A Clinical PET Imaging Tracer ([18F]DASA-23) to Monitor Pyruvate Kinase M2-Induced Glycolytic Reprogramming in Glioblastoma

PURPOSE

Pyruvate kinase M2 (PKM2) catalyzes the final step in glycolysis, a key process of cancer metabolism. PKM2 is preferentially expressed by glioblastoma (GBM) cells with minimal expression in healthy brain. We describe the development, validation, and translation of a novel PET tracer to study PKM2 in GBM. We evaluated 1-((2-fluoro-6-[18F]fluorophenyl)sulfonyl)-4-((4-methoxyphenyl)sulfonyl)piperazine ([18F]DASA-23) in cell culture, mouse models of GBM, healthy human volunteers, and patients with GBM.

EXPERIMENTAL DESIGN

[18F]DASA-23 was synthesized with a molar activity of 100.47 ± 29.58 GBq/μmol and radiochemical purity >95%. We performed initial testing of [18F]DASA-23 in GBM cell culture and human GBM xenografts implanted orthotopically into mice. Next, we produced [18F]DASA-23 under FDA oversight, and evaluated it in healthy volunteers and a pilot cohort of patients with glioma.

RESULTS

In mouse imaging studies, [18F]DASA-23 clearly delineated the U87 GBM from surrounding healthy brain tissue and had a tumor-to-brain ratio of 3.6 ± 0.5. In human volunteers, [18F]DASA-23 crossed the intact blood-brain barrier and was rapidly cleared. In patients with GBM, [18F]DASA-23 successfully outlined tumors visible on contrast-enhanced MRI. The uptake of [18F]DASA-23 was markedly elevated in GBMs compared with normal brain, and it identified a metabolic nonresponder within 1 week of treatment initiation.

CONCLUSIONS

We developed and translated [18F]DASA-23 as a new tracer that demonstrated the visualization of aberrantly expressed PKM2 for the first time in human subjects. These results warrant further clinical evaluation of [18F]DASA-23 to assess its utility for imaging therapy-induced normalization of aberrant cancer metabolism.

True ultra-low-dose amyloid PET/MRI enhanced with deep learning for clinical interpretation

PURPOSE

While sampled or short-frame realizations have shown the potential power of deep learning to reduce radiation dose for PET images, evidence in true injected ultra-low-dose cases is lacking. Therefore, we evaluated deep learning enhancement using a significantly reduced injected radiotracer protocol for amyloid PET/MRI.

METHODS

Eighteen participants underwent two separate ¹⁸F-florbetaben PET/MRI studies in which an ultra-low-dose (6.64 ± 3.57 MBq, 2.2 ± 1.3% of standard) or a standard-dose (300 ± 14 MBq) was injected. The PET counts from the standard-dose list-mode data were also undersampled to approximate an ultra-low-dose session. A pre-trained convolutional neural network was fine-tuned using MR images and either the injected or sampled ultra-low-dose PET as inputs. Image quality of the enhanced images was evaluated using three metrics (peak signal-to-noise ratio, structural similarity, and root mean square error), as well as the coefficient of variation (CV) for regional standard uptake value ratios (SUVRs). Mean cerebral uptake was correlated across image types to assess the validity of the sampled realizations. To judge clinical performance, four trained readers scored image quality on a five-point scale (using 15% non-inferiority limits for proportion of studies rated 3 or better) and classified cases into amyloid-positive and negative studies.

RESULTS

The deep learning-enhanced PET images showed marked improvement on all quality metrics compared with the low-dose images as well as having generally similar regional CVs as the standard-dose. All enhanced images were non-inferior to their standard-dose counterparts. Accuracy for amyloid status was high (97.2% and 91.7% for images enhanced from injected and sampled ultra-low-dose data, respectively) which was similar to intra-reader reproducibility of standard-dose images (98.6%).

CONCLUSION

Deep learning methods can synthesize diagnostic-quality PET images from ultra-low injected dose simultaneous PET/MRI data, demonstrating the general validity of sampled realizations and the potential to reduce dose significantly for amyloid imaging.

Cerebrovascular reactivity measurements using simultaneous 15O-water PET and ASL MRI: Impacts of arterial transit time, labeling efficiency, and hematocrit

Cerebrovascular reactivity (CVR) reflects the capacity of the brain to meet changing physiological demands and can predict the risk of cerebrovascular diseases. CVR can be obtained by measuring the change in cerebral blood flow (CBF) during a brain stress test where CBF is altered by a vasodilator such as acetazolamide. Although the gold standard to quantify CBF is PET imaging, the procedure is invasive and inaccessible to most patients. Arterial spin labeling (ASL) is a non-invasive and quantitative MRI method to measure CBF, and a consensus guideline has been published for the clinical application of ASL. Despite single post labeling delay (PLD) pseudo-continuous ASL (PCASL) being the recommended ASL technique for CBF quantification, it is sensitive to variations to the arterial transit time (ATT) and labeling efficiency induced by the vasodilator in CVR studies. Multi-PLD ASL controls for the changes in ATT, and velocity selective ASL is in theory insensitive to both ATT and labeling efficiency. Here we investigate CVR using simultaneous ¹⁵O-water PET and ASL MRI data from 19 healthy subjects. CVR and CBF measured by the ASL techniques were compared using PET as the reference technique. The impacts of blood T1 and labeling efficiency on ASL were assessed using individual measurements of hematocrit and flow velocity data of the carotid and vertebral arteries measured using phase-contrast MRI. We found that multi-PLD PCASL is the ASL technique most consistent with PET for CVR quantification (group mean CVR of the whole brain = 42 ± 19% and 40 ± 18% respectively). Single-PLD ASL underestimated the CVR of the whole brain significantly by 15 ± 10% compared with PET (p<0.01, paired t-test). Changes in ATT pre- and post-acetazolamide was the principal factor affecting ASL-based CVR quantification. Variations in labeling efficiency and blood T1 had negligible effects.

