New iodinated progestins as potential ligands for progesterone receptor imaging in breast cancer. Part 2: In vivo pharmacological characterization

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.

Progesterone receptor targeting with radiolabelled steroids: an approach in predicting breast cancer response to therapy

Steroid receptors have demonstrated to be potentially useful biological targets for the diagnosis and therapy follow-up of hormonally responsive cancers. The over-expression of these proteins in human cancer cells as well as their binding characteristics provides a favourable mechanism for the localization of malignant tumours. The need for newer and more selective probes to non-invasively assess steroid receptor expression in hormone-responsive tumours has encouraged the synthesis and the biological evaluation of several steroidal derivatives labelled with positron and gamma emitters. The physiological effects of the steroid hormone progesterone are mediated by the progesterone receptor (PR). Since PR expression is stimulated by the oestrogen receptor (ER), PR status has been considered as a biomarker of ER activity and its value for predicting and monitoring therapeutic efficacy of hormonal therapy has been studied. Imaging of PR-expressing breast cancer patients under hormonal therapy may be advantageous, since the response to therapy can be more accurately predicted after quantification of both ER and PR status. Thus, ligands for PR targeting, although much less explored than ER ligands, have gained some importance lately as potential PET and SPECT tumour imaging agents. In this review, we present a brief survey of explored approaches for progesterone targeting using radiolabelled progestins as potential clinical probes to predict responsiveness to breast cancer therapy. This article is part of a Special Issue entitled "Synthesis and biological testing of steroid derivatives as inhibitors".

Small molecule receptors as imaging targets

The aberrant expression and function of certain receptors in tumours and other diseased tissues make them preferable targets for molecular imaging. PET and SPECT radionuclides can be used to label specific ligands with high affinity for the target receptors. The functional information obtained from imaging these receptors can be used to better understand the systems under investigation and for diagnostic and therapeutic applications. This review discusses some of the aspects of receptor imaging with small molecule tracers by PET and SPECT and reviews some of the tracers for the receptor imaging of tumours and brain, heart and lung disorders.

Receptor Imaging in Oncology by Means of Nuclear Medicine: Current Status

To date, our understanding of the role of receptors and their cognate ligands in cancer is being successfully translated into the design and development of an arsenal of new, less toxic, and more specific anticancer drugs. Because most of these novel drugs are cytostatic, objective response as measured by morphologic imaging modalities (eg, computed tomography or magnetic resonance imaging) cannot be used as a surrogate marker for drug development or for clinical decision making. Positron emission tomography (PET) can be used to image and quantify the in vivo distribution of positron-emitting radioisotopes such as oxygen-15, carbon-11, and fluorine-18 that can be substituted or added into biologically relevant and specific receptor radioligands. Similarly, single-photon emission computed tomography (SPECT) can be used to image and quantify the in vivo distribution of receptor targeting compounds labeled with indium-111, technetium-99m, and iodine-123. By virtue of their whole-body imaging capacity and the absence of errors of sampling and tissue manipulation as well as preparation, both techniques have the potential to address locoregional receptor status noninvasively and repetitively. This article reviews available data on the in vivo evaluation of receptor systems by means of PET or SPECT for identifying and monitoring patients with sufficient receptor overexpression for tailored therapeutic interventions, and also for depicting tumor tissue and determining the currently largely unknown heterogeneity in receptor expression among different tumor lesions within and between patients.

Molecular imaging: The latest generation of contrast agents and tissue characterization techniques

Molecular Imaging technologies will have a profound impact on both basic research and clinical imaging in the near future. As the field covers many different specialties and scientific disciplines it is not possible to review all in a single article. In the current article we will turn our attention to those modalities that are either currently in use or in development for the medical imaging clinic.

