carbidopa. METHODS: Six different doses of l-dopa/carbidopa were investigated in this placebo-controlled, double-blind (regarding entacapone), randomized, single-dose study in 46 young healthy males. The subjects were divided into three groups (n = 14-16). Two different L-dopa/carbidopa doses were administered to each subject (50/12.5 mg and 150/37.5 mg, or 100/10 mg and 100/25 mg, or 200/50 mg and 250/25 mg). Each dose was given on two occasions; simultaneously with entacapone or with placebo, in random order, on two consecutive study visits, separated by a washout period of at least 3 weeks (four-way crossover design). Serial blood samples were drawn before dosing and up to 24 h after the dose and pharmacokinetic parameters of L-dopa, its metabolites, carbidopa, and entacapone were determined. RESULTS: Entacapone increased the AUC(0,12 h) of L-dopa to a similar extent at all doses of L-dopa/carbidopa, that is by about 30-40% compared with placebo (P < 0.001, 95% CI 0.15, 0.40). When evaluated as the ratio of geometric means, entacapone slightly decreased the mean C(max) values for L-dopa at all L-dopa/carbidopa doses compared with placebo. When given with entacapone, higher plasma concentrations of L-dopa were maintained for a longer period at all doses of L-dopa/carbidopa. Entacapone also decreased the peripheral formation of 3-O-methyldopa (3-OMD) to about 55-60% of the placebo treatment level (P < 0.001, 95% CI -0.72, -0.35) and increased the mean AUC(0,12 h) of 3,4-dihydroxy-phenylacetic acid (DOPAC) 2-2.6-fold compared with placebo (P < 0.001, 95% CI 0.60, 1.10). The mean AUC(0,12 h) of 3-methoxy-4-hydroxy-phenylacetic acid (HVA) following entacapone was approximately 65-75% of that observed with placebo (P < 0.001-0.05, 95% CI -0.76, -0.01) at each L-dopa/carbidopa dose except the 50/12.5 mg dose (P > 0.05, 95% CI -0.59, 0.05). The metabolic ratios (MR, AUC metabolite/AUC L-dopa) also confirmed that entacapone significantly decreased the proportion of 3-OMD (P < 0.001, 95% CI -0.85, -0.68) and HVA (P < 0.001, 95% CI -1.01, -0.18) in plasma at each L-dopa/carbidopa dose, whereas the AUC DOPAC/AUC L-dopa ratio was increased again at all doses (P < 0.001, 95% CI 0.26, 0.90). Entacapone did not significantly affect the pharmacokinetics of carbidopa at any of the doses, nor did L-dopa/carbidopa affect the pharmacokinetics of entacapone. CONCLUSIONS: The 200 mg dose of entacapone similarly and significantly increases the AUC of L-dopa by changing the metabolic balance of L-dopa independent of the L-dopa/carbidopa dose and therefore entacapone is likely to have a similar L-dopa potentiating effect independent of L-dopa dose.     

Re-evaluation of the L-dopa loading effect on dopamine metabolism in rat striatum

Investigation by HPLC with electrochemical detection of dopamine (DA) metabolism in rat striatum after L-dopa + benserazide treatment allowed quantification of the time course evolution of DA, 3,4-dihydroxyphenylacetic acid and homovanillic acid levels. Furthermore, four peaks which did not appear in controls, were detected in treated striatum. One was identified as 3-methoxytyrosine, the level of which was still high 9 h after treatment. 3-Methoxytyrosine, has been detected previously in plasma of parkinsonian patients treated with L-dopa, and the disturbance in DA metabolism could explain some of the side-effects induced by that treatment.     

The effect of simultaneous administration of 3,4-dihydroxyphenylpyruvic acid and L-dopa on the bioavailability of L-dopa in rat and mouse

In the rat, administration of 3-(3,4-dihydroxy)-l-phenylalanine (l-dopa) simultaneously with the corresponding α-keto acid, 3,4-dihydroxyphenylpyruvic acid (DHPPA), gives significantly higher concentrations of l-dopa in the serum and of dopamine and homovanillic acid in the brain than the same dose of l-dopa alone. Correspondingly, DHPPA potentiates the effect of l-dopa on the locomotor activity in reserpine-treated mice. DHPPA is postulated to increase the absorption of intact l-dopa from the gut by preventing the transamination reaction between l-dopa and the physiological α-keto acids.     

