Activation of the Calcium Receptor by Calcimimetic Agents Is Preserved Despite Modest Attenuating Effects of Hyperphosphatemia : Journal of the American Society of Nephrology

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Activation of the Calcium Receptor by Calcimimetic Agents Is Preserved Despite Modest Attenuating Effects of Hyperphosphatemia

Goodman, William G.1; Ward, Donald T.2; Martin, Kevin J.3; Drayer, Debra1; Moore, Carol1; Xu, Jiahong1; Lai, James1; Chon, Yun1; Nemeth, Edward. F.4

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JASN 33(1):p 201-212, January 2022. | DOI: 10.1681/ASN.2021060825
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Abstract

Calcimimetic drugs are used commonly to manage secondary hyperparathyroidism (sHPT) among patients with CKD who require treatment with dialysis.1 Cinacalcet (Sensipar, Mimpara) was the first of these drugs to be approved for use clinically.2,3 It is an oral medication given once daily. A second oral calcimimetic agent, evocalcet (Orkedia), was approved more recently by regulatory authorities in Japan.4,5 It too is given using once-daily doses. In contrast, etelcalcetide (Parsabiv) is a synthetic peptide first approved for use clinically in 2017,6 which is available only as a parenteral formulation administered intravenously at the end of thrice-weekly hemodialysis sessions. All three calcimimetic agents function as allosteric activators of the calcium receptor (CaR).7,8 As such, they increase the sensitivity of the CaR to extracellular calcium ions (Ca2+) and inhibit the release of parathyroid hormone (PTH) from the parathyroid glands. Results from clinical trials demonstrate that calcimimetic agents effectively lower plasma PTH levels regardless of disease severity among those with sHPT.3,5,6,910111213

Variations in the concentration of Ca2+ in blood are recognized generally as the principal determinant of the level of CaR activation in parathyroid tissue, which ultimately modulates PTH release.14 It is known, however, that other agents interact with the CaR and affect its level of activation. These include a variety of di-valent or tri-valent cations, organic polycations such as polyamines and aminoglycosides, and certain l-amino acids.7,15 Negatively charged anions such as sulfate and chloride can also affect CaR activation.16,17 One anion especially relevant in the setting of sHPT is inorganic phosphate.

Hyperphosphatemia has long been invoked as a cause of sHPT due to CKD and as a factor that can aggravate disease severity.18,19 The mechanisms responsible, however, have not been delineated clearly. Hyperphosphatemia and exposure to high levels of phosphorus in vitro have been reported variously, but not consistently, to affect PTH secretion in some, but not all, experimental models.202122 Indeed excess phosphorus has been reported to increase the release of PTH into incubation media from intact parathyroid tissue slices but not from dispersed parathyroid cells.23 It thus remains uncertain whether the elevated serum phosphorus levels seen commonly during in vivo studies of humans or experimental animals with impaired kidney function affect parathyroid gland function directly or whether such changes are mediated through alterations in calcium and vitamin D metabolism systemically that affect parathyroid gland function indirectly.24,25

Recent in vitro observations suggest that anionic phosphate can act directly on the CaR to modify receptor activity. Accordingly, PTH release was found to be greater, and CaR-mediated signal transduction was attenuated, when isolated mouse or human parathyroid glands were peri-fused with media containing high levels of phosphorus.26 These effects have been attributed to the binding of phosphate anions at specific sites within the CaR that modify conformation of the receptor.26

The findings from these recent in vitro studies offer a credible molecular mechanism to account for direct, phosphate-mediated alterations in CaR activation that facilitate PTH release under conditions where serum phosphorus concentrations are elevated within the range seen clinically.26 Indeed, some clinicians and providers of dialysis in the United States, citing these in vitro results,26 have suggested that calcimimetic agents are ineffective and should not be used to manage sHPT in those undergoing dialysis if serum phosphorus concentrations are elevated or if values exceed certain threshold levels. To determine whether hyperphosphatemia per se might affect CaR activation by calcimimetic agents and thus modify their therapeutic efficacy in a clinically meaningful way, the reductions in plasma PTH levels achieved in subjects with sHPT and varying degrees of hyperphosphatemia were assessed using data gathered in large clinical trials with either etelcalcetide or cinacalcet.

Methods

Study Population

The current report presents post hoc analyses of data obtained from adult subjects who participated in one of four clinical trials undertaken to assess the safety and efficacy of etelcalcetide or cinacalcet for the treatment of sHPT among subjects with CKD undergoing hemodialysis.3,6 Two studies were completed with each calcimimetic agent.

Biochemical results from subjects enrolled in two prospective, randomized, double-blinded, placebo-controlled, phase 3 clinical trials designed to assess the safety and efficacy of etelcalcetide in persons with sHPT were evaluated initially.6 In these trials, 503 subjects were assigned to treatment with etelcalcetide and received at least one dose of study drug; the duration of treatment was 26 weeks. Both studies were done during the years 2012 and 2013. All study participants had biochemical evidence of sHPT as judged by baseline, or pretreatment, plasma PTH levels >400 pg/ml using the average of two determinations obtained several days or a few weeks apart.6 Blood samples for all biochemical determinations throughout each study were obtained on the same day of the week in all subjects before scheduled hemodialysis procedures.

