The Landscape of Vascular Endothelial Growth Factor Inhibition in Retinal Diseases

Abstract

Ever since the US Food and Drug Administration (FDA) approved the first vascular endothelial growth factor (VEGF) antagonist 2 decades ago, inhibitors of VEGF have revolutionized the treatment of a variety of ocular disorders involving pathologic neovascularization and retinal exudation. In this perspective, we evaluate the current status of anti-VEGF therapies and the real-world challenges encountered with maintaining therapeutic outcomes. Finally, we describe novel VEGF-based and combinatorial approaches that are in clinical development.

Vascular endothelial growth factor (VEGF) inhibitors are the first-line therapy for patients suffering from retinal disorders involving pathologic ocular neovascularization and retinal exudation including in neovascular age-related macular degeneration (AMD) and diabetic retinopathy.¹ The initial VEGF-targeting aptamer, pegaptanib, received US Food and Drug Administration (FDA) approval in 2004 but the current VEGF-inhibiting repertoire also includes the monoclonal antibodies bevacizumab, ranibizumab, and brolucizumab; fusion protein aflibercept; and several biosimilars that are currently in clinical development and various stages of approval. Despite the remarkable impact of these VEGF-based therapies for many patients, limitations include subtherapeutic initial response in a minority of patients² and loss of initial visual acuity gains that is associated with chronic therapy.^3,4 The loss of vision related to chronic therapy can be due to atrophy,³ due to subretinal fibrosis,⁵ or a combination of the two,⁶ and this can occur despite chronic anti-VEGF pharmacotherapy. Furthermore, the real-world effectiveness of these therapeutics is also hampered by the significant treatment and caregiver burden associated with numerous clinic visits for intravitreal administration of these agents, as well as the associated financial costs and rare risks of treatment-associated complications, including endophthalmitis. There also remains ambiguity regarding the clinical effects of anti-VEGF agents, such as improvements in thickening assessed by optical coherence tomography not always correlating with improvements in visual acuity.^1,7,8 These real-world challenges associated with current anti-VEGF therapies have set the stage for the tremendous interest and opportunities to explore novel therapeutic options that may augment the significant gains associated with anti-VEGF therapies.

Novel VEGF-Targeting Approaches

Vascular Endothelial Growth Factor-C/D

All of the current therapies target VEGF-A, although aflibercept also targets VEGF-B and placental growth factor (PlGF). Currently, therapeutics in development also inhibit VEGF-C or VEGF-D (see Table 1) that have been shown to be activated as a compensatory response to VEGF-A inhibition in the eye as well as in other contexts, such as tumorigenesis.^9–12 Inhibition of these compensatory VEGF pathways may offer more durable angiogenesis-suppressing outcomes. IBI333 is a recombinant bispecific fusion protein that targets both VEGF-A and VEGF-C (NCT05639530). OPT-302 is a VEGF-C/D inhibitor and is currently being evaluated in phase III trials in conjunction with either aflibercept (COAST/NCT04757636) or ranibizumab (ShORe/NCT04757610). These new VEGF-targeting therapies with extended spectrum of action against several VEGF family proteins may offer a more durable clinical response, especially for patients that may have a suboptimal response to currently approved therapies.

Table 1.

Novel VEGF-Targeting Therapeutics in Clinical Development for Neovascular AMD or Diabetic Macular Edema Since September 2019

