Systemic clearance and demethylation of caffeine in sheep and cattle.

Pharmacokinetics of caffeine have been studied in sheep and cattle treated with caffeine (5 mg/kg) by intravenous injection. Terminal-phase elimination half-lives were 8.9 hr in sheep and 8.1 hr in cattle. Noncompartmental analyses of data collected from individual animals indicate that neither terminal-phase rate constants (beta) nor mean residence times of caffeine in plasma differed between species. Each of the three possible N-demethylated primary metabolites of caffeine was detected in plasma from each species, with theophylline predominating in sheep and paraxanthine predominating in cattle. These data indicate that hepatic capacity to clear caffeine from the systemic circulation is similar between sheep and cattle, but that the preferred routes of metabolism differ. Expression of cytochromes P4501A (CYP1A subfamily) may differ between these species.

Thin-layer chromatographic detection of ivermectin in cattle serum.

A study was conducted on the detection of ivermectin (22,23-dihydroavermectin B1) in cattle serum by thin-layer chromatography (TLC) after derivatization of this parasiticide by the known reaction with trifluoroacetic anhydride-1-methylimidazole and visual examination of the chromatograms under long-wavelength ultraviolet light. By derivatization of reference samples of ivermectin in acetonitrile, approximately 0.1 ng of a highly fluorescent material, tentatively identified as 2,5,6-tetradehydro-5,7-dideoxy-22,23-dihydroavermectin B1, could be detected on silica-gel thin layer plates. Extraction of fortified serum samples with methyl tert.-butyl ether followed by derivatization, hydrolysis, partitioning between water-hexane and TLC gave a limit of detection of 1-2 ng/ml. With these simple techniques ivermectin could be detected in cattle serum for 3-4 weeks after subcutaneous treatment of Hereford heifers.

Pharmacokinetic assessment of the dermal absorption of N,N-diethyl-m-toluamide (DEET) in cattle.

Model-independent pharmacokinetic methods based on statistical moments were applied to investigate the plasma disposition characteristics of N,N-diethyl-m-toluamide (DEET insect repellent) after single-dose treatment of experimental cattle by rapid intravenous injection (2.5-2.7 mg/kg) and by dermal application (10 mg/kg) to the back. DEET was determined in jugular blood samples by capillary GC with a nitrogen-selective detector and an internal standard of N,N-dipropyl-m-toluamide. Using weighted least squares linear regression analysis, the assay was validated over the concentration range of 19-1910 ng/ml of plasma. Comparison of areas under the plasma concentration-time curves after intravenous and dermal treatments of four Hereford heifers indicated that 72.9 +/- 8.3% (mean +/- SD) of the dermally applied dose was absorbed into the systemic circulation. The time-to-peak plasma concentrations following dermal treatments was 37.5 +/- 8.7 min. Apparent elimination rate constants were not significantly different between the two routes of administration. Linear pharmacokinetics was demonstrated with four additional cattle by comparing systemic clearance after intravenous infusion to steady-state plasma levels of approximately 0.5 and 2.5 micrograms/ml. The rapid and extensive dermal absorption of DEET observed in this study will probably contribute to a short duration of insect repellent action if ethanol-based sprays are used to protect cattle under field conditions.

Pharmacokinetics of phenolsulfonphthalein in sheep.

Pharmacokinetic variables of phenolsulfonphthalein (PSP) were determined in sheep after rapid IV injection and IV infusion to steady state. In Suffolk wethers, an average of < 75% of an IV administered dose was eliminated in urine, indicating that measures of systemic clearance overestimate renal clearance in this species. Furthermore, PSP elimination from plasma was more rapid in Suffolk than Rambouillet wethers and, in Suffolk ewes, systemic clearance decreased from mean +/- SD 7.8 +/- 0.3 ml/min/kg of body weight to 4.7 +/- 1.1 ml/min/kg at steady-state plasma concentration of 2.4 +/- 0.3 and 151.3 +/- 31.8 micrograms/ml, respectively. These observations indicate that, similar to that in other species, systemic clearance of PSP in sheep is concentration-dependent and that significant differences may exist between breeds.

Tissue levels and some pharmacological properties of an acetylated metabolite of phenelzine in the rat.

The metabolic generation of N2-acetylphenelzine by rats treated with phenelzine, and the activity of this metabolite as an inhibitor of monoamine oxidase enzymes in vivo were confirmed. The isomeric amide N1-acetylphenelzine was not a metabolic product of phenelzine and also did not inhibit monoamine oxidase enzymes. Levels of N2-acetylphenelzine in rat blood, after treatment with a dose (0.1 mmol.kg-1) of N2-acetylphenelzine sufficient to inhibit monoamine oxidase enzymes but not to increase brain levels of dopamine or noradrenaline, were higher than those generated metabolically from a higher dose (0.38 mmol.kg-1) of phenelzine which did increase brain levels of these biogenic amines. Metabolically derived N2-acetylphenelzine, therefore, probably does not contribute in any significant way to monoamine oxidase inhibition by phenelzine.

