A Feasibility Study Showing [68Ga]Citrate PET Detects Prostate Cancer

Drug-tolerant persister cells in cancer: the cutting edges and future directions

Drug-tolerant persister (DTP) cell populations were originally discovered in antibiotic-resistant bacterial biofilms. Similar populations with comparable features have since been identified among cancer cells and have been linked with treatment resistance that lacks an underlying genomic alteration. Research over the past decade has improved our understanding of the biological roles of DTP cells in cancer, although clinical knowledge of the role of these cells in treatment resistance remains limited. Nonetheless, targeting this population is anticipated to provide new treatment opportunities. In this Perspective, we aim to provide a clear definition of the DTP phenotype, discuss the underlying characteristics of these cells, their biomarkers and vulnerabilities, and encourage further research on DTP cells that might improve our understanding and enable the development of more effective anticancer therapies.

Assessing the Potential of Small Peptides for Altering Expression Levels of the Iron-Regulatory Genes FTH1 and TFRC and Enhancing Androgen Receptor Inhibitor Activity in In Vitro Prostate Cancer Models

Recent research highlights the key role of iron dyshomeostasis in the pathogenesis of prostate cancer (PCa). PCa cells are heavily dependent on bioavailable iron, which frequently results in the reprogramming of iron uptake and storage pathways. Although advanced-stage PCa is currently incurable, bioactive peptides capable of modulating key iron-regulatory genes may constitute a means of exploiting a metabolic adaptation necessary for tumor growth. Recent annual increases in PCa incidence have been reported, highlighting the urgent need for novel treatments. We examined the ability of LNCaP, PC3, VCaP, and VCaP-EnzR cells to form colonies in the presence of androgen receptor inhibitors (ARI) and a series of iron-gene modulating oligopeptides (FT-001-FT-008). The viability of colonies following treatment was determined with clonogenic assays, and the expression levels of FTH1 (ferritin heavy chain 1) and TFRC (transferrin receptor) were determined with quantitative polymerase chain reaction (PCR). Peptides and ARIs combined significantly reduced PCa cell growth across all phenotypes, of which two peptides were the most effective. Colony growth suppression generally correlated with the magnitude of concurrent increases in FTH1 and decreases in TFRC expression for all cells. The results of this study provide preliminary insight into a novel approach at targeting iron dysmetabolism and sensitizing PCa cells to established cancer treatments.

Elevated labile iron in castration-resistant prostate cancer is targetable with ferrous iron-activatable antiandrogen therapy

Clinical responses to second generation androgen signaling inhibitors (e.g., enzalutamide) in metastatic castration-resistant prostate cancer (mCRPC) are variable and transient, and are associated with dose limiting toxicities, including rare but severe CNS effects. We hypothesized that changes to iron metabolism coincident with more advanced disease might be leveraged for tumor-selective delivery of antiandrogen therapy. Using the recently described chemical probes SiRhoNox and 18F-TRX in mCRPC models, we found elevated Fe2+ to be a common feature of mCRPC in vitro and in vivo. We next synthesized ferrous-iron activatable drug conjugates of second and third-generation antiandrogens and found these conjugates possessed comparable or enhanced antiproliferative activity across mCRPC cell line models. Mouse pharmacokinetic studies showed that these prototype antiandrogen conjugates are stable in vivo and limited exposure to conjugate or free antiandrogen in the brain. Our results reveal elevated Fe2+ to be a feature of mCRPC that might be leveraged to improve the tolerability and efficacy of antiandrogen therapy.

