Enhancement of high dose cyclosporin A toxicity by frusemide.

Adult Sprague-Dawley rats were given cyclosporin A (CyA), frusemide (Fr) or both drugs daily for 14 days. The doses of CyA (50 mg/kg) and Fr (5 mg/kg) were approximately 3-6 times and twice respectively those used in man. Fr on its own produced a diuresis lasting approximately 3 hr. This was characterized by a 10-fold increase in urine flow rate, a 40-fold increase in the rate of sodium excretion, and by 2- and 4-fold increases in urea and creatinine clearance rates, respectively. In addition, there was a doubling in urinary N-acetyl-beta-D-glucosaminidase (NAG) activity. After 4 days of combination treatment with CyA and Fr, the diuretic-induced increases in urine flow rate, sodium excretion and urinary NAG activity were similar to those following frusemide alone. However, urea and creatinine clearances did not increase during the diuresis. Fr itself did not impair renal function, but rats receiving only CyA did show elevations in serum urea and creatinine, with reductions in clearance rates, which progressed with time. There was also an increase in NAG enzymuria. When the two drugs were exhibited together, renal function was more severely impaired. All animals given CyA showed proximal renal tubular cell vacuolation: in half the damage was confined to the straight segment, while the rest showed additional severe convoluted segment change. Renal function was most abnormal in those rats in which both segments were affected. All animals given both drugs showed both straight and convoluted tubular abnormalities and a 2-fold increase in serum CyA levels. CyA-induced disturbances in hepatic function and lymphoid tissue atrophy were unaffected by the addition of Fr, nor did Fr affect the immunosuppressive action of CyA.