Synthesis and pharmacological evaluation of acyl derivatives of phenelzine.

Acyl derivatives of phenelzine were required for pharmacological evaluation. Eight mono- and di-acyl derivatives were synthesized and characterized by gas chromatography, mass spectrometry, nuclear magnetic resonance and infrared spectrophotometry. Selective acylation was observed with both acetic anhydride and ethyl chloroformate. In aqueous medium, monoacylation yielded N1-acetyl- and N1-(ethoxy-carbonyl)-phenelzine exclusively, whereas in non-aqueous medium only N2-acetyl and N2-(ethoxycarbonyl) products were obtained. NMR temperature studies were conducted to ascertain the presence of rotational isomers and their ratios. At room temperature, one ethoxy-carbonyl and four phenelzine acetate derivatives were present as mixtures of rotamers. Preliminary evaluations of the MAO-inhibiting properties of acylated phenelzines indicate that a hydrogen atom on the N1-position of phenelzine and its derivatives is essential for activity.

Recent studies on the MAO inhibitor phenelzine and its possible metabolites.

Although N2-acetylphenelzine (N2AcPLZ) appears to be only a minor metabolite of phenelzine (PLZ), other investigations have demonstrated that it may be worthy of study as an antidepressant in its own right. In the present report, the possibility of ring hydroxylation as a metabolic route for PLZ was investigated in the rat. Indirect evidence for such a route was obtained using iprindole, a drug known to block ring hydroxylation. Treatment of rats with iprindole followed by PLZ was demonstrated to result in increased brain levels of PLZ and beta-phenylethylamine (control rats were treated with vehicle and then PLZ). The possibility that hydroxylation in the para-position might be a metabolic route for PLZ has led to interest in the possible use of analogues in which this position is blocked with a substituent. In preliminary acute studies at a dose of 0.1 mmol/kg p-chloro-PLZ was found to have a similar effect to PLZ on the inhibition of MAO and to lead to an elevation of catecholamines and 5-hydroxytryptamine (5-HT) in rat whole brain.

Metabolic acetylation of phenelzine in rats.

1-Acetyl-2-(2-phenylethyl)hydrazine (N2-acetylphenelzine) is identified as an acetylated metabolite of phenelzine in the rat. One hour after intraperitoneal administration of a high dose of phenelzine sulfate to rats, the blood and brain of the animals were extracted and analyzed by combined gas chromatography/electron impact mass spectrometry in the total ion and selected ion modes. This procedure provided unequivocal proof of the presence of N2-acetylphenelzine in these tissues. The other possible monoacetylated metabolite of phenelzine, 1-acetyl-1-(2-phenylethyl)hydrazine (N1-acetylphenelzine), and the diacetylated derivative, 1,2-diacetyl-2-(2-phenylethyl)hydrazine, were sought, but were not detected.

Potential prodrugs of phenelzine: N2-acetylphenelzine and N2-ethoxycarbonylphenelzine.

This report describes experiments conducted to determine whether simple monoacylated analogues of phenelzine might act as prodrug sources of phenelzine in vivo. The data suggest that hydrolysis to phenelzine may vary between analogues and that not all analogues retain the pharmacological properties of phenelzine.

Tissue levels and some in vivo responses to a monofluorinated analogue of amphetamine in the mouse.

The metabolism and some behavioral properties of each of the optical isomers of 2-amino-1-fluoro-3-phenylpropane hydrochloride (fluoroamphetamine, FAM) were examined and compared to those of the optical isomers of amphetamine (AM). Substitution of fluorine into the side-chain of AM increased the rate of elimination of drug from brain and modified the kinetics from a one- to a two-compartment model. Urinary excretion of unchanged S-(-)-FAM was reduced from that observed after R-(-)-AM, suggesting a more extensive metabolism. Fluorine substitution also modified the behavioral response to AM. Thus, each optical isomer of FAM produced paradoxical reductions in locomotor activity and body temperature.

Effects of para-substitution on tissue levels and alpha-adrenoceptor antagonist properties of tolazoline.

