By Jorge A. Alvarado, MD
New research may unlock this glaucoma treatment's full potential.
I began using selective laser trabeculoplasty (SLT) in my practice for the treatment of glaucoma as soon as it was cleared by the FDA in 2001. While it proved to be a successful therapy for many patients, I began to wonder why the
success rate was variable and also why IOP reduction achieved by the procedure was only modest in some patients.
To answer these questions, I initiated a series of experiments in the laboratory. I wanted to compare the mechanisms of action of SLT and the prostaglandin analogue (PGA) medications we prescribe to most glaucoma patients
and determine whether negative interactions exist between them. I hoped I could bring what I learned in the laboratory back to the clinic to improve and accurately predict SLT outcomes.
Thus far, the results of this work have been dramatic. I have tested the laboratory findings1-4 in a small series of patients and both reflect the following key conclusions:
- Endothelial cells in the trabecular meshwork and
Schlemm's canal are part of a signaling system that regulates
aqueous outflow by opening and closing a unique cellular
barrier formed by the endothelial cells lining the lumen of
the canal.
- SLT and PGAs share a common mechanism of action
regarding the conventional aqueous outflow pathway.
- The effects of SLT and PGAs on aqueous outflow, and
thus IOP, are not additive.
- If the mechanism of action of SLT and of PGAs are
100% equivalent, a patient's response to PGAs is predictive
of SLT outcomes. Furthermore, with a new protocol for
patient selection and treatment, an SLT success rate of near-
ly 100% is possible.
These findings also apply to argon laser trabeculoplasty (ALT), but my recent work focuses on SLT. In this article, I explain my findings and how we can use them to understand and take advantage of the true effectiveness of SLT.
Discovery of a Previously UnknownCellular Signaling System
In the 1990s I conducted experiments that showed monocytes were attracted to the trabecular meshwork by ALT. When SLT became available, I found it also resulted in monocyte recruitment.5 That led me to think that adding
monocytes to the eye might lower the IOP. However, when that experiment was done, we added monocytes as well as the media into which the monocytes had secreted various factors. We observed that the pressure-lowering effect
induced by the monocytes and the media they had conditioned occurs immediately — too quickly to be mediated by monocytes alone. I then theorized that a cell signaling system was present, and the monocytes had released factors that activated the system.
My colleagues and I decided to use the SLT laser to help us better understand the signaling system. We irradiated both trabecular meshwork endothelial cells (TMEs) and Schlemm's canal endothelial cells (SCEs). We found that mainly the TMEs became activated. It turns out the TMEs are the important cells in this cell-to-cell signaling system. They function as both transducer cells and effector cells. That means when the IOP changes, for example when the
eye focuses to read and the muscle tension increases, the TMEs detect the change. They are baroreceptors. They transduce mechanical energy that is stretching the cells into a biochemical event, which leads to the release of the aforementioned factors. When the trabecular meshwork is
stretched by any means, such as with pilocarpine, the system activates. So, either mechanical stimulation or stimulation by the laser activates the signaling system. The net ultimate event is the secretion of the factors, which are cytokines. When the cytokines are released from the TMEs, they flow
with aqueous to the barrier formed by the endothelial cells lining the lumen of Schlemm's canal and open or close the barrier as necessary. In other words, the cytokines actively regulate the conductivity, i.e., the permeability properties, of the SCEs.
We have identified several of the many cytokines released by the TMEs. In further experiments, we have added them to the Schlemm's canal barrier cells to determine which ones are intimately involved in the opening and
closing of the barrier.
A Closer Look at Schlemm's Canal Cellular Barrier
To learn more about the barrier, we transfected SCEs in
vitro with a plasmid expressing the zonula occludens-1 protein tagged with green fluorescent protein. This allows visualization of the intercellular junctions in SCEs in living cells. Wherever there is a junction, it fluoresces green. We then lasered the SCEs directly or exposed them to media conditioned by either lasered SCEs or TMEs. Both induced similar effects in the SCEs. The cellular junctions were disassembled, which increased barrier conductivity (Figures 1 and 2). We were astounded to learn that the barrier in the aqueous outflow system is much more complex than barrier functions known to exist in other parts of the body. In the vascular system, for example, the barrier is like caulking between tiles. The property of the caulking regulates the permeability of the system. The tiles themselves do not allow any fluid to pass through, but the caulking is porous so fluid can pass though. In contrast, in the aqueous outflow system, the glue that holds the cells together is not like caulking, but rather it is like zippers. Each "tooth" in the zipper extends from one cell to the other, and each cell is bound to its
neighbor by a zipper containing many teeth.