Treatment with etelcalcetide or placebo in both trials was initiated with doses of 5 mg given by intravenous injection at the end of thrice-weekly hemodialysis sessions. Serum calcium and phosphorus levels were measured at baseline and at weekly intervals during the first 20 weeks of study, then biweekly thereafter. Serum calcium concentrations also were measured 1 week after any increase in the dose of study drug to identify subjects who might experience untoward reductions in serum calcium after dosage adjustments. Plasma PTH levels were measured at baseline and every 2 weeks thereafter. Doses of etelcalcetide were raised as needed at weeks 5, 9, 13, and 17 in increments of 2.5 or 5.0 mg, using the serum total calcium value obtained 1 week previously to monitor safety, to a maximum thrice-weekly dose of 15 mg. Doses were titrated to achieve a PTH level <300 pg/ml unless albumin-corrected total serum calcium concentrations were <7.5 mg/dl, symptoms of hypocalcemia developed, or plasma PTH levels were <100 pg/ml on two consecutive study visits.6

Efficacy was assessed in both etelcalcetide trials using the mean of all PTH determinations during the last 6 weeks of study. The combined results from these trials were reported previously, and additional demographic and biochemical details are provided therein.6 For the current analyses, the percentage change in plasma PTH from baseline was evaluated at each of six 4-week intervals of follow-up and for the last 2 weeks of study.

Results obtained from adult subjects with sHPT receiving hemodialysis regularly who participated in either of two prospective, randomized, placebo-controlled clinical trials with cinacalcet also were examined.3 These trials were done between late 2001 and early 2003; both lasted 26 weeks and thus were the same length as the etelcalcetide trials. All subjects had biochemical evidence of sHPT as judged by baseline plasma PTH levels >300 pg/ml using the mean of two or more screening measurements done several days or a few weeks apart. For these studies, 365 subjects were assigned to treatment with cinacalcet and received at least one dose of study drug.3

The initial dose of cinacalcet or placebo in each trial was 30 mg given orally once daily. Doses were increased sequentially every 3 weeks during a 12-week dose-titration phase to 60, 90, 120, or 180 mg once daily with an objective of achieving plasma PTH levels <250 pg/ml. Increases in dose were permitted if PTH levels remained >200 pg/ml and if serum calcium concentrations were 7.8 mg/dl or above. Doses were not increased if subjects experienced symptoms of hypocalcemia, if serum calcium levels were <7.8 mg/dl, or if an adverse event precluded an increase in dose. Doses were reduced if PTH levels were <100 pg/ml on three consecutive study visits or if patients reported an adverse event requiring a reduction in dose.3

Plasma PTH levels and serum calcium and phosphorus concentrations were measured at baseline and at weekly intervals for the first 12 weeks of study then biweekly thereafter. Additional calcium measurements were obtained 7 days after upward adjustments to the dose of cinacalcet to monitor safety. Efficacy was assessed during the final 14 weeks of study using the mean of all determinations of plasma PTH and serum calcium during this portion of the study as reported elsewhere.3 Additional demographic and biochemical information are available in that publication.3 For the current analyses, the percentage change in plasma PTH from baseline was assessed at each of six 4-week intervals of follow-up and during the final 2 weeks of study.

Study Design

The current analyses were done to determine whether varying degrees of hyperphosphatemia modify the effect of calcimimetic agents to lower plasma PTH levels among patients with sHPT undergoing hemodialysis regularly. As such, only results from subjects receiving a calcimimetic agent in placebo-controlled clinical trials were evaluated, and the PTH-lowering effects of treatment with either etelcalcetide or cinacalcet were compared in individuals with hyperphosphatemia that differed in severity. Subjects were categorized initially according to baseline, or pretreatment, serum phosphorus concentrations that were > or ≤8.0 mg/dl, > or ≤7.0 mg/dl, or > or ≤5.5 mg/dl, respectively; the percentage change in plasma PTH from baseline was then determined at each interval of follow-up over 26 weeks for those in each category. The percentage change in PTH from baseline at each interval of follow-up was subsequently compared for subjects with a prevailing serum phosphorus level at each follow-up interval that was either above or below each of the three specified phosphorus thresholds, an approach that considered change in the severity of hyperphosphatemia over time as a potential modifier of the therapeutic response to calcimimetic agents.

As stated already, two clinical trials were done with each calcimimetic agent. The reductions in plasma PTH levels at each interval of follow-up did not differ materially between the two trials with either agent. Accordingly, the integrated results from both trials with each agent were used to determine whether hyperphosphatemia modifies the PTH-lowering effect of either etelcalcetide or cinacalcet and whether any such differences are clinically meaningful.

