Bayesian estimation of changes in transverse relaxation rates

Evaluating Imaging Biomarkers of Acquired Resistance to Targeted EGFR Therapy in Xenograft Models of Human Head and Neck Squamous Cell Carcinoma

Background: Overexpression of EGFR is a negative prognostic factor in head and neck squamous cell carcinoma (HNSCC). Patients with HNSCC who respond to EGFR-targeted tyrosine kinase inhibitors (TKIs) eventually develop acquired resistance. Strategies to identify HNSCC patients likely to benefit from EGFR-targeted therapies, together with biomarkers of treatment response, would have clinical value. Methods: Functional MRI and 18F-FDG PET were used to visualize and quantify imaging biomarkers associated with drug response within size-matched EGFR TKI-resistant CAL 27 (CALR) and sensitive (CALS) HNSCC xenografts in vivo, and pathological correlates sought. Results: Intrinsic susceptibility, oxygen-enhanced and dynamic contrast-enhanced MRI revealed significantly slower baseline R 2 ∗ , lower hyperoxia-induced Δ R 2 ∗ and volume transfer constant Ktrans in the CALR tumors which were associated with significantly lower Hoechst 33342 uptake and greater pimonidazole-adduct formation. There was no difference in oxygen-induced ΔR1 or water diffusivity between the CALR and CALS xenografts. PET revealed significantly higher relative uptake of 18F-FDG in the CALR cohort, which was associated with significantly greater Glut-1 expression. Conclusions: CALR xenografts established from HNSCC cells resistant to EGFR TKIs are more hypoxic, poorly perfused and glycolytic than sensitive CALS tumors. MRI combined with PET can be used to non-invasively assess HNSCC response/resistance to EGFR inhibition.

Preclinical transgenic and patient-derived xenograft models recapitulate the radiological features of human adamantinomatous craniopharyngioma

To assess the clinical relevance of transgenic and patient-derived xenograft models of adamantinomatous craniopharyngioma (ACP) using serial magnetic resonance imaging (MRI) and high resolution post-mortem microcomputed tomography (μ-CT), with correlation with histology and human ACP imaging. The growth patterns and radiological features of tumors arising in Hesx1Cre/+ ;Ctnnb1lox(ex3)/+ transgenic mice, and of patient-derived ACP xenografts implanted in the cerebral cortex, were monitored longitudinally in vivo with anatomical and functional MRI, and by ex vivo μ-CT at study end. Pathological correlates with hematoxylin and eosin stained sections were investigated. Early enlargement and heterogeneity of Hesx1Cre/+ ;Ctnnb1lox(ex3)/+ mouse pituitaries was evident at initial imaging at 8 weeks, which was followed by enlargement of a solid tumor, and development of cysts and hemorrhage. Tumors demonstrated MRI features that recapitulated those of human ACP, specifically, T1 -weighted signal enhancement in the solid tumor component following Gd-DTPA administration, and in some animals, hyperintense cysts on FLAIR and T1 -weighted images. Ex vivo μ-CT correlated with MRI findings and identified smaller cysts, which were confirmed by histology. Characteristic histological features, including wet keratin and calcification, were visible on μ-CT and verified by histological sections of patient-derived ACP xenografts. The Hesx1Cre/+ ;Ctnnb1lox(ex3)/+ transgenic mouse model and cerebral patient-derived ACP xenografts recapitulate a number of the key radiological features of the human disease and provide promising foundations for in vivo trials of novel therapeutics for the treatment of these tumors.

Genetically modified lentiviruses that preserve microvascular function protect against late radiation damage in normal tissues

Improvements in cancer survival mean that long-term toxicities, which contribute to the morbidity of cancer survivorship, are being increasingly recognized. Late adverse effects (LAEs) in normal tissues after radiotherapy (RT) are characterized by vascular dysfunction and fibrosis causing volume loss and tissue contracture, for example, in the free flaps used for immediate breast reconstruction after mastectomy. We evaluated the efficacy of lentivirally delivered superoxide dismutase 2 (SOD2) overexpression and connective tissue growth factor (CTGF) knockdown by short hairpin RNA in reducing the severity of LAEs in an animal model of free flap LAEs. Vectors were delivered by intra-arterial injection, ex vivo, to target the vascular compartment. LVSOD2 and LVshCTGF monotherapy before irradiation resulted in preservation of flap volume or reduction in skin contracture, respectively. Flaps transduced with combination therapy experienced improvements in both volume loss and skin contracture. Both therapies reduced the fibrotic burden after irradiation. LAEs were associated with impaired vascular perfusion, loss of endothelial permeability, and stromal hypoxia, which were all reversed in the treatment model. Using a tumor recurrence model, we showed that SOD2 overexpression in normal tissues did not compromise the efficacy of RT against tumor cells but appeared to enhance it. LVSOD2 and LVshCTGF combination therapy by targeted, intravascular delivery reduced LAE severities in normal tissues without compromising the efficacy of RT and warrants translational evaluation as a free flap-targeted gene therapy.