When the cell-signaling system is activated, the teeth of the zippers, which are finger-like filopodia, begin to contract. Large gaps start to form because the barrier is opening. As the filopodia become separated, the protein assumes a linear disposition (i.e. without teeth), and the barrier is open. We know it is open because we have conducted flow studies. By adding different factors to the cells we can reverse the opening of the barrier. For instance, adding calcium to the system closes the barrier. The system is dynamic
and we are only beginning to understand it.
Effect of Prostaglandin Analogues on Aqueous Outflow
Once we had recreated the aqueous outflow barrier system in the laboratory, we were also able to test what effect glaucoma medications have on the barrier. We added various types of medications to the cells. Non-PGA drugs, such
as brimonidine, timolol and dorzolamide induced no
changes in the barrier cells (Figure 3).
However, the PGAs activated the cells in the same way as the laser or media
conditioned by lasered SCEs or TMEs. Latanoprost, travoprost and bimatoprost linearized the cellular junctions and opened the barrier (Figure 4 and 5).
This finding runs counter to the widely held belief that PGAs work via the uveoscleral outflow pathway and matrix metalloproteinases (MMPs). It indicates that PGAs have a direct effect on the Schlemm's canal endothelial cell barrier
function and therefore the conventional trabecular meshwork aqueous outflow pathway. Based on our studies, we have concluded that SLT and PGAs share a common mechanism of action, which is regulation of both the integrity of
the intercellular junctions and the permeability of the barrier
formed by SCEs.
Other researchers also have been reconsidering the mechanism of action of PGAs. In the latest paper on this subject, the late Doug Johnson's group reported their findings using cultured anterior segments, which eliminate the
uveoscleral pathway and allow direct assessment of trabecular outflow.6 They concluded that prostaglandins increase outflow facility but do not consistently increase MMP activity. They wrote that histologic changes observed "suggest a direct trabecular meshwork effect."
Other data that support my contention that SLT and PGAs share the same mechanism of action has been presented by Steinmann and Marcellino.7 With the SLT/MED Study Group, they compared the effects of medications and SLT in a prospective, multicenter, randomized, double-arm trial. Nearly 100% of the patients were on PGAs. After at least 8 months of follow-up, mean IOP reduction was 7.6 mmHg in the patients treated with medication and 6.7 mmHg in the patients treated with SLT, indicating that
the IOP lowering effects are nearly identical.
In addition, I find it interesting that in the 1990s, the large Kaiser Permanente network of physicians in northern California noted a precipitous decline in the use of ALT. This was happening elsewhere in the country as well. No one was sure why. Perhaps doctors had tired of seeing the side effects
of the laser. Perhaps they believed ALT was not providing enough "bang for the buck." I wonder if the answer was there all along. Use of other surgical procedures for glaucoma had not declined among these 2 million patients during
that time. However, the use of the first PGA, latanoprost, increased dramatically. So, it is possible that as doctors began to use latanoprost, they became more aware that something was going on with ALT but never made the connection. To me, this further supports our contention that PGAs and SLT
share the same mechanism of action, and treating with SLT on top of PGAs will not produce any further decrease in IOP.
Testing Our Findings in the Clinic
To test what we learned in the lab, I initiated a clinical study incorporating a new SLT treatment protocol. In the study, which involved 24 consecutive patients with primary open-angle glaucoma, I stopped use of PGAs several weeks
before performing SLT. I recorded three measurements: IOP while patients were being treated with PGAs and other medications, IOP at "baseline," which was several weeks after discontinuing PGAs, and IOP approximately 90 days after
SLT. The third measurement was taken to represent the IOP
response to SLT.
The idea was to remove the PGAs in order to avoid competition with SLT over a common mechanism of action. Removal of the PGAs would also be expected to increase IOP, which we already knew was important for the success of the laser procedure. Previous studies had shown that higher baseline pressures are associated with higher success rates and an increased pressure-lowering effect.8-11
All 24 of the study patients received 360° SLT treatment, and near two-hundred laser shots averaging 0.7 mJ/burst, which represents a doubling in the number of burst typically used by most practitioners today. All 24 patients responded
to the treatment in a highly effective and predictable fashion. The results showed that discontinuing PGA therapy prior to SLT produced a greater IOP-lowering effect and a higher success rate than when PGAs were not removed. The
average IOP-lowering response of PGAs was 25.37%, and the average IOP-lowering response of SLT was 29.93%.
Furthermore, we found that in patients for whom PGAs decreased IOP, it predicted a positive response to SLT and the magnitude of SLT's pressure-lowering effect. If further studies confirm the initial findings we have summarized here, it would mean the effectiveness of SLT therapy is far superior to what we thought in the past.
