Abstract
IntroductionTrialing novel cancer therapies in the clinic would benefit from imaging agents that can detect early evidence of treatment response. The timing, extent and distribution of cell death in tumors following treatment can give an indication of outcome. We describe here an 18F-labeled derivative of a phosphatidylserine-binding protein, the C2A domain of Synaptotagmin-I (C2Am), for imaging tumor cell death in vivo using PET.MethodsA one-pot, two-step automated synthesis of N-(5-[18F]fluoropentyl)maleimide (60 min synthesis time, > 98% radiochemical purity) has been developed, which was used to label the single cysteine residue in C2Am within 30 min at room temperature. Binding of 18F-C2Am to apoptotic and necrotic tumor cells was assessed in vitro, and also in vivo, by dynamic PET and biodistribution measurements in mice bearing human tumor xenografts treated with a TRAILR2 agonist or with conventional chemotherapy. C2Am detection of tumor cell death was validated by correlation of probe binding with histological markers of cell death in tumor sections obtained immediately after imaging.Results18F-C2Am showed a favorable biodistribution profile, with predominantly renal clearance and minimal retention in spleen, liver, small intestine, bone and kidney, at 2 h following probe administration. 18F-C2Am generated tumor-to-muscle (T/m) ratios of 6.1 ± 2.1 and 10.7 ± 2.4 within 2 h of probe administration in colorectal and breast tumor models, respectively, following treatment with the TRAILR2 agonist. The levels of cell death (CC3 positivity) following treatment were 12.9–58.8% and 11.3–79.7% in the breast and colorectal xenografts, respectively. Overall, a 20% increase in CC3 positivity generated a one unit increase in the post/pre-treatment tumor contrast. Significant correlations were found between tracer uptake post-treatment, at 2 h post-probe administration, and histological markers of cell death (CC3: Pearson R = 0.733, P = 0.0005; TUNEL: Pearson R = 0.532, P = 0.023).ConclusionThe rapid clearance of 18F-C2Am from the blood pool and low kidney retention allowed the spatial distribution of cell death in a tumor to be imaged during the course of therapy, providing a rapid assessment of tumor treatment response. 18F-C2Am has the potential to be used in the clinic to assess early treatment response in tumors.
Highlights
Trialing novel cancer therapies in the clinic would benefit from imaging agents that can detect early evidence of treatment response
We have developed a PS-targeted imaging agent based on C2A, which was first used in vivo as a glutathione S-transferase–tagged dimeric construct (GSTC2A, 84 kDa) for imaging tumor cell death using MRI, where the protein was labeled with superparamagnetic iron oxide nanoparticles [7] and subsequently with Gd3+ chelates [8]
We describe a 18F-labeled derivative of C2A domain of Synaptotagmin-I (C2Am) that was tested in human xenograft models of advanced colorectal (Colo205 [16]) and triple-negative breast (MDAMB-231 [17]) cancer, treated with either conventional chemotherapy or with MEDI3039, which is a multivalent tumor necrosis factor (TNF)-related apoptosis-inducing ligand receptor-2 (TRAILR2) agonist that can induce tumor cell death at picomolar concentrations [18]
Summary
Trialing novel cancer therapies in the clinic would benefit from imaging agents that can detect early evidence of treatment response. We have developed a PS-targeted imaging agent based on C2A, which was first used in vivo as a glutathione S-transferase–tagged dimeric construct (GSTC2A, 84 kDa) for imaging tumor cell death using MRI, where the protein was labeled with superparamagnetic iron oxide nanoparticles [7] and subsequently with Gd3+ chelates [8]. This GST-tagged construct has been labeled with a 99mTc-chelate for SPECT [9] and with fluorine-18 for PET [10]
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