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12. Through its Cooperative Agreement Program in Pharmacoepidemiology, the FDA funds seven computerized research databases, giving it the capacity to study the occurrence of adverse reactions to prescription drugs within the setting of defined populations. In all databases, patient confidentiality is maintained, and personal identifiers are not released to FDA. Following the July 8 Public Health Advisory, FDA's Division of Pharmacovigilance and Epidemiology initiated an epidemiologic study using data from one of its Cooperative Agreement sites. In the first phase of this study, all patients filling prescriptions for fenfluramine were identified. There were 793 treated with fenfluramine-phentermine in combination, and 142 treated with fenfluramine monotherapy. The computerized records of these patients were searched for billing claims for echocardiography and the results of these tests were retrieved. There were 25 echocardiograms on patients from the fenfluramine cohort performed prior to their first fenfluramine prescription. These were performed for a variety of reasons such as palpitations, chest pain or for suspected congestive heart failure. Nine echocardiograms were performed on different members of this cohort subsequent to starting fenfluramine. Applying the "research" case definition, significant aortic or mitral regurgitation was identified in 0/25 pre-fenfluramine tests and in 4/9 post-fenfluramine tests (p=0.003). In the second phase of this study (which is still in progress), age-, gender-, and weight-matched untreated controls will be randomly selected from the HMO source population and results from the echocardiograms performed will be obtained. The results from the first phase of this study suggest that the background rate of significant valvular regurgitation in obese subjects is low and that the absolute and relative risk of valve dysfunction following exposure to fenfluramine with or without phentermine is elevated. These results are similar to those from the echocardiographic-prevalence surveys.