68Ga-Citrate PET of Healthy Men: Whole-Body Biodistribution Kinetics and Radiation Dose Estimates

68Ga-citrate has one of the simplest chemical structures of all 68Ga-radiopharmaceuticals, and its clinical use is justified by the proven medical applications using its isotope-labeled compound 67Ga-citrate. To support broader application of 68Ga-citrate in medical diagnosis, further research is needed to gain clinical data from healthy volunteers. In this work, we studied the biodistribution of 68Ga-citrate and subsequent radiation exposure from it in healthy men. Methods: 68Ga-citrate was prepared with an acetone-based radiolabeling procedure compliant with good manufacturing practices. Six healthy men (age 41 ± 12 y, mean ± SD) underwent sequential whole-body PET/CT scans after an injection of 204 ± 8 MBq of 68Ga-citrate. Serial arterialized venous blood samples were collected during PET imaging, and the radioactivity concentration was measured with a γ-counter. Urinary voids were collected and measured. The MIRD bladder-voiding model with a 3.5-h voiding interval was used. A model using a 70-kg adult man and the MIRD schema was used to estimate absorbed doses in target organs and effective doses. Calculations were performed using OLINDA/EXM software, version 2.0. Results: Radioactivity clearance from the blood was slow, and relatively high radioactivity concentrations were observed over the whole of the 3-h measuring period. Although radioactivity excretion via urine was rather slow (biologic half-time, 69 ± 24 h), the highest decay-corrected concentrations in urinary bladder contents were measured at the 90- and 180-min time points. Moderate concentrations were also seen in kidneys, liver, and spleen. The source organs showing the largest residence times were muscle, liver, lung, and heart contents. The heart wall received the highest absorbed dose, 0.077 ± 0.008 mSv/MBq. The mean effective dose (International Commission on Radiological Protection publication 103) was 0.021 ± 0.001 mSv/MBq. Conclusion: PET imaging with 68Ga-citrate is associated with modest radiation exposure. A 200-MBq injection of 68Ga-citrate results in an effective radiation dose of 4.2 mSv, which is in the same range as other 68Ga-labeled tracers. This suggests the feasibility of clinical studies using 68Ga-citrate imaging in humans and the possibility of performing multiple scans in the same subjects across the course of a year.

Ferrous iron-activatable drug conjugate achieves potent MAPK blockade in KRAS-driven tumors

KRAS mutations drive a quarter of cancer mortality, and most are undruggable. Several inhibitors of the MAPK pathway are FDA approved but poorly tolerated at the doses needed to adequately extinguish RAS/RAF/MAPK signaling in the tumor cell. We found that oncogenic KRAS signaling induced ferrous iron (Fe2+) accumulation early in and throughout mutant KRAS-mediated transformation. We converted an FDA-approved MEK inhibitor into a ferrous iron-activatable drug conjugate (FeADC) and achieved potent MAPK blockade in tumor cells while sparing normal tissues. This innovation allowed sustainable, effective treatment of tumor-bearing animals, with tumor-selective drug activation, producing superior systemic tolerability. Ferrous iron accumulation is an exploitable feature of KRAS transformation, and FeADCs hold promise for improving the treatment of KRAS-driven solid tumors.

The Evaluation of 68Ga-Citrate PET/CT Imaging for Dihydroartemisinin in the Treatment of Rheumatoid Arthritis

PURPOSE

We aimed to use 68Ga-citrate, a labeled product of gallium (iron analog), combined with positron emission tomography/computed tomography (PET/CT) to non-invasively evaluate the potential of the iron-responsive product dihydroartemisinin (DHA) in the treatment of rheumatoid arthritis.

PROCEDURES

From the establishment of chicken II collagen-induced arthritis (CIA) rat model over 40 days, 20 rats with one-to-one corresponding arthritis index (AI) scores were randomly divided into two groups. One group received oral DHA (at a dose of 1.5 ml/(kg day), containing 20 mg DHA per 1 ml) for 15 days; the other group received stroke-physiological saline solution (SSS, 1.5 ml/(kg day) for 15 days. 68Ga-citrate micro-PET/CT imaging was performed on day 0 (D0), day 5 (D5), day 10 (D10), and day 15 (D15) of oral administration. After data reconstruction, the cross-sectional length "d" of the ankle joint of each rat was measured on the transverse CT, and the SUVmax of the ankle joint and muscle background was measured for statistical analysis. After micro-PET/CT collection, the ankle joint tissue was observed by HE staining.