Pre-clinical imaging of transgenic mouse models of neuroblastoma using a dedicated 3-element solenoid coil on a clinical 3T platform

BACKGROUND

The use of clinical MRI scanners to conduct pre-clinical research facilitates comparisons with clinical studies. Here the utility and sensitivity of anatomical and functional MRI data/biomarkers acquired from transgenic mouse models of neuroblastoma using a dedicated radiofrequency (RF) coil on a clinical 3T scanner was evaluated.

METHODS

Multiparametric MRI of transgenic mice bearing abdominal neuroblastomas was performed at 3T, and data cross-referenced to that acquired from the same mice on a pre-clinical 7T MRI system. T2-weighted imaging, quantitation of the native longitudinal relaxation time (T1) and the transverse relaxation rate (R2*), and dynamic contrast-enhanced (DCE)-MRI, was used to assess tumour volume, phenotype and response to cyclophosphamide or cabozantinib.

RESULTS

Excellent T2-weighted image contrast enabled clear tumour delineation at 3T. Significant correlations of tumour volume (R=0.98, P<0.0001) and R2* (R=0.87, P<0.002) measured at 3 and 7T were established. Mice with neuroblastomas harbouring the anaplastic lymphoma kinase mutation exhibited a significantly slower R2* (P<0.001), consistent with impaired tumour perfusion. DCE-MRI was performed simultaneously on three transgenic mice, yielding estimates of Ktrans for each tumour (median Ktrans values of 0.202, 0.168 and 0.114 min-1). Cyclophosphamide elicited a significant reduction in both tumour burden (P<0.002) and native T1 (P<0.01), whereas cabozantinib induced significant (P<0.01) tumour growth delay.

CONCLUSIONS

Simultaneous multiparametric MRI of multiple tumour-bearing animals using this coil arrangement at 3T can provide high efficiency/throughput for both phenotypic characterisation and evaluation of novel therapeutics, and facilitate the introduction of functional MRI biomarkers into aligned imaging-embedded clinical trials.

Evaluation of the Response of Intracranial Xenografts to VEGF Signaling Inhibition Using Multiparametric MRI

Vascular endothelial growth factor A (VEGF-A) is considered one of the most important factors in tumor angiogenesis, and consequently, a number of therapeutics have been developed to inhibit VEGF signaling. Therapeutic strategies to target brain malignancies, both primary brain tumors, particularly in pediatric patients, and metastases, are lacking, but targeting angiogenesis may be a promising approach. Multiparametric MRI was used to investigate the response of orthotopic SF188^luc pediatric glioblastoma xenografts to small molecule pan-VEGFR inhibitor cediranib and the effects of both cediranib and cross-reactive human/mouse anti-VEGF-A antibody B20-4.1.1 in intracranial MDA-MB-231 LM2–4 breast cancer xenografts over 48 hours. All therapeutic regimens resulted in significant tumor growth delay. In cediranib-treated SF188^luc tumors, this was associated with lower K^trans (compound biomarker of perfusion and vascular permeability) than in vehicle-treated controls. Cediranib also induced significant reductions in both K^trans and apparent diffusion coefficient (ADC) in MDA-MB-231 LM2–4 tumors associated with decreased histologically assessed perfusion. B20-4.1.1 treatment resulted in decreased K^trans, but in the absence of a change in perfusion; a non-significant reduction in vascular permeability, assessed by Evans blue extravasation, was observed in treated tumors. The imaging responses of intracranial MDA-MB-231 LM2–4 tumors to VEGF/VEGFR pathway inhibitors with differing mechanisms of action are subtly different. We show that VEGF pathway blockade resulted in tumor growth retardation and inhibition of tumor vasculature in preclinical models of pediatric glioblastoma and breast cancer brain metastases, suggesting that multiparametric MRI can provide a powerful adjunct to accelerate the development of antiangiogenic therapies for use in these patient populations.

