OKLAHOMA CITY - Oklahoma's lethal injection protocol is constitutional and the state can proceed with the scheduled executions of four death row inmates early next year, a federal judge ruled on Monday.

U.S. District Judge Stephen Friot denied a request for a preliminary injunction that was requested by a group of Oklahoma death row inmates. The prisoners argued the use of the sedative midazolam as the first drug in a three-drug combination the state administers risks subjecting them to unconstitutionally cruel and unusual punishment.

After the ruling, Department of Corrections Director Robert Patton said the state planned to move forward with the execution of Charles Frederick Warner on Jan. 15 and three other lethal injections scheduled through March 5.

"We will now proceed with the guidelines set forth in the policy and protocol in preparation for the upcoming executions," Patton said.

The inmates sued after the April 29 execution of Clayton Lockett, who writhed on the gurney, mumbled and lifted his head during his 43-minute execution that the state tried to halt before it was over. Lockett's execution was the first in Oklahoma using midazolam, which also has been used in problematic executions in Ohio and Arizona.

In Arizona, where a combination of midazolam and the painkiller hydromorphone was used during a nearly two-hour execution in July, state officials said Monday they planned to switch to a three-drug combination similar to Oklahoma's.

Prison officials in Oklahoma and Arizona have said they would prefer to use the barbiturates pentobarbital or sodium thiopental, but both drugs have become difficult for states to obtain for executions.

Attorneys for Oklahoma maintained the problems with Lockett's execution were the result of an improperly set single IV line that wasn't properly monitored during his execution, causing the lethal drugs to be administered locally instead of directly into his blood. The protocol the state adopted after Lockett's execution calls for a fivefold increase in the amount of midazolam used, which is the same amount of the drug used in 11 successful Florida executions.