Preclinical evaluation of [225Ac]Ac-DOTA-TATE for treatment of lung neuroendocrine neoplasms

PURPOSE

There is significant interest in the development of targeted alpha-particle therapies (TATs) for treatment of solid tumors. The metal chelator-peptide conjugate, DOTA-TATE, loaded with the β-particle emitting radionuclide ¹⁷⁷Lu ([¹⁷⁷Lu]Lu-DOTA-TATE) is now standard care for neuroendocrine tumors that express the somatostatin receptor 2 (SSTR2) target. A recent clinical study demonstrated efficacy of the corresponding [²²⁵Ac]Ac-DOTA-TATE in patients that were refractory to [¹⁷⁷Lu]Lu-DOTA-TATE. Herein, we report the radiosynthesis, toxicity, biodistribution (BD), radiation dosimetry (RD), and efficacy of [²²⁵Ac]Ac-DOTA-TATE in small animal models of lung neuroendocrine neoplasms (NENs).

METHODS

[²²⁵Ac]Ac-DOTA-TATE was synthesized and characterized for radiochemical yield, purity and stability. Non-tumor-bearing BALB/c mice were tested for toxicity and BD. Efficacy was determined by single intravenous injection of [²²⁵Ac]Ac-DOTA-TATE into SCID mice-bearing human SSTR2 positive H727 and H69 lung NENs. RD was calculated using the BD data.

RESULTS

[²²⁵Ac]Ac-DOTA-TATE was synthesized with 98% yield, 99.8% purity, and displayed 97% stability after 2 days incubation in human serum at 37 °C. All animals in the toxicity study appeared healthy 5 months post injection with no indications of toxicity, except that animals that received ≥111 kBq of [²²⁵Ac]Ac-DOTA-TATE had chronic progressive nephropathy. BD studies revealed that the primary route of elimination is by the renal route. RD calculations determined pharmacokinetics parameters and absorbed α-emission dosages from ²²⁵Ac and its daughters. For both tumor models, a significant tumor growth delay and time to experimental endpoint were observed following a single administration of [²²⁵Ac]Ac-DOTA-TATE relative to controls.

CONCLUSIONS

These results suggest significant potential for the clinical translation of [²²⁵Ac]Ac-DOTA-TATE for lung NENs.

Lipophilicity Determines Routes of Uptake and Clearance, and Toxicity of an Alpha-Particle-Emitting Peptide Receptor Radiotherapy

Lipophilicity is explored in the biodistribution (BD), pharmacokinetics (PK), radiation dosimetry (RD), and toxicity of an internally administered targeted alpha-particle therapy (TAT) under development for the treatment of metastatic melanoma. The TAT conjugate is comprised of the chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate), conjugated to melanocortin receptor 1 specific peptidic ligand (MC1RL) using a linker moiety and chelation of the ²²⁵Ac radiometal. A set of conjugates were prepared with a range of lipophilicities (log D _7.4 values) by varying the chemical properties of the linker. Reported are the observations that higher log D _7.4 values are associated with decreased kidney uptake, decreased absorbed radiation dose, and decreased kidney toxicity of the TAT, and the inverse is observed for lower log D _7.4 values. Animals administered TATs with lower lipophilicities exhibited acute nephropathy and death, whereas animals administered the highest activity TATs with higher lipophilicities lived for the duration of the 7 month study and exhibited chronic progressive nephropathy. Changes in TAT lipophilicity were not associated with changes in liver uptake, dose, or toxicity. Significant observations include that lipophilicity correlates with kidney BD, the kidney-to-liver BD ratio, and weight loss and that blood urea nitrogen (BUN) levels correlated with kidney uptake. Furthermore, BUN was identified as having higher sensitivity and specificity of detection of kidney pathology, and the liver enzyme alkaline phosphatase (ALKP) had high sensitivity and specificity for detection of liver damage associated with the TAT. These findings suggest that tuning radiopharmaceutical lipophilicity can effectively modulate the level of kidney uptake to reduce morbidity and improve both safety and efficacy.