Monitoring the Vascular Response and Resistance to Sunitinib in Renal Cell Carcinoma In Vivo with Susceptibility Contrast MRI

Antiangiogenic therapy is efficacious in metastatic renal cell carcinoma (mRCC). However, the ability of antiangiogenic drugs to delay tumor progression and extend survival is limited, due to either innate or acquired drug resistance. Furthermore, there are currently no validated biomarkers that predict which mRCC patients will benefit from antiangiogenic therapy. Here, we exploit susceptibility contrast MRI (SC-MRI) using intravascular ultrasmall superparamagnetic iron oxide particles to quantify and evaluate tumor fractional blood volume (fBV) as a noninvasive imaging biomarker of response to the antiangiogenic drug sunitinib. We also interrogate the vascular phenotype of RCC xenografts exhibiting acquired resistance to sunitinib. SC-MRI of 786-0 xenografts prior to and 2 weeks after daily treatment with 40 mg/kg sunitinib revealed a 71% (P < 0.01) reduction in fBV in the absence of any change in tumor volume. This response was associated with significantly lower microvessel density (P < 0.01) and lower uptake of the perfusion marker Hoechst 33342 (P < 0.05). The average pretreatment tumor fBV was negatively correlated (R2 = 0.92, P < 0.0001) with sunitinib-induced changes in tumor fBV across the cohort. SC-MRI also revealed suppressed fBV in tumors that acquired resistance to sunitinib. In conclusion, SC-MRI enabled monitoring of the antiangiogenic response of 786-0 RCC xenografts to sunitinib, which revealed that pretreatment tumor fBV was found to be a predictive biomarker of subsequent reduction in tumor blood volume in response to sunitinib, and acquired resistance to sunitinib was not associated with a parallel increase in tumor blood volume. Cancer Res; 77(15); 4127-34. ©2017 AACR.

The effect of imatinib therapy on tumour cycling hypoxia, tissue oxygenation and vascular reactivity

Background: Several biomedical imaging techniques have recently been developed to probe hypoxia in tumours, including oxygen-enhanced (OE) and blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI). These techniques have strong potential for measuring both chronic and transient (cycling) changes in hypoxia, and to assess response to vascular-targeting therapies in the clinic. Methods: In this study, we investigated the use of BOLD and OE-MRI to assess changes in cycling hypoxia, tissue oxygenation and vascular reactivity to hyperoxic gas challenges, in mouse models of colorectal therapy, following treatment with the PDGF-receptor inhibitor, imatinib mesylate (Glivec). Results: Whilst no changes were observed in imaging biomarkers of cycling hypoxia (from BOLD) or chronic hypoxia (from OE-MRI), the BOLD response to carbogen-breathing became significantly more positive in some tumour regions and more negative in other regions, thereby increasing overall heterogeneity. Conclusions: Imatinib did not affect the magnitude of cycling hypoxia or OE-MRI signal, but increased the heterogeneity of the spatial distribution of BOLD MRI changes in response to gas challenges.

Investigating intracranial tumour growth patterns with multiparametric MRI incorporating Gd-DTPA and USPIO-enhanced imaging

High grade and metastatic brain tumours exhibit considerable spatial variations in proliferation, angiogenesis, invasion, necrosis and oedema. Vascular heterogeneity arising from vascular co-option in regions of invasive growth (in which the blood-brain barrier remains intact) and neoangiogenesis is a major challenge faced in the assessment of brain tumours by conventional MRI. A multiparametric MRI approach, incorporating native measurements and both Gd-DTPA (Magnevist) and ultrasmall superparamagnetic iron oxide (P904)-enhanced imaging, was used in combination with histogram and unsupervised cluster analysis using a k-means algorithm to examine the spatial distribution of vascular parameters, water diffusion characteristics and invasion in intracranially propagated rat RG2 gliomas and human MDA-MB-231 LM2-4 breast adenocarcinomas in mice. Both tumour models presented with higher ΔR1 (the change in transverse relaxation rate R1 induced by Gd-DTPA), fractional blood volume (fBV) and apparent diffusion coefficient than uninvolved regions of the brain. MDA-MB-231 LM2-4 tumours were less densely cellular than RG2 tumours and exhibited substantial local invasion, associated with oedema, whereas invasion in RG2 tumours was minimal. These additional features were reflected in the more heterogeneous appearance of MDA-MB-231 LM2-4 tumours on T2 -weighted images and maps of functional MRI parameters. Unsupervised cluster analysis separated subregions with distinct functional properties; areas with a low fBV and relatively impermeable blood vessels (low ΔR1 ) were predominantly located at the tumour margins, regions of MDA-MB-231 LM2-4 tumours with relatively high levels of water diffusion and low vascular permeability and/or fBV corresponded to histologically identified regions of invasion and oedema, and areas of mismatch between vascular permeability and blood volume were identified. We demonstrate that dual contrast MRI and evaluation of tissue diffusion properties, coupled with cluster analysis, allows for the assessment of heterogeneity within invasive brain tumours and the designation of functionally diverse subregions that may provide more informative predictive biomarkers.

