Advances in gene-based medicine since the 1990s have ushered in a new therapeutic
strategy of gene therapy for inborn error genetic diseases and also for cancer [1–6].
Personalized treatment strategies using stem, modified or also genetically engineered
cells are becoming a reality in clinical medicine. Allogenic or autologous cells can
be used for treatment and possibly for early diagnosis of diseases. Hematopoietic,
stromal and organ specific stem cells are under evaluation for cell-based therapies,
not only for neurological, but also cardiac, pneumological, gynecological, autoimmune
and other disorders. Such cell replacement therapy and gene transfer have provided
the basis for the development of potentially powerful new therapeutic strategies for
a broad spectrum of neurological diseases. However, currently available treatment
modalities for brain tumors, including radical surgical resection followed by radiation
and chemotherapy, have substantially improved the survival rates [7, 8]; however,
a large proportion of patients with brain tumors remain incurable [9]. Therefore,
there is substantial need for more effective therapies for patients with malignant
brain tumors, and gene therapy targeting brain tumors should fulfill this requirement.
Gene therapy for brain tumors includes many therapeutic strategies and these strategies
can be subgrouped in two major categories: (i) molecular and (ii) immunologic. The
widely used molecular gene therapy approach is suicide gene therapy based on the conversion
of non-toxic prodrugs into active anticancer agents via introduction of enzymes, and
genetic immunotherapy involves the gene transfer of immune-stimulating cytokines including
interleukin (IL)-4, IL-12 and tumor necrosis factor related apoptosis inducing ligand
(TRAIL) [10, 11]. For both molecular and immune gene therapy, stem cells can be used
as a delivery vehicle of therapeutic genes [12]. Stem cells possess an inherent tumor
tropism and have the capacity to target therapeutic genes to tumors, which supports
their use as a reliable delivery vehicle to target therapeutic gene products to primary
brain tumors and metastatic cancers throughout the brain [13, 14]. The main promise
of this emerging technology centers on the potent migratory tropism exhibited by stem
cells for disseminated foci of intracranial pathologic findings. This important characteristic,
which has been validated in a wide set of preclinical studies, forms a foundation
for the use of transplanted stem cell populations as vehicles for the delivery of
tumor-toxic molecules to sites of intracranial tumor [9, 14]. The significance of
neuronal stem cell (NSC)-based gene therapy for brain tumor is that it is possible
to exploit the tumor-tropic property of stem cells to mediate effective, tumor-selective
therapy for primary and metastatic cancers in the brain and outside, for which no
tolerated curative treatments are currently available. Before widely using stem cells
in clinical trials, translational research in experimental animal models is needed,
with a critical emphasis on developing noninvasive methods for tracking the temporal
and spatial homing of these cells to target tissues.
The current development of novel treatment strategies for brain tumors based on the
transplantation or infusion of cells that can seek out invading tumor cells demands
thorough in vivo monitoring [7]. Molecular imaging may allow modification of treatment
to maximize treatment efficacy and minimize late effects on chemotherapy and radiotherapy.
Especially, stem cells attract great interest as they show a tropism to tumor cells
and will even migrate long distances to track down single tumor cells [15, 16]. It
is thought that the migration of stem cells to neoplastic cells is mediated by the
secretion of chemical factors, such as vascular endothelial growth factor (VEGF) [7,
17], that are involved in the proliferation, growth, and maintenance of tumors. Transplantation
of unaltered stem cells has resulted in a prolonged survival of animals with experimental
tumors [18], but the insertion of anti-tumor cytokine genes (e.g., IL-12) or proapoptotic
genes (e.g., TRAIL) further improves the efficiency of this approach [19]. Little
is known about how these cells exert their beneficial effects in vivo. Despite contrary
evidence from preclinical studies, there is some concern that transplantation of stem
cells could further exacerbate tumor formation as there is mounting evidence that
brain tumors are potentially caused by a single stem cell that did not differentiate
[20]. Targeting oncogenic pathways that are essential to the survival and growth of
brain tumor stem cells represents a promising area for developing therapeutics. However,
due to the multiple oncogenic pathways involved in brain tumors, it is necessary to
determine which pathways are the essential targets for therapy.