Biochemical Determinations

Serum total calcium concentrations, adjusted for albumin, and serum phosphorus levels were measured by automated methods as reported elsewhere.3,6 In clinical trials with etelcalcetide, plasma PTH levels were measured using a dual antibody immunoradiometric assay (Advia Centaur, Covance, Indianapolis, IN).6 In earlier studies with cinacalcet, PTH determinations were done using another dual antibody immunoradiometric assay (Allegro PTH, Nichols Institute Diagnostics, San Juan Capistrano, CA), the assay employed most widely on a global basis when the clinical trials with cinacalcet were done.3

Statistical Analyses

Baseline demographic and biochemical results are presented using descriptive statistics reporting mean and SD for continuous variables and count and percentage values for categoric variable. The results for the percentage change from baseline in plasma PTH levels at each interval of follow-up during 26-week clinical trials with either etelcalcetide or cinacalcet are presented as least square mean (LSM) (SEM) differences and were assessed using a mixed effect model for repeated measures (MMRM).27 The model included baseline PTH, time in months as defined by interval of follow-up, baseline serum phosphorus as a categoric variable, serum phosphorus over time as a categoric variable, interaction between time and baseline phosphorus, and interaction between time and serum phosphorus level over time. Mean serum phosphorus levels during all intervals of follow-up represent the average of all determinations obtained during the interval, whether weekly or biweekly, in each subject. Baseline serum phosphorus and serum phosphorus over time were dichotomized as binary variables using three cut-points: >8.0 versus ≤8.0, >7.0 versus ≤7.0, and >5.5 versus ≤5.5 mg/dl, respectively. Each dichotomized phosphorus variable was used in a separate MMRM model. The LSM, SEM, difference of LSM and SEM between phosphorus groups, and associated P value at specified time intervals were obtained. For all statistical tests, P values <0.05 were considered as statistically significant. All statistical analyses were done using SAS Version 9.4.

Results

Demographic Characteristics and Biochemical Results at Baseline

Subjects enrolled in clinical trials with etelcalcetide were, on average, 4 years older than those who participated in earlier clinical trials with cinacalcet (Table 1). The percentage of subjects <65 years of age was nominally lower, whereas the percentage of subjects 65 years of age or older, or 75 years of age or older, was nominally higher, in studies with etelcalcetide compared with those with cinacalcet. Statistical testing of these differences was not done. The proportion of women and men who participated in clinical trials with each calcimimetic did not differ although the studies were done more than a decade apart (Table 1). In the more recent clinical trials with etelcalcetide, the proportion of study participants who were Black was lower compared with the earlier trials with cinacalcet. The proportion of Asian subjects enrolled in trials with either calcimimetic agent did not differ substantially, but the numbers overall were small (Table 1).

Table 1. - Demographic and biochemical results at baseline for subjects who participated in clinical trials with etelcalcetide or cinacalcet
Characteristic Etelcalcetide
(n=503)
Cinacalcet
(n=365)
Age (years), mean (SD) 58.3 (14.6) 54.3 (14.4)
Age group, n (%)
 <65 years 326 (64.8) 263 (71.2)
 ≥65 years 177 (35.2) 102 (27.9)
 ≥75 years 72 (14.3) 34 (9.3)
Sex (%)
  Male 61.2 61.4
 Female 38.8 38.6
Race (%)
 Black 26.8 35.1
 White 66.0 56.4
 Asian 3.6 2.5
 Other 3.6 6.0
Height (cm), mean (SD) 169.2 (10.8) 169.3 (10.5)
Weight (kg), mean (SD) 82.6 (22.9) 76.3 (21.2)
BMI (kg/m2), mean (SD) 28.9 (7.8) 26.4 (6.5)
Dialysis vintage, n (%)
 0–≤1 59 (11.7) 33 (9.0)
 >1–≤5 242 (48.1) 171 (46.8)
 >5 202 (40.2) 161 (44.1)
Dialysate calcium (mEq/L), n (%)
 <2.5 37 (7.4) 173 (47.4)
 ≥2.5 465 (92.4) 192 (52.6)
BP (mm Hg), mean (SD)
 Systolic 130.0 (23.5) 144.4 (24.4)
 Diastolic 70.5 (14.5) 79.4 (14.9)
Plasma PTH (pg/ml), mean (SD) 849.2 (494.9) 643.2 (356.7)
Serum calcium (mg/dl), mean (SD) 9.64 (0.66) 9.92 (0.76)
Serum phosphorus (mg/dl)
 Mean (SD) 5.84 (1.58) 6.23 (1.65)
 >5.5 n (%) 270 (53.7) 240 (65.8)
 >7.0 n (%) 106 (21.1) 97 (26.6)
 >8.0 n (%) 47 (9.3) 46 (12.6)
Age group ≥65 years includes subjects who were ≥75 years of age; the denominator is the total N at the head of each column.

Mean values for body weight and body mass index at baseline were higher among subjects enrolled in studies with etelcalcetide compared with those with cinacalcet. In contrast, mean values for BP were lower among participants in trials with etelcalcetide (Table 1). There were no substantive differences in dialysis vintage between subjects treated with either calcimimetic agent. The percentage of study participants managed with a dialysate calcium concentration <2.5 mEq/L was much lower, however, in the trials with etelcalcetide (Table1).

Biochemical Results at Baseline

Mean plasma PTH levels at baseline were higher among subjects who participated in clinical trials with etelcalcetide compared with those with cinacalcet. Mean values for serum total calcium and for serum phosphorus concentrations at baseline were slightly lower for subjects who participated in clinical trials with etelcalcetide compared with those with cinacalcet (Table 1).

