Exploring the effects of anti-VEGF therapy on retinal diseases

Take-home message: Aflibercept, an anti-VEGF agent, can enable treatment interval extensions and provide rapid and sustained vision gain in retinal vascular diseases as well as significantly reducing the severity of diabetic retinopathy.

By Lisa Stewart

The pathogenesis of diabetic macular oedema and retinal vein occlusion is associated with increased levels of vascular endothelial growth factor (VEGF), placental growth factor (PGF), and other inflammatory factors that are key targets for the treatment of retinal disease.

Aflibercept, which was specifically designed to bind VEGF and PGF with high affinity, can enable treatment interval extensions in the clinic with vision gains comparable to those seen in clinical trials. Aflibercept provides rapid and sustained vision gain in both ischaemic and non-ischaemic retinal vascular diseases and significantly reduces the severity of diabetic retinopathy.

Unravelling the role of cytokines in retinal disease

Dr Aude Ambresin from Lausanne University and the Jules Gonin Eye Hospital provided a brief description of how retinal vascular disease develops.

Impaired retinal blood flow leads to hypoxia, then the hypoxic cells release inflammatory cytokines, such as interleukin-6 (IL-6) and upregulate VEGF, PGF and VEGF receptor (VEGFR) 1. This causes activation of microglial cells, apoptosis of endothelial cells, and pericyte drop off, coupled with opening of tight junctions and neovascularisation-all of which lead to chronic inflammation and breakdown of the blood–retinal barrier.

The end results are vessel leakage, oedema, and vision loss. The pathogenesis is similar for diabetic macular oedema and retinal vein occlusion in that both involve inflammation and ischaemia leading to endothelial damage.

Dr Ambresin went on to summarise the evidence, both preclinical and clinical, for the involvement of VEGF and PGF in retinal vascular disease.

VEGF-A overexpression promotes vascular permeability and angiogenesis; PGF has also been shown to be pro-angiogenic, and promotes leukocyte infiltration and vascular inflammation.

Preclinical animal models have demonstrated that VEGF and PGF contribute to the breakdown of the blood–retinal barrier in two ways: first, via the loss of pericytes from retinal capillaries and second, by increasing vascular permeability, which leads to exudation of fluid into the subretinal space and results in macular oedema.

A preclinical model of diabetic retinopathy in mice, the Akita mouse, showed that when PGF is knocked out all measures of diabetic retinopathy decrease almost to the level seen in wild-type mice.

Experimentally induced choroidal neovascularisation (CNV) in mice is associated with significantly elevated levels of VEGF-A and PGF.

The elevation of VEGF and PGF has been further examined in clinical studies: the degree of ischaemia and severity of disease correlates with levels of VEGF-A and PGF in both retinal vein occlusion and diabetic retinopathy. Levels of PGF and VEGF are significantly higher in the vitreous of patients with active diabetic retinopathy than those with quiescent disease.

VEGF-A and PGF synergise to activate VEGFR-1, leading to angiogenesis, neovascularisation, and inflammation. This synergy has been demonstrated in several preclinical studies.

For example, whereas inhibition of VEGF-A significantly reduced the increases in vessel density in a mouse model of CNV, inhibition of PGF had no real effect. However, co-inhibition of VEGF-A and PGF reduced vessel density by significantly more than inhibition of VEGF-A alone.¹

Dr Ambresin finished by touching on galectin-1, which is a potential modulator of neovascular retinal disease. Expressed in endothelial cells, it interacts with VEGFR and its overexpression has been linked to pathological neovascularisation. Elevated levels of galectin-1 have been reported in the plasma of patients with type 2 diabetes.

In cultured retinal endothelial cells, galectin-1–mediated phosphorylation of VEGFR-2 was inhibited by aflibercept and this inhibition was associated with reduced cell proliferation.

The dual action of aflibercept

While currently available anti-VEGF agents inhibit the VEGF-A isoforms, aflibercept is up to 92 times more potent an inhibitor than bevacizumab or ranibizumab, with higher affinity and a faster association rate; moreover, unlike the other two inhibitors, aflibercept also binds PGF and VEGF-B.²

Professor Thomas Langmann of the University Hospital of Cologne described the structure and mode of action of aflibercept and discussed how they affect its clinical function.

Aflibercept is a fusion protein that comprises two VEGF-binding portions derived from the extracellular domains of human VEGFR-1 and -2, connected via a human IgG antibody fragment [Figure 1].

Aflibercept binds the active dimers of VEGF-A or PGF on both sides: once bound, they cannot interact with other molecules.

