Multimodality Molecular Imaging of Apoptosis in Oncology

Article (PDF Available)inAmerican Journal of Roentgenology 197(2):308-17 · August 2011with30 Reads
DOI: 10.2214/AJR.11.6953 · Source: PubMed
Objective: The purposes of this review are to describe the signaling pathways of and the cellular changes that occur with apoptosis and other forms of cell death, summarize tracers and modalities used for imaging of apoptosis, delineate the relation between apoptosis and inhibition of protein translation, and describe spectroscopic technologies that entail high-frequency ultrasound and infrared and midinfrared light in characterizing the intracellular events of apoptosis. Conclusion: Apoptosis is a highly orchestrated set of biochemical and morphologic cellular events. These events present many potential targets for the imaging of apoptosis in vivo. Imaging of apoptosis can facilitate early assessment of anticancer treatment before tumor shrinkage, which may increase the effectiveness of delivery of chemotherapy and radiation therapy and speed drug development.

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Available from: Francis Blankenberg, Feb 24, 2015
30 8 AJR:19 7, August 2011
then carefully packaged into small membrane-
bound packets called apoptotic bodies. Apop-
totic bodies are subsequently ingested by adja-
cent cells and phagocytes without provocation
of an inflammatory response or tissue damage.
Apoptosis is the opposite of necrotic cell death,
which is a chaotic event characterized by un-
controlled primary failure of the cell mem-
brane that frequently generates marked inflam-
mation, tissue destruction, and fibrosis.
The morphologic changes of apoptosis are
preceded by an initiation phase triggered by
an array of signals, including a lack of need-
ed growth factors, antihormonal therapy,
DNA damage, immune reactions, ischemic
injury, ionizing radiation, and chemotherapy
[3, 4]. The lag time between exposure to the
trigger and the development of observable
morphologic signs of apoptosis varies great-
ly, depending on cell type, type of trigger, in-
tensity and duration of exposure, and the lo-
cal environmental conditions of the cell.
Most apoptotic pathways converge on a
family of aspartate-specific cysteine proteases
known as the caspases [5] (Fig. 1). Each cas-
pase (initiating 8, 9, and 10 or executioner 3, 6,
and 7) crosslinks, cleaves, and activates down-
stream caspases and other apoptotic proteins
that constitute the extrinsic and intrinsic apop-
totic pathways. The extrinsic pathway is initi-
ated by specific death-inducing molecules—
tumor necrosis factor (TNF), TNF receptor
Multimodality Molecular Imaging of
Apoptosis in Oncology
Francis G. Blankenberg1
Joseph F. Norfray2
Blankenberg FG, Norfray JF
1Department of Radiology, Division of Pediatric
Radio logy, Lucil e Salter Pack ard Childr en’s Hospit al, 725
Welch Rd , Palo Alt o, CA 94 304 . Address co rrespond ence
to F. G. Blankenberg (
2Chicag o Norths ide MRI Cen ter, Chicago, I L.
Nuclear Medicine and Molecular Imaging Review
AJR 20 11; 197:3 08– 317
© Amer ican Roent gen Ray Socie ty
espite more than a decade of in-
tense investigation, a method of
imaging apoptosis has not been
fully validated. The purposes of
this article are to describe the signaling path-
ways and other cellular changes that occur
with apoptosis and other forms of cell death
that can be imaged with existing or emerging
technologies; summarize the tracers and mo-
dalities used in imaging of apoptosis in both
animal models and humans; delineate the re-
lation between apoptosis and inhibition of
protein translation, a process that can be
readily detected with proton MR spectrosco-
py (1H MRS) with existing clinical MRI
units, coils, and software without contrast
agents; and describe spectroscopic technolo-
gies that entail high-frequency ultrasound
and infrared and midinfrared light in charac-
terizing the intracellular events of apoptosis.
Caspase-Dependent Apoptosis
Apoptosis, or programmed cell death, is
the primary mechanism by which unneed-
ed or senescent cells are physiologically ab-
sorbed by healthy adjacent cells and tissues
[1, 2]. The term apoptosis (from Greek, mean-
ing the dropping or falling off of an organ or
part) describes a complex series of morpholog-
ic changes that include cytoplasmic shrinkage,
nuclear condensation, membrane blebbing, and
budding off of intracellular contents, which are
Keywords: annexin V, apoptosis, caspase, choline, lipids,
MR spectroscopy, phosphatidy lserine
DOI :10. 22 14 /AJ R.11. 69 53
Recei ved March 25 , 2011; accept ed witho ut revisi on
May16, 2 011.
J. F. Nor fray is pre sident of Re ceptomon .
OBJECTIVE. The purposes of this review are to describe the signaling pathways of
and the cellular changes that occur with apoptosis and other forms of cell death, summarize
tracers and modalities used for imaging of apoptosis, delineate the relation between apoptosis
and inhibition of protein translation, and describe spectroscopic technologies that entail high-
frequency ultrasound and infrared and midinfrared light in characterizing the intracellular
events of apoptosis.
CONCLUSION. Apoptosis is a highly orchestrated set of biochemical and morphologic
cellular events. These events present many potential targets for the imaging of apoptosis
in vivo. Imaging of apoptosis can facilitate early assessment of anticancer treatment before
tumor shrinkage, which may increase the effectiveness of delivery of chemotherapy and
radiation therapy and speed drug development.
Blankenberg and Norfray
Molec ular Imagin g of Apopt osis
Nuclear Medicine and Molecular Imaging
AJR:19 7, August 2011 309
Molecular Imaging of Apoptosis
(TNFR), TNF-related apoptosis-inducing li-
gand (TRAIL), Fas ligand (FasL) —released in
the extracellular space that on binding to their
cognate receptors recruit specialized adapter
molecules from the cytoplasm—Fas-associ-
ated death domain (FADD), TNF receptor–
associated death domain (TRADD)—along
with caspase 8 to form a death-inducing signal-
ing complex (DISC). The DISC propagates the
extrinsic death signal by proteolytic activation
of caspase 10, which cleaves and activates the
downstream executioner caspases until reach-
ing caspase 3. Once activated, caspase 3 trav-
els to the nucleus, activates poly–adenosine di-
phosphate–ribose polymerase 1 (PARP-1), a
DNA repair enzyme, and facilitates the deg-
radation of nuclear DNA into 50- to 300-kilo-
base pieces, the DNA ladder formation seen at
gel electrophoresis. Spliced DNA can also be
detected in situ and in vitro by application of
terminal deoxynucleotidyltransferase–mediat-
ed deoxyuridine triphosphate nick-end label-
ing (TUNEL), which is seen with immunohis-
tochemical staining and immunofluorescence.
