"This antidepressant might work against cancer."
Eiji said abruptly.
"What?" Lina was surprised. "An antidepressant for cancer? They're completely unrelated."
"Superficially. But at the molecular level, there are unexpected connections."
Akira showed interest. "What do you mean?"
"This antidepressant inhibits the serotonin transporter. That's its main effect."
"Yeah."
"But it also binds to the sigma receptor secondarily. This suppresses cancer cell proliferation."
Lina's eyes lit up. "Using off-target effects."
"Yes. That's the fascination of drug repositioning."
"Drug repositioning?"
Eiji explained. "A strategy of using existing drugs for diseases different from the original. Development costs are low, and safety data is already available."
"I see," Akira understood. "Faster than new drug development."
"Exactly. Sometimes you can even skip Phase I trials."
Lina searched databases. "Are there other examples?"
"Many. Like thalidomide."
"Thalidomide... that infamous drug?"
"Yes. Banned for teratogenicity. But now it's back as a treatment for multiple myeloma."
"Why?"
Eiji explained. "It had anti-angiogenic effects. Cutting off nutrient supply to cancer cells."
Akira recalled another example. "Viagra is like that too. Originally for angina."
"Yes. In clinical trials, the blood pressure lowering effect was weak, but another effect..."
"Became a huge success as an erectile dysfunction treatment."
Lina laughed. "Serendipitous discovery."
"Serendipity. But now we don't rely on chance." Eiji became serious.
"How?"
"Computational approaches. Docking existing drug structures with new target binding pockets."
"Virtual screening?"
"Yes. Testing thousands of existing drugs against new targets."
Akira supplemented. "We can also use side effect information."
"Side effects?"
"Side effects are evidence of off-target effects. We use them in reverse."
Eiji gave an example. "For instance, if a drug causes hyperglycemia as a side effect, it's possibly binding to targets involved in glucose metabolism."
"If that target is related to another disease..."
"It becomes a repositioning candidate."
Lina was impressed. "Side effects become hints for new drugs."
"Turning adversity into fortune."
Akira questioned. "But not all existing drugs can be repositioned, right?"
"Of course. Success rate is low. But worth trying."
Eiji showed a graph. Statistics on successful repositioning cases.
"Anti-inflammatory drugs, antipsychotics, antibiotics... successful cases from various classes."
"What's the common thread?"
"Multi-target activity. Polypharmacology."
"Polypharmacology?"
"The property of one drug acting on multiple targets. Once considered undesirable, now being re-evaluated."
Lina understood. "Complex diseases require hitting multiple targets simultaneously."
"Yes. Like cancer or neurodegenerative diseases."
Akira presented a new perspective. "Can't we use AI?"
"Already being used," Eiji answered. "Integrating gene expression patterns, drug response data, structural information... to predict repositioning candidates."
"Amazing era."
"But," Eiji warned, "clinical trials are ultimately necessary. Computation only gives hypotheses."
Lina said, "Still much better than searching blindly."
"Yes. Data-driven drug repositioning. That's the future."
Akira jokingly said, "So a headache medicine might work for dementia?"
Eiji smiled. "Don't know. But worth investigating."
Unexpected candidates lurk in the shadow of drug repositioning. Finding them requires both data and intuition.