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An Anonymous Coward writes:

Two clinical trials for cancer were recently halted for the best possible reason; the drugs worked so well that it would be unethical to continue. One trial was for melanoma and the other was for lung cancer, but the drug-target was the same: Programmed cell death protein 1 (PD-1).

PD-1 is a protein that inhibits responses from T cells. Cancer often takes advantage of the PD-1 pathway to prevent a productive immune response that would otherwise kill the cancer. The drugs in both the clinical trials are antibodies that bind to PD-1, thus preventing cancer-mediated inhibition of the immune response. As this is a general mechanism that cancer uses to evade the immune system, it will likely be effective at treating other forms of cancer besides melanoma and lung cancer. Also, since the drugs are targeting T cells and not the cancer directly, resistance will not develop as easily.

http://pipeline.corante.com/archives/2015/04/17/stopped_for_efficacy_again.php
http://pipeline.corante.com/archives/2015/03/24/the_best_way_to_halt_a_clinical_trial.php
https://en.wikipedia.org/wiki/Programmed_cell_death_1

We looked around briefly and also found:

http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm412802.htm
http://www.reuters.com/article/2015/03/24/us-merck-melanoma-idUSKBN0MK1FO20150324