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Review

THE ANGIOPOIETIN/TIE PATHWAY IN RETINAL VASCULAR DISEASES

A Review

Heier, Jeffrey S. MD*; Singh, Rishi P. MD; Wykoff, Charles C. MD, PHD; Csaky, Karl G. MD, PHD§; Lai, Timothy Y.Y. MD, FRCS, FRCOPHTH; Loewenstein, Anat MD**; Schlottmann, Patricio G. MD††; Paris, Liliana P. MD, PHD‡‡; Westenskow, Peter D. PHD‡‡; Quezada-Ruiz, Carlos MD§§,¶¶

Author Information
doi: 10.1097/IAE.0000000000003003

The angiopoietin/tyrosine kinase with immunoglobulin-like and endothelial growth factor–like domains (Tie) pathway plays a key role in regulating vascular stability and inflammation under healthy and pathological conditions and, as such, is a focus for the development of next-generation therapeutics for retinal vascular disorders.

Neovascular age-related macular degeneration (nAMD), diabetic macular edema (DME), diabetic retinopathy (DR), and retinal vein occlusion (RVO) are retinal vascular diseases that are among the leading causes of blindness and visual impairment worldwide.1,2 Although each has its own multifactorial etiology and pathological characteristics, one of the features of all of these conditions is the breakdown of the mature vasculature, which can be associated with the formation of new, abnormal (or immature) vessels, with vascular endothelial growth factor (VEGF)-A and the angiopoietin/Tie pathway being key players that seem to be working in conjunction.

Neovascular AMD, an advanced manifestation of AMD, is characterized by choroidal neovascularization (CNV), in which new vessels arise from the choroid, rupture Bruch's membrane, and permeate either the subretinal or subretinal pigment epithelium space.3 Diabetic retinopathy is a microvascular complication of diabetes in which chronic hyperglycemia ultimately results in damage to the retinal vasculature and pericyte loss because of inflammation, oxidative stress, hypoxia, and accumulation of advanced glycation end products.4,5 Diabetic macular edema, a vision-threatening manifestation of DR, can occur at any stage of DR when leakage from the retinal vasculature causes fluid accumulation in the macula. Retinal vein occlusion, classified as central, branch, or hemicentral RVO, occurs when the retinal vasculature is acutely occluded, causing retinal nonperfusion/ischemia, with macular edema secondary to the RVO being the primary cause of vision loss.6–9 Recent advances in ultrawidefield imaging have proven to be useful for the evaluation of RVO because of its ability to measure areas of retinal nonperfusion in the periphery.10

At present, nAMD, DME, DR, and RVO can be treated with intravitreal injections of anti–VEGF-A therapeutics, including aflibercept,11 bevacizumab (not US Food and Drug Administration approved for any ocular indication),12 brolucizumab (approved for nAMD alone),13 ranibizumab,14 and conbercept15 (approved only in China) administered every 4 weeks (Q4W) initially, followed by once every 4 up to 12 weeks. Anti–VEGF-A agents are effective in reducing macular edema, reducing neovascularization, and ultimately improving or stabilizing vision.12,16 However, analyses of real-world data have shown that the vision gains achieved in clinical trials are not being realized in clinical practice, with undertreatment being a key contributor.17–21 Long-term studies (≥3 years) have also shown that vision gains often decline over time, with injection frequency positively correlating with a mean visual acuity gain from baseline.22 Furthermore, anti–VEGF-A injections do not address all aspects of disease pathophysiology.

The need for frequent intravitreal injections to maintain clinical benefits, and an apparent ceiling on vision improvement, has led to the pursuit of new therapies more expansive than anti–VEGF-A monotherapies alone, in hopes that by addressing the multifactorial nature of the diseases, the durability and efficacy limitations of intravitreal anti–VEGF-A therapeutics can be overcome. In particular, several molecules in clinical development (AKB-9778, REGN910-3, and faricimab) targeting the angiopoietin/Tie pathway have gained increased attention in the last few years.

This review discusses the angiopoietin/Tie pathway and the additional benefits of targeting it in patients with nAMD, DME, DR, and RVO.

Methods for Background Data Search

A literature search for molecules targeting the angiopoietin/Tie pathway that have reached Phase 2 and/or Phase 3 trials was conducted by searching PubMed, Association for Research in Vision and Ophthalmology (ARVO) meeting abstracts, and ClinicalTrials.gov databases. Keywords were “(angiopoietin or Tie2 or Tie1 or VE-PTP) and (ocular or eye or retinopathy or retina* or macula* or choroid*).” Search terms were selected to ensure a broad selection of all relevant articles, and results were sorted manually to include clinical trial publications, including publications on nAMD, DR, DME, and RVO, and prioritized for relevance to clinical development. Diseases not covered in the review (e.g., glaucoma and retinopathy of prematurity) were excluded.

Association for Research in Vision and Ophthalmology meeting abstracts from 2014 to 2019, which are published annually in the Investigative Ophthalmology & Visual Science journal, were searched using the same keywords as used for the PubMed search, i.e., “(angiopoietin or Tie2 or Tie1 or VE-PTP) and (ocular or eye or retinopathy or retina* or macula* or choroid*),” primarily to identify early-development drugs that had not yet resulted in a full-article publication.

Overall, the PubMed and Association for Research in Vision and Ophthalmology meeting abstract search yielded 462 results. A total of 251 publications were not included in the review. These consisted of 84 publications relating to other diseases, and 167 publications that were deemed not relevant to the scope of this review, which included those relating to other molecules, angiopoietin-4 (Ang-4), lymphatic vasculature, corneal neovascularization, tumors, retinopathy of prematurity, and publications not focused on angiopoietin (Figure 1).

Fig. 1.
Fig. 1.:
Flow diagram showing the number of publications and trials identified and selected for analysis. ARVO, Association for Research in Vision and Ophthalmology.

ClinicalTrials.gov was searched by the keywords angiopoietin, Tie1, Tie2, and VE-PTP, and sorted for relevant therapeutic areas; of 141 results, seven trials focusing on diseases covered in the review were selected. Trials where the drug did not target the angiopoietin/Tie pathway were excluded.

For molecules identified by the above, an additional search was performed on PubMed and ClinicalTrials.gov under each identified name (i.e., both for molecule number and international nonproprietary name). Additional information on identified pipeline drugs was obtained from publicly available information on company websites.

Overview of the Angiopoietin/Tie Pathway

The angiopoietin/Tie pathway is a key regulator of vascular development and maintenance (e.g., angiogenesis and vascular permeability), homeostasis, and inflammation.23–25 Members of this pathway include the growth factors angiopoietin-1 (Ang-1), angiopoietin-2 (Ang-2), and Ang-4 (ortholog to mouse angiopoietin-3 [Ang-3]), the growth factor receptor tyrosine kinases Tie1 and Tie2, and protein tyrosine phosphatase (VE-PTP; Figure 2).

