Pharmacodynamic measures within tumors expose differential activity of PD(L)-1 antibody therapeutics.

Macromolecules such as monoclonal antibodies (mAbs) are likely to experience poor tumor penetration because of their large size, and thus low drug exposure of target cells within a tumor could contribute to suboptimal responses. Given the challenge of inadequate quantitative tools to assess mAb activity within tumors, we hypothesized that measurement of accessible target levels in tumors could elucidate the pharmacologic activity of a mAb and could be used to compare the activity of different mAbs. Using positron emission tomography (PET), we measured the pharmacodynamics of immune checkpoint protein programmed-death ligand 1 (PD-L1) to evaluate pharmacologic effects of mAbs targeting PD-L1 and its receptor programmed cell death protein 1 (PD-1). For PD-L1 quantification, we first developed a small peptide-based fluorine-18-labeled PET imaging agent, [18F]DK222, which provided high-contrast images in preclinical models. We then quantified accessible PD-L1 levels in the tumor bed during treatment with anti-PD-1 and anti-PD-L1 mAbs. Applying mixed-effects models to these data, we found subtle differences in the pharmacodynamic effects of two anti-PD-1 mAbs (nivolumab and pembrolizumab). In contrast, we observed starkly divergent target engagement with anti-PD-L1 mAbs (atezolizumab, avelumab, and durvalumab) that were administered at equivalent doses, correlating with differential effects on tumor growth. Thus, we show that measuring PD-L1 pharmacodynamics informs mechanistic understanding of therapeutic mAbs targeting PD-L1 and PD-1. These findings demonstrate the value of quantifying target pharmacodynamics to elucidate the pharmacologic activity of mAbs, independent of mAb biophysical properties and inclusive of all physiological variables, which are highly heterogeneous within and across tumors and patients.

Twist activates miR-22 to suppress estrogen receptor alpha in breast cancer.

TWIST1 (Twist) is a basic helix-loop-helix transcription factor that is overexpressed in many cancers and promotes tumor cell invasion, metastasis, and recurrence. In this study, we demonstrate that Twist upregulates expression of microRNA 22 (miR-22) which, in turn, downregulates estrogen receptor alpha (ER) expression in breast cancer. Initial analysis of miR-22 and Twist expression in a panel of breast cancer cell lines showed a direct correlation between Twist and miR-22 levels with miR-22 being highly expressed in ER negative cell lines. Overexpressing Twist caused increased miR-22 levels while downregulating it led to decreased miR-22 expression. To characterize the upstream promoter region of miR-22, we utilized rapid amplification of cDNA ends and identified the transcription start site and the putative promoter region of miR-22. Mechanistically, we determined that Twist, in combination with HDAC1 and DNMT3B, transcriptionally upregulates miR-22 expression by binding to E-boxes in the proximal miR-22 promoter. We also established that miR-22 causes an increase in growth in 3D but not 2D cultures. Importantly, we observed a direct correlation between increased breast cancer grade and Twist and miR-22 expression. We also identified two potential miR-22 binding sites in the 3'-UTR region of ER and confirmed by promoter assays that miR-22 regulates ER expression by binding to both target sites. These results reveal a novel pathway of ER suppression by Twist through miR-22 activation that could potentially promote the ER negative phenotype in breast cancers.

Process validation, current good manufacturing practice production, dosimetry, and toxicity studies of the carbonic anhydrase IX imaging agent [111 In]In-XYIMSR-01 for phase I regulatory approval.

[111 In]In-XYIMSR-01 is a promising single-photon emission computed tomography (SPECT) imaging agent for identification of tumors that overexpress carbonic anhydrase IX. To translate [111 In]In-XYIMSR-01 to phase I trials, we performed animal toxicity and dosimetry studies, determined the maximum dose for human use, and completed the chemistry, manufacturing, and controls component of a standard regulatory application. The production process, quality control testing, stability studies, and specifications for sterile drug product release were based on United States Pharmacopeia chapters <823> and <825>, FDA 21 CFR Part 212. Toxicity was evaluated by using nonradioactive [113/115 In]In-XYIMSR-01 according to 21 CFR Part 58 guidelines. Organ Level INternal Dose Assessment/EXponential Modeling (OLINDA/EXM) was used to calculate the maximum single dose for human studies. Three process validation runs at starting radioactivities of ~800 MBq were completed with a minimum concentration of 407 MBq/ml and radiochemical purity of ≥99% at the end of synthesis. A single intravenous dose of 55 µg/ml of [113/115 In]In-XYIMSR-01 was well tolerated in male and female Sprague-Dawley rats. The calculated maximum single dose for human injection from dosimetry studies was 390.35 MBq of [111 In]In-XYIMSR-01. We have completed toxicity and dosimetry studies as well as validated a manufacturing process to test [111 In]In-XYIMSR-01 in a phase I clinical trial.

