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Chapter 234 – New (Pending) Glaucoma Medical Therapy

Chapter 234 – New (Pending) Glaucoma Medical Therapy

 

WILLIAM C. STEWART

 

INTRODUCTION

Ophthalmologists are fortunate to witness the introduction in recent years of several new pharmaceutical products to help manage elevated intraocular pressure (IOP). In general, these drugs are new compounds within existing classes of medicines already in use or combinations of currently approved products. The availability of these medicines will improve physicians’ choices in the management of glaucoma patients.

For the 21st century, compounds are being evaluated that may influence the physiological function of the optic nerve, through an increase in ocular blood flow or an improvement in nerve cell physiology, without reducing IOP. Even therapeutic alteration of the human genome, the genetic delivery of neuroprotective products, and optic nerve axonal regeneration appear to be more feasible than previously believed.

This chapter reviews IOP-reducing compounds that have recently become available commercially or should be available in the near future. Also, new concepts in therapy that may be used to treat the optic nerve in the future are presented.

LATANOPROST–TIMOLOL MALEATE FIXED COMBINATION

This product is a fixed combination of latanoprost 0.005% and timolol maleate 0.5% (Xalcom, Pharmacia, Peapack, NJ) given once a day. The pharmacology of the fixed combination is essentially related to its individual components.

A multicenter regulatory trial in Germany demonstrated that the fixed combination, compared with timolol maleate given twice daily, reduced IOP an additional 1.9?mmHg on average.[1] In contrast, the fixed combination compared with latanoprost alone, dosed every morning, showed a mean improvement of 1.2?mmHg[1] ( Fig. 234-1 ). In the United States, a similar trial showed a 2.9?mmHg improvement of the fixed combination over timolol maleate 0.5% given twice daily and a 1.1?mmHg improvement over latanoprost given alone each evening.[1] In these trials, there were no safety issues related to the fixed combination over and above those associated with the individual components alone.[2]

The fixed combination also has been compared with several adjunctive therapies. Stewart[3] found in a crossover study that evening dosing of the fixed combination provided a statistically greater IOP reduction—approximately 1–2?mmHg—at 6–12 hours after dosing ( Fig. 234-2 ). In addition, the fixed combination provided a lower diurnal IOP over the 12-hour daytime curve (measured every 2 hours from 8:00 a.m. to 8:00 p.m.).[3] Further, Feldman[4] showed that the latanoprost-timolol fixed combination, dosed each morning, was more effective by 1?mmHg in a three-point diurnal curve than the dorzolamide 2%–timolol maleate 0.005% fixed combination (Cosopt, Pharmacia, Peapack, NJ). Stewart [3] also evaluated morning dosing of the fixed combination versus latanoprost and brimonidine in a crossover comparison. This showed an equal diurnal curve reduction from untreated baseline for both treatments

 

 

Figure 234-1 Latanoprost-timolol combination therapy versus monotherapy. Pfeiffer et al. showed that latanoprost-timolol fixed combination therapy provided a statistically lower intraocular pressure than either of the individual components over a three-point daytime diurnal evaluation. (From Pfeiffer N. A comparison of the fixed combination of latanoprost and timolol with its individual components. Graefes Arch Clin Exp Ophthalmol. 2002;240(11):893–9.)

 

 

Figure 234-2 Latanoprost-timolol combination versus brimonidine and timolol. Stewart et al. showed that 12 hours after dosing, the latanoprost-timolol fixed combination provided a statistically greater reduction in intraocular pressure than did brimonidine and timolol each given twice daily. (Internal data, Pharmaceutical Research Network, LLC.)

 

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over a three-point daytime diurnal curve (internal data, Pharmaceutical Research Network, LLC).

The 24-hour diurnal effect of the latanoprost-timolol fixed combination was compared with placebo by Larsson et al.[5] in 19 patients in a crossover comparison and showed an average reduction of 24% (from 19.4?mmHg to 14.7?mmHg).[5] However, at 2:00 and 4:00, there was no significant reduction from placebo. Konstas et al.[6] followed this study by evaluating latanoprost and timolol, each dosed once daily from separate bottles, and demonstrated that the unfixed combination given once daily provided an additional reduction of 4.1?mmHg, including the 2:00 and 6:00 time points Except for a 1.5?mmHg greater decrease at 6:00 a.m., evening dosing provided a statistically similar effect to morning dosing.[6]

In summary, the fixed combination appears to provide a safe additional lowering of the IOP compared with its individual components, and with other combination therapies, in the majority of patients.

