Acta pharmacol. et toxicol. 1976, 39,225-231.

From the Research and Development Laboratories of AB Draco (A subsidiary to AB Astra, Sweden), Fack, S-221 01 Lund, Sweden

Bronchospasmolytic and Cardiovascular Effects in Anaesthetized Cats of Ibuterol and Terbutaline Given

Intravenously and after Inhalation : Drug and Prodrug Compared BY

Per Andersson (Received December 3, 1975; Accepted January 19, 1976)

Abstract: The bronchospasmolytic and cardiovascular effects of terbutaline and ibuterol were tested in the anaesthetized cat after inhalation and after intra- venous infusion. A 5-min. inhalation of ibuterol in the concentrations 0.01-0.5 mM counteracted the serotonin-induced bronchoconstriction. In this respect it was 3 times as potent as terbutaline. The two compounds administered intra- venously proved equi-effective. Neither inhalation nor intravenous infusion had any significant effect on the heart rate, pulse amplitude, or arterial blood pres- sure, in doses which counteracted the induced bronchospasm. Higher doses of the compounds caused a dose-dependent increase in heart rate and pulse ampli- tude, and a decrease in diastolic blood pressure. After intravenous infusion, the cardiovascular effects of the compounds were almost identical. Inhaled ibuterol had a more pronounced effect than inhaled terbutaline. It was con- cluded that both ibuterol and terbutaline, have a good margin between the bronchospasmolytic and the cardiovascular effects after inhalation and intravenous infusion. Esterification of the OH-groups in the ring of terbutaline with di-isobutyric acid did not change this relation.

Key-words: Ibuterol - terbutaline - bronchospasmolysis - cardiovascular effect - inhalation - intravenous injection - cats.

Sympathomimetic @-receptor stimulating agents have proved very valuable in the symptomatic treatment of bronchial asthma. A new development in this type of drug has recently led to the introduction of bronchoselective agents, which according to Lands’ classification (LANDS 1967a & b) are designated fin-receptor stimulators. Terbutaline is one of these compounds that is widely used today for treating obstructive lung diseases. PERSSON & WSON (1970) described the basic pharmacology of terbutaline. OLS- SON et al. (1974) described the basic pharmacology of ibuterol[l-(3’5’-

226 PEK ANDERSON

di-isobutyryloxyphenyl)-2-(t-butylamino)-ethanol] which is the di-isobutyric acid ester of terbutaline.

Ibuterol is rapidly hydrolyzed in the sera from man, dog, and cat to terbutaline and isobutyric acid (KRISTOFFERSSON et al. 1974). The effect of ibuterol is blocked or markedly reduced by the esterase inhibitor eserine, indicating that ibuterol is inactive, and that the hydrolyzed product ter- butaline is responsible for the effect (OLSSON et al. 1974).

This study compares the bronchospasmolytic and cardiovascular effects of ibuterol and terbutaline after the inhalation of nebulized solutions and after intravenous infusion of the two compounds.

Materials and Methods

33 cats (2.8-4.5 kg) of both sexes, initially anaesthetized with pentobarbital (mebuma- lum NFN), 35 mg/kg intraperitoneally, were used. Continuous infusion of pentobarbital, 2-4 mg/hr, given in the right brachial vein throughout the experiment, maintained the necessary level of anaesthesia. A Braun constant volume respirator (frequency 26/min., volume 10 ml/kg) ensured artificial ventilation.

The Bronchospasmolytic effect of ibuterol and terbutaline was recorded according to KONZEIT & KOSSLER (1940). The intravenous injection of serotonin, 5.0-7.5 pglkg, produced the bronchosplasm. A Grass volumetric transducer (PT5) recorded the over- flow.