Fig. 2.
Fig. 2.:
An overview of the angiopoietin/Tie family. Ang-1 is a constitutive agonist of the Tie2 receptor, causing phosphorylation that leads to downstream survival signaling, vascular maturation, and stability. Ang-2 is a strong antagonist that competes with Ang-1, preventing phosphorylation of Tie2 and leading to vascular destabilization. The signaling through Tie2 is modulated by the presence of VE-PTP and the Tie1 receptor. Ang-2 can also signal through integrins independently of the Tie2 receptor, which leads to downstream effects, including endothelial cell migration and sprouting through FAK signaling. *Ang-3 (mouse) and Ang-4 (human) are interspecies orthologs. †Constitutive agonist of the Tie2 receptor. ‡Tie1 has traditionally been classified as an orphan receptor, although LECT2 has recently been identified as a functional ligand that plays a role in fibrogenesis in mice.35 Ang-1, angiopoietin-1; Ang-2, angiopoietin-2; Ang-3, angiopoietin-3; Ang-4, angiopoietin-4; FAK, focal adhesion kinase; Tie, tyrosine kinase with immunoglobulin-like and endothelial growth factor–like domains; VE-PTP, vascular endothelial protein tyrosine phosphatase.

The Angiopoietins

The angiopoietins are a family of multimeric growth factors that play a key role in normal vascular development, both under mature physiologic and pathologic conditions. Ang-1 was previously reported to be restricted to pericytes lining adult blood vessels, as demonstrated in cultured dermal pericytes (in vitro), and in transgenic mice with inducible Ang-1 expression and on wounds prepared on dorsal flanks in guinea pigs (in vivo). 25–28 However, a recent study challenged this notion by demonstrating the Ang-1 expression in pericytes lining choroidal vessels and in neurons surrounding retinal vessels in the ganglion cell and inner nuclear layers, and not by pericytes covering retinal vessels in adult mice.29 In mature vasculature, Ang-2 is generally expressed at low levels by endothelial cells in the deep vascular plexus (but not the superficial plexus) 30 and acts as a context-dependent weak agonist or antagonist of Tie2. In the deep retinal capillaries, Ang-2 primes the vasculature to respond to VEGF-A, enhancing its effects on vascular permeability and neovascularization as demonstrated in double transgenic mice with inducible VEGF-A expression in the retina.30 Ang-2 has been shown to act as a weak Tie2 agonist under baseline homeostatic conditions 31 and in some tissue/cell types (e.g., in lymphatic tissue where VE-PTP is absent [as shown in mice]32 or in tumors [demonstrated in tumor cells implanted subcutaneously in mice]).33 Ang-3 and Ang-4 are less well characterized, and their roles remain to be determined.

Tie1 and Tie2 Receptors

The transmembrane Tie receptors belong to the tyrosine kinase receptor family, and together with the angiopoietins, are key regulators of angiogenesis. Tie1 and Tie2 are expressed almost exclusively in the endothelium, clustering at endothelial cell-to-cell junctions in the presence of Ang-1 or Ang-2.23 Tie2 is a receptor for Ang-1, Ang-2, and Ang-4.34 In contrast, Tie1 signals by downregulating Tie2. Although Tie1 has long been considered an orphan receptor, recent evidence shows that LECT2 is a Tie1 ligand, with both being involved in liver fibrogenesis.35

Vascular Endothelial Protein Tyrosine Phosphatase

The expression of VE-PTP is specific to endothelial cells, where it dephosphorylates and thus inactivates Tie2, thereby contributing to vascular destabilization as described below. The expression of VE-PTP is dynamic and notably upregulated under hypoxic conditions.36

Ang-1/Ang-2 signaling through Tie2

Ang-1, a paracrine ligand constitutively expressed in many adult tissues, including smooth muscle cells and pericytes, is a strong Tie2 agonist.23,25 Binding of Ang-1 to Tie2 activates Tie2 by inducing its phosphorylation, leading to signaling through the prosurvival phosphatidylinositol 3-kinase-protein kinase B (PI3K-AKT) pathway, stabilization of the cortical actin cytoskeleton, and stabilization of vascular endothelial cadherins (VE-cadherins) at cellular junctions. A key downstream effect of PI3K-AKT pathway signaling is to suppress the expression of genes regulated by the forkhead box protein O1 (FOXO1) transcription factor, notably including Ang-2. Overall, these actions stabilize newly formed and mature vessels by promoting endothelial cell survival, pericyte recruitment, and improved endothelial barrier function. Ang-1 signaling through Tie2 in the endothelium maintains vessel stability and an antiinflammatory, homeostatic state, acting as a molecular brake that prevents leakage and inflammation.37

Conversely, Ang-2 functions as a proinflammatory cytokine,25 and its upregulation under hypoxic and hyperglycemic conditions leads to a higher Ang-2/Ang-1 ratio, which negatively regulates the Ang-1/Tie2 signaling axis.23,25 Quantification of Ang-2 and Ang-1 in retinas of diabetic rats has demonstrated relative Ang-2/Ang-1 ratios to be three-fold higher than under normal conditions.38 Ang-2 levels were also shown to be significantly elevated in the vitreous from patients with RVO, AMD, and DR compared with controls (macular hole), whereas Ang-1 levels were comparable with those in controls.39,40 Ang-2 signaling through Tie2 activates FOXO1,23 forming a positive feedback loop, resulting in an increase in vessel permeability and vascular destabilization.

Ang-2 signaling independent of Tie2

Independently of the Tie2 pathway, Ang-2 can bind to integrins (proteins that connect the extracellular matrix with cytoskeletal proteins41; e.g., αvβ1, αvβ3, α3β1, and α5β1), with a lower affinity than to Tie2, and activate these integrins to promote endothelial destabilization through inducing endothelial cell migration,42 angiogenic sprouting,42 stress fiber formation,43 and reduction of VE-cadherins at endothelial cell junctions.43 Among integrins, α5β1 was shown to have a stronger affinity for Ang-2 than αvβ1 and αvβ3 in a flow cytometric analysis,44 although the Ang-2 affinities for αvβ5, αvβ3, α3β1, and α5β1 were comparable when measured using immunoprecipitation, enzyme-linked immunosorbent assays, and Ang-2 binding assays.42,44 In mouse models of DR, Ang-2 promotes pericyte45 and astrocyte46 apoptosis under hyperglycemic conditions through α3β1 and αvβ5 integrins, respectively, independently of Tie2. In addition, αvβ3 integrin has been shown to associate with platelet-derived growth factor receptor-β (PDGFRβ) and VEGF receptor-2 (VEGFR-2) in vitro, leading to their activation and contributing to angiogenesis and vascular remodeling.41 In the presence of Tie2, Ang-2 has been shown to promote the degradation of αvβ3 integrin, indicating functional involvement in cellular mechanisms causing endothelial destabilization 41 and endothelial barrier disruption by β1 integrin under inflammatory conditions.47 The Tie2 interaction with integrins may also be disrupted by therapeutic agents, potentially changing the impact of Ang-2 from an antagonist in pathologic states to an agonist. For instance, a bispecific antagonist targeting both Tie2 and αvβ3 was able to inhibit angiogenesis and endothelial cell invasion.48

