Pharmacodynamic Model of Hepcidin Regulation of Iron Homeostasis in Cynomolgus Monkeys

Abstract

Hepcidin (H₂₅) is a hormone peptide synthesized by the liver that binds to ferroportin and blocks iron export. In this study, H₂₅ was inhibited by administration of single and multiple doses of an anti-H₂₅ monoclonal antibody Ab 12B9m in cynomolgus monkeys. The objective of this analysis was to develop a pharmacodynamic model describing the role of H₂₅ in regulating iron homeostasis and the impact of hepcidin inhibition by Ab 12B9m. Total serum H₂₅ and Ab 12B9m were determined in each animal. Corresponding measurements of serum iron and hemoglobin (Hb) were obtained. The PD model consisted of iron pools in serum (Fe_S), reticuloendothelial macrophages (Fe_M), hemoglobin (Fe_Hb), and liver (Fe_L). The iron was assumed to be transported between the Fe_S, Fe_Hb, and Fe_M unidirectionally at rates k _S, k _Hb, and k _M. H₂₅ serum concentrations were described by the previously developed PK model with the parameters fixed at their estimates. The serum iron and Hb data were fitted simultaneously. The corresponding estimates of the rate constants were k _S/Fe₀ = 0.113 h⁻¹, k _M = 0.00191 h⁻¹, and k _Hb = 0.00817 h⁻¹. The model-based IC₅₀ value for the H₂₅ inhibitory effect on ferroportin activity was 0.398 nM. The PD model predicted a negligible effect of Ab 12B9m on Hb levels for the tested doses. The presented PD model adequately described the serum iron time courses following single and multiple doses of Ab 12B9m. Ab 12B9m-induced inhibition of H₂₅ resulted in a temporal increase in serum and liver iron and a decrease in the iron stored in reticuloendothelial macrophages.

APPENDIX

PK model of anti-hepcidin monoclonal antibody Ab 12B9m.

The PK model description for Ab 12B9m has been excerpted from the original publication Xiao et al. (6). A schematic of the PK model is shown in Fig. 10. Ab_sc represents the SC depot for Ab 12B9m SC dosing. Ab 12B9m and Ab 12B9m-H₂₅ complex distribute into their central compartments (Ab and AbH), peripheral compartments (Ab_p and AbH_p), and endosome compartments (Ab_E and AbH_E). In endosome, Ab 12B9m and Ab 12B9m-H₂₅ complex binds to FcRn to form complexes (FcAb_E and FcAbH_E). The intercompartment distribution was described with a set of first-order rate constants (k _cp, k _pc, k _up, and k _R), and elimination from Ab and AbH follows first-order kinetics (k). It was assumed that Ab 12B9m and Ab 12B9m-H₂₅ complex share the same parameter as listed above. H₂₅ is produced at a constant rate (k _inH) and eliminated with first-order kinetics (k _H). In serum, the binding between Ab 12B9m and H₂₅ is governed by the association rate constant (k _onH) and the dissociation rate constant (k _offH); similarly, in endosome the binding of Ab 12B9m and Ab 12B9m-H₂₅ complex to FcRn is described by k _onA and k _offA.

$$ \frac{d{\mathrm{Ab}}_{\mathrm{SC}}}{dt}=-F\cdot {k}_{\mathrm{a}}\cdot {A}_{\mathrm{SC}} $$

(11)

$$ \begin{array}{l}\frac{d\mathrm{Ab}}{dt}=F\cdot {k}_{\mathrm{a}}\cdot {A}_{\mathrm{SC}}-\left(k+{k}_{\mathrm{up}}+{k}_{\mathrm{c}\mathrm{p}}\right)\cdot \mathrm{Ab}-{k}_{\mathrm{onH}}\cdot {\mathrm{H}}_{25}\cdot \mathrm{Ab}/{V}_{\mathrm{c}}+{k}_{\mathrm{offH}}\cdot \mathrm{A}\mathrm{b}\mathrm{H}\\ {}+{k}_{\mathrm{R}}\cdot {\mathrm{FcAb}}_{\mathrm{E}}+{k}_{\mathrm{p}\mathrm{c}}\cdot {\mathrm{Ab}}_{\mathrm{p}}\end{array} $$

(12)

$$ \frac{d{\mathrm{Ab}}_{\mathrm{p}}}{dt}={k}_{\mathrm{cp}}\cdot \mathrm{Ab}-{k}_{\mathrm{p}\mathrm{c}}\cdot {\mathrm{Ab}}_{\mathrm{p}} $$

