Hexasodium fytate exposure-response correlations in a randomized, placebo-controlled study of patients on dialysis with cardiovascular calcification

Background: Patients receiving dialysis have high cardiovascular risk in part due to extensive vascular calcification. In the CaLIPSO study, infusion of hexasodium fytate (SNF472), the hexasodium salt of inositol hexaphosphate, for 52 weeks thrice weekly during hemodialysis significantly reduced progression of coronary artery calcification (CAC). This report examines pharmacokinetic/pharmacodynamic (PK/PD) and exposure-efficacy in CaLIPSO. Methods: We measured hexasodium fytate plasma concentrations (PK) by validated liquid chromatography-mass spectroscopy, and hydroxyapatite crystallization in plasma (PD) by validated spectrophotometry. Analyses included patients evaluable for PK, PD, and CAC change (per-protocol analysis). We developed a simple E-max model for maximum concentration (C-max) and PD effect, and linear and non-linear E-max models for exposure-efficacy among individual average C-max and absolute and percent changes in CAC score from baseline to week 52. Results: Among evaluable patients receiving placebo (n = 15), 300 mg (n = 20), or 600 mg (n = 20), average C-max across visits was not quantifiable (<0.76 mu M), 15 mu M, and 46 mu M, respectively. These results suggest a more-than-proportional increase, without accumulation, with a C-max ratio of approximately 3 for the doses administered. Average inhibition of hydroxyapatite crystallization was 15%, 61%, and 75%, respectively, and similar across visits. Simple E-max models described 80% maximal effect at exposures >21.9 mu M and a plateau in exposure-efficacy above the third quartile of C-max (>= 32 mu M).
Conclusion: Hexasodium fytate has exposure-dependent effects on hydroxyapatite crystallization and progression of cardiovascular calcification. Simple E-max models show robust relations among exposure, inhibition of hydroxyapatite crystallization, and change in CAC volume.