Introduction
The aim of cancer treatment, whether this is chemo-, radio-, targeted- or immuno-therapy [
1], is to induce tumor cell death, where the two dominant forms of cell death are apoptosis and necrosis [
2]. Cell death is an important and generic target for imaging early treatment response [
3]. However, despite a long-standing unmet need there are still no reliable techniques for routine imaging of cell death in the clinic [
4].
Phosphatidylserine (PS) is externalized on the cell surface during apoptosis and is also exposed via the permeabilization of the plasma membrane that occurs during necrosis. The C2A domain of Synaptotagmin-I binds PS in a calcium-dependent manner with nanomolar affinity [
5,
6]. 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 (GST-C2A, 84 kDa) for imaging tumor cell death using MRI, where the protein was labeled with superparamagnetic iron oxide nanoparticles [
7] and subsequently with Gd
3+ chelates [
8]. This GST-tagged construct has also been labeled with a
99mTc-chelate for SPECT [
9] and with fluorine-18 for PET [
10].
Subsequently, we have used the much smaller (16-kDa) isolated C2A domain [
6], which enables better target access and tissue clearance. Introduction of a single cysteine residue, distant from the PS-binding site, using site-directed mutagenesis (S78C; C2Am), allowed the production of chemically defined derivatives in which this single cysteine was labeled with imaging tags. A near infrared-labeled derivative showed a fourfold lower binding to viable cells in vitro than a similarly labeled Annexin-V and therefore improved specificity for detecting cell death [
6]. Annexin-V is another PS-binding protein, which was tested in the clinic nearly two decades ago in the form of
99mTc-HYNIC-Annexin-V, but showed suboptimal pharmacokinetics and extensive non-specific binding [
11,
12]. A subsequent derivative,
99mTc-rh-Annexin-V-128, showed a better biodistribution profile and targeting of cell death in vivo [
13].
We recently evaluated derivatives of C2Am that had been labeled for photoacoustic imaging [
14] and SPECT [
15]. The latter were based on
99mTc and
111In chelates and could detect tumor cell death in vivo within 2 h of administration, with a tumor-to-muscle contrast of ~ 3. However, they also showed significant renal retention (> 150% IA/g), which persisted for up to 24 h post-administration.
Here, we describe a
18F-labeled derivative of C2Am that was tested in human xenograft models of advanced colorectal (Colo205 [
16]) and triple-negative breast (MDA-MB-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]. We analyzed the biodistribution and dosimetry profile of the probe and its sensitivity for detecting tumor cell death by correlating probe distribution in tumors with histological markers of tumor cell death in tumor sections. Non-specific retention of the probe was evaluated by comparing its distribution with that of a site-directed mutant,
18F-iC2Am, which is inactive in PS binding [
15].
Discussion
Imaging cell death can provide an indication of disease prognosis [
27] and in cancer can be used to detect treatment response [
3,
28]. Imaging agents that target various cell death-related events, such as cleaved caspase-3 (CC3) [
24], changes in mitochondrial membrane potential [
29], alterations in membrane permeability [
30] and exposure of PS [
31] and phosphatidylethanolamine (PE) [
32] have been described [
28], some of which have translated to the clinic [
33]. However, the success of these agents has so far been limited. A CC3 targeted agent,
18F-ICMT-11, showed a lack of sensitivity for detecting cell death in breast and lung cancer patients treated with chemotherapy [
34] and
18F-ML-10, an agent that detects changes in membrane permeability during apoptosis, failed to detect response to chemotherapy in a preclinical model of breast cancer [
35]. Exposure of PS [
31] and PE [
32], which are externalized on the outer leaflet of the plasma membrane during apoptosis, or become accessible following plasma membrane disruption during necrosis, are attractive imaging targets since they provide a sustained and abundant target [
14] that can give good image contrast [
36].
