BackgroundMicroglia are major players in the pathogenesis of multiple sclerosis (MS) and may play a dual role in disease progression. The activation status of microglia in vivo is highly dynamic and occurs as a continuum, with the pro-inflammatory and anti-inflammatory phenotypes on either end of this spectrum. Little is known about in vivo dynamics of microglia phenotypes in MS due to the lack of diagnostic tools. Positron emission tomography (PET) imaging is a powerful non-invasive technique that allows real-time imaging of microglia activation phenotypes in the central nervous system, depending on the availability of selective PET tracers. Our objective is to investigate and characterize the expression of the purinergic receptors P2Y12R and P2X7R as potential targets for PET tracer development and subsequent PET imaging in order to evaluate the dynamics of microglia status in vivo.MethodsWe used immunohistochemical analysis to explore the expression of P2Y12R and P2X7R in experimental autoimmune encephalomyelitis (EAE) post-mortem tissues and different stages of well-characterized MS lesions. We evaluated by quantitative real-time polymerase chain reaction the expression of P2Y12R and P2X7R in human polarized microglia, and we performed autoradiography binding assay with radiolabeled P2Y12R and P2X7R antagonists using MS and rat EAE tissues.ResultsHere, we demonstrate that P2X7R is associated with a pro-inflammatory phenotype of human microglia in vitro, and is highly expressed in microglia in MS lesions as well as during the peak of EAE. In contrast, P2Y12R was associated with an anti-inflammatory phenotype in human microglia in vitro and was expressed at lower levels in active inflammatory MS lesions compared to normal-appearing white matter (NAWM) and similarly in EAE, while its expression increased in the remission phase of EAE. Binding of radiolabeled tracers specific for P2Y12R and P2X7R on ex vivo tissues validated the value of these receptors as PET imaging targets for microglia phenotypes in vivo.ConclusionOur results suggest that P2Y12R and P2X7R are excellent targets for PET imaging to discriminate distinct microglia phenotypes in MS. Ultimately, this may provide insight into the role of microglia in disease progression and monitor novel treatment strategies to alter microglia phenotype.Electronic supplementary materialThe online version of this article (10.1186/s12974-017-1034-z) contains supplementary material, which is available to authorized users.

Melanoma is a malignant tumor derived from epidermal melanocytes, and it is known for its aggressiveness, therapeutic resistance, and predisposition for late metastasis. Very late antigen-4 (VLA-4; also called integrin α4β1) is a transmembrane noncovalent heterodimer overexpressed in melanoma tumors that plays an important role in tumor growth, angiogenesis, and metastasis by promoting adhesion and migration of cancer cells. In this study, we evaluated 2 conjugates of a high-affinity VLA-4 peptidomimetic ligand, LLP2A, for PET/CT imaging in a subcutaneous and metastatic melanoma tumor. Methods LLP2A was conjugated to 1,4,8,11-tetraazacyclotetradecane-1-(methane phosphonic acid)-8-(methane carboxylic acid) (CB-TE1A1P) and 2-(4,7-bis(carboxymethyl)-1,4,7-triazonan-1-yl)pentanedioic acid (NODAGA) chelators for 68Ga and 64Cu labeling. The conjugates were synthesized by solid-phase peptide synthesis, purified by reversed-phase high-performance liquid chromatography, and verified by liquid chromatography mass spectrometry. Saturation and competitive binding assays with B16F10 melanoma cells determined the affinity of the compounds for VLA-4. The biodistributions of the LLP2A conjugates were evaluated in murine B16F10 subcutaneous tumor–bearing C57BL/6 mice. Melanoma metastasis was induced by intracardiac injection of B16F10 cells. PET/CT imaging was performed at 2, 4, and 24 h after injection for the 64Cu tracers and 1 h after injection for the 68Ga tracer. Results 64Cu-labeled CB-TE1A1P-PEG4-LLP2A and NODAGA-PEG4-LLP2A showed high affinity to VLA-4, with a comparable dissociation constant (0.28 vs. 0.23 nM) and receptor concentration (296 vs. 243 fmol/mg). The tumor uptake at 2 h after injection was comparable for the 2 probes, but 64Cu-CB-TE1A1P-PEG4-LLP2A trended toward higher uptake than 64Cu-NODAGA-PEG4-LLP2A (16.9 ± 2.2 vs. 13.4 ± 1.7 percentage injected dose per gram, P = 0.07). Tumor-to-muscle and tumor-to-blood ratios from biodistribution and PET/CT images were significantly higher for 64Cu-CB-TE1A1P-PEG4-LLP2A than 64Cu-NODAGA-PEG4-LLP2A (all P values < 0.05). PET/CT imaging of metastatic melanoma with 68Ga-NODAGA-PEG4-LLP2A and 64Cu-NODAGA-PEG4-LLP2A showed high uptake of the probes at the site of metastasis, correlating with the bioluminescence imaging of the tumor. Conclusion These data demonstrate that 64Cu-labeled CB-TE1A1P/NODAGA LLP2A conjugates and 68Ga-labeled NODAGA-LLP2A are excellent imaging agents for melanoma and potentially other VLA-4–positive tumors. 64Cu-CB-TE1A1P-PEG4-LLP2A had the most optimal tumor–to–nontarget tissue ratios for translation into humans as a PET imaging agent for melanoma.