In the inhalation part of the study, solutions of ibuterol and terbutaline were nebulized by means of a nebulizer described by Room (1949). The technique was as described by OLSON (1971) for nebulizing histamine solution. During the inhalation, the nebulizer was connected to the inlet of the Rraun respirator. The bronchospasmolytic effect was studied after 5 min. inhalation of the bronchospasmolytic compounds. When broncho- spasmolytic effect was studied after intravenous administration, the compounds were constantly infused into the left femoral vein for 5 min. Three to four doses of ter- butaline and ibuterol were studied in each cat. The dose-response curves for the two drugs were made in random order. The quotient between the doses of the two compounds causing 50 % inhibition of the serotonin-response was calculated. Heart rate, pulse amplitude, and arterial blood pressure were recorded simulataneously and in addition also separately in another series of experiments in which only the cardio- vascular parameters were recorded. A Statham pressure transducer (P23A) recorded the arterial blood pressure and pulse amplitude in arteria carotis. Heart rate was recorded by means of a Grass tachograph (7P4) trigged by the ECG or the pulse wave. All registrations were made on a Grass polygraph model 7.

The compounds were inhaled or constantly infused for 5 min. The maximum effect of each dose on the blood pressure, heart rate, and pulse amplitude was determined.

Compounds used. Terbutaline sulphate (AB Draco, Lund, Sweden), ibuterol sulphate (AB Draco, Lund, Sweden), and serotonin creatinine sulphate (Koch-Light Laboratories, Bucks, England).

For inhalation, ibuterol and terbutaline were dissolved in distilled water made up to 5 YO with glycerol, ascorbic acid being added to a final concentration of 0.2 mglml. Solutions for intravenous injections were made up in 0.85 % NaCl solutions, all which were made up freshly before use. Statistical inferences were made by Student’s t-test. The quotients were tested against 1.0.

IBUTEROL AND TERBUTALINE ON HEART AND LUNG 227

Results

Inhalation and intravenous infusion of terbutaline and ibuterol brought about a concentration-dependent antagonism against the serotonin-induced bronchospasm. In the highest concentrations studied, both ibuterol and terbutaline caused a complete inhibition of the serotonin induced broncho- spasm. Fig. 1 shows the dose-response curves from an experiment using inhaled ibuterol and terbutaline in the anaesthetized cat. When inhaled, the concentration of terbutaline causing 50 olo inhibition of the serotonin-re- sponse was 0.44 5 0.16 mM (mean & S. E. M., 7 experiments). The cor- responding value for ibuterol was 0.14 k 0.04 mM (mean 5 S. E. M., 7 ex- periments). When the mean of the quotients between the EC50 values from each cat was calculated, ibuterol was found to be 3.2 k 0.3 times as active as terbutaline (mean & S. E. M., range 2.0-4.1, P < 0.0005, 7 experiments). After intravenous administration, ibuterol and terbutaline were equally ef- fective in relaxing the bronchospasm induced by serotonin. The ED50 value for terbutaline was (1.1 k 0.3) X mol/kg X min., and for ibuterol (0.9 & 0.3) X moVkg X min. (mean k S. E. M., not significant, 6 ex- periments).

The effect on the heart rate, pulse amplitude, and diastolic blood pres- sure was studied during and after a 5-min. inhalation or intravenous infusion of ibuterol and terbutaline. When inhaled, no effect on these parameters was observed for the two compounds in concentrations (0.01-0.5 mM) which counteracted the serotonin-induced bronchospasm. Inhalation in higher

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Fig. 1. Cat, 3.5 kg. Effect of inhaled ibuterol (u-n) and terbutaline (0-0) on serotonin-induced bronchospasrn. The dose-response curves are log concentration-

effect curves.

228

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PER ANDERSON

I I I

I I

I I I I

I

I

I

I I

I

Beats/m i n. mm Hg '1 ;I I

I I I

50t I

I i I I

mm Hg Fig. 2. Anaesthetized cat, Effect of ibuterol and terbutaline on heart rate, pulse

amplitude, and diastolic blood pressure after inhalation.

a = Effect on heart rate. 0 = Effect on pulse amplitude. 0 = Effect on diastolic blood pressure. Filled symbols: ibuterol Unfilled symbols: terbutaline A: 3ronchospasmolytic effect of ibuterol. B: Bronchospasmolytic effect of terbutaline (25 70 inhibition of the serotonin-

induced bronchospasm).

