Early detection of tumour immune-rejection using magnetic resonance imaging

Multiparametric MRI of early tumor response to immune checkpoint blockade in metastatic melanoma

BACKGROUND

Immune checkpoint inhibitors are now standard of care treatment for many cancers. Treatment failure in metastatic melanoma is often due to tumor heterogeneity, which is not easily captured by conventional CT or tumor biopsy. The aim of this prospective study was to investigate early microstructural and functional changes within melanoma metastases following immune checkpoint blockade using multiparametric MRI.

METHODS

Fifteen treatment-naïve metastatic melanoma patients (total 27 measurable target lesions) were imaged at baseline and following 3 and 12 weeks of treatment on immune checkpoint inhibitors using: T2-weighted imaging, diffusion kurtosis imaging, and dynamic contrast-enhanced MRI. Treatment timepoint changes in tumor cellularity, vascularity, and heterogeneity within individual metastases were evaluated and correlated to the clinical outcome in each patient based on Response Evaluation Criteria in Solid Tumors V.1.1 at 1 year.

RESULTS

Differential tumor growth kinetics in response to immune checkpoint blockade were measured in individual metastases within the same patient, demonstrating significant intertumoral heterogeneity in some patients. Early detection of tumor cell death or cell loss measured by a significant increase in the apparent diffusivity (Dapp) (p<0.05) was observed in both responding and pseudoprogressive lesions after 3 weeks of treatment. Tumor heterogeneity, as measured by apparent diffusional kurtosis (Kapp), was consistently higher in the pseudoprogressive and true progressive lesions, compared with the responding lesions throughout the first 12 weeks of treatment. These preceded tumor regression and significant tumor vascularity changes (Ktrans, ve, and vp) detected after 12 weeks of immunotherapy (p<0.05).

CONCLUSIONS

Multiparametric MRI demonstrated potential for early detection of successful response to immune checkpoint inhibitors in metastatic melanoma.

Multimodal Molecular Imaging Detects Early Responses to Immune Checkpoint Blockade

Immune checkpoint blockade (ICB) has become a standard therapy for several cancers, however, the response to ICB is inconsistent and a method for noninvasive assessment has not been established to date. To investigate the capability of multimodal imaging to evaluate treatment response to ICB therapy, hyperpolarized ¹³C MRI using [1-¹³C] pyruvate and [1,4-¹³C2] fumarate and dynamic contrast enhanced (DCE) MRI was evaluated to detect early changes in tumor glycolysis, necrosis, and intratumor perfusion/permeability, respectively. Mouse tumor models served as platforms for high (MC38 colon adenocarcinoma) and low (B16-F10 melanoma) sensitivity to dual ICB of PD-L1 and CTLA4. Glycolytic flux significantly decreased following treatment only in the less sensitive B16-F10 tumors. Imaging [1,4-¹³C2] fumarate conversion to [1,4-¹³C2] malate showed a significant increase in necrotic cell death following treatment in the ICB-sensitive MC38 tumors, with essentially no change in B16-F10 tumors. DCE-MRI showed significantly increased perfusion/permeability in MC38-treated tumors, whereas a similar, but statistically nonsignificant, trend was observed in B16-F10 tumors. When tumor volume was also taken into consideration, each imaging biomarker was linearly correlated with future survival in both models. These results suggest that hyperpolarized ¹³C MRI and DCE MRI may serve as useful noninvasive imaging markers to detect early response to ICB therapy. SIGNIFICANCE: Hyperpolarized ¹³C MRI and dynamic contrast enhanced MRI in murine tumor models provide useful insight into evaluating early response to immune checkpoint blockade therapy.See related commentary by Cullen and Keshari, p. 3444.