RESULTS

The ankle joint swelling in the DHA group was significantly suppressed, but the SSS group showed no significant suppression. 68Ga-citrate micro-PET/CT imaging results and microscope observation confirmed this finding. Statistical analysis indicated that the time tendency of AI score (Binteraction = 0.495, P < 0.001) and T/NT (Binteraction = 1.345, P < 0.001) were discrepant between DHA and SSS groups. The AI score and T/NT of the DHA group gradually increased with time, while the SSS group score gradually decreased. Furthermore, the Spearman correlation coefficient was used to describe the relationship between "d" and T/NT, which was positively correlated (r = 0.855, P < 0.001).

CONCLUSIONS

This study showed that the anti-inflammatory effect of the iron-responsive product DHA in arthritis can be monitored by an iron-like radioactive tracer (68Ga-citrate).

Metabolic regulation of prostate cancer heterogeneity and plasticity

Metabolic reprogramming is one of the main hallmarks of cancer cells. It refers to the metabolic adaptations of tumor cells in response to nutrient deficiency, microenvironmental insults, and anti-cancer therapies. Metabolic transformation during tumor development plays a critical role in the continued tumor growth and progression and is driven by a complex interplay between the tumor mutational landscape, epigenetic modifications, and microenvironmental influences. Understanding the tumor metabolic vulnerabilities might open novel diagnostic and therapeutic approaches with the potential to improve the efficacy of current tumor treatments. Prostate cancer is a highly heterogeneous disease harboring different mutations and tumor cell phenotypes. While the increase of intra-tumor genetic and epigenetic heterogeneity is associated with tumor progression, less is known about metabolic regulation of prostate cancer cell heterogeneity and plasticity. This review summarizes the central metabolic adaptations in prostate tumors, state-of-the-art technologies for metabolic analysis, and the perspectives for metabolic targeting and diagnostic implications.

Manipulating the In Vivo Behaviour of 68Ga with Tris(Hydroxypyridinone) Chelators: Pretargeting and Blood Clearance

Pretargeting is widely explored in immunoPET as a strategy to reduce radiation exposure of non-target organs and allow the use of short-lived radionuclides that would not otherwise be compatible with the slow pharmacokinetic profiles of antibodies. Here we investigate a pretargeting strategy based on gallium-68 and the chelator THPMe as a high-affinity pair capable of combining in vivo. After confirming the ability of THPMe to bind 68Ga in vivo at low concentrations, the bifunctional THPMe-NCS was conjugated to a humanised huA33 antibody targeting the A33 glycoprotein. Imaging experiments performed in nude mice bearing A33-positive SW1222 colorectal cancer xenografts compared pretargeting (100 μg of THPMe-NCS-huA33, followed after 24 h by 8-10 MBq of 68Ga3+) with both a directly labelled radioimmunoconjugate (89Zr-DFO-NCS-huA33, 88 μg, 7 MBq) and a 68Ga-only negative control (8-10 MBq of 68Ga3+). Imaging was performed 25 h after antibody administration (1 h after 68Ga3+ administration for negative control). No difference between pretargeting and the negative control was observed, suggesting that pretargeting via metal chelation is not feasible using this model. However, significant accumulation of "unchelated" 68Ga3+ in the tumour was found (12.9 %ID/g) even without prior administration of THPMe-NCS-huA33, though tumour-to-background contrast was impaired by residual activity in the blood. Therefore, the 68Ga-only experiment was repeated using THPMe (20 μg, 1 h after 68Ga3+ administration) to clear circulating 68Ga3+, producing a three-fold improvement of the tumour-to-blood activity concentration ratio. Although preliminary, these results highlight the potential of THPMe as a 68Ga clearing agent in imaging applications with gallium citrate.

