Bimodal Fluorescence-Magnetic Resonance Contrast Agent for Apoptosis Imaging

Article abstract of DOI:10.1021/jacs.8b13376

Effective cancer therapy largely depends on inducing apoptosis in cancer cells via chemotherapy and/or radiation. Monitoring apoptosis in real-time provides invaluable information for evaluating cancer therapy response and screening preclinical anticancer

Full text of DOI:10.1021/jacs.8b13376

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Article
A Bimodal Fluorescence-Magnetic Resonance
Contrast Agent for Apoptosis Imaging
Hao Li, Giacomo Parigi, Claudio Luchinat, and Thomas J. Meade
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13376 • Publication Date (Web): 28 Mar 2019
Downloaded from http://pubs.acs.org on March 28, 2019
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A Bimodal Fluorescence-Magnetic Resonance Contrast Agent for Apoptosis Imaging
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Hao Li,1 Giacomo Parigi,2 Claudio Luchinat,2 Thomas J. Meade1*
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1Departments of Chemistry; Molecular Biosciences; Neurobiology and Physiology; and
Radiology, Northwestern University, Evanston, IL 60208
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2Department of Chemistry and Magnetic Resonance Center (CERM), University of Florence,
and Consorzio Interuniversitario Risonanze Magnetiche di Metalloproteine (CIRMMP), Via
L. Sacconi 6, 50019 Sesto Fiorentino, Italy
Email: tmeade@northwestern.edu
■ ABSTRACT
Effective cancer therapy largely depends on inducing apoptosis in cancer cells via
chemotherapy and/or radiation. Monitoring apoptosis in real-time provides invaluable
information for evaluating cancer therapy response and screening preclinical anticancer drugs.
In this work, we describe the design, synthesis, characterization and in vitro evaluation of
caspase probe 1 (CP1), a bimodal fluorescence-magnetic resonance (FL-MR) probe that
exhibits simultaneous FL-MR turn-on response to caspase-3/7. Both caspases exist as
inactive zymogens in normal cells but are activated during apoptosis and are unique
biomarkers for this process. CP1 has three distinct components: a DOTA-Gd(III) chelate that
provides the MR signal enhancement, tetraphenylethylene as the aggregation induced
emission luminogen (AIEgen), and DEVD peptide which is a substrate for caspase-3/7. In
response to caspase-3/7, the water-soluble peptide DEVD is cleaved and the remaining Gd(III)-
AIEgen (Gad-AIE) conjugate aggregates leading to increased FL-MR signals. CP1 exhibited
sensitive and selective dual FL-MR turn-on response to caspase-3/7 in vitro and was
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successfully tested by fluorescence imaging of apoptotic cells. Remarkably, we were able to
use the FL response of CP1 to quantify the exact concentrations of inactive and active agents
and accurately predict the MR signal in vitro. We have demonstrated that the aggregation-
driven FL-MR probe design is a unique method for MR signal quantification. This probe
design platform can be adapted for a variety of different imaging targets, opening new and
exciting avenues for multimodal molecular imaging.
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■ INTRODUCTION
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Apoptosis is a highly regulated biochemical process that irreversibly eliminates
dysfunctional cells.1 Along with controlled proliferation, apoptosis establishes the basis for
tissue homeostasis. In contrast, cancer is unrestricted cell growth and often acquires
oncogenic mutations to evade apoptosis.2 To effectively treat cancer, anticancer therapy
largely depends on inducing apoptosis in cancer cells.3 As a result, the field of apoptosis
imaging was developed to monitor therapeutic response at the molecular level. The ultimate
goal of these studies is to improve patient management as well as develop new therapies .4
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Caspase-3 and caspase-7 are the major executioner caspases carrying out mass
proteolysis that ultimately leads to apoptosis. 1c, 5 Both caspases exist as inactive zymogens in
normal cells but are activated during apoptosis. As a result, elevated caspase-3/7 activities
serve as unique biomarkers for apoptosis and monitoring their activities provides invaluable
information for tumor therapy as well as preclinical anticancer drug selection. Through the
development of molecular imaging agents sensitive to caspase-3/7, a variety of biomedical
imaging techniques such as fluorescence, MRI, PET have been used to monitor apoptosis in
vivo.4
MR imaging is at the forefront of experimental and clinical radiology because it has
unlimited tissue penetration depth, excellent soft tissue contrast and spatiotemporal resolution
without the use of ionizing irradiation. The use of paramagnetic Gd(III) based chelates further
enhances the MR image contrast and improves diagnostic accuracy. Based on these Gd(III)
chelates, our group pioneered the design of bioresponsive or activatable MR probes6, a
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unique class of molecular probes that alter the MR signal in response to biological stimuli
such as pH,7 metal ion binding,8 endogenous molecule,9 enzyme activities,6b, 10 and more.11
They represent a frontier in clinical molecular imaging with a focus on probing diseases at
the molecular level and developing tailored treatment options accordingly.