Engineered Fragments of the PSMA-Specific 5D3 Antibody and Their Functional Characterization.

Prostate-Specific Membrane Antigen (PSMA) is an established biomarker for the imaging and experimental therapy of prostate cancer (PCa), as it is strongly upregulated in high-grade primary, androgen-independent, and metastatic lesions. Here, we report on the development and functional characterization of recombinant single-chain Fv (scFv) and Fab fragments derived from the 5D3 PSMA-specific monoclonal antibody (mAb). These fragments were engineered, heterologously expressed in insect S2 cells, and purified to homogeneity with yields up to 20 mg/L. In vitro assays including ELISA, immunofluorescence and flow cytometry, revealed that the fragments retain the nanomolar affinity and single target specificity of the parent 5D3 antibody. Importantly, using a murine xenograft model of PCa, we verified the suitability of fluorescently labeled fragments for in vivo imaging of PSMA-positive tumors and compared their pharmacokinetics and tissue distribution to the parent mAb. Collectively, our data provide an experimental basis for the further development of 5D3 recombinant fragments for future clinical use.

Development of [18F]FPy-WL12 as a PD-L1 Specific PET Imaging Peptide.

Expression of programmed cell death ligand 1 (PD-L1) within tumors is an important biomarker for guiding immune checkpoint therapies; however, immunohistochemistry-based methods of detection fail to provide a comprehensive picture of PD-L1 levels in an entire patient. To facilitate quantification of PD-L1 in the whole body, we developed a peptide-based, high-affinity PD-L1 imaging agent labeled with [18F]fluoride for positron emission tomography (PET) imaging. The parent peptide, WL12, and the nonradioactive analog of the radiotracer, 19FPy-WL12, inhibit PD-1/PD-L1 interaction at low nanomolar concentrations (half maximal inhibitory concentration [IC50], 26-32 nM). The radiotracer, [18F]FPy-WL12, was prepared by conjugating 2,3,5,6-tetrafluorophenyl 6-[18F]fluoronicotinate ([18F]FPy-TFP) to WL12 and assessed for specificity in vitro in 6 cancer cell lines with varying PD-L1 expression. The uptake of the radiotracer reflected the PD-L1 expression assessed by flow cytometry. Next, we performed the in vivo evaluation of [18F]FPy-WL12 in mice bearing cancer xenografts by PET imaging, ex vivo biodistribution, and blocking studies. In vivo data demonstrated a PD-L1-specific uptake of [18F]FPy-WL12 in tumors that is reduced in mice receiving a blocking dose. The majority of [18F]FPy-WL12 radioactivity was localized in the tumors, liver, and kidneys indicating the need for optimization of the labeling strategy to improve the in vivo pharmacokinetics of the radiotracer.

177Lu-labeled low-molecular-weight agents for PSMA-targeted radiopharmaceutical therapy.

PURPOSE: To develop a prostate-specific membrane antigen (PSMA)-targeted radiotherapeutic for metastatic castration-resistant prostate cancer (mCRPC) with optimized efficacy and minimized toxicity employing the ß-particle radiation of 177Lu. METHODS: We synthesized 14 new PSMA-targeted, 177Lu-labeled radioligands (177Lu-L1-177Lu-L14) using different chelating agents and linkers. We evaluated them in vitro using human prostate cancer PSMA(+) PC3 PIP and PSMA(-) PC3 flu cells and in corresponding flank tumor models. Efficacy and toxicity after 8 weeks were evaluated at a single administration of 111 MBq for 177Lu-L1, 177Lu-L3, 177Lu-L5 and 177Lu-PSMA-617. Efficacy of 177Lu-L1 was further investigated using different doses, and long-term toxicity was determined in healthy immunocompetent mice. RESULTS: Radioligands were produced in high radiochemical yield and purity. Cell uptake and internalization indicated specific uptake only in PSMA(+) PC3 cells. 177Lu-L1, 177Lu-L3 and 177Lu-L5 demonstrated comparable uptake to 177Lu-PSMA-617 and 177Lu-PSMA-I&T in PSMA-expressing tumors up to 72 h post-injection. 177Lu-L1, 177Lu-L3 and 177Lu-L5 also demonstrated efficient tumor regression at 8 weeks. 177Lu-L1 enabled the highest survival rate. Necropsy studies of the treated group at 8 weeks revealed subacute damage to lacrimal glands and testes. No radiation nephropathy was observed 1 year post-treatment in healthy mice receiving 111 MBq of 177Lu-L1, most likely related to the fast renal clearance of this agent. CONCLUSIONS: 177Lu-L1 is a viable clinical candidate for radionuclide therapy of PSMA-expressing malignancies because of its high tumor-targeting ability and low off-target radiotoxic effects.

