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SAN DIEGO -- Certain factors predicted which patients gained weight on the atypical antipsychotic brexpiprazole (Rexulti), according to a study presented here.

While weight gain is common while on antipsychotic treatment, previous data reported patients on brexpiprazole was tied to an average 4.6 lb (2.1 kg) weight gain after 52 weeks. In this post-hoc analysis of the open-label extension study, led by Mette Josiassen, PhD, of Danish drugmaker Lundbeck, baseline BMI and geographic location were two of the strongest predictors for identifying which patients gained a clinically relevant amount of weight.

Presented as a poster at Psych Congress 2019, the analysis looked at 26-week data on 815 adult patients on brexpiprazole for schizophrenia. Approved in July 2015 for schizophrenia and adjunct to antidepressants for major depressive disorder, brexpiprazole acts as a partial agonist of the serotonin 5-HT1A receptor and the dopamine D2 and D3 receptors, while acting as an antagonist of serotonin 5-HT2A and certain noradrenaline receptors.

This included 561 individuals who participated in one of two prior phase III parent studies or were continuers in the open-label study, as well as 254 de novo patients. This data only included participants who gained at least 7% of their body weight -- how the researchers defined "clinically relevant" -- after starting treatment.

The majority of these participants were from the U.S., Canada, or Europe, while only about 14% were from other locations such as Asia or Latin America. With a total average BMI of 28 at baseline, about 61% of the entire cohort were considered overweight or to have obesity.

However, having obesity or being overweight at the initiation of brexpiprazole treatment was found to be protective against this clinical relevant weight gain.

Compared with those who were under-weight or normal weight at baseline (a BMI under 25), patients who had obesity while starting treatment (BMI of 30 or greater), had a significantly lower odds of gaining 7% of their body weight (OR 0.38, 95% CI 0.26-0.57).

On the other hand, underweight or normal weight patients had an over 2.6-fold higher chance of gaining 7% or more of their body weight versus patients with obesity (OR 2.62, 95% CI 1.76-3.90).

People who were overweight (BMI from 25 to <30) also had a significantly lower risk of gaining a clinically relevant amount of weight while on brexpiprazole versus underweight or normal weight patients (OR 0.61, 95% CI 0.42-0.90).

Baseline BMI wasn't the only factor for predicting weight gain. Josiassen's group also found that patients living in Europe were less likely to gain weight with brexpiprazole compared with patients living in the U.S. or Canada (OR 0.78, 95% CI 0.55-1.12) after controlling with baseline BMI.

But compared with American and Canadian patients, those living in other areas like Asia and Latin America were even more likely to gain weight on brexpiprazole (OR 1.42, 95% CI 0.90-2.24).

"No interaction was seen between region and BMI category," the research group explained, adding that other factors assessed such as age, sex, and baseline weight weren't reliable predictors of weight gain, either.

Not so surprising, early weight gain within the first 2 weeks of treatment was also a predictor of continued weight gain while on brexpiprazole. Most people who continued to gain weight on the treatment gained from 2.2-3.3 lb (1-1.5 kg) within these first two treatment weeks. However at week 50, weight gain not only tapered off but some patients started to lose this treatment-related weight gain.

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