Review and perspectives on pharmacological vitreolysis

Mutational Analysis of Ocriplasmin to Reduce Proteolytic and Autolytic Activity in Pichia pastoris

BACKGROUND

Ocriplasmin (Jetrea) is using for the treatment of symptomatic vitreomacular adhesion. This enzyme undergoes rapid inactivation and limited activity duration as a result of its autolytic nature after injection within the eye. Moreover, the proteolytic activity can cause photoreceptor damage, which may result in visual impairment in more serious cases.

RESULTS

The present research aimed to reduce the disadvantages of ocriplasmin using site-directed mutagenesis. To reduce the autolytic activity of ocriplasmin in the first variant, lysine 156 changed to glutamic acid and, in the second variant for the proteolytic activity reduction, alanine 59 mutated to threonine. The third variant contained both mutations. Expression of wild type and three mutant variants of ocriplasmin constructs were done in the Pichia pastoris expression system. The mutant variants were analyzed in silico and in vitro and compared to the wild type. The kinetic parameters of ocriplasmin variants showed both variants with K156E substitution were more resistant to autolytic degradation than wild-type. These variants also exhibited reduced Kcat and Vmax values. An increase in their Km values, leading to a decreased catalytic efficiency (the Kcat/Km ratio) of autolytic and mixed variants. Moreover, in the variant with A59T mutation, Kcat and Vmax values have reduced compared to wild type. The mix variants showed the most increase in Km value (almost 2-fold) as well as reduced enzymatic affinity to the substrate. Thus, the results indicated that combined mutations at the ocriplasmin sequence were more effective compared with single mutations.

CONCLUSIONS

The results indicated such variants represent valuable tools for the investigation of therapeutic strategies aiming at the non-surgical resolution of vitreomacular adhesion.

Induction of posterior vitreous detachment (PVD) by non-enzymatic reagents targeting vitreous collagen liquefaction as well as vitreoretinal adhesion

Induction of posterior vitreous detachment (PVD) by pharmacologic vitreolysis has been largely attempted through the use of enzymatic reagents. Ocriplasmin has been the only FDA-approved clinical reagent so far. Several adverse effects of ocriplasmin have emerged, however, and the search for alternative PVD-inducing reagents continues. Since i) collagen forms an important structural component of the vitreous, and ii) strong vitreo-retinal adhesions exist between the cortical vitreous and the internal limiting membrane (ILM) of the retina, an effective PVD-inducing reagent would require both, vitreous liquefaction, and concurrent dehiscence of vitreoretinal adhesion, without being toxic to retinal cells. We designed a combination of two reagents to achieve these two objectives; a triple helix-destabilizing collagen binding domain (CBD), and a fusion of RGD (integrin-binding) tripeptide with CBD (RCBD) to facilitate separation of posterior cortical vitreous from retinal surface. Based on in vitro, ex-vivo, and in vivo experiments, we show that a combination of CBD and RCBD displays potential for safe pharmacologic vitreolysis. Our findings assume significance in light of the fact that synthetic RGD-containing peptides have already been used for inhibition of tumor cell invasion. Proteins such as variants of collagen binding domains could have extended therapeutic uses in the future.

[Effects of urokinase plasminogen activator on cultured human retinal epithelial cells]

AIM

to study the effects of urokinase plasminogen activator (UPA) on the human retinal pigment epithelium (hRPE) cell culture.

MATERIAL AND METHODS

The toxicity of 50 U/ml UPA was studied with the trypan blue exclusion test. Cell migration was assessed by the wound healing and modified Boyden chamber assays. Additionally, cell morphology, trypsin resistance, and Ki67 expression were investigated.

RESULTS

Trypan exclusion test did not reveal any cytotoxicity of 50 U/ml UPA against hRPE cells. The agent appeared able to induce cellular cluster formation and increase the number of spindle-shaped cells (6.4±2.4 cells/field and 67.3±3.2 cells/field in the controls and in the presence of 50 U/ml UPA, respectively, p<0.001). Cell migration in the Boyden chamber also showed a statistically significant increase (1.75-fold, p=0.012). Monolayer wounds were found to heal at an accelerated rate (p<0.05). This effect was dose-dependent, just like the increase in Ki67-positive cells (from 2.5 to 50 U/ml). Moreover, there was a reduction in trypsin resistance of the hRPE cells (the number of resistant cells in the control and 50 U/ml UPA cultures was 5.2±1.7 cells/field and 0.46±0.32 cells/field, respectively, p<0.001).

CONCLUSION

UPA, at concentrations of 50 U/ml or less, demonstrates no cytotoxicity against the hRPE cells. The effects of UPA on hRPE include stimulation of epithelial-mesenchymal transition, migration, proliferation, and intercellular interaction. At that, changes in migratory and proliferative activity are dose-dependent.

Pharmacologic Vitreolysis in Vascular Diseases of the Retina

Vascular diseases of the retina such as diabetic retinopathy and vascular occlusions account for a large proportion of visual morbidity and blindness worldwide. The role of vitreous in the pathogenesis of these conditions has been increasingly recognized. Despite advances in the surgical technique of pars plana vitrectomy, the use of intravitreal agents for the lysis of vitreous has received attention, guided largely by promising results from the trials involving patients with non-vascular retinal diseases such as vitreomacular traction. The purpose of this review is to provide a comprehensive summary of the present knowledge on pathophysiologic basis of pharmacologic vitreolysis and its efficacy in vascular diseases of the retina. A review of completed and ongoing clinical trials will be presented, along with insights into future directions of this therapy.