**4. Renal filtration**

Renal filtration of small molecule drugs is a phenomenon that needs to be overcome for efficient therapeutic use. Owing to their small size, aptamers also undergo this challenge. An aptamer of 6–30 kDa mass has a size of <5 nm [34]. When an unmodified aptamer is administered intravenously, even using stabilizing backbone modifications, they are subjected to rapid excretion through renal filtration, hence, reduced circulation time. To overcome this challenge, aptamers are functionalized with bulky moieties *viz.*, polyethylene glycol (PEG), liposomes, proteins, cholesterol, organic or inorganic nanomaterials, or multimerized to reach a mass above the threshold of glomerulus cut-off (30–50 kDa) [34].