FUTURE GLAUCOMA THERAPY

Current Problems

Currently, typical treatment for glaucoma consists of lowering the IOP to a level that the ophthalmologist believes will protect the optic nerve. To determine whether the treatment is successful, the patient must be followed long term with routine IOP, optic disc, and visual field examinations to rule out progressive glaucomatous damage.

However, some patients continue to have damage, even though the IOP has been lowered to a level considered safe by most studies.[7] [8] [9] In addition, some patients present with visual field or visual acuity loss from glaucoma, and current therapy offers no opportunity for the reversal of these deficits and a return to a normal condition. Future glaucoma therapy should be directed, at least in part, to addressing these problems.

New Types of Glaucoma Agents

Future glaucoma therapy may focus on directly protecting the optic nerve using at least four potential clinical approaches:

• Improved optic nerve head blood flow.

• Neuroprotection.

• Gene therapy.

• Optic nerve regeneration.

Much work needs to be performed in all these areas to determine the potential benefits of direct treatment of the optic nerve in glaucoma patients, as well as to deliver these medicines in a safe and effective way to the target tissues.

IMPROVED OPTIC NERVE HEAD BLOOD FLOW.

Increased optic nerve head blood flow in some, if not all, glaucoma patients may help protect the optic nerve. Current studies indicate that mean blood flow velocity is reduced in patients who have primary open-angle and normal-tension glaucoma.[10] Also, vasospasm may occur in certain patients who have normal-tension glaucoma.[10] Additionally, some evidence exists that ocular hypertensive patients who have progressed are more likely to have reduced blood flow velocity and increased vascular resistance. [11] Consequently, an increase in blood flow in these patients could help stabilize glaucomatous progression through a reversal of any optic nerve head ischemia induced by elevated IOP or other unknown vascular factors.

Despite research over the past decade showing reduced blood flow in primary open-angle and normal-tension glaucoma patients, no ocular blood flow product has been forthcoming. A number of pharmaceutical companies have investigated such products, but these projects have been dropped from consideration. A number of problems exist in developing a blood flow product, and these problems could influence the development of other types of new glaucoma medicines, such as those offering neuroprotection.

Theoretical Issues.

Unfortunately, despite the evidence that reduced blood flow exists in patients with primary open-angle or normal-tension glaucoma, it is not known if this is a primary or secondary effect. In other words, did the reduced blood flow cause the glaucomatous damage, or is it a secondary effect from the decreased number of axons present, which require less oxygen for survival? Further, the ocular location of any ischemic change has not been described; nor is there any clinical data showing improved visual retention resulting from increased ocular blood flow.

Development Issues.

Drug delivery remains a problem for an ocular blood flow agent. It is difficult to dose an ocular drug topically and attain sufficient drug levels in the optic nerve, retina, or choroid to achieve a clinical response. Consequently, systemic delivery would most likely be required to reach the posterior segment of the eye and improve blood flow. Systemically delivered vasoactive medicines, however, most likely would demonstrate cardiovascular side effects. Such events probably would be intolerable for a prophylactic medication given in an essentially symptomless disease.

Several other development issues exist. First, because the anatomical location of reduced blood flow related to the pathogenesis of glaucoma has not been described, any method that demonstrates a clinically important effect cannot be precisely analyzed. Because blood flow instruments are costly and they each generally measure a different location in or behind the eye (e.g., color Doppler measures the retrobulbar blood flow), developing a study to evaluate a blood flow product could be cost prohibitive. Second, the patient population that should be studied to show a clinically important blood flow effect remains unclear (e.g., older primary open-angle glaucoma patients with vascular disease, normal-tension glaucoma patients, or all glaucoma patients). Last, clinical end points for blood flow remain unclear. Regulatory agencies and the pharmaceutical industry must work together to develop new clinical end points, apart from the IOP, that would describe the benefit of a blood flow product.

Cost.

Unfortunately, showing the utility of a blood flow product as an end point most likely would require showing visual stability over 3–5 years. Along with the blood flow instrumentation required, this factor would greatly increase the cost of performing multicenter regulatory trials.

NEUROPROTECTION.

Because of the difficulties of developing a blood flow treatment, much attention has turned to developing a neuroprotective product that would work directly on the optic nerve to improve its health. Unfortunately, the development of such a product suffers from many of the same problems associated with creating a blood flow compound.

