Ranibizumab and Other VEGF Antagonists for Diabetic Macular Edema

loss, these efforts alone often do not cause significant improvement of DME. Thus, the development of treatments is critical to relieve the burden that DME causes on society. This chapter will discuss the recent development of ranibizumab and other therapy directed against vascular endothelial growth factor (VEGF) for the treatment of DME.

PATHOGENESIS OF DME AND CURRENT STANDARD OF CARE

DME occurs when plasma leaks out of vessels and accumulates within the retinal tissue of the macula. Hyperglycemia leads to impairment of pericytes and subsequent loss of the normal integrity of the retinal vasculature [5] and the blood-retina barrier [6]. As visualized by fluorescein angiography (Fig. 1A, B), this plasma leakage can be focal leakage from microaneurysms or more diffuse leakage. In the latter case, there are no structural abnormalities seen on angiography, and nonfocal leakage from retinal capillaries occurs. For both focal and diffuse edema, the interstitial fluid distorts the structure of the macula, leading to visual dysfunction. This fluid accumulation can be imaged by optical coherence tomography (OCT) (Fig. 1C). Normal vision can be restored if the leakage ceases and the excess fluid resorbs. However, the chronic presence of fluid can lead to permanent damage to neurons, thus limiting the potential for vision restoration. Treatments for DME are aimed at reducing leakage by physically changing the vasculature or by reducing permeability through molecular signaling pathways.

While the pathogenesis of DME at the molecular level is not completely understood, VEGF is thought to have a significant role. VEGF contributes to the pathogenesis of numerous retinal diseases that involve abnormal vessels, including choroidal neovascularization from age-related macular degeneration (AMD). There has been an explosion of basic science knowledge about this protein that ultimately has led to therapeutic development. VEGF is a 45-kDa glycoprotein that is a potent stimulator of vascular permeability and neovascularization [7, 8]. VEGF is a family of growth factors, and the most studied member is VEGF-A, which was known simply as VEGF before the discovery of other members. Different exon splicing pathways of the human VEGF-A gene can lead to the production of at least nine isoforms of VEGF-A [9]. In particular, the VEGF 165 isoform is thought to have a key part in disease development [10, 11]. The expression of VEGF is associated with DME, as it has been found that vitreous levels of VEGF are significantly higher in patients with DME as compared to patients without diabetes [12]. Hypoxia contributes to the development of DME [13], and VEGF expression is stimulated by hypoxia as it is regulated by the transcription factor hypoxia-inducible factor-1 [14]. Hyperglycemia also stimulates VEGF expression [15]. Once there is increased VEGF production in the setting of poorly controlled diabetes, the VEGF can affect the tight junction complexes that are a significant component of the blood-retina barrier. Specifically, it has been shown that VEGF may cause vascular permeability by downregulating the tight junctional protein occludin in retinal endothelial cells [16]. Using a primate model, Ozaki et al. found that intravitreal injection of a sustained release VEGF pellet causes breakdown of the blood-retinal barrier. This finding was accompanied by significant leakage of fluorescein from vessels and macular edema [17]. Together, these studies suggested that VEGF antagonists may serve as a treatment for DME.

The first study examining this possibility involved an orally active nonselective blocker of VEGF receptors called PKC412 [18]. PKC412 is a kinase inhibitor that blocks VEGF receptors 1 and 2, platelet-derived growth factor, the receptors for stem cell factor, and several isoforms of protein kinase C [19, 20]. In a dose-dependent manner, PKC412 reduced macular thickening and modestly improved visual acuity in patients with DME. However, the therapeutic dose also caused liver toxicity, and thus the study pointed to a need for a more selective and locally administered medication. Before anti-VEGF therapy for DME is discussed further, it is important to discuss other treatments in use to understand the potential impact that anti-VEGF therapy may have on patient care.

Currently, the standard of care for the treatment of DME is focal/grid laser. With this treatment, laser is applied focally to leaking microaneurysms or applied in a grid pattern to areas of diffuse leakage. Laser was the first evidence-based treatment developed for DME as delineated by the Early Treatment of Diabetic Retinopathy Study (ETDRS) [21]. The ETDRS demonstrated that laser treatment reduces the risk of moderate vision loss by 50% (30–15%) at 3 years. As one considers other treatments, it is worthwhile to consider that the ETDRS showed reduction in vision loss with as much as 3 years of follow-up. This demonstration of the long-term benefit of laser treatment cannot be ignored. Nevertheless, the prevailing thought has been that laser treatment effectively reduces the risk of vision loss, but laser is not effective at improving vision. Yet it must be noted that many of the patients in the ETDRS study had good visual acuities, and 85% of the patients had vision better than 20/40 [21]. This stems from the fact that the study was not designed originally to demonstrate visual improvement from laser. To investigate this question of vision improvement from laser treatment, a subset of 114 eyes in the ETDRS was examined. These eyes had definite center thickening on photographs, visual acuity worse than 20/32, and mild to moderate nonproliferative retinopathy at baseline. It was found that laser treatment led to a median change from baseline visual acuity of +4 letters at 2 years and that 29% improved ten or more letters [22]. Thus, laser treatment reduces the risk of vision loss and, in some cases, can lead to vision improvement. Since the ETDRS, laser treatment has been the gold standard to which new treatments must be compared.

In addition to laser, intravitreal triamcinolone is a commonly used treatment for DME. The rationale and evidence supporting its use is important to touch upon. It is known that inflammation can lead to vascular permeability through leukocyte-mediated mechanisms [23]. These mechanisms involve the increased expression of leukocyte adhesion proteins such as intercellular adhesion molecule-1 (ICAM-1) and CD18. Corticosteroids can reduce inflammatory signals that lead to vascular permeability as well as reduce the expression of VEGF [24, 25]. These effects of corticosteroids form the basis for the use of intravitreal triamcinolone as a treatment for DME. Gillies et al. conducted a randomized clinical trial in which 69 eyes were treated with 4 mg of intravitreal triamcinolone or a placebo injection of subconjunctival saline [26]. All eyes entered into the study were considered to have DME that persisted or recurred despite previous laser treatment. Eyes received additional injections every 6 months as needed. After 2 years, it was found that 56% of the treated eyes gained five or more letters compared to 26% of the placebo eyes. The most concerning side effects were glaucoma and cataract. An increase of intraocular pressure by 5 mmHg or more was seen in 68% of the treated