VCAM-1 Density and Tumor Perfusion Predict T-cell Infiltration and Treatment Response in Preclinical Models

Cancer immunotherapies have demonstrated durable responses in a range of different cancers. However, only a subset of patients responds to these therapies. We set out to test if non-invasive imaging of tumor perfusion and vascular inflammation may be able to explain differences in T-cell infiltration in pre-clinical tumor models, relevant for treatment outcomes. Tumor perfusion and vascular cell adhesion molecule (VCAM-1) density were quantified using magnetic resonance imaging (MRI) and correlated with infiltration of adoptively transferred and endogenous T-cells. MRI biomarkers were evaluated for their ability to detect tumor rejection 3 days after T-cell transfer. Baseline levels of these markers were used to assess their ability to predict PD-L1 treatment response. We found correlations between MRI-derived VCAM-1 density and infiltration of endogenous or adoptively transferred T-cells in some preclinical tumor models. Blocking T-cell binding to endothelial cell adhesion molecules (VCAM-1/ICAM) prevented T-cell mediated tumor rejection. Tumor rejection could be detected 3 days after adoptive T-cell transfer prior to tumor volume changes by monitoring the extracellular extravascular volume fraction. Imaging tumor perfusion and VCAM-1 density before treatment initiation was able to predict the response of MC38 tumors to PD-L1 blockade. These results indicate that MRI based assessment of tumor perfusion and VCAM-1 density can inform about the permissibility of the tumor vasculature for T-cell infiltration which may explain some of the observed variance in treatment response for cancer immunotherapies.

Analysis of 13 C and 14 C labeling in pyruvate and lactate in tumor and blood of lymphoma-bearing mice injected with 13 C- and 14 C-labeled pyruvate

Measurements of hyperpolarized 13 C label exchange between injected [1-13 C]pyruvate and the endogenous tumor lactate pool can give an apparent first-order rate constant for the exchange. The determination of the isotope flux, however, requires an estimate of the labeled pyruvate concentration in the tumor. This was achieved here by measurement of the tumor uptake of [1-14 C]pyruvate, which showed that <2% of the injected pyruvate reached the tumor site. Multiplication of this estimated labeled pyruvate concentration in the tumor with the apparent first-order rate constant for hyperpolarized 13 C label exchange gave an isotope flux that showed good agreement with a flux determined directly by the injection of non-polarized [3-13 C]pyruvate, rapid excision of the tumor after 30 s and measurement of 13 C-labeled lactate concentrations in tumor extracts. The distribution of labeled lactate between intra- and extracellular compartments and the blood pool was investigated by imaging, by measurement of the labeled lactate concentration in blood and tumor, and by examination of the effects of a gadolinium contrast agent and a lactate transport inhibitor on the intensity of the hyperpolarized [1-13 C]lactate signal. These measurements showed that there was significant export of labeled lactate from the tumor, but that labeled lactate in the blood pool produced by the injection of hyperpolarized [1-13 C]pyruvate showed only relatively low levels of polarization. This study shows that measurements of hyperpolarized 13 C label exchange between pyruvate and lactate in a murine tumor model can provide an estimate of the true isotope flux if the concentration of labeled pyruvate that reaches the tumor can be determined.

Effects of fasting on serial measurements of hyperpolarized [1-(13) C]pyruvate metabolism in tumors

Imaging of the metabolism of hyperpolarized [1-(13) C]pyruvate has shown considerable promise in preclinical studies in oncology, particularly for the assessment of early treatment response. The repeatability of measurements of (13) C label exchange between pyruvate and lactate was determined in a murine lymphoma model in fasted and non-fasted animals. The fasted state showed lower intra-individual variability, although the [1-(13) C]lactate/[1-(13) C]pyruvate signal ratio was significantly greater in fasted than in non-fasted mice, which may be explained by the higher tumor lactate concentrations in fasted animals. These results indicate that the fasted state may be preferable for the measurement of (13) C label exchange between pyruvate and lactate, as it reduces the variability and therefore should make it easier to detect the effects of therapy. © 2016 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.

