Most clinical trials do not reflect the racial and ethnic diversity of America

Most clinical trials do not reflect the racial and ethnic diversity of America

A new study finds that only 6% of clinical trials for new drug approvals in the U.S. reflect the racial and ethnic makeup of the country, with a growing trend of black and Hispanic people being underrepresented in the trials.

The findings come amid pushes for personalized medicine, which develops treatments specifically tailored to a person's genetic makeup.

Researchers at UC Riverside and UC Irvine examined data from 341 pivotal trials between 2017 and 2023 - the large, late-stage trials used to gain FDA approval for new drugs. They observed a decline in Black and Hispanic enrollment starting in 2021, even as calls for greater equity in science and medicine grew louder. Asian representation increased during this period, while white participation remained largely stable.

Precision medicine relies on understanding how genetic differences influence treatment outcomes. If large portions of human genetic variation are not adequately captured in clinical trials, important signals of safety and effectiveness may be missed.”

Sophie Zaaijer, a geneticist at both UCR and UC Irvine and co-lead author of the study

Zaaijer and co-author Simon “Niels” Groen, a UCR geneticist, argue that while ancestry alone should not guide clinical treatment decisions, it plays a critical role in the early stages of drug development. People from different backgrounds often carry different versions of genes, called alleles, that influence the body's response to medications.

“If a study includes only a small portion of humanity, we cannot be sure that a drug will work – or be safe – for everyone it is intended to help,” Groen said.

Clinical trials for drug approval in the United States are conducted both in the United States and in other countries that follow International Council for Harmonization (ICH) standards. While this ensures consistency between studies and speeds approval, it also concentrates the evidence in a few regions such as the US, Europe, China and Japan.

Sub-Saharan Africa and much of Latin America, where fewer than 3% of crucial trials take place, are often left out of the data that influence the drugs used by millions of Americans.

This could change for Hispanics. Brazil joined the ICH in 2016, followed by Mexico in 2021 and Argentina in 2024. Expanding study networks to these and other underrepresented regions could help future studies better capture the genetic variation of patients worldwide.

Zaaijer began this line of research as a postdoctoral fellow at Cornell Tech, studying how little human genetic diversity is taken into account in preclinical drug development, when patient-derived cells are used to model diseases and test potential therapies.

“I kept asking myself,” Zaaijer said: “If our preclinical models are so distorted, what happens when these drugs get into clinical trials?” Biases in preclinical models are an early warning sign, but biases in clinical trials are becoming part of medical practice, she noted.

Her collaboration with Groen's lab developed naturally. His lab at UCR studies how tiny worms metabolize plant toxins, and the parallels to human biology are striking. “Many of the same genes that are used to break down chemicals in worms are also involved in drug metabolism in humans,” Groen said.

“The genes that worms use to detoxify chemicals are ancient,” Groen continued. "We carry a lot of them. But small natural variations in the shape of these genes can have a big impact."

Published inCommunication medicineThe study offers several recommendations: set diversity goals early in the drug development pipeline in the preclinical phase, select testing sites that reflect the health needs and genetic backgrounds of local populations, and collect biological samples such as blood or saliva that can help researchers understand how people's bodies respond to a drug.

Even as DNA testing becomes more common in doctor's offices, researchers emphasize that realizing the full potential of personalized medicine depends on more meaningful, ancestry-related data from the start.

“Precision medicine will only be possible if clinical studies reflect the biology of all patients and not just a subgroup,” said Groen. “Our analysis could provide a roadmap for getting there.”

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