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A fundamental challenge using bioresponsive MR probe for molecular imaging is
signal validation.12 Because the local concentration of the probe is unknown, MR signal
enhancement cannot be specifically assigned to activated probes versus pooling of the
inactive agents. This uncertainty stems from the relatively low sensitivity of MR imaging and
the limited dynamic range of most bioresponsive MR probes.12
A solution to this problem is bimodal imaging by creating a fluorescence-magnetic
resonance (FL-MR) bioresponsive probe that exhibits simultaneous FL-MR signal
enhancement after activation. Because fluorogenic probes have excellent sensitivity and large
dynamic range, the FL signal change can be used to substantiate the MR signal enhancement
in response to the biological stimulus. While a few examples of bimodal FL-MR probes have
been reported, none have provided accurate MR signal quantification using FL
measurements.13
To overcome these challenges, we seek a common activation mechanism for both the
FL and MR components of the probe. One such mechanism is the responsive self-assembly
of aggregation induced emission luminogens (AIEgens), which are fluorophores that become
fluorescent upon aggregation.14 The aggregation-driven florescence mechanism has been
exploited to develop AIE-based bioresponsive FL probes.15 Prior to activation, these AIE-
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based bioprobes remain molecularly dispersed in water. In this state, the “propeller-shaped”
AIEgens have access to nonradiative decay pathways due to rotations of the phenyl rings that
effectively dissipate the excited state energy and quench the fluorescence.14a Post activation,
these probes form aggregates through pi-pi stacking that restricts the intramolecular motions,
decreasing the nonradiative decay rate and increasing fluorescence.
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Self-assembly is also a promising approach to enhance the MR signal of Gd(III)-
based MR probes.10d, 16 Gd(III) chelates are able to reduce the longitudinal relaxation time (T1)
and the transverse relaxation time (T2) of water protons, which translates to brighter (T1-
weighted) or darker (T2-weighted) MR images, respectively. The degree to which 1 mM
Gd(III) chelate can decrease the T1/T2 of water protons is expressed as relaxivity r1/r2 (mM-1s-
1). At clinically relevant field strengths (0.5–3 T), the relaxivity r1 of low molecular weight
Gd(III)-based CAs is positively correlated with its rotational correlation time ( ). As such, if
휏R
the Gd(III) MR probe forms larger aggregates that tumble slower in solution, the R will be
휏
lengthened allowing for more efficient Gd(III)-induced relaxation of water protons, hence
enhanced T1 and T2 MR signals.17
Here we exploit the AIE aggregation as a common mechanism for both FL and MR
probe activations. We describe the design, synthesis, characterization and in vitro evaluation
of Caspase Probe 1 (CP1), a bimodal FL-MR probe that exhibits simultaneous FL-MR turn-
on response to caspase-3/7 (Scheme 1). CP1 provides fast and robust dual turn-on response to
caspase-3/7 in vitro as well as in apoptotic cells, representing the first smart FL-MR imaging
probe that reports on enzyme activities. Remarkably, we were able to use the FL response of
CP1 to quantify the percentage of activated probe and then quantify the MR signal. We
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envision that the modular design of CP1 provides a general FL-MR bimodal platform where
different bioresponsive moieties can be installed to generate a variety of different bimodal
molecular probes.
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Scheme 1. (A) Chemical structures of CP1 and Gad-AIE. Note that Gad-AIE is a mimic of the activated CP1.
