June Chen

Novel ocular antihypertensive compounds in clinical trials.

Introduction: Glaucoma is a multifactorial disease characterized by progressive optic nerve injury and visual field defects. Elevated intraocular pressure (IOP) is the most widely recognized risk factor for the onset and progression of open-angle glaucoma, and IOP-lowering medications comprise the primary treatment strategy. IOP elevation in glaucoma is associated with diminished or obstructed aqueous humor outflow. Pharmacotherapy reduces IOP by suppressing aqueous inflow and/or increasing aqueous outflow. Purpose: This review focuses on novel non-FDA approved ocular antihypertensive compounds being investigated for IOP reduction in ocular hypertensive and glaucoma patients in active clinical trials within approximately the past 2 years. Methods: The mode of IOP reduction, pharmacology, efficacy, and safety of these new agents were assessed. Relevant drug efficacy and safety trials were identified from searches of various scientific literature databases and clinical trial registries. Compounds with no specified drug class, insufficient background information, reformulations, and fixed-combinations of marketed drugs were not considered. Results: The investigational agents identified comprise those that act on the same targets of established drug classes approved by the FDA (ie, prostaglandin analogs and β-adrenergic blockers) as well as agents belonging to novel drug classes with unique mechanisms of action. Novel targets and compounds evaluated in clinical trials include an actin polymerization inhibitor (ie, latrunculin), Rho-associated protein kinase inhibitors, adenosine receptor analogs, an angiotensin II type 1 receptor antagonist, cannabinoid receptor agonists, and a serotonin receptor antagonist. Conclusion: The clinical value of novel compounds for the treatment of glaucoma will depend ultimately on demonstrating favorable efficacy and benefit-to-risk ratios relative to currently approved prostaglandin analogs and β-blockers and/or having complementary modes of action.

Identification of a prostanoid FP receptor population producing endothelium-dependent vasorelaxation in the rabbit jugular vein.

1 Prostaglandin F2α (PGF2α) and its synthetic analogue, fluprostenol, potently relaxed the precontracted isolated jugular vein of the rabbit (RJuV). The vasorelaxant activity of PGF2α and fluprostenol was dependent upon an intact vascular endothelium. Although removal of the vascular endothelium abolished activity associated with PGF2α‐like agonists, it did not significantly alter the relaxant effects of prostaglandin E2 (PGE2). 2 The nitric oxide synthase inhibitor, NG‐nitro‐L‐arginine methyl ester (l‐NAME), at 100 μm significantly inhibited the endothelium‐dependent relaxations induced by PGF2α. Lower doses (1 μm, 10 μm) of l‐NAME had little or no effect. The relaxant effects of PGE2 were not affected by l‐NAME (1–100 μm). D‐NAME at 100 μm was without effect on the vasorelaxant responses to either PGF2α or PGE2. 3 The potassium (K)‐channel blockers tetraethylammonium (TEA, 1 mm), barium (1 mm) and quinine (100 μm), each tested in the presence of the inactive enantiomer D‐NAME (100 μm) did not significantly affect the response to PGF2α. Unexpectedly, both TEA and barium significantly and partially reversed the inhibitory effects of 100 μm l‐NAME, whereas quinine had no effect. In similar studies, none of the three potassium channel blockers had any effect on relaxations elicited by PGE2 when given with D‐NAME or l‐NAME. 4 These results indicate that the PGF2α‐sensitive prostanoid receptors found in the vascular endothelium of the rabbit jugular vein are of the FP‐receptor subtype. Nitric oxide (NO) appears to be the predominant messenger involved in PGF2α‐induced relaxation of the rabbit jugular vein. Potassium channels may have a minor role in mediating the vasorelaxation response to PGF2α. When both NO synthesis and K‐channels are simultaneously blocked, inhibition of PGF2α‐induced vasorelaxation by l‐NAME is opposed by K‐channel blockers. This diminution of the inhibitory effect of l‐NAME by TEA and barium suggests that K‐channels may possibly serve a compensatory role via the NO pathway.

Studies using isolated uterine and other preparations show bimatoprost and prostanoid FP agonists have different activity profiles.

