Biology-guided radiotherapy: redefining the role of radiotherapy in metastatic cancer

The emerging biological understanding of metastatic cancer and proof-of-concept clinical trials suggest that debulking all gross disease holds great promise for improving patient outcomes. However, ablation of multiple targets with conventional external beam radiotherapy systems is burdensome, which limits investigation and utilization of complete metastatic ablation in the majority of patients with advanced disease. To overcome this logistical hurdle, technical innovation is necessary. Biology-guided radiotherapy (BgRT) is a new external beam radiotherapy delivery modality combining positron emission tomography-computed tomography (PET-CT) with a 6 MV linear accelerator. The key innovation is continuous response of the linear accelerator to outgoing tumor PET emissions with beamlets of radiotherapy at subsecond latency. This allows the deposited dose to track tumors in real time. Multiple new hardware and algorithmic advances further facilitate this low-latency feedback process. By transforming tumors into their own fiducials after intravenous injection of a radiotracer, BgRT has the potential to enable complete metastatic ablation in a manner efficient for a single patient and scalable to entire populations with metastatic disease. Future trends may further enhance the utility of BgRT in the clinic as this technology dovetails with other innovations in radiotherapy, including novel dose painting and fractionation schemes, radiomics, and new radiotracers.

Clinical development of new drug-radiotherapy combinations

In countries with the best cancer outcomes, approximately 60% of patients receive radiotherapy as part of their treatment, which is one of the most cost-effective cancer treatments. Notably, around 40% of cancer cures include the use of radiotherapy, either as a single modality or combined with other treatments. Radiotherapy can provide enormous benefit to patients with cancer. In the past decade, significant technical advances, such as image-guided radiotherapy, intensity-modulated radiotherapy, stereotactic radiotherapy, and proton therapy enable higher doses of radiotherapy to be delivered to the tumour with significantly lower doses to normal surrounding tissues. However, apart from the combination of traditional cytotoxic chemotherapy with radiotherapy, little progress has been made in identifying and defining optimal targeted therapy and radiotherapy combinations to improve the efficacy of cancer treatment. The National Cancer Research Institute Clinical and Translational Radiotherapy Research Working Group (CTRad) formed a Joint Working Group with representatives from academia, industry, patient groups and regulatory bodies to address this lack of progress and to publish recommendations for future clinical research. Herein, we highlight the Working Group's consensus recommendations to increase the number of novel drugs being successfully registered in combination with radiotherapy to improve clinical outcomes for patients with cancer.

Neural correlates of ideomotor effect anticipations

How does our mind produce physical, goal-directed action of our body? For about 200years, philosophers and psychologists hypothesized the transformation from mind to body to rely on the anticipation of an action's sensory consequences. Whereas this hypothesis received tremendous support from behavioral experiments, the neural underpinnings of action control via such ideomotor effect anticipations are virtually unknown. Using functional magnetic resonance imaging, the present study identified the inferior parietal cortex and the parahippocampal gyrus as key regions for this type of action control - setting the stage for a neuroscientific framework for explaining action control by ideomotor effect anticipations and thus enabling a synthesis of psychological and neuroscientific approaches to human action.

18F-EF5 PET imaging as an early response biomarker for the hypoxia-activated prodrug SN30000 combined with radiation treatment in a non-small cell lung cancer xenograft model

Hypoxia is a significant therapeutic problem for solid tumors because hypoxic cells are treatment-resistant and more aggressive. Hypoxia-activated prodrugs such as SN30000 use a mechanism of activation in hypoxic cells similar to that of 2-nitroimidazole hypoxia PET tracers. Therefore, we have evaluated the usefulness of 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-(18)F-pentafluoropropyl)-acetamide ((18)F-EF5) PET to monitor and predict tumor response to SN30000 plus radiation treatment (RT).

METHODS

Human non-small cell lung cancer xenografts (H460) in athymic rats were imaged with (18)F-EF5 PET before and after treatment with SN30000 (90 mg/kg), with or without 15-Gy RT. The feasibility of imaging early changes in hypoxia in response to SN30000 was examined 24 h after treatment, followed by ex vivo γ-counting and immunohistochemical examination to study drug-induced apoptosis. Subsequently, the therapeutic effects of SN30000 with or without RT were evaluated in tumor growth delay studies and compared with early treatment-induced changes observed by (18)F-EF5 PET. Changes in tumor hemoglobin oxygen saturation as a function of time after treatment measured by optical spectroscopy were compared with PET data.

