High-affinity Near-infrared Fluorescent Small-molecule Contrast Agents for In Vivo Imaging of Prostate-specific Membrane Antigen

PSMA-targeted SMART molecules outfitted with SN38

Herein, we report the modular synthesis and evaluation of a prostate-specific membrane antigen (PSMA) targeted small molecule drug conjugate (SMDC) carrying the chemotherapeutic agent, SN38. Due to the fluorogenic properties of SN38, payload release kinetics from the platform was observed in buffers representing the pH conditions of systemic circulation and cellular internalization. It was found that this platform is stable with minimal payload release at physiological pH with most rapid payload release observed at pH values representing the endosome complex. We confirmed selective payload release and chemotherapeutic efficacy for PSMA(+) prostate cancer cells over PSMA(-) cells. These results demonstrate that chemotherapeutic agents with limited solubility can be conjugated to a water-soluble targeting and linker platform without attenuating efficacy.

Fluorescent Conjugates Based on Prostate-Specific Membrane Antigen Ligands as an Effective Visualization Tool for Prostate Cancer

Fluorescent dyes are widely used in histological studies and in intraoperative imaging, including surgical treatment of prostate cancer (PC), which is one of the most common types of cancerous tumors among men today. Targeted delivery of fluorescent conjugates greatly improves diagnostic efficiency and allows for timely correct diagnosis. In the case of PC, the protein marker is a prostate-specific membrane antigen (PSMA). To date, a large number of diagnostic conjugates targeting PSMA have been created based on modified urea. The review focuses on the conjugates selectively binding to PSMA and answers the following questions: What fluorescent dyes are already in use in the field of PC diagnosis? What factors influence the structure-activity ratio of the final molecule? What features should be considered when selecting a fluorescent tag to create new diagnostic conjugates? And what could be suggested to further development in this field at the present time?

Imaging of prostate cancer: optimizing affinity to prostate specific membrane antigen by spacer modifications in a tumor spheroid model

Early diagnosis of prostate cancer (PCa) is crucial for staging, treatment and management of patients. Prostate specific membrane antigen (PSMA), highly over-expressed on PCa cells, is an excellent target for selective imaging of PCa. In recent years, various scaffolds have been explored as potential carriers to target diagnostic and therapeutic agents to PSMA⁺ tumour cells. Numerous fluorescent or radioisotope probes linked via a peptide linker have been developed that selectively binds to PCa cells. However, there are very few reports that examine the effects of chemical modifications in the peptide linker of an imaging probe on its affinity to PSMA protein. This report systematically investigates the impact of hydrophobic aromatic moieties in the peptide linker on PSMA affinity and in vitro performance. For this, a series of fluorescent bioconjugates 12-17 with different aromatic spacers were designed, synthesized, and their interactions within the PSMA pocket were first analysed in silico. Cell uptake studies were then performed for 12-17 in PSMA⁺ cell lines and 3D tumour models in vitro. Binding affinity values of 12-17 were found to be in the range of 36 to 157.9 nM, and 12 with three aromatic groups in the spacer exhibit highest affinity (K_D = 36 nM) compared to 17 which is devoid of aromatic groups. These studies suggest that aromatic groups in the spacer region can significantly affect deep tissue imaging of fluorescent bioconjugates. Bioconjugate 12 can be a promising diagnostic tool, and conjugation to near-infrared agents would further its applications in deep-tissue imaging and surgery. Communicated by Ramaswamy H. Sarma.

A Prostate-Specific Membrane Antigen-Targeted Near-Infrared Conjugate for Identifying Pulmonary Squamous Cell Carcinoma during Resection

Pulmonary squamous cell carcinoma is the second most common lung cancer subtype and has a low 5-year survival rate at 17.6%. Complete resection with negative margins can be curative, but a high number of patients suffer early postoperative recurrence due to inadequate disease clearance at the index operation. Intraoperative molecular imaging (IMI) with tumor-targeted optical contrast agents is effective in improving resection completeness for other tumor types, but there are no IMI tracers targeted to pulmonary squamous cell carcinoma. In this report, we describe the use of a novel prostate-specific membrane antigen (PSMA)-targeted near-infrared conjugate (OTL78) to identify pulmonary squamous cell carcinoma. We identified PSMA as a viable target by examining its expression in human lung tumor specimens from a surgical cohort. Ninety-four percent of tumors expressed PSMA in either the pulmonary squamous cells or the tumor neovasculature. Using in vitro and in vivo models, we found that OTL78 reliably localized pulmonary squamous cell carcinoma in a PSMA-dependent manner. Finally, we found that IMI with OTL78 markedly improved surgeons' ability to identify residual disease after surgery in a preclinical model. Ultimately, this novel optical tracer may aid surgical resection of pulmonary squamous cell carcinoma and potentially improve long-term outcomes.

Prostate-Specific Membrane Antigen-Targeted Turn-on Probe for Imaging Cargo Release in Prostate Cancer Cells

The tunable nature of phosphoramidate linkers enables broad applicability as pH-triggered controlled-release platforms, particularly in the context of antibody- and small-molecule-drug conjugates (ADCs and SMDCs), where there remains a need for new linker technology. Herein, we explored in-depth the release of turn-on fluorogenic payloads from a homoserinyl-based phosphoramidate acid-cleavable linker. Kinetics of payload release from the scaffold was observed in buffers representing the pH conditions of systemic circulation, early and late endosomes, and lysosomes. It was found that payload release takes place in two key consecutive steps: (1) P-N bond hydrolysis and (2) spacer immolation. These two steps were found to follow pseudo-first-order kinetics and had opposite dependencies on pH. P-N bond hydrolysis increased with decreasing pH, while spacer immolation was most rapid at physiological pH. Despite the contrasting release kinetics of these two steps, maximal payload release was observed at the mildly acidic pH (5.0-5.5), while minimal payload release occurred at physiological pH. We integrated this phosphoramidate-payload linker system into a PSMA-targeted fluorescent turn-on probe to study the intracellular trafficking and release of a fluorescent payload in PSMA-expressing prostate cancer cells. Results showed excellent turn-on and accumulation of the coumarin payload in the late endosomal and lysosomal compartments of these cells. The release properties of this linker mark it as an attractive alternative in the modular design of ADCs and SMDCs, which demand selective intracellular payload release triggered by the pH changes that accompany intracellular trafficking.