PET imaging of distinct brain uptake of a nanobody and similarly-sized PAMAM dendrimers after intra-arterial administration.

INTRODUCTION: We have recently shown that intracerebral delivery of an anti-VEGF monoclonal antibody bevacizumab using an intra-arterial (IA) infusion is more effective than intravenous administration. While antibodies are quickly emerging as therapeutics, their disadvantages such as large size, production logistics and immunogenicity motivate search for alternatives. Thus we have studied brain uptake of nanobodies and polyamidoamine (PAMAM) dendrimers. METHODS: Nanobodies were conjugated with deferoxamine (DFO) to generate NB(DFO)2. Generation-4 PAMAM dendrimers were conjugated with DFO, and subsequently primary amines were capped with butane-1,2-diol functionalities to generate G4(DFO)3(Bdiol)110. Resulting conjugates were radiolabeled with zirconium-89. Brain uptake of 89ZrNB(DFO)2 and 89ZrG4(DFO)3(Bdiol)110 upon carotid artery vs tail vein infusions with intact BBB or osmotic blood-brain barrier opening (OBBBO) with mannitol in mice was monitored by dynamic positron emission tomography (PET) over 30 min to assess brain uptake and clearance, followed by whole-body PET-CT (computed tomography) imaging at 1 h and 24 h post-infusion (pi). Imaging results were subsequently validated by ex-vivo biodistribution. RESULTS: Intravenous administration of 89ZrNB(DFO)2 and 89ZrG4(DFO)3(Bdiol)110 resulted in their negligible brain accumulation regardless of BBB status and timing of OBBBO. Intra-arterial (IA) administration of 89ZrNB(DFO)2 dramatically increased its brain uptake, which was further potentiated with prior OBBBO. Half of the initial brain uptake was retained after 24 h. In contrast, IA infusion of 89ZrG4(DFO)3(Bdiol)110 resulted in poor initial accumulation in the brain, with complete clearance within 1 h of administration. Ex-vivo biodistribution results reflected those on PET-CT. CONCLUSIONS: IA delivery of nanobodies might be an attractive therapeutic platform for CNS disorders where prolonged intracranial retention is necessary.

MRI Assessment of Prostate-Specific Membrane Antigen (PSMA) Targeting by a PSMA-Targeted Magnetic Nanoparticle: Potential for Image-Guided Therapy.

Magnetic nanoparticle (MNP)-induced hyperthermia is currently being evaluated for localized prostate cancer. We evaluated the feasibility of tumor-selective delivery of prostate-specific membrane antigen (PSMA)-targeted MNPs in a murine model with high-resolution magnetic resonance imaging (MRI) after intravenous administration of MNPs at a concentration necessary for hyperthermia. A PSMA-targeted MNP was synthesized and evaluated using T2-weighted MRI, after intravenous administration of 50 mg/kg of the MNP. Significant contrast enhancement ( P < 0.0002, n = 5) was observed in PSMA(+) tumors compared to PSMA(-) tumors 24 h and 48 h after contrast agent administration. Mice were also imaged with near-infrared fluorescence imaging, to validate the MRI results. Two-photon microscopy revealed higher vascular density at the tumor periphery, which resulted in higher  peripheral accumulation of PSMA-targeted MNPs. These results suggest that the delivery of PSMA-targeted MNPs to PSMA(+) tumors is both actively targeted and passively mediated.

Evaluation of PSMA-Targeted PAMAM Dendrimer Nanoparticles in a Murine Model of Prostate Cancer.