Biodistribution and Multicompartment Pharmacokinetic Analysis of a Targeted α Particle Therapy

Targeted α particle therapy (TAT) is ideal for treating disease while minimizing damage to surrounding nontargeted tissues due to short path length and high linear energy transfer (LET). We developed a TAT for metastatic uveal melanoma, targeting the melanocortin-1 receptor (MC1R), which is expressed in 94% of uveal melanomas. Two versions of the therapy are being investigated: ²²⁵Ac-DOTA-Ahx-MC1RL (²²⁵Ac-Ahx) and ²²⁵Ac-DOTA-di-d-Glu-MC1RL (²²⁵Ac-di-d-Glu). The biodistribution (BD) from each was studied and a multicompartment pharmacokinetic (PK) model was developed to describe drug distribution rates. Two groups of 16 severe combined immunodeficient (SCID) mice bearing high MC1R expressing tumors were intravenously injected with ²²⁵Ac-Ahx or ²²⁵Ac-di-d-Glu. After injection, four groups (n = 4) were euthanized at 24, 96, 144, and 288 h time points for each cohort. Tumors and 13 other organs were harvested at each time point. Isomeric γ spectra were measured in tissue samples using a scintillation γ detector and converted to α activity using factors for γ ray abundance per α decay. Time activity curves were calculated for each organ. A five-compartment PK model was built with the following compartments: blood, tumor, normal tissue, kidney, and liver. This model is characterized by a system of five ordinary differential equations using mass action kinetics, which describe uptake, intercompartmental transitions, and clearance rates. The ordinary differential equations were simultaneously solved and fit to experimental data using a genetic algorithm for optimization. The BD data show that both compounds have minimal distribution to organs at risk other than the kidney and liver. The PK parameter estimates had less than 5% error. From these data, ²²⁵Ac-Ahx showed larger and faster uptake in the liver. Both compounds had comparable uptake and clearance rates for other compartments. The BD and PK behavior for two targeted radiopharmaceuticals were investigated. The PK model fit the experimental data and provided insight into the kinetics of the compounds systematically.

Narrowing the focus: Therapeutic cell surface targets for refractory triple-negative breast cancer

Triple-negative breast cancer (TNBC) is defined as a type of breast cancer with lack of expression of estrogen receptor, progesterone receptor and human epidermal growth factor 2 protein. In comparison to other types of breast cancer, TNBC characterizes for its aggressive behavior, more prone to early recurrence and a disease with poor response to molecular target therapy. Although TNBC is identified in only 25%-30% of American breast cancer cases annually, these tumors continue to be a therapeutic challenge for clinicians for several reasons: Tumor heterogeneity, limited and toxic systemic therapy options, and often resistance to current standard therapy, characterized by progressive disease on treatment, residual tumor after cytotoxic chemotherapy, and early recurrence after complete surgical excision. Cell-surface targeted therapies have been successful for breast cancer in general, however there are currently no approved cell-surface targeted therapies specifically indicated for TNBC. Recently, several cell-surface targets have been identified as candidates for treatment of TNBC and associated targeted therapies are in development. The purpose of this work is to review the current clinical challenges posed by TNBC, the therapeutic approaches currently in use, and provide an overview of developing cell surface targeting approaches to improve outcomes for treatment resistant TNBC.

