Preparation of F-18 labeled annexin V: A potential PET radiopharmaceutical for imaging cell death

The clinical response to antitumor therapy is measured using imaging, such as CT or MRI, 6-12 weeks following chemotherapy treatment. The images at that time reflect both tumor cell death and new growth. Therefore, the amount of tumor cell death caused by chemotherapy cannot be efficiently quantified with current imaging modalities. A quantitative measurement of tumor cell death immediately following chemotherapy is needed to help validate both new agents and to optimize administration of existing therapies. Annexin V is a 36kD protein that binds to exposed phosphatidylserine (PS) on dying cells. In order to synthesize a probe that can detect cell death in vivo, the positron emitter F-18 was conjugated to annexin V via the compound N- succinimidyl-4-[ ¹⁸F]fluorobenzoate, [¹⁸F]SFB. The decay corrected radiochemical yield of F-18 labeled annexin V from ¹⁸F fluoride was 17.6±5.6 % (n=4) in three hours. The stepwise radiochemical yield of the conjugation step with annexin V was as high as 70% when a protein concentration of 5 mg/ml was used. Cancer cells treated with the chemotherapeutic agent, etoposide, showed an 88% increase in the binding of F-18 labeled annexin V compared to untreated cells. We conclude that [¹⁸F] labeled annexin V can be readily prepared by the conjugation of annexin V with [ ¹⁸F]SFB and that the positron-emitting compound is biologically active in detecting apoptosis.