Target	Drug	Entity	Formulation	Delivery	Clinical Trial Status (Trial Name/ClinicalTrials.gov ID)
Serine/arginine-rich protein-specific kinase 1	EXN407	Exonate	Small molecule inhibitor targeting serine/arginine-rich protein-specific kinase 1	Topical	Phases I and II(NCT04565756)
Tyrosine kinase	OTX-TKI	Ocular Therapeutix	Small molecule inhibitor of tyrosine kinase (axitinib)	Intravitreal implant	Phase III(NCT06495918 and NCT06223958)
Tyrosine kinase	CLS-AX	Clearside Biomedical	Small molecule inhibitor of tyrosine kinase (axitinib)	Suprachoroidal	Phase II(ODYSSEY/NCT05891548)
Tyrosine kinase	AR-14034 SR	Alcon Research	Small molecule inhibitor of tyrosine kinase (axitinib)	Intravitreal implant	Phases I and II(NOVA-1/NCT05769153)
Tyrosine kinase	AIV007	AiViva BioPharma	Small molecule inhibitor of tyrosine kinase (lenvatinib)	Periocular	Phase I(NCT05698329)
Tyrosine kinase	GB-102	Graybug Vision	Small molecule inhibitor of tyrosine kinase (sunitinib)	Intravitreal	Phase II(ALTISSIMO/NCT03953079)
Tyrosine kinase	EYP-1901	EyePoint Pharmaceuticals	Small molecule inhibitor of tyrosine kinase (vorolanib)	Intravitreal implant	Phase II(VERONA/NCT06099184, DAVIO2/NCT05381948, and NCT05383209)
Tyrosine kinase	D-4517.2	Ashvattha Therapeutics	Hydroxyl dendrimer inhibiting tyrosine kinase (dendranib)	Subcutaneous	Phase II(Tejas/NCT05387837)
Tyrosine kinase	KHK4951	Kyowa Kirin	Small molecule inhibitor of tyrosine kinase (tivozanib)	Topical	Phase II(NCT06116890 and NCT06116916)
Vascular endothelial growth factor	MG-O-1002	Theratocular Biotek	Unspecified	Topical	Phase II(NCT05390840)
Vascular endothelial growth factor-C	IBI333	Innovent Biologics	Bispecific fusion protein targeting vascular endothelial growth factor-A and -C	Intravitreal	Phase I(NCT05639530)
Vascular endothelial growth factor-C and -D	OPT-302	Opthea	Fusion protein targeting vascular endothelial growth factor-C and -D	Intravitreal	Phase III(COAST/NCT04757636 and ShORe/NCT04757610)

Tyrosine Kinase Inhibitors

The VEGF receptors responsible for VEGF's angiogenic effects are tyrosine kinase receptors. Tyrosine kinase inhibitors offer broad inhibition of all VEGF receptors 1 to 3, potentially providing more durable action beyond targeted VEGF-A inhibition. Tyrosine kinase inhibitors also have the potential to inhibit non-VEGF receptors, such as platelet-derived growth factor receptor (axitinib,¹³ AIV007,¹⁴ D-4517.2,¹⁵ sunitinib,¹⁵ and EYP-1901¹⁵) and fibroblast growth factor receptor (AIV007¹⁴). OTX-TKI, an axitinib implant, is being studied in phase III trials with aflibercept as a comparator (NCT06495918 and NCT06223958). Two other axitinib extended-release formulations are in phase I or II testing: CLS-AX (ODYSSEY/NCT05891548) and AR-14034 SR (NOVA-1/NCT05769153). AIV007 is a lenvatinib formulation in phase I testing (NCT05698329). GB-102, a sunitinib formulation, completed phase II testing in 2021 (ALTISSIMO/NCT03953079). EYP-1901 is a tyrosine kinase inhibitor currently in phase II testing (VERONA/NCT06099184, DAVIO2/NCT05381948, and NCT05383209). D-4517.2 is a hydroxyl dendrimer therapy administered subcutaneously that can specifically target choroidal neovascular lesions to inhibit tyrosine kinase receptors and is also in phase II testing (Tejas/NCT05387837).

Topical Delivery

There are several topical delivery systems in development to offer anti-VEGF therapies in more simplified, patient-centered regimens (see Table 1). Previous testing of topical tyrosine kinase inhibitors pazopanib¹⁶ and regorafenib¹⁷ failed to demonstrate clinical efficacy likely given poor bioavailability of these therapeutics at the sites of choroidal neovascularization (CNV). Despite these prior failed trials, interest in topical anti-VEGF agents remains high. MG-O-1002 is a topical anti-VEGF agent in phase II testing (NCT05390840). KHK4951 is a topical tyrosine kinase inhibitor (tivozanib) formulation also in phase II testing (NCT06116890 and NCT06116916). EXN407 is a topical drug that inhibits splicing of VEGF transcript by targeting serine/arginine-rich protein-specific kinase 1.¹⁸ EXN407 has completed phase I and phase II testing (NCT04565756).

