Imagine a house where cells live in different rooms. With age, a fire starts in some rooms: the cells grow old. Rapamycin works like a firefighter who enters the house and puts out the fire, but only where it has already flared up.
Female flies had more of these 'burning' cells, so rapamycin helped them more: their muscles worked better, and the flies climbed faster. In males, the protection appeared later and was weaker. And since the drug barely reached the brain, nerve cells hardly changed.
Previously, it was unclear why rapamycin extends life, but not equally for everyone. Now it's clear: it puts out the general fire of aging, which is stronger in females. This knowledge will help create drugs that target aging cells precisely and adjust the dose based on sex.
Rapamycin was discovered in a soil sample from Easter Island and named after the island's local name, Rapa Nui. Its molecule weighs 914.2 daltons, which is quite a lot for a drug, and this may be what keeps it from getting into the brain.
The idea of a shared aging state that can be suppressed by a drug echoes Aldous Huxley's novel Brave New World, where people take soma to maintain youth and stability, though in reality rapamycin does not act that totally.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.