The DISC can provoke translocation of trun-
cated Bcl interacting domain (BID), a pro-
apoptotic Bcl-2 family protein, to mitochon-
dria. This protein induces oligomerization of
the proapoptotic proteins Bcl-2 antagonist/
killer (BAK) and Bcl-2–associated X (BAX)
proteins. These proteins are required to form
holes and channels in the outer mitochondrial
membrane in a process known as mitochondri-
al outer membrane permeabilization (MOMP).
These channels allow the escape of multiple
proteins, including cytochrome c, from the mi-
tochondrial intermembrane space to the cyto-
plasm. The release of cytochrome c into the
cytoplasm is the hallmark of the intrinsic (mi-
tochondrial) pathway of apoptosis. This pro-
cess is accompanied by loss of the normally
high negative mitochondrial membrane poten-
tial (ΔΨ m). Cytochrome c interacts with apop-
tosis-activating factor 1 (Apaf-1), adenosine tri-
phosphate (ATP), and procaspase 9 to form a
structure known as the apoptosome. Cleavage
of the apoptosome causes activation of caspase
9, which leads to activation of caspases 3, 6, and
7. Activation of caspase 9 without involvement
of the apoptosome has been described [6].
After caspase 3 activation, rapid redistri-
bution occurs, and the anionic phospholip-
id phosphatidylserine is exposed on the cell
surface [7, 8] (Fig. 2). Phosphatidylserine is
normally restricted to the inner surface (in-
ner leaflet) of the lipid bilayer by an ATP-de-
pendent enzyme called flippase (translocase).
Flippase in concert with a second ATP-depen-
dent enzyme, floppase, which pumps cationic
phospholipids such as phosphatidylcholine
and sphingomyelin to the cell surface, main-
tains an asymmetric distribution of differ-
ent phospholipids between the inner and out-
er leaflets of the plasma membrane [9]. The
rapid redistribution across the cell membrane,
measured in minutes, is facilitated by calci-
um-dependent deactivation of flippase and ac-
tivation of a third enzyme, called scramblase.
The net result of apoptosis is the order-
ly breakdown of a cell by compression and
self-packaging of cellular proteins, including
cytoskeleton and nuclear matrix (chromatin
clumping and condensation that can be seen
with bright light or fluorescent microscopy).
Other Signaling Pathways That Can
Induce Cell Death
The endoplasmic reticulum can trigger
apoptosis (endoplasmic reticular stress-
induced cell death) [10]. Normally, the endo-
plasmic reticulum is the site of protein synthe-
sis, conformational maturation, and quality
control for correctly folded proteins. Proteins
that do not adopt a stable conformation are dis-
located into the cytosol, where they are tar-
geted for ubiquitinylation (a tag to identify a
protein for elimination) and proteosomal deg-
radation. Certain conditions and drugs can
cause abnormal accumulation of unfolded pro-
teins, resulting in endoplasmic reticular stress.
During endoplasmic reticular stress, cells can
reachieve homeostasis by initiating a series of
orchestrated events known as the unfolded pro-
tein response. If this response is unsuccessful,
endoplasmic reticular stress can directly initi-
ate a specific ubiquitin E3 ligase that tags an-
tiapoptotic proteins (Bcl-2 and others) with
ubiquitin. The proteosome degrades these an-
tiapoptotic molecules, tipping the balance be-
tween proapoptotic and antiapoptotic factors.
Fig. 1— Ext rinsic and in trinsic p athways o f apoptot ic cell death . Diagram show s component s of extr insic
pathway (red ); in trinsic , or mitochon drial, pat hway (blue); an d common set of e vents in apo ptosis (green ).
Ex trinsic pa thway is in itiated b y binding of dea th molecule s such as tumor nec rosis fact or (TNF -related
apopt osis inducin g ligand [T RAIL] ), Fas ligand ( FasL) to s pecific recep tors on cell s urface tha t initiat e events .
Ex trinsic pa thway via c aspase 8 can also ac tivate in trinsic pa thway by ac tivatio n of apopto tic protei n Bcl
inte racting dom ain (BID). In trinsic pa thway cent ers of acti vation of ap optotic p roteins BI D and Bcl-2– associat ed
X (BA X) and ot her protein s, such as thos e from auto phagy or mit otic catas trophe, t hat permeab ilize oute r
mit ochondrial me mbrane releas ing cyto chrome c into cyto plasm. Cy tochrome c f orms apopt osome wi th other
fact ors and caspase 9 and acti vates commo n execution p athway th at it shares w ith ext rinsic pat hway thro ugh
acti vation of la st caspase, cas pase 3. Casp ase 3 then acti vates pol y-ADP- ribose pol ymerase (PA RP-1) and
slices D NA. Mass ive damage to DN A by free rad ical DNA-da maging agent s such as oxidat ive str ess, radiat ion,
and doxo rubicin and cis platin the rapy, can induce mas sive acti vation of PA RP-1, which can k ill cell by PA RP-1–
media ted cell deat h (black). DIS C = death-i nducing signal ing complex , TRA IL = TNF-r elated apop tosis-i nducing
ligand , TRAD D = TNF recept or–asso ciated deat h domain, FA DD = Fas-ass ociated death domai n, MOMP =
mitochondrial outer membrane permeabilization, ATP = adenosine triphosphate, ADP = adenosine diphosphate,
TUNEL = terminal deoxynucleotidylt ransferase–mediated deox yuridine triphosphate nick-end labeling.
310 AJR:19 7, August 2011
Blankenberg and Norfray
Autophagy and Its Relation to
Apoptotic Cell Death
Apoptotic cells externalize phosphati-
dylserine, but other forms of cell death can
have this feature, including necrosis-onco-
sis, mitotic catastrophe, cell senescence,
pyroptosis, PARP-1–mediated cell death,
and autophagy [11]. Autophagy, or self-eat-
ing, is a highly regulated form of cell death
that has considerable overlap with apoptosis
[12]. Unlike apoptosis, autophagy normally
serves a housekeeping function by removing
unneeded, senescent, and damaged cytoplas-
mic contents as opposed to the cell itself. It
also enables a cell to survive periods of cel-
lular famine through autodigestion of intra-
cellular DNA and RNA, proteins, and lipids
into free nucleotides and amino and fatty ac-
ids, respectively. These free nucleotides and
amino and fatty acids can be reused by a cell
to maintain vital functions, such as macro-
molecular synthesis and energy production.