A targeted near-infrared nanoprobe for deep-tissue penetration and imaging of prostate cancer

The current challenge in fluorescence guided surgery (FGS) for prostate cancer (PCa) is in the design of imaging probes with high selectivity, clear visualization of tumour margins, and minimal toxicity. This report aims to design and develop a novel NIR-nanoprobe, and evaluate its potential in the penetration of PCa tumour tissues. The PSMA receptor-targeted quantum dot (PSMA-QD655) is a NIR, deep-tissue imaging agent, which has the potential for intraoperative navigation during surgery and improved detection specificity for PCa. The probe was designed and synthesized by conjugating functionalized amino-PEG quantum dots (QDs) through a heterobifunctional linker to a DUPA targeted polypeptide construct. The nanoprobe was evaluated in vitro in PSMA⁺ PCa cell lines for specificity and its binding affinity was determined by flow cytometric analysis. The penetration efficacy was tested further on large PCa 3D tumour spheroids (dia ∼1200 μm, thickness ∼450 μm) by deep tissue multiphoton imaging. PSMA-QD655 was found to be an efficient deep tissue intra-operative guided surgical tool with a high affinity (K_D = 15.3 nM) and penetrative capacity. The results have been demonstrated in vitro in 2D and 3D tissue models, mimicking cancer lesions in vivo. In summary, we have developed a deep-tissue imaging NIR nanoprobe targeting prostatic lesions that (i) binds to PSMA⁺ tumour with sub-nanomolar affinity and high specificity, (ii) shows an excellent safety profile in primary cell lines in vitro and (iii) shows high penetrative capacity in a 3D prostate tumour model (∼450 μm tissue depth).

Recent Advances in Prostate-Specific Membrane Antigen-Based Radiopharmaceuticals

BACKGROUND

Prostate cancer (PCa) is the most common sex-related malignancy with high mortality in men worldwide. Prostate-specific membrane antigen (PSMA) is overexpressed on the surface of most prostate tumor cells and considered a valuable target for both diagnosis and therapy of prostate cancer. A series of radiolabeled agents have been developed based on the featured PSMA ligands in the previous decade and have demonstrated promising outcomes in clinical research of primary and recurrent PCa. Furthermore, the inspiring response and safety of lutetium-177-PSMA-617 (177Lu-PSMA-617) radiotherapy represent the potential for expanded therapeutic options for metastatic castration-resistant PCa. Retrospective cohort studies have revealed that radiolabeled PSMA agents are the mainstays of the current success, especially in detecting prostate cancer with metastasis and biochemical recurrence.

OBJECTIVE

This review is intended to present a comprehensive overview of the current literature on PSMA ligand-based agents for both radionuclide imaging and therapeutic approaches, with a focus on those that have been clinically adopted.

CONCLUSION

PSMA-based diagnosis and therapy hold great promise for improving the clinical management of prostate cancer.

Comparison of prostate-specific membrane antigen ligands in clinical translation research for diagnosis of prostate cancer

BACKGROUND

Prostate-specific membrane antigen (PSMA), overexpressed on prostate cancer (PCa), is a well-characterized cell surface protein to selectively diagnose PCa. PSMA's unique characteristics and its 1000-fold higher expression in PCa compared with other tissues renders it as a suitable biomarker for detection of PCa in its early stage. In this report, we critically analyze and recommend the requirements needed for the development of variety of PSMA-targeted molecular imaging agents based on antibodies, small molecule ligands, peptides, and aptamers. The targeting moieties are either conjugated to radionuclear isotopes or near-infrared agents for efficient diagnosis of PCa.

RECENT FINDINGS

From the analysis, it was found that several small molecule-derived PCa imaging agents are approved for clinical trials in Europe and the United States, and few are already in the clinical use for diagnosis of PCa. Even though ¹¹¹In-labeled capromab pendetide was approved by the Food and Drug Administration (FDA) and other engineered antibodies are available for detection of PCa, but high production cost, low shelf life (less than 1 month at 4°C), possibility of human immuno reactions, and low blood clearance rate necessitated a need for developing new imaging agents, which are serum stable, cost-effective, and possesses longer shelf life (6 months), have fast clearance rate from nontargeted tissues during the diagnosis process. It is found that small molecule ligand-derived imaging agents possesses most of the desired properties expected for an ideal diagnostic agent when compared with other targeting moieties.

CONCLUSION

This report discusses in detail the homing moieties used in the development of targeted diagnostic tools for detection of PCa. The merits and demerits of monoclonal antibodies, small molecule ligands, peptides, and aptamers for imaging of PCa and intraoperative guided surgery are extensively analyzed. Among all, urea-based ligands were found to be most successful in preclinical and clinical trials and show a major promise for future commercialization.

Prostate cancer: updates on current strategies for screening, diagnosis and clinical implications of treatment modalities

Prostate cancer is the most common cancer in men by way of diagnosis and a leading cause of cancer-related deaths. Early detection and intervention remains key to its optimum clinical management. This review provides the most updated information on the recent methods of prostate cancer screening, imaging and treatment modalities. Wherever possible, clinical trial data has been supplemented to provide a comprehensive overview of current prostate cancer research and development. Considering the recent success of immunotherapy in prostate cancer, we discuss cell, DNA and viruses based, as well as combinatorial immunotherapeutic strategies in detail. Furthermore, the potential of nanotechnology is increasingly being realized, especially in prostate cancer research, and we provide an overview of nanotechnology-based strategies, with special emphasis on nanotheranostics and multifunctional nanoconstructs. Understanding these recent developments is critical to the design of future therapeutic strategies to counter prostate cancer.

18F-Positron Emitting/Trimethine Cyanine-Fluorescent Contrast for Image-Guided Prostate Cancer Management

[^18/19F]-4, an anionic GCPII/PSMA inhibitor for image-guided intervention in prostate cancer, is described. [¹⁹F]-4 is radiolabeled with a radiochemical yield that is ≥27% and a molar activity of 190 ± 50 mCi/μmol in a <1 h, one-step, aqueous isotopic exchange reaction. [¹⁹F]-4 allows PSMA expression to be imaged by fluorescence (FL) and [¹⁸F]-PET. PC3-PIP (PSMA-positive, EC₅₀ = 6.74 ± 1.33 nM) cancers are specifically delineated in mice that bear 3 million (18 mg) PC3-PIP and PC3 (control, PSMA-negative) cells. Colocalization of [^18/19F]-4 PET, fluorescence, scintillated biodistribution, and PSMA expression are observed.

