PhITT is a Phase IIb, multicenter, randomized, double-blind, placebo-controlled clinical trial evaluating the efficacy and safety of phentermine (16 mg daily) in adolescents aged 12 to <18 years with obesity. Conducted across approximately 10 sites within the IDeA States Pediatric Clinical Trials Network (ISPCTN), the study aims to enroll up to 240 participants and then randomize up to 198 who meet eligibility criteria, randomized in a 2:1 ratio to phentermine or placebo over a 52-week treatment period, followed by a 2-week withdrawal assessment.

Inclusion criteria* include, but are not limited to:

• Age ≥ 12 years and < 18 years at time of consent;
• Tanner Staging ≥ 2 at the time of screening;
• Obesity (BMI ≥ 95th age- and sex-specific CDC percentile or BMI ≥ 30 kg/m2);
• Biological females must agree to use adequate contraception, defined as double barrier methods, stable hormonal contraception plus single barrier method, tubal ligation, or abstinence.

Exclusion criteria* include, but are not limited to:

• Contraindications to phentermine in adults such as:

  • A history of cardiovascular disease (e.g., coronary artery disease, stroke, clinically significant congenital heart disease, clinically significant cardiac arrhythmias, and congestive heart failure);
  • History of glaucoma;
  • Current or recent (within 14 days of screening) use of a monoamine oxidase (MAO) inhibitor;
  • Any previous history of drug dependency or current use of an illicit substance (positive urine drug screen);
  • Current pregnancy or breastfeeding;
  • Plans to become pregnant within the study duration;
  • Known hypersensitivity to sympathomimetic amines;

• Current nicotine use or nicotine cessation within 3 months of screening;

• Stage 2 hypertension (or greater) or taking any medication to treat hypertension;

• Current type 1 or type 2 diabetes mellitus or taking any medication to treat diabetes or prediabetes;

• Current or recent (within 3 months of screening) use of prescribed weight loss medication(s)/medically prescribed diets (e.g., low calorie, meal replacement/herbal agents/dietary supplements or weight loss program);

• Current or recent (within 3 months of screening) use of other sympathomimetic amines such as stimulants to treat attention- deficit/hyperactive disorder;

• Use of chronic systemic glucocorticoid therapy (consecutive use of 3 months or more) or other steroid hormone therapy other than oral contraceptives;

• Use of tricyclic antidepressants, lithium, levodopa, or dopamine receptor agonists;

• History of bariatric surgery;

• History or current diagnosis of schizophrenia, psychosis, bipolar disorder, or mental illness requiring hospitalization within 12 months of screening;

• History of suicide attempt within 2 years or self-harm within 3 months of screening;

• Current Patient Health Questionnaire-9 (PHQ-9) score of ≥ 10;

• Current suicidal ideation type 4 or 5 on Columbia Suicide Severity Rating Scale (C-SSRS);

• Current Eating Attitudes Test-26 (EAT-26) score ≥ 20 or any history of anorexia or bulimia.

• Current condition or disease interfering with metabolism, such as untreated hypo- or hyperthyroidism, Cushing's syndrome;

• Current clinically significant hepatic aspartate transaminase (AST) or alanine transaminase (ALT) > 3x upper limit of age- and sex-specific normal range or renal disease (creatinine clearance < 60 mL/minute); hypertriglyceridemia (triglyceride ≥ 400 mg/dL) or syndromic or monogenic obesity;

• Any clinically significant abnormalities on a standard 12-lead electrocardiogram at baseline;

• Heart rate > 100 bpm at screening;

• Current or recent use of any investigational medication or device or participation in an interventional clinical trial within 30 days of screening;

• Any clinically significant medical or psychiatric condition or laboratory abnormality that may increase the risk associated with trial participation, investigational product administration, or interpretation of trial results.

De-identified individual-level data including baseline characteristics, outcome measures and adverse events.

This is a double-blind study. Participants, care providers, investigators, and outcomes assessors are blinded to treatment assignment. Only the site pharmacist is unblinded to facilitate dispensing of phentermine or placebo. Randomization is centralized and stratified by sex using a permuted block design. Blinding is maintained throughout the study unless unblinding is required for safety reasons.