The Angiogenic Switch

Under homeostatic conditions, Ang-1 is constitutively expressed at higher levels than Ang-2.28,49 However, during disease states (conditions including ischemia/hypoxia, hyperglycemia, and inflammation), the so-called angiogenic switch occurs. Activation of the switch induces overexpression of vascular growth factors and Ang-2, whereas Ang-1 continues to be expressed at a relatively consistent level.23,39,40 Elevated Ang-2 levels activate endothelial cells and increase their sensitivity to VEGF. This leads to excessive permeability and neovascularization.23,39,40

Ang-2 can function as a weak agonist or an antagonist of Tie2 depending on the presence or absence of inflammation.31 When the inflammatory cytokine tumor necrosis factor-α (TNF-α) triggers Tie1 inactivation, and Ang-2 levels surpass Ang-1, Ang-2 acts as competitive antagonist of Ang-1 by binding to the Tie2 receptor without inducing its phosphorylation.31,50 Ang-2 thereby disrupts Ang-1/Tie2 signaling, leading to vascular destabilization through pericyte loss, increase in proangiogenic and inflammatory cytokines, and weakening of endothelial cell junctions. In addition, Ang-2 expression, which is suppressed by Ang-1/Tie2 signaling, increases because of the dephosphorylation and activation of the FOXO1 transcription factor, creating a positive feedback loop.31,51

Furthermore, Ang-2 is a modulator and amplifier of inflammation, with a vital role in the full response to VEGF30,52 and proinflammatory mediators (TNF-α, bradykinin, and histamine), as shown in Ang-2 knockout mice.49,52 The absence of Ang-2 attenuates cytokine-induced leukocyte adhesion,49 cytokine-induced or VEGF-induced vascular leakage,52 and retinal neovascularization.30 Meanwhile, concomitantly modulating the angiopoietin/Tie and VEGF pathways has been shown to have synergistic effects in vitro and in vivo.36,39,40,53,54 Furthermore, increased levels of Ang-2 promote vessel leakage, inflammation, and neovascularization.

In summary, the activation of Tie2 through Ang-1/Tie2 signaling works as a “molecular brake” against inflammation and vascular destabilization, whereas in pathological conditions, Ang-2 upregulation releases this brake by displacing Ang-1, thus inhibiting Ang-1/Tie2 activation (Figure 3 and see Video, Supplemental Digital Content 1, http://links.lww.com/IAE/B337).

Fig. 3.
Fig. 3.:
The angiopoietin/Tie pathway under homeostatic and disease conditions. ABIN, A20-binding inhibitor of nuclear factor-κB; AKT, protein kinase B; Ang-1, angiopoietin-1; Ang-2, angiopoietin-2; EC, endothelial cell; FOXO1, forkhead box protein O1; NF-κB, nuclear factor-κB; P, phosphorylation; Tie, tyrosine kinase with immunoglobulin-like and endothelial growth factor–like domains; VE-cadherin, vascular endothelial cadherin; VEGFR, vascular endothelial growth factor receptor; VE-PTP, vascular endothelial protein tyrosine phosphatase.

Angiopoietin/Tie Pathway in Retinal Vascular Diseases

There is a need for treatments that address the multifactorial pathophysiology of retinal vascular diseases beyond VEGF with the hope of improving outcomes and providing sustained efficacy to reduce treatment burden. The angiopoietin/Tie pathway is specifically implicated in the pathophysiology of retinal vascular diseases, including DME/DR, nAMD, and RVO. Of note, elevated levels of Ang-2, indicative of a pathological state of vascular destabilization, have been found in the vitreous of patients with nAMD, DR, PDR, and RVO compared with controls,39,40,55,56 whereas Ang-1 levels are not significantly different from controls (see Figure, Supplemental Digital Content 2, http://links.lww.com/B335).39,40 In addition, both the ANGPT2 and TIE2 genes were recently identified as potential susceptibility genes for nAMD and polypoidal choroidal vasculopathy.57,58

Evidence from preclinical experiments has also demonstrated a role for the angiopoietin/Tie pathway in maintaining endothelial cell stability, suggesting a role in the pathophysiology of DME/DR, nAMD, and RVO. In rodent models of diabetes, overexpression of Ang-1 alleviated DR pathology (inhibited leukocyte adhesion, endothelial cell injury, and blood-retinal barrier breakdown).59,60 By contrast, Ang-2 has been implicated in pericyte dropout45,61–65 and retinal astrocyte apoptosis46 under hyperglycemic conditions. Increased leukocyte adhesion and endothelial cell damage, which is implicated in retinal ischemia in both RVO and DR,8,59,66 is promoted by Ang-2, which sensitizes endothelial cells to TNF-α and increases the expression of cellular adhesion molecules such as intracellular and vascular cell adhesion molecules, and others. In mice lacking Ang-2, reduced leukocyte adhesion to the inflamed endothelium prevents transmigration into inflamed tissues.49 Inhibiting Ang-223 or VE-PTP67 can also decrease leukocyte infiltration, stabilizing the endothelial barrier. Furthermore, combined inhibition of Ang-2 and VEGF-A was shown to inhibit leukocyte infiltration into the retina compared with untreated controls, anti–Ang-2 alone, or anti–VEGF-A alone in a uveitis mouse model.39,40 In laser-induced CNV rodent models of nAMD, increasing Ang-1 levels,26,68,69 inhibiting VE-PTP,36 or activating Tie270 can decrease VEGF-A–induced vascular leakage and reduce CNV lesion size. Similarly, in the JR5558 mouse model of spontaneous CNV, anti–Ang-2 or anti–VEGF-A/anti–Ang-2 reduced CNV lesion number and vascular leakage.39,40,71 The combination treatment demonstrated a potential additive effect superior to either anti–VEGF-A or anti–Ang-2 alone.71 Recent evidence in a mouse model of autoimmune encephalomyelitis also suggests a neuroprotective effect of the anti–Ang-2 antibody, treatment with which resulted in a decrease in neuroinflammation, spinal cord demyelination, and leukocyte infiltration into the central nervous system.72

Angiopoietin/Tie Pathway and Fibrotic Response

Besides its role in vascular stability and inflammation, the involvement of the angiopoietin/Tie pathway in regulating fibrosis was shown by the heightened fibrotic response in a diabetic mouse model deficient in Ang-1. ANGPT-1/ANGPT-2 are genes that control the expression of Ang-1 and Ang-2, respectively. Increased ANGPT-2/ANGPT-1 ratios in glomerular cell fractions were found in an ANGPT-1/2 conditional knockdown mouse model, in which higher mortality rates due to kidney failure from enhanced glomerular scarring compared with controls suggested that a dysregulated angiopoietin/Tie pathway enhanced fibrotic response.73 Additional data are needed to evaluate whether angiopoietin/Tie-dependent fibrosis plays a role in retinal diseases.