(B) Caspase-3/7 sensing mechanism of CP1. CP1 is soluble and well-dispersed in aqueous solution, hence
having low FL/MR signal. In the presence of caspase-3/7, the water-soluble peptide DEVD is enzymatically
removed, and the remaining Gad-AIE conjugate aggregates to “turn on” both FL and MR signals.
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■ RESULTS AND DISCUSSION
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Synthesis of Caspase Probe 1 (CP1). CP1 has three key components: (1) a Gd-DOTA MR
agent that shortens T1, (2) tetraphenylethylene as the AIEgen and (3) a DEVD peptide as the
caspase-3/7 substrate (Scheme 1A). In the absence of caspase-3/7, CP1 remains water soluble
and monomeric, hence giving low FL/MR signal. In the presence of caspase-3/7, the water-
soluble peptide DEVD is cleaved, and the remaining Gd(III)-AIEgen (Gad-AIE) conjugate
will aggregate to enhance both FL and MR signals (Scheme 1B). This modular design
provides the ability to generate many bimodal FL-MR probes to image other enzyme of
interest with ease by inserting different hydrophilic enzyme substrate instead of the DEVD
peptide.
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The syntheses of CP1 and CP1-ctrl (control probe) are convergent (Scheme 1).
Diphenylmethane was deprotonated with n-butyllithium and subsequently reacted with 4,4'-
dimethoxybenzophenone to afford the tertiary alcohol 1, which was dehydrated in the
presence of pTSA to furnish the AIE core 2. Mono-propargylated intermediate 4 was
obtained after sequential BBr3 demethylation and alkylation with propargyl bromide. The
other phenol group in 4 was alkylated with 3-(Boc-amino)propyl bromide and subjected to
TFA for Boc deprotection to afford 5. Under amide coupling conditions, 5 was conjugated
with Gd-DOTAGA (1,4,7,10-tetraazacyclododececane,1-(glutaric acid)-4,7,10-triacetic acid),
which was synthesized separately in 5 steps, to give Gad-AIE conjugate after RP-HPLC
purification.18 Finally, the azide-functionalized DEVDK synthesized via solid phase peptide
synthesis was coupled with Gad-AIE using Cu(I)-catalyzed click reaction to give CP1 after
RP-HPLC purification. CP1-ctrl was synthesized with a free C-terminus in the DEVDK
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sequence. Since DEVD peptide with charged residue at the P1’ position has very low kcat/KM,
we assumed CP1-ctrl to have very weak FL/MR turn-on response in the presence of caspase-
3/7.19 We used CP1-ctrl in caspase-3 assays to show that CP1 responds only to caspase-3 and
not the other chemicals present in the buffer mixture. The purity of CP1, Gad-AIE and CP1-
ctrl were confirmed with analytical HPLC and ESI-MS.
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O
pTSA
BBr3
DCM
n-BuLi
THF
+
Toluene
O
O
OH
O
O
O
O
82.7%
over 2 steps
1
2
1. Cs2CO3, DMF
2. DCM/TFA
K2CO3
+
+
BocHN
Br
Br
ACN
1 equiv
HO
O
HO
OH
81.6%
over 2 steps
3
37.5%
4
O
O
O
O
O
N
O
DIC, HOBt,
DIPEA
O
N
O
N
OH
+
N
N
HN
O
O
DMF
N
O
O
N
O
O
N
H2N
O
O
O
O
O
O
O
5
34.6%
Gad-AIE
Gd-DOTAGA
O
Gad-AIE
+
O
O
CuSO4,
Na ascorbate
O
O
N
HOOC
H
N
O
O
HOOC
H
N
O
N3
N
HN
O
O
O
H
N
O
O
O
N
N
H
N
H2N
N
N
THF/H2O
N
N
H
N
H2N
H
N
H
N
H
H
N
N
O
O
O
O
H
O
HOOC
O
O
O
O
HOOC
COOH
61.5%
CP1
O
COOH
Gad-AIE
+
O
O
CuSO4,
Na ascorbate
O
O
HOOC
H
N
N
O
O
O
N3
HOOC
H
N
H
N
N
HN
O
O
O
O
O
O
N
N
HO
H
N
N
N
H
THF/H2O
55.5%
N
H
N
N
H
HO
N
H
N
H
N
N
H
O
O
O
O
HOOC
O
O
O
O
O
HOOC
COOH
CP1-ctrl
O
COOH
Scheme 2. Synthesis of Gad-AIE, CP1 and CP1-ctrl. CP1 is the “off” state prior to DEVD cleavage while Gad-
AIE is a chemical mimic of the “on” state post DVED cleavage. CP1-ctrl is a control probe that is not
responsive to caspase-3/7 in vitro because it has a charged C-terminus (blue). We used CP1-ctrl in caspase-3
assays to show that CP1 responds only to caspase-3 and not to the other chemicals present in the buffer
mixture.
