At the start, it’s always hard to know what methods will work best for something as complex as the development of disease-modifying treatments for fragile X. But, we’ve always tried to let the science lead us down the right path. At this point, the results are unequivocal, and they have shaped how we are looking for the Next Great Thing in fragile X treatments.
As a bit of background, it’s worth noting that there are two basic ways of approaching treatment research for any disease: rational drug discovery vs. high-throughput screening.
Rational drug discovery means exploring the basic mechanism of disease and identifying specific “treatment targets” that might be expected to correct the underlying problem. Usually, the target is an enzyme (a protein which facilitates biochemical reactions in the cell) or a receptor (a protein, usually on the cell surface, which detects small amounts of a chemical messenger, such as a neurotransmitter, and reacts in various ways.) Once a potential therapeutic target is identified, small molecules (i.e. drugs) which affect the target in the desired way can be tested in animal models. It’s usually best to look for targets which need to be inhibited for a therapeutic effect, since it’s usually easier to find enzyme inhibitors or receptor antagonists (there are lots of ways to interrupt biological functions with small molecules, but fewer ways to enhance them.)
High-throughput screening (HTS) means finding a simple assay, usually based on single cells or even ground up cells, but placing thousands of them in small wells in test plates, then randomly adding thousands (sometimes hundreds of thousands!) of different drugs and looking for a response (via automated equipment), without worrying about the specific mechanism of action. The hope is that many pleasant surprises could emerge from this method, that existing drugs could show unexpected therapeutic effects. The system needs to be set up properly, with some thought given to the “readout” being examined (what specific reaction you’re looking for in those miniature test tubes, like activation of one specific gene), and it is understood that many false positives (as well as false negatives) will be generated, so any “hits” from the system need to be validated later in other model systems. This is one of the standard methods that drug companies have used to look for new treatments, and it is being used increasingly in academic labs as well. A related technique, high-content screening (HCS), uses more sophisticated robotics and computers to look at more complex readouts, like dendrite shape (for an example relevant to fragile X.)
Over the years, we’ve funded a lot of both kinds of research. Certainly, basic research into the mechanisms of disease is necessary early on for both approaches, and we started out by focusing on that. At a certain point, however, we were able to sponsor HTS projects, as well as projects looking at preclinical (animal model) efficacy of potential therapeutic compounds identified by the rational approach. Looking back on the results from the past 15 years, it is clear that the rational drug discovery approach has been far more productive, and that HTS has been a big disappointment. It turns out that the fragile X experience has mirrored the general experience with HTS in pharma and academia, where the overall results have simply not justified the investment. In our case, the rational approach has produced many notable successes like lithium (and other GSK3 inhibitors), ampakines, mGluR5 antagonists, minocycline, GABA (A&B) agonists, MEK and ERK inhibitors, BK channel openers, and many others—some you’ve heard a lot about, others which we’ll be talking about much more in the near future. There is now a large pipeline of new disease-modifying treatments in development for fragile X, and all of them have come from the rational drug discovery approach. Many of these therapeutic strategies have important cross-application in the treatment of other disorder (especially autism, but the list of potential applications in other brain disorders is quite long, indeed.)
Why no success with HTS? Well, it is becoming apparent (at least in retrospect) that the biggest problem results from the nature of fragile X itself. For example, when scientists have tried to find a simple cell-based readout, they find that fragile X cells (stem cells, in this case) display increase proliferation. Run that through a high-throughput screen, and you get lots of “hits” which rescue this particular phenotype. The problem is that they’re all cancer drugs which are far too toxic to use in fragile X. As it turns out, the same signaling pathways which are over-active in fragile X neurons are really just re-configured versions of the signaling pathways which regulate cell growth and division in other cells. Part of the differentiation process, which makes one cell a liver cell and another, a neuron, is just a minor tweaking of these same pathways to accomplish radically different things in various parts of the body. All cells have similar building blocks, and drugs can’t always distinguish between them. So, it is very important to choose the readout of the HTS system very carefully, and with some consideration of this basic nature of fragile X pathology. We are hopeful that high-content screening may allow us to find novel agents which can rescue very specific defects found in fragile X (like abnormally developed dendrites,) but this is still an emerging technology.
Again, we must go where the science leads us, and learn from these valuable experiences. While fragile X is turning out to be a different sort of problem from what we might have originally expected, we are learning ways to fix the abnormalities caused by this disorder, and better ways to look for new treatments. In the end, fragile X research and rational drug discovery will ultimately teach us a great deal about how the brain works, and about what’s wrong in a host of other disorders like autism, Alzheimer’s Disease, and schizophrenia. Even though we’ve always known this fragile X research was important work, because we care so deeply about our children’s futures, it may turn out to be even more important than we could have imagined.