A substance being studied in the treatment and diagnosis of some types of cancer. It builds up in some cancer cells, which may make them easier to kill with radiation therapy and chemotherapy. Gadolinium texaphyrin is also used in magnetic resonance imaging (MRI) to help find cancer cells in the body. It is a type of radiosensitizing agent, a type of chemosensitizing agent, and a type of contrast agent. Also called motexafin gadolinium.