Evaluation of a bromine-76-labeled progestin 16alpha,17alpha-dioxolane for breast tumor imaging and radiotherapy: in vivo biodistribution and metabolic stability studies

PARP Theranostic Auger Emitters Are Cytotoxic in BRCA Mutant Ovarian Cancer and Viable Tumors from Ovarian Cancer Patients Enable Ex-Vivo Screening of Tumor Response

Theranostics are emerging as a pillar of cancer therapy that enable the use of single molecule constructs for diagnostic and therapeutic application. As poly adenosine diphosphate (ADP)-ribose polymerase 1 (PARP-1) is overexpressed in various cancer types, and is localized to the nucleus, PARP-1 can be safely targeted with Auger emitters to induce DNA damage in tumors. Here, we investigated a radioiodinated PARP inhibitor, [¹²⁵I]KX1, and show drug target specific DNA damage and subsequent killing of BRCA1 and non-BRCA mutant ovarian cancer cells at sub-pharmacological concentrations several orders of magnitude lower than traditional PARP inhibitors. Furthermore, we demonstrated that viable tumor tissue from ovarian cancer patients can be used to screen tumor radiosensitivity ex-vivo, enabling the direct assessment of therapeutic efficacy. Finally, we showed tumors can be imaged by single-photon computed tomography (SPECT) with PARP theranostic, [¹²³I]KX1, in a human ovarian cancer xenograft mouse model. These data support the utility of PARP-1 targeted radiopharmaceutical therapy as a theranostic option for PARP-1 overexpressing ovarian cancers.

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.

Radioiodinated progesterone derivative for progesterone receptor targeting with enhanced nucleus uptake via phenylboronic acid conjugation

A novel ¹³¹ I-radiolabeled probe with aromatic boronate motif (¹³¹ I-EIPBA) was designed to target progesterone receptor (PR)-positive breast cancer with enhanced nucleus uptake. Acetylene progesterone was conjugated with pegylated phenylboronic acid via click reaction and radiolabeled with ¹³¹ I to afford ¹³¹ I-EIPBA. Meanwhile, ¹³¹ I-EIPB without boronate was prepared as control agent. After determination of the lipophilicity and stability of these tracers, in vitro cell uptake studies and in vivo biodistribution in rats were performed to verify the enhanced nucleus uptake and PR targeting ability of ¹³¹ I-EIPBA. ¹³¹ I-EIPBA was obtained with moderate radiochemical yield (40.35 ± 3.52%) and high radiochemical purity (>98%). As expected, the high binding affinity (39.58 nM) of ¹³¹ I-EIPBA for PR was determined by cell binding assay. The internalization ratio of ¹³¹ I-EIPBA was remarkably higher than that of ¹³¹ I-EIPB in PR-positive MCF-7 cells. Furthermore, the enhanced nucleus uptake of ¹³¹ I-EIPBA (0.59 ± 0.02%) was found to be significantly higher than that of ¹³¹ I-EIPB (0.13 ± 0.01%) in MCF-7 cells. A novel ¹³¹ I-EIPBA compound was developed for PR targeting with improved cellular nucleus uptake. Furthermore, the introduction of aromatic boronate motif provides a worthwhile strategy for enhancing the nuclear receptor targeting of tracers.

Molecular imaging using PET and SPECT for identification of breast cancer subtypes

Breast cancer is a major disease with high morbidity and mortality in women. As a highly heterogeneous tumor, it contains different molecular subtypes: luminal A, luminal B, human epidermal growth factor 2-positive, and triple-negative subtypes. As each subtype has unique features, it may not be universal to the optimal treatment and expected response for individual patients. Therefore, it is critical to identify different breast cancer subtypes. Targeting subcellular levels, molecular imaging, especially PET and single photon emission computed tomography, has become a promising means to identify breast cancer subtypes and monitor treatment. Different biological processes between various subtypes, including changes correlated with receptor expression, cell proliferation, or glucose metabolism, have the potential for imaging with PET and single photon emission computed tomography radiopharmaceuticals. Receptor imaging, with radiopharmaceuticals targeting estrogen receptor, progesterone receptor, or human epidermal growth factor 2, is available to distinguish receptor-positive tumors from receptor-negative ones. Cell proliferation imaging with fluorine-18 fluorothymidine PET aids identification of luminal A and B subtypes on the basis of the correlation with the immunohistochemical biomarker Ki-67. Glucose metabolism imaging with fluorine-18 fluorodeoxyglucose PET may have potential to discriminate triple-negative subtypes from others. With increasing numbers of novel radiopharmaceuticals, noninvasive molecular imaging will be applied widely for the identification of different subtypes and provide more in-vivo information on individualized management of breast cancer patients.

