The metabolism of a bronchodilator procaterol HCL in the rat in vitro and in vivo

Synthesis and biodistribution of [11c]procaterol, a beta2-adrenoceptor agonist for positron emission tomography

The potent, subtype-selective radioligand (+/-)-erythro-5-(1-hydroxy-2-[11C]isopropyl-aminobutyl)-8-hydroxy-car bostyril ([11C]procaterol) was synthesized and evaluated for visualization of pulmonary beta2-adrenoceptors with positron emission tomography (PET). Procaterol was labelled by reductive alkylation of the desisopropyl precursor with [11C]acetone under the influence of NaCNBH3 and acetic acid. Synthesis and HPLC purification were performed in 34 min. Specific activities ranged from 26.5-39.3 TBq (about 700-1000 Ci)/mmol and the radiochemical yield was 2.4-8.6% (corrected for decay). Biodistribution studies were performed in male Wistar rats which were either untreated or predosed with (D,L)-propranolol hydrochloride (beta-adrenoceptor antagonist, 2.5 mg/kg), ICI 118551 (beta2-adrenoceptor antagonist, 0.15 mg/kg), CGP 20712A (beta1-adrenoceptor antagonist, 0.15 mg/kg) or isoprenaline (beta1-adrenoceptor agonist, 15 mg/kg). Specific binding was observed in lungs, spleen and red blood cells, tissues known to contain beta2-adrenoceptors. Pulmonary binding was blocked by propranolol, ICI 118551 and isoprenaline, but not by CGP 20712A. This binding pattern is consistent with the beta2 selectivity of the radioligand. The clearance of [11C]procaterol was biphasic, with a rapid distribution phase (t1/2 0.17 min) representing 90% of the injected dose followed by an elimination phase (t1/2 18.1 min). About 45% of the plasma radioactivity was unmetabolized procaterol at 15 min postinjection. In a dynamic PET-study, the lungs of untreated control rats could barely be detected and total/non-specific binding ratios rose to only 1.2 at 20 min postinjection. Although labelling and administration of (-) erythroprocaterol, the most active of 4 stereoisomers, may produce better results, [11C]procaterol seems unsuitable for beta-adrenoceptor imaging.

Beta2-agonist abuse in food producing animals: use of in vitro liver preparations to assess biotransformation and potential target residues for surveillance

1. The biotransformation of [3H]clenbuterol, [3H]salbutamol, [14C]salmeterol and 7-ethoxycoumarin by bovine liver was investigated by incubation with freshly prepared microsomes, suspension and monolayer cultures of isolated hepatocytes, precision-cut (250 microm) and chopped (600 microm) tissue slices. 2. Radio-HPLC analysis indicated that the saligenin beta2-agonists salmeterol and salbutamol were extensively metabolized by all intact cell preparations. A single major product (SmM1) was evident for salmeterol and two unresolved products for salbutamol (SbM1 and SbM2). Differential enzyme hydrolysis studies with Helix pomatia beta-glucuronidase/aryl sulphatase indicated that the main metabolites were glucuronide conjugates. Consistent with this, analysis of metabolites by liquid chromatography-mass spectrometry showed molecular ions ([M+H]+) at m/z 592 for Sm1 and 416 for both Sb1 and Sb2. 3. Comparable studies with clenbuterol revealed three minor metabolites. Prolonged incubations generated products representing, at maximum, 27% biotransformation. Two of the products have been identified as a glucuronide ([M+H]+, m/z 453) and hydroxyclenbuterol ([M+H]+, m/z 293). 4. These findings indicate that in vitro studies provide simple and cost-effective means of evaluating xenobiotic metabolism, and thus of identifying potential target residues to enable surveillance of use of unlicensed veterinary drugs, or prohibited substances in farm animals.