Autophagic cell death, however, can be an al-
ternative to apoptosis if the classic apoptotic
mechanisms are damaged or inhibited. Mas-
sive induction of autophagy also can occur to
the extent that a cell can eat itself to death.
The hallmark of autophagy is formation of
isolation membranes that engulf targeted cy-
toplasmic material, or organelles. The result
is double-membraned vesicles called autopha-
gosomes (autophagic vacuoles) [13]. Autopha-
gosomes then undergo maturation by fusion
with lysosomes to produce autolysosomes. In
an autolysosome, the inner membrane and lu-
minal contents of the autophagic vacuole are
degraded without inciting inflammation.
PolyAdenosine DiphosphateRibose
Polymerase 1Mediated Cell Death
PARP-1 normally functions as a DNA
damage sensor, and its activation repairs low
levels of DNA damage [14]. With high levels
of DNA damage, however, massive activation
of PARP-1 induces cell death by consuming
all available stores of oxidized nicotinamide
adenine dinucleotide (NAD+), its primary
substrate, forming large amounts of free ad-
enosine diphosphate (ADP) ribose, which can
directly inhibit protein translation (discussed
later). Because NAD+ can be regenerated only
by cleavage of ATP, the cell runs out of energy
and dies. Once the cell can no longer maintain
its ATP-dependent membrane functions, the
outer membrane becomes freely permeable to
all sizes of molecules, and necrotic or oncotic
cell death occurs.
Radiotracers and Modalities for the
Imaging of Apoptosis
Phosphatidylserine-Binding Agents:
Radiolabeled Annexin V
Induction of apoptosis is believed to be the
major mechanism through which most anti-
cancer treatments reduce or elim inate tumors
in humans. Selective exposure of phosphati-
dylserine on the cell surface therefore repre-
sents an attractive target for the imaging of
apoptosis in the tumors of patients undergo-
ing anticancer treatment. The first phosphati-
dylserine-binding agent to be used extensive-
ly in humans was the radiolabeled protein
known as annexin V. Annexin V is a ubiqui-
tous intracellular human protein (molecular
weight, ~36,000; 319 amino-acid residues)
with a nanomolar affinity for membrane-
bound phosphatidylserine [15]. The func-
tions of annexin V, despite its widespread in-
tracellular distribution, are unknown, but a
variety of in vitro and in vivo properties have
been described [16, 17].
The first oncologic imaging trials of annex-
in V were conducted with recombinant human
annexin V labeled with 99mTc by the penthioate
radioligand (N2S2) method in patients with pri-
mary and metastatic lung tumors [18]. Despite
the labor required for this radiolabeling meth-
od and nonspecific excretion of radiolabeled li-
gand into bile, Belhocine et al. [19] found that
patients with no change in radiotracer uptake
after the first dose of therapy had no subse-
quent objective clinical response. All patients
with a subsequent objective tumor response had
increased tumor uptake after initial treatment
compared with baseline scans. Five patients
(one with non-Hodgkin lymphoma, one with
Hodgkin lymphoma, one with small cell lung
cancer [SCLC], and two with non–small cell
lung cancer [NSCLC]) had increased annexin
V uptake 40–48 hours after chemotherapy, and
two patients had increased uptake 20–24 hours
after treatment (one with NSCLC, one with
SCLC). These results suggested marked vari-
ability in optimal timing with regard to tumor
type and therapeutic regimen [20].
Fig. 2 —Changes i n cell membrane c onfigurat ion with
apopt osis. Diagr am shows cell sp ends much res ting
energ y to maintai n normal asymmetr y of plasma
membrane, restricting anionic phospholipids such
as phosphatidylserine (PS) (orange spheres) to inner
membrane leaflet. Of all membrane phospholipids, PS
is mos t highly regu lated (by aden osine triphospha te
[ATP]-req uiring enz yme known as flippas e [orange
pore ]), mo re than 99 9 of 1000 P S molecules be ing
acti vely rest ricted wi th inner lea flet of lipid b ilayer.
With apoptosis, flippase and ATP-requiring floppase
(blue pore) are s hut off, an d third enz yme, scrambl ase
(gray pore), is ac tivated . Unlike flipp ase and floppase,
scramb lase does not re quire ATP, and it allo ws even
redistribution of PS and phosphatidyle thanolamine
(PE) (orange spheres), phosphatidylcholine (PC), and
sphingomyelin (blue spheres) across lipid bilayer. This
rapid (m inutes) redistr ibution w ith expo sure of PS on
cell sur face is signal t hat ident ifies cell as apoptot ic.
Redistribution of phospholipids assists in formation
of memb rane blebs and ultima tely apoptotic b odies.
Also o f note is rele ase of 1H histo ne protein as
DNA is s pliced by caspas e 3. Free 1H histones are
translocated to cell su rface by as of y et undescrib ed
AJR:19 7, August 2011 311
Molecular Imaging of Apoptosis
An improved labeling method based on
the bifunctional agent hydrazinonicotinamide
was selected for further clinical trials [21, 22].
Numerous studies have confirmed the poten-
tial clinical utility of hydrazinonicotinamide–
annexin V in determining the efficacy of che-
motherapy [23–26]. Kartachova et al. [27]
found that the degree of tumor response to
platinum-based chemotherapy in patients with
NSCLC directly correlated with the percent-
age increase in annexin V tumor uptake (com-
pared with pretreatment uptake) 48 hours after
the first injection of cisplatin. A less success-
ful treatment response (stable disease) was as-
sociated with slightly increased, unchanged,
or even slightly decreased annexin V tumor
uptake (r2 = 0.86, p < 0.001). In contrast, pa-
tients with progressive disease had a marked
decrease in annexin V tumor uptake. Karta-
chova et al. [28] also systematically examined
both quantitative and visual assessments of
annexin V uptake after treatment and found
excellent correlation with therapeutic outcome
according to the Response Evaluation Criteria
in Solid Tumors (r = 0.99, p < 0.0001; r = 0.9 7,
p < 0.0001) with excellent intraobserver repro-
ducibility (κ = 0.82 to κ = 0.90) and interob-
server variability of 0.82. The results of that
study suggest that chemotherapy-induced in-
creases in annexin V uptake seen at SPECT
can be consistently and routinely applied to
the early (24–48 hours) assessment of effica-
cy of therapy for NSCLC. Another study [29]
showed that uptake of annexin V in normal
tissues, such as spleen, bone marrow, kidneys,
liver, and the whole body, is not significantly
changed with chemotherapy or previous ad-
ministration of radiolabeled annexin V within
a 48-hour period.