Development of Targeted Alpha Particle Therapy for Solid Tumors

Targeted alpha-particle therapy (TAT) aims to selectively deliver radionuclides emitting α-particles (cytotoxic payload) to tumors by chelation to monoclonal antibodies, peptides or small molecules that recognize tumor-associated antigens or cell-surface receptors. Because of the high linear energy transfer (LET) and short range of alpha (α) particles in tissue, cancer cells can be significantly damaged while causing minimal toxicity to surrounding healthy cells. Recent clinical studies have demonstrated the remarkable efficacy of TAT in the treatment of metastatic, castration-resistant prostate cancer. In this comprehensive review, we discuss the current consensus regarding the properties of the α-particle-emitting radionuclides that are potentially relevant for use in the clinic; the TAT-mediated mechanisms responsible for cell death; the different classes of targeting moieties and radiometal chelators available for TAT development; current approaches to calculating radiation dosimetry for TATs; and lead optimization via medicinal chemistry to improve the TAT radiopharmaceutical properties. We have also summarized the use of TATs in pre-clinical and clinical studies to date.

Melanocortin 1 Receptor-Targeted α-Particle Therapy for Metastatic Uveal Melanoma

New effective therapies are greatly needed for metastatic uveal melanoma, which has a very poor prognosis with a median survival of less than 1 y. The melanocortin 1 receptor (MC1R) is expressed in 94% of uveal melanoma metastases, and a MC1R-specific ligand (MC1RL) with high affinity and selectivity for MC1R was previously developed. Methods: The ²²⁵Ac-DOTA-MC1RL conjugate was synthesized in high radiochemical yield and purity and was tested in vitro for biostability and for MC1R-specific cytotoxicity in uveal melanoma cells, and the lanthanum-DOTA-MC1RL analog was tested for binding affinity. Non-tumor-bearing BALB/c mice were tested for maximum tolerated dose and biodistribution. Severe combined immunodeficient mice bearing uveal melanoma tumors or engineered MC1R-positive and -negative tumors were studied for biodistribution and efficacy. Radiation dosimetry was calculated using mouse biodistribution data and blood clearance kinetics from Sprague-Dawley rat data. Results: High biostability, MC1R-specific cytotoxicity, and high binding affinity were observed. Limiting toxicities were not observed at even the highest administered activities. Pharmacokinetics and biodistribution studies revealed rapid blood clearance (<15 min), renal and hepatobillary excretion, MC1R-specific tumor uptake, and minimal retention in other normal tissues. Radiation dosimetry calculations determined pharmacokinetics parameters and absorbed α-emission dosages from ²²⁵Ac and its daughters. Efficacy studies demonstrated significantly prolonged survival and decreased metastasis burden after a single administration of ²²⁵Ac-DOTA-MC1RL in treated mice relative to controls. Conclusion: These results suggest significant potential for the clinical translation of ²²⁵Ac-DOTA-MC1RL as a novel therapy for metastatic uveal melanoma.

Abstract 5198: Targeted alpha particle therapy for uveal melanoma

New effective therapies are greatly needed for uveal melanoma, the most common ocular malignancy, which has a very poor prognosis with a median survival of less than one year when metastatic disease develops. The melanocortin 1 receptor (MC1R) is expressed in 94% of uveal melanomas but is not expressed in normal tissues of concern for toxicity. Previously, we developed an MC1R specific ligand (MC1RL) with high affinity and selectivity for MC1R. We used the MC1RL as a targeting scaffold for development of a radiopharmaceutical by conjugation of 225Ac chelate to form 225Ac -MC1RL-DOTA. 225Ac is a therapeutic alpha-emitting radionuclide. 225Ac -MC1RL-DOTA was synthesized with high radiochemical yield and purity (&gt;95 and &gt;99 % respectively) with an excellent in vitro stability, i.e. 90% after 10 days in human serum at 37ºC. An MTT cytotoxicity study demonstrated MC1R specific antiproliferative effect in cutaneous, ocular and uveal melanoma cell lines. A maximum tolerated dose (MTD) study was completed, where a range of 0-148 kBq (0-4 µCi) of 225Ac -MC1RL-DOTA was administered to groups of BALB/c mice. The radiopharmaceutical was well tolerated at even the highest doses and animals did not reach any clinical endpoints, such as weight loss, loss of kidney function or abnormal pathology. Biodistribution studies on MC1R expressing tumor bearing mice revealed tumor selectivity and a combination of renal and hepatic clearance with minimal retention in other normal tissues. A blood PK study in rats showed rapid clearance of the agent from the blood in &lt;60 min. To determine radiodosimetry, gamma spectra were acquired for tumors, blood and other tissues and organs over a 21-day time period and activity curves generated for each tissue for 225Ac and other gamma-emitting daughter products in the decay chain. An exponential line-fitting of these curves allowed the estimation of clearance kinetics, tissue biological half-life and total absorbed dose (Gy) for each tissue. Doses were highest in MC1R positive tumors and clearance organs (liver, kidney and spleen) and low in all other tissues. In vivo efficacy studies were performed in human A375/MC1R cutaneous and MEL20 uveal melanoma tumors in SCID mice, or syngeneic mouse B16 melanoma tumors in C57BL/6 mice (n=10/group) injected with either 225Ac -MC1RL-DOTA, 225Ac-scrambled-MC1RL-DOTA, La-MC1RL-DOTA (non-radioactive control agent) or saline. In every case a tumor growth delay was observed in treated mice relative to controls, and a percentage of mice bearing the human tumors had complete remission. In conclusion, we evaluated the in vitro and in vivo properties of 225Ac-MC1RL-DOTA in this study. This agent demonstrated a significant MC1R-specific in vivo therapeutic efficacy in a mouse model of uveal melanoma with low systemic toxicity.