Most early attention has been directed toward either a trophic factor or a glutamate inhibitor. However, the molecular mechanisms by which axons degenerate in glaucoma are not precisely known. Quigley[12] showed in a monkey model that axons degenerate in glaucoma by apoptosis as a final common end pathway. However, the exact trigger and process leading to apoptosis in glaucoma remain undescribed.

The ischemic model of apoptosis is the most descriptive model of this process from the systemic literature.[13] Theoretically, during ischemia, there is a period of axonal dysfunction before death occurs. If therapeutic intervention occurs during dysfunction, the axon could theoretically be rescued and the optic nerve stabilized. In this model, glaucomatous damage is hypothesized to occur from chronic ischemia, as opposed to a cerebrovascular accident, which happens acutely.

Many of the sequential events leading to ischemia-induced apoptosis have been described. It is believed that ischemia causes depolarization of the axon. The associated potassium release into the extracellular space cannot be actively reversed back into the intracellular space because of the disrupted cellular metabolism. The depolarized axon also releases its normal neurotransmitters, glutamate and aspartate, which causes extracellular calcium to be driven into the cell through the N-methyl-d-aspartate

 

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Figure 234-3 Theoretical mechanism of ischemic injury in glaucoma. This is the proposed mechanism of ischemia leading to apoptosis in glaucoma. Ischemia causes axonal depolarization, with potassium and neurotransmitter release from the cell. Extracellular calcium then enters the cell directly through L-type (gated) receptors and through N-methyl-d-aspartate (NMDA) and non-NMDA receptors mediated by glutamate and aspartate, respectively. The increased intracellular calcium leads to protein phosphorylation, production of proteases, free radical formation, nitric oxide synthetase (NOS), and release of intracellular calcium. These events cause disruption of energy-producing mechanisms and cell membranes and induction of the enzymatic processes leading to apoptosis. The neuroprotective agent memantine would theoretically act to inhibit the effect of glutamate.

(NMDA) and non-NMDA receptors, respectively. In addition, the L-type (gated) calcium channel receptors are opened after the axonal depolarization that allows extracellular calcium directly into the cells.

The increased intracellular calcium leads to protein phosphorylation, production of proteases, free radical formation, and release of intracellular calcium stores. These events cause disruption of energy-producing mechanisms and cell membranes and induction of the enzymatic processes leading to apoptosis ( Fig. 234-3 ).[13]

At least one drug, memantine (Allergan, Irvine, CA) is in clinical development as a neuroprotectant for glaucoma. It has been approved in Germany for the past two decades as an anti-Parkinson’s drug (Merz Pharmaceuticals GmbH, Frankfurt, Germany). Following its approval, memantine was shown to have antiglutamate properties that would theoretically help prevent calcium from entering the cell during ischemic stress (see Fig. 234-3 ). The results of regulatory trials may be available by mid-decade.

Recently, research has described several steps leading to the initiation of apoptosis that could be related directly to elevated IOP. Nitric oxide synthetase type 2 is neurotoxic and is induced from pericytes and astrocytes by high IOP in rats.[14] [15] Nitric oxide synthetase is not released from optic nerve head pericytes and astrocytes when the IOP is normal. Further, inhibition of nitric oxide synthetase has been shown to be protective of the optic nerve in rats (see Fig. 234-3 ).

In addition, caspase enzymes have been shown to be associated with apoptosis in rats that have increased IOP.[16] Further, brain-derived neurotropic factor has been demonstrated to diminish the caspase II enzyme during ischemic stress.[17]

Neuroprotection will be an important future therapy to ensure the stabilization of vision in glaucoma patients. However, much work needs to be performed to describe the exact mechanism of neurodegeneration in glaucoma on a molecular basis, so that a specific neuroprotective therapy for glaucoma can be developed.

GENE THERAPY.

Gene therapy could be used in two ways in glaucoma: as a drug delivery system, and as a basis for developing new therapies and treatment end points based on the genetic mutations that cause glaucoma. Gene therapy could be used as a drug delivery system by encoding external genes (e.g., a viral genome) to the patient’s own genome, which would produce protective trophic factors intracellularly. Problems with gene therapy in the eye include delivery of the genome to the target tissue (i.e., optic nerve) and potential carcinogenic effects of the implantation of new genomes inside the cell.[17]

The human genome could also be used as a basis for treatment decisions. InSite Vision (Alameda, CA) recently released a diagnostic kit for primary open-angle glaucoma (Ocugene) based on the TIGR/MYOC mt1 variant in the promoter region of the gene. A recent study found that the TIGR/MYOC–positive gene mutation was associated with more rapid progression in 147 primary open-angle glaucoma patients followed over an average of 15 years (internal data, InSite Vision). Further, Colomb et al.[18] demonstrated in 142 patients a greater prevalence of glaucomatous disc damage and higher IOP in TIGR/MYOC–positive patients. In contrast, Alward et al.[19] found no differences in optic disc, visual field, or IOP findings between TIGR/MYOC–positive and –negative patients among 779 subjects with a variety of glaucomas, ocular hypertensive patients, and normal subjects.

