Not surprisingly, I have several physicist friends with whom I often talk shop. It's enormously interesting to hear about their work ranging from cosmology to solid-state physics. Yet I find that I sometimes I have trouble explaining my own work to them. And this is certainly not because they lack the capacity to understand it. It's because the nature of drug discovery is sometimes rather alien to physicists and especially to theoretical physicists. The physicists have trouble understanding drug discovery not because it's hard but because it seems too messy, unrigorous, haphazard, subject to serendipity.
But drug discovery and design is indeed all this and more, and that's precisely why it works. Success in drug discovery demands a diverse mix of skills that range from highly rigorous analysis to statistical extrapolation, gut feeling and intuition, and of course, a healthy dose of good luck. All of these are an essential part of the cocktail (to borrow a drug metaphor). No wonder that models play an integral role in the discovery of new drugs. In this sense drug discovery is very much like chemistry which Roald Hoffmann has trouble explaining to physicists for similar reasons. For a theoretical physicist, anything that cannot be accurately expressed as a differential equation subject to numerical if not analytical solution is suspect. True success in physics is exemplified by quantum electrodynamics, the most accurate theory that we know which agrees with experiment to 12 decimal places. While not as stunningly accurate as QED, most of theoretical physics in the twentieth century consisted of rigorously solving equations and getting answers that agreed with experiment to an unprecedented degree. The goal of many physicists was, and still is, to find three laws that account for at least 99% of the universe. But the situation in drug discovery is more akin to the situation in finance described by the physicist-turned-financial modeler Emanuel Derman; we drug hunters would consider ourselves lucky to find 99 laws that describe 3% of the drug discovery universe.
Physics strives to find universal laws, drug discovery thrives on exceptions. While there certainly are general principles dictating the binding of a drug to its target protein, every protein-drug system is like a human being, presenting its own quirky personality and peculiar traits that we have to deconvolute by using every tool at our disposal, whether rigorous or not. In fact as anyone in the field would know, drug discovery scientists take great satisfaction in understanding these unique details, knowing what makes that particular molecule and that particular protein tick. Try to convince any scientist working in drug discovery that you have found an equation that would allow you to predict the potency, selectivity and side-effects of a drug starting from its chemical structure and which would be universally applicable to any drug and any protein, and you will be met with ridicule.
Physicists also have to understand that in drug discovery, understanding is much more important than accuracy. There's very little point in calculating or measuring binding affinity to four decimal places, but calculating relative trends in binding affinity could be very useful, even if there are errors in the individual numbers. Far more important than calculation however is in explaining why; why a small change in a drug causes a large change in its activity, why one enantiomer causes side-effects while another does not, why making a molecule mimicking the natural substrate of a protein failed, why adding that fluorine adversely affected solubility. "Why" in turn can lead to "what I should make next", which is really what a drug hunter wants to know. In most of these cases the number of variables is so large that calculation would be hopelessly impossible in any case, but even if it were possible, dissecting every factor quantitatively is not half as important as explanation. And here's the key point; the explanation can come from any quarter and from any method of inquiry, from calculation to intuition.
This brings us to reductionism which we have discussed on this blog before. Part of the reason drug discovery can be challenging to physicists is because they are steeped in a culture of reductionism. Reductionism is the great legacy of twentieth-century physics, but while it worked spectacularly well for particle physics it doesn't quite work for drug design. A physicist may see the human body or even a protein-drug system as a complex machine whose understandings we can completely understand once we break it down into its constituent parts. But the chemical and biological systems that drug discoverers deal with are classic examples of emergent phenomena. A network of proteins displays properties that are not obvious from the behavior of the individual proteins. An aggregate of neurons displays behavior that completely belies the apparent simplicity of neuronal structure and firing. At every level there are fundamental laws governing a particular system which we have to understand. Reductionism certainly doesn't work in drug discovery in practice since the systems are so horrendously complicated, but it may not even work in principle. Physicists need to understand that drug discovery presents reductionism in a straitjacket; it can help you a little bit at every level, but it has very little wiggle room beyond that level.
Physicists may also sometimes find themselves bewildered by the inherently multidisciplinary nature of pharmaceutical research. It is impossible to discover a new drug without the contribution of people from a variety of different fields, and no one scientist can usually claim the credit for a novel therapeutic. This concept is somewhat alien especially to theoretical physicists who are used to sitting in a room with pencil and paper and uncovering the great mysteries of the universe. To be sure, there are areas of physics like experimental particle physics which now require enormous team effort (with the LHC being the ultimate incarnation of such teamwork), but even in those cases the scientists involved have been mostly physicists.
So are physicists doomed to look at drug discoverers with a jaundiced eye? I don't think so. The nature of physics itself has significantly changed in the last thirty years or so. New fields of inquiry have presented physicists with the kind of complex systems opaque to first-principles approaches that chemists and biologists are familiar with. This is apparent in disciplines like biophysics, nonlinear dynamics, atmospheric physics, and the physics of large disordered systems. Many phenomena that physicists study today, from clouds to strange new materials are complex phenomena that don't succumb to reductionist approaches. In fact, as the physicist Philip Anderson reminds us, reductionism does not even help us fully understand well-known properties like superconductivity.
The new fields demand new approaches and their complexity means that physicists have to abandon strictly first-principles approaches and indulge in the kind of modeling familiar to chemists and biologists. Even cosmology is now steeped in model-building due to the sheer complexity of the events it studies. In addition, physicists are now often required to build bridges with other disciplines. Fields like biophysics are often as interdisciplinary as anything found in drug discovery. And just like in drug discovery, physicists now have to accept the fact that a novel solution to their problem may come from a non-physicist.
All this can only be a good augury if it means that more physicists are going to join the working ranks of drug discoverers. And it will all work out splendidly as long as they are willing to occasionally hang their reductionist hats at the door, supply pragmatic solutions and not insist on getting answers right to twelve decimal places.