"This drug is approved as an anticancer agent, but..."
Eiji pointed at the screen.
"Is there a problem?" Sena asked.
"Opposite. It might also work for Alzheimer's disease."
Akira showed interest. "Drug repositioning?"
"Yes. A strategy to find new indications for existing drugs."
Sena asked, "But why would an anticancer drug work for neurological diseases?"
Eiji explained. "Not by chance. Analyzing the target protein network revealed common pathways."
Akira opened his notebook. "Off-target effects?"
"That too. But this time, it's an intentional multi-target strategy."
Eiji drew a diagram. A network connecting drugs to multiple targets.
"This drug's main target is kinase A, but it also weakly binds kinase B."
"Isn't that a side effect?" Sena confirmed.
"It can be. But if kinase B is in the Alzheimer's disease pathway, it becomes therapeutic."
Akira understood. "Potential for killing two birds with one stone."
"More precisely, calculated versatility."
Sena wrote in her notebook. "Advantages of repositioning are..."
"First, safety data already exists. Can skip phase 1 clinical trials."
Akira added, "Development time is also shortened. Ten years can become five."
"Costs decrease too?"
"Significantly. New drug development costs hundreds of billions, repositioning costs tens of billions."
Eiji gave another example. "This was a rheumatoid arthritis drug, but now it's also used for inflammatory bowel disease."
"The mechanism?"
"Inhibition of inflammatory cytokines. Same target, different disease."
Sena asked, "But how do we find candidates?"
Eiji explained methodology. "Several approaches. First, phenotypic screening."
"Phenotypic?"
"Directly observe effects in cells or animal models. Even without knowing the target, if it works, it's good."
Akira supplemented. "A hypothesis-free approach."
"Next, structure-based methods. Dock existing drugs to new targets."
"Test everything?" Sena was surprised.
"With virtual screening. Can calculate thousands of drugs against hundreds of targets, all combinations."
Akira asked, "Computational cost?"
"Recently, machine learning makes it efficient. Pre-filter likely combinations."
Eiji introduced another approach. "And network pharmacology."
"Network?"
"Represent diseases and drugs as molecular networks. Search for common nodes or pathways."
Akira looked at the screen. "This is graph theory?"
"Yes. Calculate shortest paths between disease genes and drug targets, or cluster them."
Sena got excited. "Mathematics connects to drug design!"
"Everything is connected," Eiji said. "Informatics, structural biology, clinical medicine."
Akira asked a practical question. "But what's the success rate?"
"Honestly, still low. If one gets approved from 10 to 100 candidates, that's good."
"But still worth doing?"
"Yes. Especially for rare diseases or emergencies. During COVID-19, repurposing existing drugs was urgent."
Sena remembered. "Remdesivir was originally for Ebola, right?"
"Correct. A repositioning success story."
Eiji showed another candidate. "Don't you think this is surprising?"
"An antidepressant for... diabetes?"
"Serotonin receptors are also in pancreatic β-cells. Modulating them changes insulin secretion."
Akira was impressed. "The complexity of biology."
"Yes. One receptor has different roles in multiple tissues."
Sena said, "So unexpected combinations emerge."
"That's the appeal of repositioning. Known drugs with unknown possibilities."
Eiji opened a database. "This is DrugBank. Information on all approved drugs."
"Structure, targets, side effects, all recorded."
Akira proposed. "Can we infer new indications from side effect profiles?"
"We can. Side effects are unintended pharmacological actions. But they can become main actions for other diseases."
Sena understood. "Transform the 'shadow' side of drugs into light."
"Beautiful expression," Eiji smiled. "Unexpected candidates lurking in shadows. That's the essence of repositioning."
Akira said calmly, "But there are intellectual property issues."
"True. Existing drugs often have expired patents. Weak corporate motivation."
"But still valuable for patients."
"Yes. So the role of academia and public institutions is large."
Sena closed her notebook. "Revisiting existing resources with new perspectives. Repositioning is creative."
"A bridge connecting past and future," Eiji said.
The three continued searching databases of existing drugs for yet-unseen possibilities.