(Bloomberg) -- Verve Therapeutics Inc. shares tumbled after it cited safety concerns for pausing enrollment in a study of its gene-editing treatment for people with high cholesterol, delivering a setback to the promising new field of medicine.

The Boston, Massachusetts-based company said the experimental approach lowered cholesterol levels in the first five participants who received a certain dose in the early-stage study, but the sixth developed abnormal liver enzymes and thrombocytopenia, a condition involving low blood platelet counts. The shares fell as much as 34% when US markets opened Tuesday. 

Verve attributed the lab abnormalities, which resolved in a few days, to lipid nanoparticles, or tiny balls of fat, used to deliver the treatment, and said it would prioritize development of a second treatment using a different method.

That study has been cleared for clinical trials by regulators in the UK and Canada and is expected to start in the second quarter of this year, Verve said in a statement Tuesday. 

The news is a blow to Verve, which aims to create a one-time treatment to prevent heart attacks by altering a cholesterol-raising gene. It’s one of a handful of closely-watched companies in the burgeoning field of gene editing, which makes permanent changes in human DNA to treat — and potentially cure — diseases.

The news will disappoint investors as the paused study was more advanced than Verve’s others, and “timelines are important for these cash constrained companies,” said Bloomberg Intelligence analyst Ann-Hunter van Kirk. The news also highlights the importance of how gene-editing tools are delivered to patients’ tissues, she said. 

(Updates with shares, analyst comment starting in second paragraph.)

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