Complex sentence: Some photosensitisers naturally accumulate in the endothelial cells of vascular tissue allowing'vascular targeted' PDT, but there is also research to target the photosensitiser to the tumour (usually by linking it to antibodies or antibody fragments).
Split into simpler sentences: Some photosensitisers naturally accumulate in the endothelial cells of vascular tissue allowing'vascular targeted' PDT. Research are to target the photosensitiser to tumours (usually by linking it to antibodies or antibody fragments).