title: Design and preclinical evaluation of a 99mTc-labelled diabody of mAb J591 for SPECT imaging of prostate-specific membrane antigen (PSMA).
abstract: BACKGROUND
  Sensitive and specific detection of nodal status, sites of metastases and low-volume recurrent disease could greatly improve management of patients with advanced prostate cancer. Prostate-specific membrane antigen (PSMA) is a well-established marker for prostate carcinoma with increased levels of expression in high-grade, hormone-refractory and metastatic disease. The monoclonal antibody (mAb) J591 is directed against an extracellular epitope of PSMA and has been shown to efficiently target disseminated disease including metastases in lymph nodes and bone. Its use as a diagnostic imaging agent however is limited due to its slow pharmacokinetics. In this study a diabody derived from mAb J591 was developed as a single photon emission computed tomography (SPECT) tracer with improved pharmacokinetics for the detection of PSMA expression in prostate cancer.
  
  
  METHODS
  A diabody in VH-VL orientation and with a C-terminal cysteine was expressed in HEK293T cells and purified by a combination of metal ion affinity and size exclusion chromatography. Specificity and affinity were determined in cell binding studies. For SPECT imaging, the diabody was site-specifically labelled with [99mTc(CO)3]+ via the C-terminal His tag and evaluated in a subcutaneous DU145/DU145-PSMA prostate carcinoma xenograft model.
  
  
  RESULTS
  J591C diabody binds to PSMA-expressing cells with low nanomolar affinity (3.3 ± 0.2 nM). SPECT studies allowed imaging of tumour xenografts with high contrast from 4 h post injection (p.i.). Ex vivo biodistribution studies showed peak tumour uptake of the tracer of 12.1% ± 1.7% injected dose (ID)/g at 8 h p.i. with a tumour to blood ratio of 8.0. Uptake in PSMA-negative tumours was significantly lower with 6.3% ± 0.5% at 8 h p.i. (p < 0.001).
  
  
  CONCLUSIONS
  The presented diabody has favourable properties required to warrant its further development for antibody-based imaging of PSMA expression in prostate cancer, including PSMA-specific uptake, favourable pharmacokinetics compared to the parental antibody and efficient site-specific radiolabelling with 99mTc.
authors.affiliation: Division of Imaging Sciences and Biomedical Engineering, King's College London, 4th Floor Lambeth Wing, St, Thomas' Hospital, London SE1 7EH, UK. florian.kampmeier@kcl.ac.uk.
authors.forename: Florian
authors.initials: F
authors.lastname: Kampmeier
authors.forename: Jennifer D
authors.initials: JD
authors.lastname: Williams
authors.forename: John
authors.initials: J
authors.lastname: Maher
authors.forename: Gregory E
authors.initials: GE
authors.lastname: Mullen
authors.forename: Philip J
authors.initials: PJ
authors.lastname: Blower