Phosphorus Restriction Prevents Rapamycin-Induced Kidney Damage in Rats

There are conflicting reports about the effect or rapamycin on the kidneys. Rapamycin is known to promote phosphaturia that may be associated to renal injury.

Detailed histopathological studies were performed on the kidneys of rats with normal (control) and reduced (Nx) renal mass that were treated with rapamycin (1.3 mg/kg for 22 days) or placebo. The effect of rapamycin was also evaluated in control and Nx rats fed different amounts of phosphorus: 0.6% P (NP), 1.2% P (HP), and 0.2% P (LP). Quantitative scores of kidney lesions were obtained for interstitial nephritis (IN), tubular damage (TD), and nephrocalcinosis (NC).

When compared with placebo, rapamycin administration to Nx rats resulted in significant increases in IN (4.17 ± 0.74 vs. 1.51 ± 0.53%) and TD (14.45 ± 1.51 vs. 8.61 ± 1.83%). Rapamycin also increased NC both in control (0.86 ± 0.23 vs. 0.14 ± 0.06%) and Nx (0.86 ± 0.32 vs. 0.15 ± 0.14%) rats. In control rats receiving rapamycin, feeding HP aggravated IN (3.25 ± 0.48%), TD (22.47 ± 4.56%), and NC (3.66 ± 0.75%), while feeding LP prevented development of any renal lesions. In Nx rats treated with rapamycin, HP intake also increased IN (8.95 ± 1.94%), TD (26.86 ± 3.95%), and NC (2.77 ± 0.60%), whereas feeding LP reduced all lesions to lower levels than in rats fed NP. Rapamycin treatment increased fractional excretion of P (FEP), and an excellent correlation between scores for renal lesions and FEP was found.

Rapamycin has deleterious effects on kidney pathology causing lesions that are located mainly at tubular and tubulointerstitial level. Rapamycin-induced kidney damage is more evident in rats that already have decreased renal function and seems to be related to the phosphaturic effect of the drug. Dietary P restriction prevents kidney damage in rats treated with rapamycin.