Gene Therapies

Gene therapies can induce the expression of anti-VEGF molecules in native ocular cells, such as the retinal pigment epithelium to offer more durable VEGF inhibition as compared with current molecules delivered by intravitreal injection. The majority of these therapeutics utilize adeno-associated viral vectors to deliver gene therapies, but lentiviral and CRISPR/Cas methodologies can also be employed. Some gene therapeutics that are under current clinical testing are: LX102 in phase I and phase II testing (NCT06198413, NCT05831007, and VENUS/NCT06196840), ADVM-022 in phase II testing (LUNA/NCT05536973, INFINITY/NCT04418427, OPTIC-EXT/NCT04645212, and INFINITY-EXT/NCT05607810), SKG0106 in phase I and phase II testing (NCT06213038, NCT05986864, NCT06237777, and NCT06346600), RRG001 in phase I and phase II testing (NCT06141460 and NCT06412224), EXG102-031 in phase I and phase II testing (NCT06183814 and Everest/NCT05903794), FT-003 in phase I and phase II testing (NCT06492876 and NCT06492863), KH658 in phase I and phase II testing (NCT06458595), KH631 in phase I and phase II testing (NCT05657301 and NCT05672121), RGX-314 in phase II and phase III testing (AAVIATE/NCT04514653, ASCENT/NCT05407636, ATMOSPHERE/NCT04704921, ALTITUDE/NCT04567550, and SRLTFU/NCT03999801), 4D-150 in phase I and phase II testing (NCT05197270 and NCT05930561), HG202 in early phase I testing (SIGHT-I/NCT06031727), ABI-110 in phase I and phase II testing (NCT06550011), BD311 in early phase I testing (NCT05099094), and LX109 (NCT06022744).

Drug Targets Beyond VEGF

Suboptimal visual gains and the subtherapeutic response associated with chronic VEGF targeted therapies suggests that there may be VEGF-independent pathways involved in neovascular AMD offering novel therapeutic opportunities. In the following section, we summarize some of the main drug targets under clinical investigation (delineated in  Table 2).

Table 2.

Non-VEGF Drug Targets in Clinical Development for Neovascular AMD or Diabetic Macular Edema Since September 2019