Targeting PSMA with a Cu-64 Labeled Phosphoramidate Inhibitor for PET/CT Imaging of Variant PSMA-Expressing Xenografts in Mouse Models of Prostate Cancer

PURPOSE

Prostate-specific membrane antigen (PSMA) is highly up-regulated in prostate tumor cells, providing an ideal target for imaging applications of prostate cancer. CTT-1297 (IC50 = 27 nM) is an irreversible phosphoramidate inhibitor of PSMA that has been conjugated to the CB-TE1K1P chelator for incorporation of Cu-64. The resulting positron emission tomography (PET) agent, [(64)Cu]ABN-1, was evaluated for selective uptake both in vitro and in vivo in PSMA-positive cells of varying expression levels. The focus of this study was to assess the ability of [(64)Cu]ABN-1 to detect and distinguish varying levels of PSMA in a panel of prostate tumor-bearing mouse models.

PROCEDURES

CTT-1297 was conjugated to the CB-TE1K1P chelator using click chemistry and radiolabeled with Cu-64. Internalization and binding affinity of [(64)Cu]ABN-1 was evaluated in the following cell lines having varying levels of PSMA expression: LNCaP late-passage > LNCaP early passage ≈ C4-2B > CWR22rv1 and PSMA-negative PC-3 cells. PET/X-ray computed tomography imaging was performed in NCr nude mice with subcutaneous tumors of the variant PSMA-expressing cell lines.

RESULTS

[(64)Cu]ABN-1 demonstrated excellent uptake in PSMA-positive cells in vitro, with ∼80 % internalization at 4 h for each PSMA-positive cell line with uptake (fmol/mg) correlating to PSMA expression levels. The imaging data indicated significant tumor uptake in all models. The biodistribution for late-passage LNCaP (highest PSMA expression) demonstrated the highest specific uptake of [(64)Cu]ABN-1 with tumor-to-muscle and tumor-to-blood ratios of 30 ± 11 and 21 ± 7, respectively, at 24 h post-injection. [(64)Cu]ABN-1 cleared through all tissues except for PSMA-positive kidneys.

CONCLUSION

[(64)Cu]ABN-1 demonstrated selective uptake in PSMA-positive cells and tumors, which correlated to the level of PSMA expression. The data reported herein suggest that [(64)Cu]ABN-1 will selectively target and image variant PSMA expression and in the future will serve as a non-invasive method to follow the progression of prostate cancer in men.

Next generation NIR fluorophores for tumor imaging and fluorescence-guided surgery: A review

Cancer is a group of diseases responsible for the major causes of mortality and morbidity among people of all ages. Even though medical sciences have made enormous growth, complete treatment of this deadly disease is still a challenging task. Last few decades witnessed an impressive growth in the design and development of near infrared (NIR) fluorophores with and without recognition moieties for molecular recognitions, imaging and image guided surgeries. The present article reviews recently reported NIR emitting organic/inorganic fluorophores that targets and accumulates in organelle/organs specifically for molecular imaging of cancerous cells. Near infrared (NIR probe) with or without a tumor-targeting warhead have been considered and discussed for their applications in the field of cancer imaging. In addition, challenges persist in this area are also delineated in this review.

PSMA-targeted contrast agents for intraoperative imaging of prostate cancer

Prostate-specific membrane antigen (PSMA) can serve as a molecular cell surface target for the detection and treatment of prostate cancer. Near-infrared (NIR) fluorescence imaging enables highly sensitive, rapid, and non-radioactive imaging of PSMA, though specific targeting still remains a challenge because no optimized contrast agents exist.

Fluorescent chemical probes for accurate tumor diagnosis and targeting therapy

This review focuses on small molecular ligand-targeted fluorescent imaging probes and fluorescent theranostics, including their design strategies and applications in clinical tumor treatment.

Selecting Targets for Tumor Imaging: An Overview of Cancer-Associated Membrane Proteins

Tumor targeting is a booming business: The global therapeutic monoclonal antibody market accounted for more than $78 billion in 2012 and is expanding exponentially. Tumors can be targeted with an extensive arsenal of monoclonal antibodies, ligand proteins, peptides, RNAs, and small molecules. In addition to therapeutic targeting, some of these compounds can also be applied for tumor visualization before or during surgery, after conjugation with radionuclides and/or near-infrared fluorescent dyes. The majority of these tumor-targeting compounds are directed against cell membrane-bound proteins. Various categories of targetable membrane-bound proteins, such as anchoring proteins, receptors, enzymes, and transporter proteins, exist. The functions and biological characteristics of these proteins determine their location and distribution on the cell membrane, making them more, or less, accessible, and therefore, it is important to understand these features. In this review, we evaluate the characteristics of cancer-associated membrane proteins and discuss their overall usability for cancer targeting, especially focusing on imaging applications.

Structure-Activity Relationship of (18)F-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.

Simultaneous Activation of Induced Heterodimerization between CXCR4 Chemokine Receptor and Cannabinoid Receptor 2 (CB2) Reveals a Mechanism for Regulation of Tumor Progression

The G-protein-coupled chemokine receptor CXCR4 generates signals that lead to cell migration, cell proliferation, and other survival mechanisms that result in the metastatic spread of primary tumor cells to distal organs. Numerous studies have demonstrated that CXCR4 can form homodimers or can heterodimerize with other G-protein-coupled receptors to form receptor complexes that can amplify or decrease the signaling capacity of each individual receptor. Using biophysical and biochemical approaches, we found that CXCR4 can form an induced heterodimer with cannabinoid receptor 2 (CB2) in human breast and prostate cancer cells. Simultaneous, agonist-dependent activation of CXCR4 and CB2 resulted in reduced CXCR4-mediated expression of phosphorylated ERK1/2 and ultimately reduced cancer cell functions such as calcium mobilization and cellular chemotaxis. Given that treatment with cannabinoids has been shown to reduce invasiveness of cancer cells as well as CXCR4-mediated migration of immune cells, it is plausible that CXCR4 signaling can be silenced through a physical heterodimeric association with CB2, thereby inhibiting subsequent functions of CXCR4. Taken together, the data illustrate a mechanism by which the cannabinoid system can negatively modulate CXCR4 receptor function and perhaps tumor progression.