Response to Treatment with Etelcalcetide or Cinacalcet

Plasma PTH levels, expressed as a percentage of baseline values, declined progressively during 26 weeks of treatment with etelcalcetide independent of the degree of hyperphosphatemia at baseline, and values were incrementally lower at each interval of follow-up (Figure 1, Table 2). Progressive reductions in plasma PTH levels at each interval of follow-up also were seen during 26-week trials with cinacalcet, and these too did not differ by the severity of hyperphosphatemia at baseline except for several comparisons at selected intervals of follow-up where the percentage decrease in plasma PTH was greater among subjects with higher serum phosphorus levels at baseline (Figure 2, Table 3).

F1
Figure 1.:
Serum phosphorus levels at baseline do not alter the PTH-lowering effect etelcalcetide. The percentage change in plasma PTH from baseline at each interval of follow-up in 26-week clinical trials with etelcalcetide given thrice weekly among subjects with baseline, or pretreatment, serum phosphorus concentrations that were either above (solid circles) or below (open circles) one of three specified threshold values for serum phosphorus. Error bars have not been included for purposes of clarity, but error estimates and statistical assessments are provided in Table 2.
F2
Figure 2.:
Serum phosphorus levels at baseline do not alter the PTH-lowering effect of cinacalcet. The percentage change in plasma PTH from baseline at each interval of follow-up in 26-week clinical trials with cinacalcet given once daily among subjects with baseline, or pretreatment, serum phosphorus concentrations that were either above (solid circles) or below (open circles) one of three specified threshold values for serum phosphorus. Error bars have not been included for purposes of clarity, but error estimates and statistical assessments are provided in Table 3.
Table 2. - The percentage change in plasma PTH levels from baseline by serum phosphorus concentration at baseline and by category of serum phosphorus as determined at each interval of follow-up in 26-week clinical trials with etelcalcetide given thrice weekly
Variable Interval of Follow-Up (weeks)
1–4 5–8 9–12 13–16 17–20 21–24 25–26
Baseline phosphorus
 >8.0 mg/dl −16.4 (3.4) −30.3 (4.1) −41.4 (4.4) −48.1 (4.5) −57.4 (4.4) −57.1 (4.8) −58.5 (5.3)
 ≤8.0 mg/dl −12.0 (2.8) −31.1 (3.0) −41.8 (2.7) −47.4 (2.5) −50.0 (2.5) −50.3 (2.3) −53.1 (2.5)
 >7.0 mg/dl −15.7 (2.2) −30.0 (2.8) −39.8 (2.9) −45.9 (3.0) −52.9 (3.0) −52.9 (3.2) −54.2 (3.4)
 ≤7.0 mg/dl −12.5 (1.9) −32.0 (2.1) −42.5 (2.1) −50.5 (2.1) −53.1 (2.1) −52.5 (2.1) −52.8 (2.3)
 >5.5 mg/dl −16.5 (1.5) −31.5 (1.7) −43.2 (1.8) −48.7 (1.8) a −53.1 (1.8) −56.0 (2.0) −57.1 (2.1)
 ≤5.5 mg/dl −17.0 (1.8) −35.9 (2.1) −47.2 (2.1) −54.8 (2.1) a −57.6 (2.1) −55.5 (2.2) −58.4 (2.4)
Phosphorus over time
 >8.0 mg/dl −8.7 (4.5) a −26.5 (4.8) −35.6 (4.7) a −41.2 (4.3) b −47.6 (4.3) b −46.3 (3.9) c −50.2 (4.5) a
 ≤8.0 mg/dl −19.7 (2.1) a −34.8 (2.4) −47.6 (2.4) a −54.3 (2.4) b −59.7 (2.4) b −61.0 (2.6) c −61.4 (2.8) a
 >7.0 mg/dl −8.3 (2.7) c −26.8 (3.1) a −34.4 (3.1) c −43.4 (3.0) b −49.0 (3.0) b −46.5 (2.9) c −46.2 (3.2) c
 ≤7.0 mg/dl −19.9 (1.5) c −35.1 (1.8) a −47.8 (1.7) c −53.1 (1.8) b −56.9 (1.7) b −58.9 (1.9) c −60.8 (2.0) c
 >5.5 mg/dl −11.8 (1.8) c −28.9 (2.0) c −38.7 (1.9) c −45.8 (1.9) c −52.4 (1.9) b −50.4 (1.9) c −53.7 (2.1) c
 ≤5.5 mg/dl −21.7 (1.4) c −38.6 (1.5) c −51.7 (1.5) c −57.6 (1.5) c −58.3 (1.5) b −61.1 (1.6) c −61.9 (1.8) c
Values are LSM (SEM).
aP<0.05.
bP<0.01.
cP<0.001.