Each point represents the mean 5 S. E. M. (n = 6). The dose-response curves are log concentration-effect curves.

concentrations (2.8-80 mM) caused a dose-dependent increase in heart rate and pulse amplitude, and a decrease in diastolic blood pressure (fig. 2). In these high concentrations, ibuterol and terbutaline could only be compared at one dose level in each animal. In the concentrations 2.8 mM and 10 mM, the difference in increase in heart rate for ibuterol and terbutaline was not significant. However, in 40 mM solution, ibuterol was 2.4 times more potent than terbutaline in increasing the heart rate, 2.6 times more potent in in- creasing the pulse amplitude, and 2.2 times more potent in lowering the diastolic blood pressure (significant differences: 0.005 > P > 0.001; n = 6).

After intravenous infusion, the cardiovascular effects of the two compounds were identical, and qualitatively the same as those obtained after inhalation (fig. 3). In bronchospasmolytic effective doses [(0.3-1.0) X mol/kg X

lBUTEROL AND TERBUTALINE ON HEART AND LUNG 229

min.] none of the compounds produced any cardiovascular effects, but in higher doses (> 2 X mol/kg x min.) both compounds increased heart rate and pulse amplitude and decreased diastolic blood pressure.

Discussion

In the present study the bronchospasmolytic and cardiovascular effects in anaesthetized cats of ibuterol and terbutaline after inhalation and intra- venous infusion were determined. Ibuterol is the di-isobutyric acid ester of terbutaline, and is per se inactive (for ref. see OLSON et al. 1974). The compound is rapidly hydrolyzed by esterases into isobutyric acid and ter- butaline, which is responsible for the effect. After intravenous infusion, ibute- rol and and terbutaline were equally effective, on a molar basis, in counter- acting the serotonin-induced bronchospasm, and the influence on heart rate

x min

Fig. 3. Anaesthetized cat. Effect of ibuterol and terbutaline on heart rate, pulse amplitude, and diastolic blood pressure after intravenous infusion.

a = Effect on heart rate. @ = Effect on pulse amplitude.

= Effect on diastolic blood pressure. Filled symbols: ibuterol. Unfilled symbols: terbutaline. A: Bronchospasmolytic effect of ibuterol. B: Bronchospasmolytic effect of terbutaline (25 % inhibition of the serotonine-induced

bronchospasm).

Each point represents the mean f S . E. M. (n = 6). The dose-response curves are log concentration-effect curves.

230 PER ANDERSSON

and arterial blood pressure was also identical for the two compounds. These results agree well with those reported by OLSON et al. (1974), and indicate that terbutaIine is responsible for the effects after infusion of ibuterol. When inhaled, ibuterol was 3 times as effective as terbutaline in counteracting serotonin-induced bronchospasm. The effect on heart rate, pulse amplitude, and diastolic blood pressure was also more pronounced after the inhalation of ibuterol than after terbutaline. Neither after inhalation nor after intra- venous infusion, were there any significant effects on the heart rate, pulse amplitude, or arterial blood pressure in doses which counteracted the induced bronchospasm.

A more pronounced effect of ibuterol than of terbutaline after inhalation of the compounds, indicates that ibuterol penetrates more easily the tissue. Terbutaline in the organism is inactivated by conjugation with sulphuric acid or glucuronic acid (NILSSON et al. 1972 & 1973; CONWAY et al. 1973; SINGHVI et al. 1974). This kind of inactivation does not seem to take place in the lung. Perfusion of the isolated dog lung (BRIANT et al. 1974) and the isolated rat lung (RYRFELDT & BODIN 1975) with a solution of terbutaline did not reveal any inactivation of the compound.