Near-infrared fluorescence heptamethine carbocyanine dyes mediate imaging and targeted drug delivery for human brain tumor

Brain tumors and brain metastases are among the deadliest malignancies of all human cancers, largely due to the cellular blood-brain and blood-tumor barriers that limit the delivery of imaging and therapeutic agents from the systemic circulation to tumors. Thus, improved strategies for brain tumor visualization and targeted treatment are critically needed. Here we identified and synthesized a group of near-infrared fluorescence (NIRF) heptamethine carbocyanine dyes and derivative NIRF dye-drug conjugates for effective imaging and therapeutic targeting of brain tumors of either primary or metastatic origin in mice, which is mechanistically mediated by tumor hypoxia and organic anion-transporting polypeptide genes. We also demonstrate that these dyes, when conjugated to chemotherapeutic agents such as gemcitabine, significantly restricted the growth of both intracranial glioma xenografts and prostate tumor brain metastases and prolonged survival in mice. These results show promise in the application of NIRF dyes as novel theranostic agents for the detection and treatment of brain tumors.

Synthesis and evaluation of constrained phosphoramidate inhibitors of prostate-specific membrane antigen

Prostate-specific membrane antigen (PSMA) is a cell-surface enzyme-biomarker that is actively pursued for targeted delivery of imaging and therapeutic agents for prostate cancer. Our lab has developed PSMA inhibitors based on a phosphoramidate scaffold, which has shown both high selectivity for PSMA-positive tumors and rapid clearance in vivo when radiolabeled with (18)F. However, this scaffold exhibits hydrolytic instability under low pH and high temperature conditions, barring the use of other imaging or therapeutic radionuclides such as (68)Ga or (177)Lu. Previous studies in our lab have shown a trend in increasing acid stability as the distance between the phosphoramidate core and the α-carboxylate of the P1 residue is increased. Therefore, a new generation of phosphoramidate inhibitors was developed based on trans-4-hydroxyproline as the P1 residue to restrict the interaction of the α-carboxylate to the phosphoramidate core. These hydroxyproline inhibitors demonstrated comparable IC50 values to earlier generations as well as enhanced thermal and acid stability.

Development of targeted near-infrared imaging agents for prostate cancer

Prostate cancer is the most common noncutaneous malignancy affecting men in North America. Radical prostatectomy remains a definitive treatment for prostate cancer. However, prostate surgeries are still performed "blindly" with the extent of tumor infiltration past the margins of the surgery only being determined postoperatively. An imaging modality that can be used during surgery is needed to help define the tumor margins. With its abundant expression in prostate cancer, prostate-specific membrane antigen (PSMA) is an ideal target for detection of prostate cancer. The purpose of this study was to develop PSMA-targeted near-infrared (NIR) optical imaging probes for intraoperative visualization of prostate cancer. We synthesized a high-affinity PSMA ligand (PSMA-1) with low molecular weight and further labeled it with commercially available NIR dyes IRDy800 and Cy5.5. PSMA-1 and PSMA-1-NIR conjugates had binding affinities better than the parent ligand Cys-CO-Glu. Selective binding was measured for each of the probes in both in vitro and in vivo studies using competitive binding and uptake studies. Interestingly, the results indicated that the pharmacokinetics of the probes was dependent of the fluorophore conjugated to the PSMA-1 ligand and varied widely. These data suggest that PSMA-targeted probes have the potential to be further developed as contrast agents for clinical intraoperative fluorescence-guided surgery.

Near-infrared fluorescence imaging of cancer mediated by tumor hypoxia and HIF1α/OATPs signaling axis

Near-infrared fluorescence (NIRF) imaging agents are promising tools for noninvasive cancer imaging. Here, we explored the mechanistic properties of a specific group of NIR heptamethine carbocyanines including MHI-148 dye we identified and synthesized, and demonstrated these dyes to achieve cancer-specific imaging and targeting via a hypoxia-mediated mechanism. We found that cancer cells and tumor xenografts exhibited hypoxia-dependent MHI-148 dye uptake in vitro and in vivo, which was directly mediated by hypoxia-inducible factor 1α (HIF1α). Microarray analysis and dye uptake assay further revealed a group of hypoxia-inducible organic anion-transporting polypeptides (OATPs) responsible for dye uptake, and the correlation between OATPs and HIF1α was manifested in progressive clinical cancer specimens. Finally, we demonstrated increased uptake of MHI-148 dye in situ in perfused clinical tumor samples with activated HIF1α/OATPs signaling. Our results establish these NIRF dyes as potential tumor hypoxia-dependent cancer-targeting agents and provide a mechanistic rationale for continued development of NIRF imaging agents for improved cancer detection, prognosis and therapy.

Evaluation of phage display discovered peptides as ligands for prostate-specific membrane antigen (PSMA)

The aim of this study was to identify potential ligands of PSMA suitable for further development as novel PSMA-targeted peptides using phage display technology. The human PSMA protein was immobilized as a target followed by incubation with a 15-mer phage display random peptide library. After one round of prescreening and two rounds of screening, high-stringency screening at the third round of panning was performed to identify the highest affinity binders. Phages which had a specific binding activity to PSMA in human prostate cancer cells were isolated and the DNA corresponding to the 15-mers were sequenced to provide three consensus sequences: GDHSPFT, SHFSVGS and EVPRLSLLAVFL as well as other sequences that did not display consensus. Two of the peptide sequences deduced from DNA sequencing of binding phages, SHSFSVGSGDHSPFT and GRFLTGGTGRLLRIS were labeled with 5-carboxyfluorescein and shown to bind and co-internalize with PSMA on human prostate cancer cells by fluorescence microscopy. The high stringency requirements yielded peptides with affinities KD~1 µM or greater which are suitable starting points for affinity maturation. While these values were less than anticipated, the high stringency did yield peptide sequences that apparently bound to different surfaces on PSMA. These peptide sequences could be the basis for further development of peptides for prostate cancer tumor imaging and therapy.

