Some TMNT stuff really isn't for little kids.
In the IDW continuity, the psychotropic compound is an experimental chemical refined by StockGen from the ooze that General Krang supplied to Baxter Stockman. The compound, when injected, could enhance a laboratory specimen's neurology to such a degree that they could attain human-like intelligence and social skills. Mutagen, another substance manufactured by StockGen refined from the ooze, could mutate a laboratory animal, increasing their physical size and giving them a more humanoid body plan, and it would also typically give the specimen a degree of sentience and could also potentially grant them intelligence and mental stability, but it was not possible to guarantee this outcome in a mutant using mutagen alone.
The original project at StockGen, overseen by Lindsey Baker and Chet Allen, injected the compound into a laboratory rat named Splinter, drastically increasing his intellectual faculties, and causing his physiology to manufacture more of the compound through his blood. The project had also planned a controlled experiment whereby a snapping turtle, Specimen 6 (later named Slash), would be physically mutated with the mutagen, and he would have been injected with Splinter's blood to confirm whether it would have the same neurological enhancement effects as the original compound that was injected into Splinter.
The project was interrupted in the Change is Constant arc when the Foot Clan, secretly being informed by Chet Allen, broke into StockGen's facilities in New York City to steal ooze-based technology and test specimens. Unbeknownst to anyone involved at the time, Splinter was actually the reincarnated form of Hamato Yoshi, a human man and former Foot Ninja himself who had lived centuries ago in feudal Japan. Additionally, four laboratory turtles, which StockGen had used to develop a separate technology, terrapin/human exo-armor synthesis, were reincarnations of Hamato Yoshi's own four sons. Despite his limited physical abilities, Splinter was able to pull the facility's fire alarm and alert security, helping save the life of StockGen intern April O'Neil. The ninja team stole the turtles and some mutagen, but Splinter single-handedly attacked them, causing them to spill their loot in an alley, causing the unplanned physical mutation of Splinter, the four turtles and a random feral cat named Old Hob. Splinter and three of the turtles escaped into the sewer, and the fourth turtle escaped into the city on his own, while the newly sentient Hob was hired by Stockman to locate and retrieve them.
Without being able to continue the project, StockGen did covertly draw samples of Hob's blood, though his mutation outside controlled laboratory conditions and his fully functional yet mediocre human intellect made much less valuable a research specimen than Splinter had been. In the Shadows of the Past arc, Hob was ultimately successful in recapturing Splinter, but Splinter swiftly was abducted during a second Foot Clan break-in. Having completely failed to recover and secure the specimen, Stockman shot and fatally-wounded Hob soon before Krang abducted Stockman to Burnow Island. But Hob did not die, as the Old Hob one-shot issue revealed that he had been pilfering vials from StockGen, including a vial of the healing compound along with the samples of Splinter's pre-mutation blood containing the psychotropic compound. When Hob was shot, the bullet shattered the healing compound vial Hob was secretly holding near what would be the location of the wound, and Hob later woke up without injury.
As Splinter and his blood samples proved to be irreplaceable components in the experiment, StockGen was unable to complete it, and the mutant Slash physically developed as intended but without the mental stability and intelligence that the compound was supposed to have provided him. Slash proved violently unstable and developmentally delayed, and managed to escape StockGen's facilities during the April one-shot issue. All of StockGen's experiments were later abruptly cancelled when Hob bombed StockGen's laboratories during the City Fall arc, completely destroying the facility.
With both Baxter Stockman and Chet Allen having disappeared, Lindsey Baker was the only remaining scientist who had once been researching the psychotropic compound, and StockGen's destruction and no reference from Stockman left her chronically unemployed, unable to secure a new job in her field of expertise. In the Monsters, Misfits, and Madmen arc, Hob and Slash kidnapped Lindsey, seeking her help with Hob's desired creation of a new mutant army. Hob had already used a supply of mutagen to randomly mutate a common pigeon named Pete. Like Slash, Pete's mutation did not bestow him an ideal level of intelligence, though Pete's instabilities and development delays differed from Slash's and he was not as violently unstable. Lindsey was immediately interested upon learning that Hob had kept a sample of Splinter's blood, and was finally able to complete StockGen's original psychotropic compound project when Slash injected some of the blood into himself, and the compound in the blood immediately granted him both high intelligence and finally a measure of mental stability. With this, Lindsey was finally able to use the completed experiment's cumulative data to further her research, and she decided to remain with Hob.
In the New Mutant Order arc, Hob and Lindsey unveiled two new mutants also psychotropically-enhanced by Splinter's remaining blood samples. Herman the Hermit Crab and Mondo Gecko both possessed the full normal range of human mental faculties, presenting as sociable, largely well-adjusted individuals.
The mutants Splinter, Slash, Herman the Hermit Crab and Mondo Gecko were specifically treated with the psychotropic compound in addition to their mutation. Splinter was treated first with the compound, and only mutated some time later by accidental encounter with mutagen.
Other unauthorized mutations created from StockGen's mutagen supplies, including the four turtles Leonardo, Raphael, Donatello and Michelangelo as well as Old Hob and Pigeon Pete, were not treated with the compound, and their intelligence and stability were the result of chance. Slash was initially not treated with the compound because of the disappearance of both Splinter and the samples of his blood, but Slash was later stabilized after being injected with Splinter's blood.
StockGen was not the only entity trying to produce mutants, as both the Foot Clan and Null Group collaborated their own ooze mutation program—the Foot providing the stolen substances, and Null providing the scientists and laboratories. While their scientists did not participate in StockGen's original psychotropic compound experiments, they produced their own successfully psychotropically-enhanced mutant animals, such as the Foot's Alopex, Koya and Bludgeon and Null's Mutagen Man, Sally Pride, Man Ray, Zodi and Krisa. The story does not go further into how the Foot/Null collaboration managed to ensure these mutants' psychotropic enhancements.