Imaging Brain Injury in Former National Football League Players

Importance

Pilot studies that involved early imaging of the 18 kDa translocator protein (TSPO) using positron emission tomography (PET) indicated high levels of TSPO in the brains of active or former National Football League (NFL) players. If validated further in larger studies, those findings may have implications for athletes involved in collision sport.

Objective

To test for higher TSPO that marks brain injury and repair in a relatively large, unique cohort of former NFL players compared with former elite, noncollision sport athletes.

Design, Setting, and Participants

This cross-sectional study used carbon 11-labeled N,N-diethyl-2-(4-methoxyphenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidine-3-acetamide positron emission tomography ([11C]DPA-713 PET) data from former NFL players within 12 years of last participation in the NFL and elite noncollision sport athletes from across the US. Participants were enrolled between April 2018 and February 2023.

Main outcomes and measures

Regional [11C]DPA-713 total distribution volume from [11C]DPA-713 PET that is a measure of regional brain TSPO; regional brain volumes on magnetic resonance imaging; neuropsychological performance, including attention, executive function, and memory domains.

Results

This study included 27 former NFL players and 27 former elite, noncollision sport athletes. Regional TSPO levels were higher in former NFL players compared with former elite, noncollision sport athletes (unstandardized β coefficient, 1.08; SE, 0.22; 95% CI, 0.65 to 1.52; P < .001). The magnitude of the group difference depended on region, with largest group differences in TSPO in cingulate and frontal cortices as well as hippocampus. Compared with noncollision sport athletes, former NFL players performed worse in learning (mean difference [MD], -0.70; 95% CI, -1.14 to -0.25; P = .003) and memory (MD, -0.77; 95% CI, -1.24 to -0.30; P = .002), with no correlation between total gray matter TSPO and these cognitive domains.

Conclusions and relevance

In this cross-sectional study using [11C]DPA-713 PET, higher brain TSPO was found in former NFL players compared with noncollision sport athletes. This finding is consistent with neuroimmune activation even after cessation of NFL play. Future longitudinal [11C]DPA-713 PET and neuropsychological testing promises to inform whether neuroimmune-modulating therapy may be warranted.

Preclinical Evaluation of a New Series of Albumin-Binding 177Lu-Labeled PSMA-Based Low-Molecular-Weight Radiotherapeutics

Prostate-specific membrane antigen (PSMA)-based low-molecular-weight agents using beta(β)-particle-emitting radiopharmaceuticals is a new treatment paradigm for patients with metastatic castration-resistant prostate cancer. Although results have been encouraging, there is a need to improve the tumor residence time of current PSMA-based radiotherapeutics. Albumin-binding moieties have been used strategically to enhance the tumor uptake and retention of existing PSMA-based investigational agents. Previously, we developed a series of PSMA-based, β-particle-emitting, low-molecular-weight compounds. From this series, 177Lu-L1 was selected as the lead agent because of its reduced off-target radiotoxicity in preclinical studies. The ligand L1 contains a PSMA-targeting Lys-Glu urea moiety with an N-bromobenzyl substituent in the ε-amino group of Lys. Here, we structurally modified 177Lu-L1 to improve tumor targeting using two known albumin-binding moieties, 4-(p-iodophenyl) butyric acid moiety (IPBA) and ibuprofen (IBU), and evaluated the effects of linker length and composition. Six structurally related PSMA-targeting ligands (Alb-L1-Alb-L6) were synthesized based on the structure of 177Lu-L1. The ligands were assessed for in vitro binding affinity and were radiolabeled with 177Lu following standard protocols. All 177Lu-labeled analogs were studied in cell uptake and selected cell efficacy studies. In vivo pharmacokinetics were investigated by conducting tissue biodistribution studies for 177Lu-Alb-L2-177Lu-Alb-L6 (2 h, 24 h, 72 h, and 192 h) in male NSG mice bearing human PSMA+ PC3 PIP and PSMA- PC3 flu xenografts. Preliminary therapeutic ratios of the agents were estimated from the area under the curve (AUC0-192h) of the tumors, blood, and kidney uptake values. Compounds were obtained in >98% radiochemical yields and >99% purity. PSMA inhibition constants (Kis) of the ligands were in the ≤10 nM range. The long-linker-based agents, 177Lu-Alb-L4 and 177Lu-Alb-L5, displayed significantly higher tumor uptake and retention (p < 0.001) than the short-linker-bearing 177Lu-Alb-L2 and 177Lu-Alb-L3 and a long polyethylene glycol (PEG) linker-bearing agent, 177Lu-Alb-L6. The area under the curve (AUC0-192h) of the PSMA+ PC3 PIP tumor uptake of 177Lu-Alb-L4 and 177Lu-Alb-L5 were >4-fold higher than 177Lu-Alb-L2, 177Lu-Alb-L3, and 177Lu-Alb-L6, respectively. Also, the PSMA+ PIP tumor uptake (AUC0-192h) of 177Lu-Alb-L2 and 177Lu-Alb-L3 was ~1.5-fold higher than 177Lu-Alb-L6. However, the lowest blood AUC0-192h and kidney AUC0-192h were associated with 177Lu-Alb-L6 from the series. Consequently, 177Lu-Alb-L6 displayed the highest ratios of AUC(tumor)-to-AUC(blood) and AUC(tumor)-to-AUC(kidney) values from the series. Among the other agents, 177Lu-Alb-L4 demonstrated a nearly similar ratio of AUC(tumor)-to-AUC(blood) as 177Lu-Alb-L6. The tumor-to-blood ratio was the dose-limiting therapeutic ratio for all of the compounds. Conclusions: 177Lu-Alb-L4 and 177Lu-Alb-L6 showed high tumor uptake in PSMA+ tumors and tumor-to-blood ratios. The data suggest that linker length and composition can be modulated to generate an optimized therapeutic agent.

ME3BP-7 is a targeted cytotoxic agent that rapidly kills pancreatic cancer cells expressing high levels of monocarboxylate transporter MCT1

Nearly 30% of Pancreatic ductal adenocarcinoma (PDAC)s exhibit a marked overexpression of Monocarboxylate Transporter 1 (MCT1) offering a unique opportunity for therapy. However, biochemical inhibitors of MCT1 have proven unsuccessful in clinical trials. In this study we present an alternative approach using 3-Bromopyruvate (3BP) to target MCT1 overexpressing PDACs. 3BP is a cytotoxic agent that is known to be transported into cells via MCT1, but its clinical usefulness has been hampered by difficulties in delivering the drug systemically. We describe here a novel microencapsulated formulation of 3BP (ME3BP-7), that is effective against a variety of PDAC cells in vitro and remains stable in serum. Furthermore, systemically administered ME3BP-7 significantly reduces pancreatic cancer growth and metastatic spread in multiple orthotopic models of pancreatic cancer with manageable toxicity. ME3BP-7 is, therefore, a prototype of a promising new drug, in which the targeting moiety and the cytotoxic moiety are both contained within the same single small molecule.

One Sentence Summary

ME3BP-7 is a novel formulation of 3BP that resists serum degradation and rapidly kills pancreatic cancer cells expressing high levels of MCT1 with tolerable toxicity in mice.

First-in-human imaging using [11C]MDTC: a radiotracer targeting the cannabinoid receptor type 2

PURPOSE

We report findings from the first-in-human study of [11C]MDTC, a radiotracer developed to image the cannabinoid receptor type 2 (CB2R) with positron emission tomography (PET).

METHODS

Ten healthy adults were imaged according to a 90-min dynamic PET protocol after bolus intravenous injection of [11C]MDTC. Five participants also completed a second [11C]MDTC PET scan to assess test-retest reproducibility of receptor-binding outcomes. The kinetic behavior of [11C]MDTC in human brain was evaluated using tissue compartmental modeling. Four additional healthy adults completed whole-body [11C]MDTC PET/CT to calculate organ doses and the whole-body effective dose.

RESULTS

[11C]MDTC brain PET and [11C]MDTC whole-body PET/CT was well-tolerated. A murine study found evidence of brain-penetrant radiometabolites. The model of choice for fitting the time activity curves (TACs) across brain regions of interest was a three-tissue compartment model that includes a separate input function and compartment for the brain-penetrant metabolites. Regional distribution volume (VT) values were low, indicating low CB2R expression in the brain. Test-retest reliability of VT demonstrated a mean absolute variability of 9.91%. The measured effective dose of [11C]MDTC was 5.29 μSv/MBq.

CONCLUSION

These data demonstrate the safety and pharmacokinetic behavior of [11C]MDTC with PET in healthy human brain. Future studies identifying radiometabolites of [11C]MDTC are recommended before applying [11C]MDTC PET to assess the high expression of the CB2R by activated microglia in human brain.

Hetero-bivalent agents targeting FAP and PSMA

PURPOSE

We developed a theranostic radiopharmaceutical that engages two key cell surface proteases, fibroblast activation protein alpha (FAP) and prostate-specific membrane antigen (PSMA), each frequently overexpressed within the tumor microenvironment (TME). The latter is also expressed in most prostate tumor epithelium. To engage a broader spectrum of cancers for imaging and therapy, we conjugated small-molecule FAP and PSMA-targeting moieties using an optimized linker to provide ⁶⁴Cu-labeled compounds.