Clinical Development of Potential Therapeutics Targeting the Angiopoietin/Tie Pathway

There are several potential approaches to manipulate the angiopoietin/Tie pathway in retinal diseases, including increasing Ang-1 levels, inhibiting VE-PTP, or neutralizing Ang-2. To date, three molecules targeting the angiopoietin/Tie pathway have been studied in Phase 2 trials for DME, DR, and nAMD, namely the VE-PTP inhibitor AKB-9778 (Aerpio Pharmaceuticals, Inc.), the anti–Ang-2 antibody nesvacumab and aflibercept combination therapy (Regeneron Pharmaceuticals, Inc.), and the bispecific anti–Ang-2/anti–VEGF-A antibody faricimab (F. Hoffmann La Roche Ltd.; Table 1).74–84 Several other pipeline molecules are in early clinical or preclinical development (see Table, Supplemental Digital Content 3, http://links.lww.com/IAE/B336).60,68,85,86

Table 1. - Completed Clinical Trials for Molecules Targeting the Angiopoietin/Tie2 Pathway for Retinal Vascular Diseases
Clinical Trial Study Design Arms Key Outcomes
BCVA (Mean Change in ETDRS Letter Score) CST (Mean Change From Baseline, µm) Others
AKB-9778 Aerpio Pharmaceuticals, Inc.; VE-PTP small molecule inhibitor; subcutaneous administration
 Phase 1 NCT01702441, DME76 4-week,* open-label, dose-escalation clinical trial, N = 24 AKB-9778 5 mg BID
AKB-977 15 mg BID
AKB-977 22.5 mg BID
AKB-977 30 mg BID
1.83 ± 6.2 (mean ± SD)*
6.7 ± 7.1*
6.0 ± 3.2*
6.7 ± 8.7*
All doses well tolerated
Modest, transient reduction in blood pressure, and vasodilatory- related AEs with higher doses
 Phase 2a TEVO NCT02387788, RVO,80 no results published 84-day open-label study, N = 16 AKB-9778 15 mg BID
 Phase 2a TIME-2 NCT02050828, DME,75 primary endpoint met 12 weeks of treatment,* followed by 8 weeks of observation, N = 144 AKB-9778 15 mg BID



AKB-9778 15 mg BID + monthly ranibizumab 0.3 mg
Monthly ranibizumab 0.3 mg
1.5 ± 1.2 (SEM)



6.3 ± 1.3 (SEM)


5.7 ± 1.2 (SEM)
6.2 ± 13.0 µm*



−164.4 ± 24.2 µm*


−110.4 ± 17.2 µm*
P = 0.008 vs. combination
% With at least 2-step DRSS score improvement: 10.0%

