Identity a macrocyclic lactone (macrolide) small molecule that inhibits mTOR — the cell's master growth-and-nutrient sensor. It is an approved prescription drug (an immunosuppressant), and simultaneously the single most-studied and best-evidenced compound in longevity science. It is emphatically not a peptide — it's included as the register's central longevity anchor. ## Mechanism (as proposed) mTOR (mechanistic target of rapamycin) is a kinase that acts as the cell's nutrient-and-growth sensor, integrating signals to decide whether the cell should grow and divide (protein synthesis, proliferation — high mTOR) or conserve and repair (low mTOR). Rapamycin binds FKBP12, and that complex inhibits mTORC1 (the growth-driving arm). The downstream effects map onto major aging hallmarks: autophagy increases (cells clear damaged proteins/organelles), inflammation ("inflammaging") decreases, mitochondrial function improves, and senescent-cell burden may fall. The longevity strategy hinges on selectivity and schedule — intermittent low dosing to hit mTORC1 (growth/aging) while largely sparing mTORC2 (whose inhibition drives the glucose/immune side effects) — the pharmacological tightrope that separates "geroprotector" from "immunosuppressant."
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See also: Does rapamycin work for longevity? The A and the F.