METHODS

We synthesized FP-L1 and FP-L2 using two linker constructs attaching the FAP and PSMA-binding pharmacophores. We determined in vitro inhibition constants (K_i) for FAP and PSMA. Cell uptake assays and flow cytometry were conducted in human glioma (U87), melanoma (SK-MEL-24), prostate cancer (PSMA + PC3 PIP and PSMA - PC3 flu), and clear cell renal cell carcinoma lines (PSMA + /PSMA - 786-O). Quantitative positron emission tomography/computed tomography (PET/CT) and tissue biodistribution studies were performed using U87, SK-MEL-24, PSMA + PC3 PIP, and PSMA + 786-O experimental xenograft models and the KPC genetically engineered mouse model of pancreatic cancer.

RESULTS

⁶⁴Cu-FP-L1 and ⁶⁴Cu-FP-L2 were produced in high radiochemical yields (> 98%) and molar activities (> 19 MBq/nmol). K_i values were in the nanomolar range for both FAP and PSMA. PET imaging and biodistribution studies revealed high and specific targeting of ⁶⁴Cu-FP-L1 and ⁶⁴Cu-FP-L2 for FAP and PSMA. ⁶⁴Cu-FP-L1 displayed more favorable pharmacokinetics than ⁶⁴Cu-FP-L2. In the U87 tumor model at 2 h post-injection, tumor uptake of ⁶⁴Cu-FP-L1 (10.83 ± 1.02%ID/g) was comparable to ⁶⁴Cu-FAPI-04 (9.53 ± 2.55%ID/g). ⁶⁴Cu-FP-L1 demonstrated high retention 5.34 ± 0.29%ID/g at 48 h in U87 tumor. Additionally, ⁶⁴Cu-FP-L1 showed high retention in PSMA + PC3 PIP tumor (12.06 ± 0.78%ID/g at 2 h and 10.51 ± 1.82%ID/g at 24 h).

CONCLUSIONS

⁶⁴Cu-FP-L1 demonstrated high and specific tumor targeting of FAP and PSMA. This compound should enable imaging of lesions expressing FAP, PSMA, or both on the tumor cell surface or within the TME. FP-L1 can readily be converted into a theranostic for the management of heterogeneous tumors.

Polymeric nanoparticles for dual-targeted theranostic gene delivery to hepatocellular carcinoma

Hepatocellular carcinoma (HCC) develops predominantly in the inflammatory environment of a cirrhotic liver caused by hepatitis, toxin exposure, or chronic liver disease. A targeted therapeutic approach is required to enable cancer killing without causing toxicity and liver failure. Poly(beta-amino-ester) (PBAE) nanoparticles (NPs) were used to deliver a completely CpG-free plasmid harboring mutant herpes simplex virus type 1 sr39 thymidine kinase (sr39) DNA to human HCC cells. Transfection with sr39 enables cancer cell killing with the prodrug ganciclovir and accumulation of 9-(4-18F-fluoro-3-hydroxymethylbutyl)guanine (18F-FHBG) for in vivo imaging. Targeting was achieved using a CpG-free human alpha fetoprotein (AFP) promoter (CpGf-AFP-sr39). Expression was restricted to AFP-producing HCC cells, enabling selective transfection of orthotopic HCC xenografts. CpGf-AFP-sr39 NP treatment resulted in 62% reduced tumor size, and therapeutic gene expression was detectable by positron emission tomography (PET). This systemic nanomedicine achieved tumor-specific delivery, therapy, and imaging, representing a promising platform for targeted treatment of HCC.

A Series of PSMA-Targeted Near-Infrared Fluorescent Imaging Agents

We have synthesized a series of 10 new, PSMA-targeted, near-infrared imaging agents intended for use in vivo for fluorescence-guided surgery (FGS). Compounds were synthesized from the commercially available amine-reactive active NHS ester of DyLight800. We altered the linker between the PSMA-targeting urea moiety and the fluorophore with a view to improve the pharmacokinetics. Chemical yields for the conjugates ranged from 51% to 86%. The K_i values ranged from 0.10 to 2.19 nM. Inclusion of an N-bromobenzyl substituent at the ε-amino group of lysine enhanced PSMA⁺ PIP tumor uptake, as did hydrophilic substituents within the linker. The presence of a polyethylene glycol chain within the linker markedly decreased renal uptake. In particular, DyLight800-10 demonstrated high specific uptake relative to background signal within kidney, confirmed by immunohistochemistry. These compounds may be useful for FGS in prostate, renal or other PSMA-expressing cancers.

A Unique Core–Shell Structured, Glycol Chitosan-Based Nanoparticle Achieves Cancer-Selective Gene Delivery with Reduced Off-Target Effects

The inherent instability of nucleic acids within serum and the tumor microenvironment necessitates a suitable vehicle for non-viral gene delivery to malignant lesions. A specificity-conferring mechanism is also often needed to mitigate off-target toxicity. In the present study, we report a stable and efficient redox-sensitive nanoparticle system with a unique core–shell structure as a DNA carrier for cancer theranostics. Thiolated polyethylenimine (PEI-SH) is complexed with DNA through electrostatic interactions to form the core, and glycol chitosan-modified with succinimidyl 3-(2-pyridyldithio)propionate (GCS-PDP) is grafted on the surface through a thiolate-disulfide interchange reaction to form the shell. The resulting nanoparticles, GCS-PDP/PEI-SH/DNA nanoparticles (GNPs), exhibit high colloid stability in a simulated physiological environment and redox-responsive DNA release. GNPs not only show a high and redox-responsive cellular uptake, high transfection efficiency, and low cytotoxicity in vitro, but also exhibit selective tumor targeting, with minimal toxicity, in vivo, upon systemic administration. Such a performance positions GNPs as viable candidates for molecular-genetic imaging and theranostic applications.

Targeted delivery of cytotoxic proteins to prostate cancer via conjugation to small molecule urea-based PSMA inhibitors

Prostate cancer cells are characterized by a remarkably low proliferative rate and the production of high levels of prostate-specific proteases. Protein-based toxins are attractive candidates for prostate cancer therapy because they kill cells via proliferation-independent mechanisms. However, the non-specific cytotoxicity of these potent cytotoxins must be redirected to avoid toxicity to normal tissues. Prostate-Specific Membrane Antigen (PSMA) is membrane-bound carboxypeptidase that is highly expressed by prostate cancer cells. Potent dipeptide PSMA inhibitors have been developed that can selectively deliver and concentrate imaging agents within prostate cancer cells based on continuous PSMA internalization and endosomal cycling. On this basis, we conjugated a PSMA inhibitor to the apoptosis-inducing human protease Granzyme B and the potent Pseudomonas exotoxin protein toxin fragment, PE35. We assessed selective PSMA binding and entrance into tumor cell to induce cell death. We demonstrated these agents selectively bound to PSMA and became internalized. PSMA-targeted PE35 toxin was selectively toxic to PSMA producing cells in vitro. Intratumoral and intravenous administration of this toxin produced marked tumor killing of PSMA-producing xenografts with minimal host toxicity. These studies demonstrate that urea-based PSMA inhibitors represent a simpler, less expensive alternative to antibodies as a means to deliver cytotoxic proteins to prostate cancer cells.

Scalable Purification of Plasmid DNA Nanoparticles by Tangential Flow Filtration for Systemic Delivery

Plasmid DNA (pDNA) nanoparticles synthesized by complexation with linear polyethylenimine (lPEI) are one of the most effective non-viral gene delivery vehicles. However, the lack of scalable and reproducible production methods and the high toxicity have hindered their clinical translation. Previously, we have developed a scalable flash nanocomplexation (FNC) technique to formulate pDNA/lPEI nanoparticles using a continuous flow process. Here, we report a tangential flow filtration (TFF)-based scalable purification method to reduce the uncomplexed lPEI concentration in the nanoparticle formulation and improve its biocompatibility. The optimized procedures achieved a 60% reduction of the uncomplexed lPEI with preservation of the nanoparticle size and morphology. Both in vitro and in vivo studies showed that the purified nanoparticles significantly reduced toxicity while maintaining transfection efficiency. TFF also allows for gradual exchange of solvents to isotonic solutions and further concentrating the nanoparticles for injection. Combining FNC production and TFF purification, we validated the purified pDNA/lPEI nanoparticles for future clinical translation of this gene nanomedicine.

Preclinical Evaluation of 213Bi- and 225Ac-Labeled Low-Molecular-Weight Compounds for Radiopharmaceutical Therapy of Prostate Cancer

Prostate-specific membrane antigen (PSMA)-targeted radiopharmaceutical therapy is a new option for patients with advanced prostate cancer refractory to other treatments. Previously, we synthesized a β-particle-emitting low-molecular-weight compound, ¹⁷⁷Lu-L1 which demonstrated reduced off-target effects in a xenograft model of prostate cancer. Here, we leveraged that scaffold to synthesize α-particle-emitting analogs of L1, ²¹³Bi-L1 and ²²⁵Ac-L1, to evaluate their safety and cell kill effect in PSMA-positive (+) xenograft models. Methods: The radiochemical synthesis, cell uptake, cell kill, and biodistribution of ²¹³Bi-L1 and ²²⁵Ac-L1 were evaluated. The efficacy of ²²⁵Ac-L1 was determined in human PSMA+ subcutaneous and micrometastatic models. Subacute toxicity at 8 wk and chronic toxicity at 1 y after administration were evaluated for ²²⁵Ac-L1. The absorbed radiation dose of ²²⁵Ac-L1 was determined using the biodistribution data and α-camera imaging. Results: ²¹³Bi- and ²²⁵Ac-L1 demonstrated specific cell uptake and cell kill in PSMA+ cells. The biodistribution of ²¹³Bi-L1 and ²²⁵Ac-L1 revealed specific uptake of radioactivity within PSMA+ lesions. Treatment studies of ²²⁵Ac-L1 demonstrated activity-dependent, specific inhibition of tumor growth in the PSMA+ flank tumor model. ²²⁵Ac-L1 also showed an increased survival benefit in the micrometastatic model compared with ¹⁷⁷Lu-L1. Activity-escalated acute and chronic toxicity studies of ²²⁵Ac-L1 revealed off-target radiotoxicity, mainly in kidneys and liver. The estimated maximum tolerated activity was about 1 MBq/kg. α-Camera imaging of ²²⁵Ac-L1 revealed high renal cortical accumulation at 2 h followed by fast clearance at 24 h. Conclusion: ²²⁵Ac-L1 demonstrated activity-dependent efficacy with minimal treatment-related organ radiotoxicity. ²²⁵Ac-L1 is a promising therapeutic for further clinical evaluation.

