By studying immune cells from melanoma tumours in mice, a UCLA team discovered that immune cells that had invaded the tumours showed high activity of the gene monoamine oxidase A. The protein that gene produces, MAO-A, is the target of monoamine oxidase inhibitor (MAOIs) drugs.
Mice that did not produce MAO-A in tumours showed better control of melanoma and colon cancer. When they were treated with either phenelzine, clorgyline or mocolobemide, tumour growth slowed. The treatment worked even better in combination with drugs that block the immune checkpoint PD-1, the researchers reported.
So how to MAOIs fight cancer? The UCLA team discovered that by blocking MAO-A, the antidepressants boost the activity of the immune system's T-cells. The drugs also inhibit tumour-associated macrophages, which normally function to help tumours evade immune destruction.