 11.4%


 8.8%
 Phase 2 TIME-2b NCT03197870, NPDR, Aerpio Pharmaceuticals, Inc., 2019,74 primary endpoint not met 48-week double-masked, placebo-controlled, multicenter study, N = 167 AKB-9788 15 mg QD
AKB-9788 15 mg BID
Placebo BID
%With at least two-step DRSS score improvement*: not reported
9.6%*
3.8%*
P = 0.270 vs. AKB-9778 15 mg QD
REGN910-3 Regeneron Pharmaceuticals, Inc; nesvacumab (anti–Ang-2 antibody) + aflibercept (recombinant protein inhibitor of VEGF); intravitreal administration
 Phase 1 NCT01271972,
 advanced solid tumor
 Papadopoulos et al 81
4-week, nonrandomized, open- label, multicenter, ascending, multiple-dose study, N = 47 Acceptable safety profile
No dose-limiting toxicities
AEs included fatigue (23.4%), peripheral edema (21.3%), decreased appetite (10.6%), and diarrhea (10.6%): all grade ≤2
 Phase 2 RUBY NCT02712008, DME,78 primary endpoint not met 36-week, randomized, double-masked, active-controlled study, N = 302 12 weeks
REGN910-3 (3 mg: 2 mg) Q4W for three doses then Q8W at 16 weeks up to 32 weeks 6.8 ± 7.30* (mean ± SD), P = 0.1368 vs. aflibercept 2 mg Q4W −169.4 ± 155.86 µm (mean ± SD) % With at least 2-step DRSS score improvement: 13.3%
REGN910-3 (6 mg: 2 mg) Q4W for three doses up to 12 weeks 8.5 ± 6.89,*P = 0.9716 vs. aflibercept 2 mg Q4W −184.0 ± 143.69 µm 21.3%
Aflibercept (2 mg) Q4W for three doses up to 12 weeks 8.8 ± 9.71* −174.6 ± 160.36 µm 15.2%
36 weeks
REGN910-3 (3 mg: 2 mg) Q4W for three doses then Q8W at 16 weeks up to 32 weeks 9.8 ± 9.92* (mean ± SD), P = 0.1665 vs. aflibercept 2 mg Q4W to aflibercept Q8W −210.4 ± 164.06 µm % With at least 2-step DRSS improvement: 26.7%
REGN910-3 (6 mg: 2 mg) Q4W for three doses then at 12 weeks rerandomized to receive REGN910-3 (6 mg: 2 mg) Q8W at 16 weeks up to 32 weeks 10.3 ± 8.20,*P = 0.2223 vs. aflibercept 2 mg Q4W to aflibercept Q8W, P = 0.3159 vs. REGN910-3 Q4W to REGN910-3 Q12W −223.4 ± 145.35 µm 34.1%
REGN910-3 (6 mg: 2 mg) Q4W for three doses then at 12 weeks rerandomized to receive REGN910-3 (6 mg: 2 mg) Q12W at 20 weeks up to 32 weeks 8.5 ± 7.74,*P = 0.7943 vs. aflibercept 2 mg Q4W to aflibercept Q8W, P = 0.8537 vs. aflibercept Q4W to aflibercept Q12W −193.7 ± 158.29 µm 34.0%
Aflibercept (2 mg) Q4W for three doses then at 12 weeks rerandomized to receive aflibercept (2 mg) Q8W at 16 weeks up to 32 weeks 8.7 ± 10.65,*P = 0.6655 vs. aflibercept 2 mg Q4W to aflibercept Q12W, P = 0.1278 vs. aflibercept Q4W to REGN910-3 (6 mg: 2 mg) Q8W −161.9 ± 125.99 µm 26.1%
Aflibercept (2 mg) Q4W for three doses then at 12 weeks rerandomized to receive aflibercept (2 mg) Q12W at 20 weeks up to 32 weeks 10.0 ± 10.37* −210.6 ± 202.15 µm 25.5%
Aflibercept (2 mg) Q4W for three doses then at 12 weeks rerandomized to receive REGN910-3 (6 mg: 2 mg) Q8W at 16 weeks up to 32 weeks 11.9 ± 12.01* −203.7 ± 163.08 µm 35.4%
 Phase 2 ONYX NCT02713204, nAMD,79 primary endpoint not met 36-week, randomized, double-masked, active-controlled study, N = 365 12 weeks
REGN910-3 (3 mg: 2 mg) Q4W for three doses then Q8W at 16 weeks up to 32 weeks 5.2 ± 10.51* (mean ± SD), P = 0.9894 vs. aflibercept 2 mg −182.2 ± 172.73 µm (mean ± SD)
REGN910-3 (6 mg: 2 mg) Q4W for three doses up to 12 weeks 5.6 ± 10.59,*P = 0.8346 vs. aflibercept 2 mg −200.0 ± 152.82 µm
Aflibercept (2 mg) Q4W for three doses up to 12 weeks 5.4 ± 9.85* −178.6 ± 138.85 µm
36 weeks
REGN910-3 (3 mg: 2 mg) Q4W for three doses then Q8W at 16 weeks up to 32 weeks 5.9 ± 11.95* (mean ± SD), P = 0.4611 vs. aflibercept 2 mg Q4W to aflibercept Q8W −174.6 ± 165.22 µm (mean ± SD)
REGN910-3 (6 mg: 2 mg) Q4W for three doses then at 12 weeks rerandomized to receive REGN910-3 (6 mg: 2 mg) Q8W at 16 weeks up to 32 weeks 6.0 ± 12.00,*P = 0.225 vs. aflibercept 2 mg Q4W to aflibercept Q8W, P = 0.4690 vs. REGN910-3 Q4W to REGN910-3 Q12W −216.6 ± 187.40 µm
REGN910-3 (6 mg: 2 mg) Q4W for three doses then at 12 weeks rerandomized to receive REGN910-3 (6 mg: 2 mg) Q12W at 20 weeks up to 32 weeks 6.4 ± 12.24,*P = 0.6063 vs. aflibercept Q4W to aflibercept Q8W, P = 0.1267 vs. aflibercept 2 mg Q4W to aflibercept Q12W −181.3 ± 148.71 µm
Aflibercept (2 mg) Q4W for three doses then at 12 weeks rerandomized to receive aflibercept (2 mg) Q8W at 16 weeks up to 32 weeks 6.9 ± 12.49,*P = 0.0426 vs. aflibercept Q4W to aflibercept Q12W, P = 0.0073 vs. aflibercept Q4W to REGN910-3 Q8W −198.4 ± 155.72 µm
Aflibercept (2 mg) Q4W for three doses then at 12 weeks rerandomized to receive aflibercept (2 mg) Q12W at 20 weeks up to 32 weeks 4.2 ± 12.50* −169.7 ± 129.74 µm
Aflibercept (2 mg) Q4W for three doses then at 12 weeks rerandomized to receive REGN910-3 (6 mg: 2 mg) Q8W at 16 weeks up to 32 weeks 2.9 ± 12.16* −187.3 ± 161.72 µm
Faricimab
F. Hoffmann-La Roche Ltd., bispecific antibody targeting both Ang-2 and VEGF-A; intravitreal administration
 Phase 1 NCT01941082, nAMD77 16- to 24-week, open-label, single, and multiple ascending dose study, N = 24 Single-dose faricimab 0.5, 1.5, 3, and 6 mg
Three doses of faricimab 3 mg monthly
Three doses of faricimab 6 mg monthly
No dose-limiting toxicities
Mild AEs
 Phase 2 BOULEVARD NCT02699450, DME,82 primary endpoint met 36-week, prospective, multiple-center, multiple-dose, randomized, active comparator–controlled, double-masked study, N = 229 Anti-VEGF treatment naïve
Ranibizumab 0.3 mg Q4W up to Week 20 observation to Week 36 10.3 (80% CI, 8.8–11.9)* −204.7 µm (80% CI, −219.6 to −189.8) 12.2%
Faricimab 1.5 mg Q4W up to Week 20 observation to Week 36 11.7 (80% CI, 10.1–13.3)* −217.1 µm (80% CI, −233.0 to −201.2) 27.7%
Faricimab 6 mg Q4W up to Week 20 observation to Week 36 13.9 (80% CI, 12.2–15.6),*P = 0.03 vs. ranibizumab 0.3 mg Q4W −225.8 µm (80% CI, −242.5 to −209.1) 38.6%
Previous anti-VEGF treatment
Ranibizumab 0.3 mg Q4W up to Week 20 observation to Week 36 8.3 (80% CI, 5.7–10.8)* −148.0 µm (80% CI, −167.7 to −128.4) 23.1%
Faricimab 6 mg Q4W up to Week 20 observation to Week 36 9.6 (80% CI, 7.0–12.3)* −186.6 µm (80% CI, −206.9 to −166.4) 22.7%
 Phase 2 AVENUE NCT02484690, nAMD,84 primary endpoint not met 36-week,* multicenter, randomized, double-masked, active comparator–controlled study, N = 273 Ranibizumab 0.5 mg Q4W up to Week 32 7.6* −176 µm
Faricimab 1.5 mg Q4W up to Week 32 9.2* −157 µm
Faricimab 6 mg Q4W up to Week 32 6.0* −173 µm
Faricimab 6 mg Q4W up to Week 12, Q8W up to Week 28 6.1* −148 µm
Ranibizumab 0.5 mg Q4W up to Week 8, faricimab 6 mg Q4W up to Week 32 7.2* −185 µm
 Phase 2 STAIRWAY NCT03038880, nAMD,83 primary endpoint met 52-week,* multicenter, randomized, active comparator–controlled study, N = 76 Ranibizumab 0.5 mg Q4W
Faricimab 6.0 mg Q12W
Faricimab 6.0 Q16W flex
9.6*
10.1*
11.4*
*Primary outcome.
Study not included in PubMed results on Ang-2.
AE, adverse event; Ang-2, angiopoietin-2; BCVA, best-corrected visual acuity; BID, twice daily; CI, confidence interval; CST, central subfield thickness; DME, diabetic macular edema; DRSS, Diabetic Retinopathy Severity Scale; ETDRS, Early Treatment Diabetic Retinopathy Study; nAMD, neovascular age-related macular degeneration; NPDR, nonproliferative diabetic retinopathy; RVO, retinal vein occlusion; Q4W, every 4 weeks; Q8W, every 8 weeks; Q12W, every 12 weeks; Q16W, every 16 weeks; QD, once daily; SEM, standard error of the mean; Tie, tyrosine kinase with immunoglobulin-like and endothelial growth factor–like domains; VEGF, vascular endothelial growth factor; VE-PTP, vascular endothelial protein tyrosine phosphatase.