Mutant Forms of Radiolabeled Annexin V for
Radionuclide Imaging
Alternatives to radiolabeled annexin V in-
clude the self-chelating annexin V mutants
V-117 [30] and V-128 [31, 32]. These pro-
teins have an endogenous site for 99mTc che -
lation consisting of a six-amino-acid tag add-
ed at the N-terminus followed by amino acids
1–320 of wild-type annexin V; the amino acid
cystine 316 is also mutated to serine. The pu-
rified protein is reduced to prevent formation
of disulfide bonds at the single cysteine resi-
due near the N terminal and stored for later
labeling with 99mTc and glucoheptonate as an
exchange reagent. Technetium-99m chelation
is thought to occur by formation of an N3S
structure involving the N-terminal cysteine
and the immediately adjacent amino acids.
Annexins V-117 and V-128 both have ma-
jor advantages over hydrazinonicotinamide–
annexin V, including 50–75% decreased
renal uptake of radiotracer and a marked-
ly improved specific localization to sites of
apoptosis in animal models [33]. Two com-
panies in partnership are developing a kit for
the preparation of 99mTc–recombinant hu-
man annexin V-128. This agent is expected
to be tested in human studies soon and to be
made available for investigator-sponsored
preclinical and clinical studies.
Compared with SPECT, PET has ma-
jor advantages for quantitative imaging that
have spurred the development of several ap-
proaches to labeling a nnexin V with 18F-FDG
[34]. In one method, N-succinimidyl-4-fluo-
robenzoate is used to synthesize 18F–annexin
V. The fluorine-labeled agent has less uptake
in the liver, spleen, and kidney than does hy-
drazinonicotinamide–annexin V. Another
method involves site-specific derivatization
with an 18F-maleimide–labeled compound to
mutant annexin V-117 or annexin V-128 [35].
Both of these methods need more preclinical
study before further development for imag-
ing of apoptosis.
FDG PET of Apoptosis
FDG PET has not been systematically
compared with annexin V SPECT in clinical
trials. Studies of tumor models have shown
an enhanced apoptotic reaction (increased ra-
diolabeled annexin V uptake) that correlat-
ed with suppressed tumor glucose utilization
(decreased FDG uptake) 48 hours after the
start of cytotoxic chemotherapy [36]. In a tri-
al in which 45 patients with breast cancer re-
ceived three cycles of neoadjuvant chemother-
apy before and after surgical removal of the
primary tumor, FDG PET scans showed sig-
nificant decreases in radiotracer uptake cou-
pled with marked increases in TUNEL-pos-
itive (apoptotic) tumor cells [37]. These data
Fig. 3 —Major biochemical events behind protein
translation and its inhibition with apoptosis.
Char t shows pros ynthes is pathway s, factor s, and
enzy mes (green ) and apoptosis and protein inhibition
path ways, fact ors, and enz ymes (brown). Arrows
repre sent activation of fact ors or enzy mes. In normal
syn thesis mode , mammalian tar get of rapamycin
(mTOR) activity phosphorylat es 4E-binding proteins
(4E-BPs) preventing their inhibitory binding to
eukar yotic ini tiation f actor 4E (eI F4E) (me ssenger
RNA [m RNA] cap BP cr itical for g rabbing of mR NA
in proce ss known as ci rcularization of m RNA). W ith
apoptosis, however, constitutive phosphor ylation
of tuberous sclerosis complexe s (TSC2 , TSC1) by
phosphatidylinositol 3-kinase (PIK3)/Akt signaling
path way is lost , and complexe s are free to fo rm
inhibitory complex that binds to and inhibits mTOR.
Loss of mTOR phosphorylation releases 4E-BPs,
which bind and inhibit eIF4E. Pancreatic reticulum
kinase–like endoplasmic reticulum kinase (PERK) is
acti vated by caspa ses 3 and 7 during apopto sis that
phosp horylat es eIF2, effectiv ely trappi ng it in eIF2 B/
guanosine 5’-diphosphate/eIF2 complex incapable
of par ticipati ng in protei n synthesis. Caspases 3
and 7 also cleave eIF 4G major sca ffoldin g protein
on which prote in-init iating pro teins asse mble to
form 8 0S ribos ome. GTP = gu anosine tri phosphate ,
Met-tRN A= methiony l transfe r RNA init iator. P = free
inorganic phosphate.
312 AJR:197, August 2011
Blankenberg and Norfray
suggest that neoadjuvant chemotherapy may
effectively induce apoptosis in breast tumors
and decrease glucose uptake, possibly by inhi-
bition of protein translation (discussed later).
Decreases in FDG uptake have been con-
firmed in several studies, including a study
of human gastric tumor cells treated with
epirubicin, cisplatin, and 5-fluorouracil [38];
a study of effective anticancer therapy with
the selective tyrosine kinase inhibitor ima-
tinib mesylate (STI571, Gleevec, Novartis)
in the care of patients with gastrointestinal
stromal tumors [39]; and study of epider-
mal growth factor receptor kinase inhibi-
tion of NSCLC with gefitinib [40]. Howev-
er, because apoptosis consumes energy, at
least initially, glucose demand can increase
temporarily in some clinical situations [41].
One example appears to be the metabolic
flare often observed on FDG PET images af-
ter hormonal therapy for estrogen receptor–
positive human breast cancer [42]. The met-
abolic flare in this select patient population
may be a useful indicator of responsiveness
to antiestrogen therapy as opposed to non–
hormone-based anticancer treatments.
In summary, although apoptosis is an en-
ergy-requiring process, it appears that except
in hormonal therapy for breast cancer, short-
term tumor response is directly correlated
with a decrease in FDG uptake.
