Relevance of the OCT1 transporter to the antineoplastic effect of biguanides

► siRNA knockdown of OCT1 reduced sensitivity of EOC cells to metformin, but not to another biguanide, phenformin. ► Suppression of OCT1 also affects the activation of AMP kinase in response to metformin, but not to phenformin. ► Direct actions of metformin may be limited by low OCT1 expression in EOC tumors. ► Phenformin could be used as an alternative biguanide.