Process validation, current good manufacturing practice production, dosimetry, and toxicity studies of the carbonic anhydrase IX imaging agent [111 In]In-XYIMSR-01 for phase I regulatory approval

[¹¹¹ In]In-XYIMSR-01 is a promising single-photon emission computed tomography (SPECT) imaging agent for identification of tumors that overexpress carbonic anhydrase IX. To translate [¹¹¹ In]In-XYIMSR-01 to phase I trials, we performed animal toxicity and dosimetry studies, determined the maximum dose for human use, and completed the chemistry, manufacturing, and controls component of a standard regulatory application. The production process, quality control testing, stability studies, and specifications for sterile drug product release were based on United States Pharmacopeia chapters <823> and <825>, FDA 21 CFR Part 212. Toxicity was evaluated by using nonradioactive [^113/115 In]In-XYIMSR-01 according to 21 CFR Part 58 guidelines. Organ Level INternal Dose Assessment/EXponential Modeling (OLINDA/EXM) was used to calculate the maximum single dose for human studies. Three process validation runs at starting radioactivities of ~800 MBq were completed with a minimum concentration of 407 MBq/ml and radiochemical purity of ≥99% at the end of synthesis. A single intravenous dose of 55 μg/ml of [^113/115 In]In-XYIMSR-01 was well tolerated in male and female Sprague-Dawley rats. The calculated maximum single dose for human injection from dosimetry studies was 390.35 MBq of [¹¹¹ In]In-XYIMSR-01. We have completed toxicity and dosimetry studies as well as validated a manufacturing process to test [¹¹¹ In]In-XYIMSR-01 in a phase I clinical trial.

Nanoparticle-mediated tumor cell expression of mIL-12 via systemic gene delivery treats syngeneic models of murine lung cancers

Treatment of cancers in the lung remains a critical challenge in the clinic for which gene therapy could offer valuable options. We describe an effective approach through systemic injection of engineered polymer/DNA nanoparticles that mediate tumor-specific expression of a therapeutic gene, under the control of the cancer-selective progression elevated gene 3 (PEG-3) promoter, to treat tumors in the lungs of diseased mice. A clinically tested, untargeted, polyethylenimine carrier was selected to aid rapid transition to clinical studies, and a CpG-free plasmid backbone and coding sequences were used to reduce inflammation. Intravenous administration of nanoparticles expressing murine single-chain interleukin 12, under the control of PEG-3 promoter, significantly improved the survival of mice in both an orthotopic and a metastatic model of lung cancer with no marked symptoms of systemic toxicity. These outcomes achieved using clinically relevant nanoparticle components raises the promise of translation to human therapy.

A prostate-specific membrane antigen (PSMA)-targeted prodrug with a favorable in vivo toxicity profile

Prostate-specific membrane antigen (PSMA) is a promising target for the treatment of advanced prostate cancer (PC) and various solid tumors. Although PSMA-targeted radiopharmaceutical therapy (RPT) has enabled significant imaging and prostate-specific antigen (PSA) responses, accumulating clinical data are beginning to reveal certain limitations, including a subgroup of non-responders, relapse, radiation-induced toxicity, and the need for specialized facilities for its administration. To date non-radioactive attempts to leverage PSMA to treat PC with antibodies, nanomedicines or cell-based therapies have met with modest success. We developed a non-radioactive prodrug, SBPD-1, composed of a small-molecule PSMA-targeting moiety, a cancer-selective cleavable linker, and the microtubule inhibitor monomethyl auristatin E (MMAE). SBPD-1 demonstrated high binding affinity to PSMA (K_i = 8.84 nM) and selective cytotoxicity to PSMA-expressing PC cell lines (IC₅₀ = 3.90 nM). SBPD-1 demonstrated a significant survival benefit in two murine models of human PC relative to controls. The highest dose tested did not induce toxicity in immunocompetent mice. The high specific targeting ability of SBPD-1 to PSMA-expressing tumors and its favorable toxicity profile warrant its further development.

First-in-human neuroimaging of soluble epoxide hydrolase using [18F]FNDP PET

PURPOSE

Soluble epoxide hydrolase (sEH) is an enzyme with putative effect on neuroinflammation through its influence on the homeostasis of polyunsaturated fatty acids and related byproducts. sEH is an enzyme that metabolizes anti-inflammatory epoxy fatty acids to the corresponding, relatively inert 1,2-diols. A high availability or activity of sEH promotes vasoconstriction and inflammation in local tissues that may be linked to neuropsychiatric diseases. We developed [¹⁸F]FNDP to study sEH in vivo with positron emission tomography (PET).

METHODS

Brain PET using bolus injection of [¹⁸F]FNDP followed by emission imaging lasting 90 or 180 min was completed in healthy adults (5 males, 2 females, ages 40-53 years). The kinetic behavior of [¹⁸F]FNDP was evaluated using a radiometabolite-corrected arterial plasma input function with compartmental or graphical modeling approaches.

RESULTS

[¹⁸F]FNDP PET was without adverse effects. Akaike information criterion favored the two-tissue compartment model (2TCM) in all ten regions of interest. Regional total distribution volume (V_T) values from each compartmental model and Logan analysis were generally well identified except for corpus callosum V_T using the 2TCM. Logan analysis was assessed as the choice model due to stability of regional V_T values from 90-min data and due to high correlation of Logan-derived regional V_T values with those from the 2TCM. [¹⁸F]FNDP binding was higher in human cerebellar cortex and thalamus relative to supratentorial cortical regions, which aligns with reported expression patterns of the epoxide hydrolase 2 gene in human brain.

CONCLUSION

These data support further use of [¹⁸F]FNDP PET to study sEH in human brain.

Synthesis and preliminary evaluation of 211At-labeled inhibitors of prostate-specific membrane antigen for targeted alpha particle therapy of prostate cancer

INTRODUCTION

The high potency and short tissue range of α-particles are attractive features for targeted radionuclide therapy, particularly for cancers with micro-metastases. In the current study, we describe the synthesis of a series of ²¹¹At-labeled prostate-specific membrane antigen (PSMA) inhibitors and their preliminary evaluation as potential agents for metastatic prostate cancer treatment.

METHODS

Four novel Glu-urea based PSMA ligands containing a trialkyl stannyl group were synthesized and labeled with ²¹¹At, and for comparative purposes, ¹³¹I, via halodestannylation reactions with N-chlorosuccinimide as the oxidant. A PSMA inhibitory assay was performed to evaluate PSMA binding of the unlabeled, iodinated compounds. A series of paired-label biodistribution experiments were performed to compare each ²¹¹At-labeled PSMA ligand to its ¹³¹I-labeled counterpart in mice bearing subcutaneous PC3 PSMA+ PIP xenografts.

RESULTS

Radiochemical yields ranged from 32% to 65% for the ²¹¹At-labeled PSMA inhibitors and were consistently lower than those obtained with the corresponding ¹³¹I-labeled analogue. Good localization in PC3 PSMA+ PIP but not control xenografts was observed for all labeled molecules studied, which exhibited a variable degree of in vivo dehalogenation as reflected by thyroid and stomach activity levels. Normal tissue uptake and in vivo stability for several of the compounds was markedly improved compared with the previously evaluated compounds, [²¹¹At]DCABzL and [*I]DCIBzL.

CONCLUSIONS AND IMPLICATIONS FOR PATIENT CARE

Compared with the first generation compound [²¹¹At]DCABzL, several of the novel ²¹¹At-labeled PSMA ligands exhibited markedly improved stability in vivo and higher tumor-to-normal tissue ratios. [²¹¹At]GV-620 has the most promising characteristics and warrants further evaluation as a targeted radiotherapeutic for prostate cancer.

Triptolide: reflections on two decades of research and prospects for the future

Covering: 2000 to 2020 Triptolide is a bioactive diterpene triepoxide isolated from Tripterygium wilfordii Hook F, a traditional Chinese medicinal plant whose extracts have been used as anti-inflammatory and immunosuppressive remedies for centuries. Although triptolide and its analogs exhibit potent bioactivities against various cancers, and inflammatory and autoimmune diseases, none of them has been approved to be used in the clinic. This review highlights advances in material sourcing, molecular mechanisms, clinical progress and new drug design strategies for triptolide over the past two decades, along with some prospects for the future course of development of triptolide.