AKB-9778

AKB-9778 is a small molecule inhibitor of VE-PTP and thereby activates Tie2 independently of the presence of Ang-1 or Ang-2.24,76 AKB-9778, which is delivered by self-administered subcutaneous injections, was studied in a Phase 1 trial (NCT01702441) in patients with DME,76 followed by Phase 2 trials for RVO (TEVO; NCT02387788; no results published), DME (TIME-2), and nonproliferative DR (NPDR; TIME-2b).

TIME-2 (NCT02050828) was a Phase 2 study in 144 patients with DME of twice-daily AKB-9778 15 mg, with or without monthly intravitreal ranibizumab 0.3 mg for 3 months followed by 2 months of observation.75 At Week 12, patients receiving AKB-9778 and ranibizumab had a significantly greater mean decrease from baseline in central subfield thickness (CST), the primary endpoint, compared with ranibizumab monotherapy (−164.4 ± 24.2 µm vs. −110.4 ± 17.2 µm, respectively; P = 0.008). No difference in vision gains at Week 12 was observed between the AKB-9778 and ranibizumab combination arm versus ranibizumab monotherapy (6.3 ± 1.3 vs. 5.7 ± 1.2 letters, respectively; P = 0.74). No notable mean changes from baseline at Week 12 in CST or vision from baseline were observed with AKB-9778 monotherapy (CST, 6.2 ± 13.0 µm; best-corrected visual acuity [BCVA], 1.5 ± 1.2 letters). Among the eyes with DR at baseline, the proportion of eyes with an at least two-step improvement on the Diabetic Retinopathy Severity Scale (DRSS) was similar across treatment groups. AKB-9778 appeared to be well tolerated, with no differences in serious adverse events across arms.

TIME-2b (NCT03197870) followed up on the potential for AKB-9778 monotherapy to affect DR. This 48-week Phase 2 trial compared AKB-9778 15 mg once or twice daily versus placebo in 167 patients with moderate to severe NPDR.74 The proportion of patients with at least two-step DRSS score improvement in the study eye at Week 48 (primary endpoint) was not statistically significantly different between the AKB-9778 and placebo arms (AKB-9778 twice daily, 9.6%; placebo, 3.8%; P = 0.270).

No further study of AKB-9778 in DME or DR has been announced since the completion of TIME-2b. However, because of encouraging findings in secondary endpoints in TIME-2 and TIME-2b (i.e., change from baseline BCVA; change in DRSS score in the study eye, or fellow eye, or both; safety; and pharmacokinetic analysis), other applications are being pursued.87 A topical AKB-9778 reformulation is being assessed for primary open-angle glaucoma because of an observed intraocular pressure–lowering effect,87 whereas a decreased urinary albumin creatinine ratio with AKB-9778 may indicate a potential to improve kidney function.74

REGN910-3

REGN910-3 is a coformulation of nesvacumab, a monoclonal antibody that selectively targets and neutralizes Ang-2, and aflibercept administered as a single intravitreal injection. REGN910-3 was evaluated for nAMD in a Phase 1 trial,88 and for DME and nAMD in the Phase 2 RUBY (Anti-vasculaR Endothelial Growth Factor plUs Anti-angiopoietin 2 in Fixed comBination therapY; NCT02712008)78 and ONYX (Anti-angiOpoeitin 2 Plus Anti-vascular eNdothelial Growth Factor as a therapY for Neovascular Age Related Macular Degeneration: Evaluation of a fiXed Combination Intravitreal Injection; NCT02713204)79 trials, respectively.

RUBY (N = 302) was a 36-week study comparing aflibercept 2.0 mg with a low-dose combination of nesvacumab 3.0 mg/aflibercept 2.0 mg and high-dose combination of nesvacumab 6.0 mg/aflibercept 2.0 mg, administered Q4W for 12 weeks in patients with DME. After Week 12, patients were rerandomized (Table 1) to assess additional dosing regimens, including up to every 12 weeks (Q12W) administration of the high-dose combination, and aflibercept monotherapy up to Week 36. The primary endpoint was change from baseline in BCVA (Early Treatment Diabetic Retinopathy Study [ETDRS] letter score) at Weeks 12 and 36. At Week 12, there were no significant differences in mean BCVA gains from baseline between aflibercept monotherapy (+8.8 letters) and the low-dose (+6.8 letters; P = 0.1368 vs. aflibercept; 95% confidence interval [CI], −4.84 to 0.67) or high-dose combinations (+8.5 letters; P = 0.9716 vs. aflibercept; 95% CI, −2.10 to 2.18). However, mean central retinal thickness change from baseline was significantly lower in the high-dose combination group at Week 12 versus aflibercept (−184.0 vs. −174.6 µm, respectively; P = 0.0183; 95% CI, −50.83 to −4.74). In addition, a higher proportion of patients on the high-dose combination had complete resolution of fluid at the foveal center at Week 12 (66.3% vs. 53.7%, respectively; P = 0.0489) and normalization of macular thickness (central retinal thickness ≤300 µm; 57.6% vs. 35.3%, respectively; P = 0.0006).89 The proportion of patients with at least two-step improvement in DRSS score at Week 12 was 21.3% with the high-dose combination versus 15.2% with aflibercept monotherapy (P = 0.2268; 95% CI, −4.1 to 16.3). From Weeks 12 to 36, BCVA gains were generally maintained, with no significant differences across groups. Anatomic outcomes generally continued to improve to Week 36, with the high-dose combination maintaining an advantage over aflibercept monotherapy. The safety profiles of the nesvacumab/aflibercept combinations were reportedly similar to aflibercept monotherapy in patients with DME.78

ONYX (N = 365) was a Phase 2 trial of the nesvacumab/aflibercept combination in patients with treatment-naïve nAMD. Similar to RUBY, patients were randomized to a low-dose combination, high-dose combination, or aflibercept monotherapy administered Q4W to Week 12, and then rerandomized to additional dosing regimens, including up to Q12W administration, through Week 36 (Table 1). The primary endpoint was change from baseline in BCVA (ETDRS letter score) at Weeks 12 and 36. At Week 12, there were no significant differences in mean BCVA gains from baseline between aflibercept monotherapy (+5.4 letters) and the low-dose (+5.2 letters; P = 0.9894 vs. aflibercept monotherapy; 95% CI, −2.99 to 3.03) and high-dose (+5.6 letters; P = 0.8346; 95% CI, −2.09 to 2.59) combinations. Improvements from baseline at Week 12 in anatomic outcomes were generally comparable across groups (mean change in central retinal thickness, −178.6, −182.2, and −200.0 µm, respectively), although a numerically higher proportion of patients treated with the nesvacumab/aflibercept combination achieved complete fluid resolution (percentage of patients, 44%, 49%, and 51%, respectively). From Weeks 12 to 36, there was a potential trend toward increased retinal dryness with the high-dose nesvacumab/aflibercept combination given every 8 weeks (Q8W) compared with aflibercept monotherapy. The safety profiles of the nesvacumab/aflibercept combinations were reportedly similar to aflibercept monotherapy in patients with nAMD.79