The prostate-specific membrane antigen (PSMA) is a validated target for detection and management of prostate cancer (PC). It has also been utilized for targeted drug delivery through antibody-drug conjugates and polymeric micelles. Polyamidoamine (PAMAM) dendrimers are emerging as a versatile platform in a number of biomedical applications due to their unique physicochemical properties, including small size, large number of reactive terminal groups, bulky interior void volume, and biocompatibility. Here, we report the synthesis of generation 4 PSMA-targeted PAMAM dendrimers [G4(MP-KEU)] and evaluation of their targeting properties in vitro and in vivo using an experimental model of PC. A facile, one-pot synthesis gave nearly neutral nanoparticles with a narrow size distribution of 5 nm in diameter and a molecular weight of 27.3 kDa. They exhibited in vitro target specificity with a dissociation constant ( Kd) of 0.32 ± 0.23 µm and preferential accumulation in PSMA+ PC3 PIP tumors versus isogenic PSMA- PC3 flu tumors. Positron emission tomography-computed tomography imaging and ex vivo biodistribution studies of dendrimers radiolabeled with 64Cu, [64Cu]G4(MP-KEU), demonstrated high accumulation in PSMA+ PC3 PIP tumors at 24 h post-injection (45.83 ± 20.09% injected dose per gram of tissue, %ID/g), demonstrating a PSMA+ PC3 PIP/PSMA- PC3 flu ratio of 7.65 ± 3.35. Specific accumulation of G4(MP-KEU) and [64Cu]G4(MP-KEU) in PSMA+ PC3 PIP tumors was inhibited by the known small-molecule PSMA inhibitor, ZJ-43. On the contrary, G4(Ctrl), control dendrimers without PSMA-targeting moieties, showed comparable low accumulation of ∼1%ID/g in tumors irrespective of PSMA expression, further confirming PSMA+ tumor-specific uptake of G4(MP-KEU). These results suggest that G4(MP-KEU) may represent a suitable scaffold by which to target PSMA-expressing tissues with imaging and therapeutic agents.

Salicylic Acid-Based Polymeric Contrast Agents for Molecular Magnetic Resonance Imaging of Prostate Cancer.

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) is an innovative molecular imaging technique in which contrast agents are labeled by saturating their exchangeable proton spins by radio-frequency irradiation. Salicylic acid and its analogues are a promising class of highly sensitive, diamagnetic CEST agents. Herein, polymeric agents grafted with salicylic acid moieties and a known high-affinity ligand targeting prostate-specific membrane antigen in approximately 10:1 molar ratio were synthesized to provide sufficient MRI sensitivity and receptor specificity. The proton-exchange properties of the contrast agent in solution and in an experimental murine model are reported to demonstrate the feasibility of receptor-targeted CEST MRI of prostate cancer. Furthermore, the CEST imaging data were validated with an 111 In-labeled analogue of the agent by in vivo single photon emission computed tomographic imaging and tissue biodistribution studies.

Peptide-Based 68Ga-PET Radiotracer for Imaging PD-L1 Expression in Cancer.

Tumors create and maintain an immunosuppressive microenvironment that promotes cancer cell escape from immune surveillance. The immune checkpoint protein programmed death-ligand 1 (PD-L1) is expressed in many cancers and is an important contributor to the maintenance of the immunosuppressive tumor microenvironment. PD-L1 is a prominent target for cancer immunotherapy. Guidance of anti-PD-L1 therapy is currently effected through measurement of PD-L1 through biopsy and immunohistochemistry. Here, we report a peptide-based imaging agent, [68Ga]WL12, to detect PD-L1 expression in tumors noninvasively by positron emission tomography (PET). WL12, a cyclic peptide comprising 14 amino acids, binds to PD-L1 with high affinity (IC50≈ 23 nM). Synthesis of [68Ga]WL12 provided radiochemical purity >99% after purification. Biodistribution in immunocompetent mice demonstrated 11.56 ± 3.18, 4.97 ± 0.8, 1.9 ± 0.1, and 1.33 ± 0.21 percentage of injected dose per gram (%ID/g) in hPD-L1, MDAMB231, SUM149, and CHO tumors, respectively, at 1 h postinjection, with high binding specificity noted with coinjection of excess, nonradiolabeled WL12. PET imaging demonstrated high tissue contrast in all tumor models tested.

Rapid PD-L1 detection in tumors with PET using a highly specific peptide.

