Comparison of animal models for the evaluation of radiolabeled androgens

Aberrant androgen action in prostatic progenitor cells induces oncogenesis and tumor development through IGF1 and Wnt axes

Androgen/androgen receptor (AR) signaling pathways are essential for prostate tumorigenesis. However, the fundamental mechanisms underlying the AR functioning as a tumor promoter in inducing prostatic oncogenesis still remain elusive. Here, we demonstrate that a subpopulation of prostatic Osr1 (odd skipped-related 1)-lineage cells functions as tumor progenitors in prostate tumorigenesis. Single cell transcriptomic analyses reveal that aberrant AR activation in these cells elevates insulin-like growth factor 1 (IGF1) signaling pathways and initiates oncogenic transformation. Elevating IGF1 signaling further cumulates Wnt/β-catenin pathways in transformed cells to promote prostate tumor development. Correlations between altered androgen, IGF1, and Wnt/β-catenin signaling are also identified in human prostate cancer samples, uncovering a dynamic regulatory loop initiated by the AR through prostate cancer development. Co-inhibition of androgen and Wnt-signaling pathways significantly represses the growth of AR-positive tumor cells in both ex-vivo and in-vivo, implicating co-targeting therapeutic strategies for these pathways to treat advanced prostate cancer.

The quest for improving the management of breast cancer by functional imaging: The discovery and development of 16α-[18F]fluoroestradiol (FES), a PET radiotracer for the estrogen receptor, a historical review

INTRODUCTION

16α-[18F]Fluoroestradiol (FES), a PET radiotracer for the estrogen receptor (ER) in breast cancer, was the first receptor-targeted PET radiotracer for oncology and is continuing to prove its value in clinical research, antiestrogen development, and breast cancer care. The story of its conception, design, evaluation and use in clinical studies parallels the evolution of the whole field of receptor-targeted radiotracers, one greatly influenced by the research and intellectual contributions of William C. Eckelman.

METHODS AND RESULTS

The development of methods for efficient production of fluorine-18, for conversion of [18F]fluoride ion into chemically reactive form, and for its rapid and efficient incorporation into suitable estrogen precursor molecules at high molar activity, were all methodological underpinnings required for the preparation of FES. FES binds to ER with very high affinity, and its in vivo uptake by ER-dependent target tissues in animal models was efficient and selective, findings that preceded its use for PET imaging in patients with breast cancer.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Comparisons between ER levels measured by FES-PET imaging of breast tumors with tissue-specimen ER quantification by IHC and other methods show that imaging provided improved prediction of benefit from endocrine therapies. Serial imaging of ER by FES-PET, before and after dosing patients with antiestrogens, is used to determine the efficacious dose for established antiestrogens and to facilitate clinical development of new ER antagonists. Beyond FES imaging, PET-based hormone challenge tests, which evaluate the functional status of ER by monitoring rapid changes in tumor metabolic or transcriptional activity after a brief estrogen challenge, provide highly sensitive and selective predictions of whether or not there will be a favorable response to endocrine therapies. There is sufficient interest in the clinical applications of FES that FDA approval is being sought for its wider use in breast cancer.

CONCLUSIONS

FES was the first PET probe for a receptor in cancer, and its development and clinical applications in breast cancer parallel the conceptual evolution of the whole field of receptor-binding radiotracers.

PET Imaging Agents (FES, FFNP, and FDHT) for Estrogen, Androgen, and Progesterone Receptors to Improve Management of Breast and Prostate Cancers by Functional Imaging