Table 3. - The percentage change in plasma PTH levels from baseline by serum phosphorus concentration at baseline and by category of serum phosphorus as determined at each interval of follow-up in 26-week clinical trials with cinacalcet given once daily
Variable Interval of follow-up (weeks)
1–4 5–8 9–12 13–16 17–20 21–24 25–26
Baseline phosphorus
 >8.0 mg/dl −17.2 (4.3) −33.5 (4.9) −38.2 (5.8) −46.8 (7.2) −47.5 (6.5) −49.6 (6.5) −38.9 (7.0)
 ≤8.0 mg/dl −13.6 (2.8) −29.2 (3.0) −37.8 (3.5) −35.0 (4.5) −41.2 (3.5) −37.1 (3.4) −40.3 (3.0)
 >7.0 mg/dl −17.1 (3.0) −33.4 (3.4) −42.9 (4.0) −49.0 (4.9) a −51.8 (4.4) a −50.3 (4.5) −42.9 (4.7)
 ≤7.0 mg/dl −18.6 (2.4) −31.3 (2.4) −36.4 (2.9) −34.5 (3.7) a −41.3 (3.0) a −40.0 (3.1) −42.4 (3.3)
 >5.5 mg/dl −23.7 (2.0) −36.8 (2.2) a −43.8 (2.5) a −44.9 (3.2) −48.6 (2.8) a −46.9 (2.9) −45.8 (3.0)
 ≤5.5 mg/dl −17.0 (2.8) −27.7 (3.0) a −34.1 (3.6) a −41.2 (4.6) −38.8 (3.9) a −42.6 (4.0) −46.4 (4.1)
Phosphorus over time
 >8.0 mg/dl −7.7 (4.2) b −26.7 (4.5) −34.5 (5.6) −32.0 (7.2) a −39.9 (5.7) −34.7 (5.6) b −32.1 (6.6) a
 ≤8.0 mg/dl −23.1 (2.7) b −36.0 (2.9) −41.5 (3.3) −49.8 (4.1) a −48.8 (3.5) −52.0 (3.5) b −47.1 (3.7) a
 >7.0 mg/dl −14.3 (2.9) a −29.7 (3.2) −35.7 (3.9) a −33.3 (5.0) b −44.1 (3.8) −40.3 (4.0) a −36.7 (4.5) a
 ≤7.0 mg/dl −21.5 (2.1)* −35.0 (2.1) −43.6 (2.5) a −50.2 (3.1) b −49.0 (2.7) −50.0 (2.8) a −48.7 (2.9) a
 >5.5 mg/dl −13.2 (2.2) c −27.9 (2.3) c −33.9 (2.8) c −38.3 (3.7) a −39.3 (2.9) b −40.1 (3.0) b −41.1 (3.2) b
 ≤5.5 mg/dl −27.5 (2.1) c −36.6 (2.1) c −44.1 (2.4) c −47.8 (3.2) a −48.2 (2.5) b −49.4 (2.8) b −51.1 (3.0) b
Values are LSM (SEM).
aP<0.05.
bP<0.01.
cP<0.001.

In contrast, the percentage reduction in plasma PTH from baseline was consistently less at each interval of follow-up during 26 weeks of treatment with etelcalcetide among subjects with serum phosphorus concentrations that were above each of three prespecified threshold values compared with those with serum phosphorus levels that were below each of these thresholds (Figure 3, Table 2). Similar results were observed among subjects treated with cinacalcet in trials that were done more than a decade ago (Figure 4, Tables 3 and 4). Differences in calcimimetic dose did not account for the smaller percentage decreases in plasma PTH levels for subjects with serum phosphorus levels that were above rather than below each of three threshold values (Tables 5 and 6). Indeed, the doses of etelcalcetide or cinacalcet tended to be higher overall among subjects in the upper stratum for serum phosphorus at each phosphorus threshold evaluated with some differences being significant (Tables 5 and 6).

