Chemo 'undermines itself' through rogue response: Chemotherapy can undermine itself by causing a rogue response in healthy cells, which could explain why people become resistant, a study suggests. The treatment loses effectiveness for a significant number of patients with secondary cancers. Writing in Nature Medicine, US experts said chemo causes wound-healing cells around tumours to make a protein that helps the cancer resist treatment. A UK expert said the next step would be to find a way to block this effect. Around 90% of patients with solid cancers, such as breast, prostate, lung and colon, that spread - metastatic disease - develop resistance to chemotherapy.