Determination of the production and metabolic clearance rates of 1,25-dihydroxyvitamin D3 in the pregnant sheep and its chronically catheterized fetus by primed infusion technique

Because little is known regarding the determinants of plasma 1,25-dihydroxyvitamin D3(1,25(OH)2D3), or its fate in the fetus, we used a primed infusion technique, using physiologic amounts of high specific activity 3H-1,25(OH)2D3 to study the in vivo production rate (PR) and metabolic clearance rate (MCR) of 1,25(OH)2D3 in chronically catheterized maternal and fetal sheep during the last month of gestation (term = 145 d). The fetal MCR of 1,25(OH)2D3 was calculated at steady state, achieved within 2 h, and was found to be 2.53 +/- 0.19 mL/min (mean +/- SEM) compared to the maternal value of 15.9 +/- 0.94 mL/min. When expressed on a body wt basis, the fetal MCR of 1.22 +/- 0.09 mL/min/kg was more than 4-fold higher than the corresponding maternal value of 0.27 +/- 0.02 mL/min/kg. Measurement of endogenous plasma 1,25(OH)2D3 by RIA revealed mean fetal values of 89 +/- 10 pg/mL compared to the maternal value of 65 +/- 9 pg/mL. Fetal daily PR of 0.33 +/- 0.024 micrograms/d was 22% of the maternal PR of 1.49 +/- 0.11 micrograms/d. However, calculation of PR on a body wt basis revealed a fetal value of 0.159 +/- 0.012 micrograms/d/kg that was more than 6-fold higher than the maternal value of 0.025 +/- 0.002 micrograms/d/kg. Thus, fetal plasma concentrations of 1,25(OH)2D3 are sustained in the face of a high clearance rate of the hormone. The high MCR may be related to the high in vivo circulating concentrations of 1,25(OH)2D3, fetal to maternal placental transfer or target tissue uptake. In view of the high turnover of 1,25(OH)2D3, we suggest that this hormone has a biologic importance that warrants further investigation.

Characterization of 1,25-dihydroxyvitamin D3 receptors and in vivo targeting of [3H]-1,25(OH)2D3 in the sheep placenta

We sought to detect the presence of receptors for 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] in placental tissues of five late gestational pregnant sheep and to quantitate their biochemical properties and abundance. Cytosol prepared from cotyledonary tissue was found to contain two [3H]-1,25(OH)2D3 binding macromolecules that sedimented at 3.2 S and 4.1 S, respectively, on linear (4-20 per cent) hypertonic sucrose gradients. The 4.1 S component cosedimented with serum that had been prelabelled with [3H]-25-hydroxyvitamin D3 (25-OHD3) and was present in cytosols despite extensive washing of the tissue prior to homogenization. Concurrent incubation of the cytosol with [3H]-1,25(OH)2D3 and a tenfold molar excess of radioinert 25-OHD3 resulted in complete resolution of the 3.2 S macromolecule and disappearance of the 4.1 S binding component. The binding of [3H]-1,25(OH)2D3 to the 3.2 S component was completely abolished by coincubation with a 100-fold molar excess of radioinert 1,25(OH)2D3 and was replaced by a well resolved peak in the 4.1 S region. Scatchard analysis of cytosol binding to [3H]-1,25(OH)2D3 in the presence of a tenfold molar excess of radioinert 25OHD3 revealed a single class of non-interacting saturable binding site in the cotyledon and the endometrium of high affinity and low capacity. The mean +/- s.e. of the dissociation constant of the cotyledonary receptor of 0.21 +/- 0.06 nM was not different from that of 0.16 +/- 0.03 nM for the endometrial receptor. However, the abundance of the cotyledonary receptor was fourfold higher than that in the endometrium (110 +/- 20 versus 28 +/- 7 fmol/mg protein). Since it is not possible to completely separate endometrial tissue from cotyledonary tissue, the low abundance of receptor in endometrial cytosols may merely represent contamination of endometrial tissue with cotyledonary tissue. Further analysis of the [3H]-1,25(OH)2D3 occupied receptor in cotyledonary cytosols showed that it bound to DNA cellulose and was eluted with 0.16 M KCl. This in vitro binding of [3H]-1,25(OH)2D3 to DNA was confirmed in vivo by the finding of preferential nuclear targetting of [3H]-1,25(OH)2D3 (56 per cent of total cellular activity), 4 h after fetal intravenous administration of [3H]-1,25(OH)2D3 to five chronically catheterized fetal sheep. Total placental uptake of [3H]-1,25(OH)2D3 at this time amounted to 3.7 +/- 0.9 per cent of the injected dose. Preliminary analysis of ovine placental cytosols revealed a calcium binding protein of similar molecular weight to that found in the ovine intestine and in the intestine and placenta of rodents.(ABSTRACT TRUNCATED AT 400 WORDS)