A Glucose-Triptolide Conjugate Selectively Targets Cancer Cells under Hypoxia

A major hurdle in the treatment of cancer is chemoresistance induced under hypoxia that is characteristic of tumor microenvironment. Triptolide, a potent inhibitor of eukaryotic transcription, possesses potent antitumor activity. However, its clinical potential has been limited by toxicity and water solubility. To address those limitations of triptolide, we designed and synthesized glucose-triptolide conjugates (glutriptolides) and demonstrated their antitumor activity in vitro and in vivo. Herein, we identified a lead, glutriptolide-2 with an altered linker structure. Glutriptolide-2 possessed improved stability in human serum, greater selectivity toward cancer over normal cells, and increased potency against cancer cells. Glutriptolide-2 exhibits sustained antitumor activity, prolonging survival in a prostate cancer metastasis animal model. Importantly, we found that glutriptolide-2 was more potent against cancer cells under hypoxia than normoxia. Together, this work provides an attractive glutriptolide drug lead and suggests a viable strategy to overcome chemoresistance through conjugation of cytotoxic agents to glucose.

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A second-generation glucose-triptolide conjugate (glutriptolide-2) was developed

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Glutriptolide-2 exhibits selective toxicity to cancer cells over normal cells

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Glutriptolide-2 possesses sustained antitumor activity in vivo

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Glutriptolide-2 shows greater potency against cancer cells under hypoxia

PET imaging of soluble epoxide hydrolase in non-human primate brain with [18F]FNDP

Purpose

Soluble epoxide hydrolase (sEH) is a promising candidate positron emission tomography (PET) imaging biomarker altered in various disorders, including vascular cognitive impairment (VCI), Alzheimer’s disease (AD), Parkinson’s disease (PD), stroke, and depression, known to regulate levels of epoxyeicosatrienoic acids (EETs) and play an important role in neurovascular coupling. [¹⁸F]FNDP, a PET radiotracer for imaging sEH, was evaluated through quantitative PET imaging in the baboon brain, radiometabolite analysis, and radiation dosimetry estimate.

Methods

Baboon [¹⁸F]FNDP dynamic PET studies were performed at baseline and with blocking doses of the selective sEH inhibitor AR-9281 to evaluate sEH binding specificity. Radiometabolites of [¹⁸F]FNDP in mice and baboons were measured by high-performance liquid chromatography. Regional brain distribution volume (V_T) of [¹⁸F]FNDP was computed from PET using radiometabolite-corrected arterial input functions. Full body distribution of [¹⁸F]FNDP was studied in CD-1 mice, and the human effective dose was estimated using OLINDA/EXM software.

Results

[¹⁸F]FNDP exhibited high and rapid brain uptake in baboons. AR-9281 blocked [¹⁸F]FNDP uptake dose-dependently with a baseline V_T of 10.9 ± 2.4 mL/mL and a high-dose blocking V_T of 1.0 ± 0.09 mL/mL, indicating substantial binding specificity (91.70 ± 1.74%). The V_ND was estimated as 0.865 ± 0.066 mL/mL. The estimated occupancy values of AR-9281 were 99.2 ± 1.1% for 1 mg/kg, 88.6 ± 1.3% for 0.1 mg/kg, and 33.8 ± 3.8% for 0.02 mg/kg. Murine biodistribution of [¹⁸F]FNDP enabled an effective dose estimate for humans (0.032 mSv/MBq). [¹⁸F]FNDP forms hydrophilic radiometabolites in murine and non-human primate plasma. However, only minute amounts of the radiometabolites entered the animal brain (< 2% in mice).

Conclusions

[¹⁸F]FNDP is a highly sEH-specific radiotracer that is suitable for quantitative PET imaging in the baboon brain. [¹⁸F]FNDP holds promise for translation to human subjects.

Novel β- and γ-Amino Acid-Derived Inhibitors of Prostate-Specific Membrane Antigen

Prostate-specific membrane antigen (PSMA) is an excellent biomarker for the early diagnosis of prostate cancer progression and metastasis. The most promising PSMA-targeted agents in the clinical phase are based on the Lys-urea-Glu motif, in which Lys and Glu are α-(l)-amino acids. In this study, we aimed to determine the effect of β- and γ-amino acids in the S1 pocket on the binding affinity for PSMA. We synthesized and evaluated the β- and γ-amino acid analogues with (S)- or (R)-configuration with keeping α-(l)-Glu as the S1'-binding pharmacophore. The structure-activity relationship studies identified that compound 13c, a β-amino acid analogue with (R)-configuration, exhibited the most potent PSMA inhibitory activity with an IC₅₀ value of 3.97 nM. The X-ray crystal structure of PSMA in complex with 13c provided a mechanistic basis for the stereochemical preference of PSMA, which can guide the development of future PSMA inhibitors.

High Availability of the α7-Nicotinic Acetylcholine Receptor in Brains of Individuals with Mild Cognitive Impairment: A Pilot Study Using 18F-ASEM PET

Emerging evidence supports a hypothesized role for the α7-nicotinic acetylcholine receptor (α7-nAChR) in the pathophysiology of Alzheimer's disease. 18F-ASEM (3-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-6-18F-fluorodibenzo[b,d]thiophene 5,5-dioxide) is a radioligand for estimating the availability of α7-nAChR in the brain in vivo with PET. Methods: In this cross-sectional study, 14 patients with mild cognitive impairment (MCI), a prodromal stage to dementia, and 17 cognitively intact, elderly controls completed 18F-ASEM PET. For each participant, binding in each region of interest was estimated using Logan graphical analysis with a metabolite-corrected arterial input function. Results: Higher 18F-ASEM binding was observed in MCI patients than in controls across all regions, supporting higher availability of α7-nAChR in MCI. 18F-ASEM binding was not associated with verbal memory in this small MCI sample. Conclusion: These data support use of 18F-ASEM PET to examine further the relationship between α7-nAChR availability and MCI.

Auger radiopharmaceutical therapy targeting prostate-specific membrane antigen in a micrometastatic model of prostate cancer

Auger radiopharmaceutical therapy is a promising strategy for micrometastatic disease given high linear energy transfer and short range in tissues, potentially limiting normal tissue toxicities. We previously demonstrated anti-tumor efficacy of a small-molecule Auger electron emitter targeting the prostate-specific membrane antigen (PSMA), 2-[3-[1-carboxy-5-(4-[¹²⁵I]iodo-benzoylamino)-pentyl]-ureido]-pentanedioic acid), or ¹²⁵I-DCIBzL, in a mouse xenograft model. Here, we investigated the therapeutic efficacy, long-term toxicity, and biodistribution of ¹²⁵I-DCIBzL in a micrometastatic model of prostate cancer (PC).

Methods: To test the therapeutic efficacy of ¹²⁵I-DCIBzL in micrometastatic PC, we used a murine model of human metastatic PC in which PSMA+ PC3-ML cells expressing firefly luciferase were injected intravenously in NSG mice to form micrometastatic deposits. One week later, 0, 0.37, 1.85, 3.7, 18.5, 37, or 111 MBq of ¹²⁵I-DCIBzL was administered (intravenously). Metastatic tumor burden was assessed using bioluminescence imaging (BLI). Long-term toxicity was evaluated via serial weights and urinalysis of non-tumor-bearing mice over a 12-month period, as well as final necropsy.

Results: In the micrometastatic PC model, activities of 18.5 MBq ¹²⁵I-DCIBzL and above significantly delayed development of detectable metastatic disease by BLI and prolonged survival in mice. Gross metastases were detectable in control mice and those treated with 0.37-3.7 MBq ¹²⁵I-DCIBzL at a median of 2 weeks post-treatment, versus 4 weeks for those treated with 18.5-111 MBq ¹²⁵I-DCIBzL (P<0.0001 by log-rank test). Similarly, treatment with ≥18.5 MBq ¹²⁵I-DCIBzL yielded a median survival of 11 weeks, compared with 6 weeks for control mice (P<0.0001). At 12 months, there was no appreciable toxicity via weight, urinalysis, or necropsy evaluation in mice treated with any activity of ¹²⁵I-DCIBzL, which represents markedly less toxicity than the analogous PSMA-targeted α-particle emitter. Macro-to-microscale dosimetry modeling demonstrated lower absorbed dose in renal cell nuclei versus tumor cell nuclei due to lower levels of drug uptake and cellular internalization in combination with the short range of Auger emissions.

Conclusion: PSMA-targeted radiopharmaceutical therapy with the Auger emitter ¹²⁵I-DCIBzL significantly delayed development of detectable metastatic disease and improved survival in a micrometastatic model of PC, with no long-term toxicities noted at 12 months, suggesting a favorable therapeutic ratio for treatment of micrometastatic PC.

Kinetic Control in Assembly of Plasmid DNA/Polycation Complex Nanoparticles

Polyelectrolyte complex (PEC) nanoparticles assembled from plasmid DNA (pDNA) and polycations such as linear polyethylenimine (lPEI) represent a major nonviral delivery vehicle for gene therapy tested thus far. Efforts to control the size, shape, and surface properties of pDNA/polycation nanoparticles have been primarily focused on fine-tuning the molecular structures of the polycationic carriers and on assembly conditions such as medium polarity, pH, and temperature. However, reproducible production of these nanoparticles hinges on the ability to control the assembly kinetics, given the nonequilibrium nature of the assembly process and nanoparticle composition. Here we adopt a kinetically controlled mixing process, termed flash nanocomplexation (FNC), that accelerates the mixing of pDNA solution with polycation lPEI solution to match the PEC assembly kinetics through turbulent mixing in a microchamber. This achieves explicit control of the kinetic conditions for pDNA/lPEI nanoparticle assembly, as demonstrated by the tunability of nanoparticle size, composition, and pDNA payload. Through a combined experimental and simulation approach, we prepared pDNA/lPEI nanoparticles having an average of 1.3 to 21.8 copies of pDNA per nanoparticle and average size of 35 to 130 nm in a more uniform and scalable manner than bulk mixing methods. Using these nanoparticles with defined compositions and sizes, we showed the correlation of pDNA payload and nanoparticle formulation composition with the transfection efficiencies and toxicity in vivo. These nanoparticles exhibited long-term stability at -20 °C for at least 9 months in a lyophilized formulation, validating scalable manufacture of an off-the-shelf nanoparticle product with well-defined characteristics as a gene medicine.