A multivalent approach of imaging probe design to overcome an endogenous anion binding competition for noninvasive assessment of prostate specific membrane antigen

2[(3-Amino-3-carboxypropyl)(hydroxy)(phosphinyl)methyl]pentane-1,5-dioic acid) (GPI) is a highly potent inhibitor of prostate specific membrane antigen (PSMA) with a rapid in vivo clearance profile from nontarget organs including kidneys, but its use for imaging of PSMA is impeded by an endogenous anion (serum phosphate) competition, which compromises its specific binding to the antigen. Multipresentation of a targeting molecule on a single entity has been recognized as a practical way for imaging sensitivity enhancement. Herein, we demonstrate a multivalent approach based on a (64)Cu-specific bifunctional chelator scaffold to overcome the endogenous phosphate competition thus enabling the utility of GPI conjugates for in vivo detection of PSMA and imaging quantification. Both monomeric (H2CBT1G) and dimeric (H2CBT2G) conjugates were synthesized and labeled with (64)Cu for in vitro and in vivo evaluations. A 4-fold enhancement of PSMA binding affinity was observed for H2CBT2G as compared to H2CBT1G from the PSMA competitive binding assays performed on LNCaP cells. In vivo PET imaging studies were conducted on mouse xenograft models established with a PSMA(+) cell line, LNCaP, and PSMA(-) PC3 and H2009 cell lines. (64)Cu-CBT2G showed significantly higher LNCaP tumor uptake than (64)Cu-CBT1G at 1, 4, and 24 h postinjection (p.i.) (p < 0.05). In addition, tumor uptake of (64)Cu-CBT2G remained steady out to 24 h p.i. (1.46 ± 0.54, 1.12 ± 0.56, and 1.00 ± 0.50% ID/g at 1, 4, and 24 h p.i., respectively), while (64)Cu-CBT1G showed a great decrease from 1 to 4 h p.i. The PSMA imaging specificity of both H2CBT1G and H2CBT2G was demonstrated by their low uptake in PSMA(-) tumors (PC3 and H2009) and further confirmed by a significant signal reduction in PSMA(+) LNCaP tumors in the blockade study. In addition, the LNCaP tumor uptake (% ID/g) of (64)Cu-CBT2G was found to be in a positive linear correlation with the tumor size (R(2) = 0.92, 0.94, and 0.93 for 1 h, 4 h, and 24 h p.i.). This may render the probe with potential application in the management of patients with prostate cancer.

Exogenous near-infrared fluorophores and their applications in cancer diagnosis: biological and clinical perspectives

INTRODUCTION

Near-infrared fluorescent (NIRF) imaging is a rapidly growing research field which has the potential to be an important imaging modality in cancer diagnosis. Various exogenous NIR fluorophores have been developed for the technique, including small molecule fluorophores and nanoparticles. NIRF imaging has been used in animal models for the detection of cancer overthe last twenty years and has in recent years been used in human clinical trials.

AREAS COVERED

This article describes the types and characteristics of exogenous fluorophores available for in vivo fluorescent cancer imaging. The article also discusses the progression of NIRF cancer imaging over recent years and its future challenges, from both a biological and clinical perspective. in The review also looks at its application for lymph node mapping, tumor targeting and characterization, and tumor margin definition for surgical guidance.

EXPERT OPINION

NIRF imaging is not in routine clinical cancer practice; yet, the authors predict that techniques using NIR fluorophores for tumor margin definition and lymph node mapping will enter clinical practice in the near future. The authors also anticipate that NIRF imaging research will lead to the development of flurophores with 'high brightness' that will overcome the limited penetration of this modality and be better suited for non invasive tumor targeting.

PSMA-targeted SPECT agents: mode of binding effect on in vitro performance

BACKGROUND

The enzyme-biomarker prostate-specific membrane antigen (PSMA) is an active target for imaging and therapeutic applications for prostate cancer. The internalization of PSMA has been shown to vary with inhibitors' mode of binding: irreversible, slowly reversible, and reversible.

METHODS

In the present study, PSMA-targeted clickable derivatives of an irreversible phosphoramidate inhibitor DBCO-PEG(4) -CTT-54 (IC(50) = 1.0 nM) and a slowly reversible phosphate inhibitor, DBCO-PEG(4) -CTT-54.2 (IC(50) = 6.6 nM) were clicked to (99m) Tc(CO)(3) -DPA-azide to assemble a PSMA-targeted SPECT agent. The selectivity, percent uptake, and internalization of these PSMA-targeted SPECT agents were evaluated in PSMA-positive and PSMA-negative cells.

RESULTS

In vitro studies demonstrated that PSMA-targeted SPECT agents exhibited selective cellular uptake in the PSMA-positive LNCaP cells compared to PSMA-negative PC3 cells. More importantly, it was found that (99m) Tc(CO)(3) -DPA-DBCO-PEG(4) -CTT-54 based on an irreversible PSMA inhibitor core, exhibited greater uptake and internalization than (99m) Tc(CO)(3) -DPA-DBCO-PEG(4) -CTT-54.2 constructed from a slowly reversible PSMA inhibitor core.

CONCLUSIONS

We have demonstrated that a PSMA-targeted SPECT agent can be assembled efficiently using copper-less click chemistry. In addition, we demonstrated that mode of binding has an effect on internalization and percent uptake of PSMA-targeted SPECT agents; with the irreversible targeting agent demonstrating superior uptake and internalization in PSMA+ cells. The approach demonstrated in this work now supports a modular approach for the assembly of PSMA-targeted imaging and therapeutic agents.

Synthesis and biological evaluation of low molecular weight fluorescent imaging agents for the prostate-specific membrane antigen

Targeted near-infrared (NIR) optical imaging can be used in vivo to detect specific tissues, including malignant cells. A series of NIR fluorescent ligands targeting the prostate-specific membrane antigen (PSMA) was synthesized and each compound was tested for its ability to image PSMA+ tissues in experimental models of prostate cancer. The agents were prepared by conjugating commercially available active esters of NIR dyes, including IRDye800CW, IRDye800RS, Cy5.5, Cy7, or a derivative of indocyanine green (ICG) to the terminal amine group of (S)-2-(3-((S)-5-amino-1-carboxypentyl)ureido)pentanedioic acid 1, (14S,18S)-1-amino-8,16-dioxo-3,6-dioxa-9,15,17-triazaicosane-14,18,20-tricarboxylic acid 2 and (3S,7S)-26-amino-5,13,20-trioxo-4,6,12,21-tetraazahexacosane-1,3,7,22-tetracarboxylic acid 3. The K(i) values for the dye-inhibitor conjugates ranged from 1 to 700 pM. All compounds proved capable of imaging PSMA+ tumors selectively to varying degrees depending on the choice of fluorophore and linker. The highest tumor uptake was observed with IRDye800CW employing a poly(ethylene glycol) or lysine-suberate linker, as in 800CW-2 and 800CW-3, while the highest tumor to nontarget tissue ratios were obtained for Cy7 with these same linkers, as in Cy7-2 and Cy7-3. Compounds 2 and 3 provide useful scaffolds for targeting of PSMA+ tissues in vivo and should be useful for preparing NIR dye conjugates designed specifically for clinical intraoperative optical imaging devices.

