about zafirlukast and other drugs

why do theophylline or erythromycin decrease the blood level of zafirlukast?

And aspirin increases the blood level of zafirlukast. Why?

I've checked drugs.com and it explains that this is what happens when these drugs are given with zafirlukast.
With other drugs, erythromycin (an inhibitor) increases the effects (of bronchodilators)