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Photophysical properties of CP1 and Gad-AIE. The Gad-AIE was used as a mimic for the
activated CP1 product. The UV-Vis spectrum shows the maximum absorptions for both CP1
and Gad-AIE are located at ~313 nm, which are close to that of a typical TPE fluorophore
(Table S2). We evaluated the fluorescence properties of both CP1 and Gad-AIE in caspase
assay buffer. A relatively high concentration of 50 µM was used to resemble the high
concentration required for the MR imaging. A 50 µM solution of CP1 was not fluorescent,
but the same concentration of Gad-AIE was highly fluorescent (Figure 1A). This confirms the
design principle that upon cleavage of the DEVD peptide CP1 aggregates and becomes
fluorescent. The AIE behavior of Gad-AIE was further studied by measuring its FL in
different H2O/DMSO mixtures (Figure 1B). When the water fractions were below 90%, Gad-
AIE remained non-fluorescent. However, as the water fraction approached 100%, Gad-AIE
became highly emissive, a characteristic behavior of AIEgens. To quantify the concentration-
dependent aggregation behavior of Gad-AIE, we measured its critical micelle concentration
to be 43 µM by acquiring the FL spectra of Gad-AIE at different concentrations in water.20
This value is much lower than that of CP1 as the latter remained non-aggregated even at
concentrations as high as 200 µM (Figure S1). As a result, we concluded that Gad-AIE is
much more prone to aggregation than CP1.
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Figure 1. (A) Fluorescence spectra of CP1 (50 µM) and Gad-AIE (50 µM) in caspase-3 buffer. CP1’s
fluorescence is quenched as it is the “off” state; whereas Gad-AIE is highly fluorescent as it is the “on” state
(B) Relative FL intensities of Gad-AIE (50 µM) in different H2O/DMSO mixtures. Gad-AIE is highly emissive
at high water fraction. (C) FL intensity of Gad-AIE at different concentrations. The critical micelle
concentration (CMC) is 43 µM which is much lower compared to that of CP1 (CMC > 200 µM)
In vitro fluorescence detection of caspase-3/7 activities by CP1. The distinct fluorescence
properties of CP1 and Gad-AIE prompted us to use CP1 for caspase-3/7 detection. When CP1
was incubated with caspase-3 (0.4 µg/mL, ca.11.4 nM), it showed a significant fluorescence
increase over time with a ca. 45-fold fluorescence increase only after 1 hr incubation at 37 °C
(Figure 2A). The cleaved product peak was clearly identified with HPLC-MS(Figure S2).
While CP1 has cross-reactivity with caspase-7 (a known DEVDase),19 its fluorescence
response to caspase-3/7 was abolished when co-incubated with 50 µM of caspase-3/7
inhibitor (Figure 2B). In contrast, CP1-ctrl showed no apparent fluorescence change in the
presence of caspase-3/7, indicating that it is not responsive to caspase-3/7 (Figure 2B). We
then evaluated the selectivity of CP1 towards caspase-3/7 and revealed that CP1 has no
fluorescence response towards cathepsin B, a possible competing enzyme or lysozyme which
is very abundant in cells (Figure 2C). Collectively, these results showed that CP1 exhibited
sensitive and selective FL turn-on response to caspase-3/7 in vitro.