1 The pharmacology of bimatoprost, a synthetic prostaglandin‐amide, was examined in prostaglandin F2α (PGF2α)‐sensitive preparations. Bimatoprost potently contracted the rabbit isolated uterus (pEC50=7.92±0.16). In contrast, bimatoprost exhibited weak excitatory activity in human myometrium from pregnant and nonpregnant donors, mouse uterus, rat uterus, and endothelium‐intact rabbit jugular veins, and did not stimulate DNA synthesis in mouse fibroblasts. 2 The possibility that the effects of bimatoprost may reflect partial agonism at prostanoid FP receptors was examined and the contractile effects of full agonists, 17‐phenyl PGF2α (FP) and U‐46619 (TP, a control), were determined in the absence and presence of 1 μM bimatoprost on the mouse uterus. Analyses of the agonist–agonist functional studies showed no antagonism, indicating that bimatoprost is not a partial agonist. 3 Bioassay metabolism studies of bimatoprost and latanoprost (FP receptor agonist prodrug) in the rabbit uterus were conducted using recipient mouse uterus. Results indicated that the potent responses to bimatoprost in the rabbit uterus are produced by the intact molecule and not by its putative free acid metabolite, 17‐phenyl PGF2α. Some hydrolysis of latanoprost to latanoprost free acid appears to have occurred in the rabbit uterus, according to biological detection. 4 The pharmacology of bimatoprost could not be explained by its interaction with known prostanoid FP receptors and was independent of species‐, tissue‐, or preparation‐related factors. The potent contractile effects of bimatoprost in the rabbit uterus provide further pharmacological evidence for the presence of a novel receptor population that preferentially recognises bimatoprost.

Fixed-combination and emerging glaucoma therapies.

Ocular hypotensive agents are the only approved pharmacotherapy for glaucoma. Despite significant advances during the past two decades, a large proportion of glaucoma patients require more than one drug. The most recent additions to the armamentarium of antiglaucoma drugs are fixed-combination products for the glaucoma patient who is insufficiently responsive to monotherapy. Fixed-combination products have the combined efficacy of two ocular hypotensive drugs, and the convenience of a two-drug treatment regimen in a single container, which may aid patient adherence to treatment. Available fixed-combination products consist of timolol 0.5% as an invariant with brimonidine 0.2%, dorzolamide 2%, travoprost 0.004%, latanoprost 0.005% or bimatoprost 0.03%. Research on more advanced antiglaucoma medications continues. Promising new directions appear to be the Rho-kinase inhibitors, microtubule-disrupting agents, serotonergics and cannabimimetics. Efforts continue to improve existing antiglaucoma drugs in an attempt to design second-generation cholinomimetics, adrenergics, prostaglandins and prostamides.

An EP receptor with a novel pharmacological profile in the T‐cell line Jurkat

1 Comparison of the rank order of potency of the natural prostanoids prostaglandin E2 (PGE2), PGD2, PGF2a and carbaprostacyclin in stimulating cyclic AMP in Jurkat cells is consistent with the presence of an EP receptor. 2 Lack of responsiveness to the EP1/EP3 selective agonist, sulprostone, and the EP2 agonists, butaprost and AH 13205, indicates that this receptor is not of the EP1, EP2 or EP3 subtypes. 3 Inhibition of PGE2‐stimulated cyclic AMP by the EP4 antagonist, AH 23848 is non‐competitive, unlike the competitive antagonism reported in the pig saphenous vein EP4 preparation. Furthermore, 16,16‐dimethyl PGE2 is 100 fold less potent than PGE2 in Jurkat cells, while these agonists are equipotent in the rabbit jugular vein purported EP4 preparation. In addition, 1‐OH PGE1 which also is active in the rabbit jugular vein preparation, is inactive in Jurkat cells at concentrations up to 1 × 10−4 m. These data are not wholly consistent with any adenylate cyclase coupled EP receptor described to date. 4 It is postulated that an EP receptor, positively coupled to adenylate cyclase, with a unique pharmacological profile is present in Jurkat cells.