RESULTS

The uptake of (18)F-EF5 was significantly lower in SN30000-treated tumors than in saline controls 24 h after treatment (mean standardized uptake value, 0.44 ± 0.08 vs. 0.56 ± 0.08 for control group; P < 0.05). Apoptosis was significantly higher in SN30000-treated tumors than in controls. Early treatment-induced changes in (18)F-EF5 uptake were indicative of tumor response in growth delay studies at the group level. SN30000 plus RT significantly decreased (18)F-EF5 uptake relative to baseline and resulted in complete tumor remission in 5 of 7 animals. SN30000 alone decreased (18)F-EF5 uptake, generally in tumors with high initial standardized uptake values, and showed a minor tumor growth delay effect. The changes induced by SN30000 with or without RT in (18)F-EF5 uptake correlated with baseline hypoxia levels. RT caused significant increases in tumor oxygen concentration and hemoglobin oxygen saturation.

CONCLUSION

A hypoxia PET imaging agent can measure changes in tumor hypoxic fraction in response to SN30000. These results suggest the utility of (18)F-EF5 PET for monitoring early response to tumor treatment with SN30000 plus RT in the clinical development of this novel hypoxia-activated prodrug.

The neural substrate of the ideomotor principle revisited: evidence for asymmetries in action-effect learning

Ideomotor theory holds that the perception or anticipatory imagination of action effects activates motor tendencies toward the action that is known to produce these effects, herein referred to as ideomotor response activation (IRA). IRA presupposes that the agent has previously learned which action produces which effects, and that this learning process has created bidirectional associations between the sensory effect codes and the motor codes producing the sensory effects. Here, we refer to this process as ideomotor learning. In the presented fMRI study, we adopted a standard two-phase ideomotor learning paradigm; a mixed between/within-subjects design allowed us to assess the neural substrate of both, IRA and ideomotor learning. We replicated earlier findings of a hand asymmetry in ideomotor processing with significantly stronger IRA by left-hand than right-hand action effects. Crucially, we traced this effect back to more pronounced associative learning for action-contingent effects of the left hand compared with effects of the right hand. In this context, our findings point to the caudate nucleus and the angular gyrus as central structures of the neural network underlying ideomotor learning.

GABAB receptor antagonist SGS742 improves spatial memory and reduces protein binding to the cAMP response element (CRE) in the hippocampus

Memory storage in the brain requires protein synthesis initiated through signaling pathways that control transcription. Such mechanisms are under active investigation for therapies in disorders involving cognitive dysfunction. Long-term memory can be improved by inhibiting activation or reducing expression of transcription factors such as ATF4/CREB2 and some C/EBP family members which appear to serve as memory suppressors. Here, we provide evidence that GABAB receptor antagonists may enhance cognition, at least in part, by this mechanism. We tested a GABAB receptor antagonist, SGS742 (CGP36742), on hippocampal-dependent memory and hippocampal nuclear CRE-binding activity in rats. As a result, acute in vivo administration of SGS742 both improved memory and reduced total hippocampal CRE-binding activity of which a large proportion in the basal state could be immunoneutralized with CREB2 antibodies. Consistent with its activity on information storage mechanisms, acute SGS742 effectively improved long-term memory in retrograde protocols, in which drug was given at times when memory formation can be interrupted by blocking new protein production. In conclusion, GABAB antagonists may provide a pharmacological therapy for cognitive impairment, sharing mechanistic features with genetic approaches to reduce CREB2 activity and to augment long-term memory.

