The pathogenic role of platelet-activating factor (PAF) in asthma has been
questioned due to the limited or negative efficacy of PAF antagonists; howe
ver, in acute asthma (AA), where the endogenous release of PAF may be enhan
ced, the effects of PAF antagonist receptors have not been investigated. It
was postulated that inhaled PAF provokes gas exchange defects in mild asth
ma likely to be related to airway vascular leakage.
The response to a potent, selective PAF receptor antagonist, SR 27471A, on
pulmonary gas exchange was studied, more specifically ventilation-perfusion
(VA'/Q') distributions, in patients with AA within 48 h of hospitalization
. A randomized, double-blind, placebo-controlled, parallel group (n=6, each
) design was used. After baseline measurements, either placebo or SR 27417A
(20 mg, orally) was administered and measurements were repeated 3 h later.
Conventional anti-asthma medication was not interrupted.
Despite a near-complete inhibition of the in vitro, platelet aggregation te
sts by 40 nM PAF (mean+/-SEM from 72+/-9 to 6+/-2%) and 80 nM PAF (from 81/-7 to 6+/-3%both p<0.01) by SR 27471A indicating a good bioactivity of the
compound, no significant changes in baseline forced expiratory volume in o
ne second, (40+/-6%), respiratory system resistance (6.2+/-0.7 cmH(2)O.L-1.
s), alveolar-arterial pressure difference for oxygen (5.2+/-0.4 kPa), arter
ial oxygen tension (9.0+/-0.5 kPa) or VA'/Q' distributions, as expressed by
the dispersion of pulmonary blood flow (LogSD Q, 1.07+/-0.09; normal value
s <0.60), were observed.
It is concluded that SR 27417A has limited value when added to the conventi
onal treatment of acute asthma. These findings minimize the potential patho
genic role of endogenous platelet-activating factor as a relevant mediator
of airway inflammation during acute asthma.