Deciphering the Complement Cascade
This complex mosaic of interdependent processes, once understood, may yield therapeutic interventions.
Robert Murphy, Contributing Editor
Complement inhibition is being tested in several clinical trials as a therapeutic strategy for age-related macular degeneration. In a lecture given earlier this year at the Retinal Physician Symposium in Las Vegas, Jason Slakter, MD, reviewed the rationale for complement inhibition and updated the progress of recent and current trials. This article summarizes Dr. Slakter's discussion of the topic.
THE COMPLEMENT SYSTEM
Part of the innate immune system, the complement system consists of numerous interacting molecules. Certain chemicals carefully control the inflammatory process that derives from the complement system. There are three main activating pathways: classical, lectin and alternative.
The complement system's many molecular interactions all converge on C3. The activation of C3 with what is called C3 convertase is a key step in converting C3 into C3a and C3b. The real problem lies with C3b, which does two things. One, it binds other molecules to cleave C5 and activate the rest of this pathway. Also, C3b acts as its own stimulator to once again break down C3. So you get what is called the amplification loop, with some C3b looping back to cleave more C3, breaking it down and leading to an accelerating cycle of activation.
Once the complement pathways are triggered, it begins to initiate inflammatory and destructive activities that must then be regulated and controlled. This is where complement factor H intervenes, which along with other molecules serves to modulate or reduce the complement cascade. Here we may run into a problem. We know that critical genetic defects in factor H contribute to the risk of AMD. If the inflammation generated by complement activity causes damage in AMD, and if something triggers complement activation and factor H molecules are defective, you may then get unregulated or under-regulated complement activity. Consequently, you get chronic ongoing damage to the tissue in the back of the eye, which leads to drusen, RPE damage and choroidal neovascularization. There is evidence not just in genetic research but also from drusen composition and even CNV models suggesting that complement activation and its byproducts can have a direct effect on AMD development.
Therapeutically, we can target different points in the complement pathway (Figure 1). Several companies are now exploring ways to do precisely that.
Figure 1. Therapeutically, we can target several different points along the complement pathway.
WHAT WE DON'T KNOW
But first, consider what we don't know. We don't know the source of complement activation. There are many theories, one of which is that it is stimulated by oxidative breakdown products. One source of possible oxidative damage is carboxyethylpyrrole (CEP), which in animal models has been shown to induce complement deposition and generate AMD-like findings. Other theories suggest that complement pathways are stimulated by infectious processes, or a build-up of antibodies or toxins. In fact, all of these processes have been shown to induce complement activation in experimental studies. What we don't know is what role they play in humans.
There are further gaps in our knowledge. For example, why should a certain treatment work in a particular patient? What specific pathway is involved, and will that have a bearing on which drugs we want to use and which targets we wish to pursue? Is this an acute or lifelong problem? What are a particular treatment's risks and benefits?
Considerable evidence suggests that the alternative pathway is the key area of activity in AMD. However, other studies suggest that the classical pathway may be involved as well. Thus, for some individuals it may be insufficient to target just one step in the pathway. It also underscores the importance of genetic analysis before starting a patient on a complement inhibitor.
Other questions are equally pressing. Is AMD a lifelong process? If so, do we need to initiate treatment at an early age before any damage occurs or can we wait until patients are older, already manifesting the disease? The literature does not offer us any guidance yet. The critical question is whether the damage is done by the time we give a complement inhibitor or can we, in fact, intervene in advance of some acute processes?
Another issue has to do with how much complement suppression is needed to achieve clinical results. Do you need to block all the complement activity? Complement is preserved throughout almost all species and thus plays a critical role in overall health. Therefore, how much complement inhibition is too much? And what about the risk of infection or other adverse events?
Now consider the potential benefits. Name any aspect of AMD mentioned so far, and we see a potential role for complement modulation. Essentially, you can look at complement as a potential intervention point for every stage of AMD. In patients with geographic atrophy, you can try to prevent its progression. You can try to attack the conversion process from dry to wet AMD or advanced disease. Treat the condition early enough, and you may reduce drusen formation or prevent them altogether. These are all potential targets for complement inhibition.
COMPLEMENT INHIBITORS CURRENTLY UNDER STUDY
Researchers currently are testing a number of compounds designed to suppress complement as treatments for AMD. All are early-stage studies which so far have yielded preliminary data.
POT-4. Most of the following discussion centers on POT-4, for which a phase 1 trial has been completed. A derivative of compstatin, POT-4 blocks the conversion of C3 to C3a and C3b, and shuts down the amplification loop. Importantly, POT-4 does not destroy C3. It simply inhibits C3 from binding to the convertase and breaking down. That may be important in preventing a rebound phenomenon if the body detects C3 being depleted.