After the results of the Phase 2 trials, Regeneron has reportedly discontinued development of the nesvacumab/aflibercept coformulation for DME and nAMD, according to a press release by Regeneron.90

Faricimab

Faricimab (RG7716) is a bispecific, immunoglobulin G1–based antibody targeting both Ang-2 and VEGF-A, designed using Roche's proprietary CrossMAb technology. Faricimab can bind Ang-2 and VEGF-A at the same time, without steric hindrance, and does not require binding of either molecule to permit binding of the other. The fragment crystallizable (Fc) portion was engineered to reduce potential for inflammation and to allow a faster systemic clearance by eliminating binding interactions with neonatal Fc and Fcγ receptors.39,40 Faricimab was studied in >500 patients across a Phase 1 trial in patients with nAMD (NCT01941082)77 and in three Phase 2 trials, BOULEVARD (A Study of Faricimab in Participants With Center-involving DME), AVENUE (A Proof-of-Concept Study of Faricimab in Participants With CNV Secondary to AMD), and STAIRWAY (A Study to Evaluate Faricimab for Extended Durability in the Treatment of nAMD).

BOULEVARD (NCT02699450) was a 36-week Phase 2 trial in patients with treatment-naïve (primary population; N = 168) and previously anti–VEGF-treated DME (exploratory population; N = 61). Patients were randomized to faricimab 6.0 mg, faricimab 1.5 mg (treatment naïve only), or ranibizumab 0.3 mg Q4W for 24 weeks with an additional cohort of previously treated patients who were randomized to receive faricimab 6.0 mg or ranibizumab 0.3 mg, followed by an off-treatment observation period through Week 36.82 The primary endpoint was mean change in BCVA from baseline at Week 24 with faricimab versus ranibizumab in treatment-naïve patients. In the treatment-naïve population, faricimab 6.0 mg–treated patients achieved statistically greater mean BCVA gains than ranibizumab 0.3 mg at Week 24 (adjusted mean BCVA change from baseline, faricimab 6.0 mg, +13.9; faricimab 1.5 mg, +11.7; ranibizumab 0.3 mg, +10.3 letters; faricimab 6.0 mg vs. ranibizumab 0.3 mg, +3.6 letters; P = 0.03; 80% CI, 1.5–5.6 letters; prespecified significance level P < 0.2). The proportion of patients achieving ≥10-letter gains at Week 24 was 72%, 61%, and 59%, respectively, in the faricimab 6.0 mg, faricimab 1.5 mg, and ranibizumab 0.3 mg treatment arms. At Week 24, adjusted mean change in CST from baseline was −225.8, −217.1, and −204.7 µm, respectively, with a higher percentage of faricimab 6.0 mg–treated patients achieving CST ≤325 µm (faricimab 6.0 mg, 77.3%; faricimab 1.5 mg, 63.3%; and ranibizumab 0.3 mg, 61.2%). The proportion of patients with at least two-step improvement in DRSS score at Week 24 was 38.6%, 27.7%, and 12.2%, respectively.

In the previously anti–VEGF-treated cohort, the faricimab 6.0 mg and ranibizumab 0.3 mg arms had similar mean BCVA gains (+9.6 and +8.3 letters, respectively) and at least two-step DR severity improvements (22.7% and 23.1%), whereas CST changes (−186.6 and −148.0 µm, respectively) and proportion of patients achieving ≥10-letter gains (60% and 43%, respectively) and CST ≤325 µm (87.0% and 53.6%, respectively) were numerically greater in the faricimab 6.0 mg arm at Week 24.

Durability was assessed by Kaplan–Meier analyses using strict retreatment criteria based on BCVA and CST measurements in the off-treatment observation period. If these criteria were met, the patient was administered ranibizumab and exited the study. In both patient populations, faricimab 6.0 mg–treated patients had a longer time to retreatment versus ranibizumab-treated patients. The ocular and systemic safety profile of faricimab was comparable with ranibizumab in patients with DME, with no new or unexpected safety outcomes.

The nAMD Phase 2 program consisted of two studies, AVENUE (NCT02484690) and STAIRWAY (NCT03038880), with AVENUE testing the potential for Q8W dosing of faricimab and superior BCVA gains compared with monthly ranibizumab, and STAIRWAY for sustained efficacy through extended durability up to every 16 weeks (Q16W). AVENUE was a Phase 2 trial that enrolled 273 patients with treatment-naïve nAMD in a 36-week assessment of faricimab at different dosing regimens compared with ranibizumab. The primary endpoint was mean change in BCVA from baseline to Week 36. Patients were randomized to receive faricimab 1.5 mg Q4W, faricimab 6.0 mg Q4W, and faricimab 6.0 mg Q4W up to Week 12, followed by faricimab 6.0 mg Q8W, and ranibizumab 0.5 mg Q4W for 8 weeks, followed by faricimab 6.0 mg Q4W and ranibizumab 0.5 mg Q4W.84 Superiority was not achieved at Week 36, with all arms achieving comparable vision gains from baseline (mean BCVA change from baseline, ETDRS letters: ranibizumab Q4W, + 7.6; faricimab 1.5 mg Q4W, +9.2; faricimab 6.0 mg Q4W, +6.0; faricimab 6.0 Q8W, +6.1; and ranibizumab 0.5 mg/faricimab 6.0 mg, +7.2) and comparable CST reductions (mean CST change from baseline, µm: −176, −157, −173, −148, and −185, respectively), with no new or unexpected safety outcomes with faricimab.84

STAIRWAY (NCT03038880) was a 52-week Phase 2 trial designed to evaluate fixed Q16W and Q12W dosing with faricimab in patients with treatment-naïve nAMD. Patients (N = 76) were randomized to faricimab 6.0 mg Q16W flex, faricimab 6.0 mg Q12W, or ranibizumab 0.5 mg Q4W. The primary objective was to evaluate the efficacy of faricimab administered at 16-week and 12-week intervals, as assessed by BCVA at Week 40. At Week 24, patients were evaluated for disease activity presence based on six protocol-defined criteria that included a decrease of ≥5 BCVA letters versus average over the last two visits, a decrease of ≥10 BCVA letters versus highest BCVA over the last two visits (due to nAMD), an increase of >50 µm versus average CST over the last 2 visits on spectral-domain optical coherence tomography, an increase of ≥75 µm over the lowest CST over the last two visits on spectral-domain optical coherence tomography, and presence of new macular hemorrhage, as well as any other disease activity based on investigator opinion. Those in the Q16W flex arm with active disease received faricimab 6.0 mg at 12-week intervals until study end.83 At Week 24, 65% (36/55) of faricimab-treated patients had no active disease according to the prespecified criteria. At Week 52, faricimab 6.0 mg Q16W flex, faricimab 6.0 mg Q12W, and ranibizumab 0.5 mg Q4W achieved comparable mean BCVA gains from baseline (ETDRS letters: +11.4, +10.1, and +9.6, respectively), and comparable CST reductions (mean CST change from baseline, µm: −123, −139, and −130, respectively).83 There were no new or unexpected safety outcomes with faricimab.

