Brenna was the key expert witness in the action of the United States Anti-Doping Agency (USADA) against 2006 Tour de France first place finisher Floyd Landis, adjudicated in a hearing of the Tribunal Arbitral du Sport/Court for Arbitration in Sport held in Malibu, California. He testified to the accuracy of Carbon Isotope Ratio (CIR) tests conducted by the French Antidoping laboratory at Châtenay-Malabry. Landis conducted a high profile, multimillion dollar defense but lost the 2007 original hearing  with the decision relying for technical opinion on Brenna’s testimony. Landis later lost a 2008 de novo case before the American Arbitration Association also relying on Brenna’s testimony and was stripped of his title. Years later Landis revealed he was doping and filed a whistle-blower lawsuit under the federal False Claims Act against Lance Armstrong that was settled with multimillion dollar payments by Armstrong.