Comment by tripletao

This one says four months and the participant "had his toes and parts of his fingers amputated", indeed not ideal.

https://www.bbc.com/news/magazine-35766627

That was an antibody against human CD28, and I'd heard speculation at the time that the monkey trials failed to predict the human response due to differences between monkey and human CD28. That was apparently not correct, since "the sponsor had not only determined the relevant extracellular sequence (which is 100% identical to the human one) but also the binding affinities of the mAb, which were identical for human and monkey". That author goes on to propose a different explanation, though I don't know if any explanation is generally accepted.

https://pmc.ncbi.nlm.nih.gov/articles/PMC2990150/

It's funny to recall this in the context of Terence Tao's question of whether you'd take an AI-designed drug with no mechanistic insight. The sponsors of this trial had reason to believe they had strong mechanistic insight, but they were wrong. A pharmacologist from the nineteenth century would have probably run the trial more safely (lower starting dose, one participant at a time) without that false confidence.