Management of secondary hyperparathyroidism: the gap between diagnosis and treatment. The Renal Osteodystrophy Multicenter Enquiry

Population Pharmacokinetic and Pharmacodynamic Modeling of Etelcalcetide in Patients with Chronic Kidney Disease and Secondary Hyperparathyroidism Receiving Hemodialysis

INTRODUCTION

Etelcalcetide is a novel calcimimetic that binds and activates calcium-sensing receptors (CaSRs) for the treatment of secondary hyperparathyroidism (SHPT).

METHODS

To assess titrated dosing regimens, population pharmacokinetic (PK) and PK/pharmacodynamic (PKPD) modeling of etelcalcetide was performed using NONMEM 7.2. In this analysis, plasma etelcalcetide, serum parathyroid hormone (PTH) and calcium (Ca) concentration-time data were collected from five phase I, II, and III clinical trials following single or multiple intravenous doses of etelcalcetide ranging from 2.5 to 60 mg. A semi-mechanistic model was used to describe the relationship between etelcalcetide, PTH, and Ca. This model included the role of PTH in Ca regulation, the feedback of Ca onto PTH production via the CaSR, and the activity of etelcalcetide plasma levels in increasing the sensitivity of the CaSR to Ca via the cooperative binding model. The impact of relevant covariates was evaluated by stepwise forward/backward selection. Model evaluation was based on standard goodness-of-fit plots and prediction-corrected visual predictive checks (pcVPCs). Simulation was conducted to evaluate titrated dosing regimens.

RESULTS AND DISCUSSION

The time courses of etelcalcetide, PTH, and Ca were well-described by the model. The clearance and central volume of distribution (Vc) of etelcalcetide were 0.472 L/h and 49.9 L, respectively, while estimates of the turnover half-lives of PTH and Ca were 0.36 and 23 h, respectively. The extent of interindividual variability in model parameters was low to moderate (6-67%), and no covariates were identified as significant predictors of PK and PD variability. pcVPCs confirmed the predictive ability of the model.

CONCLUSIONS

The current analysis confirms the putative mechanism of action of etelcalcetide as an allosteric activator of CaSR. Simulations showed that dose titration of etelcalcetide, rather than fixed dose, is needed to effectively decrease the PTH level in patient populations.

Population Pharmacokinetics and Pharmacodynamics of the Calcimimetic Etelcalcetide in Chronic Kidney Disease and Secondary Hyperparathyroidism Receiving Hemodialysis

Etelcalcetide is a novel calcimimetic in development for the treatment of secondary hyperparathyroidism (SHPT). A population pharmacokinetic/pharmacodynamic (PK/PD) model was developed relating etelcalcetide exposures to markers of efficacy (parathyroid hormone [PTH]) and safety (calcium) using data from three clinical studies. The semimechanistic model was developed that included allosteric activation pharmacology and understanding of calcium homeostasis. The temporal profiles for all biomarkers were well described by the model. The cooperativity constant was 4.94, confirming allosteric activation mechanism. Subjects with more severe disease (higher PTH baseline) were predicted to experience less pronounced reduction in PTH (percentage change from baseline), but more reduction in calcium (Ca; percentage change from baseline). There was no evidence that dose adjustment by any covariate was needed. Model‐based simulations provided quantitative support to several elements of dosing, such as starting dose, monitoring, and titration timing for registration trials.

Doxercalciferol treatment of secondary hyperparathyroidism

OBJECTIVE

To review the pharmacology, pharmacokinetics, effectiveness, safety, and therapeutic considerations related to the use of doxercalciferol in the treatment of secondary hyperparathyroidism.

DATA SOURCES

A MEDLINE search (1966-June 2006) was conducted using the key words vitamin D, ergocalciferols, and secondary hyperparathyroidism. Text word searches were also performed for the terms 1-alpha-hydroxy-vitamin D(2), 1-alpha-hydroxyergocalciferol, Hectorol, and doxercalciferol. Searches were limited to studies published in English and conducted in human subjects.

STUDY SELECTION AND DATA EXTRACTION

All published clinical studies evaluating the safety and effectiveness of doxercalciferol in secondary hyperparathyroidism were reviewed, and anecdotal patient reports were also evaluated. Selected clinical studies involving the use of calcitriol and/or paricalcitol in the treatment of secondary hyperparathyroidism were also included.

DATA SYNTHESIS

Doxercalciferol effectively reduces parathyroid hormone levels in patients with chronic kidney disease (CKD). Both oral and intravenous administration can significantly increase serum calcium and/or phosphorus levels as evidenced by placebo-controlled clinical trials. This agent has not been studied comparatively with calcitriol or paricalcitol to assess relative safety.

CONCLUSIONS

Doxercalciferol is approved for and effective in the treatment of secondary hyperparathyroidism related to CKD, both before and during dialysis, but has not demonstrated a lower incidence of hypercalcemia and/or hyperphosphatemia in relation to other vitamin D therapies. The drug is available in both oral and intravenous dosage forms. Doxercalciferol should be maintained as a formulary alternative for patients unresponsive to or intolerant of other vitamin D therapies, but comparative randomized studies are needed to differentiate its place in therapy.

Advantages of adjusting the initial dose of intravenous calcitriol according to PTH levels

BACKGROUND

To assess the usefulness of starting calcitriol therapy with a dose proportional to the degree of hyperparathyroidism, 141 patients from 28 centers were treated with intravenous calcitriol for 6 months. The aim was to achieve a final PTH between 125 and 250 pg/mL. Patients with serum PTH>250 pg/mL were included in the study and divided into 4 groups according to baseline PTH levels.

METHODS

The study was completed by 100 patients, a third of which were treated strictly according to the protocol, labeled "compliants"; thus, calcitriol was started according to baseline PTH levels. Two thirds of patients, labeled "noncompliants," showed one or more violation in the dosage regimen.

RESULTS

After 2 months of treatment with calcitriol, 59% of the "compliants" and 35% of the "noncompliants" decreased their PTH levels>40% (P = 0.022), 70%, and 49%, respectively after 3 months of treatment. After 3 months of treatment, 67% of the "compliants" reached the target (PTH 125 to 250 pg/mL) in contrast with 23% of the "noncompliants" (P < 0.001). The number of hypercalcemic and hyperphosphatemic episodes was significantly lower in the "compliants" group (P < 0.006).

CONCLUSION

These results demonstrate several advantages when calcitriol therapy is started with a dose proportional to the severity of hyperparathyroidism.