SEVELAMER CARBONATE (Page 3 of 4)

12 CLINICAL PHARMACOLOGY

12.1 Mechanism of Action

Sevelamer carbonate for oral suspension contains sevelamer carbonate, a non-absorbed phosphate-binding cross-linked polymer, free of metal and calcium. It contains multiple amines separated by one carbon from the polymer backbone. These amines exist in a protonated form in the intestine and interact with phosphate molecules through ionic and hydrogen bonding. By binding phosphate in the gastrointestinal tract and decreasing absorption, sevelamer carbonate lowers the phosphate concentration in the serum (serum phosphorus).

12.2 Pharmacodynamics

In addition to effects on serum phosphorus levels, sevelamer hydrochloride has been shown to bind bile acids in vitro and in vivo in experimental animal models. Because sevelamer binds bile acids, it may interfere with normal fat absorption and thus may reduce absorption of fat soluble vitamins such as A, D and K. In clinical trials of sevelamer hydrochloride, both the mean total and LDL cholesterol declined by 15% to 31%; the clinical significance of this finding, which was observed after 2 weeks, is unclear. Triglycerides, HDL cholesterol, and albumin did not change.

12.3 Pharmacokinetics

A mass balance study using 14 C-sevelamer hydrochloride, in 16 healthy male and female volunteers showed that sevelamer hydrochloride is not systemically absorbed. No absorption studies have been performed in patients with renal disease.

Drug Interactions

In vivo

Sevelamer carbonate has been studied in human drug-drug interaction studies (9.6 grams once daily with a meal) with warfarin and digoxin. Sevelamer hydrochloride, which contains the same active moiety as sevelamer carbonate, has been studied in human drug-drug interaction studies (2.4 to 2.8 grams single dose or three times daily with meals or two times daily without meals) with ciprofloxacin, digoxin, enalapril, iron, metoprolol, mycophenolate mofetil, and warfarin.

Coadministered single dose of 2.8 grams of sevelamer hydrochloride in fasted state decreased the bioavailability of ciprofloxacin by approximately 50% in healthy subjects.

Concomitant administration of sevelamer and mycophenolate mofetil in adult and pediatric patients decreased the mean MPA Cmax and AUC0-12h by 36% and 26%, respectively.

Sevelamer carbonate or sevelamer hydrochloride did not alter the pharmacokinetics of enalapril, digoxin, iron, metoprolol, and warfarin when coadministered.

13 NONCLINICAL TOXICOLOGY

13.1 Carcinogenesis, Mutagenesis, Impairment of Fertility

Standard lifetime carcinogenicity bioassays were conducted in mice and rats. Rats were given sevelamer hydrochloride by diet at 0.3, 1, or 3 g/kg/day. There was an increased incidence of urinary bladder transitional cell papilloma in male rats of the high dose group (human equivalent dose twice the maximum clinical trial dose of 13 g). Mice received dietary administration of sevelamer hydrochloride at doses of up to 9 g/kg/day (human equivalent dose 3 times the maximum clinical trial dose). There was no increased incidence of tumors observed in mice.

In an in vitro mammalian cytogenetic test with metabolic activation, sevelamer hydrochloride caused a statistically significant increase in the number of structural chromosome aberrations. Sevelamer hydrochloride was not mutagenic in the Ames bacterial mutation assay.

14 CLINICAL STUDIES

The ability of sevelamer to control serum phosphorus in CKD patients on dialysis was predominantly determined from the effects of the hydrochloride salt to bind phosphate. Six clinical trials used sevelamer hydrochloride and three clinical trials used sevelamer carbonate. The sevelamer hydrochloride studies include one double-blind, placebo-controlled 2-week study (sevelamer N=24); two open-label, uncontrolled, 8-week studies (sevelamer N=220); and three active-controlled open-label studies with treatment durations of 8 to 52 weeks (sevelamer N=256). The sevelamer carbonate studies include one open-label, active-controlled, cross-over study with two 4-week treatment periods using sevelamer carbonate powder (N=31); and one randomized, parallel, open-label study using sevelamer carbonate powder (N=144) dosed once daily or sevelamer hydrochloride tablets (N=73) dosed three times daily for 24 weeks. Five of the active-controlled studies are described here (two sevelamer carbonate and three sevelamer hydrochloride studies).