Effect of route administration and liposome entrapment on the metabolism and disposition of adriamycin in the rat

The plasma clearance, tissue distribution, metabolism, and excretion of free Adriamycin (Adr) and liposome-entrapped Adr (Adr/L) was examined after iv and ip administration to rats. When given iv, free Adr was cleared more rapidly from plasma than was Adr/L. In contrast, Adr and Adr/L were cleared from plasma at similar rates when given by the ip route and peak plasma concentrations were significantly lower than observed after iv treatment. Irrespective of the route of administration, tissue distribution of Adr was altered after liposome entrapment, with increased uptake of Adr equivalents into liver and spleen and decreased uptake into kidney, heart, and lung. However, tissue concentrations of Adr were generally lower in rats treated ip with Adr or Adr/L than in animals dosed iv. Furthermore, an enhanced uptake of Adr/L relative to free Adr in lymph nodes draining the peritoneal cavity was observed only in animals treated by the ip route. The rates of both biliary and urinary excretion of Adr were 1/3-1/2 of control after liposome entrapment regardless of the route of administration, although excretion rates in rats dosed ip were half of those observed in animals treated by the iv route. Similarly, liposome entrapment of bromosulfophthalein was found to decrease its biliary excretion rate to 1/3-1/2 of control. Bile and urine of rats given Adr/L iv contained a higher proportion of Adr metabolites and less unchanged Adr than was found in animals receiving the free drug. This finding suggests that liposome entrapment of Adr leads to a modification in the metabolism of this drug in rats.     

The influences of the route of administration on the metabolism and excretion of bitolterol, a new bronchodilator, in the rat.

1. [3H]Bitolterol, an ester prodrug to colterol (N-t-butyl-arterenol), when administered orally, was excreted mostly in the urine; approx. equal amounts of 3H were found in urine and faeces after intraperitoneal or intravenous injection. 2. Half the dose was excreted in the bile following parenteral administration, while only a small amount of radioactivity was found in bile after oral dosage. The biliary-excreted material consisted mainly of glucuronides, for all routes of administration. 3. The glucuronides of colterol and 3-O-methyl-colterol were excreted in urine after oral administration of bitolterol. In addition to the glucuronides, free colterol and 3-O-methyl-colterol were excreted in urine following parenteral administration. 4. A part of bitolterol was hydrolysed to colterol in rat stomach, and bitolterol was more rapidly hydrolysed to colteral with homogenates of intestinal mucosa than with stomach homogenates in vitro.     

The renal handling of dopamine originating from L-dopa and gamma-glutamyl-L-dopa.

1. The formation and outflow of dopamine and its deaminated metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) was studied in cortical fragments of the rat kidney loaded with L-beta-3,4-dihydroxyphenylalanine (L-dopa) or gamma-glutamyl-L-dopa (GluDOPA). Dopamine and DOPAC in the tissues and in the effluent were assayed by means of h.p.l.c. with electrochemical detection. 2. In rats given 30 mg kg-1 L-dopa, tissue and outflow levels of both dopamine and DOPAC were 3 fold those observed with a lower dose of L-dopa (10 mg kg-1). In rats given GluDOPA (16.7 mg kg-1) levels of dopamine in renal tissues and in perifusate samples were found to be higher than those obtained with an equimolar dose of L-dopa (10 mg kg-1); however, no significant difference was observed for DOPAC. The outflow of both dopamine and DOPAC in kidney slices of rats injected with L-dopa (10 and 30 mg kg-1) or GluDOPA (16.7 mg kg-1) was found to decline monophasically with similar slopes of decline. The rate constants of loss (k, min-1) of DOPAC (10 mg kg-1 L-DOPA, k = 0.0070; 30 mg kg-1 L-DOPA, k = 0.0087; 16.7 mg kg-1 GluDOPA, k = 0.0080) were 2 to 3 fold those of dopamine (10 mg kg-1 L-dopa, k = 0.0027; 30 mg kg-1 L-DOPA, k = 0.0034; 16.7 mg kg-1 GluDOPA, k = 0.0030). With both precursors the DOPAC/dopamine ratio in perifusate samples were 2.0 fold those in the tissues.(ABSTRACT TRUNCATED AT 250 WORDS)     

Bioavailability assessment of cyclosporine in the rat. Influence of route of administration