Citation Format: Narges K. Tafreshi, Nella C. Delva, Christopher J. Tichacek, Michael L. Doligalski, Darpan N. Pandya, Nikunj B. Bhatt, HyunJoo Kil, Mikalai M. Budzevich, Epifanio Ruiz, Thaddeus J. Wadas, Mark L. McLaughlin, David L. Morse. Targeted alpha particle therapy for uveal melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5198. doi:10.1158/1538-7445.AM2017-5198

Abstract 1709: Targeting immune checkpoint therapy to the lung tumor microenvironment

Lung cancer is the leading cause of cancer deaths worldwide. Many lung cancer patients are diagnosed with advanced disease. These patients have a low 5-year survival rate and limited treatment options. Novel therapies, which target biomarkers that are overexpressed in lung cancer but have basal expression in benign tissue, are desperately needed. The goal of this work was to develop a targeted agent for immune checkpoint therapy of lung cancer. Recently, immune checkpoint inhibitors have been approved for use in lung cancer and many more are being tested in clinical trials. The current immune checkpoint inhibitor agents are not tumor-targeted. Targeting the immune checkpoint inhibitor to tumor cell-surface markers should concentrate the conjugate in the tumor microenvironment and enhance the immune response in the tumor while reducing the systemic dosages needed, resulting in lower systemic toxicity.

The delta opioid receptor (DOR) is expressed in some lung cancers, but is not expressed or is expressed only at basal levels in normal tissues outside the brain. We have previously synthesized fluorescently-labeled DOR-targeted imaging agents based on a synthetic peptide antagonist (DORL). These targeted fluorescent agents have high affinity and selectivity for DOR, and exhibit good pharmacokinetic (PK) and biodistribution (BD) profiles, i.e. specific tumor uptake with rapid systemic clearance and no uptake in tissues of concern, e.g. brain. We are now developing lung cancer-specific immunotherapy agents that target the DOR by conjugating DORL to immunomodulatory molecules. In the current work, we synthesized a fluorescently-labeled DOR targeting ligand and conjugated it to an anti-PD1 antibody (DORL-PD1). We synthesized immunoconjugates with several targeting ligand-to-antibody ratios (TARs). We engineered murine lung cancer cells to constitutively express the DOR. By lanthanide time-resolved fluorescence (LTRF) competitive binding assays, we have shown that the agents have high avidity for the DOR in vitro with higher TARs resulting in higher binding avidity. We characterized the uptake of DORL-PD1 in vitro using live-cell fluorescence microscopy. Using syngeneic engraftment tumor models in immunocompetent mice, we performed longitudinal fluorescence imaging studies to determine the agent circulation time (PK), tumor selectivity and tissue distribution (BD). Immune checkpoint efficacy studies were performed using the DOR negative mouse tumor models.

