Furthermore, there have been some reports suggesting that certain PDE4 inhibitors can inhibit cough to inhaled capsaicin in normal guinea pigs32,33 and in allergic guinea pig models of asthma.32,34-36 However, in contrast, a lack of effect of PDE4 inhibition on citric acid–induced cough in normal guinea pigs has also been reported.35 This evidence concerns the gene PDE4A and asthma.