Investigating the Vascular Phenotype of Subcutaneously and Orthotopically Propagated PC3 Prostate Cancer Xenografts Using Combined Carbogen Ultrasmall Superparamagnetic Iron Oxide MRI

The aim of this study was to use the combined carbogen-ultrasmall superparamagnetic iron oxide (CUSPIO) magnetic resonance imaging (MRI) method, which uses spatial correlations in independent susceptibility imaging biomarkers, to investigate and compare the impact of tumor size and anatomical site on vascular structure and function in vivo. Mice bearing either subcutaneous or orthotopic PC3 LN3 prostate tumors were imaged at 7 T, using a multi-gradient echo sequence to quantify R2, before and during carbogen (95% O2/5% CO2) breathing, and subsequently following intravenous administration of USPIO particles. Carbogen and USPIO-induced changes in R2 were used to inform on hemodynamic vasculature and fractional blood volume (%), respectively. The CUSPIO imaging data were also segmented to identify and assess five categories of R2 response. Small and large subcutaneous and orthotopic tumor cohorts all exhibited significantly (P < 0.05) different median baseline R2, ΔR2carbogen, and fractional blood volume. CUSPIO imaging showed that small subcutaneous tumors predominantly exhibited a negative ΔR2carbogen followed by a positive ΔR2USPIO, consistent with a well perfused tumor vasculature. Large subcutaneous tumors exhibited a small positive ΔR2carbogen and relatively low fractional blood volume, suggesting less functional vasculature. Orthotopic tumors revealed a large, positive ΔR2carbogen, consistent with vascular steal, and which may indicate that vascular function is more dependent on site of implantation than tumor size. Regions exhibiting significant ΔR2carbogen, but no significant ΔR2USPIO, suggesting transient vascular shutdown over the experimental timecourse, were apparent in all 3 cohorts. CUSPIO imaging can inform on efficient drug delivery via functional vasculature in vivo, and on appropriate tumor model selection for pre-clinical therapy trials.

Acute tumour response to a bispecific Ang-2-VEGF-A antibody: insights from multiparametric MRI and gene expression profiling

BACKGROUND

To assess antivascular effects, and evaluate clinically translatable magnetic resonance imaging (MRI) biomarkers of tumour response in vivo, following treatment with vanucizumab, a bispecific human antibody against angiopoietin-2 (Ang-2) and vascular endothelial growth factor-A (VEGF-A).

METHODS

Colo205 colon cancer xenografts were imaged before and 5 days after treatment with a single 10 mg kg(-1) dose of either vanucizumab, bevacizumab (anti-human VEGF-A), LC06 (anti-murine/human Ang-2) or omalizumab (anti-human IgE control). Volumetric response was assessed using T2-weighted MRI, and diffusion-weighted, dynamic contrast-enhanced (DCE) and susceptibility contrast MRI used to quantify tumour water diffusivity (apparent diffusion coefficient (ADC), × 10(6) mm(2) s(-1)), vascular perfusion/permeability (K(trans), min(-1)) and fractional blood volume (fBV, %) respectively. Pathological correlates were sought, and preliminary gene expression profiling performed.

RESULTS

Treatment with vanucizumab, bevacizumab or LC06 induced a significant (P<0.01) cytolentic response compared with control. There was no significant change in tumour ADC in any treatment group. Uptake of Gd-DTPA was restricted to the tumour periphery in all post-treatment groups. A significant reduction in tumour K(trans) (P<0.05) and fBV (P<0.01) was determined 5 days after treatment with vanucizumab only. This was associated with a significant (P<0.05) reduction in Hoechst 33342 uptake compared with control. Gene expression profiling identified 20 human genes exclusively regulated by vanucizumab, 6 of which are known to be involved in vasculogenesis and angiogenesis.

CONCLUSIONS

Vanucizumab is a promising antitumour and antiangiogenic treatment, whose antivascular activity can be monitored using DCE and susceptibility contrast MRI. Differential gene expression in vanucizumab-treated tumours is regulated by the combined effect of Ang-2 and VEGF-A inhibition.

Repeatability and sensitivity of T2* measurements in patients with head and neck squamous cell carcinoma at 3T

PURPOSE

To determine whether quantitation of T2* is sufficiently repeatable and sensitive to detect clinically relevant oxygenation levels in head and neck squamous cell carcinoma (HNSCC) at 3T.

MATERIALS AND METHODS

Ten patients with newly diagnosed locally advanced HNSCC underwent two magnetic resonance imaging (MRI) scans between 24 and 168 hours apart prior to chemoradiotherapy treatment. A multiple gradient echo sequence was used to calculate T2* maps. A quadratic function was used to model the blood transverse relaxation rate as a function of blood oxygenation. A set of published coefficients measured at 3T were incorporated to account for tissue hematocrit levels and used to plot the dependence of fractional blood oxygenation (Y) on T2* values, together with the corresponding repeatability range. Repeatability of T2* using Bland-Altman analysis, and calculation of limits of agreement (LoA), was used to assess the sensitivity, defined as the minimum difference in fractional blood oxygenation that can be confidently detected.