Kinetic Control in Assembly of Plasmid DNA/Polycation Complex Nanoparticles

Polyelectrolyte complex (PEC) nanoparticles assembled from plasmid DNA (pDNA) and polycations such as linear polyethylenimine (lPEI) represent a major nonviral delivery vehicle for gene therapy tested thus far. Efforts to control the size, shape, and surface properties of pDNA/polycation nanoparticles have been primarily focused on fine-tuning the molecular structures of the polycationic carriers and on assembly conditions such as medium polarity, pH, and temperature. However, reproducible production of these nanoparticles hinges on the ability to control the assembly kinetics, given the nonequilibrium nature of the assembly process and nanoparticle composition. Here we adopt a kinetically controlled mixing process, termed flash nanocomplexation (FNC), that accelerates the mixing of pDNA solution with polycation lPEI solution to match the PEC assembly kinetics through turbulent mixing in a microchamber. This achieves explicit control of the kinetic conditions for pDNA/lPEI nanoparticle assembly, as demonstrated by the tunability of nanoparticle size, composition, and pDNA payload. Through a combined experimental and simulation approach, we prepared pDNA/lPEI nanoparticles having an average of 1.3 to 21.8 copies of pDNA per nanoparticle and average size of 35 to 130 nm in a more uniform and scalable manner than bulk mixing methods. Using these nanoparticles with defined compositions and sizes, we showed the correlation of pDNA payload and nanoparticle formulation composition with the transfection efficiencies and toxicity in vivo. These nanoparticles exhibited long-term stability at -20 °C for at least 9 months in a lyophilized formulation, validating scalable manufacture of an off-the-shelf nanoparticle product with well-defined characteristics as a gene medicine.

177Lu-labeled low-molecular-weight agents for PSMA-targeted radiopharmaceutical therapy

PURPOSE

To develop a prostate-specific membrane antigen (PSMA)-targeted radiotherapeutic for metastatic castration-resistant prostate cancer (mCRPC) with optimized efficacy and minimized toxicity employing the β-particle radiation of ¹⁷⁷Lu.

METHODS

We synthesized 14 new PSMA-targeted, ¹⁷⁷Lu-labeled radioligands (¹⁷⁷Lu-L1-¹⁷⁷Lu-L14) using different chelating agents and linkers. We evaluated them in vitro using human prostate cancer PSMA(+) PC3 PIP and PSMA(-) PC3 flu cells and in corresponding flank tumor models. Efficacy and toxicity after 8 weeks were evaluated at a single administration of 111 MBq for ¹⁷⁷Lu-L1, ¹⁷⁷Lu-L3, ¹⁷⁷Lu-L5 and ¹⁷⁷Lu-PSMA-617. Efficacy of ¹⁷⁷Lu-L1 was further investigated using different doses, and long-term toxicity was determined in healthy immunocompetent mice.

RESULTS

Radioligands were produced in high radiochemical yield and purity. Cell uptake and internalization indicated specific uptake only in PSMA(+) PC3 cells. ¹⁷⁷Lu-L1, ¹⁷⁷Lu-L3 and ¹⁷⁷Lu-L5 demonstrated comparable uptake to ¹⁷⁷Lu-PSMA-617 and ¹⁷⁷Lu-PSMA-I&T in PSMA-expressing tumors up to 72 h post-injection. ¹⁷⁷Lu-L1, ¹⁷⁷Lu-L3 and ¹⁷⁷Lu-L5 also demonstrated efficient tumor regression at 8 weeks. ¹⁷⁷Lu-L1 enabled the highest survival rate. Necropsy studies of the treated group at 8 weeks revealed subacute damage to lacrimal glands and testes. No radiation nephropathy was observed 1 year post-treatment in healthy mice receiving 111 MBq of ¹⁷⁷Lu-L1, most likely related to the fast renal clearance of this agent.

CONCLUSIONS

¹⁷⁷Lu-L1 is a viable clinical candidate for radionuclide therapy of PSMA-expressing malignancies because of its high tumor-targeting ability and low off-target radiotoxic effects.

Enhancing CAR T-cell therapy through cellular imaging and radiotherapy

Chimeric antigen receptor (CAR) T-cell therapy is one of the most remarkable advances in cancer therapy in the last several decades. More than 300 adoptive T-cell therapy trials are ongoing, which is a testament to the early success and hope engendered by this line of investigation. Despite the enthusiasm, application of CAR T-cell therapy to solid tumours has had little success, although positive outcomes are increasingly being reported for these diseases. In this Series paper, we discuss the short-term strategies to improve CAR T-cell therapy responses, particularly for solid tumours, by combining CAR T-cell therapy with radiotherapy through the use of careful monitoring and non-invasive imaging. Through the use of imaging, we can gain greater mechanistic insights into the cascade of events that must unfold to enable tumour eradication by CAR T-cell therapy.

Imaging CAR T cell therapy with PSMA-targeted positron emission tomography

Chimeric antigen receptor (CAR) T cell therapy for hematologic malignancies is fraught with several unknowns, including number of functional T cells that engage target tumor, durability and subsequent expansion and contraction of that engagement, and whether toxicity can be managed. Non-invasive, serial imaging of CAR T cell therapy using a reporter transgene can address those issues quantitatively. We have transduced anti-CD19 CAR T cells with the prostate-specific membrane antigen (PSMA) because it is a human protein with restricted normal tissue expression and has an expanding array of positron emission tomography (PET) and therapeutic radioligands. We demonstrate that CD19-tPSMA^(N9del) CAR T cells can be tracked with [¹⁸F]DCFPyL PET in a Nalm6 model of acute lymphoblastic leukemia. Divergence between the number of CD19-tPSMA^(N9del) CAR T cells in peripheral blood and bone marrow and those in tumor was evident. These findings underscore the need for non-invasive repeatable monitoring of CAR T cell disposition clinically.

Preclinical Evaluation of 203/212Pb-Labeled Low-Molecular-Weight Compounds for Targeted Radiopharmaceutical Therapy of Prostate Cancer

Targeted radiopharmaceutical therapy (TRT) using α-particle radiation is a promising approach for treating both large and micrometastatic lesions. We developed prostate-specific membrane antigen (PSMA)-targeted low-molecular-weight agents for ²¹²Pb-based TRT of patients with prostate cancer (PC) by evaluating the matching γ-emitting surrogate, ²⁰³Pb. Methods: Five rationally designed low-molecular-weight ligands (L1-L5) were synthesized using the lysine-urea-glutamate scaffold, and PSMA inhibition constants were determined. Tissue biodistribution and SPECT/CT imaging of ²⁰³Pb-L1-²⁰³Pb-L5 were performed on mice bearing PSMA(+) PC3 PIP and PSMA(-) PC3 flu flank xenografts. The absorbed radiation dose of the corresponding ²¹²Pb-labeled analogs was determined using the biodistribution data. Antitumor efficacy of ²¹²Pb-L2 was evaluated in PSMA(+) PC3 PIP and PSMA(-) PC3 flu tumor models and in the PSMA(+) luciferase-expressing micrometastatic model. ²¹²Pb-L2 was also evaluated for dose-escalated, long-term toxicity. Results: All new ligands were obtained in high yield and purity. PSMA inhibitory activities ranged from 0.10 to 17 nM. ²⁰³Pb-L1-²⁰³Pb-L5 were synthesized in high radiochemical yield and specific activity. Whole-body clearance of ²⁰³Pb-L1-²⁰³Pb-L5 was fast. The absorbed dose coefficients (mGy/kBq) of the tumor and kidneys were highest for ²⁰³Pb-L5 (31.0, 15.2) and lowest for ²⁰³Pb-L2 (8.0, 4.2). The tumor-to-kidney absorbed dose ratio was higher for ²⁰³Pb-L3 (3.2) and ²⁰³Pb-L4 (3.6) than for the other agents, but with lower tumor-to-blood ratios. PSMA(+) tumor lesions were visualized through SPECT/CT as early as 0.5 h after injection. A proof-of-concept therapy study with a single administration of ²¹²Pb-L2 demonstrated dose-dependent inhibition of tumor growth in the PSMA(+) flank tumor model. ²¹²Pb-L2 also demonstrated an increased survival benefit in the micrometastatic model compared with ¹⁷⁷Lu-PSMA-617. Long-term toxicity studies in healthy, immunocompetent CD-1 mice revealed kidney as the dose-limiting organ. Conclusion: ²⁰³Pb-L1-²⁰³Pb-L5 demonstrated favorable pharmacokinetics for ²¹²Pb-based TRT. The antitumor efficacy of ²¹²Pb-L2 supports the corresponding ²⁰³Pb/²¹²Pb theranostic pair for PSMA-based α-particle TRT in advanced PC.