F3
Figure 3.:
The decrease in plasma PTH is less at each interval of follow-up among subjects with serum phosphorus levels above rather than below each of three specified thresholds. The percentage change in plasma PTH from baseline for subjects with serum phosphorus concentrations that were either above (solid circles) or below (open circles) one of three specified threshold values for serum phosphorus at each interval of follow-up in 26-week clinical trials with etelcalcetide given thrice weekly. Error bars have not been included for purposes of clarity, but error estimates and statistical assessments are provided in Table 2.maller decreases
F4
Figure 4.:
The decrease in plasma PTH is less at each interval of follow-up among subjects with serum phosphorus levels above rather than below each of three specified thresholds. The percentage change in plasma PTH from baseline for subjects with serum phosphorus concentrations that were either above (solid circles) or below (open circles) one of three specified threshold values for serum phosphorus at each interval of follow-up in 26-week clinical trials with cinacalcet given once daily. Error bars have not been included for purposes of clarity, but error estimates and statistical assessments are provided in Table 3.
Table 4. - LSM percentage changes from baseline values for plasma PTH levels at each interval of follow-up during 26 weeks of treatment with etelcalcetide or cinacalcet
Interval of Follow-Up Cinacalcet Etelcalcetide Difference P value Cinacalcet Etelcalcetide Difference P Value
Phosphorus threshold >8.0 mg/dl ≤8.0 mg/dl
 1–4 −5.6531 −10.1709 4.5178 0.4495 −22.8283 −19.9314 −2.8959 0.1247
 5–8 −26.7660 −27.6595 0.8934 0.8883 −34.1349 −36.0511 1.9161 0.3768
 9–12 −31.2044 −38.1500 6.9456 0.3096 −41.0753 −48.0792 7.0039 0.0034
 13–16 −31.2054 −42.4974 11.2920 0.0551 −48.3005 a −56.4653 a 8.1647 a 0.0008 a
 17–20 −37.9316 −50.0757 12.1441 0.0558 −48.4937 a −59.7635 a 11.2698 a <0.0001 a
 21–24 −32.8027 a −48.2502 a 15.4476 a 0.0113 a −50.0941 a −62.4424 a 12.3483 a <0.0001 a
 25–26 −29.1272 a −51.2249 a 22.0977 a 0.0027 a −48.2943 a −60.4776 a 12.1833 a <0.0001 a
Phosphorus threshold >7.0 mg/dl ≤70.0 mg/dl
 1–4 −11.6684 −10.5382 −1.1332 0.7651 −22.2604 −19.3156 −2.9447 0.1351
 5–8 −28.8359 −27.7495 −1.0864 0.8004 −33.9189 −36.0667 2.1479 0.3392
 9–12 −33.1086 −35.5063 2.3977 0.6066 −42.1485 a −49.5614 a 7.4149 a 0.0023 a
 13–16 −42.1486 −44.2285 2.0799 0.6320 −46.1415 a −55.3716 a 9.2301 a 0.0002 a
 17–20 −41.8348 a −50.2693 a 8.4345 a 0.0628 a −47.1485 a −58.7358 a 11.5873 a <0.0001 a
 21–24 −39.045 a −47.7841 a 8.7796 a 0.0569 a −47.7986 a −60.5913 a 12.7927 a <0.0001 a
 25–26 −34.2415 a −47.6846 a 13.4431 a 0.0111 a −48.5905 a −60.9266 a 12.3361 a <0.0001 a
Phosphorus threshold >5.5 mg/dl ≤5.5 mg/dl
 1–4 −13.0684 −11.9729 −1.0955 0.6408 −27.8351 −22.0802 −5.8549 0.0159
 5–8 −29.1779 −28.3873 −0.7906 0.7729 −36.9513 −39.1445 2.1932 0.3954
 9–12 −34.7117 −38.4548 3.7431 0.2130 −45.0769 −52.0865 a 7.0096 a 0.0092 a
 13–16 −41.4667 −45.8840 4.4173 0.1336 −48.9638 −57.9175 a 8.9537 a 0.0008 a
 17–20 −40.3514 a −52.1941 a 11.8427 a 0.0001 a −48.8464 a −58.6222 a 9.7758 a 0.0005 a
 21–24 −39.3696 a −51.2943 a 11.9247 a 0.0002 a −49.7453 a −61.0547 a 11.3094 a 0.0002 a
 25–26 −39.3919 a −54.4785 a 14.8866 a <0.0001 a −51.6663 a −61.7764 a 10.1101 a 0.0023 a
Results are presented separately for subjects with serum phosphorus concentrations above or below one of three threshold values used to categorize individuals by degree of hyperphosphatemia. Probability values represent comparisons at each interval of follow-up between subjects treated with etelcalcetide or cinacalcet.
aDifferences between treatments that are statistically significant.

Table 5. - The mean thrice-weekly dose of etelcalcetide at each interval of follow-up during 26 weeks of treatment for subjects with serum phosphorus concentrations above or below one of three threshold values used to categorize individuals by degree of hyperphosphatemia
Interval of Follow-Up Mean Dose (SD) of Etelcalcetide by Serum Phosphorus Category (mg/dl)
>8.0 mg/dl,
P value
≤8.0 mg/dl,
P value
>7.0 mg/dl,
P value
≤7.0 mg/dl,
P value
>5.5 mg/dl,
P value
≤5.5 mg/dl,
P value
1–4 4.9 (0.5)
0.0549
5.0 (0.3) 5.0 (0.3)
0.7400
5.0 (0.3) 5.0 (0.2)
0.4700
5.0 (0.4)
5–8 8.2 (2.6)
0.0656
7.2 (2.5) 8.0 (2.5) a
0.0133
7.1 (2.5) a 7.7 (2.6) a
0.0018
7.0 (2.5) a
9–12 9.9 (4.1)
0.1160
8.4 (3.9) 9.8 (4.4) a
0.0185
8.3 (3.8) a 8.9 (4.1)
0.1037
8.3 (3.8)
13–16 9.0 (4.3)
0.7136
8.6 (4.3) 9.0 (4.2)
0.5500
8.6 (4.3) 8.8 (4.4)
0.6121
8.6 (4.2)
17–20 9.9 (4.8)
0.2288
8.7 (4.4) 9.9 (4.2)
0.0510
8.8 (4.4) 9.2 (4.4)
0.1242
8.5 (4.4)
21–24 8.0 (4.5)
0.6929
8.4 (4.4) 8.2 (4.4)
0.8037
8.4 (4.4) 8.7 (4.4)
0.2080
8.1 (4.4)
25–26 7.5 (4.2)
0.3106
8.5 (4.5) 8.0 (4.3)
0.5593
8.5 (4.5) 8.8 (4.4)
0.2866
8.2 (4.4)
Probability values represent comparisons between the doses of either etelcalcetide or cinacalcet at each interval of follow-up among subjects with varying degrees of hyperphosphatemia.
aDifferences that are statistically significant.