In conclusion, we have synthesized fluorescent DOR-targeted immune checkpoint therapy agents, DORL-PD1; demonstrated avidity and selectivity for the DOR in vitro and in vivo; and immune checkpoint therapy efficacy in vivo. Future studies will evaluate the efficacy of DORL-PD1 in immune competent mice bearing DOR positive tumors. These agents could be useful for increasing the efficacy and reducing systemic toxicity of immune-checkpoint therapy of lung cancer.

Citation Format: Allison S. Cohen, Michael L. Doligalski, Hong Zheng, Narges K. Tafreshi, Veronica Estrella, Nella Delva, Jonathan Nguyen, Amer Beg, Mark L. McLaughlin, David L. Morse. Targeting immune checkpoint therapy to the lung tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1709. doi:10.1158/1538-7445.AM2017-1709

Abstract 517: An MC1R targeted 225Ac radiopharmaceutical agent for treatment of uveal melanoma

Prognosis in metastatic uveal melanoma is poor with median survival being less than one year. There is no approved therapy for metastatic disease. Hence, new targeted therapies are needed. The melanocortin 1 receptor (MC1R) is expressed in 94% of uveal melanomas but is not expressed in normal tissues of concern for toxicity. We have developed a MC1R specific ligand (MC1RL), conjugated imaging contrast agents to it, demonstrated high selectivity for MC1R expressing tumors in mice, and demonstrated rapid systemic clearance, without retention in tissues of concern for toxicity. The aim of this study is to use MC1RL as a targeting scaffold for development of a radiopharmaceutical by conjugation of 225Ac chelate. 225Ac is a therapeutic alpha emitting radionuclide. A targeted approach to deliver 225Ac to uveal melanoma without causing toxicity in normal tissues would provide clinicians with a novel and powerful therapeutic option for treatment of metastatic disease.

Here, we conjugated 1,4,7,10-tetraazacyclo-dodecane-1,4,7,10-tetraacetic acid (DOTA) to the MC1R specific scaffold (DOTA-MC1RL) and chelated the nonradioactive surrogate 139La (substitute for 225Ac) and demonstrated high binding affinity to MC1R (0.2 nM Ki). We anticipate that the radioactive conjugates will have comparable affinities. We also synthesized 225Ac-DOTA-MC1RL and showed a radiochemical yield greater than 95% and high radiochemical purity (≥99.8%) as determined by radio-TLC, radio-HPLC, and gamma-counter quantification. Moreover, 225Ac -DOTA-MC1RL showed excellent in vitro stability, i.e. 90% after 10 days in human serum at 37ºC. A maximum tolerated dosage (MTD) study was performed by administration of 0, 9, 18, 28, 37, 56, 74 and 148 kBq of 225Ac-DOTA-MC1RL. Animals were followed for 120 days and there were no signs of altered behavior among the groups. The group that received the highest dosage had a slightly but significantly lower increase in body weight over the course of the study, suggesting that 225Ac-DOTA-MC1RL is tolerated extremely well. Blood work and organ pathology did not show any significant deleterious effects even at the highest dose. In vitro cytotoxicity assays showed significant cell death in uveal and cutaneous cell lines in an MC1R dependent manner. Biodistribution studies showed tumor selectivity and a combination of renal and hepatic clearance with minimal retention in other normal tissues. In vivo efficacy studies in SCID mice bearing MC1R expressing human A375 subcutaneous xenograft tumors showed complete loss of tumor within one week of intravenous administration of 225Ac-DOTA-MC1RL relative to controls that had continued tumor growth.

In conclusion, we have demonstrated radiosynthesis of 225Ac-DOTA-MC1RLwith high yield, purity and stability. In vitro studies demonstrated MC1R dependent cytotoxicity in uveal melanoma cells. In vivo studies demonstrated favorable biodistribution and significant antitumor efficacy.

