A new blood test could speed up development of "smart" cancer drugs by showing doctors how well they work in individual patients.
Chemical markers in blood can be used to judge whether a drug is hitting its intended target, scientists have found.
Levels of the chemicals, known as "metabolites", were measured in 41 trial patients with advanced cancers.
The mix of markers accurately indicated how well cancers were responding to the experimental drug pictilisib.
The drug specifically targets a molecular pathway called P13 kinase that is defective in a range of different cancers.
As cancers with defects in the pathway grow, they can cause levels of bloodstream metabolites to drop.
An increase in metabolites indicates that the drug is working.
Lead scientist Dr Florence Raynaud, from The Institute of Cancer Research in London, said: "We have shown that assessing a patient's metabolites can be a quick and simple way of assessing whether a cancer drug is specifically hitting its intended target in the body.
"Our study is an important step in the development of new precision cancer therapies, and is the first to show that blood metabolites have real potential to monitor the effects of novel agents.
"Our method was developed specifically for pictilisib but could now be adapted to discover metabolite markers for other cancer treatments."
The findings are reported in the journal Molecular Cancer Therapeutics.