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

BACKGROUND

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

METHODS

In the present study, an irreversible phosphoramidate inhibitor, CTT-54 (IC50 = 14 nM), has been modified to deliver 99mTc-(CO)3-DTPA as a SPECT imaging payload to PSMA+ cells in vivo and in vitro. Percent uptake, competitive binding, and internalization will evaluate the imaging agent in vitro. Preliminary biodistribution and imaging will be utilized for in vivo evaluation.

RESULTS

In vitro studies demonstrate that the radiotracer 99mTc-(CO)3-DTPA-CTT-54 exhibits increasing cellular uptake in the PSMA+ LNCaP cells over time. More importantly, it was found that 99mTc-(CO)3-DTPA-CTT-54 is rapidly internalized into LNCaP cells, presumably through the PSMA enzyme-inhibitor complex. In a pilot biodistribution study, increasing accumulation of the radiotracer in LNCaP xenografts was observed from 2 to 4 hr and significant clearance from non-target tissues.

CONCLUSIONS

While DTPA may not represent the ideal chelate structure for 99mTc(CO)3, the data provides proof-of-concept support for the development of a next-generation phosphoramidate-based PSMA inhibitor-conjugates for use as SPECT imaging agents.

N-substituted glutamyl sulfonamides as inhibitors of glutamate carboxypeptidase II (GCP2)

Glutamate carboxypeptidase II (GCP2) is a membrane-bound cell-surface peptidase which is implicated in several neurological disorders and is also over-expressed in prostate tumor cells. There is a significant interest in the inhibition of GCP2 as a means of neuroprotection, while GCP2 inhibition as a method to treat prostate cancer remains a topic of further investigation. The key zinc-binding functional group of the well-characterized classes of GCP2 inhibitors (phosphonates and phosphoramidates) is tetrahedral and negatively charged at neutral pH, while glutamyl urea class of inhibitors possesses a planar and neutral zinc-binding group. This study introduces a new class of GCP2 inhibitors, N-substituted glutamyl sulfonamides, which possess a neutral tetrahedral zinc-binding motif. A library containing 15 secondary sulfonamides and 4 tertiary (N-methyl) sulfonamides was prepared and evaluated for inhibitory potency against purified GCP2 enzyme activity. While most inhibitors lacked potency at 100 μm, short alkyl sulfonamides exhibited promising low micromolar potency, with the optimal inhibitor in this series being glutamyl N-(propylsulfonamide) (2g). Lastly, molecular docking was used to develop a model to formulate an explanation for the relative inhibitory potencies employed for this class of inhibitors.

Flow cytometric detection of prostate tumor cells using chemoaffinity labels

BACKGROUND

The enzyme-biomarker prostate-specific membrane antigen (PSMA) is an emerging target for imaging and therapeutic applications for prostate cancer. However, the use of PSMA for detecting circulating prostate tumor cells remains under-explored. The present study focuses on the specific labeling of PSMA+ prostate cancer cells with a fluorescent PSMA inhibitor and the quantitation of PSMA+ cells in blood by flow cytometry (FC) using a gating strategy to separate labeled PSMA+ cells from peripheral blood mononuclear cells.

METHODS

Suspensions of PSMA+ (LNCaP) and PSMA- (DU145) cells were incubated with the fluorescent PSMA inhibitor FAMX-CTT-54. Incubation parameters (time, temperature, and label concentration) were varied to optimize cell labeling. A gating protocol based on double fluorescent labeling of CD45 and PSMA was developed for the quantitiation of LNCaP cells in the presence of white blood cells from bovine blood. Nonfluorescent beads were added to the labeled cell mixture and served as internal standard for precise cellular quantification of LNCaP cells by flow cytometry.

RESULTS

The fluorescent PSMA inhibitor FAMX-CTT-54 was specific for PSMA+ cells. The minimum time and concentration of FAMX-CTT-54 for effective labeling of PSMA+ cell suspensions at 37°C was 7.5 min and 35 nM, respectively; no labeling was observed on PSMA- cells. Co-incubation or pre-incubation of PSMA+ cells with the unlabeled PSMA inhibitor CTT-54 resulted in a concentration-dependent reduction in fluorescent labeling with FAMX-CTT-54 thereby confirming that the labeling was specific for PSMA. In blood samples in which LNCaP cells were added, an average of five cells were detected in a 115 µl sample of the most dilute sample examined (29 cells/ml); three cells were expected theoretically. The greater loss of labeling of PSMA+ cells with FAMX-CTT-54 when pre-incubated with CTT-54 is consistent with the irreversible mode of binding of CTT-54 to PSMA and subsequent internalization of the PSMA-inhibitor complex.

CONCLUSIONS

The results suggest that fluorescent PSMA inhibitors can be utilized to effectively detect and quantify PSMA+ cells by FC. These results support the use of such compounds in the application of FC to detect, quantify, and characterize circulating prostate tumor cells.

Integrated bioinformatics analysis for cancer target identification

The exponential growth of high-throughput Omics data has provided an unprecedented opportunity for new target identification to fuel the dried-up drug discovery pipeline. However, the bioinformatics analysis of large amount and heterogeneous Omics data has posed a great deal of technical challenges for experimentalists who lack statistical skills. Moreover, due to the complexity of human diseases, it is essential to analyze the Omics data in the context of molecular networks to detect meaningful biological targets and understand disease processes. Here, we describe an integrated bioinformatics analysis strategy and provide a running example to identify suitable targets for our in-house Enzyme-Mediated Cancer Imaging and Therapy (EMCIT) technology. In addition, we go through a few key concepts in the process, including corrected false discovery rate (FDR), Gene Ontology (GO), pathway analysis, and tissue specificity. We also describe popular programs and databases which allow the convenient annotation and network analysis of Omics data. We provide a practical guideline for researchers to quickly follow the protocol described and identify those targets that are pertinent to their work.