Preparation of diastereomeric beta-d-glucuronides of the bronchodilator procaterol using immobilized rabbit liver microsomal enzymes

Liver microsomal pellets from a phenobarbital induced New Zealand white male rabbit were prepared, solubilized, and reacted with cyanogen bromide activated Sepharose beads to form an immobilized microsomal enzyme preparation. Viability of bound enzymes was determined using established methods. Racemic erythro-[3H]-procaterol hydrochloride was incubated with immobilized microsomal enzymes at room temperature in 0.1 M potassium phosphate buffer (pH 7.4) together with cofactors magnesium chloride and UDPGA for 16 h. The incubation mixture was filtered and the filtrate fractionated by a C18 solid phase extraction procedure. Two polar reaction products were isolated and characterized by TLC, HPLC-RAM (homogeneous flow-through radioactivity detection), [1H]-NMR, and Fast-Atom Bombardment mass spectrometry as diastereomeric aryl-O-glucuronides of erythro-procaterol. Based on HPLC-RAM analysis, glucuronidation of racemic procaterol proceeds with regiostereoselectivity. In addition, both diastereomers are formed in nearly equal amounts indicating lack of enantioselectivity in this reaction. Combined yield of both diastereomers was approx. 60%.

The metabolism of cilostamide in the rat in vitro and in vivo

The metabolism of N-cyclohexyl'N-methyl-4-(1,2-dihydro-2-oxo-6-quinolyloxy)butyramide (cilostamide) was studied in vitro and in vivo after oral administration to rats. Recovery of orally administered 14C-cilostamide (3 mg/kg) and its metabolites in 48 h was 50.5% dose in urine and 47.6% dose in faeces. The drug was extensively metabolized. The metabolites of cilostamide in rats wee identified by t.l.c., g.l.c.-mass spectrometry and n.m.r. The major route of metabolism is hydroxylation at the cyclohexyl ring.

Disposition and metabolism of formoterol fumarate, a new bronchodilator, in rats and dogs

1. The disposition and metabolism of formoterol fumarate, a highly potent beta 2-adrenoceptor stimulant, were studied in rats and dogs. 2. After oral administration of [3H] formoterol fumarate to dogs, unchanged formoterol accounted for greater than 60% of the plasma radioactivity immediately after dosage; greater than 20% was due to the unchanged drug until 12 h after dosage. In contrast, only 1-3% of the radioactivity was present as unchanged drug in rat plasma. After i.v. dosage, unchanged drug was much higher in both species. The elimination half-life of formoterol was 4-6 h in dogs and 1.7 h in rats. 3. In both species, 36-45% of the dose was excreted in urine and 50-56% in faeces in 72 h, irrespective of the administration route. Biliary excretion after oral dosage amounted to 65 and 31% in rats and dogs, respectively. 4. T.l.c. before and after enzymic hydrolysis revealed that the drug was excreted in urine and bile of rats mostly as a conjugate. Dog urine also contained the conjugate but the unchanged drug was much higher than in rats. The conjugated metabolite was purified from rat urine and identified as the 2-O-glucuronide. The glucuronide was the only metabolite detected in the urine and bile of rats and in the urine of dogs.

Quantitative determination of the beta-adrenoceptor stimulant formoterol in urine by gas chromatography mass spectrometry

A method for the quantitative determination of the beta-stimulant formoterol in urine, using a gas chromatograph--mass spectrometer, is described. Formoterol can be analyzed after the addition of a deuterium-labelled internal standard and conversion to a mixed bis-pentafluoropropionyl-methyl derivative for selected ion monitoring. The detection limit was 5 ng/ml. Urinalysis after the oral administration of formoterol fumarate, using a combined enzymic hydrolysis method, revealed that the drug was conjugated with glucuronic acid in rats, dogs and humans.

The metabolites of procaterol HCl in urine and faeces of dog and man

1. The metabolites of procaterol HCl in dog urine and faeces and in human urine were qualitatively analysed by an improved g.l.c.-mass spectrometric method. 2. Trimethylsilylated derivatives of procaterol metabolites were identified by mass fragmentography as procaterol glucuronide, 5-(2-amino-1-hydroxybutyl)-8-hydroxycarbostyril (desisopropylprocaterol), 5-formyl-8-hydroxycarbostyril, 8-hydroxycarbostyril and unchanged procaterol. 3. The metabolic pattern of procaterol HCl was species-independent in rats, dogs and man.