APTX 4869 (四八六九 ,Yon-Hachi-Roku-Kyuu), code-named Prototype Detective, is a fictional toxin and derivative of an experimental immortality formula developed by Shiho Miyano, which is used by the Black Organization as an untraceable, lethal poison.
APTX is an abbreviation of "Apoptoxin," and, as the name implies, it involves the activation of uncontrolled apoptosis which causes death by the destruction of cells. In addition, it can also activate telomerase. The original purpose of the compound, as hinted by Haibara herself, is "to raise the dead", indicating an intent to control biological life.
However, in very rare cases, it causes the apoptosis of non-neural cells only and the apoptosis would stabilize at some point, causing the victim's apparent age to be decreased. Shinichi, the series' protagonist, is such a victim; later Shiho Miyano, who chooses to attempt suicide by using her own drug, was added to the two known people to have survive after taking the APTX-4869 throughout the series