Study Products: Three study products administered sequentially and estimated to approximate TFV 1% gel (Product A), 3 times the concentration and dose of Product A (Product B), and 2 times concentration and dose of Product B (Product C) as defined below. At JHU only, the study product will also be radiolabeled with Technetium-99m-DTPA (99mTc-DTPA) for SPECT/CT imaging. Take-home enemas consisting of normal saline (NS) or ½ normal saline (½ NS) will be self-administered at home.