Quantitative and Qualitative Improvement of Low-Count [68Ga]Citrate and [90Y]Microspheres PET Image Reconstructions Using Block Sequential Regularized Expectation Maximization Algorithm

PURPOSE

There are several important positron emission tomography (PET) imaging scenarios that require imaging with very low photon statistics, for which both quantitative accuracy and visual quality should not be neglected. For example, PET imaging with the low photon statistics is closely related to active efforts to significantly reduce radiation exposure from radiopharmaceuticals. We investigated two examples of low-count PET imaging: (a) imaging [90Y]microsphere radioembolization that suffers the very small positron emission fraction of Y-90's decay processes, and (b) cancer imaging with [68Ga]citrate with uptake time of 3-4 half-lives, necessary for visualizing tumors. In particular, we investigated a type of penalized likelihood reconstruction algorithm, block sequential regularized expectation maximization (BSREM), for improving both image quality and quantitative accuracy of these low-count PET imaging cases.

PROCEDURES

The NEMA/IEC Body phantom filled with aqueous solution of Y-90 or Ga-68 was scanned to mimic the low-count scenarios of corresponding patient data acquisitions on a time-of-flight (TOF) PET/magnetic resonance imaging system. Contrast recovery, background variation, and signal-to-noise ratio were evaluated in different sets of count densities using both conventional TOF ordered subset expectation (TOF-OSEM) and TOF-BSREM algorithms. The regularization parameter, beta, in BSREM that controls the tradeoff between image noise and resolution was evaluated to find a value for improved confidence in image interpretation. Visual quality assessment of the images obtained from patients administered with [68Ga]citrate (n = 6) was performed. We also made preliminary visual image quality assessment for one patient with [90Y]microspheres. In Y-90 imaging, the effect of 511-keV energy window selection for minimizing the number of random events was also evaluated.

RESULTS

Quantitatively, phantom images reconstructed with TOF-BSREM showed improved contrast recovery, background variation, and signal-to-noise ratio values over images reconstructed with TOF-OSEM. Both phantom and patient studies of delayed imaging of [68Ga]citrate show that TOF-BSREM with beta = 500 gives the best tradeoff between image noise and image resolution based on visual assessment by the readers. The NEMA-IQ phantom study with [90Y]microspheres shows that the narrow energy window (460-562 keV) recovers activity concentrations in small spheres better than the regular energy window (425-650 keV) with the beta value of 2000 using the TOF-BSREM algorithm. For the images obtained from patients with [68Ga]citrate using TOF-BSREM with beta = 500, the visual analogue scale (VAS) was improved by 17 % and the Likert score was increased by 1 point on average, both in comparison to corresponding scores for images reconstructed using TOF-OSEM.

CONCLUSION

Our investigation shows that the TOF-BSREM algorithm improves the image quality and quantitative accuracy in low-count PET imaging scenarios. However, the beta value in this algorithm needed to be adjusted for each radiopharmaceutical and counting statistics at the time of scans.

NMR-based metabolomics analysis identifies discriminatory metabolic disturbances in tissue and biofluid samples for progressive prostate cancer

BACKGROUND

Prostate cancer (PCa) is one of the most common cancers in men, but its metabolic characteristics during tumor progression are still far from being fully understood.

METHODS

The metabolic profiles of matched tissue, serum and urine samples from the same patients were analyzed using a ¹H NMR-based metabolomics approach. We identified several important metabolites that significantly altered at different stages of PCa, including benign prostatic hyperplasia (BPH), early PCa (EPC), advanced PCa (APC), metastatic PCa (MPC) and castration-resistant PCa (CRPC). Metabolic correlation networks among tissue, serum and urine samples were examined using Pearson's correlation.

