OKLAHOMA CITY -- Two death row inmates in Oklahoma are asking a federal judge to stop their pending executions because the state plans to substitute one of the drugs used during lethal injections.

Earlier this year, the Oklahoma Department of Corrections ran out of sodium thiopental, a key component in the three-drug cocktail that causes unconsciousness. Instead, the state planned to substitute a similar drug called pentobarbital. That drug is used in animal euthanasia.

Death row inmates Jeffrey David Matthews and John David Duty both are challenging the use of the new drug, alleging it would violate their constitutional rights to be free from "cruel and unusual punishment."

A federal judge is conducting the preliminary injunction hearing on Friday in Oklahoma City.