Interestingly, primate studies showed that injection of POT-4 into the vitreous produces a glob of debris, a spherical intravitreal deposit. Laboratory studies of this deposit showed that it was nothing more than the drug and vitreous proteins. These pellets shrunk over time, yet when removed turned out to be biologically viable POT-4 — the drug was still active. When injected into the vitreous, POT-4 essentially becomes its own sustained-release delivery device (Figure 2).
Figure 2. When injected into the vitreous, POT-4 essentially becomes its own sustained-release delivery device.
A phase 1 clinical trial was designed — the first of its kind in which a complement inhibitor was used in human subjects for an ophthalmic indication. The investigators looked for a population they could reasonably study, and therefore focused on advanced end-stage wet AMD patients with disciform scarring. Some had leakage, some merely fibrotic scarring. The study's endpoints were to obtain a safety profile and assess the pharmacokinetics.
The trial enrolled 27 patients, all described as having “advanced scars” in the macula. None were being treated with any drugs, including anti-VEGF therapy. The trial included six sites, with seven doses and a single intraocular injection of study drug.
Safety was superb. Investigators saw no visual problems. (Some patients even enjoyed a short-term visual improvement attributed to a placebo effect.) Some minor adverse effects common to intravitreal injections, such as redness and the occasional subconjunctival hemorrhage, did occur. But there were no cases of endophthalmitis or inflammation.
As with the primates, the humans likewise developed intravitreal deposits at the two higher doses, 450 µg and 1,000 µg, beginning one day after injection. Some of the pellets lasted six months before gradually fading away. Looking at dose-related serum build-up, investigators found that the highest dose with a sustained depot seemed to carry on past 30 days after injection.
The phase 1 study achieved its goal: it indicated safety and it conveyed what seemed to be a durable effect.
Potentia has partnered with Alcon to develop POT-4. Investigators have initiated a phase 2 trial in wet AMD. Researchers will then turn to further trials in dry AMD, looking at geographic atrophy. Ultimately, investigators hope to pursue a risk-reduction strategy.
Eculizumab. This drug is already approved for a different indication, nocturnal hemoglobinuria. Eculizumab is a monoclonal antibody that works against C5, so we've descended the cascade a bit. Given by intravenous infusion, the drug is a proven inhibitor of terminal complement activation at the C5 level.
A phase 2 study at Bascom Palmer is looking at two groups, those with drusen and those with geographic atrophy, both groups randomized two-to-one, drug vs. placebo. The researchers will follow them for a year, measuring drusen volume and geographic atrophy and using both autofluorescence and SD-OCT imaging techniques.
ARC-1905. This is a pegylated aptamer from Ophthotech with a high affinity and binding tendency to C5. ARC-1905 prevents the formation of the active metabolite C5a and the membrane attack complex. It spares C3 and its breakdown products, a quality that is both good and bad. Its effect may be limited since you're not touching the amplification loop.
But evidence suggests that C3 breakdown products may be involved in some form of antiinfective action. Therefore, more selective blockade of the complement cascade may be beneficial. Answers to these issues will come as the trial results are reviewed.
We do know that ARC-1905 has a favorable toxicology profile. In vitro studies were positive. A phase 1 study in wet AMD appeared safe. Investigators are now doing a phase 1 trial in geographic atrophy, enrolling two different dose groups (Figure 3). Following a loading dose, patients will receive further treatment over a year. Endpoints will be changes in geographic atrophy, drusen area and morphology.
Figure 3. Study design of pegylated aptamer therapy for geographic atrophy.
Anti-Factor D. Genentech has developed an anti-factor D monoclonal antibody, given by intravitreal injection and designed to inhibit the alternative pathway as a trigger point for C3 and C5. Investigators have completed a phase 1 single-injection dose escalation trial that produced excellent safety data. The work has moved onto a phase 2 trial for dry AMD.
Anti-Factor H. Significant evidence indicates that factor H deficiency and malfunction can stimulate overactivation of the pathway. With this in mind, investigators are looking at a fusion protein of complement receptor 2 and factor H. The goal is to restore complement factor H activity by regenerating the portion of factor H that the patient genetically is unable to manage. That project is in preclinical testing.
No one can dispute that complement plays a role in AMD's pathogenesis. Many questions remain on exactly how and where it acts, and what triggers it. Complement inhibitors and complement-factor inhibitors will likely have to be given chronically, which calls for creativity in how to deliver the drug. This is especially so as we move into the era of preventive therapy and risk reduction, where lifetime therapy requires that we deliver safe and reasonable drugs in a harmless manner.
Many people are looking at all these issues. In the next few years, expect to see many of these drugs move up the development pipeline, for both wet and dry AMD. And expect to see them play a critical role in your future clinical practice. RP