Turns out they did on both mice and primates, and the side effect is also evaluated:

Another potential concern, however, emerged about 1 month before Mei was treated: the final conclusions of a primate toxicology study conducted at PriMed. The company’s report, dated 17 February 2025, concluded all four monkeys that received the therapy, whether at low or high doses, developed moderate to severe liver damage.

However, the ethics board did not review the safety report:

An ethics panel at Xinhua Hospital in China approved a gene-editing trial, but had not reviewed primate data suggesting safety concerns. Another hospital panel later concluded the experimental treatment was “definitely related” to Mei’s death.

Even worse, the brain development data from the primate study is entirely unconvincing:

But four of the experts Science asked to assess this figure, including one of the original reviewers of the Nature manuscript, expressed concerns about how well the primate proof-of-concept study was conducted. The submitted paper had no control samples to show the stain was selectively highlighting the gene editor. “It’s completely unconvincing,” says David Sanders, a biochemist at Purdue University who has studied gene therapy. “One can’t have the confidence that one isn’t mostly looking at background staining.”