The identification of genetic and biochemical mechanisms underlying tumor growth and
progression along with the unraveling of the human genome provided a plethora of new
targets for cancer detection, treatment and monitoring. Simultaneously, the extraordinary
development of a number of imaging technologies, including hybrid systems, allowed
the visualization of biochemical, molecular and physiological aberrations linked to
underlying mutations in a given tumor. In vivo evaluation of complex biological processes
such as proliferation, apoptosis, angiogenesis, metastasis, gene expression, receptor-ligand
interactions, transport of substrates and metabolism of nutrients in human cancers
is feasible by molecular imaging using positron emission tomography/computed tomography
(PET/CT) and radiolabeled molecular probes. Some of these compounds are in preclinical
phases of evaluation whereas others have already been applied in clinical settings.
Molecular imaging of brain tumors provides a prominent example of how some biological
processes and target expression can be visualized by PET/CT in animal tumor models
and also in patients with various cancers – such as lung, breast or even brain tumors
– for the non-invasive detection of well-known markers of tumor aggressiveness, invasiveness
and resistance to treatment and for the evaluation of tumor response to therapy while
reducing side effects or even for evaluation of prognosis. Such ability to monitor
cell therapy in vivo is therefore desirable to potentially provide more control over
the activity of stem cells [21, 22]. This strategy is relatively new for brain tumors,
but is already well established for other cancers [23]. One possibility is that stem
cells will be engineered with a suicide gene [24] that could be activated if transplanted
cells do not behave in a therapeutic manner. However, the ability to control the activity
of transplanted cells presupposes that it is possible to visualize the presence, location,
and activity of stem cells in vivo. Although magnetic resonance imaging (MRI) has
been extensively used to track cells repeatedly in vivo [25], it is the higher specificity
of positron emission tomography (PET) ligands and its ability to detect reporter genes
that are attractive features to develop long-term in vivo monitoring of transplanted
cells [26]. This new “image and treat” strategy, involving assessment of target presence
and distribution in an individual patient followed by optimized, target-specific drug
delivery, may potentially improve efficacy of brain tumors while reducing side effects.
However, this strategy is a great step towards a personalized medicine.
The reporter gene approach has also been used with MRI to facilitate iron uptake [27,
28]. The contrast agents used to detect transplanted cells from the background of
the brain are incorporated into the cell in vitro before transplantation [22, 29].
The presence of the contrast agent inside the cell therefore allows the prolonged
visualization of grafted cells on MRI scans. However, the continued presence of the
contrast agent inside the cell can also affect cellular functions [25, 30]. Although
this approach has been demonstrated with PET/single photon emission computed tomography
(SPECT) ligands and allowed monitoring for up to 14 days [31], it is the short half-life
of PET ligands that compromises the long-term visualization of cells. A more promising
method is to engineer reporter genes inside the cells before grafting and to systemically
inject the PET ligands to detect transplanted cells [32]. The use of reporter genes
avoids most issues pertaining to the long-term effects the contrast agent might exert
on cellular functions. The repeated application of a contrast agent complements a
flexible imaging approach that can target various relevant targets, such as fludeoxyglucose
(FDG)-PET to investigate tumor metabolism [29, 33, 34]. Especially for the continued
assessment of tumor evaluation in response to treatment, it is important to be able
to assess the malignancy of the tumor noninvasively at various time points. The main
concerns with this approach regard the potential immunogenic properties of reporter
genes that might lead to graft rejection or the down-regulation of the reporter gene
abolishing graft detection. Unfortunately, at present little research is dedicated
to the development of PET techniques that would allow the continued assessment of
cell therapy. The use of radioligands to a reporter gene will provide maximum flexibility
and further integration into a wider molecular imaging strategy of brain tumors. Currently,
many studies utilize the combination of two or more reporter genes that would enable
the use of different imaging modalities to overcome the drawbacks associated with
a single reporter gene and/or associated detection system [35].
Apart from in vivo monitoring, PET will also be instrumental to gain a greater mechanistic
understanding of how cell therapy exerts its therapeutic effects. As stem cells can
be engineered to express particular genes and serve as Bsmart delivery vehicles [36],
their effects on tumor cells could be through inhibition of angiogenesis, induction
of apoptosis in tumor cells, or induction of differentiation of tumor cells [37, 38].