Citation Format: Narges K. Tafreshi, Michael L. Doligalski, Darpan N. Pandya, Hyun Joo Kil, Mikalai Budzevich, Thaddeus J. Wadas, Mark L. McLaughlin, David Morse. An MC1R targeted 225Ac radiopharmaceutical agent for treatment of uveal melanoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 517.

The role of toll-like receptors in colorectal cancer progression: evidence for epithelial to leucocytic transition

Toll-like receptors (TLRs) are expressed by immune cells, intestinal epithelium, and tumor cells. In the homeostatic setting, they help to regulate control over invading pathogens and maintain the epithelial lining of the large and small intestines. Aberrant expression of certain TLRs by tumor cells can induce growth inhibition while others contribute to tumorigenesis and progression. Activation of these TLRs can induce inflammation, tumor cell proliferation, immune evasion, local invasion, and distant metastasis. These TLR-influenced behaviors have similarities with properties observed in leukocytes, suggesting that tumors may be hijacking immune programs to become more aggressive. The concept of epithelial to leucocytic-transition (ELT) is proposed, akin to epithelial to mesenchymal transition, in which tumors develop the ability to activate leucocytic traits otherwise inaccessible to epithelial cells. Understanding the mechanisms of ELT could lead to novel therapeutic strategies for inhibiting tumor metastasis.

Carbonic anhydrase IX as an imaging and therapeutic target for tumors and metastases

Carbonic anhydrase IX (CAIX) which is a zinc containing metalloprotein, efficiently catalyzes the reversible hydration of carbon dioxide. It is constitutively up-regulated in several cancer types and has an important role in tumor progression, acidification and metastasis. High expression of CAIX generally correlates with poor prognosis and is related to a decrease in the disease-free interval following successful therapy. Therefore, it is considered as a prognostic indicator in oncology.In this review, we describe CAIX regulation and its role in tumor hypoxia, acidification and metastasis. In addition, the molecular imaging of CAIX and its potential for use in cancer detection, diagnosis, staging, and for use in following therapy response is discussed. Both antibodies and small molecular weight compounds have been used for targeted imaging of CAIX expression. The use of CAIX expression as an attractive and promising candidate marker for systemic anticancer therapy is also discussed.

In vivo and in silico pharmacokinetics and biodistribution of a melanocortin receptor 1 targeted agent in preclinical models of melanoma

The melanocortin 1 receptor (MC1R) is overexpressed in most melanoma metastases, making it a promising target for imaging of melanomas. In this study, the expression of MC1R in a large fraction of patients with melanoma was confirmed using mRNA and tissue microarray. Here, we have characterized the in vivo tumor and tissue distribution and pharmacokinetics (PK) of uptake and clearance of a MC1R specific peptidomimetic ligand conjugated to a near-infrared fluorescent dye. We propose an interdisciplinary framework to bridge the different time and space scales of ligand-tumor-host interactions: intravital fluorescence microscopy to quantify probe internalization at the cellular level, a xenograft tumor model for whole body pharmacokinetics, and a computational pharmacokinetic model for integration and interpretation of experimental data. Administration of the probe into mice bearing tumors with high and low MC1R expression demonstrated normalized image intensities that correlated with expression levels (p < 0.05). The biodistribution study showed high kidney uptake as early as 30 min postinjection. The PK computational model predicted the presence of receptors in the kidneys with a lower affinity, but at higher numbers than in the tumors. As the mouse kidney is known to express the MC5R, this hypothesis was confirmed by both coinjection of a ligand with higher MC5R affinity compared to MC1R and by injection of lower probe concentrations (e.g., 1 nmol/kg), both leading to decreased kidney accumulation of the MC1R ligand. In addition, through this interdisciplinary approach we could predict the rates of ligand accumulation and clearance into and from organs and tumors, and the amount of injected ligand required to have maximum specific retention in tumors. These predictions have potential to aid in the translation of a targeted agent from lab to the clinic. In conclusion, the characterized MC1R-specific probe has excellent potential for in vivo detection of melanoma metastases. The process of cell-surface marker validation, targeted imaging probe development, and in vitro, in vivo, and in silico characterization described in this study can be generally applied to preclinical development of targeted agents.