RESULTS

T2* LoA for 22 outlined tumor volumes were 13%. The T2* dependence of fractional blood oxygenation increases monotonically, resulting in increasing sensitivity of the method with increasing blood oxygenation. For fractional blood oxygenation values above 0.11, changes in T2* were sufficient to detect differences in blood oxygenation greater than 10% (Δ T2* > LoA for ΔY > 0.1).

CONCLUSION

Quantitation of T2* at 3T can detect clinically relevant changes in tumor oxygenation within a wide range of blood volumes and oxygen tensions, including levels reported in HNSCC. J. Magn. Reson. Imaging 2016;44:72-80.

Acute changes in liver tumour perfusion measured non-invasively with arterial spin labelling

BACKGROUND

Non-invasive measures of tumour vascular perfusion are desirable, in order to assess response to vascular targeting (or modifying) therapies. In this study, hepatic arterial spin labelling (ASL) magnetic resonance imaging (MRI) was investigated to measure acute changes in perfusion of colorectal cancer in the liver, in response to vascular disruption therapy with OXi4503.

METHODS

SW1222 and LS174T tumours were established in the liver of MF1 nu/nu mice via intrasplenic injection. Perfusion and R2(*) MRI measurements were acquired with an Agilent 9.4T horizontal bore scanner, before and at 90 min after 40 mg kg(-1) OXi4503.

RESULTS

A significant decrease in SW1222 tumour perfusion was observed (-43±33%, P<0.005). LS174T tumours had a significantly lower baseline level of perfusion. Intrinsic susceptibility MRI showed a significant increase in R2(*) in LS174T tumours (28±25%, P<0.05). An association was found between the change in tumour perfusion and the proximity to large vessels, with pre-treatment blood flow predictive of subsequent response. Histological evaluation confirmed the onset of necrosis and evidence of heterogeneous response between tumour deposits.

CONCLUSIONS

Hepatic ASL-MRI can detect acute response to targeted tumour vascular disruption entirely non-invasively. Hepatic ASL of liver tumours has potential for use in a clinical setting.

New approach to phase correction in multi-echo T2 relaxometry

Estimation of the transverse relaxation time, T₂, from multi-echo spin-echo images is usually performed using the magnitude of the noisy data, and a least squares (LS) approach. The noise in these magnitude images is Rice distributed, which can lead to a considerable bias in the LS-based T₂ estimates. One way to avoid this bias problem is to estimate a real-valued and Gaussian distributed dataset from the complex data, rather than using the magnitude. In this paper, we propose two algorithms for phase correction which can be used to generate real-valued data suitable for LS-based parameter estimation approaches. The first is a Weighted Linear Phase Estimation algorithm, abbreviated as WELPE. This method provides an improvement over a previously published algorithm, while simplifying the estimation procedure and extending it to support multi-coil input. The algorithm fits a linearly parameterized function to the multi-echo phase-data in each voxel and, based on this estimated phase, projects the data onto the real axis. The second method is a maximum likelihood estimator of the true decaying signal magnitude, which can be efficiently implemented when the phase variation is linear in time. The performance of the algorithms is demonstrated via Monte Carlo simulations, by comparing the accuracy of the estimates. Furthermore, it is shown that using one of the proposed algorithms enables more accurate T₂ estimates; in particular, phase corrected data significantly reduces the estimation bias in multi-component T₂ relaxometry example, compared to when using magnitude data. WELPE is also applied to a 32-echo in vivo brain dataset, to show its practical feasibility.

Bayesian analysis of transverse signal decay with application to human brain

PURPOSE

Transverse relaxation analysis with several signal models has been used extensively to determine tissue and material properties. However, the derivation of corresponding parameter values is notoriously unreliable. We evaluate improvements in the quality of parameter estimation using Bayesian analysis and incorporating the Rician noise model, as appropriate for magnitude MR images.

THEORY AND METHODS

Monoexponential, stretched exponential, and biexponential signal models were analyzed using nonlinear least squares (NLLS) and Bayesian approaches. Simulations and phantom and human brain data were analyzed using three different approaches to account for noise. Parameter estimation bias (reflecting accuracy) and dispersion (reflecting precision) were derived for a range of signal-to-noise ratios (SNR) and relaxation parameters.

RESULTS

All methods performed well at high SNR. At lower SNR, the Bayesian approach yielded parameter estimates of considerably greater precision, as well as greater accuracy, than did NLLS. Incorporation of the Rician noise model greatly improved accuracy and, to a somewhat lesser extent, precision, in derived transverse relaxation parameters. Analyses of data obtained from solution phantoms and from brain were consistent with simulations.

CONCLUSION

Overall, estimation of parameters characterizing several different transverse relaxation models was markedly improved through use of Bayesian analysis and through incorporation of the Rician noise model.