RESULTS

The changes in metabolic phenotypes during the progression of PCa were more noticeable in tissue samples when compared with serum and urine samples. Herein we identified a series of important metabolic disturbances, including decreased trends of citrate, creatinine, acetate, leucine, valine, glycine, lysine, histidine, glutamine and choline as well as increased trends of uridine and formate. These metabolites are mainly implicated in energy metabolism, amino acid metabolism, choline and fatty acid metabolism as well as uridine metabolism. We also found that energy metabolism in tumor tissues was positively associated with amino acid metabolism in serum and urine. Additionally, CRPC patients had a peculiar metabolic phenotype, especially decreased amino acid metabolism in serum.

CONCLUSIONS

The present study characterizes metabolic disturbances in both tissue and biofluid samples during PCa progression and provides potential diagnostic biomarkers and therapeutic targets for PCa.

Targeting iron metabolism in high-grade glioma with 68Ga-citrate PET/MR

Noninvasive tools that target tumor cells could improve the management of glioma. Cancer generally has a high demand for Fe(III), an essential nutrient for a variety of biochemical processes. We tested whether 68Ga-citrate, an Fe(III) biomimetic that binds to apo-transferrin in blood, detects glioma in preclinical models and patients using hybrid PET/MRI. Mouse PET/CT studies showed that 68Ga-citrate accumulates in subcutaneous U87MG xenografts in a transferrin receptor-dependent fashion within 4 hours after injection. Seventeen patients with WHO grade III or IV glioma received 3.7-10.2 mCi 68Ga-citrate and were imaged with PET/MR 123-307 minutes after injection to establish that the radiotracer can localize to human tumors. Multiple contrast-enhancing lesions were PET avid, and tumor to adjacent normal white matter ratios were consistently greater than 10:1. Several contrast-enhancing lesions were not PET avid. One minimally enhancing lesion and another tumor with significantly reduced enhancement following bevacizumab therapy were PET avid. Advanced MR imaging analysis of one patient with contrast-enhancing glioblastoma showed that metabolic hallmarks of viable tumor spatially overlaid with 68Ga-citrate accumulation. These early data underscore that high-grade glioma may be detectable with a radiotracer that targets Fe(III) transport.

Nuclear magnetic resonance spectroscopy as a new approach for improvement of early diagnosis and risk stratification of prostate cancer

Prostate cancer (PCa) is the second most common male cancer worldwide and the fifth leading cause of death from cancer in men. Early detection and risk stratification is the most effective way to improve the survival of PCa patients. Current PCa biomarkers lack sufficient sensitivity and specificity to cancer. Metabolite biomarkers are evolving as a new diagnostic tool. This review is aimed to evaluate the potential of metabolite biomarkers for early detection, risk assessment, and monitoring of PCa. Of the 154 identified publications, 27 and 38 were original papers on urine and serum metabolomics, respectively. Nuclear magnetic resonance (NMR) is a promising method for measuring concentrations of metabolites in complex samples with good reproducibility, high sensitivity, and simple sample processing. Especially urine-based NMR metabolomics has the potential to be a cost-efficient method for the early detection of PCa, risk stratification, and monitoring treatment efficacy.

Imaging Hepatocellular Carcinoma With 68Ga-Citrate PET: First Clinical Experience

While cross-sectional imaging with computed tomography (CT) and magnetic resonance imaging is the primary method for diagnosing hepatocellular carcinoma (HCC), they provide little biological insight into this molecularly heterogeneous disease. Nuclear imaging tools that can detect molecular subsets of tumors could greatly improve diagnosis and management of HCC. To this end, we conducted a patient study to determine whether HCC can be resolved using ⁶⁸Ga-citrate positron emission tomography (PET). One patient with recurrent HCC was injected with 300 MBq of ⁶⁸Ga-citrate and imaged with PET/CT 249 minutes post injection. Four (28%) of 14 hepatic lesions were avid for ⁶⁸Ga-citrate. One extrahepatic lesion was not PET avid. The average maximum standardized uptake value (SUV_max) for the lesions was 7.2 (range: 6.2-8.4), while the SUV_max of the normal liver parenchyma was 4.7 and blood pool was 5.7. The avid lesions were not significantly larger than the quiescent lesions, and a prior contrast CT showed uniform enhancement among the lesions, suggesting that tumor signals are due to specific binding of the radiotracer to the transferrin receptor, rather than enhanced vascularity in the tumor microenvironment. Further studies are required in a larger patient cohort to verify the molecular basis of radiotracer uptake and the clinical utility of this tool.

