Renal effects of trimethoprim in ciclosporin- and azathioprine-treated kidney-allografted patients.

Sixteen stable renal transplant patients on chronic ciclosporin (CS, n = 8) or azathioprine (AZA, n = 8) treatment were given trimethoprim (TMP) 160 mg twice a day for 7 days. This TMP dose did not affect glomerular filtration, as 99mTc-DTPA clearance was unchanged in both groups. TMP did, however, increase serum creatinine and reduce creatinine clearance in all patients. This effect was most pronounced in CS-treated patients. We conclude that even at the moderate dosage, as employed presently, TMP blocks tubular secretion of creatinine. This route for creatinine excretion is quite important as the clearance ratio between creatinine and DTPA averaged 1.21 in CS-treated patients and 1.05 in AZA-treated patients before TMP treatment. During TMP the ratio was reduced to 0.95 and 0.99, respectively, suggesting a complete cessation of tubular creatinine secretion by TMP. TMP did not, however, significantly affect other markers of renal tubular function.