In the isolated perfused rat lung, ibuterol is more rapidly absorbed than terbutaline from the respiratory tract. Ibuterol is also extensively hydrolyzed to terbutaline (RYRFELDT & BODIN 1975). As the ester is much more lipo- philic (cf. RYRFELDT & Bodin 1975) than the non-esterified compound, it might more easily penetrate into the tissue. This could explain the higher efficacy of ibuterol than of terbutaline after inhalation. The more pronounced effect of the ester than of the non-esterified compound also shows that the ester is rapidly hydrolyzed in lung tissue to terbutaline. A more pronounced effect of ibuterol than of terbutaline is also reported after oral administration (OLSON et al. 1974).

Both ibuterol and terbutaline have a good margin between the broncho- spasmolytic and the cardiovascular effects after inhalation and intravenous infusion.

R E F E R E N C E S

Briant, R. H., E. W. Blackwell, F. M. Williams, D. S. Davies & C. T. Dollery: The metabolism of sympathomimetic bronchodilator drugs by the isolated perfused dog lung. Xenobiotica 1974, 3, 787-799.

Conway, W. D., S. M. Singhvi, M. Gibaldi & R. N. Boyes: The effect of route of administration on the metabolic fate of terbutaline in the rat. Xenobiotica 1973,

Konzett, H. & R. Rossler: Versuchsanordnung zu Untersuchungen an den Bronkial- muskulatur. Naunyn-Schmiedeberg’s Arch. exp. Path. Pharm. 1940, 195, 71-74.

3, 813-821.

IBUTEROL AND TERBUTALINE ON HEART AND LUNG 23 1

Kristoffersson, J., L. A. Svensson & K. TegnBr: Lung latentiation of terbutaline. In vitro serum esterase catalyzed hydrolysis of a series of acyl-substituted mono- and diesters of terbutaline. Acta Pharm. Suec. 1974, 12, 427-438.

Lands, A. M., A. Arnold, J . P. McAuliff, F. P. Luduena & T. G. Brown Jr.: Differentia- tion of receptor jystems activated by sympathomimetic amines. Nature 1967a, 214,

Lands, A. M., F. P. Luduena & H. J. Buzzo: Differentiation of receptors responsive to isoproterenol. Life Sci. 1967b, 6, 2241-2249.

Nilsson, H. T., K. Persson & K. Tegnkr: The metabolism of terbutaline in man. Xenobiotica 1972, 2, 363-375.

Nilsson, H. T., C. G. A. Persson, K. Persson, K. Tegndr & A. Ryrfeldt: The metabolism of terbutaline in dog and rat. Xenobioticn 1973, 3, 615-623.

Olsson, 0. A. T.: Histamine induced bronchospasm in unanaesthetized guinea-pigs. I. Aerosol technique. Acta Allerg. 1971, 26, 438-447.

Olsson, 0. A. T., C. G. A. Persson, H. Persson & L. Sorenby: Pharmacological properties of 1-(3’5’-di-isobutyryloxyphenyl)-2-(t-butylamino)-ethanol-hydrochloride, KWD 2058, a new sympathomimetic bronchodilator. Acta pharnzacol. et toxicol.

Persson, H. & 0. A. T. Olsson: Some pharmacological properties of terbutaline,l-(3’5’- dihydroxyphenyl)-2-(t-butylamino)-ethanol. A new sympathomimetic p-receptor stimulating agent. Acta Med. Scand. 1970, suppl. 512, 11-19.

5 97-5 9 8.

1974, 35, 76-90.

Rooth, G.: Inhalation of liquid aerosol. Acta Med. Scund. 1949, suppl. 228, 1. Ryrfeldt, A. & N. 0. Bodin: The physiological disposition of ibuterol, terbutaline and

isoproterenol after endotracheal installation to rats. Xenobiotica 1975, 5, 521-529. Singhvi, S. M., W. D. Conway, M. Gibaldi & R. N. Boyes: Influence of dose on the

metabolism and excretion of terbutaline in the rat. Xenobiotica 1974, 4, 563-570.