PET/CT to detect adverse reactions to metal debris in patients with metal-on-metal hip arthroplasty: an exploratory prospective study

Metal-on-metal (MoM) bearings in total hip arthroplasties and hip resurfacing arthroplasties have recently shown a new type of complication: adverse reactions to metal debris (ARMD). ARMD is characterized by local severe inflammation and tissue necrosis leading to implant failures. The gluteal muscle region is important for the patient outcome after revision surgery. This prospective positron emission tomography/computed tomography (PET/CT) study was undertaken to evaluate the characteristics of 2-deoxy-2-[¹⁸ F]fluoro-d-glucose ([¹⁸ F]FDG) and [⁶⁸ Ga]Gallium citrate ([⁶⁸ Ga]Citrate) PET/CT in ARMD patients. [¹⁸ F]FDG and [⁶⁸ Ga]Citrate PET/CT were performed in 18 hip arthroplasty patients: 12 ARMD patients (with 16 MoM hips) and six arthroplasty controls without ARMD. Tracer uptake was evaluated visually, and maximum standardized uptake (SUV_max ) was measured in the gluteal muscle region. ARMD severity was graded by metal artefact reduction sequence-magnetic resonance imaging (MARS-MRI). Periprosthetic [¹⁸ F]FDG uptake was observed in 15 of 16 hips, [⁶⁸ Ga]Citrate uptake in three of 16 hips, respectively. The distribution of tracer uptake resembled infection in three hips. In the gluteal muscle region, the SUV_max of [¹⁸ F]FDG was significantly greater in hips with moderate and severe ARMD compared with the controls (P = 0·009 for [¹⁸ F]FDG and P = 0·217 for [⁶⁸ Ga]Citrate). In patients who needed revision surgery, an intraoperative finding of gluteal muscle necrosis was associated with increased local SUV_max as detected by preoperative [¹⁸ F]FDG (P = 0·039), but not by [⁶⁸ Ga]Citrate (P = 0·301). In conclusion, the inflammatory reaction to metal debris in hip arthroplasty patients is best visualized with [¹⁸ F]FDG.

Real-Time Transferrin-Based PET Detects MYC-Positive Prostate Cancer

Noninvasive biomarkers that detect the activity of important oncogenic drivers could significantly improve cancer diagnosis and management of treatment. The goal of this study was to determine whether 68Ga-citrate (which avidly binds to circulating transferrin) can detect MYC-positive prostate cancer tumors, as the transferrin receptor is a direct MYC target gene. PET imaging paired with 68Ga-citrate and molecular analysis of preclinical models, human cell-free DNA (cfDNA), and clinical biopsies were conducted to determine whether 68Ga-citrate can detect MYC-positive prostate cancer. Importantly, 68Ga-citrate detected human prostate cancer models in a MYC-dependent fashion. In patients with castration-resistant prostate cancer, analysis of cfDNA revealed that all patients with 68Ga-citrate avid tumors had a gain of at least one MYC copy number. Moreover, biopsy of two PET avid metastases showed molecular or histologic features characteristic of MYC hyperactivity. These data demonstrate that 68Ga-citrate targets prostate cancer tumors with MYC hyperactivity. A larger prospective study is ongoing to demonstrate the specificity of 68Ga-citrate for tumors with hyperactive MYC.Implications: Noninvasive measurement of MYC activity with quantitative imaging modalities could substantially increase our understanding of the role of MYC signaling in clinical settings for which invasive techniques are challenging to implement or do not characterize the biology of all tumors in a patient. Moreover, measuring MYC activity noninvasively opens the opportunity to study changes in MYC signaling in patients under targeted therapeutic conditions thought to indirectly inhibit MYC. Mol Cancer Res; 15(9); 1221-9. ©2017 AACR.