Other Phosphatidylserine-Binding and Related
Agents in Preclinical and Early Clinical Trials
Several peptides and proteins have been
found that can recognize membrane-bound
phosphatidylserine [43]. These include the
C2A domain of synaptotagmin I. C2A binds
to negatively charged phospholipids, includ-
ing phosphatidylserine, in membranes in a
calcium-dependent manner. C2A and its mu-
tants have been labeled with 99mTc via 2-imi-
nothiolane thiolation of a fusion protein (mo-
lecular weight, ~39,000) [44] for SPECT and
with iron particles (superparamagnetic iron
oxide) [45, 46] and Gd3+ [47] for MRI. Al-
though C2A complexes have yet to be test-
ed in humans, the significantly higher disas-
sociation constant of the latest form of these
tracers for phosphatidylserine of 55.4–71.0
nM (wild-type C2A, ~20 nM) compared
with various types of derivatized annexin V,
including hydrazinonicotinamide (1–2 nM),
are a cause for concern [48].
Other novel approaches include the small-
molecule zinc-dipicolylamine coordination
complex [49] and cationic liposomes [50].
One report [51] describes a 12-residue phos-
phatidylserine-binding peptide discovered by
screening of a phage library of random pep-
tides. The molecular basis of interaction be-
tween the identified peptide CLSYYPSYC
and the phosphatidylserine molecule has yet to
be elucidated. Alternative approaches to imag-
ing phosphatidylserine exposure are possible
[52], but the considerable experience acquired
with radiolabeled annexin V in both humans
and animal models of apoptosis is lacking.
The same group of investigators also identi-
fied a novel probe ApoPep-1, a six-amino-ac-
id CQRPPR peptide [53]. ApoPep-1 targeted
apoptotic cells in tumor tissue and in culture as
seen with fluorescent imaging in vitro and in
vivo by binding to 1H histone exposed on the
surface of apoptotic cells. ApoPep-1 also rec-
ognized necrotic cells by entering the cells and
binding to 1H histone in the nucleus of necrotic
cells. Results of preliminary imaging experi-
ments with 124I–ApoPep-1 in treated murine
tumors are encouraging.
Imaging of Caspase 3 Activity
Imaging the activity of the caspase cascade
may be quite informative because most forms
of apoptosis converge on the terminal execu-
tion enzyme, caspase 3 [54, 55]. The back-
bone of these tracers is based on the 5-pyr-
rolidinylsulfonyl isatin class of nonpeptidyl
caspase inhibitors. The dicarbonyl function of
isatins covalently binds to the cysteine residue
of the active site of a given caspase. Although
this work is promising, Rosado et al. [56] and
Spires-Jones et al. [57] found that caspase 3 ac-
tivation is not necessarily unique to apoptosis.
Those authors found that caspase 3 is required
for full activation of a number of physiolog-
ic cellular functions, including aggregation of
platelets and secretion of enzymes from pan-
creatic acinar cells. The early physiologic ac-
tivation of caspase 3 (though at a lower level
than associated with apoptosis) was found to
be independent of cytosolic calcium ion levels,
mitochondrial cytochrome c release, and sub-
sequent activation of caspase 9 and phosphati-
dylserine exposure.
Additional technical problems are the
need to generate isatin sulfonamides that
have higher metabolic stability and more
moderate lipophilicity while retaining se-
lectivity and affinity for caspases 3 and 7
[58]. To address these issues, a 2′-fluoroeth-
yl-1,2,3-triazole has been identified that has
subnanomolar affinity for caspase 3 and with
click labeling has relatively high labeling
efficiencies and high stability in vivo with
rapid uptake and elimination in healthy tis-
sues and in tumors. More preclinical work is
needed with 18 F-labeled isatin analogues to
determine their potential as radiotracers for
imaging apoptosis, particularly because of
the small increases (twofold) in radiotracer
uptake in models of tumor apoptosis with the
latest 18F-labeled isatin 18F–isoprenylcyste-
ine carboxyl methyltransferase 11 (ICMT-11)
labeled by click radiochemistry [59]. Other
isatin analogues may exhibit improved local-
ization to sites of apoptosis in vivo [60, 61]
but have not been studied in tumor models.
Uncategorized Radiotracers for the Imaging of
Another class of apoptosis-imaging radio-
tracers is the ApoSense family of ra-
diopharmaceuticals [62–65], such as
N,N- didansyl-l-cystine (DDC) and
α-ethyl-fluoroalanine (NST-732). These
small molecules (molecular weight, 300
700 depending on the formulation) have an
amphipathic structure, having both specific
hydrophobic and charged moieties. The pub-
lished doses of these agents on a molar ba-
sis are 100- to 1000-fold as high as those for
other classes of agents. The most recently de-
veloped of these radiotracers, butyl-2-meth-
yl-malonic acid (ML-9; molecular weight,
173), has been applied to the study of apop-
totic tumor cells in vitro and in vivo [66].
Despite improvement in tumor uptake in vi-
tro and in vivo, the absolute uptake values,
even in tumors treated with extremely high
doses of chemotherapy (median lethal dose
of intraperitoneal doxorubicin, 19.6 mg/kg
[67]; carmustine [BCNU], 30 mg/kg [68,
69]; 5-fluorouracil, 200 mg/kg [70]) spaced
48 hours apart, remain well below 1.5% in-
jected dose/g and are seen only after observ-
able decreases in tumor weight compared
with control values. Supranormal doses of
chemotherapeutic agents are known to in-
duce necrosis rather than apoptosis, raising
the question of the mechanisms of radiotrac-
er localization in this study. Results on in
vivo tumor imaging with 18F and other iso-
topes have yet to be published.
Assessment of Mobile Lipids and Choline With
Proton MR Spectroscopy in the Imaging of
The initial reports of studies of in vitro
water-suppressed lipid 1H MRS described
apoptosis-specific changes, including a se-
lective increase in –CH2– (methylene) rel-
ative to –CH3 (methyl) mobile lipid proton
AJR:19 7, August 2011 313
Molecular Imaging of Apoptosis
signal intensities at 1.3 and 0.9 ppm [71–
73]. The increase in –CH2– resonance oc-
curred with a wide range of apoptotic drugs
and serum growth factor deprivation. In con-
trast, necrosis was characterized by a com-
pletely different 1H MRS profile, in which
the concentrations of most of the metabo-
lites examined increased significantly. The
exception was CH2 mobile lipids, which re-
mained unchanged, coupled with a decrease
in reduced glutathione. The ratio of CH2 to
CH3 signal intensity had a strong linear and
temporal correlation with other markers of
apoptosis, including fluorescent annexin V
cytometry and DNA ladder formation. The
increase in –CH2– resonance signal intensi-
ty observed with apoptosis was not accompa-
nied by changes in total lipid composition or
synthesis. This finding suggested an increase
in membrane lipid mobility or organization.
