47 Enzymatic vitrectomy and pharmacologic vitreodynamics

INTRODUCTION AND HISTORY

The field of vitreoretinal surgery has evolved substantially over the last several decades. Scientific advances have improved our understanding of disease pathophysiology and new surgical adjuncts and techniques have decreased surgical time and improved patient outcomes. Pharmacologic agents have recently been developed for intraocular use in order to enhance vitreous removal. Pharmacologic vitreodynamics is a new concept which describes the use of pharmacologic agents to manipulate the vitreous. In addition, and perhaps more importantly, it also describes the complex interactions that take place at the vitreoretinal interface.

Pharmacologic compounds that are developed for manipulation of the vitreous are designed to cleave the vitreoretinal junction and/or to increase vitreous liquefaction. This concept of lysing the vitreoretinal juncture and core vitreous has been described as pharmacologic vitreolysis.1 Two enzymatic agents have been shown to cause both vitreous liquefaction and complete separation of the vitreoretinal interface, leading to the development of a posterior vitreous detachment (PVD).2–5 These substances can eliminate vitreoschisis and lead to a clean internal limiting membrane (ILM), as shown in animal and human studies.6,7 Enzymatic vitrectomy is an evolving modality that will likely be used more frequently for therapy and preventive measures.8

For many years, ophthalmologists have researched and utilized enzymatic agents, one of the earliest being alpha-chymotripsin and its effect on zonular proteins during intracapsular cataract extraction. Many substances have been proposed to cleave the vitreoretinal juncture and liquefy the central vitreous as an adjunct to vitreoretinal surgery. Both enzymatic and nonenzymatic agents have been developed to resolve vitreous traction and accelerate the clearance of a vitreous opacity, such as vitreous hemorrhage. The search for an appropriate agent to manipulate the vitreous has included several enzymes, most of which are autologous enzymes that activate other endogenous enzymes. Many of the earliest-studied agents were fraught with failure due to lack of efficacy, retinal toxicity, or difficult preparation. Nevertheless, several enzymes appear to be promising for pharmacologic manipulation of the vitreous. Significant progress has been made in the field of pharmacologic vitreodynamics, and this chapter will provide a summary of recent findings.

PHARMACOLOGY AND BIOCHEMISTRY

Recent advances in biochemistry have improved our understanding of the complex arrangement of the vitreoretinal interface. Until recently, the vitreous gel was long seen as unimportant and noncontributory to retinal pathology.9 It is now commonly believed that the vitreous gel has a significant influence on retinal diseases. The vitreous is a clear, semisolid gel containing hyaluronic acid interspersed in a framework of parallel collagen fibrils.9 The vitreous cortex contains the greatest concentrations of collagen and hyaluronic acid. These collagen fibrils form an outer layer of the vitreous cortex which is adherent to the ILM.10 The glycoproteins laminin and fibronectin bind vitreous collagen fibers between the posterior vitreous cortex and the ILM,11 and they are

thought to play a role in areas of vitreoretinal adhesion. An age-related PVD is thought to occur as the vitreous becomes syneretic, such that the gel consistency is lost to become partially or completely fluid. Pools of fluid collect in the premacular region, and a tear in the posterior cortical vitreous facilitates this fluid then to pass through the vitreous cortex and separate the surrounding cortical vitreous from the retina. Similar to the progression of an age-related PVD, two events must occur in order to ensure successful pharmacologic vitreolysis: (1) vitreous liquefaction and (2) complete vitreoretinal interface separation (PVD).8 Multiple pharmacologic compounds have been designed with this goal in mind, and they have been met with variable success.9,12

Chondroitinase, one of the earliest-studied enzymes, lyses the proteoglycan chrondroitin sulfate which is associated with the vitreoretinal interface.13 A recent masked, placebo-controlled, in vivo study concluded that a dose of 1 IU chondroitinase failed to produce a PVD.14 Dispase was initially thought to be a good candidate for pharmacologic vitreolysis due to its ability to hydrolyze type IV collagen and fibronectin. Dispase is able to create a PVD at a dose of 0.025 IU15,16; however, it also causes intraocular inflammation, retinal hemorrhages, electroretinogram (ERG) amplitude reductions, epiretinal membranes, cataract, and ultrastructural damage to the retina.15–17 The retinal toxicity associated with dispase is thought to be due to the broad range of proteins that are subject to its action,9 and this toxicity has limited its clinical use.

Hyaluronidase (Vitrase, ISTA Pharmaceuticals) is commercially available for off-label use as an adjunct to increase the absorption and dispersion of other injected drugs, such as local anesthetics. It works by primarily digesting the proteoglycan hyaluronan which makes up a large component of the vitreous body. Hyaluronidase has been shown to cause vitreous liquefaction in animal models,9 and a recent phase III trial was conducted.18,19 In this study, the percentage of patients achieving clearance of vitreous hemorrhage sufficient to see underlying pathology by 3 months was 33% in patients receiving hyaluronidase 55 IU and 26% for saline-treated controls (P = 0.025).19 Despite the authors’ conclusions that hyaluronidase is safe, approximately onethird of patients were noted to have moderate or severe iritis and approximately 10% developed a retinal detachment following injection with hyaluronidase 55 IU. Furthermore, multiple studies have concluded that an intravitreal injection of hyaluronidase cannot induce a PVD in animal models.20–22 As mentioned earlier, successful enzymatic vitrectomy requires both vitreous gel liquefaction and complete vitreoretinal separation. Sebag1,9 has used the term “anomalous PVD” to describe the situation when vitreous liquefaction occurs without a PVD, as is seen with hyaluronidase. Anomalous PVD may lead to traction at the vitreoretinal interface with the development of multiple pathologic conditions.1,9 Thus, hyaluronidase has not been considered an ideal agent for enzymatic vitrectomy, and it has not been approved for intraocular use by the Food and Drug Administration.

Plasmin enzyme is a nonspecific, endogenous protease capable of creating a PVD by hydrolyzing laminin and fibronectin at the vitreoretinal interface.23,24 Its ability to cause vitreous liquefaction is thought to be due to its activity on collegenases25 and endogenous matrix metal- loproteinase-2.26 In animal models, a single injection of plasmin enzyme creates a PVD without causing morphological changes to the retina, and the degree of vitreoretinal separation is doseand time-dependent, with a peak activity at approximately 30 minutes.6,24 Autologous plasmin enzyme (APE) can be isolated from the patient’s serum via