Pharmacologic Vitreolysis: The Goal Defines the Means

Employing a variety of methods, this therapy has an emerging role in retinal therapeutics.

MARC D. DE SMET, MD, PhD, FRCSC

Pharmacologic vitreolysis hides under 1 heading and 2 separate processes: separation of the vitreous from the retinal surface, and effective liquefaction of the vitreous gel.¹ The vitreoretinal interface plays an important role in several vitreoretinal disorders, some of which (for example, diabetic retinopathy) are reaching epidemic proportions. Despite major advances in vitreoretinal surgical techniques and instrumentation, relief of vitreoretinal traction remains a critical, often difficult step. Both anecdotal and published evidence support the protective role conferred by a posterior hyaloid detachment (PVD) in proliferative disease and maculopathies.^2-4 Therefore, achieving an atraumatic PVD by surgical or pharmacologic means is an area of intense research and interest.

For a spontaneous PVD to occur, 2 concurrent processes are required: liquefaction of the vitreous gel (syneresis), and separation of the posterior hyaloid (synchesis). Each process must occur in a fine equilibrium, as too much liquefaction may prevent the posterior hyaloid from separating, leading to an anomalous PVD and retinal traction.⁵ Anomalous PVD is at the pathogenetic root of many vitreoretinopathies, from peripheral tears and detachments to macular traction syndromes and holes. Thus, pharmacological PVD must address this balancing act between liquefaction and separation either in its entirety, as will be required in prophylactic applications, or to enhance an existing process as in cases with partial PVD.

While gel liquefaction is an inherent component of PVD formation, its occurrence away from the retinal interface answers to other pharmacologic dynamics. Central vitreous cortex has less densely packed collagen fibrils and a lower hyaluran concentration. Physiologically, its liquefaction leads to the formation of pools of liquid vitreous, from which debris and cells can rapidly escape. Clearance of vitreous hemorrhage may answer to other dynamics and requirements than vitreomacular traction. Evidence so far from clinical trials and published literature certainly point in this direction.

PHARMACOLOGIC PVD

The interface can be targeted in several ways. Chondroitin sulphate is a major component of the vitreoretinal interface. Its cleavage should favor a posterior vitreous separation without vitreous liquefaction.^1,6 While some studies argue in favor of its use as a surgical adjunct, others find little benefit.⁷ Dispase, which cleaves fibronectin from type 4 collagen, facilitates PVD induction in animal models, but also affects the basement membranes of Müller cells and the ILM, and is not without risk.^8,9 Intraretinal hemorrhages, ILM disruptions, reduced ERGs, even the induction of PVR, have been observed with some adverse events appearing within 15 minutes of administration in primates and human volunteers.^10-12

Plasmin enzyme, a serine protease, acts on a variety of glycoproteins including laminin and fibronectin.¹³ Over the past 10 years, several articles have demonstrated its clinical usefulness as a surgical adjunct.^14-18 Its major inconvenience is the difficulty in preparing the autologous compound, characterizing it, and maintaining its activity prior to administration. More convenient would be the use of a recombinant protein with a well-characterized activity profile.

MICROPLASMIN

Plasmin enzyme is a large molecule with the catalytic domain located close to the C-terminal end of the molecule.¹⁹ Adjacent are 5 protein loops called cringles used to bind the enzyme to substrate, but devoid of any enzymatic activity. Microplasmin, a recombinant enzyme, contains only the catalytic domain of human plasmin.²⁰ Preclinical testing in several species has clearly demonstrated its ability to consistently induce a complete PVD at dose around 125 μg (adjusted to the size of a human eye), without having adverse side effects.^21-23 As with plasmin enzyme, the retinal surface following the administration of microplasmin has a smooth appearance on electron microscopy that cannot be achieved with surgery alone (Figure 1). The whole posterior vitreous face detaches, remaining adherent to the retina only at the level of the pars plana. Microplasmin does not only promote posterior hyaloid detachment, it also causes some liquefaction of the gel as shown by electron microcopy and light-scattering experiments.^21,24 However, gel liquefaction is not a uniform process and is limited to the area surrounding the site of injection.²⁵ Recent pharmacokinetic studies using a chromophore release assay determined that microplasmin is rapidly degraded within the eye, subject to a second-order kinetic process. Most of the activity was gone within a few hours of administration.