Intrinsic susceptibility MRI identifies tumors with ALKF1174L mutation in genetically-engineered murine models of high-risk neuroblastoma

The early identification of children presenting ALK(F1174L)-mutated neuroblastoma, which are associated with resistance to the promising ALK inhibitor crizotinib and a marked poorer prognosis, has become a clinical priority. In comparing the radiology of the novel Th-ALK(F1174L)/Th-MYCN and the well-established Th-MYCN genetically-engineered murine models of neuroblastoma using MRI, we have identified a marked ALK(F1174L)-driven vascular phenotype. We demonstrate that quantitation of the transverse relaxation rate R2* (s(-1)) using intrinsic susceptibility-MRI under baseline conditions and during hyperoxia, can robustly discriminate this differential vascular phenotype, and identify MYCN-driven tumors harboring the ALK(F1174L) mutation with high specificity and selectivity. Intrinsic susceptibility-MRI could thus potentially provide a non-invasive and clinically-exploitable method to help identifying children with MYCN-driven neuroblastoma harboring the ALK(F1174L) mutation at the time of diagnosis.

Evaluation and immunohistochemical qualification of carbogen-induced ΔR₂ as a noninvasive imaging biomarker of improved tumor oxygenation

PURPOSE

To evaluate and histologically qualify carbogen-induced ΔR2 as a noninvasive magnetic resonance imaging biomarker of improved tumor oxygenation using a double 2-nitroimidazole hypoxia marker approach.

METHODS AND MATERIALS

Multigradient echo images were acquired from mice bearing GH3 prolactinomas, preadministered with the hypoxia marker CCI-103F, to quantify tumor R2 during air breathing. With the mouse remaining positioned within the magnet bore, the gas supply was switched to carbogen (95% O2, 5% CO2), during which a second hypoxia marker, pimonidazole, was administered via an intraperitoneal line, and an additional set of identical multigradient echo images acquired to quantify any changes in tumor R2. Hypoxic fraction was quantified histologically using immunofluorescence detection of CCI-103F and pimonidazole adduct formation from the same whole tumor section. Carbogen-induced changes in tumor pO2 were further validated using the Oxylite fiberoptic probe.

RESULTS

Carbogen challenge significantly reduced mean tumor R2 from 116 ± 13 s(-1) to 97 ± 9 s(-1) (P<.05). This was associated with a significantly lower pimonidazole adduct area (2.3 ± 1%), compared with CCI-103F (6.3 ± 2%) (P<.05). A significant correlation was observed between ΔR2 and Δhypoxic fraction (r=0.55, P<.01). Mean tumor pO2 during carbogen breathing significantly increased from 6.3 ± 2.2 mm Hg to 36.0 ± 7.5 mm Hg (P<.01).

CONCLUSIONS

The combined use of intrinsic susceptibility magnetic resonance imaging with a double hypoxia marker approach corroborates carbogen-induced ΔR2 as a noninvasive imaging biomarker of increased tumor oxygenation.

Exploring ΔR(2) * and ΔR(1) as imaging biomarkers of tumor oxygenation

PURPOSE

To investigate the combined use of hyperoxia-inducedΔR(2) * and ΔR(1) as a noninvasive imaging biomarker of tumor hypoxia.

MATERIALS AND METHODS

MRI was performed on rat GH3 prolactinomas (n = 6) and human PC3 prostate xenografts (n = 6) propagated in nude mice. multiple gradient echo and inversion recovery truefisp images were acquired from identical transverse slices to quantify tumor R(2) * and R(1)before and during carbogen (95% O2 /5% CO2 ) challenge, and correlates of ΔR(2) * and ΔR(1) assessed.

RESULTS

Mean baseline R(2) * and R(1) were 119 ± 7 s(-1) and 0.6 ± 0.03 s(-1) for GH3 prolactinomas and 77 ± 12 s(-1) and 0.7 ± 0.02 s(-1) for PC3 xenografts, respectively. During carbogen breathing, mean ΔR(2) * and ΔR(1) were -20 ± 8 s(-1) and 0.08 ± 0.03 s(-1) for GH3 and -0.5 ± 1 s(-1) and 0.2 ± 0.08 s(-1) for the PC3 tumors, respectively. A pronounced relationship betweenΔR(2) * and ΔR(1) was revealed.

CONCLUSION

Considering the blood oxygen-hemoglobin dissociation curve, fast R2 * suggested that GH3 prolactinomas were more hypoxic at baseline, and their carbogen response dominated by increased hemoglobin oxygenation, evidenced by highly negative ΔR(2) *. PC3 tumors were less hypoxic at baseline, and their response to carbogen dominated by increased dissolved oxygen, evidenced by highly positive ΔR(1) . Because the two biomarkers are sensitive to different oxygenation ranges, the combination of ΔR(2) * and ΔR(1) may better characterize tumor hypoxia than each alone.