Molecular imaging can report on the status of the tumor immune microenvironment and guide immunotherapeutic strategies to enhance the efficacy of immune modulation therapies. Imaging agents that can rapidly report on targets of immunomodulatory therapies are few. The programmed death ligand 1 (PD-L1) is an immune checkpoint protein over-expressed in several cancers and contributes to tumor immune suppression. Tumor PD-L1 expression is indicative of tumor response to PD-1 and PD-L1 targeted therapies. Herein, we report a highly specific peptide-based positron emission tomography (PET) imaging agent for PD-L1. We assessed the binding modes of the peptide WL12 to PD-L1 by docking studies, developed a copper-64 labeled WL12 ([64Cu]WL12), and performed its evaluation in vitro, and in vivo by PET imaging, biodistribution and blocking studies. Our results show that [64Cu]WL12 can be used to detect tumor PD-L1 expression specifically and soon after injection of the radiotracer, to fit within the standard clinical workflow of imaging within 60 min of administration.

PD-L1 Detection in Tumors Using [(64)Cu]Atezolizumab with PET.

The programmed death protein 1 (PD-1) and programmed death-ligand 1 (PD-L1) pair is a major immune checkpoint pathway exploited by cancer cells to develop and maintain immune tolerance. With recent approvals of anti-PD-1 and anti-PD-L1 therapeutic antibodies, there is an urgent need for noninvasive detection methods to quantify dynamic PD-L1 expression in tumors and to evaluate the tumor response to immune modulation therapies. To address this need, we assessed [(64)Cu]atezolizumab for the detection of PD-L1 expression in tumors. Atezolizumab (MPDL3208A) is a humanized, human and mouse cross-reactive, therapeutic PD-L1 antibody that is being investigated in several cancers. Atezolizumab was conjugated with DOTAGA and radiolabeled with copper-64. The resulting [(64)Cu]atezolizumab was assessed for in vitro and in vivo specificity in multiple cell lines and tumors of variable PD-L1 expression. We performed PET-CT imaging, biodistribution, and blocking studies in NSG mice bearing tumors with constitutive PD-L1 expression (CHO-hPD-L1) and in controls (CHO). Specificity of [(64)Cu]atezolizumab was further confirmed in orthotopic tumor models of human breast cancer (MDAMB231 and SUM149) and in a syngeneic mouse mammary carcinoma model (4T1). We observed specific binding of [(64)Cu]atezolizumab to tumor cells in vitro, correlating with PD-L1 expression levels. Specific accumulation of [(64)Cu]atezolizumab was also observed in tumors with high PD-L1 expression (CHO-hPD-L1 and MDAMB231) compared to tumors with low PD-L1 expression (CHO, SUM149). Collectively, these studies demonstrate the feasibility of using [(64)Cu]atezolizumab for the detection of PD-L1 expression in different tumor types.

Preclinical Comparative Study of (68)Ga-Labeled DOTA, NOTA, and HBED-CC Chelated Radiotracers for Targeting PSMA.

(68)Ga-labeled, low-molecular-weight imaging agents that target the prostate-specific membrane antigen (PSMA) are increasingly used clinically to detect prostate and other cancers with positron emission tomography (PET). The goal of this study was to compare the pharmacokinetics of three PSMA-targeted radiotracers: (68)Ga-1, using DOTA-monoamide as the chelating agent; (68)Ga-2, containing the macrocyclic chelating agent p-SCN-Bn-NOTA; and (68)Ga-DKFZ-PSMA-11, currently in clinical trials, which uses the acyclic chelating agent, HBED-CC. The PSMA-targeting scaffold for all three agents utilized a similar Glu-urea-Lys-linker construct. Each radiotracer enabled visualization of PSMA+ PC3 PIP tumor, kidney, and urinary bladder as early as 15 min post-injection using small animal PET/computed tomography (PET/CT). (68)Ga-2 demonstrated the fastest rate of clearance from all tissues in this series and displayed higher uptake in PSMA+ PC3 PIP tumor compared to (68)Ga-1 at 1 h post-injection. There was no significant difference in PSMA+ PC3 PIP tumor uptake for the three agents at 2 and 3 h post-injection. (68)Ga-DKFZ-PSMA-11 demonstrated the highest uptake and retention in normal tissues, including kidney, blood, spleen, and salivary glands and PSMA-negative PC3 flu tumors up to 3 h post-injection. In this preclinical evaluation (68)Ga-2 had the most advantageous characteristics for PSMA-targeted PET imaging.

