CytomX has a tox wobble

It's a measure of how nervously investors view CytomX's attempt to develop a masked therapeutic that the unconfirmed rumour of a diarrhoea toxicity, floated on social media, caused its stock to fall 19% on Tuesday. To its credit, CytomX moved quickly to stem the bleeding, issuing a statement pre-market on Wednesday to confirm the death in the phase 1 study of its lead project, the conditionally activated anti-EpCAM ADC varsetatug masetecan. The colorectal cancer patient died of treatment-related kidney injury believed to have been the result of nausea, vomiting and diarrhoea, CytomX saying it became aware of this on 11 July, and reported it to the FDA seven days later. Its stock recovered slightly, probably because the company confirmed that the trial was continuing towards its aim of reporting updated results in the first quarter of 2026. However, toxicity is front and centre again, especially as the first data showed serious diarrhoea and acute kidney injury, albeit none above grade 3. Loperamide prophylaxis was earlier implemented to mitigate diarrhoea, and it's notable that this didn't avert the death. After various ups and downs, varseta-M became central to CytomX's investment case when the group pivoted to it in January.