The P2X7 receptor plays a significant role in microglial activation, and as a potential drug target, the P2X7 receptor is also an interesting target in positron emission tomography. The current study aimed at the development and evaluation of a potent tracer targeting the P2X7 receptor, to which end four adamantanyl benzamide analogues with high affinity for the human P2X7 receptor were labelled with carbon-11. All four analogues could be obtained in excellent radiochemical yield and high radiochemical purity and molar activity, and all analogues entered the rat brain. [11C]SMW139 showed the highest metabolic stability in rat plasma, and showed high binding to the hP2X7 receptor in vivo in a hP2X7 receptor overexpressing rat model. Although no significant difference in binding of [11C]SMW139 was observed between post mortem brain tissue of Alzheimer’s disease patients and that of healthy controls in in vitro autoradiography experiments, [11C]SMW139 could be a promising tracer for P2X7 receptor imaging using positron emission tomography, due to high receptor binding in vivo in the hP2X7 receptor overexpressing rat model. However, further investigation of both P2X7 receptor expression and binding of [11C]SMW139 in other neurological diseases involving microglial activation is warranted.

Purpose Almost all radiolabellings of antibodies with 89 Zr currently employ the hexadentate chelator desferrioxamine (DFO). However, DFO can lead to unwanted uptake of 89 Zr in bones due to instability of the resulting metal complex. DFO*-NCS and the squaramide ester of DFO, DFOSq, are novel analogues that gave more stable 89 Zr complexes than DFO in pilot experiments. Here, we directly compare these linker-chelator systems to identify optimal immuno-PET reagents. Methods Cetuximab, trastuzumab and B12 (non-binding control antibody) were labelled with 89 Zr via DFO*-NCS, DFOSq, DFO-NCS or DFO*Sq. Stability in vitro was compared at 37°C in serum (7 days), in formulation solution (24 h ± chelator challenges) and in vivo with N87 and A431 tumour-bearing mice. Finally, to demonstrate the practical benefit of more stable complexation for the accurate detection of bone metastases, [ 89 Zr]Zr-DFO*-NCS and [ 89 Zr]Zr-DFO-NCS-labelled trastuzumab and B12 were evaluated in a bone metastasis mouse model where BT-474 breast cancer cells were injected intratibially. Results [ 89 Zr]Zr-DFO*-NCS-trastuzumab and [ 89 Zr]Zr-DFO*Sq-trastuzumab showed excellent stability in vitro, superior to their [ 89 Zr]Zr-DFO counterparts under all conditions. While tumour uptake was similar for all conjugates, bone uptake was lower for DFO* conjugates. Lower bone uptake for DFO* conjugates was confirmed using a second xenograft model: A431 combined with cetuximab. Finally, in the intratibial BT-474 bone metastasis model, the DFO* conjugates provided superior detection of tumour-specific signal over the DFO conjugates. Conclusion DFO*-mAb conjugates provide lower bone uptake than their DFO analogues; thus, DFO* is a superior candidate for preclinical and clinical 89 Zr-immuno-PET. Keywords Bone metastasis model. DFO. DFO*. DFOSq. DFO*Sq. 89 Zr-immuno-PET This article is part of the Topical Collection on Preclinical Imaging