Target	Drug	Entity	Formulation	Delivery	Clinical Trial Status (Trial Name/ClinicalTrials.gov ID)
Angiopoietin 1 receptor	AXT107	AsclepiX Therapeutics	Collagen peptide that targets VEGF & angiopoietin 1 receptor	Suprachoroidal	Phases I and II(DISCOVER/NCT05859776)
Angiopoietin 1 receptor	PMC-403	PharmAbcine	Monoclonal antibody targeting angiopoietin 1 receptor	Intravitreal	Phase I(NCT05953012)
Angiopoietin 2	Faricimab	Hoffmann-La Roche	Bispecific antibody against VEGF & angiopoietin 2	Intravitreal	Approved by FDA in 2022
Angiopoietin 2	ASKG712/AM712	AskGene Pharma, AffaMed Therapeutics	Bispecific molecule against VEGF and angiopoietin 2	Intravitreal	Phase I(NCT05940428, NCT05456828, and NCT05345769)
Angiopoietin 2	Zifibancimig	Hoffmann-La Roche	Bispecific antibody against VEGF & angiopoietin 2	Intravitreal, port delivery system	Phases I and II(BURGUNDY/NCT04567303)
Angiopoietin 2	BI 836880	Boehringer Ingelheim	Bispecific nanobody against VEGF and angiopoietin 2	Intravitreal	Phases I and II(NCT03861234)
B-cell lymphoma-extra large (senolytic)	UBX1325	Unity Biotechnology	Small molecule inhibitor	Intravitreal	Phase II(BEHOLD/NCT04857996, ASPIRE/NCT06011798, and ENVISION/NCT05275205)
Complement	Iptacopan	Novartis Pharmaceuticals	Small molecule inhibitor targeting factor B	Oral	Phase II(NCT05230537)
Complement	IBI302	Innovent Biologics	Bispecific fusion protein targeting VEGF & complement cascade	Intravitreal	Phase III(STAR/NCT05972473)
CXC chemokine receptor 3	OCU-10-C-110	Ocugenix Corporation	Small molecule drug activating CXC chemokine receptor 3	Intravitreal	Phase I(NCT05904691)
Fibroblast growth factor 2	RBM-007	Ribomic USA	Aptamer targeting fibroblast growth factor 2	Intravitreal	Phase II(TOFU/NCT04200248, RAMEN/NCT04640272, TEMPURA/NCT04895293, and SUSHI/NCT03633084)
Fibroblast growth factor 2	RC-28E	RemeGen Co	Bispecific fusion protein targeting VEGF & fibroblast growth factor 2	Intravitreal	Phase III(NCT05885503)
Integrin	AG-73305	Allgenesis Biotherapeutics	Bispecific fusion protein targeting VEGF & integrins	Intravitreal	Phase II(NCT05301751)
Integrin	OCU200	Ocugen	Fusion protein of tumstatin & transferrin	Intravitreal	Phase I(NCT05802329)
Interleukin 6	KSI-501	Kodiak Sciences	Bispecific protein/biopolymer conjugate targeting VEGF & interleukin 6	Intravitreal	Phase III(DAYBREAK/NCT06556368)
Interleukin 6	Vamikibart	Hoffmann-La Roche	Monoclonal antibody against interleukin 6	Intravitreal	Phase II(NCT05151731 and NCT05151744)
NLR family pyrin domain containing 3/4 (inflammasome)	K8	Inflammasome Therapeutics	Modified nucleoside reverse transcriptase inhibitor that targets NLR family pyrin domain containing 3/4	Intravitreal implant	Phase I(NCT05699759)
NOTCH regulated ankyrin repeat protein	SYL1801	Sylentis	Small interfering RNA targeting NOTCH regulated ankyrin repeat protein	Topical	Phase II(NCT05637255)
Wnt	EYE103	EyeBiotech	Trispecific antibody Wnt agonist	Intravitreal	Phases II and III(AMARONE/NCT05919693 and BRUNELLO/NCT06571045)

Angiopoietin

Faricimab received FDA approval in 2022, which, unlike previous anti-VEGF agents, is a bispecific inhibitor that also targets angiopoietin 2. Activation of the angiopoietin pathway is thought to stabilize neovascular lesions and desensitize the effects of VEGF. In the phase III clinical trials (YOSEMITE/NCT03622580, RHINE/NCT03622593, TENAYA/NCT03823287, and LUCERNE/NCT03823300), faricimab demonstrated similar clinical efficacy to aflibercept.^19–22 Although clinical trial results indicated that patients treated with faricimab could be extended to longer inter-injection intervals, it remains unclear whether dual inhibition of VEGF and angiopoietin 2 offers more durable anti-neovascular effects as compared with current anti-VEGF monotherapies which were not administered in a treat-and-extend protocol in clinical trials. This is also similar to clinical trials of high dose aflibercept (8 mg)²³ in which the comparator—aflibercept (2 mg)—was administered at a fixed dosing interval. The comparator does not reflect current clinical practice as most providers use treat-and-extend regimens as the neovascular lesions stabilize on clinical examination and imaging.²⁴ As such, the claim of increased inter-treatment interval for these clinical trial designs may need further evaluation.²¹ Nonetheless, angiopoietin remains a therapeutic target of high interest. Three additional drugs that, like faricimab, target both VEGF and angiopoietin 2 (ASKG712/AM712, zifibancimig, and BI 836880) are in phase I or II clinical testing (see Table 2). Angiopoietin 2 is an inhibitor of angiopoietin 1 receptor; thus, an alternative strategy would be to activate the angiopoietin 1 receptor directly. AXT107 and PMC-403 are angiopoietin 1 receptor agonists currently in phase I or II testing (see Table 2).

