(T0930-02-10) A Mechanistic Ocular PK/PD Model with Systemic Target-Mediated Drug Disposition to Predict Human PK for Intravitreally Administered Anti-Tie2 Agonist

Purpose: Diabetic macular edema (DME) results from fluid accumulation and swelling in the macular region of the retina due to leaky blood vessels. Endothelial tyrosine kinase Tie2 receptor promotes cell junction integrity and reduces vascular permeability. Activation of the Tie2 pathway is expected to complement current anti-vascular endothelial growth factor (VEGF) strategies in the treatment of DME where vascular stabilization is crucial. Herein, pharmacokinetic/pharmacodynamic (PK/PD) properties of an intravitreally (ITV) administered anti-Tie2 agonist hexamer Fab were estimated in cynomolgus monkeys and translated to humans using an ocular PK/PD model with systemic target-mediated drug disposition (TMDD).

Methods: In two independent studies, 3 single, ascending doses (SAD) of anti-Tie2 agonist hexamer Fab were administered to cynomolgus monkeys (cynos) via bilateral ITV injection, and ocular and serum PK were followed for 21 or 28 days. In a separate multiple-dose study in cynos, the serum PK was characterized after bilateral ITV administration of a total of four doses to two groups of cynos where one group received the highest SAD dose and the other group received the maximum feasible dose once every four weeks (Q4W). The total Fab concentration was measured in the collected serum, aqueous and vitreous humor samples using either an ELISA or a hybrid immunoaffinity (IA) LC-MS/MS assays. Angiopoietin-2 (Ang-2) concentration was monitored in plasma at baseline and at pre-defined post-dose timepoints in all animals tested as a potential PD biomarker to assess target engagement.

A mechanistic PK/PD model describing drug disposition in the vitreous humor, aqueous humor, retina, and systemic compartments was fitted simultaneously to the observed PK profiles in cynos. TMDD was assumed to occur in the central (serum) compartment and 100% bioavailability from the vitreous humor into the systemic circulation was assumed. Observed on-treatment changes in plasma Ang-2 concentration were simulated via an indirect response model, where increasing pAKT levels suppress Ang-2 synthesis as described by an n^th order Hill function calibrated to in vitro data relating pAKT levels to increasing anti-Tie2 agonist concentrations. The calibrated model was used to predict human ocular and serum PK/PD and to provide an efficacious clinical dose range and dosing frequencies for anti-Tie2 agonist hexamer Fab. Vitreal exposure (Total Fab C_max and AUC) in humans was predicted by assuming the distribution volume of Fab in vitreous humor is approximately 4 mL, based on the typical physiological vitreous humor volume. The assumed human vitreal half-life (t_1/2) of 8 days was calculated by doubling vitreal t_1/2 values observed in cynos based on previously observed inter-species differences in t_1/2 of other ocular therapeutics. The compartmental systemic PK parameters (CL and V) were scaled up from cynos based on body weight using allometric scaling. In addition, the sensitivity of assumed drug/target retinal permeabilities on anti-Tie2 agonist activity at the site of action was explored with the model. All simulations were performed in SimBiology version 6.0, the product of Matlab version 9.9 (2020b) (MathWorks, Natick, MA, USA).

Results: Following ITV administration to cynos, anti-Tie2 agonist hexamer Fab showed absorption-rate limited PK with the release and absorption from the vitreous into aqueous and systemic compartments constituting the rate-limiting process. Exposure to anti-Tie2 agonist hexamer Fab increased with dose in a dose-proportional manner for vitreous and aqueous humor, and the observed half-life was 3.2-4.2 days in both compartments. A greater than dose-proportional increase in exposure was observed in serum, suggesting occurrence of TMDD. Receptor saturation appeared to occur at the higher doses. Based on the compartmental PK/PD model fitted to serum PK in cyno, non-specific, non-saturable CL was estimated to be 30 mL/day for the anti-Tie2 hexamer. No accumulation of anti-Tie2 agonist was observed upon repeated-dose administration. In addition, a transient, dose-dependent reduction in systemic Ang-2 levels was shown to correlate with the serum PK profiles.

Projected human PK/PD simulations support evaluation of doses covering a broad range of vitreal exposures above the in vitro EC50 and thoroughly exploring the relationship between exposure and activity. The simulations also support a target dosing frequency of Q8W. However, the model suggests that the onset of activity in the retina is highly dependent on retinal permeability and might be delayed if the bell-shaped activity curve observed in vitro for anti-Tie2 agonist hexamer Fab translates in the clinic.

Conclusion: Anti-Tie2 agonist hexamer Fab showed a relatively long ocular t_1/2 compared with typical Fab t_1/2 values reported in cynomolgus monkey vitreous humor, indicating size-based PK of the hexamer conjugate in ocular compartments. The compartmental PK/PD model with systemic TMDD successfully described nonlinear PK observed in serum, and helped select the optimal dose range for Phase 1 study.