Many breast and prostate cancers are driven by the action of steroid hormones on their cognate receptors in primary tumors and in metastases, and endocrine therapies that inhibit hormone production or block the action of these receptors provide clinical benefit to many but not all of these cancer patients. Because it is difficult to predict which individuals will be helped by endocrine therapies and which will not, positron emission tomography (PET) imaging of estrogen receptor (ER) and progesterone receptor (PgR) in breast cancer, and androgen receptor (AR) in prostate cancer can provide useful, often functional, information on the likelihood of endocrine therapy response in individual patients. This review covers our development of three PET imaging agents, 16α-[18F]fluoroestradiol (FES) for ER, 21-[18F]fluoro-furanyl-nor-progesterone (FFNP) for PgR, and 16β-[18F]fluoro-5α-dihydrotestosterone (FDHT) for AR, and the evolution of their clinical use. For these agents, the pathway from concept through development tracks with an emerging understanding of critical performance criteria that is needed for successful PET imaging of these low-abundance receptor targets. Progress in the ongoing evaluation of what they can add to the clinical management of breast and prostate cancers reflects our increased understanding of these diseases and of optimal strategies for predicting the success of clinical endocrine therapies.

Radiobromine and radioiodine for medical applications

The halogens bromine and iodine have similar chemical properties and undergo similar reactions due to their closeness in Group 17 of the periodic chart. There are a number of bromine and iodine radionuclides that have properties useful for diagnosis and therapy of human diseases. The emission properties of radiobromine and radioiodine nuclides with half-lives longer than 1 h are summarized along with properties that make radionuclides useful in PET/SPECT imaging and β/Auger therapy, such that the reader can assess which of the radionuclides might be useful for medical applications. An overview of chemical approaches that have been used to radiolabel molecules with radiobromine and radioiodine nuclides is provided with examples. Further, references to a large variety of different organ/cancer-targeting agents utilizing the radiolabeling approaches described are provided.

Specific 18F-FDHT Accumulation in Human Prostate Cancer Xenograft Murine Models Is Facilitated by Prebinding to Sex Hormone-Binding Globulin

Tremendous efforts are currently dedicated to the development of novel therapies targeting the androgen receptor (AR), the major driver of prostate cancer disease and its progression to castration resistance. The ability to noninvasively interrogate AR expression over time in murine models of prostate cancer would permit longitudinal preclinical analysis of novel compounds that could not otherwise be accomplished ex vivo. Although PET imaging with 16β-¹⁸F-fluoro-5α-dihydrotestosterone (¹⁸F-FDHT) has successfully quantified AR levels clinically, no rodent model of ¹⁸F-FDHT imaging has been reported so far. One difference between humans and rodents is the absence in the latter of the sex hormone-binding globulin (SHBG), a glycoprotein that binds to testosterone in the bloodstream, Here, we explore the role of SHBG in developing a working model of rodent AR imaging. Methods: Three human prostate cancer cell lines and xenografts (LNCaP, 22Rv1, and PC3) were used to examine the uptake of free ¹⁸F-FDHT and SHBG-bound ¹⁸F-FDHT. Both ligands were examined for stability and competitive binding to AR over time in vitro before in vivo studies. PET/CT imaging was used to dynamically measure the uptake of both tracers over 4 h, whereas specificity was determined by competitive binding with the AR antagonist enzalutamide. Results: AR levels correlated with the uptake of both ¹⁸F-FDHT and SHBG-¹⁸F-FDHT in prostate cancer cell lines. Interestingly, whereas both free and SHBG-bound ¹⁸F-FDHT had a similar cellular accumulation at 1 and 2.5 h, SHBG-¹⁸F-FDHT accumulated at significantly higher levels after 4 h-evidence that receptor-mediated uptake of SHBG accounted for later time-point differences. This observation was also seen in 22Rv1 tumor-bearing mice, in which SHBG-¹⁸F-FDHT exhibited a significantly increased uptake (average tumor-to-background ratio [TBR], 1.62 ± 0.62) in comparison to unbound ¹⁸F-FDHT (TBR, 0.81 ± 0.08) at 4 h. Furthermore, the specificity of the SHBG-¹⁸F-FDHT accumulation at 4 h was demonstrated by a reduced tumor uptake after AR blockade with enzalutamide (TBR, 1.07 ± 0.13). Conclusion: Prebinding of ¹⁸F-FDHT to SHBG allows accurate and quantitative PET imaging of AR levels in murine models of prostate cancer. This procedure may permit the use of PET imaging to study the longitudinal effects of AR-targeting therapies, accelerating novel-drug development.

