A Missouri inmate is asking the U.S. Supreme Court to spare his life because he says the execution drug could trigger severe pain and convulsions due to the remnants of a brain tumor and damage caused by surgery to remove it.

Ernest Lee Johnson is scheduled to die Tuesday for using a claw hammer to kill three Columbia convenience store workers in 1994.

A portion of Johnson's benign tumor was removed in 2008, but some of it remains. Johnson's attorney says the operation also required removal of about 20 percent of Johnson's brain tissue.

The lawyer says Johnson is prone to seizures.

The state attorney general's office says in court filings that Missouri has carried out "rapid and painless" executions since going to the one-drug method using pentobarbital in 2013.

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