PET of glial metabolism using 2-18F-fluoroacetate.

UNLABELLED: Imaging of the glial activation that occurs in response to central nervous system trauma and inflammation could become a powerful technique for the assessment of several neuropathologies. The selective uptake and metabolism of 2-(18)F-fluoroacetate ((18)F-FAC) in glia may represent an attractive strategy for imaging glial metabolism. METHODS: We have evaluated the use of (18)F-FAC as a specific PET tracer of glial cell metabolism in rodent models of glioblastoma, stroke, and ischemia-hypoxia. RESULTS: Enhanced uptake of (18)F-FAC was observed (6.98 +/- 0.43 percentage injected dose per gram [%ID/g]; tumor-to-normal ratio, 1.40) in orthotopic U87 xenografts, compared with healthy brain tissue. The lesion extent determined by (18)F-FAC PET correlated with that determined by MRI (R(2) = 0.934, P = 0.007). After transient middle cerebral artery occlusion in the rat brain, elevated uptake of (18)F-FAC (1.00 +/- 0.03 %ID/g; lesion-to-normal ratio, 1.90) depicted the ischemic territory and correlated with infarct volumes as determined by 2,3,5-triphenyltetrazolium chloride staining (R(2) = 0.692, P = 0.010) and with the presence of activated astrocytes detected by anti-glial fibrillary acidic protein. Ischemia-hypoxia, induced by permanent ligation of the common carotid artery with transient hypoxia, resulted in persistent elevation of (18)F-FAC uptake within 30 min of the induction of hypoxia. CONCLUSION: Our data support the further evaluation of (18)F-FAC PET for the assessment of glial cell metabolism associated with neuroinflammation.

Preparation and evaluation of L- and D-5-[18F]fluorotryptophan as PET imaging probes for indoleamine and tryptophan 2,3-dioxygenases.

Indoleamine and tryptophan 2,3-dioxygenases (IDO1 and TDO2) are pyrrolases catalyzing the oxidative cleavage of the 2,3-double bond of L-tryptophan in kynurenine pathway. In the tumor microenvironment, their increased activity prevents normal immune function, i.e. tumor cell recognition and elimination by cytotoxic T-cells. Consequently, inhibition of the kynurenine pathway may enhance the activity of cancer immunotherapeutics by reversing immune dysfunction. We sought to investigate the properties of radiolabeled 5-[18F]fluorotryptophan with respect to its ability for measuring IDO1 and TDO2 activity by positron emission tomography (PET). RESULTS: L-5-[18F]fluorotryptophan and D-5-[18F]fluorotryptophan were synthesized by Cu(I) catalyzed [18F]fluorodeboronylation of Boc/tBu protected precursors in moderate yields (1.5±0.6%) sufficient for pre-clinical studies. The specific activity of the product was 407-740GBq/µmol, radiochemical purity >99% and enantiomeric excess 90-99%. Enzymatic assay confirmed that L-5-fluorotryptophan is an IDO1 and TDO2 substrate whereas the D-isomer is not. In-vitro cell uptake experiments using CT26 cells with doxycycline-induced overexpression of human-IDO1 and human-TDO2 revealed an elevated cell uptake of L-5-[18F]fluorotryptophan upon induction of IDO1 or TDO2 enzymes compared to baseline; however, the uptake was observed only in the presence of low L-tryptophan levels in media. PET imaging experiments performed using tumor bearing mouse models expressing IDO1 at various levels (CT26, CT26-hIDO1, 17082A, 17095A) showed tumor uptake of the tracer elevated up to 8%ID/g; however, the observed tumor uptake could not be attributed to IDO1 activity in the tumor tissue. The metabolism of L- and D- isomers was markedly different in vivo, the D-isomer was excreted by a combination of hepatobiliary and renal routes, the L-isomer underwent extensive metabolism to [18F]fluoride. CONCLUSION: The observed in vivo tumor uptake of the tracer could not be attributed to IDO1 or TDO2 enzyme activity in the tumor, presumably due to competition with endogenous tryptophan as well as rapid tracer metabolism.

Preclinical Efficacy of an Antibody-Drug Conjugate Targeting Mesothelin Correlates with Quantitative 89Zr-ImmunoPET.

Antibody-drug conjugates (ADC) use monoclonal antibodies (mAb) as vehicles to deliver potent cytotoxic drugs selectively to tumor cells expressing the target. Molecular imaging with zirconium-89 (89Zr)-labeled mAbs recapitulates similar targeting biology and might help predict the efficacy of these ADCs. An anti-mesothelin antibody (AMA, MMOT0530A) was used to make comparisons between its efficacy as an ADC and its tumor uptake as measured by 89Zr immunoPET imaging. Mesothelin-targeted tumor growth inhibition by monomethyl auristatin E (MMAE), ADC AMA-MMAE (DMOT4039A), was measured in mice bearing xenografts of ovarian cancer OVCAR-3×2.1, pancreatic cancers Capan-2, HPAC, AsPC-1, and HPAF-II, or mesothelioma MSTO-211H. Ex vivo analysis of mesothelin expression was performed using immunohistochemistry. AMA-MMAE showed the greatest growth inhibition in OVCAR-3×2.1, Capan-2, and HPAC tumors, which showed target-specific tumor uptake of 89Zr-AMA. The less responsive xenografts (AsPC-1, HPAF-II, and MSTO-211H) did not show 89Zr-AMA uptake despite confirmed mesothelin expression. ImmunoPET can demonstrate the necessary delivery, binding, and internalization of an ADC antibody in vivo and this correlates with the efficacy of mesothelin-targeted ADC in tumors vulnerable to the cytotoxic drug delivered. Mol Cancer Ther; 16(1); 134-42. ©2016 AACR.