Systemic availability determines the net loss of drug through poor dosage form design, incomplete absorption, first-pass metabolism, and other reasons for drug destruction. The present study demonstrates the wide variability in assessment of single-dose bioavailability of cyclosporine A (CSA) based on the route of parenteral administration. Rats were administered CSA (10 or 25 mg/kg) orally or via the ip route, and the systemic availability was estimated from drug administered via the penile, femoral, or jugular route. CSA delivered via the jugular vein resulted in a significantly greater AUC (22,253 +/- 8,087 ng-hr/ml), compared to the AUC estimated after femoral, penile, or ip routes (11,919 +/- 1,760, 9,718 +/- 1,113, and 4,233 +/- 1,741 ng-hr/ml, respectively; p less than 0.05). There were no differences in elimination rate constants between groups. However, there was significant delay observed in peak concentration following both femoral vein and ip administration, similar to oral absorption. The bioavailability estimated for each oral dose did not significantly differ within groups but was widely variable, depending upon the intravascular standard used in the equation. The present data illustrate the specificity of route of nonenteral administration in the assessment of systemic availability in the rat model.     

Stereoselective pharmacokinetics of ketoprofen in the rat. Influence of route of administration

The 2-arylpropionic acid nonsteroidal anti-inflammatory drugs are usually administered as racemates. The enantiomers may have different pharmacokinetics and the R-isomer may metabolically invert to the S-isomer. To pinpoint the kinetics of the inversion and the location in which this metabolic process takes place, racemic ketoprofen (KT) was administered to the rat. Using a stereospecific HPLC assay, the pharmacokinetics of KT were studied following 10 mg/kg iv, po, and ip doses of racemic KT to male Sprague-Dawley rats. Plasma concentrations were always greater for (S)-KT than for (R)-KT. The mean +/- SD AUC S/R ratios were 11.8 +/- 9.93 (N = 5), 11.0 +/- 2.64 (N = 4), and 33.7 +/- 11.2 (N = 5) after iv, ip, and po doses, respectively. Bile duct-cannulated rats (N = 4) had AUCs of 85.4 +/- 58.6 and 22.8 +/- 18.4 (mg/liter) hr for (S)- and (R)-KT, respectively. The percentage of conjugated drug recovered in bile was 77.9 +/- 3.77 and 11.4 +/- 2.48% of the dose as (S)- and (R)-KT, respectively. The data indicate 1) substantial stereoselectivity, 2) presystemic gastrointestinal and systemic inversion of (R)- to (S)-KT, 3) extensive elimination of drug through bile in rats, and 4) extensive reabsorption subsequent to biliary excretion.     

In vitro benzo[a]pyrene metabolism from lindane-treated rat liver: effect of oral and acute administration, and comparison with phenobarbital and methylcholanthrene pretreatment.

Lindane (γ-hexachlorocyclohexane) was given orally to Sprague-Dawley male rats in their basal diet, at 24, 120, and 240 ppm, for 4 weeks. Acute intoxication was obtained with ip injection, at 20 mg/kg/day for 3 consecutive days, and the inductive effect was compared to phenobarbital (ip, 80 mg/kg/day for 3 days) and methylcholanthrene (ip, 20 mg/kg/day for 3 days) pretreatment. The spectrofluorometric measurement of arylhydrocarbon hydroxylase (AHH) activity did not show any induction after lindane and phenobarbital pretreatment, while the quantitative determination of benzo[a]pyrene (BP) metabolites by thin-layer chromatography indicated an increase of 3-hydroxy-BP (3-OH-BP). Differences in the BP/protein ratio could explain this discrepancy. Consequently, lindane appeared as a weaker inducer of AHH activity than phenobarbital. The in vitro binding of [¹⁴C]BP metabolites to DNA was largely enhanced by the oral administration of lindane from as low as 24 ppm in the diet and seemed dose related until 120 ppm, reaching two or three times the control value. The acute intoxication by lindane resulted in an increase in BP metabolite binding to DNA similar to that obtained with 120 ppm in the diet. From the chromatographic pattern of BP metabolites, it is concluded that lindane induced a large formation of 4,5-BP-dihydrodiol, which was related to a two-fold increase in epoxide-hydratase activity. From these results, lindane could be considered as a phenobarbital-like inducer.     