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Figure 2. (A) Time-dependent FL turn-on response of CP1(50 µM) with caspase-3 (0.4 µg/mL). (B) Specificity
study of CP1 towards caspase-3, -7. From left to right are normalized FL turn-on responses of CP1(50 µM)
with caspase-3 (0.4 µg/mL); caspase-7 (0.4 µg/mL); caspase-3 (0.4 µg/mL) and its inhibitor Ac-DEVD-CHO
(50 µM); caspase-7 (0.4 µg/mL) and its inhibitor Ac-DEVD-CHO (50 µM), normalized FL turn-on responses
of CP1-ctrl (50 µM) with caspase-3 (0.4 µg/mL). (C) Selectivity study of CP1. From left to right are
normalized FL turn-on responses of CP1 (50 µM) with caspase-3 (0.4 µg/mL); caspase-7 (0.4 µg/mL);
cathepsin B (0.4 µg/mL); lysozyme (0.4 µg/mL). FL intensities were measured at 20 min, 40 min and 60 min.
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In vitro fluorescence imaging of apoptotic HeLa cells with CP1. After demonstrating
CP1’s excellent sensitivity and selectivity to caspase-3/7 in vitro, we evaluated the
cytotoxicity of CP1 in HeLa cells using MTS assay and found no toxicity observed in HeLa
cells after 24 hrs incubation with 0.2 mM CP1 (Figure S3), indicating CP1 is biocompatible.
We further evaluated the cellular uptake of CP1 in HeLa cells. After 24 hrs incubation at 50
µM, CP1 has a cellular uptake of 0.05 fmol/cell, which is abundant for FL imaging
experiments (Figure S4 & S5). As a result, we tested CP1 for visualizing apoptotic cells.
HeLa cells were incubated with 50 µM CP1 for 2 hrs at 37 °C and treated with medium alone
or medium containing 1 µM staurosporine (STS, a known apoptosis inducer) for 1 hr.21 Prior
to imaging, apoptotic and nonapoptotic cells were stained with Annexin V-Alexa Fluor 594
conjugate that fluorescently labels the exposed phosphatidylserine of apoptotic cells.
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As expected, very low CP1 and Annexin V Alexa Fluor fluorescence were observed
for non-apoptotic HeLa cells, indicative of little caspase-3/7 activity for these healthy cells
(Figure 3B & 3C). In contrast, the apoptotic cells showed strong fluorescence signal in both
CP1 and Annexin V-Alexa Fluor channels (Figure 3F & 3G), indicating elevated caspase-3/7
activities in these cells. We also performed time-lapse fluorescence imaging of apoptotic
HeLa cells with CP1 (Figure 4). Healthy HeLa cells were incubated with CP1 (50 µM) for
2hrs, then with STS (1 µM) and immediately placed under the fluorescence microscope.
Images were acquired every three mins over three hours. As shown in Figure 4, the
fluorescence of HeLa cells increased over time, indicating increased caspase-3/7 activity
during apoptosis.
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Figure 3. Fluorescence microscope images of HeLa cells. Top row (A-D) are non-apoptotic Hela cells treated
with CP1 (50 µM) and co-stained with Annexin V-Alexa Fluor 594 conjugate. Annexin V-Alexa Fluor 594
labels the exposed phosphatidylserine in apoptotic cells and serves as a positive control. Bottom row (E-H) are
apoptotic HeLa cells treated with CP1 (50 µM) and co-stained with Annexin V-Alexa Fluor 594. Co-
localization of CP1 and Annexin V-Alexa Fluor 594 confirms CP1’s ability to identify apoptotic cells. Yellow,
CP1 fluorescence. Pink, Annexin V-Alexa Fluor 594 fluorescence.
Figure 4. Time-lapse fluorescence imaging of apoptotic HeLa cells incubated with CP1 (50 µM) prior to
apoptosis induction with STS (1 µM). The fluorescence of HeLa cells increased over time, indicating elevated
caspase-3/7 activity in HeLa cells during apoptosis.
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In vitro MR detection of caspase-3/7 activities with CP1. Based on the excellent FL
response of CP1 with caspase-3/7, we tested its MR response to caspase-3/7. The T1 and T2
changes of CP1 solution (200 µM) in the presence of caspase-3 (0.4 µg/mL, ca. 11.4 nM)
were measured with relaxometry at 1.41 T and 37 °C, and the results showed that CP1 has
faster kinetics compared to a previously reported Gd(III) caspase-responsive MR probe.10d
Indeed, a 45.3% increase in 1/T1 was observed after 1 hr incubation, and an 84.1% increase
after 5 hrs (Figure 5A). The T2 change was even more significant, as a 59.0% increase in 1/T2
was observed after 1 hr incubation, and a remarkable 111.8% increase after 5 hrs (Figure 5B).
