RHO KINASE INHIBITORS IN GLAUCOMA

Guest author

RAMSHA ANWAR

Ajmal Khan Tibbiya College, Aligarh, India

INTRODUCTION:

Rho Kinase is a serine/threonine protein kinase involved in the regulation and modulation of cell shape and size via its action on the cytoskeleton.

FUNCTIONS OF RHO KINASE:

As downstream effectors of Rho GTPase, Rho Kinases are involved in calcium independent regulation of smooth muscle contraction.

(A small GTP-binding protein, called Rho, acts on ROCKs downstream, to produce certain changes in the tissues)

Rho Kinases are also linked with: 

(1) the control of cytoskeleton dynamics 
(2) actinomyosin contractile forces 
(3) cell adhesion 
(4) cell stiffening 
(5) extracellular matrix and re-organization 
(6) cell morphology.

The above mentioned factors regulate aqueous humor outflow via the trabecular pathway resulting in lowered intra-ocular pressure (IOP).

Rho Kinases (ROCK) exist in 2 isoforms: ROCK 1 and ROCK 2.

ROCK 1 is located on chromosome 18 and contains 1354 aminoacids. 
ROCK 2 is located on chromosome 12 and encodes a 1388 aminoacid product.

In humans ROCKs are expressed in majority of tissues, including trabecular meshwork and ciliary muscle cells.

Structurally, ROCKs are composed of 3 major domains:

1. An N-terminal kinase domain (which phosphorylates protein targets).
2. A C-terminal kinase domain (which limits kinase activity via intramolecular interactions)
3. A coiled-coil Rho-binding domain (which facilitates the switch from the inactive to active conformation).

On binding to Rho, the catalytic activity of ROCKs is moderately enhanced.

ROCKs inhibit myosin light chain phosphatase thereby mediating actin cytoskeletal changes. These directly affect the contractile properties of trabecular meshwork outflow tissue.

ROCK promotes the assembly of actin stress fibers and focal adhesions.

ROCK also regulates cell contraction and motility.

Inhibition of ROCK improves aqueous outflow through the trabecular meshwork resulting in lowered IOP.

RHO KINASE SIGNALING PATHWAY:

1.  Rho Kinase is a downstream effector of the RhoA protein, a small GTPase.
2. GTPase interchange between two conformations (a) A Guanosine triphosphate (GTP)-bound ACTIVE conformation and a Guanosine Diphosphate (GDP)-bound INACTIVE conformation.
3. A number of factors control GTPase activation. These regulators include:

·         Guanine nucleotide Exchange Factors (GEFs)

·         GTPase Activating Proteins (GAP)

·         Guanine nucleotide Dissociation Inhibitors (GDIs)

·         Myosin Light Chain

·         LIM kinase

The above mentioned substrates interact to control actinomyosin contractility, membrane permeability, cellular adhesion, cell stiffening, cell morphological changes, extracellular matrix organization and DNA synthesis.

EFFECTS OF RHO KINASE INHIBITORS AS IOP LOWERING AGENTS

No new class of anti-glaucoma drugs has been introduced since 1996, when FDA approved latanoprost.

Therefore, this group of Rho Kinase inhibitors are an exciting new class of drugs available to lower IOP.

Two drugs are commonly available commercially. These include: Ripasudil (K-115) and Netarsudil (AR-13503).

DRUGS	RIPASUDIL	NETARSUDIL
COMMERCIAL NAME	Glanatec (Derivative of Fasudil)	Rhopressa
CHEMICAL FORMULA	C15H18FN3O2S	C28H27N3O3
IUPAC NAME	4-fluoro-5-(((2S)-2-methyl-1,4-diazepan-1-yl)sulfonyl)isoquinoline	(4-((1S)-1-(Aminomethyl)-2-(isoquinolin-6-ylamino)-2-oxoethyl)pheny)methyl2,4-dimethylbenzoate
DURATION IT TAKES TO LOWER IOP AFTER INSTILLATION	2 hours	2 hours
DOSAGE	0.4% drug instilled twice daily	0.02% solution used once daily
SIDE EFFECTS	Conjunctival hyperemia Conjunctival haemorrhage (Hyperemia usually resolves spontaneously over few hours) ROCK inhibitors usually lower BP and vascular resistance. ROCK inhibition reduced the intraocular penetration of concurrent timolol instillation (presumably by increasing the elimination through the dilated conjunctival vasculature).	Conjunctival hyperemia Cornea verticillatae (Usually asymptomatic and did not reduce visual function) Pain at instillation site Conjunctival haemorrhage

