Angiogenesis
The growth of new blood vessels. A tumour bigger than a pinhead needs its own blood supply and sends out signals (mainly VEGF) to recruit one; anti-angiogenic drugs cut that supply.
Starved of oxygen, tumour cells switch on the HIF transcription factor and secrete VEGF and other growth factors that make nearby blood vessels sprout towards them; the resulting vessels are leaky and disorganised, which raises pressure inside the tumour and hampers drug delivery. 'Inducing angiogenesis' is one of the hallmarks of cancer, and drugs that block VEGF (bevacizumab) or its receptors (sunitinib, lenvatinib, cabozantinib) are standard in kidney, liver, colorectal and other cancers, though they slow rather than cure. Normalising rather than destroying tumour vessels may improve delivery of chemotherapy and immune cells, and VEGF also suppresses immune cells, which is one rationale for combining anti-angiogenics with immunotherapy.