SGS742: the first GABA(B) receptor antagonist in clinical trials

The GABA(B) receptor antagonist SGS742 (CGP36742) displays pronounced cognition enhancing effects in mice, young and old rats and in Rhesus monkeys in active and passive avoidance paradigms, in an eight-arm radial maze and a Morris water maze and in a social learning task. SGS742 blocks the late inhibitory postsynaptic potential and the paired-pulse inhibition of population spikes recorded from CA1 pyramidal neurons of the hippocampus of rats in vitro and in vivo. SGS742 significantly enhances the release of glutamate, aspartate, glycine and somatostatin in vivo. Chronic administration of SGS742 causes an up-regulation of GABA(B) receptors in the frontal cortex of rats. Single doses cause a significant enhancement of the mRNA and protein levels of NGF and BDNF in the cortex and hippocampus of rats. The observed antidepressant effects of SGS742 in rats may be explained by these findings. SGS742 was well tolerated in experimental animals as well as in young and elderly human volunteers with an absolute bioavailability in humans of 44%. In a Phase II double-blind, placebo-controlled study in 110 patients with mild cognitive impairment (MCI), oral administration of SGS742 at a dose of 600 mg t.i.d. for 8 weeks significantly improved attention, in particular choice reaction time and visual information processing as well as working memory measured as pattern recognition speed. A second Phase II clinical trial in 280 Alzheimer's disease patients is underway.

A role for receptor tyrosine phosphataseζ in glioma cell migration

Glioblastomas (GBM) are the most frequent and malignant human brain tumor type. Typically striking in adulthood, tumor progression is rapid, relentless, and ultimately leads to the patient's death within a year of diagnosis. The identification of transcriptionally regulated genes can lead to the discovery of targets for antibody or small-molecule-mediated therapy, as well as diagnostic markers. We prepared cDNA arrays that are specifically enriched for genes expressed in human brain tumors and profiled gene expression patterns in 14 individual tumor samples. Out of 25,000 clones arrayed, greater than 200 genes were found transcriptionally induced in glioblastomas compared to normal human brain tissue including the receptor tyrosine phosphatasezeta (RPTPzeta) and one of its ligands, pleiotrophin (Ptn). We confirmed by Northern blot analysis and immunohistochemistry that RPTPzeta is enriched in tumor samples. Knockdown of RPTPzeta by RNA interference studies established a functional role of RPTPzeta in cell migration. Our results suggest a novel function for RPTPzeta in regulating glioblastoma cell motility and point to the therapeutic utility of RPTPzeta as a target for antibody-mediated therapy of brain tumors.

Uncoupling protein-2 prevents neuronal death and diminishes brain dysfunction after stroke and brain trauma

Whereas uncoupling protein 1 (UCP-1) is clearly involved in thermogenesis, the role of UCP-2 is less clear. Using hybridization, cloning techniques and cDNA array analysis to identify inducible neuroprotective genes, we found that neuronal survival correlates with increased expression of Ucp2. In mice overexpressing human UCP-2, brain damage was diminished after experimental stroke and traumatic brain injury, and neurological recovery was enhanced. In cultured cortical neurons, UCP-2 reduced cell death and inhibited caspase-3 activation induced by oxygen and glucose deprivation. Mild mitochondrial uncoupling by 2,4-dinitrophenol (DNP) reduced neuronal death, and UCP-2 activity was enhanced by palmitic acid in isolated mitochondria. Also in isolated mitochondria, UCP-2 shifted the release of reactive oxygen species from the mitochondrial matrix to the extramitochondrial space. We propose that UCP-2 is an inducible protein that is neuroprotective by activating cellular redox signaling or by inducing mild mitochondrial uncoupling that prevents the release of apoptogenic proteins.

Candidate editases for GluR channels in single neurons of rat hippocampus and cerebellum

RNA editing by site selective adenosine deamination changes codons in several nuclear transcripts in the mammalian brain and affects critical properties of the encoded proteins, as exemplified by the calcium permeability of AMPA receptor channels. The recently cloned RNA dependent adenosine deaminases ADAR1, ADAR2 and ADAR3 form a small family of sequence-related candidate editases which are expressed in brain and other tissues at distinct levels and patterns. We have employed single-cell polymerase chain reaction of hippocampal CA1 and CA3 pyramidal neurons and cerebellar Purkinje and Bergmann glial cells in an attempt to evaluate the expression of these enzymes at a cellular level. We found ADAR2 expressed in all cells analyzed; approximately 50% of the cells co-expressed ADAR1 or ADAR3. The differential ADAR expression revealed by our study might underlie the distinct editing efficiencies and selectivities in different GluR subunit transcripts.