MECHANISM:

ROCK inhibition lowers IOP by:

1. Increasing aqueous humor outflow.
2. Reducing aqueous humor production.
3. Decreasing episcleral venous pressure (EVP). This can be understood by the following equation=

IOP= EVP + 1/ C(F-U)

Here, C= Facility of outflow.

F= Formation of aqueous humor.

U= Resorption rate of aqueous humor.

IOP= Intra-ocular pressure.

As the metabolic pathway of Rho Kinase controls many aspects of cell morphology, when it is inhibited following changes occur:

1. Change of cell shape in the trabecular meshwork.
2. Change in actin cytoskeletal structure
3. Rounding of cell bodies
4. Disruption of actin production.

All these changes allow greater outflow of aqueous humor through the trabecular meshwork which ultimately results in decreased IOP.

NOR-EPINEPHRINE TRANSPORT INHIBITION:

Many ROCK inhibiting drugs chemically include a nor-epinephrine transport (NET) inhibition component.

This nor-epinephrine inhibitor helps to reduce aqueous production and decreases EVP.

Nor-epinephrine transport inhibition lowers aqueous humor production by (i) vasoconstriction (ii) reducing blood flow to the ciliary processes.

Aqueous humor production may be reduced by 20-23% by nor-epinephrine transport inhibition.

NEUROPROTECTION:

ROCK inhibitors increase blood flow in the optic nerve head by relaxation of the vascular endothelial smooth muscle. Nitric oxide synthase inhibitor induced impairement of optic nerve blood flow was also prevented by ROCK inhibition.

ROCK inhibitors influence neuron survival and axon regeneration. Fasudil protected against glutamate-related excitotoxicity in the retina and better preserved cells of the ganglion cell layer on exposure to N-methyl-D-aspartate.

CLINICAL TRIALS:

Phase 1,2,3 clinical trials of netarsudil were conducted in more than 2000 patients. 0.02% once daily in the evening was found to be the most efficacious and well tolerated dosing regimen. The phase 2 clinical trial included comparison with latanoprost and four phase 3 trials comparing netarsudil with timolol.

When netarsudil and latanoprost were compared, IOP reductions were similar [-5.8 mmHg in Netarsudil group vs -5.9 mmHG in the latanoprost group]. Among all patients netarsudil was 1 mmHg less effective than latanoprost and did not meet the statistical analysis for non-inferiority of netarsudil to latanoprost criteria.

Rocket 1,2,4 phase 3 trials compared netarsudil to timolol. All studies showed comparable IOP reduction with both drugs.

A fixed-dose combination of netarsudil and latanoprost (Roclatan) has been tested with individual components in two clinical studies (Mercury 1 and Mercury 2). Mercury 3 is an ongoing trial comparing Roclatan and Ganfort.

Mercury 1 & 2 have shown 1-3 mmHg greater IOP lowering with Roclatan compared to the individual components. IOP reductions of atleast 30% were achieved in 65% of patients treated with Roclatan, compared to 40% when individual components were used.

Roclatan treated patients achieved 16 mmHg or less IOP in 61% cases, while ≤ 14 mmHg was achieved in 33% cases. This was comparable to those treated with individual components who achieved similar results in 40% or less and 15% or less patients respectively.

CONCLUSION:

Use of Rho Kinase inhibitors in treating glaucoma has been proven to be of benefit. Although there is evidence showing that Rho Kinase inhibitors are related to increased conjunctival hyperemia.