Effect of paricalcitol and cinacalcet on serum phosphate, FGF-23, and bone in rats with chronic kidney disease

Abstract

Calcimimetics activate the calcium-sensing receptor (CaR) and reduce parathyroid hormone (PTH) by increasing the sensitivity of the parathyroid CaR to ambient calcium. The calcimimetic, cinacalcet, is effective in treating secondary hyperparathyroidism in dialysis patients [chronic kidney disease (CKD 5)], but little is known about its effects on stage 3-4 CKD patients. We compared cinacalcet and paricalcitol in uremic rats with creatinine clearances "equivalent" to patients with CKD 3-4. Uremia was induced in anesthetized rats using the 5/6th nephrectomy model. Groups were 1) uremic control, 2) uremic + cinacalcet (U+Cin; 15 mg x kg(-1) x day(-1) po for 6 wk), 3) uremic + paricalcitol (U+Par; 0.16 microg/kg, 3 x wk, ip for 6 wk), and 4) normal. Unlike U+Par animals, cinacalcet promoted hypocalcemia and marked hyperphosphatemia. The Ca x P in U+Cin rats was twice that of U+Par rats. Both compounds suppressed PTH. Serum 1,25-(OH)(2)D(3) was decreased in both U+Par and U+Cin rats. Serum FGF-23 was increased in U+Par but not in U+Cin, where it tended to decrease. Analysis of tibiae showed that U+Cin, but not U+Par, rats had reduced bone volume. U+Cin rats had similar bone formation and reduced osteoid surface, but higher bone resorption. Hypocalcemia, hyperphosphatemia, low 1,25-(OH)(2)D(3), and cinacalcet itself may play a role in the detrimental effects on bone seen in U+Cin rats. This requires further investigation. In conclusion, due to its effects on bone and to the hypocalcemia and severe hyperphosphatemia it induces, we believe that cinacalcet should not be used in patients with CKD without further detailed studies.

Fig. 1.

Effect of paricalcitol and cinacalcet on plasma phosphorus in uremic rats. At the onset of uremia, rats were given vehicle (100 μl of propylene glycol), paricalcitol (0.16 μg/kg body wt ip, 3 × wk), or cinacalcet (15 mg/kg body wt daily by gavage) for 6 wk. Normal rats served as control. Results are expressed as means ± SE (*P < 0.01 vs. all other groups); n = 6 for normal, n = 14 for uremic control, n = 11 uremic + paricalcitol, and n = 12 for uremic + cinacalcet.

Fig. 2.

Effect of paricalcitol and cinacalcet on filtered load of phosphorus (FLPO₄) and urinary phosphorus excretion (Uv.P; A) and on filtered load of calcium (FLCa) and urinary calcium excretion (Uv.Ca; B) in uremic rats. At the onset of uremia, rats were given vehicle (100 μl of propylene glycol), paricalcitol (0.16 μg/kg body wt ip, 3 × wk), or cinacalcet (15 mg/kg body wt daily by gavage) for 6 wk. At the end of the study, 24-h urinary collections were analyzed. Results are the average of 2 24-h urinary collections and are expressed as means ± SE (*P < 0.001 vs. uremic control and uremic + paricalcitol, **P < 0.01 vs. uremic control and uremic + paricalcitol); n = 14 for uremic control, n = 11 uremic + paricalcitol, and n = 12 for uremic + cinacalcet.

Fig. 3.

Effect of paricalcitol and cinacalcet on serum FGF-23 in uremic rats. At the onset of uremia, rats were given vehicle (100 μl of propylene glycol), paricalcitol (0.16 μg/kg body wt ip, 3 × wk), or cinacalcet (15 mg/kg body wt daily by gavage) for 6 wk. Normal rats served as control. Results are expressed as means ± SE; n = 6 for normal, n = 14 for uremic control, n = 11 uremic + paricalcitol, and n = 12 for uremic + cinacalcet.

Fig. 4.

Effect of paricalcitol and cinacalcet on serum FGF-23 in normal and uremic rats. Normal or uremic rats were treated with 1 dose of paricalcitol (300 ng/rat ip) or cinacalcet (30 mg/rat by gavage). Blood was drawn before and 24 h after treatment. Results were analyzed by paired t-test. Each line represents an individual animal; n = 7 for normal + paricalcitol, n = 6 for normal + cinacalcet, n = 6 for uremic + paricalcitol, and n = 5 for uremic + cinacalcet. Notice the different scales used, which clearly demonstrate the differences in response between paricalcitol- and cinacalcet-treated groups, especially in uremic animals. NS, not significant.

Fig. 5.

Effect of paricalcitol and cinacalcet on serum 1,25-(OH)₂D₃ in uremic rats. At the onset of uremia, rats were given vehicle (100 μl of propylene glycol), paricalcitol (0.16 μg/kg body wt ip, 3 × wk), or cinacalcet (15 mg/kg body wt daily by gavage) for 6 wk. Normal rats served as control. Results are expressed as means ± SE; n = 6 for normal, n = 14 for uremic control (UC), n = 11 uremic + paricalcitol (U+Par), and n = 12 for uremic + cinacalcet (U+Cin).

Fig. 6.

von Kossa and tetrachrome staining of 5-μm sections of proximal tibial metaphyses from normal and uremic rats treated with vehicle, paricalcitol, or cinacalcet. At the onset of uremia, rats were given vehicle (100 μl of propylene glycol), paricalcitol (0.16 μg/kg body wt ip, 3 × wk), or cinacalcet (15 mg/kg body wt daily by gavage) for 6 wk. Yellow arrows indicate osteoblast surface and red arrows indicate osteoclast surface.

Fig. 7.

Fluorochrome bases analysis of 8-μm sections of proximal tibial metaphyses from normal and uremic rats treated with vehicle, paricalcitol, or cinacalcet. At the onset of uremia, rats were given vehicle (100 μl of propylene glycol), paricalcitol (0.16 μg/kg body wt ip, 3 × wk), or cinacalcet (15 mg/kg body wt daily by gavage) for 6 wk. Normal rats served as control. On days 10 and 2 before death, the rats received an injection of calcein (10 mg/ml in PBS, 0.1 ml/100 g body wt). Bright red staining represents the osteoid seam (undermineralized tissue). Single and double green fluorescent labels on the bone surface represent increased bone formation.