Tad1p, a yeast tRNA-specific adenosine deaminase, is related to the mammalian pre-mRNA editing enzymes ADAR1 and ADAR2

We have identified an RNA-specific adenosine deaminase (termed Tad1p/scADAT1) from Saccharomyces cerevisiae that selectively converts adenosine at position 37 of eukaryotic tRNAAla to inosine. The activity of purified recombinant Tad1p depends on the conformation of its tRNA substrate and the enzyme was found to be inactive on all other types of RNA tested. Mutant strains in which the TAD1 gene is disrupted are viable but lack Tad1p enzyme activity and their tRNAAla is not modified at position A37. Transformation of the mutant cells with the TAD1 gene restored enzyme activity. Tad1p has significant sequence similarity with the mammalian editing enzymes which act on specific precursor-mRNAs and on long double-stranded RNA. These findings suggest an evolutionary link between pre-mRNA editing and tRNA modification.

Mammalian RNA-dependent deaminases and edited mRNAs

The past year has witnessed major progress in the field of mammalian nuclear RNA editing. Two new sequence-related RNA-dependent adenosine deaminases, distantly related to the previously characterized double-stranded RNA adenosine deaminase DRADA/dsRAD, have been molecularly characterized. One of these deaminases edits in vitro with precision for the molecular determinant that controls the Ca2+ permeability of fast synaptic glutamate-gated cation channels. This deaminase, like DRADA, is expressed in many tissues and the search is now on for more substrates of these RNA-editing enzymes. Moreover, the physiological role of the apolipoprotein B RNA editing enzyme APOBEC-1 has been investigated in genetically manipulated mice.

RED2, a brain-specific member of the RNA-specific adenosine deaminase family

The mammalian RNA-specific adenosine deaminases DRADA/dsRAD (alias ADAR) and RED1 (alias ADARB1) have been implicated in the site-selective editing of brain-expressed pre-mRNAs for glutamate receptor subunits and of antigenomic RNA of hepatitis delta virus. These enzymes are expressed in many if not all tissues, predicting an as yet unappreciated significance for adenosine deamination-mediated recoding of gene transcripts in the mammalian organism. We now report the molecular cloning of cDNA for RED2 (alias ADARB2), a third member of the RNA-specific adenosine deaminase family in the rodent. RED2 is closely sequence-related to RED1 but appears to be expressed only in the brain, where expression is widespread reaching highest levels in olfactory bulb and thalamus. RED2 further differs from RED1 in having a 54-residue amino-terminal extension which includes an arginine-rich motif. Different from DRADA and RED1, recombinantly expressed RED2 did not deaminate adenosines in extended synthetic dsRNA or in GluR-B pre-mRNA. However, a chimera of RED1 and RED2 edited the GluR-B Q/R and R/G sites with moderate efficiency. Our data suggest that RED2 may edit brain-specific transcripts with distinct structural features.

Structural requirements for RNA editing in glutamate receptor pre-mRNAs by recombinant double-stranded RNA adenosine deaminase

Pre-mRNAs for brain-expressed ionotropic glutamate receptor subunits undergo RNA editing by site-specific adenosine deamination, which alters codons for molecular determinants of channel function. This nuclear process requires double-stranded RNA structures formed by exonic and intronic sequences in the pre-mRNA and is likely to be catalyzed by an adenosine deaminase that recognizes these structures as a substrate. DRADA, a double-stranded RNA adenosine deaminase, is a candidate enzyme for L-glutamate-activated receptor channel (GluR) pre-mRNA editing. We show here that DRADA indeed edits GluR pre-mRNAs, but that it displays selectivity for certain editing sites. Recombinantly expressed DRADA, both in its full-length form and in an N-terminally truncated version, edited the Q/R site in GluR6 pre-mRNA and the R/G site but not the Q/R site of GluR-B pre-mRNA. This substrate selectivity correlated with the base pairing status and sequence environment of the editing-targeted adenosines. The Q/R site of GluR-B pre-mRNA was edited by an activity partially purified from HeLa cells and thus differently structured editing sites in GluR pre-mRNAs appear to be substrates for different enzymatic activities.

A mammalian RNA editing enzyme

Editing of RNA by site-selective adenosine deamination alters codons in brain-expressed pre-messenger RNAs for glutamate receptor (GluR) subunits including a codon for a channel determinant (Q/R site) in GluR-B, which controls the Ca2+ permeability of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors. Editing of GluR pre-mRNAs requires a double-stranded RNA (dsRNA) structure formed by exonic and intronic sequences and is catalysed by an unknown dsRNA adenosine deaminase. Here we report the cloning of complementary DNA for RED1, a dsRNA adenosine deaminase expressed in brain and peripheral tissues that efficiently edits the Q/R site in GluR-B pre-mRNA in vitro. This site is poorly edited by DRADA, which is distantly sequence-related to RED1. Both deaminases edit the R/G site in GluR-B pre-mRNA, indicating that members of an emerging gene family catalyse adenosine deamination in nuclear transcripts with distinct but overlapping substrate specificities.

