For many antiviral drugs, the efflux at the luminal membrane is rate limiting, occasionally resulting in intracellular accumulation. Therefore, drugs such as cidofovir and adefovir may produce concentration-dependent renal toxicity [18]. Because tenofovir has structural similarity to these compounds (all are nucleotide analogues), its accumulation within tubular epithelial cells may interfere with renal function. Moreover, transporter expression may modulate the extent of tubular damage.