ABBOTT's PROTIRELIN (THYMONE) NEEDS ADDITIONAL CLINICAL TRIAL TO SUPPORT USE

Executive Summary

ABBOTT's PROTIRELIN (THYMONE) NEEDS ADDITIONAL CLINICAL TRIAL TO SUPPORT USE in symptomatic treatment of muscle weakness secondary to amyotrophic lateral sclerosis (ALS), FDA's Peripheral and Central Nervous System Drugs Advisory Cmte. concluded at its Feb. 14 meeting. The cmte. agreed that existing studies, while supportive of the indication, were not properly designed to act as the only basis of approval. "Sample size, statistical level of significance, and blinding," are issues to consider for design of a new study, consultant and former cmte. member Sid Gilman, MD, University of Michigan, noted. Paul Leber, MD, director of FDA's Div. of Neuropharmacological Drug Products, explained that the purpose of the meeting was to suggest to Abbott "what must be done" before the agency could approve an NDA for the ALS indication of thymone, a thyrotropin-releasing hormone (TRH). Leber said the company had submitted "only one positive study," which was insufficient evidence for approval. TRH currently is marketed as an adjunctive agent in the diagnostic assessment of thyroid functions, though the dosage for this indication is much lower than that used in treating muscle weakness for ALS. Russell Katz, MD, deputy director of FDA's Div. of Neuropharmacologic Drug Products, said the agency had "no problem with the firm's conclusion" that the drug is safe. However, the orphan status of the product for ALS, Katz said, does not justify diminished efficacy standards for drug approval. In a five-to-one vote, the cmte. endorsed the "only positive" investigation accepted by the FDA as "adequately demonstrating efficacy." The randomized placebocontrolled study compared the effects of TRH with saline solution on measures of strength and neurological function in 12 patients. Benjamin Brooks, MD, University of Wisconsin, reported that the study found that intravenous TRH transiently improves muscle strength in patients with mild to moderate weakness, though the drug is not useful in patients with advanced illness. Concerning the issue of study "blindness," however, the cmte. found Brooks' research "flawed" because all patients in the study had received TRH one time prior to the start of the investigation. Thus, the placebo and drug-treated patients both could determine if they received TRH by noticing whether or not they experienced the drug's side-effects. Stanley Fahn, MD, Columbia University, the only cmte. member who rejected Brooks' study, suggested a double-blind design in which a low dose of TRH, which would produce side effects but not therapeutic benefit, be compared to a high drug dose. Cmte. members also recommended a study that would test the effectiveness of subcutaneous, rather than IV injection of TRH.