Cyclosporin does not inhibit the tubular secretion of creatinine

Background. The immunosuppressive drug cyclosporin is known to impair renal function. The degree of renal dysfunction is usually estimated from the clearance of creatinine (C-cr). Theoretically however, a fall in C-cr can be caused by a decrease of GFR, an inhibition of the tubular secretion of creatinine, or the combination of both. CsA has convincingly been shown to decrease GFR, but detailed information on the effects of CsA on tubular secretion of creatinine is lacking. Methods. We performed two studies to investigate the influence of CsA on tubular creatinine secretion. In study A we simultaneously measured C-cr and GFR (using inulin) immediately before and 4 weeks after cessation of CsA therapy in 17 renal transplant patients. In study B, the rise in serum creatinine after administration of cimetidine, which blocks the tubular secretion of creatinine, was compared in renal transplant patients treated with either CsA (in whom secretion might already be inhibited) or azathioprine. Results. Study A: After cessation of CsA there was an increase of GFR (54 +/- 15 vs 63 + 16 ml/min/1.73 m(2); P<0.01) and of C-cr (71 +/- 21 vs 82 +/- 23 ml/min/1.73; m(2). P<0.01), but the ratio between C-cr and GFR (a measure of the relative contribution of tubular secretion to the clearance of creatinine) did not change significantly (1.33 +/- 0.21 vs 1.32 +/- 0.30). Study B: In nine couples of patients matched for GFR the relative rises in serum creatinine after administration of cimetidine were 26 +/- 21% and 22 +/- 7% for the CsA and azathioprine treated patients respectively (NS). Conclusion. CsA does not substantially inhibit the tubular secretion of creatinine. A rise in serum creatinine after administration of CsA can thus be attributed completely to a fall in GFR.