Synthesis and characterization of time-resolved fluorescence probes for evaluation of competitive binding to melanocortin receptors

Probes for use in time-resolved fluorescence competitive binding assays at melanocortin receptors based on the parental ligands MSH(4), MSH(7), and NDP-α-MSH were prepared by solid phase synthesis methods, purified, and characterized. The saturation binding of these probes was studied using HEK-293 cells engineered to overexpress the human melanocortin 4 receptor (hMC4R) as well as the human cholecystokinin 2 receptor (hCCK2R). The ratios of non-specific binding to total binding approached unity at high concentrations for each probe. At low probe concentrations, receptor-mediated binding and uptake was discernable, and so probe concentrations were kept as low as possible in determining Kd values. The Eu-DTPA-PEGO-MSH(4) probe exhibited low specific binding relative to non-specific binding, even at low nanomolar concentrations, and was deemed unsuitable for use in competition binding assays. The Eu-DTPA-PEGO probes based on MSH(7) and NDP-α-MSH exhibited Kd values of 27±3.9nM and 4.2±0.48nM, respectively, for binding with hMC4R. These probes were employed in competitive binding assays to characterize the interactions of hMC4R with monovalent and divalent MSH(4), MSH(7), and NDP-α-MSH constructs derived from squalene. Results from assays with both probes reflected only statistical enhancements, suggesting improper ligand spacing on the squalene scaffold for the divalent constructs. The Ki values from competitive binding assays that employed the MSH(7)-based probe were generally lower than the Ki values obtained when the probe based on NDP-α-MSH was employed, which is consistent with the greater potency of the latter probe. The probe based on MSH(7) was also competed with monovalent, divalent, and trivalent MSH(4) constructs that previously demonstrated multivalent binding in competitive binding assays against a variant of the probe based on NDP-α-MSH. Results from these assays confirm multivalent binding, but suggest a more modest increase in avidity for these MSH(4) constructs than was previously reported.

Abstract 5519: In vivo fluorescence lifetime imaging differentiates the bound versus unbound status of a cell surface receptor targeted ligand

In vivo fluorescence lifetime imaging allows for the isolation of probe-specific fluorescence lifetime from surrounding tissue auto-fluorescence. Fluorescence lifetime is not affected by agent concentration or depth, or image acquisition parameters. However, fluorescence lifetime can be affected by the in vivo tissue microenvironment, e.g. pH or hypoxia. In this work, fluorescence lifetime is used to characterize the presence and fate of a high-affinity peptidyl delta opioid receptor (δOR) ligand conjugated to the near-infrared fluorescent IRDye800CW (Li-Cor).

HCT116 colorectal cancer cells engineered to over-express the δOR and parental non-expressing HCT116 cells were bilaterally xenografted subcutaneously into the flanks of athymic nude mice. At a series of time-points following tail vein injection of the ligand, in vivo fluorescence images images were acquired using the Optix MX3 scanner (Advanced Research Technologies).

For 24 h, the ligand fluoresence lifetime was significantly different (p&lt;0.05) in the δOR positive tumors relative to the negative tumors. At 48 hours, the fluorescence lifetime in the negative tumor decreased and was no longer different. We hypothesize that the early difference in fluorescence lifetime is attributed to the bound state of the ligand on receptor positive tumors relative to the unbound state in negative tumors. Over time, unbound ligand clears from the negative tumor leaving only a low concentration of ligand retained by non-specific interactions.

