A 72-year-old patient after operation of holecystectomia was prescribed gentamicyn (80 mg every 8 hours) and cephalothin (2 g every 6 hours) because of fever. In 10 days there was an increase of creatinine up to 310 mu mol/L. BP - 130/80 mm Hg, daily quantity of the urine is 1200 ml. Urine tests are without pathology. Ultrasound: the size of kidneys is normal. What is the most probable reason for renal failure?