The specificity of CP1 towards caspase-3/7 was also evaluated through relaxometry, and the
results mirror that from the fluorescence assays (Figure 5C). Collectively, these results
confirmed our design principle utilizing AIE aggregation as a common mechanism for both
FL and MR probe activation.
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MR imaging at higher field strength is beneficial as it offers higher signal-to-noise
ratio, better resolution and decreased imaging time.22 As such, T1- and T2-weighed MR
images were acquired at 7 T for CP1 solution phantoms with and without caspase-3 incubated
(Figure 5D). While R- mediated increase of r1 is well known at low field, it does not
휏
necessarily apply at high field where a narrow R range (0.5-4 ns) is required to achieve high
휏
r1.23 This was confirmed by the observed minimal change of T1 from 619.0 ms to 692.7 ms
when CP1 was incubated with caspases-3 (Figure 5D, top row). Nonetheless, a significant
decrease in T2 from 147.2 ms to 85.1 ms was seen at 7 T when CP1 was incubated with
caspase-3 overnight.(Figure 5D, bottom row). Furthermore, the T1 and T2 of CP1-ctrl
solution phantom only changed by 0.7 % and 12.9 % respectively in the presence of caspase-
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3, indicating that CP1-ctrl is not responsive towards caspase-3 in vitro as designed (Figure
5D). Finally, to evaluate whether enough CP1 enters the cells to produce a change in MR
signal, we imaged HeLa cells incubated with and without CP1 (Figure S6). The T1 relaxation
time of HeLa cells decreased from 1744.2 ms to 1468.0 ms when incubated with CP1,
indicating that CP1 can effectively accumulate inside the cells.
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Figure 5. (A) Time-dependent T1 decrease of caspase-3 buffer solution in the presence of CP1 (200 µM) and
caspase-3 (0.4 µg/mL) measured at 1.41 T. (B) Time-dependent T2 decrease of caspase-3 buffer solution in the
presence of CP1 (200 µM) and caspase-3 (0.4 µg/mL) measured at 1.41 T. (C) Selectivity study of CP1 towards
caspase-3/7 measured at 1.41 T. From left to right are %T1 increase of CP1(200 µM) with caspase-3 (0.4
µg/mL), caspase-7, caspase-3 (0.4 µg/mL) and its inhibitor Ac-DEVD-CHO (50 µM); caspase-7and its
inhibitor Ac-DEVD-CHO (50 µM); CP1-ctrl with caspase-3. (D) Top row: T1-weighted MR images of CP1 and
CP1-ctrl solution incubated with and without caspase-3 overnight. Bottom row: T2-weighted MR images of
CP1 and CP1-ctrl solution incubating with and without caspase-3 overnight. MR images were acquired at 7 T,
ambient temperature. These results indicate that CP1 can be used to detect caspase-3 activity using T2-weighted
MR imaging at 7 T.
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Table 1. Relaxivities of MR probes at 1.41 T (37 °C) and 7 T (25 °C)
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r1 at 1.41 T
(mM−1 s-1)
r2 at 1.41 T
(mM−1 s-1)
r1 at 7 T
(mM−1 s-1)
r2 at 7 T
(mM−1 s-1)
MR Probe
CP1
8.1
16.7
8.8
9.7
5.8
5.9
8.0
6.2
5.8
10.9
30.6
20.9
18.7
32.8
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CP1+
caspase3a
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10.8
30.5
CP1-ctrl
CP1-ctrl +
Caspase3
8.1
Gad-AIE
18.9
aApproximately 60 % conversion was observed after 200 µM CP1 was incubated with caspase-3 (0.4 µg/mL) overnight.
