Use of Medication to Improve Weight Loss in Suboptimal Early Responders to Behavioral Treatment

Subjects will be a total of 150 adults, aged 21-70 years, with a body mass index (BMI) of 31 kg/m2 or above (28 kg/m2 with an obesity-related comorbidity). In phase 1, eligible subjects will complete questionnaires and an in-person baseline assessment of obesity-related behavioral characteristics (satiety, hunger, the relative reinforcing value of food \[RRVfood\], and impulsivity \[delay discounting\]), neuropeptides, and gastric emptying. After this baseline assessment, participants will begin an initial 4-week behavioral treatment (BT) "run-in" delivered individually in 20-30 minute weekly sessions (delivered virtually). The primary goal of phase 1 will be to evaluate baseline satiety, postprandial change in GLP-1, and gastric emptying as predictors of percent weight loss after 4 weeks of BT. We will also examine whether these variables predict categorization as a suboptimal early responder to BT (e.g., \<2.0% loss; co-primary outcome). Secondary endpoints of phase 1 are percent weight loss from the start of the BT run-in (week -4) to randomization (week 0) and categorization as a suboptimal early responder, as predicted by additional behavioral characteristics (hunger as measured by VAS ratings, RRVfood as measured using a computer task, and impulsivity as measured using a delay discounting computer task) and neuropeptides (higher fasting ghrelin, lower fasting leptin, and lower postprandial changes in insulin and PYY). In phase 2, suboptimal early responders (based on weight loss during the BT run-in) will be randomly assigned to 24 weeks of: 1) BT plus placebo (BT+P); or 2) BT plus medication (BT+M; phentermine 15.0 mg). Both treatment groups will continue to attend 20-30 minute individual BT sessions (delivered virtually), weekly for the first 12 weeks and every other week for the last 12 weeks (total of 18 visits). Both treatment groups will also take once daily study medication (placebo or phentermine 15.0 mg) for the duration of the intervention period. Early BT responders identified during the run-in will receive the same 24-week BT program, but will not receive study medication or be included in the randomized trial. The assessments administered at baseline - questionnaires, including behavioral testing, blood draws, and measurements of body weight - will be repeated at randomization (week 0) and at week 24. The primary endpoint of phase 2 is change in body weight (i.e., % reduction in initial weight), as measured from randomization to week 24, among suboptimal early responders assigned to BT+P vs. BT+M. A randomized sample size of 50 non-responders (25 per group), assuming a 20% attrition rate, will give us 81.5% power to detect between-treatment group differences at week 24 of 4.5% (effect size: d = 0.82). Secondary endpoints of phase 2 will include change in body weight in kg from randomization to week 24, as well as the portion of suboptimal early responders who achieve a post-randomization loss of ≥ 5% and ≥ 10% of initial body weight. We will also examine differences between suboptimal early responders treated with BT+M vs. BT+P in changes in hunger, satiety, the reinforcing efficacy of food, and impulsivity between randomization and week 24. A comparison will also be made in percent weight loss from randomization to week 24 between suboptimal early responders treated with BT+M and early responders treated with BT alone. If you are interested in participating in this study, information and a link to contact the research team can be found here: https://clinicalresearch.itmat.upenn.edu/3XOX/ or you can call us at the numbers listed below.