Lung (1990) Suppl: 115-119

New York Inc. 1990

The Pharmacology of Salmeterol

M. Johnson

Glaxo Group Research, Ltd., Ware, Hertfordshire, United Kingdom

Abstract. The pharmacology of salmeterol hydroxynaphthoate (SALM) has been investigated in respiratory tissues in vitro and in animal models in vivo. In guinea pig trachea and human bronchial smooth muscle, SALM was more potent than isoprenaline (ISO), salbutamol (SALB), and clenbuterol (CLEN). The duration of action was >7 h, whereas that for ISO, SALB, and CLEN was 2, 1 I, and 45 min, respectively. The sustained activity of SALM was reversed by sotalol, but was reestablished when the /3-blocker was removed. SALM was >3000-fold weaker than ISO in cardiac tissues, indi- cating high/32-adrenoceptor selectivity. In the conscious guinea pig, aeroso- lized SALM, SALB, and CLEN caused dose-related bronchodilatation. The activity of SALM persisted for at least 6 h, compared with <2 h for SALB and CLEN. SALM is also a potent inhibitor of mediator release from human lung, this effect being sustained for up to 20 hours. In guinea pig airways in vivo, SALM inhibited histamine-induced plasma protein extravasation for - 8 h. Salmeterol is a potent and selective/32-adrenoceptor agonist with a unique profile of action. It induces persistent bronchodilatation, sustained suppression of mediator release, and long-lasting inhibition of edema forma- tion. This combination of properties may represent an important new ad- vance in the treatment of bronchial asthma.

Key words: Salmeterol--Bronchial asthma--Bronchodilatation--Suppres- sion of mediator release--Inhibition of edema formation.

Salmeterol hydroxynaphthoate is a novel, long-acting/3-adrenoceptor agonist [1]. The pharmacology of the drug has been investigated in a range of animal and human respiratory tissues in vitro and in animal models in vivo.

Offprint requests to: Dr. M. Johnson, Peripheral Pharmacology, Glaxo Group Research, Priory Street, Ware, Hertfordshire SG12 ODJ, UK.

116

Table 1. Potency and rates of onset and offset of fl-adrenoceptor agonists in the electrically stimulated guinea pig trachea prepara- tion a,b

Agonist ECso c Onset time d Offset time ¢ (nM) (mill) (min)

Isoprenaline 20 I -2 t -3 (n = 4) (13-29)

Salbutamol 25 3.2 11 (n = 8) (11-56) (2.0-5.4) (8-15)

Clenbuterol 10 5.5 45 (n = 6) (4-25) (2.8-10.2) (29-68)

Salmeterol 4 29.0 >420 (n = 14) (2-11) (24-36)

Guinea pig isolated trachea preparations were electrically field-stimulated under superfusion conditions according to the method of Coleman and Nials (1986). b 95% confidence limits are shown in parentheses. c ECs0 = concentration of ~-agonist required to inhibit con- tractile responses by 50%. d Onset time is the time required for 50% of the response to an ECs0 concentration to be achieved. • O f f s e t time is the time required for 50% recovery from an ECs0 concentration.

ME. Johnson

In relaxation of guinea pig trachea and human bronchial smooth muscle, salmeterol was at least equipotent with isoprenaline (ECs0 values: 4.2 and 5.7 nM), and markedly more potent than salbutamol and clenbuterol. The rates of onset and offset of action of salmeterol have been evaluated in the superfused, electrically field-stimulated, guinea pig isolated trachea preparation, exhibiting cholinergically mediated contractions [2]. The onset of salmeterol inhibition was delayed compared with other/3-adrenoceptor agonists under these condi- tions, but the duration of action was markedly prolonged, being >7 h, whereas that for isoprenaline, salbutamol, and clenbuterol was 2, 11, and 45 min, re- spectively (Table 1).

The sustained activity of salmeterol on respiratory smooth muscle was, however, rapidly and fully reversed by propranolol (pA2 9.0) and sotalol, but was reestablished when the fl-adrenoceptor antagonist was removed [3].

In contrast to its potency on respiratory smooth muscle, salmeterol was >3000-fold weaker than isoprenaline in cardiac tissue, indicating high selectiv- ity for/32-adrenoceptors. In the conscious guinea pig, nebulized aerosols of salbutamol, clenbuterol, formoterol, and salmeterol caused dose-related pro- tection against histamine-induced bronchoconstriction [4]. However, whereas salbutamol, clenbuterol, and formoterol at threshold effective doses (0.2 and 0.01 mM, respectively) had a duration of action of <2 -4 h in this model, the activity of salmeterol (0.12 raM) persisted with little or no decline for at least 6

Pharmacology of Salmeterol 117

o 1 0 0

~ 8o

60

"= 40

2 0

:5 0 .=

-20 1 I I I

0 0 . 5 1 . 5 3 6

Time (h)

Fig. 1. Bronchodilator activity in the conscious guinea pig. Aerosol administration--threshold effective dose: E~, salmeterol; O, salbutamol; A, clenbuterol; I , formoterol.

