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The indirect impact of the known comparator drug in double-blind comparative clinical trials of novel agents is underappreciated, despite its potentially pernicious effects. This hypothesis-generating analysis illustrates potential spillover effects of a comparator (amphotericin) in the evaluation of the first member (caspofungin) of a novel class (echinocandins) of antifungal drugs. Reported rates of drug-related fever in the first 3 studies of caspofungin for the treatment of mucosal candidiasis in patients with advanced human immunodeficiency virus infection were retrospectively analyzed. We compared patients who received 50 mg of caspofungin per day in a double-blind trial that used fluconazole as the comparator with patients who received the corresponding dosage in 2 similar earlier studies that used amphotericin as the comparator. With respect to the incidence of drug-related fever, the difference between the concurrent caspofungin and fluconazole groups was less than the difference between caspofungin groups from studies that used different comparators. In phase II/III blinded, active-control trials, the reporting of adverse experiences attributed to a first-in-class drug might be confounded to a variable degree by expectations regarding a well-known comparator.