Further Positive Results with New Protocol
In the meantime, I use the new patient selection and treatment protocol in all patients for whom I am considering SLT. For patients taking no glaucoma medications, I first test their response to a PGA. If the PGA decreases IOP by 30-35%, I proceed with SLT. If the PGA does not reduce IOP, I do not perform SLT because I do not expect it to be effective. For patients taking a PGA and/or other medications, I discontinue the PGA for 4-6 weeks while replacing it with a non-P GA drug, and monitor IOP every 2 weeks. If the IOP rises, I expect SLT to work. The pressure tends to increase to a maximum level at approximately 4 weeks in most cases, after which time I perform SLT. After the laser treatment, I usually reintroduce a PGA to protect the eye from the IOP increase that often occurs in the early postoperative period. After 2-4 weeks, the SLT becomes fully functional and the PGA can be safely discontinued.
I perform the SLT procedure at a low energy level, 0.66 mJ, beginning at 12 o'clock and working clockwise to deliver 150-200 laser spots in 360°.
Following this protocol, I have been obtaining, on average, a 29-30% decrease in IOP in my SLT patients. This is a far greater response than what I had been achieving. Most patients see a significant drop in IOP at 1 month after the laser procedure and on average are able to eliminate one medication from their regimen.
I hope the results of our laboratory and clinical work prompt other ophthalmologists to rethink the use of PGAs in conjunction with SLT and consider alternative medications in patients who have already undergone SLT treatment. It is my firm belief that taking these steps will improve outcomes and better protect our patients' vision.
Dr. Alvarado is one of the few full-time faculty members at the University of California San Francisco who belong to the "blue and gold" society, consisting of individuals who received their undergraduate education, medical school, specialty and subspecialty training at the University of California Berkeley and the University of California San Francisco. Dr. Alvarado has devoted his entire career to basic research on cellular mechanisms of aqueous outflow regulation and the pathogenesis of glaucoma.
REFERENCES
- Alvarado JA, Iguchi R, Martinez J, et al. Similar effects of selective laser trabeculoplasty and prostaglandin analogs on the permeability of cultured Schlemm canal cells. Am J Ophthalmol 2010;150(2):254-264.
- Alvarado JA, Iguchi R, Juster R, et al. From the bedside to the bench and back again: predicting and improving the outcomes of SLT glaucoma therapy. Trans Am Ophthalmol Soc 2009;107:167-183.
- Alvarado JA, Yeh RF, Franse-Carman L, et al. MJ. Interactions between endothelia of the trabecular meshwork and of Schlemm's canal: a new insight into the regulation of aqueous outflow in the eye. Trans Am Ophthalmol Soc 2005;103:148-162.
- Alvarado JA, Alvarado RG, Yeh RF, Franse-Carman L, et al. A new insight into the cellular regulation of aqueous outflow: how trabecular meshwork endothelial cells drive a mechanism that regulates the permeability of Schlemm's canal endothelial cells. Br J Ophthalmol 2005;89(11):1500-1505.
- Alvarado JA, Katz LJ, Trivedi S, Shifera AS. Monocyte modulation of aqueous outflow and recruitment to the trabecular meshwork following selective laser trabeculoplasty. Arch Ophthalmol 2010;128:731-737.
- Bahler CK, Howell KG, Hann CR, et al. Prostaglandins increase trabecular meshwork outflow facility in cultured human anterior segments. Am J Ophthalmol 2008;145(1):114-119.
- Katz LJ, Steinmann WC, Marcellino G, SLT/MED Study Group. Comparison of selective laser trabeculoplasty (SLT) vs. medical therapy for initial therapy for glaucoma or ocular hypertension. Paper presented at the International Glaucoma Symposium, Athens, Greece, March 28-31, 2007.
- Song J, Lee PP, Epstein DL, et al. High failure rate associated with 180 degrees selective laser trabeculoplasty. J Glaucoma 2005;14:400-408.
- Johnson PB, Katz LJ, Rhee DJ. Selective laser trabeculoplasty: predictive value of early intraocular pressure measurements for success at 3 months. Br J Ophthalmol 2006;90:741-743.
- Hodge WG, Damji KF, Rock W, et al. Baseline IOP predicts selective laser trabeculoplasty success at 1 year post-treatment: results from a randomised clinical trial. Br J Ophthalmol 2005;89:1157-1160.
- Singh D, Coote MA, O'Hare F, et al. Topical prostaglandin analogues do not affect selective laser trabeculoplasty outcomes. Eye 2009;23:2194-2199.