Table 2. Inhibition of mediator release from human lung by/3-adreno- ceptor agonists

Agonist ECso (nM) --- seM (n)

Histamine LTC4/D4 PGD2

Salmeterol 3.0 --- 1.9 (11) 0.9 ~ 0.5 (11) 1.5 --- 0.5 (4) Isoprenaline 1.1 --- 0.3 (24) 0.4 --- 0.1 (24) 1.4 - 0.3 (6) SalbutamoI 35.8 --- 14.6 (24) 11.5 --- 2.7 (24) 51.7 ± 32.0 (6) Formoterol 0.25 (2) 0.03 (2) NT

NT, not tested

hours (Fig. 1). Salmeterol therefore produces long-lasting relaxation of a i rways smooth muscle in vitro and persistent bronchodilatat ion in vivo.

In addition to its bronchodilator propert ies, salmeterol exhibits o ther phar- macologic activities that may be of value in the t reatment of bronchial asthma. For example, it is a potent inhibitor of the release of spasmogenic and inflam- matory mediators , such as histamine, leukotrienes C4 and D4, and prostaglan- din ]32 (Table 2) f rom sensitized human lung fragments challenged with antigen [5].

/3-adrenoceptor agonists were preincubated with human lung fragments , sensitized overnight by incubation in atopic serum for 30 min before challenge with a submaximal concentrat ion of antigen. Histamine, LTC4/D4, and PGDz were assayed by specific radioimmunoassay.

118

A

m 100 u3 ~O

80 @

6O "6 c 40 0

z3 2O c-

_c 0

om -20

4

J:

0 4 8 12 16 20 Time (hr)

B I00

o~ 80

~ 6O

c 40 0 . :. . .:

~5 2O c -

_c 0

~ - 2 0 4 8 12 16 20

Time (hr)

M. Johnson

Fig. 2. Duration of action of fl-adrenoceptor agonists as inhibitors of mediator release from human lung. A, Histamine: , , formoterol 4 nM (n = 4); Q, salmeterol 40 nM (n = 12); A, isoprenaline 20 nM (n = 6); II, salbutamol 200 nM (n = 7). B, Leukotriene CJD4: symbols as in A.

While inhibition of mediator release by other fl2-agonists such as salbuta- tool and formoterol in this system was rapidly reversed (<6 h), tha t for an equieffective concentrat ion of salmeterol was sustained for up to 20 h (Fig. 2). Salmeterol also inhibited histamine-induced plasma protein extrava.sation in guinea pig lung in vivo (Fig. 3). When given by inhalation at aerosol concent ra- tions of 0.01-1 mg/ml, this effect of salmeterol was apparent for 6-8 hours. In contrast, other/32-adrenoceptor agonists such as salbutamol were ineffect ive after 2 h. Finally, salmeterol has inhibitory effects on inflammatory ceils, which are significantly more marked than those observed with classic/3-agonists such as isoprenaline and salbutamol. In vitro, salmeterol (10-8-10 -6 M) inhibited LTB4 release from human neutrophils, whereas isoprenaline and salbutamol were without effect over this concentrat ion range. This anti-inflamrnatory ef- fect was also apparent in vivo in the guinea pig lung, where l ipopolysaccharide (100/~g/ml)-induced neutrophil accumulation was inhibited by inhaled salme- terol (0.1 mg/ml). Similarly, zymosan stimulation of inflammatory cell influx into guinea pig skin was prevented by local or oral (10 mg/kg) administrat ion of salmeterol.

Salmeterol is therefore a potent and selective fl2-adrenoceptor agonist with

Pharmacology of Salmeterol 119

60"

50 " .< cta - - 40' E

"" 30 '

uJ 20" 13. n

10'

0 Control H i s t

0.5 rng/ml

Hist + Hist + Hist + Hist + salm salrn salrn salm

O.O01mg/rnl O.01mg/ml 0.1mg/ml 1.0rng/ml

Hist + salm

0.1mglml + proprano[o[ lmg/kg sc

Fig. 3. Inhibition of histamine-induced plasma protein extravasation (PPE) in guinea pig lung by salmeterol.

a unique profile of action. It induces persistent bronchodilatation and has sig- nificant anti-inflammatory activity, reflected in sustained suppression of media- tor release, long-lasting inhibition of edema formation, and modulation of in- flammatory cells. This combination of properties potentially represents an important new advance in the treatment and management of bronchial asthma.

References

1. Bradshaw J, Brittain RT, Coleman RA, Jack D, Kennedy I, Lunts LHC, Skidmore IF (1987) The design of salmeterol, a long-acting selective fl:-adrenoceptor agonist. Br J Pharmacol 92:590P

2. Coleman RA, Nials AT (1986) The characterisation and use of the electrically-stimulated, supeffused guinea-pig tracheal strip preparation. Br J Pharmacol 88:409P

3. Brittain RT (1990) Approaches to a long-acting, selective fl2-adrenoceptor stimulant. Lung 168 Suppl:ll 1-114

4. Ball DI, Coleman RA, Denyer LH, Nials AT, Sheldrick KE (1987) In vitro characterisation of the fl:-adrenoceptor agonist, salmeterol. Br J Pharrnacol 92:591P

5. Butchers PR, Cousins SA, Vardey CJ (1987) Salmeterol: a potent and long-acting inhibitor of the release of inflammatory and spasmogenic mediators from human lung. Br J Pharmacol 92:745P