Subsequent studies showed that the source
of mobile lipid resonance is the osmophil-
ic lipid droplets (0.2–2.0 µm) observed in
the cytoplasm of apoptotic cells [74]. These
droplets contain variable amounts of polyun-
saturated fatty acids associated largely with
18:1 and 18:2 lipid moieties and an accumu-
lation of triacylglycerides generated by apop-
tosis-induced phospholipase A2 activity and
ceramide (a regulatory molecule in mediat-
ing membrane-related apoptotic events with
a long –CH2– chain) produced from apop-
tosis-mediated hydrolysis of sphingomyelin
by the membrane enzyme sphingomyelinase.
Results of more recent studies have
suggested normal ization of –CH2– resona nce
to the average intensity of a broad region of
the spectrum 1.6–4.7 ppm to avoid the risk
of an increase in –CH3 (or any other single
resonance peak) that would preclude the use
of the CH2 to CH3 ratio to quantify apoptosis
[75]. By this normalization approach, it
is possible to detect a fourfold to fivefold
increase in –CH2– signal intensity in breast
carcinoma cell lines undergoing apoptosis in
response to docetaxel that is not affected by
the liposomal delivery of the drug in culture.
Significant decreases in choline (including
choline, choline-containing compounds, and
phospholipids) resonance 6–12 hours after
the increase in mobile lipid signal intensity
in vitro have been found with 1H MRS [73].
Lindskog [76] proposed leveraging the polar
opposite changes of mobile lipid and choline
that occur with apoptosis by calculation of
the ratio of –CH2– to choline signal intensity
for the quantification of programmed cell
death in murine models and in children with
neuroblastoma undergoing chemotherapy.
The ratio of –CH2– to choline signal
intensity may therefore be a useful method
of increasing the dynamic range of 1H MRS
measurement of apoptosis.
The biology behind the empiric observa-
tion of decreases in choline with 1H MRS
was not well understood until Clemens et al.
[77] and Morley et al. [78] described the in-
verse relation between protein translation and
apoptosis. Those investigators found that the
initiation of apoptosis is accompanied by an
abrupt halt in global protein synthesis, includ-
ing that of choline and choline-containing lip-
ids and other compounds. The interruption of
global protein synthesis is therefore directly
related to decreases in choline resonance ob-
served with 1H MRS. Results of numerous in-
vestigations have shown that choline signal
intensity rapidly declines (in as little as 12–
24 hours) with effective therapy in a variety
of clinical settings [79–84] and animal mod-
els [85–88]. Furthermore, 1H MRS for the
assessment of changes in choline signal in-
tensity in response to therapy has been suc-
cessfully performed with clinical 1.5- and 3-T
MRI units and coils not only on the brain but
also on the breast, lower extremities, and liv-
er. Measurement of radiation-induced apopto-
sis of cervical carcinoma with 1H MRS also is
possible with clinical MRI units and standard
endovaginal coils [89, 90].
The events behind the sudden cessation of
global protein translation are biochemical-
ly complex but can be narrowed to several
key events (Fig. 3): inhibition of eukaryotic
initiation factor 2 (eIF2), proteolysis of the
initiation factors eIF4GI and eIF4GII, acti-
vation of endoplasmic reticulum transmem-
brane stress sensors, and inactivation of the
progrowth mammalian target of rapamycin
(mTOR) pat hway.
Movement of capped messenger RNA
chains through the ribosomes during pro-
tein translation requires eIF2. This factor is
also extremely sensitive to phosphorylation
by activated pancreatic reticulum kinase–
like endoplasmic reticulum kinase (PERK),
an enzyme that inhibits eIF2 binding to the
eIF2B guanosine 5’-diphosphate–guano-
sine triphosphate (GDP-GTP) exchange en-
zyme. Without recharging of eIF2-GDP
back to eIF2-GTP by eIF2B, translation of
most proteins ceases. Nonenzymatic ADP
ribosylation also can inactivate eIF2—a re-
sult of the presence of large amounts of free
ADP ribose produced by PARP-1, a DNA re-
pair enzyme directly activated by caspase 3
during apoptosis. Caspase 3 can also cleave
the protein translational initiation factors eI-
F4GI, eIF4GII, eIF2, and the eIF3-p35 sub-
unit and thereby inhibit protein synthesis.
The endoplasmic reticulum transmem-
brane stress sensors, including PERK, ac-
tivating transcription factor 6 (ATF6), and
inositol-requiring enzyme 1 (IRE1) can be
activated by radiation, oxidative stress, and
chemotherapeutic drugs. Normally these
sensors are part of a prosurvival mechanism
that removes damaged or misfolded proteins
by ubiquitinylation and proteasome degra-
dation and reduces global protein synthesis
as part of the unfolded protein response un-
til homeostasis of the cell is achieved. How-
ever, under prolonged endoplasmic reticular
stress, as with chemotherapy or radiation,
these sensors induce apoptosis by activation
of the transcription factor CCAAT enhanc-
er-binding protein (C/EBP) homologous pro-
tein (CHOP). CHOP alters the balance of
synthesis from the prosurvival to the pro-
apoptotic members of the Bcl-2 family of
proteins, promoting apoptosis through the
mitochondrial (intrinsic) pathway.
The upstream enzymes phosphatidylinosi-
tol 3-kinase (PI3K) and Akt activate mTOR
in concert with a complex set of signaling
events that recognize growth factors, mito-
gens, hormones, amino acids, and glucose.
Binding of these molecules to their unique
receptors on the cell surface normally stim-
ulates protein synthesis. Signaling through
mTOR causes changes in the phosphory-
lation status of the eIF4E-binding proteins
(4E-BPs). In the hypophosphorylated state
these proteins sequester eIF4E (the initiation
factor that binds to the messenger RNA cap),
reduce its bioavailability, and reduce protein
synthesis. With phosphorylation, release of
eIF4E from 4E-BPs allows eIF4E to interact
with the scaffolding protein eIF4G, increas-
ing formation of the eIF4F complex that ini-
tiates protein translation. With apoptosis the
activity of Akt is reduced and the normal
Akt substrates tuberous sclerosis complex 2
(TSC2)/tuberin and TSC1/harmatin (prod-
ucts of the tuberous sclerosis oncogene) are
no longer phosphorylated and are free to as-
sociate and inhibit mTOR and its effects on
protein synthesis as part of their normal tu-
mor-suppressive functions.