Biodistribution and Radiation Dosimetry of 124I-DPA-713, a PET Radiotracer for Macrophage-Associated Inflammation

Whole-body PET/CT was performed using 124I-DPA-713, a radioligand for the 18-kDa translocator protein (TSPO), to determine biodistribution and radiation dosimetry. Methods: Healthy subjects aged 18-65 y underwent whole-body PET/CT either at 4, 24, and 48 h or at 24, 48, and 72 h after intravenous injection of 124I-DPA-713. Time-activity curves were generated and used to calculate organ time-integrated activity coefficients for each subject. The resulting time-integrated activity coefficients provided input data for calculation of organ absorbed doses and effective dose for each subject using OLINDA. Subjects were genotyped for the TSPO polymorphism rs6971, and plasma protein binding of 124I-DPA-713 was measured. Results: Three male and 3 female adults with a mean age of 40 ± 19 y were imaged. The mean administered activity and mass were 70.5 ± 5.1 MBq (range, 62.4-78.1 MBq) and 469 ± 34 ng (range, 416-520 ng), respectively. There were no adverse or clinically detectable pharmacologic effects in any of the 6 subjects. No changes in vital signs, laboratory values, or electrocardiograms were observed. 124I-DPA-713 cleared rapidly (4 h after injection) from the lungs, with hepatic elimination and localization to the gastrointestinal tract. The mean effective dose over the 6 subjects was 0.459 ± 0.127 mSv/MBq, with the liver being the dose-limiting organ (0.924 ± 0.501 mGy/MBq). The percentage of free radiotracer in blood was approximately 30% at 30 and 60 min after injection. Conclusion:124I-DPA-713 clears rapidly from the lungs, with predominantly hepatic elimination, and is safe and well tolerated in healthy adults.

Prostate-specific membrane antigen-targeted photoacoustic imaging of prostate cancer in vivo

A sensitive, noninvasive method to detect localized prostate cancer, particularly for early detection and repetitive study in patients undergoing active surveillance, remains an unmet need. Here, we propose a molecular photoacoustic (PA) imaging approach by targeting the prostate-specific membrane antigen (PSMA), which is over-expressed in the vast majority of prostate cancers. We performed spectroscopic PA imaging in an experimental model of prostate cancer, namely, in immunocompromised mice bearing PSMA+ (PC3 PIP) and PSMA- (PC3 flu) tumors through administration of the known PSMA-targeted fluorescence agent, YC-27. Differences in contrast between PSMA+ and isogenic control tumors were observed upon PA imaging, with PSMA+ tumors showing higher contrast in average of 66.07-fold with 5 mice at the 24-hour postinjection time points. These results were corroborated using standard near-infrared fluorescence imaging with YC-27, and the squared correlation between PA and fluorescence intensities was 0.89. Spectroscopic PA imaging is a new molecular imaging modality with sufficient sensitivity for targeting PSMA in vivo, demonstrating the potential applications for other saturable targets relevant to cancer and other disorders.

Salicylic Acid-Based Polymeric Contrast Agents for Molecular Magnetic Resonance Imaging of Prostate Cancer

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) is an innovative molecular imaging technique in which contrast agents are labeled by saturating their exchangeable proton spins by radio-frequency irradiation. Salicylic acid and its analogues are a promising class of highly sensitive, diamagnetic CEST agents. Herein, polymeric agents grafted with salicylic acid moieties and a known high-affinity ligand targeting prostate-specific membrane antigen in approximately 10:1 molar ratio were synthesized to provide sufficient MRI sensitivity and receptor specificity. The proton-exchange properties of the contrast agent in solution and in an experimental murine model are reported to demonstrate the feasibility of receptor-targeted CEST MRI of prostate cancer. Furthermore, the CEST imaging data were validated with an 111 In-labeled analogue of the agent by in vivo single photon emission computed tomographic imaging and tissue biodistribution studies.

Brush border enzyme-cleavable linkers: Evaluation for reducing renal uptake of radiolabeled prostate-specific membrane antigen inhibitors

INTRODUCTION

Radiolabeled, low-molecular-weight prostate-specific membrane antigen (PSMA) inhibitors based on the Glu-ureido pharmacophore show promise for the detection and treatment of castration-resistant prostate cancer; however, high renal retention of activity, related in part to overexpression of PSMA in kidneys can be problematic. The goal of the current study was to investigate the use of brush border enzyme-cleavable linkers as a strategy for reducing kidney activity levels from radiolabeled PSMA inhibitors.

METHODS

PSMA-769 (6), a derivative of the prototypical PSMA inhibitor (((S)‑1‑carboxy‑5‑(4‑iodobenzamido)pentyl)carbamoyl)glutamate (12) modified to contain a Gly-Tyr linker, and its protected tin precursor (11) were synthesized starting from the basic pharmacophore molecule Lys-urea-Glu. An analogue of 6 containing d‑tyrosine in lieu of l‑tyrosine (PSMA-769-d-tyrosine) and the corresponding tin precursor (d-11) also were synthesized. Both radioiodinated and ²¹¹At-labeled 6 were synthesized by radiohalogenation of 11 and deprotection in situ. Similarly, radioiodinated d-6 was synthesized from d-11. Paired label biodistribution of [¹²⁵I]12 and [¹³¹I]6 was performed in normal mice and in SCID mice bearing both PC3 PIP (PSMA+) and PC3 flu (PSMA-) subcutaneous prostate carcinoma xenografts. The biodistribution of [¹³¹I]6 and [²¹¹At]6 was also evaluated in this tumor model. Biodistribution of the two radioiodinated diastereomers of 6 was evaluated in normal mice and urine samples were analyzed for the presence of 4‑iodohippuric acid.

RESULTS

Compounds [¹³¹I]6 and [²¹¹At]6 were synthesized from 11 in overall radiochemical yields of 32.5 ± 0.1% (n = 4) and 22% (n = 1), respectively; radiochemical purity was >95%. In normal mice, renal uptake of [¹³¹I]6 was 1.4-, 2.8- and 161-fold lower than that seen for co-injected [¹²⁵I]12 at 1 h, 4 h and 21 h, respectively. In tumor-bearing mice, kidney uptake of [¹³¹I]6 was similar to that for [¹²⁵I]12 (P > 0.05) at 1 h and 4 h but was 6- to 7-fold lower at 21 h; however, [¹³¹I]6 uptake in PC3 PIP tumors was also lower than that seen for [¹²⁵I]12 at 21 h (12.6 ± 3.4%ID/g vs. 36.8 ± 12.4%ID/g). Uptake of [²¹¹At]PSMA-769 in PC3 PIP tumors was slightly higher than that seen for [¹³¹I]PSMA-769 at 4 h (9.6 ± 1.6%ID/g versus 7.8 ± 1.6%ID/g; P = 0.002); its uptake in a number of normal tissues also was higher. In normal mice, kidney uptake of [¹²⁵I]PSMA-769 at 4 h was about 73% of that seen for [¹³¹I]PSMA-769-d-tyrosine. Activity in the urine of mice receiving [¹²⁵I]PSMA-769 contained mainly 4‑[¹²⁵I]iodohippuric acid while unmetabolized intact molecule was present in the case of [¹²⁵I]PSMA-769-d-tyrosine.

CONCLUSION

Use of this brush border enzyme-cleavable linker reduced kidney uptake and resulted in improved tumor:kidney uptake ratios. Although further structural improvements are needed, this linker approach might be useful as a component in strategies for reducing renal uptake of radiolabeled PSMA inhibitors.

18F-XTRA PET for Enhanced Imaging of the Extrathalamic α4β2 Nicotinic Acetylcholine Receptor

Reduced density of the α4β2 nicotinic acetylcholine receptor (α4β2-nAChR) in the cortex and hippocampus of the human brain has been reported in aging and patients with neurodegenerative disease. This study assessed the pharmacokinetic behavior of ¹⁸F-(-)-JHU86428 (¹⁸F-XTRA), a new radiotracer for in vivo PET imaging of the α4β2-nAChR, particularly in extrathalamic regions of interest in which the α4β2-nAChR is less densely expressed than in thalamus. ¹⁸F-XTRA was also used to evaluate the α4β2-nAChR in the hippocampus in human aging. Methods: Seventeen healthy nonsmoker adults (11 men, 6 women; age, 30-82 y) underwent PET neuroimaging over 90 or 180 min in a high-resolution research tomograph after bolus injection of ¹⁸F-XTRA. Methods to quantify binding of ¹⁸F-XTRA to the α4β2-nAChR in the human brain were compared, and the relationship between age and binding in the hippocampus was tested. Results: ¹⁸F-XTRA rapidly entered the brain, and time-activity curves peaked within 10 min after injection for extrathalamic regions and at approximately 70 min in the thalamus. The 2-tissue-compartment model (2TCM) predicted the regional time-activity curves better than the 1-tissue-compartment model, and total distribution volume (V_T) was well identified by the 2TCM in all ROIs. V_T values estimated using Logan analysis with metabolite-corrected arterial input were highly correlated with those from the 2TCM in all regions, and values from 90-min scan duration were on average within 5% of those values from 180 min of data. Parametric images of V_T were consistent with the known distribution of the α4β2-nAChR across the brain. Finally, an inverse correlation between V_T in the hippocampus and age was observed. Conclusion: Our results extend support for use of ¹⁸F-XTRA with 90 min of emission scanning in quantitative human neuroimaging of the extrathalamic α4β2-nAChR, including in studies of aging.

