Mechanistic modelling of enzyme-restoration effects of new recombinant liver-targeted proteins in acute intermittent porphyria

Abstract

Background and purpose: Acute intermittent porphyria (AIP) is a rare disease caused by a genetic mutation in the hepatic activity of the porphobilinogen-deaminase. We aimed to develop a mechanistic model of the enzymatic restoration effects of a novel therapy based on the administration of different formulations of recombinant human-PBGD (rhPBGD) linked to the ApoAI lipoprotein. This fusion protein circulates in blood, incorporating into HDL and penetrating hepatocytes.

Experimental approach: Single i.v. dose of different formulations of rhPBGD linked to ApoAI were administered to AIP mice in which a porphyric attack was triggered by i.p. phenobarbital. Data consist on 24 h urine excreted amounts of heme precursors, 5-aminolevulinic acid (ALA), PBG and total porphyrins that were analysed using non-linear mixed-effects analysis.

Key results: The mechanistic model successfully characterized over time the amounts excreted in urine of the three heme precursors for different formulations of rhPBGD and unravelled several mechanisms in the heme pathway, such as the regulation in ALA synthesis by heme. Treatment with rhPBGD formulations restored PBGD activity, increasing up to 51 times the value of the rate of tPOR formation estimated from baseline. Model-based simulations showed that several formulation prototypes provided efficient protective effects when administered up to 1 week prior to the occurrence of the AIP attack.

Conclusion and implications: The model developed had excellent performance over a range of doses and formulation type. This mechanistic model warrants use beyond ApoAI-conjugates and represents a useful tool towards more efficient drug treatments of other enzymopenias as well as for acute intermittent porphyria.

Keywords: AIP; acute intermittent porphyria; mechanistic; modelling; porphobilinogen deaminase.

FIGURE 1

Raw data used to build the porphobilinogen deaminase (PBGD) enzymatic activity and heme precursor models. (a) PBGD enzymatic activity profiles over time after a single 60 nmol·kg⁻¹ body weight intravenous administration of either recombinant human (rh) PBGD‐ apolipoprotein AI (ApoAI; n = 27), rhApoAI‐PBGD (n = 9) or rhApoAI‐human PBGD‐I129M/N340S (PBGDMS; n = 8). Points represent the individual activity values, whereas the solid line displays the median tendency. (b) Heme precursor amounts collected in 24 h urine of acute intermittent porphyria (AIP) mice for control (n = 22) and treated AIP mice with 300 (n = 8) or 600 (n = 7) nmol·kg⁻¹ body weight of rhPBGD‐ApoAI, rhApoAI‐PBGD (n = 7) or rhApoAI‐PBGDMS (n = 6). Points show the individual amount values over time, and the dashed line displays the median profile

FIGURE 2

Schematic representation of the heme precursors model with porphobilinogen deaminase (PBGD) effects. Black solid arrows represent amount transfers between compartments. Red dashed arrows show stimulation effects from either phenobarbital or PBGD in liver or in the bloodstream. Blue dot‐end dashed lines display inhibitory feedbacks from either hepatic heme amounts or hepatic PBG amounts. All the model parameters have been defined in the main text and in the supporting information

FIGURE 3

Porphobilinogen deaminase (PBGD) enzymatic activity model: Visual predictive checks (VPCs) and individual profiles. (a) VPCs. The three first panels show the different formulations administered intravenously (n = 27, 9 and 8 mice, respectively). The fourth panel is for recombinant human (rh) PBGD‐apolipoprotein AI (ApoAI; n = 31 in total for the four dose groups) after a subcutaneous administration. One thousand datasets with the same characteristics as the original one were simulated and plotted together with the observed data. Points, observations. Red solid line, observed median profile. Light red area, 95% confidence interval of the simulated median profiles. Black dashed lines, medians of the simulated 2.5th and 97.5th percentiles that provide a 95% prediction interval. (b) Individual enzymatic activity profiles from randomly selected individuals. The two panels on the left‐hand side present two mice after an intravenous administration, and the remaining two IDs received a subcutaneous administration. Dots, enzymatic activity observations. Blue dashed line, population predictions. Red solid line, individual predictions

FIGURE 4

Heme precursors model: Visual predictive checks (VPCs). (a) Control (n = 22) acute intermittent porphyria (AIP) mice. B, AIP mice treated with either 300 (n = 8) nmol·kg⁻¹ or 600 (n = 7) nmol·kg⁻¹ recombinant human (rh) porphobilinogen deaminase (PBGD)‐apolipoprotein AI (ApoAI). C, AIP mice treated with 300 (n = 7) nmol·kg⁻¹ rhApoAI‐PBGD. D, AIP mice treated with 300 (n = 6) nmol·kg⁻¹ rhApoAI‐human PBGD‐I129M/N340S (PBGDMS). One thousand datasets with the same characteristics as the original one were simulated and plotted together with the observed data. Points, observations. Red solid line, observed median profile. Light areas, 95% confidence interval of the simulated median profiles. Black dashed lines, medians of the simulated 2.5th and 97.5th percentiles that provide a 95% prediction interval

FIGURE 5

Model‐based simulated profiles. (a) Hepatic ALA and heme amounts over time with (black solid line) and without (red dashed line) phenobarbital induction, both in the absence of recombinant human (rh) porphobilinogen deaminase (PBGD) administration. (b) PBGD enzymatic activity (expressed as natural logarithm) in liver derived from the administration of 300 nmol·kg⁻¹ of either rhPBGD‐apolipoprotein AI (ApoAI) (red), rhApoAI‐PBGD (green) or rhApoAI‐human PBGD‐I129M/N340S (PBGDMS; blue)

FIGURE 6

Model exploration. Lines represent the percentage of urinary porphobilinogen (PBG) reduced with respect to the control profile at day 4 after the start of the acute attack depending on the day of recombinant human (rh) apolipoprotein AI (ApoAI)‐ human PBGD‐I129M/N340S (PBGDMS) administration: From 8 days prior to the start of the acute attack (day −8) to 2 days after the start of the attack (day 2). Each line shows a different dosing schedule (either 300 nmol·kg⁻¹ o.d. or 60 nmol·kg⁻¹ o.d. for 3 days) and route of administration [either intravenous (i.v.) or subcutaneous (s.c.)]