Diaminosulfide based polymer microparticles as cancer vaccine delivery systems

The aim of the research presented here was to determine the characteristics and immunostimulatory capacity, in vivo, of antigen and adjuvant co-loaded into microparticles made from a novel diaminosulfide polymer, poly(4,4'-trimethylenedipiperdyl sulfide) (PNSN), and to assess their potential as cancer vaccine vectors. PNSN microparticles co-loaded with the antigen, ovalbumin (OVA), and adjuvant, CpG 1826, (PNSN(OVA + CpG)) were fabricated and characterized for size (1.64 μm diameter; PDI=0.62), charge (-23.1 ± 0.3), and loading efficiencies of antigen (7.32 μg/mg particles) and adjuvant (0.95 μg/mg particles). The ability of PNSN(OVA + CpG) to stimulate cellular and humoral immune responses in vivo was compared with other PNSN microparticle formulations as well as with poly(lactic-co-glycolic acid)(PLGA)-based microparticles, co-loaded with OVA and CpG (PLGA(OVA + CpG)), an adenovirus encoding OVA (Ad5-OVA), and OVA delivered with incomplete Freund's adjuvant (IFA(OVA)). In vivo OVA-specific IgG1 responses, after subcutaneous prime/boosts in mice, were similar when PNSN(OVA + CpG) and PLGA(OVA + CpG) were compared and the presence of CpG 1826 within the PNSN microparticles demonstrated significantly improved responses when compared to PNSN microparticles loaded with OVA alone (PNSN(OVA)), plus or minus soluble CpG 1826. Cellular immune responses to all particle-based vaccine formulations ranged from being negligible to modest with PNSN(OVA + CpG) generating the greatest responses, displaying significantly increased levels of OVA-specific CD8+ T lymphocytes compared to controls and IFA(OVA) treated mice. Finally, it was shown that of all vaccination formulations tested PNSN(OVA + CpG) was the most protective against subsequent challenge with an OVA-expressing tumor cell line, E.G7. Thus, microparticles made from poly(diaminosulfide)-based macromolecules possess promising potential as vaccine vectors and, as demonstrated here, may have impact as cancer vaccines in particular.

Imaging pH with hyperpolarized 13C

pH is a fundamental physiological parameter that is tightly controlled by endogenous buffers. The acid-base balance is altered in many disease states, such as inflammation, ischemia and cancer. Despite the importance of pH, there are currently no routine methods for imaging the spatial distribution of pH in humans. The enormous gain in sensitivity afforded by dynamic nuclear polarization (DNP) has provided a novel way in which to image tissue pH using MR, which has the potential to be translated into the clinic. This review explores the advantages and disadvantages of current pH imaging techniques and how they compare with DNP-based approaches for the measurement and imaging of pH with hyperpolarized (13)C. Intravenous injection of hyperpolarized (13)C-labeled bicarbonate results in the rapid production of hyperpolarized (13)CO(2) in the reaction catalyzed by carbonic anhydrase. As this reaction is close to equilibrium in the body and is pH dependent, the ratio of the (13)C signal intensities from H(13)CO(3)(-) and (13)CO(2), measured using MRS, can be used to calculate pH in vivo. The application of this technique to a murine tumor model demonstrated that it measured predominantly extracellular pH and could be mapped in the animal using spectroscopic imaging techniques. A second approach has been to use the production of hyperpolarized (13)CO(2) from hyperpolarized [1-(13)C]pyruvate to measure predominantly intracellular pH. In tissues with a high aerobic capacity, such as the heart, the hyperpolarized [1-(13)C]pyruvate undergoes rapid oxidative decarboxylation, catalyzed by intramitochondrial pyruvate dehydrogenase. Provided that there is sufficient carbonic anhydrase present to catalyze the rapid equilibration of the hyperpolarized (13)C label between CO(2) and bicarbonate, the ratio of their resonance intensities may again be used to estimate pH, which, in this case, is predominantly intracellular. As both pyruvate and bicarbonate are endogenous molecules they have the potential to image tissue pH in the clinic.