(13)

$$ \frac{d{\mathrm{H}}_{25}}{dt}={k}_{\mathrm{inH}}-{k}_{\mathrm{H}}\cdot {H}_{25}-{k}_{\mathrm{onH}}\cdot {\mathrm{H}}_{25}\cdot {A}_{\mathrm{b}}/{V}_{\mathrm{c}}+{k}_{\mathrm{offH}}\cdot \mathrm{A}\mathrm{b}\mathrm{H} $$

(14)

$$ \frac{d\mathrm{A}\mathrm{b}\mathrm{H}}{dt}={k}_{\mathrm{onH}}\cdot {\mathrm{H}}_{25}\cdot \mathrm{Ab}/{V}_{\mathrm{c}}-\left({k}_{\mathrm{offH}}+k+{k}_{\mathrm{up}}+{k}_{\mathrm{c}\mathrm{p}}\right)\cdot \mathrm{A}\mathrm{b}\mathrm{H}+{k}_{\mathrm{p}\mathrm{c}}\cdot {\mathrm{AbH}}_{\mathrm{p}}+{k}_{\mathrm{R}}\cdot {\mathrm{FcAbH}}_{\mathrm{E}} $$

(15)

$$ \frac{d{\mathrm{AbH}}_{\mathrm{p}}}{dt}={k}_{\mathrm{cp}}\cdot \mathrm{A}\mathrm{b}\mathrm{H}-{k}_{\mathrm{p}\mathrm{c}}\cdot {\mathrm{AbH}}_{\mathrm{p}} $$

(16)

$$ \frac{d{\mathrm{Ab}}_{\mathrm{E}\mathrm{tot}}}{dt}={k}_{\mathrm{up}}\cdot \mathrm{Ab}-{k}_{\deg}\cdot {\mathrm{Ab}}_{\mathrm{E}}-{k}_{\mathrm{R}}\cdot {\mathrm{FcAb}}_{\mathrm{E}} $$

(17)

$$ \frac{d{\mathrm{AbH}}_{\mathrm{E}\mathrm{tot}}}{dt}={k}_{\mathrm{up}}\cdot \mathrm{A}\mathrm{b}\mathrm{H}-{k}_{\deg}\cdot {\mathrm{AbH}}_{\mathrm{E}}-{k}_{\mathrm{R}}\cdot {\mathrm{FcAbH}}_{\mathrm{E}} $$

(18)

where V _c denotes the central compartment volume of Ab 12B9m, which was assumed to be the same for hepcidin H₂₅ and the Ab 12B9m-H₂₅ complex; Ab_Etot and AbH_Etot are the total amounts, respectively, of Ab 12B9m and Ab 12B9m-H₂₅ in the endosomal compartments:

$$ {\mathrm{Ab}}_{\mathrm{E}\mathrm{tot}}={\mathrm{Ab}}_{\mathrm{E}}+{\mathrm{FcAb}}_{\mathrm{E}} $$

(19a)

and

$$ {\mathrm{AbH}}_{\mathrm{E}\mathrm{tot}}={\mathrm{AbH}}_{\mathrm{E}}+{\mathrm{FcAbH}}_{\mathrm{E}} $$

(19b)

Fig. 10

Schematic of pharmacokinetic model for H₂₅ and Ab 12B9m in cynomolgus monkeys

The free and bound endosomal antibodies were calculated according to the equilibrium assumption:

$$ {\mathrm{Ab}}_{\mathrm{E}}=\frac{{\mathrm{Ab}}_{\mathrm{E}\mathrm{tot}}\cdot \left({K}_{\mathrm{DA}}\cdot {V}_{\mathrm{E}}+{\mathrm{Ab}}_{\mathrm{E}}+{\mathrm{Ab}\mathrm{H}}_{\mathrm{E}}\right)}{K_{\mathrm{DA}}\cdot {V}_{\mathrm{E}}+{\mathrm{Fc}}_{\mathrm{tot}}+{\mathrm{Ab}}_E+{\mathrm{Ab}\mathrm{H}}_{\mathrm{E}}} $$

(20)

and

$$ {\mathrm{Ab}\mathrm{H}}_{\mathrm{E}}=\frac{{\mathrm{Ab}\mathrm{H}}_{\mathrm{E}\mathrm{tot}}\cdot \left({K}_{\mathrm{DA}}\cdot {V}_{\mathrm{E}}+{\mathrm{Ab}}_{\mathrm{E}}+{\mathrm{Ab}\mathrm{H}}_{\mathrm{E}}\right)}{K_{\mathrm{DA}}\cdot {V}_{\mathrm{E}}+{\mathrm{Fc}}_{\mathrm{tot}}+{\mathrm{Ab}}_{\mathrm{E}}+{\mathrm{Ab}\mathrm{H}}_{\mathrm{E}}} $$

