Clinical Trial Issues

The path forward for the clinical development of antiobesity drugs is guided by the recommendations of the various governmental agencies involved in the approval of new medicinal agents. In the US the Food and Drug Administration (FDA) issued a draft guidance in 1996 to aid in constructing appropriate clinical trials to meet the requirements for the drug approval process.30 The European Agency for the Evaluation of Medicinal Products, through its Committee for Proprietary Medicinal Products (CPMP), issued a similar guidance in 1997. A primary feature underlying the guidance in these two documents is the concern for an adequate risk-to-benefit ratio for drugs used to treat obesity. This is a prime consideration because obesity is not usually considered a life-threatening condition (at least at the time of initiation of therapy), and because pharmacotherapy will likely involve long-term drug administration. Also, of concern to the FDA is data suggesting that in the US the majority of prescriptions written for obesity drugs are for women 18-44 years of age. Table 3 summarizes some of the major recommendations from the FDA and CPMP documents. The FDA document provides some guidance as to the potential size of the efficacy trials while the CPMP does not. In reality, the number of subjects needed for a convincing phase III trial for registration may be significantly greater than that noted in the current guidance document. For the phase III studies (started in 2001) for the CB-1 receptor antagonist rimonabant, over 6600 patients were involved in the obesity trials according to information provided by Sanofi-Aventis, Inc.32

While BMI is the most commonly used measure to assess the degree of overweight, there are concerns about how well it defines adiposity among different body types, between the genders, and among different racial and age groups. Since BMI does not really measure body composition, obesity trials may include other measures. For example, accumulation of central or visceral fat might be a better index of the overall association of obesity with morbidity, and therefore, a measure of waist-to-hip ratio may be appropriate in addition to calculating BMI. Noninvasive methods of measuring body composition in humans are available, e.g., DEXA and hydrostatic weighing. However, these can add substantially to trial costs and may not be readily available at most clinical trial sites.

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