Assessing responses to cancer therapy using molecular imaging

Tumor responses to therapy in the clinic are still evaluated primarily from non-invasive imaging measurements of reductions in tumor size. This approach, however, lacks sensitivity and can only give a delayed indication of a positive response to treatment. Major advances in our understanding of the molecular mechanisms responsible for cancer, combined with new targeted clinical imaging technologies designed to detect the molecular correlates of disease progression and response to treatment, are set to revolutionize our approach to the detection and treatment of the disease. We describe here the imaging technologies available to image tumor cell proliferation and migration, metabolism, receptor and gene expression, apoptosis and tumor angiogenesis and vascular function, and show how measurements of these parameters can be used to give early indications of positive responses to treatment or to detect drug resistance and/or disease recurrence. Special emphasis has been placed on those applications that are already used in the clinic and those that are likely to translate into clinical application in the near future or whose use in preclinical studies is likely to facilitate translation of new treatments into the clinic.

Monitoring T-lymphocyte trafficking in tumors undergoing immune rejection

Activated T cells, isolated from animals that had rejected a tumor (E.G7-OVA) expressing chicken ovalbumin, were labeled with citrated superparamagnetic iron oxide nanoparticles at an intracellular iron concentration of up to 0.5 pg/cell. Injection of these labeled T cells into animals bearing E.G7-OVA tumors undergoing immune rejection resulted in tumor infiltration of these cells, which was detectable as a heterogeneous decrease in intensity in T(2)-weighted MR images. T-cell infiltration was confirmed by immunohistochemical staining of tumor sections obtained postmortem and was shown to colocalize with iron that had been stained using Prussian blue. Tumor rejection was correlated with the uptake of labeled T cells, since the infiltration of labeled T cells was only observed in those tumors that went on to regress. This technique should assist in the elucidation of those factors that are important in mediating tumor immune rejection.

Molecular imaging using magnetic resonance: new tools for the development of tumour therapy

Molecular imaging - the exploitation of specific molecules as the source of image contrast - promises new insights into disease processes in the laboratory and since the imaging modalities employed are applicable clinically, can be used to translate this knowledge into new diagnostics and treatments in the clinic. This brief review focuses on the use of MR-based molecular imaging techniques for developing tumour therapy. As examples, methods for detecting drug-induced tumour cell apoptosis; the response of tumours and their susceptibilities to an antivascular drug; early signs of tumour immune rejection and methods for detecting immune cell infiltration of tumours are described.

Uric acid promotes tumor immune rejection

Uric acid released from dying cells has been shown recently to act as a danger signal for the immune system, stimulating dendritic cell maturation and enhancing T-cell responses to foreign antigens. Stimulation of dendritic cell maturation by uric acid has been proposed as a mechanism by which the immune system could generate responses against tumors. We show here that uric acid levels are elevated in tumors undergoing immune rejection and that the inhibition of uric acid production, by systemic administration of allopurinol, or the removal of uric acid, by administration of uricase, delayed tumor immune rejection, whereas subcutaneous administration of crystalline uric acid enhanced the rejection process.

Tumor Cell-Derived Nitric Oxide Is Involved in the Immune-Rejection of an Immunogenic Murine Lymphoma

The roles played by host-derived nitric oxide (NO) in the growth and subsequent immune rejection of a immunogenic murine lymphoma were investigated by growing the tumor in mice in which the gene for either inducible NO synthase (iNOS) or endothelial NOS (eNOS) had been ablated. This showed that NO from tumor-infiltrating host cells had no significant effect on either tumor growth or immune rejection, although measurements of tumor nitrite levels and protein nitration showed that there had been significant NO production in the rejected tumors, in both the eNOS and iNOS knockout mice. Inhibition of both tumor and host NOS activities, with an iNOS-selective inhibitor (1400W), a nonselective NOS inhibitor [Nomega-nitro-L-arginine methyl ester (L-NAME)], or scavenging NO with a ruthenium-based scavenger, significantly delayed tumor rejection, while having no appreciable effect on tumor growth. Incubation of tumor cells with medium taken from cultured splenocytes, that had been isolated from immunized animals and activated by incubating them with irradiated tumor cells, resulted in an increase in tumor cell NOS activity and an increase in tumor cell apoptosis, which could be inhibited using L-NAME. We propose that, during the immune rejection of this tumor model, there is induction of tumor NOS activity by cytokines secreted by activated lymphocytes within the tumor and that this results in increased levels of tumor NO that induce tumor cell apoptosis and facilitate immune rejection of the tumor.