[64Cu]XYIMSR-06: A dual-motif CAIX ligand for PET imaging of clear cell renal cell carcinoma

Carbonic anhydrase IX (CAIX) is a cell surface enzyme that is over-expressed in approximately 95% of cases of clear cell renal cell carcinoma (ccRCC), the most common renal cancer. We synthesized and performed in vitro and in vivo evaluation of a dual-motif ligand, [64Cu]XYIMSR-06, for imaging CAIX expression on ccRCC tumors using positron emission tomography (PET). [64Cu]XYIMSR-06 was generated in yields of 51.0 ± 4.5% (n=5) and specific activities of 4.1 - 8.9 GBq/μmol (110-240 Ci/mmol). Tumor was visualized on PET images by 1 h post-injection with high tumor-to-background levels (>100 tumor-to-blood and -muscle) achieved within 24 h. Biodistribution studies demonstrated a maximum tumor uptake of 19.3% injected dose per gram of radioactivity at 4 h. Tumor-to-blood, -muscle and -kidney ratios were 129.6 ± 18.8, 84.3 ± 21.0 and 2.1 ± 0.3, respectively, at 8 h post-injection. At 24 h a tumor-to-kidney ratio of 7.1 ± 2.5 was achieved. These results indicate pharmacokinetics superior to those of previously reported imaging agents binding to CAIX. [64Cu]XYIMSR-06 is a new low-molecular-weight PET ligand targeting CAIX, which can image localized and metastatic ccRCC.

The distribution of the alpha7 nicotinic acetylcholine receptor in healthy aging: An in vivo positron emission tomography study with [18F]ASEM

Altered function of the alpha7 nicotinic acetylcholine receptor (α7-nAChR) is implicated in several neuropsychiatric diseases. Nevertheless, studies of the human cerebral α7-nAChR even in healthy aging are limited in number and to postmortem tissue.

METHODS

The distribution of the cerebral α7-nAChR was estimated in nine brain regions in 25 healthy volunteers (ages 21-86 years; median 57 years, interquartile range 52 years) using [¹⁸F]ASEM with positron emission tomography (PET) imaging. Regional total distribution volume (V_T) measurements were calculated using the Logan method from each subject's 90 min dynamic PET data and their metabolite-corrected plasma input function. Spearman's rank or Pearson's correlation analysis was used depending on the normality of the data. Correlation between age and regional 1) volume relative to intracranial volume (volume ratio) and 2) [¹⁸F]ASEM V_T was tested. Correlation between regional volume ratio and [¹⁸F]ASEM V_T was also evaluated. Finally, the relationship between [¹⁸F]ASEM V_T and neuropsychological measures was investigated in a subpopulation of 15 elderly healthy participants (those 50 years of age and older). Bonferroni correction for multiple comparisons was applied to statistical analyses.

RESULTS

A negative correlation between tissue volume ratio and age was observed in six of the nine brain regions including striatum and five cortical (temporal, occipital, cingulate, frontal, or parietal) regions. A positive correlation between [¹⁸F]ASEM V_T and age was observed in all nine brain regions of interest (ROIs). There was no correlation between [¹⁸F]ASEM V_T and volume ratio in any ROI after controlling for age. Regional [¹⁸F]ASEM V_T and neuropsychological performance on each of eight representative subtests were not correlated among the well-performing subpopulation of elderly healthy participants.

CONCLUSIONS

Our results suggest an increase in cerebral α7-nAChR distribution over the course of healthy aging that should be tested in future longitudinal studies. The preservation of the α7-nAChR in the aging human brain supports the development of therapeutic agents that target this receptor for use in the elderly. Further study of the relationship between α7-nAChR availability and cognitive impairment over aging is needed.

Imaging of Glial Cell Activation and White Matter Integrity in Brains of Active and Recently Retired National Football League Players

Importance

Microglia, the resident immune cells of the central nervous system, play an important role in the brain's response to injury and neurodegenerative processes. It has been proposed that prolonged microglial activation occurs after single and repeated traumatic brain injury, possibly through sports-related concussive and subconcussive injuries. Limited in vivo brain imaging studies months to years after individuals experience a single moderate to severe traumatic brain injury suggest widespread persistent microglial activation, but there has been little study of persistent glial cell activity in brains of athletes with sports-related traumatic brain injury.

Objective

To measure translocator protein 18 kDa (TSPO), a marker of activated glial cell response, in a cohort of National Football League (NFL) players and control participants, and to report measures of white matter integrity.

Design, Setting, and Participants

This cross-sectional, case-control study included young active (n = 4) or former (n = 10) NFL players recruited from across the United States, and 16 age-, sex-, highest educational level-, and body mass index-matched control participants. This study was conducted at an academic research institution in Baltimore, Maryland, from January 29, 2015, to February 18, 2016.

Main Outcomes and Measures

Positron emission tomography-based regional measures of TSPO using [11C]DPA-713, diffusion tensor imaging measures of regional white matter integrity, regional volumes on structural magnetic resonance imaging, and neuropsychological performance.

Results

The mean (SD) ages of the 14 NFL participants and 16 control participants were 31.3 (6.1) years and 27.6 (4.9) years, respectively. Players reported a mean (SD) of 7.0 (6.4) years (range, 1-21 years) since the last self-reported concussion. Using [11C]DPA-713 positron emission tomographic data from 12 active or former NFL players and 11 matched control participants, the NFL players showed higher total distribution volume in 8 of the 12 brain regions examined (P < .004). We also observed limited change in white matter fractional anisotropy and mean diffusivity in 13 players compared with 15 control participants. In contrast, these young players did not differ from control participants in regional brain volumes or in neuropsychological performance.

Conclusions and Relevance

The results suggest that localized brain injury and repair, indicated by higher TSPO signal and white matter changes, may be associated with NFL play. Further study is needed to confirm these findings and to determine whether TSPO signal and white matter changes in young NFL athletes are related to later onset of neuropsychiatric symptoms.

A PSMA-targeted theranostic agent for photodynamic therapy

Prostate-specific membrane antigen (PSMA) is over-expressed in the epithelium of prostate cancer and in the neovasculature of many non-prostate solid tumors. PSMA has been increasingly used as a target for cancer imaging and therapy. Here we describe a low-molecular-weight theranostic photosensitizer, YC-9, for PSMA-targeted optical imaging and photodynamic therapy (PDT). YC-9 was synthesized by conjugating IRDye700DX N-hydroxysuccinimide (NHS) ester with a PSMA targeting Lys-Glu urea through a lysine-suberate linker in suitable yield. Optical imaging in vivo demonstrated PSMA-specific tumor uptake of YC-9 with rapid clearance from non-target tissues. PSMA-specific cell kill was demonstrated with YC-9in vitro through PDT in PSMA⁺ PC3-PIP and PSMA^- PC3-flu cells. In vivo PDT in mice bearing PSMA⁺ PC3-PIP tumors at 4h post-injection of YC-9 (A total of four PDT sessions were performed, 48h apart) resulted in significant tumor growth delay, while tumors in control groups continued to grow. PDT with YC-9 significantly increased the median survival of the PSMA⁺ PC3-PIP tumor mice (56.5days) compared to control groups [23.5-30.0days, including untreated, light alone, YC-9 alone (without light) and non-targeted IRDye700DX PDT treatment groups], without noticeable toxicity at the doses used. This study proves in principle that YC-9 is a promising therapeutic agent for targeted PDT of PSMA-expressing tissues, such as prostate tumors, and may also be useful against non-prostate tumors by virtue of neovascular PSMA expression.

Targeted Delivery and Sustained Antitumor Activity of Triptolide through Glucose Conjugation

Triptolide, a key ingredient from the traditional Chinese medicinal plant thunder god vine, which has been used to treat inflammation and autoimmune diseases for centuries, has been shown to be an irreversible inhibitor of the XPB subunit of the transcription factor TFIIH and initiation of RNA polymerase II mediated transcription. The clinical development of triptolide over the past two decades has been limited by its toxicity and low water solubility. Herein, we report the development of a glucose conjugate of triptolide, named glutriptolide, which was intended to target tumor cells overexpressing glucose transporters selectively. Glutriptolide did not inhibit XPB activity in vitro but demonstrated significantly higher cytotoxicity against tumor cells over normal cells with greater water solubility than triptolide. Furthermore, it exhibited remarkable tumor control in vivo, which is likely due to sustained stepwise release of active triptolide within cancer cells. These findings indicate that glutriptolide may serve as a promising lead for developing a new mechanistic class of anticancer drugs.

Imaging of carbonic anhydrase IX with an 111In-labeled dual-motif inhibitor

We developed a new scaffold for radionuclide-based imaging and therapy of clear cell renal cell carcinoma (ccRCC) targeting carbonic anhydrase IX (CAIX). Compound XYIMSR-01, a DOTA-conjugated, bivalent, low-molecular-weight ligand, has two moieties that target two separate sites on CAIX, imparting high affinity. We synthesized [¹¹¹In]XYIMSR-01 in 73.8–75.8% (n = 3) yield with specific radioactivities ranging from 118 – 1,021 GBq/μmol (3,200–27,600 Ci/mmol). Single photon emission computed tomography of [¹¹¹In]XYIMSR-01 in immunocompromised mice bearing CAIX-expressing SK-RC-52 tumors revealed radiotracer uptake in tumor as early as 1 h post-injection. Biodistribution studies demonstrated 26% injected dose per gram of radioactivity within tumor at 1 h. Tumor-to-blood, muscle and kidney ratios were 178.1 ± 145.4, 68.4 ± 29.0 and 1.7 ± 1.2, respectively, at 24 h post-injection. Retention of radioactivity was exclusively observed in tumors by 48 h, the latest time point evaluated. The dual targeting strategy to engage CAIX enabled specific detection of ccRCC in this xenograft model, with pharmacokinetics surpassing those of previously described radionuclide-based probes against CAIX.