Relaxivity measurements. The relaxivities r1 and r2 of both CP1 and CP1-ctrl with and
without caspase-3 overnight incubation were measured at 1.41 T and 7 T (Table 1). At 1.41 T,
both the r1 and r2 of CP1 solution increased by ca. 100% in the presence of caspase-3. At 7 T,
the r1 of CP1 did not change in the presence of caspase-3 but its r2 increased by 200%. The r1
and r2 of CP1 incubated with caspase-3 closely resemble that of Gad-AIE, even though only
about 60% CP1 was converted after overnight incubation with caspase-3 (Figure S2). This
indicates that CP1 does not require 100% conversion to achieve significant MR signal
enhancement. In contrast to CP1, CP1-ctrl showed no changes in relaxivity in the presence of
caspase-3 at either field strength.
Nuclear magnetic relaxation dispersion. To confirm that the observed T1-weighted MR
1
signal enhanced at 1.41 T of CP1 is a result of increased rotational correlation time ( ), H
휏R
nuclear magnetic relaxation dispersion (NMRD) profiles of CP1 and Gad-AIE at various
concentrations were measured. The NMRD profiles report the water proton relaxivity at
different field strengths. Parameters such as τR can be estimated from the analysis of these
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profiles. The presence of a relaxivity peak around 0.5-2 T (ca. 20-90 MHz proton Larmor
frequency) immediately indicates a large τR corresponding to the formation of aggregates,
whereas the lack of such a peak is a clear indication of fast mobility. At both 25 °C and 37 °C
the relaxivity of CP1 is concentration-dependent, and higher concentration leads to greater
relaxivity most likely due to the increasing formation of aggregates at higher concentrations
(Figure 6A & B). Notably, the relaxivity profile of CP1 at the lowest concentration (0.25 mM)
shows no relaxivity peak in the high field region, indicating the occurrence of fast mobility.
In contrast, the relaxivity profiles of Gad-AIE are independent of its concentration, and the
presence of a high relaxivity peak clearly indicates the formation of large aggregates (Figure
6A & B), which was confirmed by DLS measurements (Figure S7). From the concentration-
dependent relaxivities of CP1, we extrapolated the relaxivity profile of fully non-aggregated
CP1 (see SI and Fig 6C) and analyzed it with the Solomon-Bloembergen-Morgan (SBM)
model.23c The analysis provided an estimate of τR of about 100 ns at 25 °C and 71 ns at 37 °C
(Table S3). The relaxivity profile of Gad-AIE was fit using the modified Florence NMRD
program to account for the presence of static ZFS (Figure 6C),24 and indicates a much longer
τR of 3500 ns at 25 °C and 2700 ns at 37 °C, supporting our hypothesis that the τR increase
leads to the observed MR enhancement. A detailed description for the fitting method as well
as other estimated parameters are available (Table S3).
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Figure 6. Relaxivity profiles of Gad-AIE and CP1 measured (A) at 25 °C and (B) 37 °C. At all concentrations
measured, Gad-AIE had consistent NMRD profiles while CP1 showed greater relaxivity at higher
concentrations. Presumably, this is because Gad-AIE forms large aggregates even at low concentrations while
CP1 does not. (C) Best fit profiles of Gad-AIE and of fully non-aggregated CP1, as extrapolated from its
concentration dependence.
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Correlating the FL and MR signal of CP1 to caspase-3. Quantification of the MR signal
presents a unique challenge since the MR signal intensity depends not only on the relaxivity
change due to activation but also on the concentration of the MR probe. Hence, we utilized
the FL signal to quantify the concentrations of CP1 versus Gad-AIE during caspase-3 assay
and to quantify the MR signal. To do so, a FL calibration curve was generated using different
ratios of [CP1] and [Gad-AIE] while keeping the total concentration (200 µM) constant
(Figure 7A). Then, 200 µM CP1 was incubated with caspase-3 and the fluorescence response
was monitored over time (Figure 7B, black). By fitting the fluorescence of the assay solution
to the FL calibration curve we calculated the concentrations of CP1 at each time point.