99mTc-Annexin V, which binds PS, has translated to the clinic; however, it showed poor pharmacokinetics and non-specific binding [
33]. Exposure of PE by dying cells has been exploited in the development of duramycin, a 19 amino acid tetracyclic peptide (~ 2 kDa), as a cell death imaging agent [
37,
38]. However, despite its smaller size the clearance profile and contrast generated by
99mTc-duramycin was similar to that observed here with
18F-C2Am. Following treatment of Colo205 tumors with a TRAILR2 agonist, the increase in tumor-to-muscle (T/m) contrast generated by
99mTc-duramycin was 30.8 ± 1.9 ⨉ [
37]. However, when corrected for non-specific retention using a control treatment antibody [
37], the T/m was 6.9 ± 1.5×, which is similar to that observed here with
18F-C2Am (6.1 ± 2.1×). The retention of
18F-iC2Am, which is non-specific, was ~15%. More importantly, for clinical translation,
18F-C2Am showed comparable blood half-life (
t1/2: 12.4 ± 2.2 min) to that reported recently for the small PET agent
68Ga-duramycin (
t1/2: 17.3 ± 4.12 min) [
39], but longer than that observed previously for the SPECT agent
99mTc-duramycin (
t1/2: 4.1 ± 0.3 min) [
40]. Cell death-dependent contrast was obtained earlier using
18F-C2Am, at 2 h following agent administration, as compared to 4 h for
99mTc-duramycin.
18F-C2Am also cleared more quickly than the γ-emitter labeled C2Am derivatives described previously (
99mTc-C2Am,
t1/2: 582 ± 6 min;
111In-C2Am, t
1/2: 480 ± 48 min) [
15] and gave higher T/m ratios (∼ 6–10 ×) as compared to
99mTc-C2Am (~ 4.3 ×) and
111In-C2Am (~ 2.2 ×), albeit with different tumor models (EL4 and Colo205) and treatment (chemotherapy) [
15]. The C2Am fragment (< 3 kDa) detected in serum and urine, within 30 min of administration, is likely the result of proteolytic cleavage in the liver and/or kidney, which could contribute to this rapid clearance. The mean SUV
max for
18F-C2Am at 1 h post-administration (0.66 ± 0.18) was greater than that reported previously for a much larger
18F-labeled C2A derivative (
18F-GST-C2A; 80 kDa; SUV
max = 0.47 ± 0.28) [
10]. Moreover, no inactive C2A controls were performed in this previous study, suggesting that a substantial part of the tumor signal may have been caused by non-specific retention [
10].
18F-C2Am showed greater T/m contrast (two- to threefold) for identical levels of cell death and lower (2–10 ×) non-specific retention in other tissues post-treatment when compared to
18F-ICMT-11 [
24],
18F-ML-10 [
35],
99mTc-Duramycin [
36], and
18F-Annexin V [
41].
Pre-blocking PS using unlabeled C2Am at tenfold greater molar dose than
18F-C2Am resulted in a ~ 70% reduction in tumor retention of
18F-C2Am. This reduction was much greater than expected given the high levels of PS predicted to be exposed by dying cells in these tumors (~ 30 μM), based on cell treatment experiments in vitro [
14], and suggests that the PS accessible to C2Am in vivo is much less than this.
The levels of tumor cell death observed in the clinic can vary widely, from a few percent pre-treatment to 5–16% apoptosis following neoadjuvant treatment in breast cancer [
42,
43].
18F-C2Am detects both apoptosis and necrosis and therefore should be more sensitive than those agents that detect apoptosis alone, such as
18F-ML-10 [
35]. Moreover, C2Am is also capable of binding PE [
44], which like PS, is also externalized to the surface of dying cells.
The contrast observed here, where SUV doubled within an hour of
18F-C2Am injection following an increase in cell death from 8 to 20%, suggests that this agent should detect the expected levels of tumor cell death in the clinic. Analysis of
18F-C2Am signal heterogeneity, for example using Minkowski functionals, which we have used previously to analyze the heterogeneous distribution of a gadolinium chelate-conjugated derivative of C2A in magnetic resonance images [
45], could further enhance the sensitivity of the agent for detecting treatment response.
The estimated human mean effective dose with
18F-C2Am is 12.5 ± 5.7 μSv/MBq, which was higher than that estimated for the SPECT tracer
99mTc-duramycin (3.19 ± 2.16 μSv/MBq) [
40], but similar to that observed recently for a small molecule PET tracer (
18F-PSMA-11; 12.8 ± 0.6 μSv/MBq) [
46], and for a biologic (
99mTc-Annexin-V; 9.7 ± 1.0 μSv/MBq) [
47]. The total estimated effective human dose of
18F-C2Am equates to 5.6 ± 2.6 mSv (450 MBq injected), which is less than the PET element of a [
18F]FDG PET/CT scan, 9.0 ± 1.6 mSv [
48].
In conclusion, we have demonstrated 18F-C2Am as a PET agent for imaging cell death in vivo that showed fast renal clearance, good tumor contrast post-treatment and acceptable dosimetry. 18F-C2Am has the potential to be used in the clinic to assess early treatment response in tumors, such as breast, prostate, lung and colorectal.
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