BOSTON (AP) — A former opioid sales executive admitted Wednesday to participating in a nationwide conspiracy to bribe doctors to prescribe a highly addictive fentanyl spray for people who didn't need it and will cooperate with prosecutors targeting his co-workers in the closely watched case against Insys Therapeutics.

Alec Burlakoff, 44, pleaded guilty in Boston's federal court to racketeering conspiracy and is likely to be a witness for the government when billionaire founder John Kapoor and other high-level executives at the Arizona-based company go on trial in January.

They're accused of fueling the opioid epidemic by paying kickbacks to doctors willing to write large numbers of prescriptions for the powerful medication Subsys, which is meant for cancer patients with severe pain.

The kickbacks were disguised as speaking fees for events billed as opportunities for other doctors to learn about the drug, prosecutors say. Some doctors were paid more than $200,000, Assistant U.S. Attorney Fred Wyshak said.

Burlakoff and his lawyer declined to comment after the hearing. A lawyer for Kapoor has said he looks forward to fighting the charges and believes he will be vindicated.

Prosecutors say Insys executives targeted doctors who were already prescribing large amounts of opioids and then measured the return investment of their bribes by tracking the number of Subsys prescriptions the doctors were writing. Doctors writing lots of prescriptions were rewarded with more speaking opportunities and doctors who were not were punished by being taken off the list of speakers, Wyshak said.

The scam stretched across the U.S., involving pharmacies and pain management clinics and practitioners from Saginaw, Michigan, to southwest Florida and Laredo, Texas, prosecutors say.

In one case, Burlakoff wasn't happy one doctor in Michigan was averaging only four Subys prescriptions per week, court documents say. So Burlakoff flew to Michigan, took the doctor to dinner and told his co-workers to "expect a nice 'bump' fellas," prosecutors say. In the two months after the dinner, the doctor — who received bribes and kickbacks — wrote about 120 prescriptions for the spray, according to court documents.

Burlakoff also dressed up as a life-sized bottle of Subsys for a promotional video aimed at encouraging sales reps to push doctors to prescribe higher doses of the spray, court documents say. In the video, Burlakoff, wearing the fentanyl spray costume, danced and rapped with other sales reps about the drug based on a song by rapper A$AP Rocky, prosecutors say.

Burlakoff faces up to 20 years in prison for racketeering conspiracy. He's expected to be sentenced after the trial.