(21)

where

$$ \begin{array}{l}{\mathrm{Ab}}_{\mathrm{E}}+{\mathrm{Ab}\mathrm{H}}_{\mathrm{E}}=\frac{1}{2}\left({\mathrm{Ab}}_{\mathrm{E}\mathrm{tot}}+{\mathrm{Ab}\mathrm{H}}_{\mathrm{E}\mathrm{tot}}-{K}_{\mathrm{DA}}\cdot {V}_{\mathrm{E}}-{\mathrm{Fc}}_{\mathrm{tot}}\right)\\ {}+\frac{1}{2}\sqrt{{\left({\mathrm{Ab}}_{\mathrm{E}\mathrm{tot}}+{\mathrm{Ab}\mathrm{H}}_{\mathrm{E}\mathrm{tot}}-{K}_{\mathrm{DA}}\cdot {V}_{\mathrm{E}}-{\mathrm{Fc}}_{\mathrm{tot}}\right)}^2+4\cdot \left({\mathrm{Ab}}_{\mathrm{E}\mathrm{tot}}+{\mathrm{Ab}\mathrm{H}}_{\mathrm{E}\mathrm{tot}}\right)\cdot {K}_{\mathrm{DA}}\cdot {V}_{\mathrm{E}}}\end{array} $$

(22)

$$ {\mathrm{Fc}\mathrm{Ab}}_{\mathrm{E}}=\frac{{\mathrm{Fc}}_{\mathrm{tot}}\cdot {\mathrm{Ab}}_{\mathrm{E}}}{K_{\mathrm{DA}}\cdot {V}_{\mathrm{E}}+{\mathrm{Ab}}_{\mathrm{E}}+{\mathrm{Ab}\mathrm{H}}_{\mathrm{E}}} $$

(23)

$$ {\mathrm{Fc}\mathrm{AbH}}_{\mathrm{E}}=\frac{{\mathrm{Fc}}_{\mathrm{tot}}\cdot {\mathrm{Ab}\mathrm{H}}_{\mathrm{E}}}{K_{\mathrm{DA}}\cdot {V}_{\mathrm{E}}+{\mathrm{Ab}}_{\mathrm{E}}+{\mathrm{Ab}\mathrm{H}}_{\mathrm{E}}} $$

(24)

The initial conditions for the model variables were zero except for Ab_SC, Ab, and H₂₅. The initial values of Ab_SC and Ab were the first doses administered SC or IV, whereas the initial value for H₂₅ was the baseline H₂₅ serum amount H_25,0. The steady state for Eq. 12 resulted in the following baseline equation:

$$ {k}_{\mathrm{inH}}={k}_{\mathrm{H}}\cdot {H}_{25,0} $$

(25)

The linear disposition parameters for Ab 12B9m were re-parameterized in terms of clearances and volumes:

$$ k=\frac{\mathrm{CL}}{V_{\mathrm{c}}} $$

(26)

and

$$ {k}_{\mathrm{c}\mathrm{p}}=\frac{Q}{V_{\mathrm{c}}} $$

(27a)

and

$$ {k}_{\mathrm{p}\mathrm{c}}=\frac{Q}{V_{\mathrm{p}}} $$

(27b)

where Q denotes the distributional clearance and V _p is the volume of the peripheral compartment. Total Ab 12B9m and total H₂₅ serum concentrations were expressed as

$$ {C}_{\mathrm{Ab}\mathrm{tot}}=\frac{\mathrm{Ab}+\mathrm{A}\mathrm{b}\mathrm{H}}{V_{\mathrm{c}}} $$

(28a)

and

$$ {C}_{\mathrm{H}\mathrm{tot}}=\frac{{\mathrm{H}}_{25}+\mathrm{A}\mathrm{b}\mathrm{H}}{V_{\mathrm{c}}} $$

(28b)

The serum free hepcidin concentrations were calculated as

$$ {C}_{\mathrm{H}25}=\frac{{\mathrm{H}}_{25}}{V_{\mathrm{c}}} $$

(29)

The values of the PK parameters are presented in Table III.

Table III Parameter Estimates for the PK Model of Anti-hepcidin Monoclonal Antibody Ab 12B9m Obtained from (6)