Table 6. - The mean daily dose of cinacalcet at each interval of follow-up during 26 weeks of treatment for subjects with serum phosphorus concentrations above or below one of three threshold values used to categorize individuals by degree of hyperphosphatemia
Interval of Follow-Up Mean Dose (SD) of Cinacalcet by Serum Phosphorus Category (mg/dl)
>8.0 mg/dl,
P value
≤8.0 mg/dl,
P value
>7.0 mg/dl,
P value
≤7.0 mg/dl,
P value
>5.5 mg/dl,
P value
≤5.5 mg/dl,
P value
1–4 35.9 (3.8)
0.4469
35.5 (3.3) 36.2 (3.1) a
0.0500
35.4 (3.4) a 36.1 (3.1) a
0.0002
34.8 (3.6) a
5–8 61.7 (17.5)
0.4364
64.1 (15.5) 65.1 (15.4)
0.51154
63.7 (15.8) 65.6 (14.9) a
0.0367
62.1 (16.3) a
9–12 84.8 (22.7)
0.84330
86.0 (27.8) 87.7 (24.2)
0.6323
85.6 (28.1) 90.4 (25.7) a
0.0065
82.0 (28.5) a
13–16 109.1 (50.3)
0.9503
109.8 (53.5) 107.8 (45.0)
0.7722
110.2 (54.8) 112.0 (51.7)
0.5128
107.9 (54.6)
17–20 115.7 (61.7)
0.8754
113.8 (54.5) 122.6 (51.6)
0.2103
112.0 (55.6) 117.0 (53.5)
0.3891
111.3 (56.2)
21–24 130.4 (55.5)
0.3080
117.9 (56.2) 135.3 (52.1) a
0.0211
115.2 (56.5) a 125.4 (57.0)
0.0765
113.3 (54.9)
25–26 131.1 (50.2)
0.3563
118.3 (58.5) 140.2 (50.2) a
0.0086
114.8 (58.5) a 127.0 (56.9) a
0.0476
112.4 (58.0) a
Probability values represent comparisons between the doses of either etelcalcetide or cinacalcet at each interval of follow-up among subjects with varying degrees of hyperphosphatemia.
aDifferences that are statistically significant.

Treatment with either etelcalcetide or cinacalcet effectively lowered plasma PTH levels by 40%–50% from baseline values over 26 weeks, but the reductions overall were modestly greater with etelcalcetide (Figures 3 and 4, Tables 2 and 3). The relative efficacy of these two calcimimetic agents to lower plasma PTH levels also differed by the severity of hyperphosphatemia at each interval of follow-up as judged by the mean percentage change in PTH from baseline values. Among subjects with serum phosphorus concentrations above each of three specified thresholds, greater percentage reductions were achieved among subjects given etelcalcetide compared with those given cinacalcet during the later stages of study but not at earlier time points (Table 4). In contrast, the mean percentage change in plasma PTH from baseline was somewhat greater among subjects given etelcalcetide compared with those given cinacalcet at earlier intervals of follow-up for individuals whose serum phosphorus concentrations were below each of three specified phosphorus thresholds (Table 4). These differences, which occurred early, persisted for the remainder of the study.

Discussion

The CaR interacts with a variety of organic and inorganic ligands that can increase or decrease the level of receptor activation. Calcimimetics facilitate CaR activation by interacting with orthosteric or allosteric sites on the receptor, acting as agonists or positive allosteric modulators to inhibit PTH secretion. In contrast, calcilytics attenuate CaR activation by Ca2+ and diminish CaR-mediated intracellular signaling; all known calcilytics are negative allosteric modulators and stimulate PTH secretion.7

Results from recent in vitro studies indicate that activation of the CaR by calcimimetic ligands is attenuated by phosphorus.26 Phosphate functions as a negative allosteric modulator of the CaR not unlike calcilytic compounds. These inhibitory effects have been attributed to interactions between negatively charged phosphate anions and specific sites within the extracellular domain of the CaR that serve to stabilize the CaR homodimer in a more open, inactive conformation.26

The current results confirm, for the first time in vivo and in humans, findings from in vitro experiments indicating that phosphorus attenuates CaR activation and diminishes the inhibitory effect of calcimimetics on PTH secretion.26 As such, the hyperphosphatemia seen commonly among patients undergoing dialysis likely contributes, at least in part, to the elevated PTH levels that characterize those with sHPT.

