Synthesis and Evaluation of a Monomethyl Auristatin E─Integrin αvß6 Binding Peptide-Drug Conjugate for Tumor Targeted Drug Delivery.

Many anticancer drugs exhibit high systemic off-target toxicities causing severe side effects. Peptide-drug conjugates (PDCs) that target tumor-specific receptors such as integrin αvß6 are emerging as powerful tools to overcome these challenges. The development of an integrin αvß6-selective PDC was achieved by combining the therapeutic efficacy of the cytotoxic drug monomethyl auristatin E with the selectivity of the αvß6-binding peptide (αvß6-BP) and with the ability of positron emission tomography (PET) imaging by copper-64. The [64Cu]PDC-1 was produced efficiently and in high purity. The PDC exhibited high human serum stability, integrin αvß6-selective internalization, cell binding, and cytotoxicity. Integrin αvß6-selective tumor accumulation of the [64Cu]PDC-1 was visualized with PET-imaging and corroborated by biodistribution, and [64Cu]PDC-1 showed promising in vivo pharmacokinetics. The [natCu]PDC-1 treatment resulted in prolonged survival of mice bearing αvß6 (+) tumors (median survival: 77 days, vs αvß6 (-) tumor group 49 days, and all other control groups 37 days).

Preclinical Evaluation of 68Ga- and 177Lu-Labeled Integrin αvß6-Targeting Radiotheranostic Peptides.

The integrin αvß6, an epithelium-specific cell surface receptor, is overexpressed on numerous malignancies, including the highly lethal pancreatic ductal adenocarcinomas. Here, we developed and tested a novel αvß6-targeting peptide, DOTA-5G (1) radiolabeled with 68Ga, for PET/CT imaging and 177Lu for treatment. With the goal to develop a radiotheranostic, further modifications were made for increased circulation time, renal recycling, and tumor uptake, yielding DOTA-albumin-binding moiety-5G (2). Methods: Peptides 1 and 2 were synthesized on solid phase, and their affinity for αvß6 was assessed by enzyme-linked immunosorbent assay. The peptides were radiolabeled with 68Ga and 177Lu. In vitro cell binding, internalization, and efflux of 68Ga-1 and 177Lu-2 were evaluated in αvß6-positive BxPC-3 human pancreatic cancer cells. PET/CT imaging of 68Ga-1 and 68Ga-2 was performed on female nu/nu mice bearing subcutaneous BxPC-3 tumors. Biodistribution was performed for 68Ga-1 (1 and 2 h after injection), 68Ga-2 (2 and 4 h after injection), and 177Lu-1 and 177Lu-2 (1, 24, 48, and 72 h after injection). The 177Lu-2 biodistribution data were extrapolated for human dosimetry data estimates using OLINDA/EXM 1.1. Therapeutic efficacy of 177Lu-2 was evaluated in mice bearing BxPC-3 tumors. Results: Peptides 1 and 2 demonstrated high affinity (<55 nM) for αvß6 by enzyme-linked immunosorbent assay. 68Ga-1, 68Ga-2, 177Lu-1, and 177Lu-2 were synthesized in high radiochemical purity. Rapid in vitro binding and internalization of 68Ga-1 and 177Lu-2 were observed in BxPC-3 cells. PET/CT imaging and biodistribution studies demonstrated uptake in BxPC-3 tumors. Introduction of the albumin-binding moiety in 177Lu-2 resulted in a 5-fold increase in tumor uptake and retention over time. Based on the extended dosimetry data, the dose-limiting organ for 177Lu-2 is the kidney. Treatment with 177Lu-2 prolonged median survival by 1.5- to 2-fold versus controls. Conclusion: 68Ga-1 and 177Lu-2 demonstrated high affinity for the integrin αvß6 both in vitro and in vivo, were rapidly internalized into BxPC-3 cells, and were stable in mouse and human serum. Both radiotracers showed favorable pharmacokinetics in preclinical studies, with predominantly renal excretion and good tumor-to-normal-tissue ratios. Favorable human dosimetry data suggest the potential of 177Lu-2 as a treatment for pancreatic ductal adenocarcinoma.