(Figure 7B, red). The concentration of Gad-AIE at each time point would be (200 µM –
[CP1]). Subsequently, the calculated concentrations of CP1 and Gad-AIE were used to
compute the T1 and T2 at each point according to eq 1 and eq 2,25 respectively:
(1)
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1
1
=
+ 풓ퟏ[CP1] + 풓′ퟏ[Gad ― AIE]
푇1,obs 푇1,d
1
1
(2)
=
+ 풓ퟐ[CP1] + 풓′ퟐ[Gad ― AIE]
푇2,obs 푇2,d
where r1/r2 and r'1/r'2 are the relaxivities of CP1 and Gad-AIE, respectively, measured at 1.41
T; T1,d and T2,d are the diamagnetic contribution of the caspase buffer solution. The calculated
T1 and T2 at each time point were plotted (Figure 7C & D, red) and compared to the measured
T1 and T2 (Figure 7C & D, black), and the difference was within 10 ms. To our knowledge,
this represents the first quantitative correlation between FL and MR signals of a bimodal FL-
MR activated probe in vitro. Importantly, the AIE-based probe platform can differentiate
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between an enzyme-induced MR turn-on response and a false positive MR signal
enhancement originated from the pooling of inactive agents.
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Figure 7. (A) FL intensity of different ratios of [CP1] and [Gad-AIE] with a constant total concentration of
200 µM. As expected, the concentration of CP1 is negatively correlated with the FL intensity. (B) Black: FL
intensity of CP1 (200 µM) incubated with caspase-3 over time. Red: calculated CP1 concentration over the
course of the assay. (C) Red: calculated time-dependent T1 decrease of caspase-3 buffer solution in the
presence of CP1 (200 µM) and caspase-3. Black: measured time-dependent T1 decrease of caspase-3 buffer
solution in the presence of CP1 (200 µM) and caspase-3. (D) Same as C but for T2.
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■ CONCLUSION
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We have reported the design, synthesis, and in vitro evaluations of a new bimodal
caspase-activatable imaging probe CP1 that exhibits FL-MR turn-on response in both in vitro
caspase enzymatic assays and apoptotic HeLa cells. Most importantly, the FL signal of CP1
can be used to quantify the concentrations of the active and inactive probes during caspase-3
assay hence accurately predicting the MR response in vitro. This FL imaging capability can
potentially enable ex vivo validation of the in vivo MR imaging results, which can be
challenging to interpret due to background signal and unknown local concentration of Gd(III).
One potential improvement to be made is to further increase the cellular uptake of CP1
through attachment of cancer-targeting ligands. Nonetheless, results from cellular uptake
experiment as well as MR imaging of cell pellet phantom indicated that enough CP1
accumulated in cells to produce enhanced MR signal. Finally, the unique AIE-based design
provides a modular platform for the bimodal detection of many other biomarkers. Ongoing
experiment includes in vivo MR imaging using CP1 to monitor therapy-induced tumor
apoptosis followed by ex vivo fluorescence imaging of apoptotic tumors tissue.
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■ ASSOCIATED CONTENT
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Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website at
DOI:
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Procedures for the synthesis of all compounds; characterization of synthetic intermediates by
NMR and HRMS; HPLC, ESI-MS, HRMS of CP1 and Gad-AIE; methods and materials for
enzyme assays, cell experiments, NMRD experiment and NMRD fitting.
■ AUTHOR INFORMATION
Corresponding Author
tmeade@northwestern.edu
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
H.L. and T.J.M. acknowledge support by NIH Grant P30CA060553. G.P. and C.L. would
like to thank Fondazione Cassa di Risparmio di Firenze and the COST Action CA15209
(EURELAX) for financial support. Fluorescence imaging was performed with help from Dr.
Jessica Hornick at Northwestern University Biological Imaging Facility generously supported
by the Chemistry of Life Processes Institute and the NU Office for Research. MR imaging
was performed with help from Dr. Emily Waters and Dr. Chad Haney at Center for Advanced
Molecular Imaging generously supported by NCI CCSG P30 CA060553 awarded to the
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Robert H. Lurie Comprehensive Cancer Center. Metal analysis was performed with help from
Dr. Keith MacRenaris and Rebecca Sponenburg at the Northwestern University Quantitative
Bio-elemental Imaging Center generously supported by NASA Ames Research Center
NNA06CB93G. HRMS was performed with help from Dr. Saman Shafaie from Integrated
Molecular Structure Education and Research Center (IMSERC) at Northwestern University.
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