Topical Bevacizumab Therapy in Graft Rejection After Penetrating Keratoplasty

Neovascularization is defined as formation of new vessels from vascular endothelial cells derived from existing blood vessels. These new immature vessels are friable. They have increased permeability, lack structural integrity, and can result in lipid deposition and corneal opacities. Ultrastructurally, corneal neovascularization is characterized by vessels encroaching through separated stromal lamellae. Corneal vascularization following penetrating keratoplasty may result in graft rejection.

cause a variety of symptoms, including eye pain and excessive tearing, light sensitivity, redness, intolerance to contact lenses, and decreased vision.

Role of Corneal Neovascularization in Corneal Graft Rejection
Corneal neovascularization remains a significant risk factor for corneal graft rejection and subsequent graft failure after penetrating keratoplasty.The collaborative corneal transplantation study 1 identified, in addition to several other factors, the extent of stromal vessels (quadrants) as a serious risk factor for corneal graft failure.During corneal neovascularization, an upregulation of angiogenic factors is seen in association with down regulation of antiangiogenic molecules.Studies in human and rat models have found vascular endothelial growth factor (VEGF) to be up regulated in inflamed and vascularized cornea . 2 In a mouse model, it was observed that corneal avascularity during development is redundantly regulated.Lack of the antiangiogenic factors thrombospondin (TSP)-1 and/or -2 resulted in no spontaneous corneal angiogenesis.By contrast, TSP-1, more than TSP-2, helped to suppress inflammation-induced corneal angiogenesis postnatally, implying that angiogenic privilege in the cornea is actively maintained . 3Bachmann et al 4 in a mouse model found that neutralization of VEGF-A after high-risk corneal transplantation effectively inhibited postoperative hemangiogenesis, lymphangiogenesis, and recruitment of antigen-presenting cells, thereby improving corneal graft survival.Vascular endothelial growth factor promotes several steps of angiogenesis, including proteolytic activities, endothelial cell proliferation, migration, and capillary tube formation . 5 Topical or systemic application of bevacizumab inhibited both inflammation-induced angiogenesis and lymphangiogenesis in the cornea.This finding suggestes an important role of VEGF-A in corneal lymphangiogenesis and bevacizumab may be useful in preventing immune rejections after penetrating keratoplasty.

Types of corneal vascularization
Superficial: In superificial corneal vascularization, vessels arrange in arborizing pattern in subepithelial layer and can be traced with conjunctival vessels.This type of vascularization may be observed in trachoma, and phlyctenular conjunctivitis.Deep: In deep corneal vascularization, vessels are derived from anterior ciliary artery and lie in corneal stroma.Vessels are usually straight and not anastomosing.This type of vascularization may be observed in graft rejection, interstitial keratitis, deep corneal ulcer, and chemical burn.Pannus: In pannus, extensive superficial vascularization occurs with cellular infiltration.It is of two types:  Progressive: Infiltration is ahead of vascularization  Regressive: Infiltration is behind the vascular leash.

Management
Corneal neovascularization is one of the main risk factor for immune rejection after corneal transplantation.Healthy cornea is devoid of blood vessels and is said to be immune privileged.When corneal grafts are placed into an avascular recipient corneal bed (low risk keratoplasty) the two year survival rate approaches 90% under cover of steroids.
The survival rates of corneal grafts placed over vascularized recipient beds (high risk keratoplasty) decrease significantly.Even in low risk setting, mild angiogenesis develops after keratoplasty and increase risk of immune graft rejection.For this reason aggressive treatment of neovascularization may be necessary prior to corneal transplant surgery to ensure lower risk of graft rejection.
Various antiangiogenic agents may be used in management of post keratoplasty graft rejection: factor.They also prevent intercellular adhesion leading to leukocyte adhesion and migration.They can slow the progression of neovascularization, but cannot stabilize or reverse the process.Vascular endothelial growth factor inhibitors are emerging as the new pharmacological therapy in the management of post keratoplasty graft rejection.Anti-VEGF agent has been found to be effective in reducing corneal vascularization (Fig. 1).Currently available anti-VEGF agents include:  Bevacizumab (Avastin) Bevacizumab is a humanized monoclonal antibody that inhibits all active isoforms of VEGF and is approved by USFDA since February 2004 for treatment of metastatic colorectal cancer. Ranibizumab (Lucentis) Ranibizumab is also a humanized monoclonal antibody fragment highly related to bevacizumab structurally. Pegaptanib (Macugen) Pegaptanib is a synthetic oligonucleotide that binds the pathologic isoform of VEGF, which is VEGF 165 and this binding, happens extracellular.

Role of bevacizumab in the treatment of corneal vascularization
Efficacy of bevacizumab therapy has been highlighted in several studies.Manzano et al 6 in an experimental study on rats showed that a concentration of 4 mg/mL bevacizumab limits corneal neovascularization.Dastjerdi et al 7 in their study used 1% bevacizumab for 3 weeks in 10 eyes of 10 patients and followed them up to 24 weeks.They found that short-term topical bevacizumab therapy reduced the severity of corneal neovascularization without local or systemic adverse effects.Chen et al 8 used subconjunctival injection of bevacizumab in doses 0.25-2.5 mg twice per week for 2 to 8 weeks in rabbits and concluded that it is effective in preventing corneal neovascularization in acute phase of various kinds of corneal inflammation.Carrasco 9 reported the use of bevacizumab in an 81-year old woman with corneal neovascularization secondary to herpetic stromal keratitis.Treatment with subconjunctival bevacizumab showed regression of corneal neovascularization.Bock et al 10 in their study used topical bevacizumab (5x/day; 5 mg/ml) for 15 days in corneal neovascularization of various cause and concluded that it inhibits corneal neovascularization without obvious corneal epithelial side effects.

Role of bevacizumab in the treatment of corneal vascularization after keratoplasty
Efficacy of bevacizumab therapy has been highlighted in several studies.Schollmayer et al 11 performed a retrospective case series study of nine eyes of nine patients with corneal transplant and neovascularization.Five eyes were administered topical bevacizumab 5 mg/ml 4 times daily for a month.Three eyes were administered bevacizumab subconjunctivally 2.5 mg/0.1 ml and one eye was administered bevacizumab both topically as well via subconjunctival route.After a follow up of 6 months, it was concluded that topical and subconjunctival bevacizumab is effective in regressing neovascularization in keratoplasty patients.Saxena et al 12 used topical bevacizumab (4 mg/4 ml) in dose of one drop twice daily for 15 days in a patient of corneal graft rejection and did a follow up after 1 month, 6 month and 9 months.After 1 month, his BCVA improved to 20/120 from finger counting 1meter.Corneal vascularization and stromal haze regressed.After 6 months, his BCVA improved to 20/60

Fig. 1 .
Fig. 1.Effect of topical bevacizumab in a case of corneal neovascularization.A. Pretreatment: Corneal vascularization and stromal haze is visible.B. Post-treatment: Marked regression in corneal vascularization and stromal haze has resulted.