A multi-parametric imaging investigation of the response of C6 glioma xenografts to MLN0518 (tandutinib) treatment

Angiogenesis, the development of new blood vessels, is essential for tumour growth; this process is stimulated by the secretion of numerous growth factors including platelet derived growth factor (PDGF). PDGF signalling, through its receptor platelet derived growth factor receptor (PDGFR), is involved in vessel maturation, stimulation of angiogenesis and upregulation of other angiogenic factors, including vascular endothelial growth factor (VEGF). PDGFR is a promising target for anti-cancer therapy because it is expressed on both tumour cells and stromal cells associated with the vasculature. MLN0518 (tandutinib) is a potent inhibitor of type III receptor tyrosine kinases that demonstrates activity against PDGFRα/β, FLT3 and c-KIT. In this study a multi-parametric MRI and histopathological approach was used to interrogate changes in vascular haemodynamics, structural response and hypoxia in C6 glioma xenografts in response to treatment with MLN0518. The doubling time of tumours in mice treated with MLN0518 was significantly longer than tumours in vehicle treated mice. The perfused vessel area, number of alpha smooth muscle actin positive vessels and hypoxic area in MLN0518 treated tumours were also significantly lower after 10 days treatment. These changes were not accompanied by alterations in vessel calibre or fractional blood volume as assessed using susceptibility contrast MRI. Histological assessment of vessel size and total perfused area did not demonstrate any change with treatment. Intrinsic susceptibility MRI did not reveal any difference in baseline R2* or carbogen-induced change in R2*. Dynamic contrast-enhanced MRI revealed anti-vascular effects of MLN0518 following 3 days treatment. Hypoxia confers chemo- and radio-resistance, and alongside PDGF, is implicated in evasive resistance to agents targeted against VEGF signalling. PDGFR antagonists may improve potency and efficacy of other therapeutics in combination. This study highlights the challenges of identifying appropriate quantitative imaging response biomarkers in heterogeneous models, particularly considering the multifaceted roles of angiogenic growth factors.

Evaluation of clinically translatable MR imaging biomarkers of therapeutic response in the TH-MYCN transgenic mouse model of neuroblastoma

PURPOSE

To evaluate noninvasive and clinically translatable magnetic resonance (MR) imaging biomarkers of therapeutic response in the TH-MYCN transgenic mouse model of aggressive, MYCN-amplified neuroblastoma.

MATERIALS AND METHODS

All experiments were performed in accordance with the local ethical review panel and the UK Home Office Animals Scientific Procedures Act 1986 and with the UK National Cancer Research Institute guidelines for the welfare of animals in cancer research. Multiparametric MR imaging was performed of abdominal tumors found in the TH-MYCN model. T2-weighted MR imaging, quantitation of native relaxation times T1 and T2, the relaxation rate R2*, and dynamic contrast-enhanced MR imaging were used to monitor tumor response to cyclophosphamide (25 mg/kg), the vascular disrupting agent ZD6126 (200 mg/kg), or the antiangiogenic agent cediranib (6 mg/kg, daily). Any significant changes in the measured parameters, and in the magnitude of the changes after treatment between treated and control cohorts, were identified by using Student two-tailed paired and unpaired t test, respectively, with a 5% level of significance.

RESULTS

Treatment with cyclophosphamide or cediranib induced a 54% or 20% reduction in tumor volume at 48 hours, respectively (P < .005 and P < .005, respectively; P < .005 and P < .005 versus control, respectively). Treatment with ZD6126 induced a 45% reduction in mean tumor volume 24 hours after treatment (P < .005; P < .005 versus control). The antitumor activity of cyclophosphamide, cediranib, and ZD6126 was consistently associated with a decrease in tumor T1 (P < .005, P < .005, and P < .005, respectively; P < .005, P < .005, and P < .005 versus control, respectively) and with a correlation between therapy-induced changes in native T1 and changes in tumor volume (r = 0.56; P < .005). Tumor response to cediranib was also associated with a decrease in the dynamic contrast-enhanced MR imaging-derived volume transfer constant (P = .07; P < .05 versus control) and enhancing fraction (P < .05; P < .01 versus control), and an increase in R2* (P < .005; P < .05 versus control).

CONCLUSION

The T1 relaxation time is a robust noninvasive imaging biomarker of response to therapy in tumors in TH-MYCN mice, which emulate high-risk neuroblastoma in children. T1 measurements can be readily implemented on clinical MR systems and should be investigated in translational clinical trials of new targeted therapies for pediatric neuroblastoma.

SUPPLEMENTAL MATERIAL

http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.12120128/-/DC1.