[(18)F]Fluoroethyl Triazole Substituted PSMA Inhibitor Exhibiting Rapid Normal Organ Clearance.

Prostate-specific membrane antigen (PSMA) is overexpressed in the epithelium of prostate cancer and nonprostate solid tumor neovasculature. PSMA is increasingly utilized as a target for cancer imaging and therapy. Here, we report the synthesis and in vivo biodistribution of a low-molecular-weight PSMA-based imaging agent, 2-[3-(1-carboxy-5-{3-[1-(2-[(18)F]fluoroethyl)-1H-1,2,3-triazol-yl]propanamido}pentyl)ureido]pentanedioic acid ([(18)F]YC-88), containing an [(18)F]fluoroethyl triazole moiety. [(18)F]YC-88 was synthesized from 2-[(18)F]fluoroethyl azide and the corresponding alkyne precursor in two steps using either a one- or two-pot procedure. Biodistribution and positron emission tomography (PET) imaging were performed in immunocompromised mice using isogenic PSMA(+) PC3 PIP and PSMA(-) PC3 flu xenografts. YC-88 exhibited high affinity for PSMA as evidenced by a Ki value of 12.9 nM. The non-decay corrected radiochemical yields of [(18)F]YC-88 averaged 14 ± 1% (n = 5). Specific radioactivities ranged from 320 to 2,460 Ci/mmol (12-91 GBq/µmol) with an average of 940 Ci/mmol (35 GBq/µmol, n = 5). In an immunocompromised mouse model, [(18)F]YC-88 clearly delineated PSMA(+) PC3 PIP prostate tumor xenografts on imaging with PET. At 1 h postinjection, 47.58 ± 5.19% injected dose per gram of tissue (% ID/g) was evident within the PSMA(+) PC3 PIP tumor, with a ratio of 170:1 of uptake within PSMA(+) PC3 PIP to PSMA(-) PC3 flu tumor placed in the opposite flank. The tumor-to-kidney ratio at 2 h postinjection was 4:1. At or after 30 min postinjection, minimal nontarget tissue uptake of [(18)F]YC-88 was observed. Compared to [(18)F]DCFPyL, which is currently in clinical trials, the uptake of [(18)F]YC-88 within the kidney, liver, and spleen was significantly lower at all time-points studied. At 30 min and 1 h postinjection, salivary gland uptake of [(18)F]YC-88 was significantly less than that of [(18)F]DCFPyL. [(18)F]YC-88 is a new PSMA-targeted PET agent synthesized utilizing click chemistry that demonstrates high PSMA(+) tumor uptake in a xenograft model. Because of its low uptake in the kidney, rapid clearance from nontarget organs, and relatively simple one-pot, two-step radiosynthesis, [(18)F]YC-88 is a viable new PET radiotracer for imaging PSMA-expressing lesions.

Structural characterization and in vivo evaluation of ß-Hairpin peptidomimetics as specific CXCR4 imaging agents.

The CXCR4 chemokine receptor is integral to several biological functions and plays a pivotal role in the pathophysiology of many diseases. As such, CXCR4 is an enticing target for the development of imaging and therapeutic agents. Here we report the evaluation of the POL3026 peptidomimetic template for the development of imaging agents that target CXCR4. Structural and conformational analyses of POL3026 and two of its conjugates, DOTA (POL-D) and PEG12-DOTA (POL-PD), by circular dichroism, two-dimensional NMR spectroscopy and molecular dynamics calculations are reported. In silico observations were experimentally verified with in vitro affinity assays and rationalized using crystal structure-based molecular modeling studies. [(111)In]-labeled DOTA conjugates were assessed in vivo for target specificity in CXCR4 expressing subcutaneous U87 tumors (U87-stb-CXCR4) through single photon emission computed tomography (SPECT/CT) imaging and biodistribution studies. In silico and in vitro studies show that POL3026 and its conjugates demonstrate similar interactions with different micelles that mimic cellular membrane and that the ε-NH2 of lysine(7) is critical to maintain high affinity to CXCR4. Modification of this group with DOTA or PEG12-DOTA led to the decrease of IC50 value from 0.087 nM for POL3026 to 0.47 nM and 1.42 nM for POL-D and POL-PD, respectively. In spite of the decreased affinity toward CXCR4, [(111)In]POL-D and [(111)In]POL-PD demonstrated high and significant uptake in U87-stb-CXCR4 tumors compared to the control U87 tumors at 90 min and 24 h post injection. Uptake in U87-stb-CXCR4 tumors could be blocked by unlabeled POL3026, indicating specificity of the agents in vivo. These results suggest POL3026 as a promising template to develop new imaging agents that target CXCR4.