If indeed the TIGR/MYOC gene or separate genes could be shown to be a risk factor for earlier onset or more progressive disease, a patient’s therapeutic end points could be modified based on his or her genetic profile. In the future, such a patient may start therapy earlier in life and be maintained at a lower IOP to help prevent glaucomatous progression and visual loss. Consequently, the genetic profile may help individualize patient therapy to better ensure a stable glaucomatous disease course.

In addition, new therapies may be derived from glaucoma-related genes. Proteins or enzymes produced by the abnormal gene could be identified and their deposition or action prevented to stabilize or reverse damage in the outflow system of the eye or the optic nerve.[20] The abnormal gene could potentially be turned off, as a separate mechanism, to help prevent abnormal protein or enzyme production. Ultimately, investigation into correcting the abnormal gene itself might provide a more

 

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Figure 234-4 Effect of diclofenac on intraocular pressure (IOP). Stewart et al. showed that the highest IOP over a 6-week course of topical corticosteroid treatment was statistically minimized in patients given diclofenac 0.06% compared with vehicle. Diclofenac 0.10% also showed a trend toward minimizing the IOP increase caused by corticosteroids. *, Significantly different from vehicle (P >.015). (Internal data, Pharmaceutical Research Network, LLC.)

permanent cure to the disease. Further, Tian[21] has shown that several genes can be up-regulated from the presence of the glaucoma itself, unrelated to any genetic defect. Therapeutic potential exists in down-regulating these genes to prevent their products from being deposited in the trabecular meshwork.

InSite Vision recently evaluated diclofenac in a formulation designed to achieve higher aqueous levels than those obtained by the currently available commercial product. The study evaluated whether diclofenac could blunt the ocular hypertensive response from a topical corticosteroid in first-degree relatives of patients with primary open-angle glaucoma. Patients had their highest ocular hypertensive response to topical corticosteroid minimized statistically by diclofenac (0.06%, 4.9?mmHg; 0.10%, 5.4?mmHg; vehicle, 7.2?mmHg)[20] ( Fig. 234-4 ). TIGR/MYOC gene regulation had previously been linked to corticosteroids.[22] Further, in a study performed on patients with ocular hypertension or primary open-angle glaucoma, patients who were TIGR/MYOC gene positive and were treated with 0.10% diclofenac had lower IOPs (1.3–3.4?mmHg) than those treated with placebo over the 12-hour daytime diurnal curve (internal data, InSite Vision) ( Fig. 234-5 ). Future research may provide useful clinical drugs that will help treat glaucoma on a genetic basis.

 

 

Figure 234-5 Effect of diclofenac on intraocular pressure (IOP) in TIGR/MYOC–positive patients. This study showed that patients who were TIGR/MYOC gene positive had a statistically lower IOP following treatment with 0.10% diclofenac compared with vehicle. *, Overall 0.1% diclofenac treatment effect significantly different from vehicle. (Internal data, Pharmaceutical Research Network, LLC.)

OPTIC NERVE REGENERATION.

Little work has been performed specifically with regard to optic nerve regeneration. Unlike the more sophisticated mammals, lower life-forms such as goldfish are able to regenerate their optic nerves. Uncovering the secrets in these lower life-forms may help develop a therapy to allow optic nerve regeneration in humans.[23] [24]

 

 

REFERENCES

 

1. Pfeiffer N. A comparison of the fixed combination of latanoprost and timolol with its individual components. Graefes Arch Clin Exp Ophthalmol. 2002;240(11):893–99.

 

2. Higginbotham EJ, Feldman R, Stiles M, Dubiner H. Latanoprost and timolol combination therapy vs monotherapy: one-year randomized trial. Arch Ophthalmol. 2002;120(7):915–22.

 

3. Stewart JA Day DG, Sharpe ED, Stewart WC. Efficacy and safety of latanoprost/timolol maleate fixed combination versus timolol maleate and brimonidine given twice daily. Invest Ophthalmol Vis Sci. 2002;43:E-Abstract 3430.