Comparison of Human Ocular Distribution of Bimatoprost and Latanoprost

PURPOSE This study investigated the ocular distribution of bimatoprost, latanoprost, and their acid hydrolysis products in the aqueous humor, cornea, sclera, iris, and ciliary body of patients treated with a single topical dose of 0.03% bimatoprost or 0.005% latanoprost for understanding concentration-activity relationships. METHODS Thirty-one patients undergoing enucleation for an intraocular tumor not affecting the anterior part of the globe were randomized to treatment with bimatoprost or latanoprost at 1, 3, 6 or 12 h prior to surgery. Concentrations of bimatoprost, bimatoprost acid, latanoprost, and latanoprost acid in the human aqueous and ocular tissues were measured using liquid chromatography tandem mass spectrometry. RESULTS Following topical administration, intact bimatoprost was distributed in human eyes with a rank order of cornea/sclera >iris/ciliary body >aqueous humor. Bimatoprost acid was also detected in these tissues, where its low levels in the cornea relative to that of latanoprost acid indicated that bimatoprost hydrolysis was limited. Latanoprost behaved as a prodrug that entered eyes predominantly via the corneal route. Levels of latanoprost acid were distributed as cornea >>aqueous humor>iris>sclera>ciliary body. CONCLUSIONS Our study provided experimental evidence that levels of bimatoprost in relevant ocular tissues, and not only aqueous humor, are needed to understand the mechanisms by which bimatoprost lowers intraocular pressure (IOP) in human subjects. The data suggest that bimatoprost reached the target tissues favoring the conjunctival/scleral absorption route. Findings of intact bimatoprost in the target ciliary body indicated its direct involvement in reducing IOP. However, bimatoprost acid may have only a limited contribution on the basis that bimatoprost has greater/similar IOP-lowering efficacy than latanoprost, yet bimatoprost acid levels were a fraction of latanoprost acid levels in the aqueous humor and cornea and only sporadically detectable in the ciliary body. In this report, human ocular tissues were examined concurrently with aqueous humor for the in vivo distribution of bimatoprost, bimatoprost acid, latanoprost, and latanoprost acid.

Prostaglandin ep4 antagonists

Prostaglandin antagonists, with their pharmacological effects, are well-known drugs capable of treating widely diffused illnesses, including pain and inflammation disorders. In recent years, a major research focus has been devoted to the identification of agents able to selectively antagonize each receptor with which prostaglandins interact. This review attempts to give a broad overview of molecules capable of selectively blocking the prostaglandin PGE2 EP4 receptor. Further therapeutic applications and uses have also been disccussed, including the first drug candidate to have reached clinical trials within the last few years.

Bimatoprost-Induced Calcium Signaling in Human T-Cells does not Involve Prostanoid FP or TP Receptors

Purpose: The prostamide bimatoprost and prostanoid FP receptor agonists are highly efficacious drugs for glaucoma treatment. The presence of both prostamide and prostanoid FP receptors in bimatoprost-sensitive preparations has made prostamide receptor classification difficult. This study investigated a novel bimatoprost-sensitive preparation. Methods: Human peripheral blood T lymphoblasts (Molt-3) and human osteoblasts (hFOB) were cultured for intracellular calcium signaling studies and quantitative real-time PCR analysis of RNA. Results: Bimatoprost stimulated concentration-related increases in [Ca2 +]i in a human T-cell line that does not express human FP receptor/variants, according to PCR analysis. The calcium signal induced by bimatoprost was not antagonized by prostanoid FP receptor antagonist/partial agonist AL-8810 or selective TP receptor antagonist SQ 29548. Conversely, bimatoprost did not elevate [Ca2 +]i in human osteoblasts, which were confirmed to contain RNA of human FP receptor/variants. Conclusions: Molt-3 cells have been identified as a bimatoprost-sensitive preparation in which the activity of bimatoprost is independent of prostanoid FP receptors.

Prostanoid FP Receptor Mediated, Endothelium Dependent Vasodilatation and the Ocular Surface Hyperemic Response to PGF2α and Related Compounds

Prostaglandin Flaits esters, and synthetic prostanoid FP receptor agonists causedose-related increases in ocular surface hyperemia (OSH) that may persist over the course of a day. This side effect of red eyes limited the use of PGF2aand its isopropylester (IE) for glaucoma therapy. The goal to identify a quantitative model for predictingocular OSH in humans prompted studies to correlate this effect with activity at a prostanoid receptor. Criteria for the receptor associated with OSH were that it is potently stimulated by PGF2aand linked to smooth muscle relaxation, since dilatation of theconjunctival blood vessels is a vasorelaxant response.

Prostanoid Receptor Assays

Prostanoids, which include the prostaglandins (PGs) and thromboxanes (TXs), interact with a specific family of G‐protein coupled receptors, of which there are known to be five distinct types, DP, EP, FP, IP and TP, each particularly sensitive to one of the five natural prostanoids, PGD2, PGE2, PGF2(, PGI2 and TXA2, respectively. Of these, it is known that the EP receptor comprises four well‐characterized subtypes: EP1, EP2, EP3 and EP4. These receptor subtypes are widely distributed throughout mammals and other species, and show particularly high levels of expression in smooth muscle and blood platelets. Despite the fact that few of these preparations express a single receptor type/subtype in isolation, a range of useful smooth muscle and platelet assays for the various prostanoid receptors are available and are presented in this unit.