Tolazoline and a series of para-substituted analogues were examined in mice to determine the effects of para substitution on alpha-adrenoceptor antagonist potency and tissue disposition. alpha-Adrenoceptor antagonism was measured as abilities to attenuate the hypothermic or sedative actions of clonidine. In general, para substitution by electron-withdrawing metabolically stable groups (Cl, F) resulted in increased or unchanged brain levels of drug relative to tolazoline. The para substitution by electron-donating metabolically labile groups (CH3, OCH3) leads to reduced brain levels. Effects on alpha-adrenoceptor antagonist properties did not follow a similar pattern. Thus, increased or decreased antagonism of clonidine effects by para-chloro- or para-methyl-tolazoline, respectively, could be attributed solely to increased or decreased brain levels of drug. para-Fluorotolazoline did not antagonize clonidine but was present in brain at levels equivalent to those of tolazoline. para-Methoxytolazoline on the other hand could not be detected in any tissue but antagonised hypothermia more readily than sedation. These data indicate that the factors governing the disposition or alpha-adrenoceptor antagonist properties of tolazoline analogues are different and independent of each other.

Reserpine-induced hypothermia and its reversal by dopamine agonists.

Prior treatments with reserpine altered the thermic response of mice to subsequently administered apomorphine and amphetamine. Thus, normal mice exhibited hypo- and hyper-thermic responses to apomorphine and (+)-amphetamine, respectively but did not respond to (-)-amphetamine. These responses were each readily attenuated by haloperidol. Reserpinized mice, on the other hand, exhibited hyperthermic responses to all three agonists and these responses were not attenuated by haloperidol. In addition to its hypothermic action, reserpine also produced hypoactivity which was reversed by (+)-amphetamine. This reversal of hypoactivity was attenuated by haloperidol. These data suggest that reversal of reserpine-induced hypothermia by dopamine agonists results through activation of mechanisms which are separate from those normally associated with agonist-induced thermic responses. Reversal of hypoactivity, on the other hand, appears to be due to reactivation of those systems which normally regulate locomotor activity.

Effects of chronic phenelzine in the rat: altered tissue weights and metabolism of 14C-phenelzine.

Phenelzine (2-phenylethylhydrazine, PZ, Nardil) a clinically important antidepressant, inhibits several enzyme systems including monoamine oxidase (MAO). Since PZ is itself a known substrate for MAO, it is possible that its metabolites will differ according to the functional status of MAO. We have, therefore, examined aspects of the metabolism of 14C-PZ in the rat after multiple (15 days) treatments with nonlabelled PZ and compared results to those obtained from drug naive animals. In addition, we have examined the effects of PZ treatment upon total body weight and the weights of selected organs. Total body weights and weights of lungs, livers and kidneys were reduced from controls after repeated injection with PZ. The excretion of radioactivity was also altered. The PZ-pretreated animals excreted less (p less than 0.05) radioactivity in urine (41.1 +/- 5.6 vrs 59.2 +/- 3.7% of dose in controls) and more in expired air (p less than 0.05) than did controls. These data suggest that prior treatments with PZ alter the metabolism and excretion of subsequently administered 14C-PZ.

Some in vivo responses to a side chain monofluorinated amphetamine in the rat.

Mature male or female Sprague-Dawley rats were injected intraperitoneally with various doses (0.005, 0.027 and 0.05 mM/Kg) of (+) amphetamine sulfate or (+)-alpha-fluoromethyl-beta-phenylethylamine hydrochloride. Core (rectal) body temperatures were measured at pretreatment and 30, 60, 90, 120, 180, 240 and 300 minutes after treatment. Injection of (+)amphetamine resulted in increased body temperatures in all dose groups excepting males in the low dose (0.005 mM/kg) group after which temperatures returned to and remained at control levels. Injection of (+)alpha-fluoromethylphenylethylamine resulted in initially increased temperatures in all dose groups and at 180 minutes reduced temperatures in high dose males and in medium and high dose females. Hypothermia was not observed after treatment with (+)amphetamine. These results suggest that (+)alpha-fluoromethyl-beta-phenylethylamine has chemical properties distinct from those of (+)-amphetamine.

Effect of haloperidol and d-amphetamine on cerebral tyramine and octopamine levels.

The administration of d-amphetamine or haloperidol produced a marked reduction in the rat striatum concentration of p-tyramine, an effect that was not observed in the mesolimbic system. However, the administration of d-amphetamine to haloperidol-pretreated animals produced in both brain areas a marked reduction in p-tyramine levels. Furthermore, this latter treatment produced a marked increase in the m-tyramine levels in both brain regions. Hypothalamic p-octopamine levels were reduced by d-amphetamine, but not by haloperidol or haloperidol in the presence of d-amphetamine.

Species variation with respect to the metabolism and excretion of d-amphetamine and d,l-N-hydroxyamphetamine succinate.

The metabolism of amphetamine and N-hydroxyamphetamine was studied in the rat and mouse. Extensive reduction of N-hydroxyamphetamine to amphetamine occurred in both species. In addition, para-hydroxylation of amphetamine was a common metabolic route, although it was more predominant in the rat. No appreciable difference in the 24-h excretion of amphetamine and p-hydroxyamphetamine in either species was seen after substitution of N-hydroxyamphetamine for amphetamine.