Relative abundance of subunit mRNAs determines gating and Ca2+ permeability of AMPA receptors in principal neurons and interneurons in rat CNS

Recording of glutamate-activated currents in membrane patches was combined with RT-PCR-mediated AMPA receptor (AMPAR) subunit mRNA analysis in single identified cells of rat brain slices. Analysis of AMPARs in principal neurons and interneurons of hippocampus and neocortex and in auditory relay neurons and Bergmann glial cells indicates that the GluR-B subunit in its flip version determines formation of receptors with relatively slow gating, whereas the GluR-D subunit promotes assembly of more rapidly gated receptors. The relation between Ca2+ permeability of AMPAR channels and the relative GluR-B mRNA abundance is consistent with the dominance of this subunit in determining the Ca2+ permeability of native receptors. The results suggest that differential expression of GluR-B and GluR-D subunit genes, as well as splicing and editing of their mRNAs, account for the differences in gating and Ca2+ permeability of native AMPAR channels.

Editing of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor GluR-B pre-mRNA in vitro reveals site-selective adenosine to inosine conversion

In neurons of the mammalian brain primary transcripts of genes encoding subunits of glutamate receptor channels can undergo RNA editing, leading to altered properties of the transmitter-activated channel. Editing of these transcripts is a nuclear process that targets specific adenosines and requires a double-stranded RNA structure configured from complementary exonic and intronic sequences. We show here that the two independent editing sites in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor GluR-B pre-mRNA are edited with positional accuracy by nuclear extract from HeLa cells. Nucleotide analysis by thin layer chromatography of the edited RNA sequences revealed selective adenosine to inosine conversion, most likely reflecting the participation of double-stranded RNA adenosine deaminase. Our results predict the presence of inosine-containing codons in other mammalian mRNAs.

Control of kinetic properties of AMPA receptor channels by nuclear RNA editing

AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor channels mediate the fast component of excitatory postsynaptic currents in the central nervous system. Site-selective nuclear RNA editing controls the calcium permeability of these channels, and RNA editing at a second site is shown here to affect the kinetic aspects of these channels in rat brain. In three of the four AMPA receptor subunits (GluR-B, -C, and -D), intronic elements determine a codon switch (AGA, arginine, to GGA, glycine) in the primary transcripts in a position termed the R/G site, which immediately precedes the alternatively spliced modules "flip" and "flop." The extent of editing at this site progresses with brain development in a manner specific for subunit and splice form, and edited channels possess faster recovery rates from desensitization.

Tryptophan mutants of human C5a anaphylatoxin: a fluorescence anisotropy decay and energy transfer study

Three mutants of the anaphylatoxin C5a were prepared with positions 2, 64 and 70, respectively, substituted by tryptophan. The last mutant was additionally labelled at Cys27 for fluorescence energy transfer (FET) measurements. The structural integrity and biological activity of the molecules were not affected. Fluorescence anisotropy decay (FAD) measurements showed that the rotational correlation time for tryptophan decreases in the order: [Trp2]rhC5a > [Trp64]rhC5a > [Trp70]rhC5a, indicating an increasing mobility of the side chain. Measurements of the fluorescence energy transfer from Trp70 to the 1,5-AEDANS group at Cys27 yielded a distance distribution of 2.4 +/- 0.8 nm. This value is compatible with the C-terminal chain being arranged as a slightly stretched helix pointing away from the body of the molecule.

Dysgerminoma in Turner's syndrome

The importance of the Y-chromosome for the germ cell tumour development in gonadal dysgenesis has been emphasized many times. In contrast, only two cases of dysgerminoma or gonadoblastoma had been published so far in the XO-Turner's syndrome. With this report, another case of Turner's syndrome developing a dysgerminoma in a gonadal streak is presented. No Y-chromosome containing stemline could be detected in the patient nor in the tumour. A primary genetic etiology or a mechanism related to early secondary regression or dysgenesis of the gonad are discussed as causative factors in germ cell tumour development within gonadal streaks.