New Drug May Halt Weight Gain From Antipsychotic Medications

FALLS CHURCH, VA – May 12, 2026 – For millions of individuals who rely on antipsychotic medications to manage serious mental illnesses like schizophrenia and bipolar disorder, treatment often involves a cruel trade-off: mental stability in exchange for significant, often rapid, weight gain. A Virginia-based biopharmaceutical company, however, is poised to present data on a novel drug that may finally sever this link, offering hope for better physical health without compromising psychiatric care.

Response Pharmaceuticals has announced it will present highly anticipated results from a Phase 1b clinical trial of its lead candidate, RDX-002, at the upcoming American Psychiatric Association (APA) 2026 Annual Meeting. The drug is being developed as the first therapy specifically designed to prevent antipsychotic-induced weight gain (AIWG), a pervasive and dangerous side effect that has plagued psychiatric medicine for decades.

The Crippling Side Effect of Lifesaving Treatment

Atypical antipsychotics (AAPs) are a cornerstone of modern psychiatry, providing essential treatment for an estimated 4 million patients in the United States alone. Yet, their effectiveness is frequently undermined by a constellation of metabolic side effects. Up to 70% of patients taking AAPs experience clinically significant weight gain, with some gaining 20 to 35 pounds within the first year of therapy.

This isn't merely a cosmetic issue. The weight gain is often rapid and is a primary driver of metabolic syndrome, characterized by increased blood pressure, high blood sugar, and abnormal cholesterol and triglyceride levels. These conditions dramatically elevate the risk for type 2 diabetes, heart disease, and stroke, contributing to a tragic reality where individuals with serious mental illness have a significantly reduced life expectancy compared to the general population.

The physical toll of AIWG also creates a major barrier to care. Faced with an expanding waistline and the associated health problems, patients are up to 13 times more likely to stop taking their medication, risking a relapse of their underlying psychiatric condition. Psychiatrists are left with a difficult choice: manage the mental illness while watching the patient's physical health decline, or switch to a less effective but metabolically safer drug. Currently, no therapy is specifically approved to prevent this debilitating side effect, leaving a vast unmet medical need.

A Novel Approach Targeting the Gut

Response Pharmaceuticals' RDX-002 represents a fundamentally new strategy. It is a first-in-class, gut-restricted small molecule designed to inhibit intestinal microsomal triglyceride transfer protein (iMTP). In simpler terms, it works directly in the intestine to block the protein responsible for packaging and absorbing dietary fat and cholesterol into the body. By reducing the uptake of these fats—and their associated calories—RDX-002 aims to counteract the metabolic disruption caused by antipsychotic drugs at its source.

The data to be presented at the APA meeting, from a trial involving subjects taking olanzapine—an antipsychotic notorious for causing severe weight gain—provides the first clinical evidence for this approach. According to the company's announcement, the study met its primary endpoint, with RDX-002 reducing post-meal triglycerides by a staggering 82% compared to olanzapine alone.

More importantly for patients, the drug demonstrated a direct impact on weight. Over the second week of treatment, a period of typically rapid weight gain, subjects taking RDX-002 with olanzapine gained only 0.4 kilograms (less than one pound). In contrast, those taking olanzapine alone gained 1.7 kilograms (about 3.75 pounds), a statistically significant difference.

“Highly effective antipsychotic treatments are often hampered by rapid weight gain and associated increases in cardiometabolic risk factors that can ultimately result in treatment discontinuation,” stated William Sasiela, PhD, Chief Medical Officer of Response Pharmaceuticals, in the company's press release. “In this study, we looked to shed light on one of the drivers of that weight gain and provide initial evidence for RDX-002 as a potential preventative treatment.”

Navigating the Path to Market

With these encouraging early results, Response Pharmaceuticals is charting a course through a therapeutic landscape with few competitors. While doctors sometimes prescribe the diabetes drug metformin off-label to mitigate AIWG, and newer weight-loss drugs like GLP-1 receptor agonists show promise, their high cost and limited insurance coverage are significant barriers. RDX-002's key advantage lies in its preventative, targeted mechanism.

The company, led by a team of industry veterans, licensed the worldwide rights to RDX-002 from pharmaceutical giant Sanofi, which had acquired the drug's original developer. This pedigree, combined with over $11 million in recent funding, positions the clinical-stage biotech to advance its lead candidate into the next critical phase of testing.

“Psychiatrists are on the front line of managing antipsychotic-induced weight gain, and the data we are presenting at APA represent important findings supporting the potential of RDX-002 to address this clinically meaningful side effect," said Eric Keller, Chief Executive Officer of Response Pharmaceuticals. He confirmed the company's intention to move forward, stating, “We are continuing to develop RDX-002 in antipsychotic-induced weight gain and look forward to advancing RDX-002 into a planned Phase 2 study in patients taking atypical antipsychotics.”

This larger, longer study will be crucial in confirming the durability and safety of RDX-002's effects in a broader patient population. The drug has already been studied in over 450 people for up to 252 days, where it was generally well-tolerated with mostly mild gastrointestinal side effects.

Beyond Antipsychotics: A Broader Metabolic Horizon

The potential for RDX-002 may extend well beyond the psychiatric clinic. Response Pharmaceuticals is also developing the drug to address another major emerging challenge in metabolic health: maintaining weight loss after patients stop taking popular GLP-1 drugs like Ozempic and Wegovy. Many patients experience rapid rebound weight gain upon discontinuing these treatments, and RDX-002's mechanism of blocking fat absorption could offer a sustainable long-term solution.

In a recently completed study, RDX-002 demonstrated a clinically meaningful preservation of weight loss for patients coming off these therapies. This dual-pronged strategy suggests RDX-002 could become a versatile tool in the broader fight against obesity and metabolic disease. For now, however, the focus is on the millions of patients with serious mental illness who have long awaited a solution that allows them to protect their minds without sacrificing their bodies.