Diffusion-Weighted MRI of Solid Tumors
Before and After Therapy
Diffusion-weighted MRI (DWI) may be
a useful biomarker of therapeutic efficacy in
314 AJR:197, August 2011
Blankenberg and Norfray
patients with cancer [91–94]. DWI relies on
tagging of water molecules in a given imag-
ing voxel with a strong electromagnetic pulse
and observation of the tagged water mol-
ecules over a short time (i.e., 50 ms). DWI
then is used for indirect measurement of the
distance traveled by these water molecules
in this short time (usually on the order of 30
µm). On average, water molecules restrict-
ed by their local microenvironment (inside a
cell as opposed to in the extracellular space)
travel or diffuse less than water present in re-
gions of necrosis, inflammation, edema, and
cysts. The average distances traveled by wa-
ter molecules in a voxel are referred to as the
apparent diffusion coefficient (ADC).
Tumors in general have increased cellular
density and therefore lower ADC values than
normal tissues and benign tumors [95]. With
successful therapy these low ADC values
rapidly increase over several days and appear
to correspond to tumor cell loss and expan-
sion of the extracellular space in a tumor [96,
97]. With ADC quantification, the therapeu-
tic efficacy of the apoptosis induction strate-
gy has been detected as early as 3 days after
dosing, a time when tumor volume changes
are not yet apparent [98]. In the same study,
the investigators found that ADC increases
in a tumor were dose dependent and consis-
tent with histologic markers of apoptosis.
The mean ADC increase in tumors was lin-
early proportional to the mean apoptotic cell
density and was inversely proportional to the
mean proliferating cell density.
In summary, although DWI holds promise
as a noninvasive means of early detection of
markers of treatment efficacy that does not
require administration of a contrast agent,
changes in ADC are relatively small (usual-
ly < 50% of baseline value) and can be af-
fected by inflammation, blood flow, cardiac
and respiratory motion, and the presence of
necrosis. DWI also has to be standardized in
terms of acquisition parameters, tumor type,
specific therapy, and other factors before it
gains widespread clinical use in oncology.
In addition, unlike 1H MRS and detection of
phosphatidylserine-recognition agents and
caspase-activated radiotracers, there is no di-
rect link between DWI findings and an apop-
totic or protein translation pathway. This is
not the case for DWI assessment of cerebral
ischemia, in which there is a direct biochem-
ical connection between hypoxic injury and
decreased ATP production and swelling of
neurons that generates a restricted micro-
environment for the diffusion of water mol-
ecules. The issue of the timing of DWI af-
ter therapy has not been addressed and may
prove crucial, as it is for the assessment of
ischemic injury to the brain, in which there
is a narrow time window after an insult be-
fore normalization of ADC (or even achieve-
ment of subnormal status) and the onset of
necrosis that precludes the use of this pulse
sequence after several days [99].
High-Frequency Ultrasound, Optical
Coherence Tomography, and Fourier
Transform Midinfrared Imaging of
High-frequency ultrasound (10 MHz or
greater) has been used to detect the unique
specular reflections of apoptotic cells in vitro
and in vivo [100–102]. These specular reflec-
tions arise predominately from the condensa-
tion and fragmenting of the cell nucleus in the
later phases of apoptosis. Transducers operat-
ing at 10–60 MHz generate ultrasound wave-
lengths of 25–150 µm, approaching the size
of individual cells and nuclei (10–20 µm), and
therefore are sensitive to changes in cell size
and nuclear morphology that occur with apop-
tosis. The backscatter from apoptotic nuclei is
up to sixfold as great as that of nonapoptotic
cellular nuclei and occurs in cultures treated
with a variety of drugs and radiation.
The main limitation of high-frequency ul-
trasound is the poor penetration of the beam
in soft tissue (depth, 2–5 cm for 10- to 30-
MHz transducers). This inherent limitation
can be partly overcome by application of
high-frequency ultrasound backscatter analy-
ses to relatively superficial tumors of the skin
and breast or by use of endoscopic probes for
nasopharyngeal and gastrointestinal cancers.
Optic coherence tomography (OCT) is a
form of backscatter spectroscopy. Rather than
ultrasound, laser light with a central wave-
length of 1325 nm and −3-dB bandwidth of ap-
proximately 100 nm with axial resolution of 9
µm is used [103, 104]. As with ultrasound, a
variety of microscopic scattering and reflective
interfaces within a cell arise with apoptosis.
OCT can depict marked increases in integrat-
ed backscatter from cells undergoing apopto-
sis and mitotic arrest, whereas cells undergo-
ing necrosis exhibit a decrease in backscatter.
These changes appear to be linked to structur-
al changes observed at histologic examination
of the cell samples. These results indicate that
OCT integrated backscatter and first-order en-
velope statistics can be used to detect and pos-
sibly differentiate modes of cell death in vitro.
However, because of the poor soft-tissue pen-
etration by light of the wavelengths used for
OCT, application to the clinic will be limited
to intravascular, endoscopic, and ophthalmo-
logic (retinal) use in the near future.
Midinfrared region spectroscopy (~2.5–
15.5 µm wavelength, or ~4000–650 cm–1
wavenumber) is another nondestructive mi-
croscopic imaging technique (axial resolution
< 1 µm) that generates fingerprint-like spec-
tra of the characteristic vibrational frequen-
cies of various chemical bonds and, therefore,
functional groups [105]. That changes in hy-
drogen bond lengths and angles of as little as
0.01 Å and 1° can show clear differences in a
vibrational spectrum is a major advantage of
infrared spectromicroscopy over other vibra-
tional methods, such as Raman spectrosco-
py. The high real-time sensitivity of infrared
spectroscopy to the hydrogen bond structure
that links cellular water molecules to ions and
other small molecules of interest in apoptosis,
necrosis, and proliferation is made possible
by the following conditions: more than 70%
of the cell content is highly polar water mol-
ecules; the hydrogen bond responds instanta-
neously to ions and other species, such as rad-
icals, small organic acids, and hydrogen gas,
which are expected to be present during the
functional metabolism of the stress adaptive
response and apoptosis; and results of previ-
ous biochemical studies can guide the inter-
pretation of infrared spectra of OH stretch vi-
brations in the hydride-OH region that link
the variations to the presence of ions and oth-
er small molecules in liquid and other con-
densed phases.
Synchrotron radiation–based Fourier
transform midinfrared spectromicroscopy
allows the study of individual living cells
with a high signal-to-noise ratio that exploits
the continuous and intense midinfrared light
spectrum available from a cyclotron [106].