Fibroblast Growth Factors

Fibroblast growth factors (FGFs) encompass a family of signaling ligands that regulate the endothelial cell response to pro-angiogenic cues.²⁵ Preclinical studies demonstrated that specifically FGF2 is an essential ligand promoting CNV,^26–28 although other FGF ligands may also have a role.²⁹ Some of these studies have indicated that targeting both FGF2 and VEGF can inhibit endothelial cell sprouting beyond that achieved by VEGF inhibition alone.²⁸ Two therapeutics targeting FGF2—RBM-007 and RC-28E—are currently in either phase II or III testing (see Table 2).

Integrins

Integrins are ubiquitously expressed cell surface receptors that mediate cellular adhesion to other cells and to the extracellular matrix. Integrins mediate a wide variety of cellular processes, including motility, proliferation, apoptosis, and survival. Notably, integrins may also regulate the recruitment of peripheral immune cells to the eye in AMD and their activation in situ to angiogenesis-modulating phenotypes.³⁰ AG-73305 is a bispecific fusion protein that targets integrins and VEGF, and it is in phase II testing (see Table 2). OCU200 is a fusion protein of tumstatin and transferrin, of which the former targets VEGF and integrins. It is in phase I testing (see Table 2).

Complement

Complement inhibition has long been an attractive therapeutic avenue in the treatment of AMD, spurred by foundational pathologic,^31,32 genetic,^33–41 and other preclinical studies demonstrating that complement inhibition reduces CNV in animal models.^42,43 These research efforts have come to clinical fruition in 2023 with the C3 inhibitor pegcetacoplan and the C5-inhibiting aptamer avacincaptad pegol both receiving FDA approval for the treatment of geographic atrophy (GA) secondary to non-exudative AMD.⁴⁴ Although the FDA-approved complement inhibitors both decreased the progression of GA in clinical trials, they were associated with an increased risk for conversion to neovascular AMD with clinical trial data suggesting an approximately 1.5 to 6.5-fold increase in the development of neovascular AMD.^45,46 Although this somewhat mirrors the atrophic retinal neurodegeneration that has been associated with chronic anti-VEGF therapies,^47,48 further studies are necessary to determine the etiology of increased risk of exudation following therapy with complement inhibitors. There is significant interest in targeting complement in the treatment of neovascular AMD as well. Iptacopan, an oral factor B inhibitor, is in phase II testing (see Table 2). IBI302, a bispecific fusion protein targeting VEGF and the complement cascade, is in phase III testing (see Table 2). Preclinical studies have demonstrated a role for complement proteins in regulating CNV,⁴³ highlighting the importance of understanding the relationship among complement proteins, VEGF, and CNV.

Targeting Senescence

With age, senescent cells accumulate in multiple tissues and persistently secrete inflammatory mediators which is thought to promote age-related dysfunction in a variety of organ systems.^49–51 Preclinical studies have demonstrated that senescence and senescence associated secretory proteins (SASPs) may represent a common cellular endpoint of various ocular pathologies, including following microvascular ischemia in diabetic retinopathy.^52,53 Recently, preclinical studies have also linked intracellular cholesterol accumulation to macrophage and photoreceptor senescence via depleting NAD+,^54–57 providing a possible mechanistic link between the lipid-rich drusen pathognomonic for early stage AMD and retinal neurodegeneration in late stage non-exudative AMD. Taken together, these studies have provided the foundational scientific impetus for targeting senescent cells as a possible therapy for dry AMD. There is also a possible role for cellular senescence in neovascular AMD due upregulation of inflammatory and pro-angiogenic factors and impaired autophagy.⁵⁸ Nonetheless, the senolytic drug UBX1325 was unable to demonstrate noninferiority compared to aflibercept in the treatment of neovascular AMD (ENVISION/NCT05275205). However, significant interest remains in senolytic therapy for diabetic macular edema for which phase II testing is ongoing⁵⁹ (ASPIRE/NCT06011798).