Effects of 3,4-dihydroxyphenylpyruvic acid and L-glutamic acid on some pharmacokinetic parameters of L-dopa in the rat

In the rat, various oral doses of 3,4-dihydroxyphenylpyruvic acid (DHPPA) administered simultaneously with L-dopa were shown to elevate the serum L-dopa and cerebral dopamine concentrations dose-dependently. With increasing DHPPA: L-dopa ratio the L-dopa-sparing effect became greater. Although oral and intravenous doses of DHPPA showed that it was readily converted to L-dopa in the serum, only small amounts of dopamine (DA) were detected in the brain. The high 3-O-methyldopa concentrations, measured in the serum after DHPPA administration, might be responsible for the failure of the L-dopa formed to pass the blood-brain barrier. Concomitant administration of L-dopa and glutamic acid gave significantly higher L-dopa values in the serum and DA and homovanillic acid values in the brain than did the same dose of L-dopa alone. This indicates that the transamination of L-dopa can also be effected by coadministration of L-dopa and an amino group donor in vivo.     

L-dopa induces dyskinesia in normal monkeys: behavioural and pharmacokinetic observations

RATIONALE: L-Dopa induces dyskinesias during the treatment of Parkinson's disease and also in primates with nigrostriatal lesions produced by MPTP, but it is claimed that L-dopa does not provoke dyskinesia in humans or monkeys with an intact or mildly damaged nigrostriatal system. OBJECTIVES: This study assessed the behavioural and pharmacokinetic effects of chronic oral administration of L-dopa plus carbidopa alone, or with co-administration of the peripheral COMT inhibitor entacapone, to normal macaque monkeys. Repeated high dose L-dopa administration was shown to induce marked dyskinesias in monkeys with an intact nigrostriatal system, and the threshold for dyskinesia expression was increased by peripheral catechol-O-methyltransferase inhibition with entacapone. METHODS: Six groups of normal macaque monkeys (n=8 per group; Macaca fascicularis) were treated with L-dopa (20, 40 or 80 mg/kg) plus carbidopa (5, 10 or 20 mg/kg) with or without the catechol-O-methyltransferase inhibitor entacapone (20, 40 or 80 mg/kg), or with entacapone alone (80 mg/kg), by oral administration once daily for 13 weeks. RESULTS: Eleven of 16 animals receiving high dose L-dopa (80 mg/kg plus carbidopa 20 mg/kg PO with or without entacapone 80 mg/kg PO for 13 weeks) gradually developed reproducible and idiosyncratic combinations of chorea, athetosis and dystonia maximal at 60-100 min after L-dopa administration, which progressively intensified over the course of the study. The dyskinesias observed were similar in type and distribution to L-dopa-induced dyskinesia observed in patients with Parkinson's disease and in MPTP-treated primates. The occurrence of dyskinesia correlated with plasma concentrations of L-dopa, with animals displaying the most severe dyskinesias having significantly higher plasma concentrations of L-dopa one hour after dosing than animals with mild or moderate dyskinesia or no dyskinesia. Co-administration of entacapone with L-dopa plus carbidopa significantly lowered peak plasma concentrations of L-dopa and this was reflected by a decrease in the severity of dyskinesias, with only one animal receiving entacapone and high dose L-dopa plus carbidopa showing severe dyskinesia, while four receiving high dose L-dopa plus carbidopa alone did so. CONCLUSIONS: These results show for the first time that chronic oral L-dopa administration can provoke dyskinesias in primates independently of nigrostriatal damage, and that this effect is dose related.     

Effects of 3,4-dihydroxyphenylpyruvic acid and L-glutamic acid on some pharmacokinetic parameters of L-dopa in the rat

In the rat, various oral doses of 3,4-dihydroxyphenylpyruvic acid (DHPPA) administered simultaneously with L-dopa were shown to elevate the serum L-dopa and cerebral dopamine concentrations dose-dependently. With increasing DHPPA: L-dopa ratio the L-dopasparing effect became greater. Although oral and intravenous doses of DHPPA showed that it was readily converted to L-dopa in the serum, only small amounts of dopamine (DA) were detected in the brain. The high 3-O-methyldopa concentrations, measured in the serum after DHPPA administration, might be responsible for the failure of the L-dopa formed to pass the blood-brain barrier. Concomitant administration of L-dopa and glutamic acid gave significantly higher L-dopa values in the serum and DA and homovanillic acid values in the brain than did the same dose of L-dopa alone. This indicates that the transamination of L-dopa can also be effected by coadministration of L-dopa and an amino group donor in vivo.     