MRI measurements of vessel calibre in tumour xenografts: comparison with vascular corrosion casting

Vessel size index (R_v, μm) has been proposed as a quantitative magnetic resonance imaging (MRI) derived imaging biomarker in oncology, for the non-invasive assessment of tumour blood vessel architecture and vascular targeted therapies. Appropriate pre-clinical evaluation of R_v in animal tumour models will improve the interpretation and guide the introduction of the biomarker into clinical studies. The objective of this study was to compare R_v measured in vivo with vessel size measurements from high-resolution X-ray computed tomography (μCT) of vascular corrosion casts measured post mortem from the same tumours, with and without vascular targeted therapy. MRI measurements were first acquired from subcutaneous SW1222 colorectal xenografts in mice following treatment with 0 (n = 6), 30 (n = 6) or 200 mg/kg (n = 3) of the vascular disrupting agent ZD6126. The mice were then immediately infused with a low viscosity resin and, following polymerisation and maceration of surrounding tissues, the resulting tumour vascular casts were dissected and subsequently imaged using an optimised μCT imaging approach. Vessel diameters were not measurable by μCT in the 200 mg/kg group as the high dose of ZD6126 precluded delivery of the resin to the tumour vascular bed. The mean R_v for the three treatment groups was 24, 23 and 23.5 μm respectively; the corresponding μCT measurements from corrosion casts from the 0 and 30 mg/kg cohorts were 25 and 28 μm. The strong association between the in vivo MRI and post mortem μCT values supports the use of R_v as an imaging biomarker in clinical trials of investigational vascular targeted therapies.

Non-invasive in vivo imaging of vessel calibre in orthotopic prostate tumour xenografts

Susceptibility contrast magnetic resonance imaging (MRI), utilising ultrasmall superparamagnetic iron oxide (USPIO) particles, was evaluated for the quantitation of vessel size index (Rv, μm), a weighted average measure of tumour blood vessel calibre, and fractional tumour blood volume (fBV, %), in orthotopically propagated murine PC3 prostate tumour xenografts. Tumour vascular architecture was assessed in vivo by MRI prior to and 24 hr after treatment with 200 mg/kg of the vascular disrupting agent ZD6126. A Bayesian hierarchical model (BHM) was used to reduce the uncertainty associated with quantitation of Rv and fBV. Quantitative histological analyses of the uptake of Hoechst 33342 for perfused vasculature, and haematoxylin and eosin staining for necrosis, were also performed to qualify the MRI data. A relatively large median Rv of 40.3 μm (90% confidence interval (CI90) = 37.4, 44.0 μm) and a high fBV of 5.4% (CI90 = 5.3, 5.5%) were determined in control tumours, which agreed with histologically determined vessel size index. Treatment with ZD6126 significantly (p < 0.01) reduced tumour Rv (34.2 μm, CI90 = 31.2, 38.0 μm) and fBV (3.9%, CI90 = 3.8, 4.1%), which were validated against histologically determined significant reductions in perfusion and vessel size, and increased necrosis. Together these data (i) highlight the use of a BHM to optimise the inferential power available from susceptibility contrast MRI data, (ii) provide strong evaluation and qualification of R(v) and fBV as non-invasive imaging biomarkers of tumour vascular morphology, (iii) reveal the presence of a different vascular phenotype and (iv) demonstrate that ZD6126 exhibits good anti-vascular activity against orthotopic prostate tumours.

Investigating temporal fluctuations in tumor vasculature with combined carbogen and ultrasmall superparamagnetic iron oxide particle (CUSPIO) imaging

A combined carbogen ultrasmall superparamagnetic iron oxide (USPIO) imaging protocol was developed and applied in vivo in two murine colorectal tumor xenograft models, HCT116 and SW1222, with established disparate vascular morphology, to investigate whether additional information could be extracted from the combination of two susceptibility MRI biomarkers. Tumors were imaged before and during carbogen breathing and subsequently following intravenous administration of USPIO particles. A novel segmentation method was applied to the image data, from which six categories of R(2)* response were identified, and compared with histological analysis of the vasculature. In particular, a strong association between a negative ΔR(2)*(carbogen) followed by positive ΔR(2)*(USPIO) with the uptake of the perfusion marker Hoechst 33342 was determined. Regions of tumor tissue where there was a significant ΔR(2)*(carbogen) but no significant ΔR(2)*(USPIO) were also identified, suggesting these regions became temporally isolated from the vascular supply during the experimental timecourse. These areas correlated with regions of tumor tissue where there was CD31 staining but no Hoechst 33342 uptake. Significantly, different combined carbogen USPIO responses were determined between the two tumor models. Combining ΔR(2)*(carbogen) and ΔR(2)*(USPIO) with a novel segmentation scheme can facilitate the interpretation of susceptibility contrast MRI data and enable a deeper interrogation of tumor vascular function and architecture.