Figure 1. Scanning electron micrograph of a pig eye treated with microplasmin 125 μg for 2 hours (right) or balanced salt solution (left). A smooth retinal surface is seen in the image on the right, while a meshwork of vitreous is evident in the left image.

The initial clinical study, known as the Microplasmin Vitrectomy Study (MIVI I), was carried out on 60 patients with vitreomacular traction scheduled for surgery. Patients were divided into 6 groups of 10 patients each. Two phases were defined: one with a fixed dose (25 μg) but with time escalation (2 hrs, 24 hrs, 7 days), and the other with a fixed 24-hour exposure but ascending dose (25, 50, 75, 125 μg).²⁶ Both safety and preliminary efficacy data were obtained. One patient developed a retinal detachment after receiving a dose of 25μg, with the initial symptoms starting 2 hours after administration. He ultimately developed a giant tear and required silicone oil for repair. Eleven percent of patients developed small peripheral tears as a PVD was surgically induced. This figure, while high, is in keeping with published series for noncannulated surgeries. No other safety issue was raised. Early efficacy data suggested that a prolonged exposure to microplasmin or increasing the dose was more likely to lead to a spontaneous PVD. Other studies have since been initiated and are confirming and extending these findings. Spontaneous closure of macular holes have been reported in the context of several trials (MIVI I, II, III) (Figure 2). In MIVI III, a United States–based study, 30% of patients had a spontaneous release of traction and did not require surgery (World Congress of Ophthalmology, 2008). These results suggest that pharmacologic vitreolysis may allow for nonsurgical treatment of macular traction syndromes. Other conditions resulting from an anomalous PVD may also benefit. One or more injections of microplasmin may be required to reach this goal. This will be the subject of future research.

Figure 2. Resolution of vitreomacular traction with the use of microplasmin 125 μg.

VITREOUS LIQUEFACTION

Gel liquefaction and PVD induction do not necessarily occur simultaneously. Several of the enzymes listed above have little effect on the gel itself. Both microplasmin and plasmin theoretically have some effect on the vitreous matrix, but their effect needs to be further defined and evaluated. Ovine hyaluronidase (Vitrase) is the only compound that was evaluated in 2 phase 3 randomized clinical trials for the management of vitreous hemorrhage.^27,28 The studies showed a statistically significant improvement in BCVA as early as month 1 in the treated group, with partial clearance of the vitreous blood being observed. The effect on the posterior hyaloid was limited.

An intriguing prospect is the combination of an agent causing vitreous liquefaction with one primarily acting as a PVD inducer. In an experimental rat model of diabetes, the induction of a PVD was significantly more effective when both plasmin and hyaluronidase were combined than when either drug was used on its own.²⁹ While the author's conclusion was that hyaluronidase caused an immediate accelerated dispersal of plasmin enzyme, it is also possible that a more complete vitreous liquefaction resulted from the use of hyaluronidase, setting the stage for a more intense collapse of the posterior vitreous face as the detachment occurred.

CONCLUSION

Growing interest and research in pharmacologic vitreolysis identified its 2 main components: PVD induction and vitreous liquefaction. While most drugs affect both, those identified so far appear to have a preferential activity on one or the other arm of this process. The task to be addressed — clearance of a vitreous hemorrhage, PVD induction for traction limited to the fovea, or affecting the wider posterior pole would seem to dictate a different pharmacologic strategy. The exact strategy will be defined in years to come. However, it is already clear that the management of macular disorders will rapidly change. RP

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