PET Imaging of Breast Cancer: Role in Patient Management

Breast cancer is the most common malignancy in females. Imaging plays a critical role in diagnosis, staging and surveillance, and management of disease. Fluorodeoxyglucose (FDG) PET the imaging is indicated in specific clinical setting. Sensitivity of detection depends on tumor histology and size. Whole body FDG PET can change staging and management. In recurrent disease, distant metastasis can be detected. FDG PET imaging has prognostic and predictive value. PET/MR is evolving rapidly and may play a role management, assessment of metastatic lesions, and treatment monitoring. This review discusses current PET modalities, focusing on of FDG PET imaging and novel tracers.

Longitudinal noninvasive imaging of progesterone receptor as a predictive biomarker of tumor responsiveness to estrogen deprivation therapy

PURPOSE

To investigate whether longitudinal functional PET imaging of mammary tumors using the radiopharmaceuticals [(18)F]FDG (to measure glucose uptake), [(18)F]FES [to measure estrogen receptor (ER) levels], or [(18)F]FFNP [to measure progesterone receptor (PgR) levels] is predictive of response to estrogen-deprivation therapy.

EXPERIMENTAL DESIGN

[(18)F]FDG, [(18)F]FES, and [(18)F]FFNP uptake in endocrine-sensitive and -resistant mammary tumors was quantified serially by PET before ovariectomy or estrogen withdrawal in mice, and on days 3 and 4 after estrogen-deprivation therapy. Specificity of [(18)F]FFNP uptake in ERα(+) mammary tumors was determined by competition assay using unlabeled ligands for PgR or glucocorticoid receptor (GR). PgR expression was also assayed by immunohistochemistry (IHC).

RESULTS

The levels of [(18)F]FES and [(18)F]FDG tumor uptake remained unchanged in endocrine-sensitive tumors after estrogen-deprivation therapy compared with those at pretreatment. In contrast, estrogen-deprivation therapy led to a reduction in PgR expression and [(18)F]FFNP uptake in endocrine-sensitive tumors, but not in endocrine-resistant tumors, as early as 3 days after treatment; the changes in PgR levels were confirmed by IHC. Unlabeled PgR ligand R5020 but not GR ligand dexamethasone blocked [(18)F]FFNP tumor uptake, indicating that [(18)F]FFNP bound specifically to PgR. Therefore, a reduction in FFNP tumor to muscle ratio in mammary tumors predicts sensitivity to estrogen-deprivation therapy.

CONCLUSIONS

Monitoring the acute changes in ERα activity by measuring [(18)F]FFNP uptake in mammary tumors predicts tumor response to estrogen-deprivation therapy. Longitudinal noninvasive PET imaging using [(18)F]FFNP is a robust and effective approach to predict tumor responsiveness to endocrine treatment.

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".

Mapping biological behaviors by application of longer-lived positron emitting radionuclides

With the technological development of positron emission tomography (PET) and the advent of novel antibody-directed drug delivery systems, longer-lived positron-emitting radionuclides are moving to the forefront to take important roles in tracking the distribution of biotherapeutics such as antibodies, and for monitoring biological processes and responses. Longer half-life radionuclides possess advantages of convenient on-site preparation procedures for both clinical and non-clinical applications. The suitability of the long half-life radionuclides for imaging intact monoclonal antibodies (mAbs) and their respective fragments, which have inherently long biological half-lives, has attracted increased interest in recent years. In this review, we provide a survey of the recent literature as it applies to the development of nine-selected longer-lived positron emitters with half-lives of 9-140h (e.g., (124)I, (64)Cu, (86)Y and (89)Zr), and describe the biological behaviors of radionuclide-labeled mAbs with respect to distribution and targeting characteristics, potential toxicities, biological applications, and clinical translation potentials.

