The Limits of Evidence in Drug Approval and Availability: A Case Study of Cilostazol and Naftidrofuryl for the Treatment of Intermittent Claudication

Abstract Purpose Despite numerous efforts to develop effective medications for the treatment of intermittent claudication (IC) over the past 4 decades, a gold standard medical management option has yet to be defined. Although not life-threatening, IC interferes with mobility and activities of daily living, significantly impairing quality of life and potentially causing depression. Cilostazol, the leading pharmacologic agent for IC in the United States, was approved by the US Food and Drug Administration (FDA) in 1999 based on controversial data. Meanwhile, naftidrofuryl, the first-line pharmacologic agent for IC in the United Kingdom and Europe, has never been approved by the FDA and therefore is not available in the United States. The clinical data for cilostazol and naftidrofuryl are plagued by flaws related to lack of protocol standardization, objective endpoints, and strict eligibility criteria in study subjects, making identification of a true treatment effect impossible. Furthermore, no prospective randomized trial comparing the efficacy of cilostazol and naftidrofuryl has been conducted, because the manufacturers of these agents have much to lose and little to gain from such a study. Objective This article provides an overview of the pharmacology of cilostazol and naftidrofuryl, and the clinical studies leading to their approval and clinical acceptance. It further explores the possible sources of bias in analyzing these clinical trials, some of which have been brought to light by the National Institute for Health and Clinical Excellence (NICE) of the United Kingdom in its technology appraisal guidance. It also speculates the ways in which economic incentives may affect drug-marketing decisions. Methods A literature review of pharmacology and clinical trials for cilostazol and naftidrofuryl was performed in PubMed. The majority of included clinical trials were initially identified through the most recent Cochrane review articles as well as the FDA’s approval packet for cilostazol. The technology appraisal guidance of the National Institute for Health and Care Excellence of the United Kingdom and the manufacturer’s response to this guidance document were located via an online search engine. Findings The clinical data for cilostazol and naftidrofuryl are plagued by flaws related to lack of protocol standardization, objective endpoints, and strict eligibility criteria in study subjects, making identification of a true treatment effect difficult. Furthermore, no prospective randomized trial comparing the efficacy of cilostazol and naftidrofuryl has been conducted. Implications The history of the evaluation, approval, and marketing of these drugs illustrates the limitations of data in the regulatory approval and marketing of agents whose benefit is subjective and difficult to quantify. Implementation of a standardized protocol with strict eligibility criteria, objective quantifiable measurement of drug effect, and validated endpoints will eventually allow development of an ideal pharmacotherapy for IC.