Repurposing an atherosclerosis targeting peptide for tumor imaging.

Cancer and atherosclerosis are chronic diseases that share common characteristics at both early and advanced stages and can arise from multiple factors. Both diseases are characterized by uncontrolled cell proliferation, inflammation, angiogenesis and apoptosis. Herein we investigated the ability of a peptide (CTHRSSVVC), that was previously reported to bind atherosclerotic lesions to home in the tumor microenvironment. The CTHRSSVVC peptide was synthesized on solid phase and N-terminally labeled with a sulfo-Cy5 dye. The specific binding to macrophage was evaluated in vitro with flow cytometry and immunofluorescence and in vivo for tumor targeting in BALB/c mice bearing a 4T1 tumor using optical imaging. The sulfo-Cy5-CTHRSSVVC peptide was synthesized in greater than 99% purity. No selective binding of the sulfo-Cy5-CTHRSSVVC peptide to macrophages in vitro was observed, however in vivo the sulfo-Cy5-CTHRSSVVC peptide accumulated in the 4T1 tumor, with a tumor-to-normal tissue ratio of 7.21 ± 1.44 at 2 h post injection. Ex vivo analysis of tumor tissue by confocal microscopy suggested that the sulfo-Cy5-CTHRSSVVC peptide had accumulated in the stroma of the tumor specifically, in regions of spindle shaped cells. In conclusion, although the target for the sulfo-Cy5-CTHRSSVVC peptide remains to be identified, the Cy5-CTHRSSVVC peptide warrants further investigation as a tumor imaging agent.

Evaluation of Copper-64-Labeled αvß6-Targeting Peptides: Addition of an Albumin Binding Moiety to Improve Pharmacokinetics.

The incorporation of non-covalent albumin binding moieties (ABMs) into radiotracers results in increased circulation time, leading to a higher uptake in the target tissues such as the tumor, and, in some cases, reduced kidney retention. We previously developed [18F]AlF NOTA-K(ABM)-αvß6-BP, where αvß6-BP is a peptide with high affinity for the cell surface receptor integrin αvß6 that is overexpressed in several cancers, and the ABM is an iodophenyl-based moiety. [18F]AlF NOTA-K(ABM)-αvß6-BP demonstrated prolonged blood circulation compared to the non-ABM parent peptide, resulting in high, αvß6-targeted uptake with continuously improving detection of αvß6(+) tumors using PET/CT. To further extend the imaging window beyond that of fluorine-18 (t1/2 = 110 min) and to investigate the pharmacokinetics at later time points, we radiolabeled the αvß6-BP with copper-64 (t1/2 = 12.7 h). Two peptides were synthesized without (1) and with (2) the ABM and radiolabeled with copper-64 to yield [64Cu]1 and [64Cu]2, respectively. The affinity of [natCu]1 and [natCu]2 for the integrin αvß6 was assessed by enzyme-linked immunosorbent assay. [64Cu]1 and [64Cu]2 were evaluated in vitro (cell binding and internalization) using DX3puroß6 (αvß6(+)), DX3puro (αvß6(-)), and pancreatic BxPC-3 (αvß6(+)) cells, in an albumin binding assay, and for stability in both mouse and human serum. In vivo (PET/CT imaging) and biodistribution studies were done in mouse models bearing either the paired DX3puroß6/DX3puro or BxPC-3 xenograft tumors. [64Cu]1 and [64Cu]2 were synthesized in ≥97% radiochemical purity. In vitro, [natCu]1 and [natCu]2 maintained low nanomolar affinity for integrin αvß6 (IC50 = 28 ± 3 and 19 ± 5 nM, respectively); [64Cu]1 and [64Cu]2 showed comparable binding to αvß6(+) cells (DX3puroß6: ≥70%, ≥42% internalized; BxPC-3: ≥19%, ≥12% internalized) and ≤3% to the αvß6(-) DX3puro cells. Both radiotracers were ≥98% stable in human serum at 24 h, and [64Cu]2 showed a 6-fold higher binding to human serum protein than [64Cu]1. In vivo, selective uptake in the αvß6(+) tumors was observed with tumor visualization up to 72 h for [64Cu]2. A 3-5-fold higher αvß6(+) tumor uptake of [64Cu]2 vs [64Cu]1 was observed throughout, at least 2.7-fold improved BxPC-3-to-kidney and BxPC-3-to-blood ratios, and 2-fold improved BxPC-3-to-stomach ratios were noted for [64Cu]2 at 48 h. Incorporation of an iodophenyl-based ABM into the αvß6-BP ([64Cu]2) prolonged circulation time and resulted in improved pharmacokinetics, including increased uptake in αvß6(+) tumors that enabled visualization of αvß6(+) tumors up to 72 h by PET/CT imaging.