Intranasal administration of testosterone increased immobile-sniffing, exploratory behavior, motor behavior and grooming behavior in rats

Currently, testosterone (T) replacement therapy is typically provided by oral medication, injectable T esters, surgically implanted T pellets, transdermal patches and gels. However, most of these methods of administration are still not ideal for targeting the central nervous system. Recently, therapeutic intranasal T administration (InT) has been considered as another option for delivering T to the brain. In the present study, the effects of 21-day InT treatment were assessed on open field behavior in gonadectomized (GDX) rats and intact rats. Subcutaneous injections of T at same dose were also tested in GDX rats. A total of 12 behavioral events were examined in GDX groups with or without T and in intact groups with or without InT. Significant decreases in open field activity were observed in rats after GDX without InT compared to sham-operated rats. The open field activity scores for most tests significantly increased with InT treatment in GDX rats and in intact rats compared with the corresponding GDX rats and intact rats. Intranasal administration of T improved the reduced behaviors resulted from T deficiency better than subcutaneous injection of T, demonstrating that T can be delivered to the brain by intranasal administration. Our results suggest that intranasal T delivery is an effective option for targeting the central nervous system.

Production of non-standard PET radionuclides and the application of radiopharmaceuticals labeled with these nuclides

The field of positron emission tomography (PET) has expanded dramatically over recent years. In spite of this expansion the large majority of clinical studies are carried out utilizing one radiopharmaceutical-2-fluoro-2-deoxyglucose. Many research groups are developing novel radiopharmaceuticals. A major emphasis is on other agents labeled with 18F. Several other positron emitting radionuclides can be prepared in high yields in small biomedical cyclotrons. Some of these have half-lives that make delivery significantly easier than the delivery of 18F compounds. These radionuclides include: 64Cu (half life 12.7 h), 76Br (half life 16.2 h), 86Y (half life 14.74 h) and 124I (half life 4.2 days). The method of production of these and other 'non-standard' PET radionuclides will be discussed and the method of labeling radiopharmaceuticals with these radionuclides described. Several of these radiopharmaceuticals have been studied in animal models as well and a limited number translated to the human situation.

Prostate cancer PET bioprobes: Synthesis of [18F]-radiolabeled hydroxyflutamide derivatives

Approximately 80-90% of prostate cancers are androgen dependent at initial diagnosis. The androgen receptor (AR) is present in most advanced prostate cancer specimens and is believed to have a critical role in its development. Today, treatment of prostate cancer is done by inhibition of AR using antiandrogens such as flutamide (pro-drug of hydroxyflutamide), nilutamide, and bicalutamide. However, there is currently no noninvasive imaging modalities to detect, guide, and monitor specific treatment of AR-positive prostate cancer. (R)-3-Bromo-N-(4-fluoro-3-(trifluoromethyl)phenyl)-2-hydroxy-2-methyl-propanamide [18F]-1 and N-(4-fluoro-3-(trifluoromethyl)phenyl)-2-hydroxy-2-methylpropanamide [18F]-2, derivatives of hydroxyflutamide, were synthesized as a fluorine-containing imaging agent candidates. A three-step fluorine-18 radiosynthesis route was developed, and the compounds were successfully labeled with a 10+/-3% decay corrected radiochemical yield, 95% radiochemical purity, and a specific activity of 1500+/-200 Ci/mmol end of bombardment (n = 10). These labeled biprobes not only may enable for the future quantitative molecular imaging of AR-positive prostate cancer using positron emission tomography but may also allow for image-guided treatment of prostate cancer.

Molecular imaging of cancer with positron emission tomography

The imaging of specific molecular targets that are associated with cancer should allow earlier diagnosis and better management of oncology patients. Positron emission tomography (PET) is a highly sensitive non-invasive technology that is ideally suited for pre-clinical and clinical imaging of cancer biology, in contrast to anatomical approaches. By using radiolabelled tracers, which are injected in non-pharmacological doses, three-dimensional images can be reconstructed by a computer to show the concentration and location(s) of the tracer of interest. PET should become increasingly important in cancer imaging in the next decade.