 

4. Feldman RM. A comparison of fixed combination of latanoprost and timolol with fixed combination of dorzolamide and timolol (COSOPT) in patients with elevated intraocular pressure: a three-month masked evaluator, phase IIIb, multicenter study in the United States (XALCOM vs. COSOPT). Invest Ophthalmol Vis Sci. 2002;43:E-Abstract 295.

 

5. Larsson LI, Mishima HK, Takamatsu M, et al. The effect of latanoprost on circadian intraocular pressure. Surv Ophthalmol. 2002;47(Suppl 1):S90–6.

 

6. Konstas AG, Nakos E, Tersis I, et al. A comparison of once-daily morning vs evening dosing of concomitant latanoprost/timolol. Am J Ophthalmol. 2002;133(6):753–7.

 

7. Mao LK, Stewart WC, Shields MB. Correlation between intraocular pressure control and progressive glaucomatous damage in primary open-angle glaucoma. Am J Ophthalmol. 1991;111:51–5.

 

8. Stewart WC, Chorak RP, Hunt HH, Sethuraman G. Factors associated with visual loss in patients with advanced glaucomatous changes in the optic nerve head. Am J Ophthalmol. 1993;116:176–81.

 

9. Stewart WC, Kolker AE, Sharpe ED, et al. Factors associated with long-term progression or stability in primary open-angle glaucoma. Am J Ophthalmol. 2000;130:274–9.

 

10. Stewart WC. Where are all the ocular blood flow medications? Rev Ophthalmol. 1998 May;137–40.

 

11. Hamzavi S, Stewart WC, Hamzavi SL, Stroman GL. Transcranial Doppler in progressed and stable ocular hypertensive patients. Invest Ophthalmol Vis Sci. 1996;37:S31.

 

12. Quigley HA, Nickells RW, Kerrigan LA, et al. Retinal ganglion cell death in experimental glaucoma and after axotomy occurs by apoptosis. Invest Ophthalmol Vis Sci. 1995;36:774–86.

 

13. Hickenbottom SL, Grotta J. Neuroprotective therapy. Semin Neurol. 1998;18: 485–92.

 

14. Shareef S, Sawada A, Neufeld AH. Isoforms of nitric oxide synthase in the optic nerves of rat eyes with chronic moderately elevated intraocular pressure. Invest Ophthalmol Vis Sci. 1999;40:2884–91.

 

15. Liu B, Neufeld AH. Nitric oxide synthase-2 in human optic nerve head astrocytes induced by elevated pressure in vitro. Arch Ophthalmol. 2001;119:240–5.

 

16. Kurokawa T, Arai J, Katai N, et al. Expression of Caspase-9 in rat retinal ganglion cell layer neurons after transient ischemia. Invest Ophthalmol Vis Sci. 2000;41:S17.

 

17. Hegazy KA, Dunn MW, Sarma SC. Functional human heme oxygenase has a neuroprotective effect on adult rat ganglion cells after pressure-induced ischemia. Neuroreport. 2000;11:1185–9.

 

18. Colomb E, Nguyen TD, Bechetoille A, et al. Association of a single nucleotide polymorphism in the TIGR/MYOCCILIN gene promoter with the severity of primary open-angle glaucoma. Clin Genet. 2001;60(3):220–5.

 

19. Alward WLM, Kwon YH, Khanna CL, et al. Variations in the myocilin gene in patients with open-angle glauocoma. Arch Ophthalmol. 2002;120:1189–97.

 

20. Stewart WC, Walters TR, Day DG, et al. Effects of ISV-205 (diclofenac in DuraSite) on corticosteroid-induced IOP response in glaucoma relatives. Invest Ophthalmol Vis Sci. 2000;41:S51.

 

21. Tian B, Geiger B, Epstein DL, Kaufman PL. Cytoskeletal involvement in the regulation of aqueous humor outflow. Invest Ophthalmol Vis Sci. 2000;41:619–23.

 

22. Polansky JR, Fauss DJ, Zimmerman CC. Regulation of TIGR/MYOC gene expression in human trabecular meshwork cells. Eye. 2000;14(Pt 3B):503–14.

 

23. Sivron T, Schwab ME, Schwartz M. Presence of growth inhibitors in fish optic nerve myelin: post injury changes. J Comp Neurol. 1994;343:237–46.

 

24. Cohen I, Sivron T, Lavie V, et al. Vimentin immunoreactive glial cells in the fish optic nerve: implications for regeneration. Glia. 1994;10:16–29.

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