Evaluation of a 3-hydroxypyridin-2-one (2,3-HOPO) Based Macrocyclic Chelator for (89)Zr(4+) and Its Use for ImmunoPET Imaging of HER2 Positive Model of Ovarian Carcinoma in Mice.

A novel octadentate 3-hydroxypyridin-2-one (2,3-HOPO) based di-macrocyclic ligand was evaluated for chelation of (89)Zr; subsequently, it was used as a bi-functional chelator for preparation of (89)Zr-labeled antibodies. Quantitative chelation of (89)Zr(4+) with the octadentate ligand forming (89)ZrL complex was achieved under mild conditions within 15 minutes. The (89)Zr-complex was stable in vitro in presence of DTPA, but a slow degradation was observed in serum. In vivo, the hydrophilic (89)Zr-complex showed prevalently renal excretion; and an elevated bone uptake of radioactivity suggested a partial release of (89)Zr(4+) from the complex. The 2,3-HOPO based ligand was conjugated to the monoclonal antibodies, HER2-specific trastuzumab and an isotypic anti-gD antibody, using a p-phenylene bis-isothiocyanate linker to yield products with an average loading of less than 2 chelates per antibody. Conjugated antibodies were labeled with (89)Zr under mild conditions providing the PET tracers in 60-69% yield. Despite the limited stability in mouse serum; the PET tracers performed very well in vivo. The PET imaging in mouse model of HER2 positive ovarian carcinoma showed tumor uptake of (89)Zr-trastuzumab (29.2 ± 12.9 %ID/g) indistinguishable (p = 0.488) from the uptake of positive control (89)Zr-DFO-trastuzumab (26.1 ± 3.3 %ID/g). In conclusion, the newly developed 3-hydroxypyridin-2-one based di-macrocyclic chelator provides a viable alternative to DFO-based heterobifunctional ligands for preparation of (89)Zr-labeled monoclonal antibodies for immunoPET studies.

ImmunoPET helps predicting the efficacy of antibody-drug conjugates targeting TENB2 and STEAP1.

The efficacy of antibody-drug conjugates (ADCs) targeted to solid tumors depends on biological processes that are hard to monitor in vivo. 89Zr-immunoPET of the ADC antibodies could help understand the performance of ADCs in the clinic by confirming the necessary penetration, binding, and internalization. This work studied monomethyl auristatin E (MMAE) ADCs against two targets in metastatic castration-resistant prostate cancer, TENB2 and STEAP1, in four patient-derived tumor models (LuCaP35V, LuCaP70, LuCaP77, LuCaP96.1). Three aspects of ADC biology were measured and compared: efficacy was measured in tumor growth inhibition studies; target expression was measured by immunohistochemistry and flow cytometry; and tumor antibody uptake was measured with 111In-mAbs and gamma counting or with 89Zr-immunoPET. Within each model, the mAb with the highest tumor uptake showed the greatest potency as an ADC. Sensitivity between models varied, with the LuCaP77 model showing weak efficacy despite high target expression and high antibody uptake. Ex vivo analysis confirmed the in vivo results, showing a correlation between expression, uptake and ADC efficacy. We conclude that 89Zr-immunoPET data can demonstrate which ADC candidates achieve the penetration, binding, and internalization necessary for efficacy in tumors sensitive to the toxic payload.

ImmunoPET imaging of phosphatidylserine in pro-apoptotic therapy treated tumor models.

UNLABELLED: An immunoPET imaging probe for the detection of phosphatidylserine was developed and tested in animal models of human cancer treated with pro-apoptotic therapy. We hypothesized that the relatively long plasma half-life of a probe based on a full-length antibody coupled with a residualizing radionuclide would be able to catch the wave of drug-induced apoptosis and lead to a specific accumulation in apoptotic tumor tissue. METHODS: The imaging probe is based on a 89Zr-labeled monoclonal antibody PGN635 targeting phosphatidylserine. The probe was evaluated pre-clinically in four tumor xenograft models: one studied treatment with paclitaxel to trigger the intrinsic apoptotic pathway, and three others interrogated treatment with an agonistic death-receptor monoclonal antibody to engage the extrinsic apoptotic pathway. RESULTS: High accumulation of 89Zr-PGN635 was observed in treated tumors undergoing apoptosis reaching 30 %ID/g and tumor-to-blood ratios up to 13. The tumor uptake in control groups treated with vehicle or imaged with a non-binding antibody probe was significantly lower. CONCLUSIONS: The results demonstrate the ability of 89Zr-PGN635 to image drug-induced apoptosis in animal models and corroborate our hypothesis that radiolabeled antibodies binding to intracellular targets transiently exposed on the cell surface during apoptosis can be employed for detection of tumor response to therapy.