A targeted low molecular weight near-infrared fluorescent probe for prostate cancer

Prostate-specific membrane antigen (PSMA) remains an active target for imaging and therapeutic applications for prostate cancer. Although radionuclide-based imaging is generally more sensitive and also has been deeply explored, near-infrared fluorescence imaging agents are simple to prepare and compatible with long-term storage conditions. In the present study, a near-infrared fluorescent imaging probe (Cy5.5-CTT-54.2) has been developed by chemical conjugation of Cy5.5N-hydroxysuccinimide ester (Cy5.5-NHS) with a potent PSMA inhibitor CTT-54.2 (IC(50)=144 nM). The probe displays a highly potency (IC(50)=0.55 nM) against PSMA and has demonstrated successful application for specifically labeling PSMA-positive prostate cancer cells in both two and three-dimensional cell culture conditions. These results suggest that the potent, near-infrared Cy5.5-PSMA inhibitor conjugate may be useful for the detection of prostate tumor cells by optical in vivo imaging.

Cannabinoid receptor 2 and its agonists mediate hematopoiesis and hematopoietic stem and progenitor cell mobilization

Endocannabinoids are arachidonic acid derivatives and part of a novel bioactive lipid signaling system, along with their G-coupled cannabinoid receptors (CB₁ and CB₂) and the enzymes involved in their biosynthesis and degradation. However, their roles in hematopoiesis and hematopoietic stem and progenitor cell (HSPC) functions are not well characterized. Here, we show that bone marrow stromal cells express endocannabinoids (anandamide and 2-arachidonylglycerol), whereas CB₂ receptors are expressed in human and murine HSPCs. On ligand stimulation with CB₂ agonists, CB₂ receptors induced chemotaxis, migration, and enhanced colony formation of bone marrow cells, which were mediated via ERK, PI3-kinase, and Gαi-Rac1 pathways. In vivo, the CB₂ agonist AM1241 induced mobilization of murine HSPCs with short- and long-term repopulating abilities. In addition, granulocyte colony-stimulating factor -induced mobilization of HSPCs was significantly decreased by specific CB₂ antagonists and was impaired in Cnr2(-/-) cannabinoid type 2 receptor knockout mice. Taken together, these results demonstrate that the endocannabinoid system is involved in hematopoiesis and that CB₂/CB₂ agonist axis mediates repopulation of hematopoiesis and mobilization of HSPCs. Thus, CB₂ agonists may be therapeutically applied in clinical conditions, such as bone marrow transplantation.

Near IR heptamethine cyanine dye-mediated cancer imaging

PURPOSE

Near-IR fluorescence imaging has great potential for noninvasive in vivo imaging of tumors. In this study, we show the preferential uptake and retention of two hepatamethine cyanine dyes, IR-783 and MHI-148, in tumor cells and tissues.

EXPERIMENTAL DESIGN

IR-783 and MHI-148 were investigated for their ability to accumulate in human cancer cells, tumor xenografts, and spontaneous mouse tumors in transgenic animals. Time- and concentration-dependent dye uptake and retention in normal and cancer cells and tissues were compared, and subcellular localization of the dyes and mechanisms of the dye uptake and retention in tumor cells were evaluated using organelle-specific tracking dyes and bromosulfophthalein, a competitive inhibitor of organic anion transporting peptides. These dyes were used to detect human cancer metastases in a mouse model and differentiate cancer cells from normal cells in blood.

RESULTS

These near-IR hepatamethine cyanine dyes were retained in cancer cells but not normal cells, in tumor xenografts, and in spontaneous tumors in transgenic mice. They can be used to detect cancer metastasis and cancer cells in blood with a high degree of sensitivity. The dyes were found to concentrate in the mitochondria and lysosomes of cancer cells, probably through organic anion transporting peptides, because the dye uptake and retention in cancer cells can be blocked completely by bromosulfophthalein. These dyes, when injected to mice, did not cause systemic toxicity.

CONCLUSIONS

These two heptamethine cyanine dyes are promising imaging agents for human cancers and can be further exploited to improve cancer detection, prognosis, and treatment.

Design considerations for tumour-targeted nanoparticles

Inorganic/organic hybrid nanoparticles are potentially useful in biomedicine, but to avoid non-specific background fluorescence and long-term toxicity, they need to be cleared from the body within a reasonable timescale. Previously, we have shown that rigid spherical nanoparticles such as quantum dots can be cleared by the kidneys if they have a hydrodynamic diameter of approximately 5.5 nm and a zwitterionic surface charge. Here, we show that quantum dots functionalized with high-affinity small-molecule ligands that target tumours can also be cleared by the kidneys if their hydrodynamic diameter is less than this value, which sets an upper limit of 5-10 ligands per quantum dot for renal clearance. Animal models of prostate cancer and melanoma show receptor-specific imaging and renal clearance within 4 h post-injection. This study suggests a set of design rules for the clinical translation of targeted nanoparticles that can be eliminated through the kidneys.

A low molecular weight PSMA-based fluorescent imaging agent for cancer

We synthesized YC-27 3 to provide a fluorescent imaging agent for the prostate-specific membrane antigen (PSMA), a marker for hormone-independent prostate cancer and tumor neovasculature, with suitable pharmacokinetics for use in vivo. Immediate precursor trifluoroacetate salt of 2-(3-{5-[7-(5-amino-1-carboxy-pentylcarbamoyl)-heptanoylamino]-1-carboxy-pentyl}-ureido)-pentanedioic acid 2 was conjugated with a commercially available near-infrared light-emitting dye (IRDye 800CW) to provide 3 in 72% yield. YC-27 3 demonstrated a PSMA inhibitory activity of 0.37nM and was capable of generating target-to-nontarget ratios of at least 10 in PSMA-expressing PC3-PIP vs. PSMA-negative PC3-flu tumors in vivo. YC-27 3 may be useful for study of PSMA-expressing tissue in preclinical models or for intraoperative guidance.