Synthesis and Evaluation of Gd(III) -Based Magnetic Resonance Contrast Agents for Molecular Imaging of Prostate-Specific Membrane Antigen.

Magnetic resonance (MR) imaging is advantageous because it concurrently provides anatomic, functional, and molecular information. MR molecular imaging can combine the high spatial resolution of this established clinical modality with molecular profiling in vivo. However, as a result of the intrinsically low sensitivity of MR imaging, high local concentrations of biological targets are required to generate discernable MR contrast. We hypothesize that the prostate-specific membrane antigen (PSMA), an attractive target for imaging and therapy of prostate cancer, could serve as a suitable biomarker for MR-based molecular imaging. We have synthesized three new high-affinity, low-molecular-weight Gd(III) -based PSMA-targeted contrast agents containing one to three Gd(III)  chelates per molecule. We evaluated the relaxometric properties of these agents in solution, in prostate cancer cells, and in an in vivo experimental model to demonstrate the feasibility of PSMA-based MR molecular imaging.

Imaging Axl expression in pancreatic and prostate cancer xenografts.

The receptor tyrosine kinase Axl is overexpressed in and leads to patient morbidity and mortality in a variety of cancers. Axl-Gas6 interactions are critical for tumor growth, angiogenesis and metastasis. The goal of this study was to investigate the feasibility of imaging graded levels of Axl expression in tumors using a radiolabeled antibody. We radiolabeled anti-human Axl (Axl mAb) and control IgG1 antibodies with (125)I with high specific radioactivity and radiochemical purity, resulting in an immunoreactive fraction suitable for in vivo studies. Radiolabeled antibodies were investigated in severe combined immunodeficient mice harboring subcutaneous CFPAC (Axl(high)) and Panc1 (Axl(low)) pancreatic cancer xenografts by ex vivo biodistribution and imaging. Based on these results, the specificity of [(125)I]Axl mAb was also validated in mice harboring orthotopic Panc1 or CFPAC tumors and in mice harboring subcutaneous 22Rv1 (Axl(low)) or DU145 (Axl(high)) prostate tumors by ex vivo biodistribution and imaging studies at 72h post-injection of the antibody. Both imaging and biodistribution studies demonstrated specific and persistent accumulation of [(125)I]Axl mAb in Axl(high) (CFPAC and DU145) expression tumors compared to the Axl(low) (Panc1 and 22Rv1) expression tumors. Axl expression in these tumors was further confirmed by immunohistochemical studies. No difference in the uptake of radioactivity was observed between the control [(125)I]IgG1 antibody in the Axl(high) and Axl(low) expression tumors. These data demonstrate the feasibility of imaging Axl expression in pancreatic and prostate tumor xenografts.

Heterobivalent agents targeting PSMA and integrin-αvß3.

Differential expression of surface proteins on normal vs malignant cells provides the rationale for the development of receptor-, antigen-, and transporter-based, cancer-selective imaging and therapeutic agents. However, tumors are heterogeneous, and do not always express what can be considered reliable, tumor-selective markers. That suggests development of more flexible targeting platforms that incorporate multiple moieties enabling concurrent targeting to a variety of putative markers. We report the synthesis, biochemical, in vitro, and preliminary in vivo evaluation of a new heterobivalent (HtBv) imaging agent targeting both the prostate-specific membrane antigen (PSMA) and integrin-αvß3 surface markers, each of which can be overexpressed in certain tumor epithelium and/or neovasculature. The HtBv agent was functionalized with either 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) or the commercially available IRDye800CW. DOTA-conjugated HtBv probe 9 bound to PSMA or αvß3 with affinities similar to those of monovalent (Mnv) compounds designed to bind to their targets independently. In situ energy minimization experiments support a model describing the conformations adapted by 9 that enable it to bind both targets. IRDye800-conjugated HtBv probe 10 demonstrated target-specific binding to either PSMA or integrin-αvß3 overexpressing xenografts. HtBv agents 9 and 10 may enable dual-targeted imaging of malignant cells and tissues in an effort to address heterogeneity that confounds many cancer-targeted imaging agents.