In clinical trials with etelcalcetide, the decreases in plasma PTH at each interval of follow-up over 26 weeks did not differ according to the degree of hyperphosphatemia at baseline. Similar results were obtained in trials with cinacalcet, although the reductions in PTH at some intervals of follow-up were greater among subjects with baseline serum phosphorus concentrations that were above rather than below the threshold cutoff values of 7.0 mg/dl and 5.5 mg/dl, respectively (Table 3). Subsequent assessments that categorized subjects according to the prevailing serum phosphorus concentration at each interval of follow-up demonstrated, however, that higher levels of phosphorus in serum were associated consistently with smaller reductions in plasma PTH during treatment with either calcimimetic. Specifically, subjects who had serum phosphorus concentrations that were above one of three specified threshold values at each interval of follow-up experienced smaller percentage decreases in plasma PTH from baseline compared with those with serum phosphorus levels that were below these thresholds.

In this study, relatively higher serum phosphorus concentrations as measured at each interval of follow-up rather than baseline serum phosphorus levels were associated with smaller percentage decreases in plasma PTH during treatment with each calcimimetic agent. These smaller decrements in PTH were observed despite the use of larger doses of either etelcalcetide or cinacalcet providing in vivo evidence in humans supporting the in vitro findings reported by Centeno and colleagues.26 The differences in the PTH-lowering effects of both calcimimetics at each serum phosphorus threshold were relatively small, however, ranging from 5% to 15%, and thus would be considered of marginal importance from a clinical perspective. These findings probably reflect an ongoing, dynamic interaction between the ambient level of phosphorus in serum and available anionic binding sites located in the extracellular domain of the CaR.17,26 Such interactions are likely to vary over time and to be influenced importantly by the prevailing concentration of inorganic phosphate in serum. In contrast, serum phosphorus concentrations determined at a single point in time before initiating treatment with a calcimimetic agent would not be expected to be a reliable predictor of concentration-dependent interactions between phosphate anions and the CaR that might readily influence the level of CaR activation several weeks or a few months later.

The effect of hyperphosphatemia to attenuate calcimimetic-induced reductions in plasma PTH became evident early during treatment with each calcimimetic and persisted over 26 weeks of follow-up. Although the effect of hyperphosphatemia on reductions in plasma PTH levels was small early during treatment, the differences between treatment groups became larger at later stages of study (Table 4). Only then did the separation of values prove to be statistically significant when comparing subjects receiving etelcalcetide with those treated with cinacalcet. In contrast, differences between treatment with etelcalcetide and cinacalcet not only appeared earlier, but also were somewhat greater, when the average decrease in PTH from baseline was compared at each interval of follow-up between subjects with serum phosphorus concentrations that were below each of the three specified threshold values for serum phosphorus as determined at each interval of follow-up (Table 4).

Treatment with etelcalcetide or cinacalcet lowered plasma PTH levels by approximately 50% from baseline values over 26 weeks, but the reductions from baseline were only marginally greater among those treated with etelcalcetide. As such, the current results indicate that hyperphosphatemia does not modify the PTH-lowering effect of treatment with either calcimimetic agent to a degree that is meaningful therapeutically in sHPT. It should be noted, however, that hyperphosphatemia may contribute, at least in part, to the persistent elevations in plasma PTH that occur among those with sHPT through reductions in the inhibitory effect of CaR activation on PTH secretion when calcimimetic therapy is not involved. Indeed, greater effects of phosphate on CaR activation and PTH release may occur when serum phosphorus levels are restored toward normal in hypophosphatemic disorders.

Finally, it is known that etelcalcetide and cinacalcet interact with the CaR at different molecular sites. Cinacalcet binds within the transmembrane domain whereas etelcalcetide binds within the extracellular domain of the CaR.28,29 It has yet to be determined whether differences in the binding sites for etelcalcetide and cinacalcet account for variations in the level of CaR activation by these two calcimimetic agents or for the modest differences between them with respect to the effects of hyperphosphatemia on CaR activation and PTH release.

To conclude, hyperphosphatemia can be shown to attenuate the response to calcimimetic agents among patients with sHPT. The effect overall is relatively small, however, whereas the ability to substantially lower plasma PTH levels during treatment with these agents is robust and well maintained even when serum phosphorus levels are elevated.

Disclosures

D. Drayer, C. Moore, and Y. Chon are employees of Amgen and own stock in the company. W.G. Goodman is employed currently by PRO Unlimited and is assigned under contract to work with Amgen; holds equity interests in the company as a past employee; and has consultancy agreements with Amgen. J. Lai and J. Xu are contract workers for Amgen. J. Lai is employed by Parexel International. K.J. Martin has consultancy agreements with Amgen, Ardelyx, MediBeacon, Tricida, and Vifor Pharma; has received honoraria from Ardelyx and Tricida; serves on a medical advisory board for Ardelyx; and reports scientific advisor or membership with Amgen, Applied Therapeutics, Clinical Nephrology, and Tricida. E.F. Nemeth is a consultant for Calcilytix Therapeutics, Inc.; and reports scientific advisor or membership with Calcilytix Therapeutics, Inc.

Funding

The project was done, in part, by employees of Amgen, and some of the costs of publication will be covered by Amgen.

Published online ahead of print. Publication date available at www.jasn.org.

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Keywords:

calcimimetic; etelcalcetide; cinacalcet; calcium receptor; hyperparathyroidism; hyperphosphatemia; parathyroid hormone; calcium; receptors; calcium-sensing

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