ST. LOUIS (AP) — A Missouri inmate just hours away from execution is asking the U.S. Supreme Court to step in, arguing that lethal drug could cause unconstitutional pain and suffering.

The appeal was filed Tuesday on behalf of Walter Timothy Storey. He is scheduled for lethal injection at 12:01 a.m. Wednesday for killing a female neighbor in 1990.

Missouri prison officials refuse to disclose details about how or if its execution drug, pentobarbital, is tested. Storey’s attorneys argue that the secrecy makes it impossible to know if the drug will quickly work or cause an unconstitutionally painful death.

The Missouri Attorney General’s office says 12 executions performed with the same drug have been “rapid and painless.”

If the court doesn’t step in, Storey’s execution will be the state’s first in 2015.