Detecting Tumor Metastases: The Road to Therapy Starts Here

Metastasis is the complex process by which primary tumor cells migrate and establish secondary tumors in an adjacent or distant location in the body. Early detection of metastatic disease and effective therapeutic options for targeting these detected metastases remain impediments to effectively treating patients with advanced cancers. If metastatic lesions are identified early, patients might maximally benefit from effective early therapeutic interventions. Further, monitoring patients whose primary tumors are effectively treated for potential metastatic disease onset is also highly valuable. Finally, patients with metastatic disease can be monitored for efficacy of specific therapeutic interventions through effective metastatic detection techniques. Thus, being able to detect and visualize metastatic lesions is key and provides potential to greatly improve overall patient outcomes. In order to achieve these objectives, researchers have endeavored to mechanistically define the steps involved in the metastatic process as well as ways to effectively detect metastatic progression. We presently overview various preclinical and clinical in vitro and in vivo assays developed to more efficiently detect tumor metastases, which provides the foundation for developing more effective therapies for this invariably fatal component of the cancerous process.

18F-FNDP for PET Imaging of Soluble Epoxide Hydrolase

Soluble epoxide hydrolase (sEH) is a bifunctional enzyme located within cytosol and peroxisomes that converts epoxides to the corresponding diols and hydrolyzes phosphate monoesters. It serves to inactivate epoxyeicosatrienoic acids (EETs), which are generated in the brain to couple neuronal activity and cerebral blood flow in normal and pathologic states. Altered regulation of sEH was observed previously in various neuropathologic disorders including vascular dementia and stroke. Inhibitors of sEH are pursued as agents to mitigate neuronal damage after stroke. We developed N-(3,3-diphenylpropyl)-6-¹⁸F-fluoronicotinamide (¹⁸F-FNDP), which proved highly specific for imaging of sEH in the mouse and nonhuman primate brain with PET.

METHODS

¹⁸F-FNDP was synthesized from the corresponding bromo precursor. sEH inhibitory activity of ¹⁸F-FNDP was measured using an sEH inhibitor screening assay kit. Biodistribution was undertaken in CD-1 mice. Binding specificity was assayed in CD-1 and sEH knock-out mice and Papio anubis (baboon) through pretreatment with an sEH inhibitor to block sEH binding. Dynamic PET imaging with arterial blood sampling was performed in 3 baboons, with regional tracer binding quantified using distribution volume. The metabolism of ¹⁸F-FNDP in baboons was assessed using high-performance liquid chromatography.

RESULTS

¹⁸F-FNDP (inhibition binding affinity constant, 1.73 nM) was prepared in 1 step in a radiochemical yield of 14% ± 7%, specific radioactivity in the range of 888-3,774 GBq/μmol, and a radiochemical purity greater than 99% using an automatic radiosynthesis module. The time of preparation was about 75 min. In CD-1 mice, regional uptake followed the pattern of striatum > cortex > hippocampus > cerebellum, consistent with the known brain distribution of sEH, with 5.2% injected dose per gram of tissue at peak uptake. Blockade of 80%-90% was demonstrated in all brain regions. Minimal radiotracer uptake was present in sEH knock-out mice. PET baboon brain distribution paralleled that seen in mouse, with a marked blockade (95%) noted in all regions indicating sEH-mediated uptake of ¹⁸F-FNDP. Two hydrophilic metabolites were identified, with 20% parent compound present at 90 min after injection in baboon plasma.

CONCLUSION

¹⁸F-FNDP can be synthesized in suitable radiochemical yield and high specific radioactivity and purity. In vivo imaging experiments demonstrated that ¹⁸F-FNDP targeted sEH in murine and nonhuman primate brain specifically. ¹⁸F-FNDP is a promising PET radiotracer likely to be useful for understanding the role of sEH in a variety of conditions affecting the central nervous system.

In vitro characterization of pralidoxime transport and acetylcholinesterase reactivation across MDCK cells and stem cell-derived human brain microvascular endothelial cells (BC1-hBMECs)

Background

Current therapies for organophosphate poisoning involve administration of oximes, such as pralidoxime (2-PAM), that reactivate the enzyme acetylcholinesterase. Studies in animal models have shown a low concentration in the brain following systemic injection.

Methods

To assess 2-PAM transport, we studied transwell permeability in three Madin-Darby canine kidney (MDCKII) cell lines and stem cell-derived human brain microvascular endothelial cells (BC1-hBMECs). To determine whether 2-PAM is a substrate for common brain efflux pumps, experiments were performed in the MDCKII-MDR1 cell line, transfected to overexpress the P-gp efflux pump, and the MDCKII-FLuc-ABCG2 cell line, transfected to overexpress the BCRP efflux pump. To determine how transcellular transport influences enzyme reactivation, we developed a modified transwell assay where the inhibited acetylcholinesterase enzyme, substrate, and reporter are introduced into the basolateral chamber. Enzymatic activity was inhibited using paraoxon and parathion.

Results

The permeability of 2-PAM is about 2 × 10⁻⁶ cm s⁻¹ in MDCK cells and about 1 × 10⁻⁶ cm s⁻¹ in BC1-hBMECs. Permeability is not influenced by pre-treatment with atropine. In addition, 2-PAM is not a substrate for the P-gp or BCRP efflux pumps.

Conclusions

The low permeability explains poor brain penetration of 2-PAM and therefore the slow enzyme reactivation. This elucidates one of the reasons for the necessity of sustained intravascular (IV) infusion in response to organophosphate poisoning.

Electronic supplementary material

The online version of this article (doi:10.1186/s12987-016-0035-0) contains supplementary material, which is available to authorized users.

(2S)-2-(3-(1-Carboxy-5-(4-211At-Astatobenzamido)Pentyl)Ureido)-Pentanedioic Acid for PSMA-Targeted α-Particle Radiopharmaceutical Therapy

Alpha-particle emitters have a high linear energy transfer and short range, offering the potential for treating micrometastases while sparing normal tissues. We developed a urea-based, ²¹¹At-labeled small molecule targeting prostate-specific membrane antigen (PSMA) for the treatment of micrometastases due to prostate cancer (PC).

METHODS

PSMA-targeted (2S)-2-(3-(1-carboxy-5-(4-²¹¹At-astatobenzamido)pentyl)ureido)-pentanedioic acid (²¹¹At- 6: ) was synthesized. Cellular uptake and clonogenic survival were tested in PSMA-positive (PSMA+) PC3 PIP and PSMA-negative (PSMA-) PC3 flu human PC cells after ²¹¹At- 6: treatment. The antitumor efficacy of ²¹¹At- 6: was evaluated in mice bearing PSMA+ PC3 PIP and PSMA- PC3 flu flank xenografts at a 740-kBq dose and in mice bearing PSMA+, luciferase-expressing PC3-ML micrometastases. Biodistribution was determined in mice bearing PSMA+ PC3 PIP and PSMA- PC3 flu flank xenografts. Suborgan distribution was evaluated using α-camera images, and microscale dosimetry was modeled. Long-term toxicity was assessed in mice for 12 mo.

RESULTS

²¹¹At- 6: treatment resulted in PSMA-specific cellular uptake and decreased clonogenic survival in PSMA+ PC3 PIP cells and caused significant tumor growth delay in PSMA+ PC3 PIP flank tumors. Significantly improved survival was achieved in the newly developed PSMA+ micrometastatic PC model. Biodistribution showed uptake of ²¹¹At- 6: in PSMA+ PC3 PIP tumors and in kidneys. Microscale kidney dosimetry based on α-camera images and a nephron model revealed hot spots in the proximal renal tubules. Long-term toxicity studies confirmed that the dose-limiting toxicity was late radiation nephropathy.

CONCLUSION

PSMA-targeted ²¹¹At- 6: α-particle radiotherapy yielded significantly improved survival in mice bearing PC micrometastases after systemic administration. ²¹¹At- 6: also showed uptake in renal proximal tubules resulting in late nephrotoxicity, highlighting the importance of long-term toxicity studies and microscale dosimetry.

Design, synthesis and biological evaluation of PSMA/hepsin-targeted heterobivalent ligands

Cell surface biomarkers such as prostate-specific membrane antigen (PSMA) and hepsin have received considerable attention as targets for imaging prostate cancer (PCa) due to their high cell surface expression in such tumors and easy access for imaging probes. Novel amidine-containing indole analogs (13-21) as hepsin inhibitors were designed and synthesized. These compounds showed in vitro inhibitory activity against hepsin with IC50 values from 5.9 to 70 μM. Based on the SAR of amidine-derived analogs, the novel heterobivalent compound 30, targeting both hepsin and PSMA, was synthesized by linking compound 18 with Lys-urea-Glu, the key scaffold for the specific binding to PSMA, followed by the conjugation of the optical dye SulfoCy7. Compound 30 exhibited inhibitory activities against PSMA and hepsin, with IC50 values of 28 nM and 2.8 μM, respectively. In vitro cell uptake and preliminary in vivo optical imaging studies of 30 showed selective binding and retention in both PSMA and hepsin high-expressing PC3/ML-PSMA-HPN cells as compared with low-expressing PC3/ML cells.