Effect of route of administration on the toxicity and teratogenicity of EDTA in the rat.

The toxic and teratogenic effects of EDTA were studied in the rat following administration of the compound in the diet (EDTA/do), by gastric intubation (EDTA/po), or subcutaneously (EDTA/sc). EDTA was administered on Days 7 through 14 of gestation at the following dosages: EDTA/do, 3% in the diet (954 mg/kg/day); EDTA/po, 625 mg/kg twice daily (1250 mg/kg/day) or 750 mg/kg twice daily (1500 mg/kg/day); EDTA/sc, 375 mg/kg. Appropriate controls were run for each route of exposure. EDTA/do resulted in no maternal deaths but did produce severe maternal toxicity and malformations in 71% of the offspring. EDTA/po was much more toxic to the dams (87.5% maternal death at 750 mg/kg twice daily, and 36% at 625 mg/kg twice daily) but produced fewer malformed young (20.5% at 625 mg/kg twice daily) than did the slightly lower dose of EDTA administered in the diet. EDTA/sc was lethal to 24% of the dams at a much lower dose than that given by either oral route but did not produce a significant number of malformations in the offspring. Possible factors involved in these differences in toxicity and teratogenicity are absorption into the circulation, interaction with essential metals, and the stress associated with administration of the compound.     

Effect of route of administration on the pharmacokinetic behavior of enantiomers of nefopam and desmethylnefopam

Nefopam hydrochloride is a non-narcotic analgesic used parenterally and orally as a racemic mixture for the relief of postoperative pain. However, no information is presently available on the oral kinetics of (+) and (-) nefopam in humans. Also, nefopam is metabolized by N-demethylation but it is not known whether the desmethylnefopam enantiomers (DES1 and DES2) are present in plasma following intravenous (I.V.) or oral administration of parent drug. To address these issues, 24 healthy white male subjects received two treatments using a double-blind, placebo-controlled crossover design: oral administration of 20 mg nefopam hydrochloride solution or a placebo solution on a sugar cube, simultaneously with a continuous infusion of 20 mg nefopam hydrochloride or placebo infusion. A chiral assay using LC-MS was developed for the simultaneous determination of both enantiomers of the parent drug and its metabolite in plasma and urine. Following I.V. administration, the kinetics of (+) and (-) nefopam could be fitted to a bi-exponential equation but exhibited no stereoselectivity. Both enantiomers had large clearances (53.7 and 57.5 L/hr) and volumes of distribution (390 and 381 L) and half-lives around 5 hours. Following oral administration, (+) and (-) nefopam were rapidly absorbed with bioavailabilities of 44% and 42%, respectively, probably due to a first-pass effect. After I.V. administration, the enantiomers of desmethylnefopam exhibited lower concentrations and longer half-lives (20.0 h for DES1 and 25.3 h for DES2) relative to nefopam enantiomers. Following oral administration, desmethylnefopam enantiomers' plasma concentrations peaked earlier and higher than after I.V. administration (P < 0.05). Following I.V. and oral administration, desmethylnefopam enantiomers showed stereoselectivity in AUC and Cmax values. Urinary excretion of parent and metabolite enantiomers was less than 5% of dose. This study shows that desmethylnefopam enantiomers can contribute to the analgesic effect of racemic nefopam only when it is administered orally.     

Dobutamine administration exacerbates postischaemic myocardial dysfunction in isolated rat hearts: an effect reversed by thyroxine pretreatment

The present study has investigated the effects of dobutamine on postischaemic dysfunction in the setting of global ischaemia and reperfusion in a model of isolated heart preparation. Isolated rat hearts were subjected to 20 min of zero-flow global ischaemia followed by 45 min of reperfusion. Dobutamine administration (10 μg/kg/min) during the reperfusion period resulted in deterioration of functional recovery, which was abolished by propranolol administration. Long-term thyroxine pretreatment (12.5 μg 100 g⁻¹ body weight, b.i.d., s.c., for 2 weeks) reversed the detrimental effect of dobutamine and increased postischaemic recovery of function. We conclude that the combination of thyroxine pretreatment and dobutamine administration could potentially be a new therapeutic strategy to improve postischaemic dysfunction particularly in clinical settings such as cardiopulmonary bypass and/or myocardial infarction.