Comparing duration of action, ranibizumab has been reported to suppress VEGF-A levels in the aqueous humour-VEGF levels in the aqueous humour correlate well with those in the vitreous and the retina-for a mean of 36 days.³

Accordingly, bimonthly injections are not appropriate. Aflibercept, however, suppresses VEGF-A levels for a mean of 71 days,⁴ which means that most patients are still experiencing suppression at the end of the recommended injection interval of 8 weeks.

A small crossover study of seven patients allowed aflibercept and ranibizumab to be compared directly:⁵ patients who had persistent active neovascular AMD with ranibizumab were switched to aflibercept. Suppression of VEGF levels lasted twice as long with aflibercept as with ranibizumab.

These extended cytokine suppression times suggest that treatment intervals can be extended.

This was tested in clinical studies, where bimonthly dosing of aflibercept delivered similar improvements in visual acuity to monthly ranibizumab, and is supported by real-life clinical data: patients had vision gains comparable to those seen in clinical trials, which were maintained using a treat-and-extend protocol with mean injection intervals of approximately 3 months. Treatment intervals can be extended with aflibercept even in patients with a poor response to other anti-VEGF agents.

The action of aflibercept on retinal disease

Professor Peter Kaiser from the Cole Eye Institute in Cleveland, Ohio, USA, explored aflibercept’s action on retinal disease.

He reiterated that VEGF is not the only cytokine elevated in retinal disease and diabetes, but so too are others including PGF, IL-6, and IL-8, and showed clinical data that backed up the mostly preclinical data Dr Ambresin had shared.

He emphasised that levels of VEGF, PGF, and other inflammatory cytokines correlate with increasing severity of disease, such that patients with high ischaemic loads have extremely high levels of both VEGF and PGF. 

Clinical studies in retinal vein occlusion or diabetic macular oedema using aflibercept have demonstrated a very rapid gain in visual acuity-improvements were seen after the first injection-that persists to at least 3 years. As well as the visual acuity gains, a dramatic reduction in retinal thickness is seen after just one injection. This gain is maintained such that the majority of patients actually have normal retinal thickness at 3 years.

Professor Kaiser emphasised that rapid treatment is important for central retinal vein occlusion.

“Historically we were taught that it didn’t really matter when you treated a patient because the outcomes were rather similar,” he said. “But if we look at the phase III studies with aflibercept this doesn’t hold . . . there’s a dramatic difference if you wait more than 2 months from diagnosis to institute treatment.”

Phase III studies showed a clinically significant improvement in diabetic retinopathy severity scores with aflibercept treatment, which continued to improve with longer treatment. About one-third of patients had an improvement after 1 year, which had increased to almost half by 3 years.

Retrospective studies examined the effect of switching to aflibercept in patients with retinal vein occlusion and diabetic macular oedema who were resistant to treatment with bevacizumab or ranibizumab. Patients who are hard to treat tend to have very elevated levels of VEGF and other cytokines, which is believed to be one reason why they don’t respond. Improvements in visual acuity and central retinal thickness were noted as well as extensions in the injection intervals.

In summary, aflibercept, which is a more potent inhibitor of VEGF than its competitors, with higher affinity and a faster association rate, and which also binds PGF and VEGF-B, can enable treatment interval extensions and provide rapid and sustained vision gain in retinal vascular diseases as well as significantly reducing the severity of diabetic retinopathy.

References

1. Y. Li, et al., Cell Physiol. Biochem. 2015; 35: 1787-1796. doi:10.1159/000373990.

2. N. Papadopoulos, et al., Angiogenesis 2012; 15: 171-185.

3. P.S. Muether, et al., Am. J. Ophthalmol. 2013; 156(5): 989-993.e2. doi:10.1016/j.ajo.2013.06.020.

4. S. Fauser, V. Schwabecker and P.S. Muether. Am. J. Ophthalmol. 2014; 158(3): 532-536. doi:10.1016/j.ajo.2014.05.025.

5. S. Fauser and P.S. Muether. Br. J. Ophthalmol. 2016; 100(11): 1494-1498. doi:10.1136/bjophthalmol-2015-308264.

Dr Aude Ambresin

e: aude.ambresin@fa2.ch

Professor Thomas Langmann

e: thomas.langmann@klinik.uni-regensburg.de

Professor Peter Kaiser

e: kaiserp@ccf.org

This article was adapted from physician presentations at a Bayer symposium at the 16th EURETINA congress in Copenhagen. They did not indicate any proprietary interest relevant to the subject matter. The event was chaired by Professor Francine Behar-Cohen of the Jules Gonin Eye Hospital in Switzerland.