Drug A is an anticlotting agent that is chiefly metabolized…

Drug A is an anticlotting agent that is chiefly metabolized by CYP1A2. It was found that patients who smoke >10 cigarettes per day received increased benefit of the drug at a given dose, relative to nonsmokers. This is despite a significantly lower serum concentration of the drug in smokers. Which of the following could best explain these observations?