The Effects of an Albumin Binding Moiety on the Targeting and Pharmacokinetics of an Integrin αvß6-Selective Peptide Labeled with Aluminum [18F]Fluoride.

PURPOSE: The αvß6-BP peptide selectively targets the integrin αvß6, a cell surface receptor recognized as a prognostic indicator for several challenging malignancies. Given that the 4-[18F]fluorobenzoyl (FBA)-labeled peptide is a promising PET imaging agent, radiolabeling via aluminum [18F]fluoride chelation and introduction of an albumin binding moiety (ABM) have the potential to considerably simplify radiochemistry and improve the pharmacokinetics by increasing biological half-life. PROCEDURES: The peptides NOTA-αvß6-BP (1) and NOTA-K(ABM)-αvß6-BP (2) were synthesized on solid phase, radiolabeled with aluminum [18F]fluoride, and evaluated in vitro (integrin ELISA, albumin binding, cell studies) and in vivo in mouse models bearing paired DX3puroß6 [αvß6(+)]/DX3puro [αvß6(-)], and for [18F]AlF 2, BxPC-3 [αvß6(+)] cell xenografts (PET imaging, biodistribution). RESULTS: The peptides were radiolabeled in 23.0 ± 5.7 % and 22.1 ± 4.4 % decay-corrected radiochemical yield, respectively, for [18F]AlF 1 and [18F]AlF 2. Both demonstrated excellent affinity and selectivity for integrin αvß6 by ELISA (IC50(αvß6) = 3-7 nM vs IC50(αvß3) > 10 µM) and in cell binding studies (51.0 ± 0.7 % and 47.2 ± 0.7 % of total radioactivity bound to DX3puroß6 cells at 1 h, respectively, vs. ≤ 1.2 % to DX3puro for both compounds). The radiotracer [18F]AlF 1 bound to human serum at 16.3 ± 1.9 %, compared to 67.5 ± 1.0 % for the ABM-containing [18F]AlF 2. In vivo studies confirmed the effect of the ABM on blood circulation (≤ 0.1 % ID/g remaining in blood for [18F]AlF 1 as soon as 1 h p.i. vs. > 2 % ID/g for [18F]AlF 2 at 6 h p.i.) and higher αvß6(+) tumor uptake (4 h: DX3puroß6; [18F]AlF 1: 3.0 ± 0.7 % ID/g, [18F]AlF 2: 7.2 ± 0.7 % ID/g; BxPC-3; [18F]AlF 2: 10.2 ± 0.1 % ID/g). CONCLUSION: Both compounds were prepared using standard chemistries; affinity and selectivity for integrin αvß6 in vitro remained unaffected by the albumin binding moiety. In vivo, the albumin binding moiety resulted in prolonged circulation and higher αvß6-targeted uptake.

Correction to: Evaluation of Two Optical Probes for Imaging the Integrin αvß6- In Vitro and In Vivo in Tumor-Bearing Mice.

This article was updated to correct the axes in Figures 4e and 5d.

Evaluation of Two Optical Probes for Imaging the Integrin αvß6- In Vitro and In Vivo in Tumor-Bearing Mice.