The development of peptide-based tools for the analysis of angiogenesis.

Limitations to the application of molecularly targeted cancer therapies are the inability to accurately match patient with effective treatment and the absence of a prompt readout of posttreatment response. Noninvasive agents that rapidly report vascular endothelial growth factor (VEGF) levels using positron emission tomography (PET) have the potential to enhance anti-angiogenesis therapies. Using phage display, two distinct classes of peptides were identified that bind to VEGF with nanomolar affinity and high selectivity. Co-crystal structures of these different peptide classes demonstrate that both bind to the receptor-binding region of VEGF. (18)F-radiolabelling of these peptides facilitated the acquisition of PET images of tumor VEGF levels in a HM7 xenograph model. The images obtained from one 59-residue probe, (18)F-Z-3B, 2 hr postinjection are comparable to those obtained with anti-VEGF antibody B20 72 hr postinjection. Furthermore, VEGF levels in growing SKOV3 tumors were followed using (18)F-Z-3B as a PET probe with VEGF levels increasing with tumor size.

Site-specifically 89Zr-labeled monoclonal antibodies for ImmunoPET.

UNLABELLED: Three thiol reactive reagents were developed for the chemoselective conjugation of desferrioxamine (Df) to a monoclonal antibody via engineered cysteine residues (thio-trastuzumab). The in vitro stability and in vivo imaging properties of site-specifically radiolabeled (89)Zr-Df-thio-trastuzumab conjugates were investigated. METHODS: The amino group of desferrioxamine B was acylated by bromoacetyl bromide, N-hydroxysuccinimidyl iodoacetate, or N-hydroxysuccinimidyl 4-[N-maleimidomethyl]cyclohexane-1-carboxylate to obtain thiol reactive reagents bromoacetyl-desferrioxamine (Df-Bac), iodoacetyl-desferrioxamine (Df-Iac) and maleimidocyclohexyl-desferrioxamine (Df-Chx-Mal), respectively. Df-Bac and Df-Iac alkylated the free thiol groups of thio-trastuzumab by nucleophilic substitution forming Df-Ac-thio-trastuzumab, while the maleimide reagent Df-Chx-Mal reacted via Michael addition to provide Df-Chx-Mal-thio-trastuzumab. The conjugates were radiolabeled with (89)Zr and evaluated for serum stability, and their positron emission tomography (PET) imaging properties were investigated in a BT474M1 (HER2-positive) breast tumor mouse model. RESULTS: The chemoselective reagents were obtained in 14% (Df-Bac), 53% (Df-Iac) and 45% (Df-Chx-Mal) yields. Site-specific conjugation of Df-Chx-Mal to thio-trastuzumab was complete within 1 h at pH 7.5, while Df-Iac and Df-Bac respectively required 2 and 5 h at pH 9. Each Df modified thio-trastuzumab was chelated with (89)Zr in yields exceeding 75%. (89)Zr-Df-Ac-thio-trastuzumab and (89)Zr-Df-Chx-Mal-thio-trastuzumab were stable in mouse serum and exhibited comparable PET imaging capabilities in a BT474M1 (HER2-positive) breast cancer model reaching 20-25 %ID/g of tumor uptake and a tumor to blood ratio of 6.1-7.1. CONCLUSIONS: The new reagents demonstrated good reactivity with engineered thiol groups of trastuzumab and very good chelation properties with (89)Zr. The site-specifically (89)Zr-labeled thio-antibodies were stable in serum and showed PET imaging properties comparable to lysine conjugates.

A modular platform for the rapid site-specific radiolabeling of proteins with 18F exemplified by quantitative positron emission tomography of human epidermal growth factor receptor 2.

Receptor-specific proteins produced by genetic engineering are attractive as PET imaging agents, but labeling with conventional (18)F-based prosthetic groups is problematic due to long synthesis times, poor radiochemical yields, and low specific activities. Therefore, we developed a modular platform for the rapid preparation of water-soluble prosthetic groups capable of efficiently introducing (18)F into proteins. The utility of this platform is demonstrated by the thiol-specific prosthetic group, [(18)F]FPEGMA, which was used to produce site-specifically (18)F-labeled protein ((18)F-trastuzumab-ThioFab) in 82 min with a total radiochemical yield of 13 +/- 3% and a specific activity of 2.2 +/- 0.2 Ci/micromol. (18)F-trastuzumab-ThioFab retained the biological activity of native protein and was successfully validated in vivo with microPET imaging of Her2 expression in a xenograft tumor-bearing murine model modulated by the Hsp90 inhibitor, 17-(allylamino)-17-demethoxygeldanamycin.