Complex sentence: Some, such as bevacizumab, have been approved and are in clinical use, but one of the main problems with anti-angiogenesis drugs is that many factors stimulate blood vessel growth in cells normal or cancerous.
Split into simpler sentences: Some, such as bevacizumab, have been approved and are in clinical use. One of the main problems with anti-angiogenesis drugs is that many factors stimulate blood vessel growth in cells normal or cancerous.