PURPOSE: The purpose of this study was to develop and evaluate two αvß6-targeted fluorescent imaging agents. The integrin subtype αvß6 is significantly upregulated in a wide range of epithelial derived cancers, plays a key role in invasion and metastasis, and expression is often located at the invasive edge of tumors. αvß6-targeted fluorescent imaging agents have the potential to guide surgical resection leading to improved patient outcomes. Both imaging agents were based on the bi-PEGylated peptide NH2-PEG28-A20FMDV2-K16R-PEG28 (1), a peptide that has high affinity and selectivity for the integrin αvß6: (a) 5-FAM-X-PEG28-A20FMDV2-K16R-PEG28 (2), and (b) IRDye800-PEG28-A20FMDV2-K16R-PEG28 (3). PROCEDURES: Peptides were synthesized using solid-phase peptide synthesis and standard Fmoc chemistry. Affinity for αvß6 was evaluated by ELISA. In vitro binding, internalization, and localization of 2 was monitored using confocal microscopy in DX3puroß6 (αvß6+) and DX3puro (αvß6-) cells. The in vivo imaging and ex vivo biodistribution of 3 was evaluated in three preclinical mouse models, DX3puroß6/DX3puro and BxPC-3 (αvß6+) tumor xenografts and a BxPC-3 orthotopic pancreatic tumor model. RESULTS: Peptides were obtained in > 99% purity. IC50 values were 28 nM (2) and 39 nM (3). Rapid αvß6-selective binding and internalization of 2 was observed. Fluorescent intensity (FLI) measurements extracted from the in vivo images and ex vivo biodistribution confirmed uptake and retention of 3 in the αvß6 positive subcutaneous and orthotopic tumors, with negligible uptake in the αvß6-negative tumor. Blocking studies with a known αvß6-targeting peptide demonstrated αvß6-specific binding of 3. CONCLUSION: Two fluorescence imaging agents were developed. The αvß6-specific uptake, internalization, and endosomal localization of the fluorescence agent 2 demonstrates potential for targeted therapy. The selective uptake and retention of 3 in the αvß6-positive tumors enabled clear delineation of the tumors and surgical resection indicating 3 has the potential to be utilized during image-guided surgery.

Identification, Characterization, and Optimization of Integrin αvß6-Targeting Peptides from a One-Bead One-Compound (OBOC) Library: Towards the Development of Positron Emission Tomography (PET) Imaging Agents.

The current translation of peptides identified through the one-bead one-compound (OBOC) technology into positron emission tomography (PET) imaging agents is a slow process, with a major delay between ligand identification and subsequent lead optimization. This work aims to streamline the development process of 18F-peptide based PET imaging agents to target the integrin αvß6. By directly identify αvß6⁻targeting peptides from a 9-mer 4-fluorobenzoyl peptide library using the on-bead two-color (OBTC) cell-screening assay, a total of 185 peptide beads were identified and 5 beads sequenced for further evaluation. The lead peptide 1 (VGDLTYLKK(FB), IC50 = 0.45 ± 0.06 µM, 25% stable in serum at 1 h) was further modified at the N-, C-, and bi-termini. C-terminal PEGylation increased the metabolic stability (>95% stable), but decreased binding affinity (IC50 = 3.7 ± 1 µM) was noted. C-terminal extension (1i, VGDLTYLKK(FB)KVART) significantly increased binding affinity for integrin αvß6 (IC50 = 0.021 ± 0.002 µM), binding selectivity for αvß6-expressing cells (3.1 ± 0.8:1), and the serum stability (>99% stable). Our results demonstrate the challenges in optimizing OBOC-derived peptides, indicate both termini of 1 are sensitive to modifications, and show that further modification of 1 is necessary to demonstrate utility as an 18F-peptide imaging agent.

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.

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.

Caged [(18)F]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.

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.

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.

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.

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.

The hydrazide/hydrazone click reaction as a biomolecule labeling strategy for M(CO)3 (M = Re, (99m)Tc) radiopharmaceuticals.

Facile reactivity of hydrazides and aldehydes was explored as potential coupling partners for incorporation into M(CO)(3) (M = Re, (99m)Tc) based radiopharmaceuticals. Both 'click, then chelate' and 'prelabel, then click' synthetic routes produced identical products in high yields and lacked metal-hydrazide/-hydrazone interactions, highlighting the potential of this click strategy.