One was stolen by Mawdryn and his accomplices in the hope of becoming immortal, but instead they were forced to suffer unending and forceful regenerations. Their undead-like condition was so severe that Mawdryn and his colleagues longed for death and attempted to persuade the Fifth Doctor to sacrifice his remaining regenerations so they could die.
At first the Doctor refused and nearly left in his TARDIS, but the condition Mawdryn and his followers had been afflicted with had become contagious and Nyssa and Tegan both began to age. Realising he had no choice if he was to save his companions lives, the Doctor agreed and became prepared to sacrifice his regenerations.
Another Metamorphic symbiosis regenerator was appropriated by the Time Lord healer Ophiuchus for conducting illegal research into regeneration to prolong the normal twelve regeneration limit of Time Lords. Ophiuchus used the regenerator to tap into his own regeneration energy. This act of research was immediately condemned and Ophiuchus was forced to hide in the Cloisters behind a holographic projection. After being found by the Fifth Doctor, who was persuaded by his companions to help, Ophiuchus regenerated into a female incarnation and escaped Gallifrey with a new regeneration cycle. (COMIC: Ophiuchus)