Microplasmin represents a recombinant protein that contains the catalytic domain of human plasmin but is much more stable as compared to the latter; both plasmin and microplasmin are nonspecific serine proteases cleaving a variety of glycoproteins such as fibronectin, laminin, fibrin and thrombospondin.⁶ These glycoproteins are involved in the adherence of the vitreous cortex to the ILM. Thus cleavage of these enzymes potentially induces a PVD: experimentally, microplasmin has been shown to increase vitreous diffusion coefficients in vitro using the non-invasive technique of dynamic light scattering⁷ and caused simultaneous vitreolysis and posterior vitreous separation in ex vivo and in vivo animal eye models in an apparent dose- and time-dependent fashion^8,⁹ without morphological alterations in the retina.¹⁰ In a human, double-masked, Phase II trial 60 patients were randomized in four treatment cohorts either receiving increasing doses of microplasmin (75, 125, and 175 µg) or an initial injection of 125 µg followed by an injection of 125 µg if no release of adhesion occurred 1 month later or sham injection.¹¹ The study revealed that within 28 days of sham, 75, 125, and 175 µg microplasmin administration, a nonsurgical resolution of vitreomacular adhesion was observed in 8, 25, 44, and 27% of patients; when the 125 µg dose was repeated up to 3 times, adhesion release was observed in 58% of patients 28 days after the final injection.¹¹ Another phase 2, placebo-controlled, double-masked, dose-ranging clinical trial analyzed the effect of a single injection of microplasmin (25, 75, or 125 µg) 7 days prior to scheduled vitrectomy for vitreomacular traction and observed that 125 µg microplasmin was associated with a greater likelihood of induction and progression of PVD than placebo injection.¹² Further randomized clinical trials are now ongoing to evaluate the safety and efficacy of microplasmin in association with vitrectomy or even as a monotherapy replacing the need for surgical intervention small macular holes (Fig. 127.1).

Fig. 127.1 (A) Optical coherence tomography (OCT) demonstrating a full-thickness macular hole with vitreomacular adhesion visible at the edge of the macular hole. (B) OCT 6 months after treatment with 125 µg microplasmin showing the resolution of the vitreomacular adhesion and associated macular hole closure without vitrectomy.

(Courtesy of Steve Pakola, MD, ThromboGenics)

Plasmin

Plasmin was initially used by Verstraeten and coworkers to facilitate a PVD during vitrectomy in rabbit eyes. They found a positive effect but also mentioned a transient reduction of the b-wave amplitude in the electroretinogram as a potential adverse effect.¹³ This study was followed by other experimental trials confirming the proteolytic activity of plasmin on the vitreoretinal junction. Autologous plasmin was then used in clinical case series investigating the effect of a single injection without vitrectomy as a treatment for refractory diffuse diabetic macular edema,¹⁴ tractional diabetic macular edema¹⁵ or macular edema associated with branch retinal vein occlusion.¹⁶ The reduction of retinal thickness and improvement of visual acuity in these series supported an effectiveness of plasmin and the need for further trials. Autologous plasmin-assisted vitrectomy was performed for stage 3 macular holes, traumatic macular holes, diabetic macular edema and stage 5 retinopathy of prematurity¹⁷^–²² and all of these trials underlined the potential of plasmin to facilitate vitrectomy and its good safety profile.

Hyaluronidase

Hyaluronidase has a potential to liquefy the vitreous, but does not induce a PVD in animal models.²³ Therefore, its value in humans to relieve vitreoretinal traction seems limited. A phase III trial in diabetic humans suffering from vitreous hemorrhage showed a faster resorption of the vitreal hemorrhage as compared to a control group with no treatment-related relevant side-effects noted.²⁴ However, a potential effect on the induction of a complete PVD in these patients was not further evaluated.

Dispase

Dispase represents a neutral protease that preferentially cleaves fibronectin and type IV collagen.²⁵ Although initially shown to induce a PVD without damage to underlying tissue,²⁶ subsequent trials brought up evidence for retinal bleedings, epimacular membrane formation, abnormalities in the electroretinogram responses and ultrastructural retinal damage.²⁷ Thus, this enzyme was not further evaluated for clinical use.

