Tanushree Ganguly

A phosphoramidate-based prostate-specific membrane antigen-targeted SPECT agent.

Prostate‐specific membrane antigen (PSMA) remains an active target for imaging and therapeutic applications for prostate cancer.

A high-affinity [(18)F]-labeled phosphoramidate peptidomimetic PSMA-targeted inhibitor for PET imaging of prostate cancer.

INTRODUCTION In this study, a structurally modified phosphoramidate scaffold, with improved prostate-specific membrane antigen (PSMA) avidity, stability and in vivo characteristics, as a PET imaging agent for prostate cancer (PCa), was prepared and evaluated. METHODS p-Fluorobenzoyl-aminohexanoate and 2-(3-hydroxypropyl)glycine were introduced into the PSMA-targeting scaffold yielding phosphoramidate 5. X-ray crystallography was performed on the PSMA/5 complex. [(18)F]5 was synthesized, and cell uptake and internalization studies were conducted in PSMA(+) LNCaP and CWR22Rv1 cells and PSMA(-) PC-3 cells. In vivo PET imaging and biodistribution studies were performed at 1 and 4 h post injection in mice bearing CWR22Rv1 tumor, with or without blocking agent. RESULTS The crystallographic data showed interaction of the p-fluorobenzoyl group with an arene-binding cleft on the PSMA surface. In vitro studies revealed elevated uptake of [(18)F]5 in PSMA(+) cells (2.2% in CWR22Rv1 and 12.1% in LNCaP) compared to PSMA(-) cells (0.08%) at 4 h. In vivo tumor uptake of 2.33% ID/g and tumor-to-blood ratio of 265:1 was observed at 4 h. CONCLUSIONS We have successfully synthesized, radiolabeled and evaluated a new PSMA-targeted PET agent. The crystal structure of the PSMA/5 complex highlighted the interactions within the arene-binding cleft contributing to the overall complex stability. The high target uptake and rapid non-target clearance exhibited by [(18)F]5 in PSMA(+) xenografts substantiates its potential use for PET imaging of PCa. ADVANCES IN KNOWLEDGE The only FDA-approved imaging agent for PCa, Prostascint®, targets PSMA but suffers from inherent shortcomings. The data acquired in this manuscript confirmed that our new generation of [(18)F]-labeled PSMA inhibitor exhibited promising in vivo performance as a PET imaging agent for PCa and is well-positioned for subsequent clinical trials. Implications for Patient Care Our preliminary data demonstrate that this tracer possesses the required imaging characteristics to be sensitive and specific for PCa imaging in patients at all stages of the disease.

Caged [18F]FDG Glycosylamines for Imaging Acidic Tumor Microenvironments Using Positron Emission Tomography

Solid tumors are hypoxic with altered metabolism, resulting in secretion of acids into the extracellular matrix and lower relative pH, a feature associated with local invasion and metastasis. Therapeutic and diagnostic agents responsive to this microenvironment may improve tumor-specific delivery. Therefore, we pursued a general strategy whereby caged small-molecule drugs or imaging agents liberate their parent compounds in regions of low interstitial pH. In this manuscript, we present a new acid-labile prodrug method based on the glycosylamine linkage, and its application to a class of positron emission tomography (PET) imaging tracers, termed [(18)F]FDG amines. [(18)F]FDG amines operate via a proposed two-step mechanism, in which an acid-labile precursor decomposes to form the common radiotracer 2-deoxy-2-[(18)F]fluoro-d-glucose, which is subsequently accumulated by glucose avid cells. The rate of decomposition of [(18)F]FDG amines is tunable in a systematic fashion, tracking the pKa of the parent amine. In vivo, a 4-phenylbenzylamine [(18)F]FDG amine congener showed greater relative accumulation in tumors over benign tissue, which could be attenuated upon tumor alkalinization using previously validated models, including sodium bicarbonate treatment, or overexpression of carbonic anhydrase. This new class of PET tracer represents a viable approach for imaging acidic interstitial pH with potential for clinical translation.

Development of inhibitor-directed enzyme prodrug therapy (IDEPT) for prostate cancer

Prostate cancer (PCa) is the second most common cause of cancer death among American men after lung cancer. Unfortunately, current therapies do not provide effective treatments for patients with advanced, metastatic, or hormone refractory disease. Therefore, we seek to generate therapeutic agents for a novel PCa treatment strategy by delivering a suicide enzyme (yCDtriple) to a cell membrane bound biomarker found on PCa cells (prostate-specific membrane antigen (PSMA)). This approach has resulted in a new PCa treatment strategy reported here as inhibitor-directed enzyme prodrug therapy (IDEPT). The therapeutic agents described were generated using a click chemistry reaction between the unnatural amino acid (p-azidophenylalanine (pAzF)) incorporated into yCDtriple and the dibenzylcyclooctyne moiety of our PSMA targeting agent (DBCO-PEG4-AH2-TG97). After characterization of the therapeutic agents, we demonstrate significant PCa cell killing of PSMA-positive cells. Importantly, we demonstrate that this click chemistry approach can be used to efficiently couple a therapeutic protein to a targeting agent and may be applicable to the ablation of other types of cancers and/or malignancies.

Structure–Activity Relationship of 18F-Labeled Phosphoramidate Peptidomimetic Prostate-Specific Membrane Antigen (PSMA)-Targeted Inhibitor Analogues for PET Imaging of Prostate Cancer

A series of phosphoramidate-based prostate specific membrane antigen (PSMA) inhibitors of increasing lipophilicity were synthesized (4, 5, and 6), and their fluorine-18 analogs were evaluated for use as positron emission tomography (PET) imaging agents for prostate cancer. To gain insight into their modes of binding, they were also cocrystallized with the extracellular domain of PSMA. All analogs exhibited irreversible binding to PSMA with IC50 values ranging from 0.4 to 1.3 nM. In vitro assays showed binding and rapid internalization (80-95%, 2 h) of the radiolabeled ligands in PSMA(+) cells. In vivo distribution demonstrated significant uptake in CWR22Rv1 (PSMA(+)) tumor, with tumor to blood ratios of 25.6:1, 63.6:1, and 69.6:1 for [(18)F]4, [(18)F]5, and [(18)F]6, respectively, at 2 h postinjection. Installation of aminohexanoic acid (AH) linkers in the phosphoramidate scaffold improved their PSMA binding and inhibition and was critical for achieving suitable in vivo imaging properties, positioning [(18)F]5 and [(18)F]6 as favorable candidates for future prostate cancer imaging clinical trials.