A chemotherapy drug for use in a method for treating a tumor in a target region in a subject's body, the method comprising: applying an alternating voltage at a first frequency between a first set of one or more electrode elements positioned at a first side of the target region and a second set of one or more electrode elements positioned at a second side of the target region, wherein the first side and the second side are on opposite sides of the target region; applying an alternating voltage at a second frequency between a third set of one or more electrode elements positioned at a third side of the target region and a fourth set of one or more electrode elements positioned at a fourth side of the target region, wherein the third side and the fourth sides are on opposite sides of the target region, wherein the third side is disposed circumferentially between the first side and the second side, and wherein the fourth side is disposed circumferentially between the first side and the second side; administering the chemotherapy drug to the subject, and applying an alternating voltage at a third frequency between (i) both the first and third sets of one or more electrode elements and (ii) both the second and fourth sets of one or more electrode elements, wherein the third frequency is lower than the first frequency and the third frequency is lower than the second frequency; wherein the alternating voltage at the first frequency, the alternating voltage at the second frequency, and the alternating voltage at the third frequency are each applied during different intervals of time; and wherein the method delivers the chemotherapy drug across a biological barrier in the target region. applying an alternating voltage at a first frequency between a first set of one or more electrode elements positioned at a first side of the target region and a second set of one or more electrode elements positioned at a second side of the target region, wherein the first side and the second side are on opposite sides of the target region; applying an alternating voltage at a second frequency between a third set of one or more electrode elements positioned at a third side of the target region and a fourth set of one or more electrode elements positioned at a fourth side of the target region, wherein the third side and the fourth sides are on opposite sides of the target region, wherein the third side is disposed circumferentially between the first side and the second side, and wherein the fourth side is disposed circumferentially between the first side and the second side; administering the chemotherapy drug to the subject, and applying an alternating voltage at a third frequency between (i) both the first and third sets of one or more electrode elements and (ii) both the second and fourth sets of one or more electrode elements, wherein the third frequency is lower than the first frequency and the third frequency is lower than the second frequency; wherein the alternating voltage at the first frequency, the alternating voltage at the second frequency, and the alternating voltage at the third frequency are each applied during different intervals of time; and wherein the method delivers the chemotherapy drug across a biological barrier in the target region.