What is the difference between optopharmacology and optogenetics?
Optogenetics is the engineering of proteins sensitive to light. These are naturally photosensitive proteins (e.g., algae or bacteria) that can be modified for overexpression by means of gene therapy to enable us to gain remote control over a number of biological processes. Optopharmacology achieves similar objectives, but by developing synthetic compounds that are photosensitive and act on endogenous proteins, that is, proteins that are not overexpressed but already exist in the organism. We do not need gene therapy for this to work. It is enough for the particular tissue to contain a drug designed so that its principal ingredient is activated by light. We could say that optopharmacology and optogenetics achieve roughly the same effect, but that their methods are completely different. As I see it, their development process is completely different too: a drug that is photosensitive is still a drug that has gone through the same filters of validation, toxicity and robustness that any other drug obtained through a drug development process goes through. But with a therapy using optogenetics, the genetic modification that you make has to be validated first. This can be done, but it is a much more complicated process.
Which photosensitive molecules have been used to study and manipulate biological processes like endocytosis? What immediate applications could the procedure have?
«We have created a photoswitchable peptide that could be used to inhibit the proliferation of affected cells selectively»
Endocytosis refers to cell internalization. It is a biological process by which cells—which we can picture as closed bags in which biochemical reactions take place—allow components to cross the cell membrane and enter. The process is particularly intense in altered cells, like cancer cells. One of the ways to stop cancer cells is, precisely, by controlling endocytosis. Working with Ernest Giralt (UB and IRBB), we have created a photoswitchable peptide that changes shape and affinity when it is illuminated. It is like a set of traffic lights controlling internalization. In fact, we have called it a “traffic light” because it can stop the process when illuminated and reactivate it when you change the light’s colour. We think that these peptides, which we designed as molecular tools for basic research, have real possibilities as an anti-cancer therapy: they could be used to inhibit the proliferation of affected cells selectively. Maybe this won’t be in their present form, but it is worth continuing to study them. We developed them as part of OpticalBullet, a five-year project funded by the European Research Council (ERC) and aimed at controlling cell exocytosis and endocytosis with light.
Following the same line of investigation, the ERC has lent support to the development of Theralight, a project to find therapeutic solutions to regulate molecules using light. What does the project involve?
In the Theralight project, we are looking for therapies based on the photocontrol of cellular activity. This is a competitively funded proof of concept project, which happens when researchers have already done a basic research project and found that some aspect of their research might have social, medical or industrial applications. During the OpticalBullet project, we began to collaborate with Amadeu Llebaria of the Spanish National Research Council (CSIC) and together we found a light-activated molecule that has basically three interesting practical properties.
First, it has high affinity: only a small amount of compound is needed to achieve a very large effect on metabotropic glutamate receptors (mGluR). Second, it is highly selective for receptor subtypes. There are compounds that affect many subtypes of receptors in the same protein family, and modern pharmacology is looking for compounds that affect only one target, a single type of protein. Ours does that. Of the eight subtypes of mGluR receptors, it is specific to mGlu5. So the molecule is very potent and highly selective. Third, it is bioavailable orally.
That means that you do not need to inject the drug where you want the effect. You can ingest it and it will have an effect in the right place. These three properties make it attractive as a drug, so we decided to continue our research, involving experts in advanced pharmacology and doing in vivo experiments. We are conducting the project in collaboration with the CSIC, with Amadeu Llebaria, and with Jesús Giraldo of the Universitat Autonoma de Barcelona, and Francisco Ciruela of the Universitat de Barcelona.
Does that mean that optopharmacology can make therapies more effective and lead to fewer side effects?
To make a drug more effective, normally you have to apply more. When there is more of a drug inside the body, more proteins are affected and, as a result, the drug can affect proteins that we do not want to be affected, producing non-specific effects. That is why we look for drugs that affect only a single subtype of protein. But even with very specific drugs, we can find the same protein in different parts of the body and we want the drug to affect only the part where there is a pathological process. The most perfect drug, the most highly selective one, cannot escape this reality. The only thing we can do is to apply the drug locally, but even then it could spread and affect other areas. One potential solution, therefore, is to look for a drug that is selective for a single protein and can also be activated in a controlled way in space and time.
«The optopharmacology allows us to administer drugs that are inert and only activated at the time and place we want»
This is where optopharmacology comes in: to administer drugs that are inert and only activated at the time and place we want. In other words, remote-control drugs. Ibuprofen offers an example. It is a fantastic analgesic, but high dosages or long-term use can have a very negative impact on the stomach. If we could limit its effect to the pain centres, that would solve the problem because we could control the area and dosage patterns for activating the drug. There would be no need to take it three times a day: we could take it once a day and it would circulate in the bloodstream without any effect. Then, when we applied pulses of light where needed, it would be activated. That is the concept. Logically, we do not know yet when the day will come when we can buy and use light-activated drugs. But we are now exploring this path in studies with lab animals.