Being able to study how stem cells will alter the molecular composition of the tumor
will allow in vivo monitoring of the therapeutic efficacy of cell therapy. Additionally,
PET imaging allows the serial in vivo assessment of the inflammatory and immunological
response by visualizing, for instance, a T-cell response [39]. Modulation of the immune
response could provide additional benefit to combat tumor cells. The PET imaging is
at present the only in vivo technology that can assess these various molecular aspects
of stem cell efficacy and lead to a mechanistic understanding of how stem cells attack
tumor cells.
Molecular imaging by PET is already used clinically to evaluate stem cell therapy
for different neurological diseases besides the classical brain tumors [40]. Interesting
in this context are patients with primary central nerve syndrome (CNS) lymphoma under
the age of 60 years: high dose chemotherapy with autologous stem cell therapy represents
one of the first-line treatment options in this subpopulation [41]. Additionally,
simple FDG-PET leads to a 92% positive and 88% negative prediction of treatment outcome
and correlates more strongly with disease-free survival in cerebral lymphoma than
computed tomography [42]. The FDG-PET can therefore already serve an exquisite role
in patient selection and help to determine which conditions are most suitable for
stem cell therapy [43, 44]. Serial quantitative PET will provide a means to monitor
treatment progression [45] and afford a refinement of dosage and integration with
other therapies.
Imaging of brain tumors already commonly uses FDG-PET. Translation of cell therapy
for brain tumors will be best suited in conditions where imaging technology can already
provide a robust baseline assessment that would allow patient selection. As the clinical
translation of stem cell therapy for brain tumors progresses [45], preclinical validation
studies will need to rely more on molecular imaging to ensure the safety and efficacy
of this promising therapy. Growing sophistication of molecular imaging will allow
increasingly sophisticated therapeutic approaches to be envisaged and monitored in
vivo [46–49]. The multitude of therapeutic effects that can be exerted by unaltered
and genetically engineered stem cells will need to be determined in terms of their
efficacy in relation to the molecular composition of the tumor. Being able to select
patients at an early stage based on a molecular signature of the entire tumor will
allow clinicians to determine what strategy might be best suited to remove the neoplastic
cells.
With increasing focus on the advance towards (more) curative solutions in brain tumor
therapies, it is hard not to be excited by the potential of stem cell-based therapies.
However, the efficacy of stem cell therapy continues to remain in question. Initial
clinical trials have focused on evaluation of multiple adult stem cell phenotypes
in their unaltered, native state as a “first generation” resource for repair. Though
significant strides in perfecting delivery of these biologics to the diseased brain
have been achieved, the benefits with regard to cerebral functional recovery have
been modest at best. There is a clear need for robust genetic characterization before
widely using the above described clinical stem cell therapy approaches. We have to
be certain that the genome of the starting material is stable and normal, but the
limited resolution of conventional karyotyping is unable to give us such assurance.
Advanced molecular cytogenetic technologies and single nucleotide polymorphism analysis
should be introduced [50]. As seen with pharmacotherapeutics in the last century,
successful translation of “second generation” biotherapeutics in the 21st century
will require close integration of a community of practice and research to ensure broad
application of this emerging technology in the treatment of brain tumors.
Developments in molecular imaging will therefore provide the diagnostic framework
of a molecular medicine of brain tumors [12]. This is of special importance as, despite
exciting initial reports, clinical potency of stem cell therapy in animal brain tumor
models has to date proven disappointing. Attempts to extrapolate the animal study
results to humans are stymied by the fact that stem cells are heterogeneous, resulting
in differences in their efficacy. Indeed, therapeutic success relies on an effective
strategy to select for a stem cell sub-population within some particular stage of
the development at which they are competitive and capable of targeting brain tumors.
Molecular imaging may help to better understand the specific tropic mechanisms that
govern stem cell migration toward invasive tumors and the need to identify appropriate
tissue sources and culture processes for the generation of adequate therapeutic stem
cell populations. Despite all these current limitations, the use of stem cells as
vectors for the treatment of brain tumors holds significant promise and may prove
to be an important therapeutic modality for patients with malignant brain tumors.
In combination with stem cells, molecular imaging may help to revolutionize a personalized,
molecularly targeted medicine.