We investigated potential drug-drug interactions between vandetanib and metformin -LSB- organic cation transporter @ ( OCT@ ) substrate ; NCT@ -RSB- ; digoxin -LSB- P-glycoprotein ( P-gp ) substrate ; NCT@ -RSB- ; midazolam -LSB- cytochrome P@ ( CYP ) @A@ substrate ; NCT@ -RSB- ; omeprazole ( proton pump inhibitor ) or ranitidine ( histamine H@-receptor antagonist ; both NCT@ ) .