Clear changes can be observed in the spec-
tral regions corresponding to proteins, DNA,
and RNA as a cell changes from the G1
phase to the S phase and finally into mitosis.
These spectral changes include markers for
the changing secondary structure of proteins
in the cell and variations in DNA and RNA
content and packing as the cell cycle pro-
gresses. The dying or dead cell undergoes a
shift in the protein amide I and II bands cor-
responding to changing protein morphology
and oxidative state. A substantial increase
in the intensity of an ester carbonyl C= O
peak at 1743 cm-1 can be observed. For tis-
sues with intrinsic refractive (reflective) in-
terfaces, such as atherosclerotic vessels (lip-
AJR:19 7, August 2011 315
Molecular Imaging of Apoptosis
id, calcified plaques) and the retina (pigment
epithelium), it is possible to characterize the
features of apoptosis and other forms of cell
death [107]. As with OCT, tissue penetra-
tion of midinfrared light and the presence of
free water form a barrier to whole-body to-
mographic imaging. However, intravascular
and endoscopic catheter technology can be
used in theory to reduce penetration to a few
critical wavelengths of laser light close to a
region of interest such as superficial malig-
nant tumors, malignant tumors of the skin,
and tissues with little unbound water optical
scattering, such as the vitreous of the eye in
an effort at noninvasive characterization of
tumors of the retina.
Although remarkable progress has been
made in the development of radiotracers and
apoptosis-specific MRI agents, much work
is needed to bring any of these agents into
routine clinical use. The exception is radio-
labeled annexin V-128, which should enter
clinical trials in the near future. In the short
term, 1H MRS monitoring of changes in cho-
line signal intensity appears to be the most
direct path for imaging of tumor apoptosis
in response to therapy, even in regions and
organs outside the brain, including breast,
liver, extremities, and cervix. Decreases in
choline directly reflect the silencing of pro-
tein translation induced by apoptosis and au-
tophagy. Proton (1H) MRS can be readily
performed with existing clinical MRI units,
coils, and software without an IV contrast
agent, making it an attractive method for as-
sessing therapeutic response in clinical drug
trials, especially of neoadjuvant therapy.
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    • "To assess the apoptotic effect of MWCNTs-PEG following Laser treatment, we examined the translocation of phosphatidyl serine (PS) groups from the inner to the exterior surface of the membrane as marker of early apoptosis state induction 19. Using the high affinity of annexin to PS groups we were able to detect the apoptosis rate in all samples. "
    [Show abstract] [Hide abstract] ABSTRACT: Pancreatic cancer (PC) is one of the most lethal solid tumor in humans, with an overall 5-year survival rate of less than 5%. Thermally active carbon nanotubes have already brought to light promising results in PC research and treatment. We report here the construct of a nano-biosystem based on multi-walled carbon nanotubes and polyethylene glycol (PEG) molecules validated through AFM, UV-Vis and DLS. We next studied the photothermal effect of these PEG-ylated multi-walled carbon nanotubes (5, 10 and 50 μg/mL, respectively) on pancreatic cancer cells (PANC-1) and further analyzed the molecular and cellular events involved in cell death occurrence. Using cell proliferation, apoptosis, membrane polarization and oxidative stress assays for ELISA, fluorescence microscopy and flow cytometry we show here that hyperthermia following MWCNTs-PEG laser mediated treatment (808 nm, 2W) leads to mitochondrial membrane depolarization that activates the flux of free radicals within the cell and the oxidative state mediate cellular damage in PC cells via apoptotic pathway. Our results are of decisive importance especially in regard with the development of novel nano-biosystems capable to target mitochondria and to synergically act both as cytotoxic drug as well as thermally active agents in order to overcome one of the most common problem met in oncology, that of intrinsic resistance to chemotherapeutics.
    Full-text · Article · Sep 2014
    • "Apoptotic cells put signatures or biomarkers on their surface, such as phosphatidylserine and histone H1, that are little or absent on the surface of healthy cells151617. Apoptosis imaging probes such as annexin V and dipicoyl zinc amide that bind to phosphatidylserine have been exploited for monitoring tumor cell apoptosis in vivo151617. We have previously identified ApoPep-1 that recognized apoptotic and necrotic cells through binding to histone H1 on the surface of apoptotic cells and in the nucleus of necrotic cells, respectively [18]. "
    [Show abstract] [Hide abstract] ABSTRACT: Early decision on tumor response after anti-cancer treatment is still an unmet medical need. Here we investigated whether in vivo imaging of apoptosis using linear and cyclic (disulfide-bonded) form of ApoPep-1, a peptide that recognizes histone H1 exposed on apoptotic cells, at an early stage after treatment could predict tumor response to the treatment later. Treatment of stomach tumor cells with cistplatin or cetuximab alone induced apoptosis, while combination of cisplatin plus cetuximab more efficiently induced apoptosis, as detected by binding with linear and cyclic form of ApoPep-1. However, the differences between the single agent and combination treatment were more remarkable as detected with the cyclic form compared to the linear form. In tumor-bearing mice, apoptosis imaging was performed 1 week and 2 weeks after the initiation of treatment, while tumor volumes and weights were measured 3 weeks after the treatment. In vivo fluorescence imaging signals obtained by the uptake of ApoPep-1 to tumor was most remarkable in the group injected with cyclic form of ApoPep-1 at 1 week after combined treatment with cisplatin plus cetuximab. Correlation analysis revealed that imaging signals by cyclic ApoPep-1 at 1 week after treatment with cisplatin plus cetuximab in combination were most closely related with tumor volume changes (r2 = 0.934). These results demonstrate that in vivo apoptosis imaging using Apopep-1, especially cyclic ApoPep-1, is a sensitive and predictive tool for early decision on stomach tumor response after anti-cancer treatment.
    Full-text · Article · Jun 2014
  • [Show abstract] [Hide abstract] ABSTRACT: This article discusses possible roles for emerging novel positron emission tomography (PET) radiotracers for diagnosis and follow-up of lymphomas. Novel imaging probes are being developed to fulfill the need of a more specific radiopharmaceutical to target subcomponents of tumor microenvironment to individualize management approaches. Noninvasive molecular imaging probes are being developed to revolutionize characterization of tumor biology and response to therapy in more specific ways for the host, tumor microenvironment, and therapeutic regimens. The new clinical PET probes seem promising in fostering clinical gains that would lead to better survival outcomes, although further studies are warranted to prove a role in the management of lymphomas.
    Article · Jan 2012
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