Activate or deactivate drugs using light

Scientists from the Paul Scherrer Institute PSI have used the Swiss free-electron X-ray laser SwissFEL and the Swiss light source SLS to make a film that could give a decisive impetus to the development of a new type of medicine. They advanced the field of photopharmacology, a discipline that develops active substances that can be specifically activated or deactivated using light. The study is published today in the journal Nature Communication.

Photopharmacology is a new field of medicine with a bright future ahead of it. This could help treat diseases such as cancer even more effectively than before. Photopharmacological drugs are equipped with a molecular photoswitch. The substance is activated by a light pulse, but only once it has reached the region of the body where it is supposed to act. And after doing its job, it can be extinguished again by another light pulse.

This could limit potential side effects and reduce the development of resistance to drugs – to antibiotics, for example.

Licht switchable drugs

To make conventional drugs sensitive to light, a switch is built into it. In their study, scientists led by lead authors Maximilian Wranik and Jörg Standfuss used the active molecule combretastatin A-4, which is currently being tested in clinical trials as an anticancer drug. It binds to a protein called tubulin, which forms the microtubules which are the basic structure of cells in the body and which also drive cell division. Combretastatin A-4, or “CA4” for short, destabilizes these microtubules, thereby stopping the uncontrolled division of cancer cells, i.e., it slows the growth of tumors.

In the modified CA4 molecule, a bridge consisting of two nitrogen atoms is added, which makes it particularly photoactive. In an inactive state, the so-called azo bridge stretches the molecular components to which it is attached to form an elongated chain. The pulse of light bends the link, bringing the ends of the chain together – like a muscle contracting to bend a joint. Basically, in its elongated form, the molecule does not fit inside the tubulin binding pockets – depressions on the surface of the protein where the molecule can anchor in order to exert its effect. However, when the molecule is bent, it fits perfectly – like a key in a lock. Molecules like this, which fit into corresponding binding pockets, are also called ligands.

Filming a possible cancer drug

The latest study shows that the processes involved go far beyond the simple principle of lock and key. “Contrary to what the manuals say, the key and the lock behave dynamically; they constantly change shape,” explains Maximilian Wranik. “Whole protein is anything but static.” Often the binding pockets are only half open, the ligand lodges there briefly and is released again before it can do its job. Alternatively, you can have a so-called “induced fit”, where something that isn’t quite the right shape is “made to fit”. The ligand changes the shape of the pocket so that it can properly lodge in place and stay there. The scientists have now filmed the ligand in the binding site, as it changes from the bent configuration to the straight shape after being quenched, showing how the pocket somewhat adapts to this new configuration before the ligand turns. detached. The connecting pocket then collapses and, after some time, reforms. What is clear is that the better the ligand fits, the longer it stays bound to the site.

In any case, a deeper understanding of these processes, made visible for the first time, opens up the possibility of designing new, more suitable active substances, so that the binding time and therefore the efficacy of a drug can be improved.

A new level of structure determination

The processes involved take place at the atomic level and within milliseconds. To observe them, the researchers used the large high-precision research facilities at PSI, the combination of which is unprecedented in the world: the Swiss light source SLS and the Swiss free-electron X-ray laser SwissFEL can not only record individual images on tiny timescales and tiny dimensions, but an entire sequence of images that can then be stitched together to create a movie. “We took nine snapshots between one nanosecond and 100 milliseconds after the active molecule had turned off,” explains project leader Jörg Standfuss. Photobiologically relevant processes take place during this period.

Among other things, his team used SLS to analyze the structure of the molecules involved, down to the atomic level, and SwissFEL to measure the processes down to 100 femtoseconds, or one tenth of a trillionth of a second. “Without the excellent support and collaboration of the experts from SwissFEL and SLS, the realization of such a unique project would not have been possible,” emphasizes Standfuss.

Filming actives against gout or Covid-19

The possibility of filming photoactive substances at work also opens up the possibility of collecting many other important information in the field of medicine.

“Of course, we would also like to track the exact sequence of events when the active substance is activated,” says Standfuss. “It’s a bit more complicated, though – so we won’t be tackling that until the next stage.”

Other than that, the study only looks at one of the many known binding pockets in tubulin. Even this doesn’t just serve as a docking site for cancer drugs. Colchicine, which is used to treat gout and other inflammatory rheumatic diseases, and the new Covid 19 drug sabizabulin, which is still under development, also bind to the same pocket. The new method could therefore be used to examine other drugs or other binding sites. The hope is that this method will help clinical research come up with more effective therapies for a wide variety of diseases, Standfuss says. “With the help of our large research facilities, we want to open up time as a new dimension when determining the structure of active substances, allowing us to understand and optimize them even further.”