Based on the results of BOULEVARD, AVENUE, and STAIRWAY, four global Phase 3 trials were initiated: YOSEMITE and RHINE for DME, and TENAYA and LUCERNE for nAMD. YOSEMITE (NCT03622580) and RHINE (NCT03622593) commenced in 2018 to assess safety and efficacy of faricimab Q8W, faricimab per personalized treatment interval (extended dosing intervals tailored to needs of individual patients), and aflibercept Q8W in ∼2010 patients with DME. The primary outcome is mean BCVA change from baseline at Year 1 as an average of Weeks 48, 52, and 56. TENAYA (NCT03823287) and LUCERNE (NCT03823300) commenced in 2019 to assess the safety and efficacy of faricimab at Q16W, Q12W, and Q8W dosing regimens compared with aflibercept Q8W in ∼1,280 patients with nAMD. The primary endpoint is mean change in BCVA at Week 48 averaged over Weeks 40, 44, and 48. The YOSEMITE, RHINE, TENAYA, and LUCERNE trials were fully enrolled by the end of 2019 and are estimated to be completed in 2021 for YOSEMITE/RHINE91,92 and 2022 for TENAYA/LUCERNE.93,94

Discussion

To date, of the three molecules targeting the angiopoietin/Tie pathway assessed in Phase 2 trials for both DME and nAMD, only faricimab has progressed into Phase 3 trials. Although the other molecules in Phase 2 (AKB-9778 and REGN910-3) failed to meet their primary endpoints, signs of additional benefit were seen in their trials with modulation of the angiopoietin/Tie pathway plus anti–VEGF-A versus anti–VEGF-A monotherapy. Whether the theoretical antifibrotic, neuroprotective, antiinflammatory, and/or vascular-stabilizing properties of blocking Ang-2/activating Tie2 will provide clinical benefit is yet to be determined.

Although AKB-9778, REGN910-3, and faricimab all target the angiopoietin/Tie pathway, the approaches to the drug and Phase 2 trial designs were notably different. The Phase 2 trials for AKB-9778 and nesvacumab/aflibercept focused on early (12-week) improvements in vision and anatomical outcomes, whereas the faricimab trials had later primary endpoints (24–40 weeks) and additionally assessed the potential for longer treatment intervals, including Q16W in STAIRWAY.

AKB-9778 was delivered by subcutaneous injection, not intravitreal injection, which may have limited its intraocular availability and therefore potentially limited its efficacy in DME and NPDR. A new bispecific antibody for intravitreal injection that inhibits VEGF and activates Tie2 is currently being investigated by Aerpio for the treatment of nAMD and DME.87 Another VE-PTP inhibitor, the monoclonal antibody ARP-1536, in combination with anti-VEGF therapy for the treatment of diabetic complications, remains in Aerpio's early-stage pipeline,87 suggesting that there may be further clinical development.

Although the nesvacumab/aflibercept coformulation did not improve vision outcomes (primary outcome) over aflibercept at Week 12, there was an apparent advantage in anatomical outcomes with the high-dose combination. However, Regeneron has not pursued Phase 3 trials of the nesvacumab/aflibercept coformulation for DME and nAMD at this time. Furthermore, the observed clinical response of a combination therapy may vary as additive, synergistic, or antagonistic because of the nature of interactions between the antibodies, as seen in a clinical trial on the combination of anti–epidermal growth factor receptor and anti-VEGF antibodies in patients with colorectal metastatic cancer.95 The variable pharmacokinetics and pharmacodynamics of the constituent antibodies may also have a negative effect on the efficacy of the combination or even lead to toxicity.95 In general, many combination trials do not progress to later phases because of insufficient safety and efficacy shown.96

To date, anatomic assessments of improvement in DME, DR, nAMD, and RVO have focused on the VEGF-A–driven pathologies of leakage and neovascularization, largely measured through spectral-domain optical coherence tomography, fundus photography, and fluorescein angiography. Although modulation of the angiopoietin/Tie pathway would also be expected to affect these outcomes, the benefits over blocking only VEGF-A may be difficult to discern and may not be fully apparent using these tools alone because of some inherent limitations associated with them, such as the inability to directly measure vessel stabilization and integrity, crude assessment of flow dynamics, the difficulties encountered in studying retinal function, and the invasive nature of the injection of a dye. Other, newer imaging modalities, such as optical coherence tomography angiography, adaptive optics, and fluorescent lifetime imaging ophthalmoscopy, among others, in which the contrast is based on the lifetime of individual fluorophores rather than their emission spectra, may be better suited to provide a more comprehensive assessment of the status of the retina and response to treatment, and potentially visualize the benefits of modulating the angiopoietin/Tie pathway. A further advantage of fluorescent lifetime imaging ophthalmoscopy is that fluorescence lifetime is dependent on the metabolic environment, so the technique could be beneficial in investigating metabolic changes in DME and the impact of Ang-2 inhibition. In contrast, fluorescent lifetime imaging ophthalmoscopy may be more difficult to interpret because of lack of standardization between the already limited devices and sites.

Although DR/DME, RVO, and nAMD have different pathophysiologic mechanisms and involved vasculatures, there are overlapping goals of therapy, with the objective of improving vision through stabilizing blood vessels, preventing/stabilizing fibrosis, decreasing inflammation, and enhancing neuroprotection. Longer trials (possibly ≥1–2 years) and novel imaging methodologies that more completely elucidate functional, in addition to anatomical, retinal characteristics, may be required to determine whether the theoretical antifibrotic, neuroprotective, antiinflammatory, and/or vascular stabilizing properties of blocking Ang-2 will provide clinical benefit. Altogether, evidence from preclinical to Phase 2 studies suggests there is activity and/or additional benefit of modulating the angiopoietin/Tie pathway in addition to targeting VEGF-A.

Conclusions

The angiopoietin/Tie pathway plays a key role in physiological and pathological vascular instability and angiogenesis in retinal vascular diseases, and as such is of interest for the development of new therapeutics. Clinical and preclinical evidence suggests a potential benefit of a multipathway approach by targeting the angiopoietin/Tie pathway and VEGF-A over anti–VEGF-A monotherapy alone, in part because of the synergistic nature of the pathways.

Acknowledgments

Third-party writing assistance (article draft preparation and revision per author direction) was provided by Kathryn Condon, PhD, and Priyanka Narang, PhD, of Envision Pharma Group and funded by F. Hoffmann-La Roche Ltd.

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Keywords:

angiopoietins; angiopoietin/Tie pathway; Ang-1; Ang-2; retinal vascular diseases; Tie2; vascular stability; VEGF-A

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