Development of a 68Ge/68Ga Generator System Using Polysaccharide Polymers and Its Application in PET Imaging of Tropical Infectious Diseases

Gallium-68 (⁶⁸Ga) is a positron emitter for clinical positron emission tomography (PET) applications that can be produced by a ⁶⁸Ge/⁶⁸Ga generator without cyclotron. However, commercially available ⁶⁸Ge/⁶⁸Ga generator systems require multiple steps for the preparation of ⁶⁸Ga radiopharmaceuticals and are sometimes plagued by metallic impurities in the ⁶⁸Ga eluent. We developed a ⁶⁸Ge/⁶⁸Ga generator system using polysaccharide-based adsorbents and direct application of the generator-eluted ⁶⁸Ga-citrate to PET imaging of tropical infectious diseases. N-Methylglucamine (MG) as a ⁶⁸Ge-adsorbing unit (Sepha-MGs) was introduced to a series of Sephadex G-10, G-15, G-25, G-50, and G-75. In the batch method, over 97% of the ⁶⁸Ge in the solution was adsorbed onto the Sepha-MG series within 15 min. In particular, ⁶⁸Ge was effectively adsorbed on the Sepha(15)-MG packed columns and 70-80% of the ⁶⁸Ga was eluted by 1 mL of 0.1 M trisodium citrate with low ⁶⁸Ge contamination (<0.001%). The chemical form of the generator-eluted ⁶⁸Ga solution was identified as ⁶⁸Ga-citrate. In PET studies, affected regions in mice infected with Leishmania and severe fever with thrombocytopenia syndrome virus were clearly visualized using the ⁶⁸Ga-citrate. Sepha-MGs are useful adsorbents for ⁶⁸Ge/⁶⁸Ga generator systems with high ⁶⁸Ga elution efficiency and minimal ⁶⁸Ge breakthrough. These results indicated that eluted ⁶⁸Ga-citrate can be directly used for PET imaging of infectious sites in mice. This novel generator system may be useful for straightforward PET imaging of infection in clinical practice.

Noninvasive Measurement of mTORC1 Signaling with 89Zr-Transferrin

Purpose: mTOR regulates many normal physiological processes and when hyperactive can drive numerous cancers and human diseases. However, it is very challenging to detect and quantify mTOR signaling noninvasively in clinically relevant animal models of disease or man. We hypothesized that a nuclear imaging tool measuring intracellular mTOR activity could address this unmet need.Experimental Design: Although the biochemical activity of mTOR is not directly amenable to nuclear imaging probe development, we show that the transferrin receptor can be used to indirectly measure intracellular changes in mTOR activity.Results: After verifying that the uptake of radiolabeled transferrin (the soluble ligand of the transferrin receptor) is stimulated by active mTORC1 in vitro, we showed that ⁸⁹Zr-labeled transferrin (Tf) can measure mTORC1 signaling dynamics in normal and cancerous mouse tissues with PET. Finally, we show that ⁸⁹Zr-Tf can detect the upregulation of mTORC1 by tumor cells to escape the antitumor effects of a standard-of-care antiandrogen, which is to our knowledge the first example of applying PET to interrogate the biology of treatment resistant cancer.Conclusions: In summary, we have developed the first quantitative assay to provide a comprehensive measurement of mTOR signaling dynamics in vivo, in specific normal tissues, and during tumor development in genetically engineered animal models using a nuclear imaging tool that is readily translatable to man. Clin Cancer Res; 23(12); 3045-52. ©2016 AACR.