In vitro selectivity, in vivo biodistribution and tumour uptake of annexin V radiolabelled with a positron emitting radioisotope

The availability of a noninvasive method to detect and quantify apoptosis in tumours will enable tumour response to several cancer therapies to be assessed. We have synthesised two radiotracers, annexin V and the N-succinimidyl-3-iodobenzoic acid (SIB) derivative of annexin V, labelled with radio-iodine (I-124 and I-125) and provided proof of the concept by assessing specific binding and biodistribution of these probes to apoptotic cells and tumours. We have also assessed the tumour uptake of [I-124] annexin V in a mouse model of apoptosis. RIF-I cells induced to undergo apoptosis in vitro showed a drug concentration-dependent increased binding of [I-125] annexin V and [I-125] SIB-annexin V. In the same model system, there was an increase in terminal deoxynucleotidyl transferase-mediated nick end labelling (TUNEL)-positive cells and a decrease in clonogenic survival. Radiotracer binding was completely inhibited by preincubation with unlabelled annexin V. In RIF-1 tumour-bearing mice, rapid distribution of [I-125]SIB annexin V-derived radioactivity to kidneys was observed and the radiotracer accumulated in urine. The binding of [I-125]SIB-annexin V to RIF-1 tumours increased by 2.3-fold at 48 h after a single intraperitoneal injection of 5-fluorouracil ( 165 mg kg(-1) body weight), compared to a 4.4- fold increase in TUNEL-positive cells measured by immunostaining. Positron emission tomography images with both radiotracers demonstrated intense localisation in the kidneys and bladder. Unlike [I-124]SIB-annexin V, [I-124] annexin V also showed localisation in the thyroid region presumably due to deiodination of the radiolabel. [I-124]SIB-annexin V is an attractive candidate for in vivo imaging of apoptosis by PET.