New Methods for Labeling RGD Peptides with Bromine-76

Direct bromination of the tyrosine residues of peptides and antibodies with bromine-76, to create probes for PET imaging, has been reported. For peptides that do not contain tyrosine residues, however, a prosthetic group is required to achieve labeling via conjugation to other functional groups such as terminal α-amines or lysine ε-amines. The goal of this study was to develop new strategies for labeling small peptides with Br-76 using either a direct labeling method or a prosthetic group, depending on the available functional group on the peptides. A new labeling agent, N-succinimidyl-3-[(76)Br]bromo-2,6-dimethoxybenzoate ([(76)Br]SBDMB) was prepared for cyclic RGD peptide labeling. N-succinimidyl-2, 6-dimethoxybenzoate was also used to pre-attach a 2, 6-dimethoxybenzoyl (DMB) moiety to the peptide, which could then be labeled with Br-76. A competitive cell binding assay was performed to determine the binding affinity of the brominated peptides. PET imaging of U87MG human glioblastoma xenografted mice was performed using [(76)Br]-BrE[c(RGDyK)](2) and [(76)Br]-BrDMB-E[c(RGDyK)](2). An ex vivo biodistribution assay was performed to confirm PET quantification. The mechanisms of bromination reaction between DMB-c(RGDyK) and the brominating agent CH(3)COOBr were investigated with the SCRF-B3LYP/6-31G* method with the Gaussian 09 program package. The yield for direct labeling of c(RGDyK) and E[c(RGDyK)](2) using chloramine-T and peracetic acid at ambient temperature was greater than 50%. The yield for [(76)Br]SBDMB was over 60% using peracetic acid. The conjugation yields for labeling c(RGDfK) and c(RGDyK) were over 70% using the prosthetic group at room temperature. Labeling yield for pre-conjugated peptides was over 60%. SDMB conjugation and bromination did not affect the binding affinity of the peptides with integrin receptors. Both [(76)Br]Br-E[c(RGDyK)](2) and [(76)Br]BrDMB-E[c(RGDyK)](2) showed high tumor uptake in U87MG tumor bearing mice. The specificity of the imaging tracers was confirmed by decreased tumor uptake after co-administration of unlabeled dimeric RGD peptides. The energy barrier of the transition state of bromination for the dimethoxybenzoyl group was about 9 kcal/mol lower than that for the tyrosine residue. In conclusion, the newly developed N-succinimidyl-2, 6-dimethoxybenzoate molecule can be used either for one step labeling through pre-conjugation or as the precursor for a Br-76 labeled prosthetic group for indirect labeling. Radiobromination on a dimethoxybenzoyl group has selectivity over radiobromination on tyrosine. The energy barrier difference of the transition states of bromination between the dimethoxybenzoyl group and the tyrosine residue may account for the reaction selectivity when both groups are present in the same molecule.

Development of [F-18]fluorine-substituted Tanaproget as a progesterone receptor imaging agent for positron emission tomography

The level of progesterone receptors (PRs) in breast tumors can be used to guide the selection of endocrine therapies for breast cancer patients. To this end, we have prepared a fluorine-18 labeled analogue of Tanaproget, a nonsteroidal progestin with very high PR binding affinity and low affinity for androgen and glucocorticoid receptors, and have studied its tissue distribution in estrogen-primed rats to evaluate its potential for imaging PR levels by positron emission tomography. 4-[(18)F]Fluoropropyl-Tanaproget ([(18)F]9, FPTP) was prepared in three steps, within 140 min at an overall decay-corrected yield of 5% and effective specific activity of >550 Ci/mmol. In biodistribution studies, [(18)F]9 uptake was high in target tissues at both 1 and 3 h (uterus, 4.55 and 5.26%ID/g; ovary, 2.32 and 2.20%ID/g, respectively) and was cleanly blocked by coinjection of excess unlabeled compound. Uterus to blood and muscle activity ratios were 9.2 and 5.2 at 1 h and 32 and 26 at 3 h, respectively. The biodistribution of [(18)F]9 compares favorably to that of previously prepared F-18 labeled steroidal progestins, FENP and FFNP. Its high target tissue uptake efficiency and selectivity, and prolonged retention, suggest that it has excellent promise as a PET imaging agent for PR-positive breast tumors.