Antiproliferative agents in the management of proliferative vitreoretinopathy

Proliferative vitreoretinopathy (PVR) is a serious and major complication of primary rhegmatogenous retinal detachment, severe diabetic retinopathy, and severe intraocular trauma. The pathophysiology of this disease implies a very complex cascade of events resulting in a subsequent proliferative response within the retina (see Chapter 97, Pathogenesis of PVR). Experimental studies described an intraretinal cellular response to retinal detachment, which is triggered by serum and growth factors present in the vitreous cavity²⁸ and includes all types of non-neural cells such as RPE cells, astrocytes, microglia, macrophages, but predominantly Muller cells²⁹^–³² which is then followed by a fibrocellular growth on the subretinal or epiretinal surface.³³ Both, subretinal fibrosis and epiretinal proliferation, have an impact on functional results and anatomical failure of retinal detachment surgery.^33,³⁴ As these cellular responses to retinal detachment may occur early in the development of the disease and may continue even in the longer term following (successful) surgical intervention, pointing to the need for an early pharmacological intervention and a long-term drug release or pharmacological effect.³⁵

Other contributing factors include a relative hypoxia due to the distance between the neurosensory retina and the choriocapillaris,³⁶ which leads to glial changes and photoreceptor deconstruction. These mechanisms are the target of recent and current approaches for an adjuvant pharmacologic intervention in PVR as these cellular responses can not be controlled by surgery alone.

Antiproliferative and anti-inflammatory agents have been the subject of in vitro investigations including substances such as colchicine, daunomycin and 5-fluorouracil.^37,³⁸ However, none of these agents have become part of any standardized procedure to treat or prevent PVR. Only daunomycin and 5-fluorouracil combined with low-molecular heparin were tested in clinical studies, which revealed no clinically relevant impact on PVR.³⁹ Some 20 years ago, steroids applied intravitreally were used to suppress the process of PVR in clinical settings and in experimental studies.⁴⁰ However, the effect was not lasting enough to avoid PVR.

New concepts include other agents such as alkylphosphocholines (APCs), which are synthetic phospholipid derivatives that have been shown to effectively inhibit cellular proliferation.⁴¹ Their mechanism of action involves binding to the membrane-bound G-protein PKC (protein kinase C), which is part of a major intracellular second-messenger system that regulates cell attachment, spreading, migration and proliferation. It was described in the literature that APCs have a potential to inhibit RPE cell spreading and migration as well as proliferation and cell-mediated membrane contraction at nontoxic concentrations in vitro and did not display any toxic effect of APCs to retinal tissue in in-vivo animal models.⁴² Besides prevention and treatment of PVR, APCs might have the potential for topical application as a single-dose agent to prevent posterior capsule opacification formation following cataract surgery.⁴³

In addition to those above-mentioned substances, a variety of other drugs are currently evaluated. However, none so far has reached a clinically relevant potential.

Tissue plasminogen activator in vitreoretinal surgery

This may be related to toxic effects of iron released from subretinal hemoglobin as well as an increased physical barrier for retinal diffusion and fibrotic changes.⁴⁴^–⁴⁶ To date, there is no consensus on how to treat subretinal hemorrhages associated with neovascular age-related macular degeneration and most surgical treatment strategies seem insufficient to restore or improve vision.⁴⁷

A displacement of the hemorrhage may be achieved by intravitreally applied expansile gas combined with intravitreal injection of recombinant plasminogen activator (rtPA). However, the subretinal lytic effect of intravitreally applied rtPA was challenged by some authors due to its molecular size and the reduced retinal diffusion.^48,⁴⁹ Therefore, a combination of intravitreal VEGF inhibitors to treat the underlying neovascular process combined with an injection of expansible gas is favored by some authors.⁵⁰

In our own experience, intravitreal rtPA in combination with expansile gas was in the long-term more effective if combined with subsequent anti-VEGF treatment than intravitreal bevacizumab in combination with expansile gas alone (data submitted). Other groups have suggested triple therapies using rtPA, bevacizumab or ranibizumab and have shown a successful management of the disease with this approach.^51,⁵² If rTPA and pneumatic displacement combination is contraindicated, an anti-VEGF monotherapy may be performed to prevent further visual loss.⁵³ However, after intravitreal injection of any agent combined with expansible gas a vitreous hemorrhage may occur, very often due to a displacement of the subretinal blood into the vitreous cavity. In these cases, vitrectomy is indicated and may lead to good functional results.

It has also been suggested to perform a vitrectomy and apply rtPA subretinally followed by fluid gas exchange ⁵⁴ to displace the hemorrhage. This approach led to visual improvement in some cases. The authors also reported that vitrectomy with subretinal injection of rtPA and intravitreal gas tamponade was more effective than vitrectomy with intravitreal injection of rtPA and gas in terms of complete displacement of the submacular blood.⁵⁵ Although functional improvement in the majority of patients suggests the absence of direct retinal toxicity of subretinally applied rtPA, vitrectomy and subretinal injection of rtPA bares a greater risk for postoperative complications.⁵⁵

Despite good experiences in clinical use, experimental studies demonstrated dose-dependent negative effects of rtPA as seen by a significant and potentially irreversible reduction of the b-wave amplitude in ERG examination in bovine retinas.⁵⁶