Ketoconazole should not be given to a patient being treated with astemizole because:
Ketoconazole induces the metabolism of astemizole
Dangerous ventricular arrhythmias can occur
Astemizole inhibits the metabolism of ketoconazole
Astemizole antagonizes the antifungal action of ketoconazole
Ketoconazole, erythromycin and clarithromycin are potent inhibitors of microsomal enzymes. Astemizole, terfenadine and cisapride are the drugs that can block cardiac K+ channels at high concentration. If administered along with microsomal enzyme inhibitors, these drugs can result in torsades de pointes, a serious polymorphic ventricular tachycardia.
Histamine, 5-HT and PGs