Molecular imaging: new applications for biochemistry

Molecular imaging can reveal in vivo analysis and quantification of biochemical reactions. To enable cell-surface imaging of receptors, novel ligands have been developed which can be radiolabeled or imaged by bioluminescence. Specific examples include somatostatin receptors, estrogen and progesterone receptors, receptors involved in adhesion and externalization of phosphatidyl serine as an indicator of apoptosis. Central nervous system imaging can be carried out using ligands for receptors including dopamine, serotonin and Gamma amino butyric acid (GABA). In addition, tumor and metabolic imaging can be carried out with the Na-K ATPase pump using the tracer thallium-201 for SPECT or F-18 FDG for PET imaging. Finally, novel receptors or endogenous metabolic pathways can be analyzed combining cell-gene therapy to create specific tracer targets in cells that can be studied by molecular imaging. The challenge of molecular imaging is to first identify key pathways that are unique for a specific disease processes, such as atherosclerosis, cancer, CNS disorders, immunologic and arthritis disorders and next to devise a high-affinity specific small molecular ligand that can be adapted to be a radiolabeled tracer to study this pathway. Advances in genomics and proteomics combine with new peptide-chemistry approaches should provide a large number of targets and tracers in the near future to achieve these imaging objectives.

Nuclear medicine applications in molecular imaging

With the emergence of the new field of molecular imaging, there is an increasing demand for development of sensitive and safe novel imaging agents that can be rapidly translated from small animal models into patients. Nuclear medicine and positron emission tomography (PET) techniques have the ability to detect and serially monitor a variety of biologic and pathophysiologic processes, usually with tracer quantities of radiolabeled peptides, drugs, and other molecules at doses free of pharmacologic side effects, unlike the current generation of intravenous agents required for magnetic resonance (MR) and computed tomography (CT) scanning. In this article, we will review a representative sampling of the wide array of radiopharmaceuticals developed specifically for nuclear medicine radionuclide imaging that have been approved for clinical use, and those in pre-clinical trials. We will also review the existing strategies used to select the appropriate biologic markers and targets for radionuclide labeling that have been employed in the development of novel radiotracers and the imaging of small animals with new microSPECT (single photon emission computed tomography) technologies.

New iodinated progestins as potential ligands for progesterone receptor imaging in breast cancer. Part 1: Synthesis and in vitro pharmacological characterization

Five putative iodinated progesterone receptor (PR) binding ligands were synthesized and evaluated as potential imaging agents for PR-positive human breast tumours. Two compounds (E- and Z-17-hydroxy-21-iodo-19-nor-17alpha-pregna-4,20-dien-3-one; E- and Z-IPG1) were previously described, but are re-evaluated. The other three were novel compounds: two nortestosterone analogues derived from ORG 3236 (E- and Z-13-ethyl-17-hydroxy-21-iodo-11-methylene-18,19-dinor-17alpha-pre gna-4,20-diene-3-one; E- and Z-IPG2) and one norprogesterone analogue derived from ORG 2058 (21-[4-iodophenoxy]-16alpha-ethyl-19-norpregn-4-ene-3, 20-dione; IPG3). The E-iodovinyl nortestosterone compounds were obtained by a new route of synthesis. Competitive binding studies were performed to determine their binding affinities for the PR in three types of tissue (human MCF-7 breast tumour cells and rat uterine and mammary tumour tissue) and for the androgen receptor (AR) in human MCF-7 breast tumour cells, as well as for the sex hormone-binding globulin (SHBG) and corticosteroid-binding globulin (CBG) in human plasma. All four 17alpha-iodovinyl nortestosterone derivatives displayed high binding affinity for the human PR, that of Z-IPG1 and E- and Z-IPG2 being even higher than that of ORG2058. Their affinities for the rat PR were somewhat lower, especially those of both E-isomers. The affinity of IPG3 was lower for both the human and rat PR. The nortestosterone derivatives also showed AR binding, the relative binding affinities ranging from 4.3 to 17.0% as compared with 5alphaDHT. Additionally, neither of these steroids displayed any significant binding to either SHBG or CBG in human plasma. We conclude that the in vitro binding properties of all four 17alpha-iodovinyl nortestosterone derivatives warrant evaluation of the distribution characteristics of their 123I-labelled analogues to determine their usefulness as PR imaging agents.