14.2 Cross-Over Study of Sevelamer Carbonate Powder and Sevelamer Hydrochloride Tablets

Stage 5 CKD patients on hemodialysis were entered into a four-week sevelamer hydrochloride run-in period and 31 patients received, in random order, sevelamer carbonate powder and sevelamer hydrochloride tablets for four weeks each with no intervening washout. Study dose during the cross-over period was determined based on the sevelamer hydrochloride dose during the run-in period on a gram-per-gram basis. The phosphorus levels at the end of each of the two cross-over periods were similar. Average actual daily dose was 6 g/day divided among meals for sevelamer carbonate powder and 6.4 g/day divided among meals for sevelamer hydrochloride tablets.

14.3 Clinical Study of Sevelamer Carbonate Powder in Pediatric Patients

A clinical study with sevelamer carbonate was conducted in 101 patients 6 to 18 years of age with chronic kidney disease. This study included a washout period for patients on a phosphate binder, a 2-week, double-blind, fixed-dose period (FDP) in which patients were randomized to sevelamer carbonate (n=50) or placebo (n=51), and a 26-week, open-label, sevelamer carbonate dose titration period (DTP). Most patients were 13 to 18 years of age (73%) and had a BSA ≥1.2 m2 (84%). Approximately 78% of patients were CKD patients on dialysis.

Sevelamer carbonate significantly reduced serum phosphorus through Week 2 (primary endpoint) by an LS Mean difference of -0.90 (SE 0.27) mg/dL compared to placebo (p=0.001). A similar treatment response was observed in patients who received sevelamer carbonate during the 6-month open-label DTP. Approximately 30% of subjects reached their target serum phosphorus. The median prescribed daily dose was approximately 7 g per day during the titration period. The results of the primary efficacy endpoint were consistent by BSA subgroup. In contrast, a treatment effect was not observed in subjects with a baseline serum phosphorus below 7 mg/dL, many of whom were the subjects 6 to <13 years of age or the subjects not on dialysis (Figure 2).

Figure 2: Change in Serum Phosphorus (mg/dL) from Baseline to Week 2 by Subgroup

Figure 2. Change in serum phosphorus (mg/dL) from baseline to Week 2 by subgroup
(click image for full-size original)

[a]: LS Mean difference of Sevelamer Carbonate – Placebo, based on ANCOVA within subgroup and with treatment as fixed effect and screening BSA and baseline serum phosphorus as covariates.

14.4 Sevelamer Hydrochloride versus Active-Control, Cross-Over Study in Hemodialysis Patients

Eighty-four CKD patients on hemodialysis who were hyperphosphatemic (serum phosphorus >6.0 mg/dL) following a two-week phosphate binder washout period were randomized in a cross-over design to receive in random order sevelamer hydrochloride and active control for eight weeks each. Treatment periods were separated by a two-week phosphate binder washout period. Patients started on treatment three times per day with meals. Over each eight-week treatment period, at three separate time points the dose of sevelamer hydrochloride could be titrated up to control serum phosphorus, the dose of active control could also be altered to attain phosphorus control. Both treatments significantly decreased mean serum phosphorus by about 2 mg/dL (Table 6).

Table 6: Mean Serum Phosphorus (mg/dL) at Baseline and Endpoint
* p<0.0001, within treatment group comparison
Sevelamer Hydrochloride (N=81) Active Control (N=83)
Baseline at End of Washout8.48.0
Endpoint6.45.9
Change from Baseline at Endpoint(95% Confidence Interval)-2.0*(-2.5, -1.5)-2.1*(-2.6, -1.7)

The distribution of responses is shown in Figure 3. The distributions are similar for sevelamer hydrochloride and active control. The median response is a reduction of about 2 mg/dL in both groups. About 50% of subjects have reductions between 1 and 3 mg/dL.
Figure 3: Percentage of Patients (Y-axis) Attaining a Phosphorus Reduction from Baseline (mg/dL) at Least as Great as the Value of the X-axis

Figure 3: Percentage of Patients (Y-axis) Attaining a Phosphorus Reduction from Baseline (mg/dL) at Least as Great as the Value of the X-axis
(click image for full-size original)
Average daily sevelamer hydrochloride dose at the end of treatment was 4.9 g (range of 0 to 12.6 g).

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