In other words, drugs will be very potent and, at the same time, very precise?
Right now, several approaches are being pursued to achieve this combination of effectiveness and localization. One is the controlled release of drugs. Highly specific active ingredients are put into capsules and only released where they are needed. But they could still end up spreading through the body as well. Optopharmacology offers an alternative route: to construct a remote control inside the molecule that can be activated. A part of a molecule would act like an antenna that turns on when you irradiate it with light.
Two problems have been found in the development process for photosensitive drugs: one is to avoid altering the drug’s active ingredient when you manipulate the molecule and the other is to make light reach the inside of the body. Do these problems have solutions?
It is true that we cannot make light reach just anywhere. At least, not easily. We would have to introduce optical fibre or LEDs inside the organism. These methods are more invasive. That is why we think that the first areas of application will be exposed tissue, such as the skin, the retina and mucous membranes. But this is simply because all the medical equipment is currently in development, both for taking images and for illumination. Maybe with the same tools that are used to do a colonoscopy, we could selectively illuminate, for example, an intestinal tumour. From a molecular perspective, optopharmacology could be used for anything. In some areas, the medical equipment is already available and applications pose fewer difficulties.
So are you ruling out other applications?
No, we are not ruling them out because we are also enhancing the molecules so that they become sensitive to wavelengths that penetrate more deeply, like red and infrared. Infrared can go ten times deeper than ultraviolet and it is less toxic for cells. Infrared can penetrate to a depth of millimetres and that means that we can equally reach certain nerves or certain depths of the cerebral cortex. But to advance from stimulation with violet light to stimulation with infrared light, we have to change molecules a great deal: we have to introduce groups that would have the same effect when stimulated by light of a different colour. Doing pharmacology with infrared is a line of research that we are now pursuing with Jordi Hernando (Universitat Autonoma de Barcelona,) and Rafael Yuste (Columbia University, New York) and we want to apply it to other molecules.
In the future, could this method enable a chronic or long-term patient to manage his own disease?
«The method would give the patient some autonomy over drug dosage and allow us to personalize the therapy»
In principle, we could, provided that the treatment is supervised. The method would give the patient some autonomy over drug dosage, but it would also, and no less importantly, allow us to personalize the therapy. There are patients who need a different amount of medication depending on the degrees or subdivisions of a pathology. If no two organisms are alike, the guidelines need to be adapted to each individual. With the regulation of light, you could immediately test dosage effects to find the point at which the treatment for an individual is suited to his threshold. For example, people’s pain thresholds vary hugely. The drug we have developed in the Theralight project has an analgesic effect. Perhaps one day we could consider inserting a small optical fibre or light source in a patient’s spinal cord and on days when the pain is great, we could—under medical supervision—increase the effect of the drug by adjusting pulses of light. Having a “photocontrolled module” inside a molecule that permits remote regulation opens a major gateway to personalized treatment.
Right now, are there any drugs on the market that use this technique?
Not using the light-activation of a specific protein, no. But there are light-based therapies: they are called photodynamic and they involve inert drugs that become locally toxic when illuminated. They can be used, for example, to treat small tumours or vascularizations of the retina. But this is not the same process that we are using. Instead, toxic molecules are released by light in order to destroy cells. It could be seen as a preliminary step: it does not involve pharmacological selectivity, but rather targeted destruction. It is equivalent to radiation therapy. It resembles what we are doing in that we use similar medical equipment, but the molecular principle is different.
We have ventured into science fiction by talking about the therapeutic possibilities of light in the future. What steps need to be taken before a new generation of photoactive drugs reaches the market?
«I would say that the first photoregulated drug applications will be on the retina, skin and mucous membranes»
A “normal” lab-tested drug takes approximately ten years to come to market. This is a costly process driven by the private sector. We are at an earlier stage, looking at new principles like photoregulation and exploring the potential advantages over existing technologies. If a photoregulated drug is able to solve a significant therapeutic problem, it will logically have the characteristics that we have uncovered: highly specific, very potent, high affinity and bioavailable orally. I would say that the first applications will be on the retina, skin and mucous membranes, because that is logical: it is where the intervention will be least invasive and of greatest interest, both for patients and for pharmaceutical companies. We continue doing experiments with light on the retina and skin. We are also testing the effects on mice with Parkinson’s and we need to evaluate them. Independently of this project, our basic research continues to produce value-added drugs. That is, light-dependent molecules that have an effect on endogenous proteins.