Inflammatory Signaling

There is abundant basic science and clinical evidence that the immune system plays an important role in the pathogenesis of neovascular AMD.^56,60–72 Pathologic studies of AMD eyes indicates that ocular immune privilege is breached in both nonexudative and neovascular AMD with accumulation of immune cells, especially macrophages, near sites of drusen and neovascularization.^62–68 The exact function of these accumulated immune cells in promoting or inhibiting the disease process remains ambiguous. It is likely that immune cell ontogeny, activation state, age-related dysfunction, or other yet-unknown factors contribute to whether immune cells promote or inhibit ocular neovascularization.^30,70,73 For example, early preclinical studies demonstrated that macrophage depletion reduced the severity of angiogenesis in animal models of disease.^74,75 These seminal studies were corroborated by more recent studies demonstrating that there are specific macrophage subclasses that promote angiogenesis.^73,76,77 The macrophage activation state, driven by interactions with the local ocular milieu, also plays a key role, with proinflammatory macrophages (i.e. M1-like) thought to be anti-angiogenic.^30,70 Despite this ambiguity, several chemokine- or cytokine-targeting therapeutics are under study including those aimed at the CXC chemokine receptor 3 and interleukin 6 (see Table 2).

Wnt Signaling

Wnt signaling is a highly conserved pathway important for a wide variety of developmental processes.⁷⁸ Several studies have pointed to increased activation of Wnt signaling in AMD,^79–84 although whether this activation is a protective or pathologic mechanism is unknown. It is likely that the role of Wnt signaling is multifaceted and may have different disease roles in a ligand-, cell-, and context-dependent manner. EYE103 is a trispecific Wnt agonist and is in phase II and III testing (AMARONE/NCT05919693 and BRUNELLO/NCT06571045).

Conclusions

For the past 2 decades, VEGF inhibition has served as the mainstay of treating various forms of ocular neovascularization and edema and will likely remain a cornerstone of the retinal specialist's therapeutic armamentarium for many decades to come. In this perspective, we have summarized novel therapeutic targets in neovascular AMD other than VEGF. The major challenges with chronic VEGF antagonists remain treatment burden, under-responsiveness in some patients, risk of endophthalmitis, loss of initial visual gains with long term therapy due to atrophy and fibrosis, and a ceiling effect on efficacy. Given the wide variety of novel compounds currently in clinical development, it remains unclear which—if any—of these novel drug targets offer clinical effects independent of and synergistic with VEGF inhibition. For example, the ENVISION trial failed to demonstrate that the senolytic UBX1325 monotherapy was noninferior to aflibercept for the treatment of neovascular AMD. On the other hand, for diabetic macular edema, the BEHOLD trial showed that UBX1325 increased the median time to rescue anti-VEGF treatment by >30 weeks as compared with sham injections highlighting that non-VEGF drug targets may augment the durability of existing anti-VEGF monotherapy. Many therapies in clinical development are being tested in combination with VEGF inhibition, such as the bispecific therapies targeting angiopoietin, complement, fibroblast growth factor, integrins, or interleukin 6 in addition to VEGF inhibition (see Table 2).

Despite the successes of anti-VEGF monotherapy, the stage has been set for the discovery and development of pathways to be targeted. Preclinical studies continue to identify novel cellular, metabolic, and molecular pathways that could play a role in the development of inappropriate ocular neovascularization,^85–89 as well as novel therapeutic approaches.⁹⁰ Even now, the FDA approval of non-VEGF-targeting therapeutics has provided early clues that we must proceed with caution especially because these therapies may influence VEGF drive and the initiation of neovascular disease in previously non-exudative cases. New compounds, if used in combination with VEGF antagonists, must demonstrate short- and long-term superiority and comparable safety. Elucidating the complex interplay between VEGF and novel drug targets and how their potential synergism alters the pathogenesis of AMD—both non-exudative and neovascular—is within reach.