Prostate-specific membrane antigen targeted imaging and therapy of prostate cancer using a PSMA inhibitor as a homing ligand

Prostate cancer (PCa) is a major cause of mortality and morbidity in Western society today. Current methods for detecting PCa are limited, leaving most early malignancies undiagnosed and sites of metastasis in advanced disease undetected. Major deficiencies also exist in the treatment of PCa, especially metastatic disease. In an effort to improve both detection and therapy of PCa, we have developed a PSMA-targeted ligand that delivers attached imaging and therapeutic agents selectively to PCa cells without targeting normal cells. The PSMA-targeted radioimaging agent (DUPA-(99m)Tc) was found to bind PSMA-positive human PCa cells (LNCaP cell line) with nanomolar affinity (K(D) = 14 nM). Imaging and biodistribution studies revealed that DUPA-(99m)Tc localizes primarily to LNCaP cell tumor xenografts in nu/nu mice (% injected dose/gram = 11.3 at 4 h postinjection; tumor-to-muscle ratio = 75:1). Two PSMA-targeted optical imaging agents (DUPA-FITC and DUPA-rhodamine B) were also shown to efficiently label PCa cells and to internalize and traffic to intracellular endosomes. A PSMA-targeted chemotherapeutic agent (DUPA-TubH) was demonstrated to kill PSMA-positive LNCaP cells in culture (IC(50) = 3 nM) and to eliminate established tumor xenografts in nu/nu mice with no detectable weight loss. Blockade of tumor targeting upon administration of excess PSMA inhibitor (PMPA) and the absence of targeting to PSMA-negative tumors confirmed the specificity of each of the above targeted reagents for PSMA. Tandem use of the imaging and therapeutic agents targeted to the same receptor could allow detection, staging, monitoring, and treatment of PCa with improved accuracy and efficacy.

The FLARE intraoperative near-infrared fluorescence imaging system: a first-in-human clinical trial in breast cancer sentinel lymph node mapping

BACKGROUND

Invisible NIR fluorescent light can provide high sensitivity, high-resolution, and real-time image-guidance during oncologic surgery, but imaging systems that are presently available do not display this invisible light in the context of surgical anatomy. The FLARE imaging system overcomes this major obstacle.

METHODS

Color video was acquired simultaneously, and in real-time, along with two independent channels of NIR fluorescence. Grayscale NIR fluorescence images were converted to visible "pseudo-colors" and overlaid onto the color video image. Yorkshire pigs weighing 35 kg (n = 5) were used for final preclinical validation of the imaging system. A six-patient pilot study was conducted in women undergoing sentinel lymph node (SLN) mapping for breast cancer. Subjects received (99m)Tc-sulfur colloid lymphoscintigraphy. In addition, 12.5 microg of indocyanine green (ICG) diluted in human serum albumin (HSA) was used as an NIR fluorescent lymphatic tracer.

RESULTS

The FLARE system permitted facile positioning in the operating room. NIR light did not change the look of the surgical field. Simultaneous pan-lymphatic and SLN mapping was demonstrated in swine using clinically available NIR fluorophores and the dual NIR capabilities of the system. In the pilot clinical trial, a total of nine SLNs were identified by (99m)Tc- lymphoscintigraphy and nine SLNs were identified by NIR fluorescence, although results differed in two patients. No adverse events were encountered.

CONCLUSIONS

We describe the successful clinical translation of a new NIR fluorescence imaging system for image-guided oncologic surgery.

In vitro targeted photodynamic therapy with a pyropheophorbide--a conjugated inhibitor of prostate-specific membrane antigen

BACKGROUND

The lack of specific delivery of photosensitizers (PSs), represents a significant limitation of photodynamic therapy (PDT) of cancer. The biomarker prostate-specific membrane antigen (PSMA) has attracted considerable attention as a target for imaging and therapeutic applications for prostate cancer. Although recent efforts have been made to conjugate inhibitors of PSMA with imaging agents, there have been no reports on PS-conjugated PSMA inhibitors for targeted PDT of prostate cancer. The present study focuses on the use of a PSMA inhibitor-conjugate of pyropheophorbide-a (Ppa-conjugate 2) for targeted PDT to achieve apoptosis in PSMA+ LNCaP cells.

METHODS

Confocal laser scanning microscopy with a combination of nuclear staining and immunofluorescence methods were employed to monitor the specific imaging and PDT-mediated apoptotic effects on PSMA-positive LNCaP and PSMA-negative (PC-3) cells.

RESULTS

Our results demonstrated that PDT-mediated effects by Ppa-conjugate 2 were specific to LNCaP cells, but not PC-3 cells. Cell permeability was detected as early as 2 hr by HOE33342/PI double staining, becoming more intense by 4 hr. Evidence for the apoptotic caspase cascade being activated was based on the appearance of poly-ADP-ribose polymerase (PARP) p85 fragment. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay detected DNA fragmentation 16 hr post-PDT, confirming apoptotic events.

CONCLUSIONS

Cell permeability by HOE33342/PI double staining as well as PARP p85 fragment and TUNEL assays confirm cellular apoptosis in PSMA+ cells when treated with PS-inhibitor conjugate 2 and subsequently irradiated. It is expected that the PSMA targeting small-molecule of this conjugate can serve as a delivery vehicle for PDT and other therapeutic applications for prostate cancer.

Molecular imaging of prostate cancer

Prostate carcinoma is the most common non-cutaneous malignancy in males. Imaging of prostatic lesions is of great importance and aids in oncologic management and monitoring of therapy response. Particularly molecular imaging based on positron emission tomography (PET) and single photon emission computerized tomography (SPECT) has great potential. Using radio-labelled molecular probes, these approaches are highly sensitive and can provide key molecular and functional information on tumours. The identification of suitable targets based on unique genetic and biochemical features of cancer lesions is one of the core activities driving progress in molecular imaging of pathological processes. Nowadays, mainly metabolic probes are being used routinely for detection and staging of prostate cancer. The development of new specific receptor ligands and targeted probes and antibodies holds great promise to further enhance the performance of molecular imaging and to further improve the diagnosis and monitoring of prostate cancer.

Multivalent scaffolds for affinity maturation of small molecule cell surface binders and their application to prostate tumor targeting

Adamantane scaffolds for affinity maturation of prostate cancer specific ligands of low molecular mass are described. These scaffolds are modular and can be used for conjugation of up to three ligands and an additional effector molecule by standard peptide coupling techniques. The potential of the scaffolds is demonstrated with the multimerization of GPI 1, a prostate cancer specific small molecule. A detailed study of multimerized GPI conjugates with near-infrared fluorophores and their binding properties to different prostate cancer cell lines shows the specific binding of these conjugates to cell types positive for prostate specific membrane antigen (PSMA). We demonstrate that these conjugates allow the sensitive imaging of prostate cancer cells with NIR methodology and suggest that our adamantane scaffolds might be generally useful for affinity maturation of small molecules targeting cell surface epitopes.