Preparation and preliminary bioevaluation of 99mTc(CO)3-11β-progesterone derivative prepared via click chemistry route

INTRODUCTION

Progesterone receptors (PRs) overexpressed in breast cancers serve as potential targets for developing radiotracers for use in nuclear medicine. Hence, suitably derivatized progesterone can be envisaged as a potential vector for targeting overexpression of receptors in breast cancer. In the present article, we report the preparation of a (99m)Tc(CO)(3)-progesterone triazole using the Cu(I)-catalyzed novel click chemistry route. Preliminary evaluation of the radiolabeled derivative has been carried out in binding studies with MCF 7 cell lines.

METHODS

11-Hydroxyprogesterone has been synthetically derivatized to 11-azidoprogesterone. Subsequently, the cycloaddition reaction between progesterone azide and propargyl glycine was carried out to prepare 1,4-bifunctionalized progesterone triazole analogue. The clicked progesterone triazole derivative was radiolabeled with (99m)Tc and characterized by HPLC. The chemical characterization of (99m)Tc(CO)(3)-progesterone triazole has been carried out by preparing its corresponding rhenium complex using the [NEt(4)](2)[Re(CO)(3)Br(3)] precursor. While in vitro studies were carried out in MCF7 cell lines, in vivo distribution studies were performed in female Swiss mice.

RESULTS

The radiolabeled complex could be prepared in >95% radiochemical yield as determined by HPLC. In vitro studies of (99m)Tc(CO)(3)-progesterone complex in MCF7 cell lines overexpressing receptors for breast cancer showed binding up to 30%. In vivo distribution studies in female Swiss mice have shown uterine uptake of 0.41 (0.06) % ID/g at 3 h postinjection (pi) and retention therein till 24 h pi.

CONCLUSION

The present study demonstrates a novel and facile route for preparation of (99m)Tc-labeled progesterone complex using click chemistry. This strategy can be further extended towards preparation of radiolabeled complexes of other steroidal derivatives.

Imaging progesterone receptor in breast tumors: synthesis and receptor binding affinity of fluoroalkyl-substituted analogues of tanaproget

The progesterone receptor (PR) is estrogen regulated, and PR levels in breast tumors can be used to predict the success of endocrine therapies targeting the estrogen receptor (ER). Tanaproget is a nonsteroidal progestin agonist with very high PR binding affinity and excellent in vivo potency. When appropriately radiolabeled, it might be used to image PR-positive breast tumors noninvasively by positron emission tomography (PET). We describe the synthesis and PR binding affinities of a series of fluoroalkyl-substituted 6-aryl-1,4-dihydrobenzo[d][1,3]oxazine-2-thiones, analogues of Tanaproget. Some of these compounds have subnanomolar binding affinities, higher than that of either Tanaproget itself or the high affinity PR ligand R5020. Structure-binding affinity relationships can be rationalized by molecular modeling of ligand complexes with PR, and the enantioselectivity of binding has been predicted. These compounds are being further evaluated as potential diagnostic PET imaging agents for breast cancer, and enantiomerically pure materials of defined stereochemistry are being prepared.

Bromination from the macroscopic level to the tracer radiochemical level: (76)Br radiolabeling of aromatic compounds via electrophilic substitution

No-carrier-added (NCA) (76)Br labeling of 4-(5-acetoxy-7-bromobenzoxazol-2-yl)phenyl acetate, a diacetate-protected estrogen-receptor beta (ERbeta) selective ligand, was carried out successfully using [(76)Br]bromide ion. The labeling was achieved via oxidative electrophilic destannylation of an organotin precursor molecule by modification of the leaving group (from Bu(3)Sn to Me(3)Sn) and the addition of methanol to the reaction mixture. The differences between the oxidative bromination reaction under small-scale macroscopic vs tracer level radiochemical conditions were explored in terms of effective brominating agents, which depend greatly on the nature of the solvent during the radiochemical bromination, and the potential interference by trace levels of highly reactive impurities in the reaction that compete for the desired bromination at the NCA level. Our observations, and our development of experimental protocols for successful radiobromination at the tracer NCA-scale, should be applicable to the synthesis of other radiobromine-labeled organic compounds of potential interest as PET radiopharmaceuticals and radiotherapy agents.