In vitro studies demonstrated that the fluoresecence lifetime of this ligand is not altered within the biological pH range. However, the tumor microenvironment is complex and multiple factors may be influencing the observed differences. Future plans include studies of 3D cell cultures using varied microenvironmental conditions and studies on δOR endogenous cell lines.

Citation Format: Yoli JeuneSmith, Allison S. Cohen, Narges K. Tafreshi, Josef Vagner, Robert Gillies, David Morse. In vivo fluorescence lifetime imaging differentiates the bound versus unbound status of a cell surface receptor targeted ligand. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5519. doi:10.1158/1538-7445.AM2013-5519

Synthesis and characterization of a melanoma-targeted fluorescence imaging probe by conjugation of a melanocortin 1 receptor (MC1R) specific ligand

The incidence of malignant melanoma is rising more rapidly than that of any other cancer in the United States. The melanocortin 1 receptor (MC1R) is overexpressed in most human melanoma metastases, thus making it a promising target for imaging and therapy of melanomas. We have previously reported the development of a peptidomimetic ligand with high specificity and affinity for MC1R. Here, we have conjugated near-infrared fluorescent dyes to the C-terminus of this ligand via lysine-mercaptopropionic acid linkers to generate MC1R specific optical probes (MC1RL-800, 0.4 nM K(i); and MC1RL-Cy5, 0.3 nM K(i)). Internalization of the imaging probe was studied in vitro by fluorescence microscopy using engineered A375/MC1R cells and B16F10 cells with endogenous MC1R expression. The in vivo tumor targeting of MC1RL-800 was evaluated by intravenous injection of probe into nude mice bearing bilateral subcutaneous A375 xenograft tumors with low MC1R expression and engineered A375/MC1R tumors with high receptor expression. Melanotic B16F10 xenografts were also studied. Fluorescence imaging showed that the agent has higher uptake values in tumors with high expression compared to low (p < 0.05), demonstrating the effect of expression levels on image contrast-to-noise. In addition, tumor uptake was significantly blocked by coinjection of excess NDP-α-MSH peptide (p < 0.05). In conclusion, the MC1R-specific imaging probe developed in this study displays excellent potential for the intraoperative detection of regional node involvement and for margin detection during melanoma metastasis resection.

Abstract 4557: Development of a melanocortin receptor 1 targeted probe for molecular imaging of melanoma

Melanoma is curable by means of surgical excision if diagnosed in early stages, but once the metastatic stage is reached, prognosis is poor because the tumor is resistant to most cures. Therefore, a melanoma targeted probe that can deliver therapy to metastases at high dosages could increase the chances of treatment. Melanocortin 1 receptor (MC1R) is overexpressed in most human melanoma metastases, thus making it a promising target for imaging and therapy of melanomas. To confirm that MC1R is a specific melanoma marker, mRNA and protein expression was confirmed and quantified in tumor and normal unaffected patient tissue samples. MC1R mRNA expression was highly and generally expressed among melanoma samples surveyed. In contrast, MC1R expression was not elevated in other skin cancers, normal skin and organs involved in toxicity and clearance, i.e. heart, spleen, liver and kidney. To determine MC1R protein expression in patient samples, immunohistochemistry was performed on a melanoma tissue microarray containing 267 samples. None of the normal skin samples (n = 19) had staining with a pathology score of α4. Benign lesions (n = 65), samples of local invasion to regional lymph nodes (n = 35) and metastatic melanoma (n=40) had moderate to high staining in 15, 33 and 47% of the samples, respectively. We have previously reported the development of a peptidomimetic ligand with high specificity and affinity for MC1R. In this study, we have conjugated this ligand to an infrared dye to generate a MC1R specific optical probe (named ML-800). Our whole-cell binding assay using A375/MC1R human melanoma engineered cells was used to determine the high binding affinity of ML-800 (0.4± 0.1 nM Ki). The cellular uptake of the probe was studied in A375/MC1R cells by fluorescence microscopy both in vitro and in vivo using a dorsal skin-fold window-chamber mouse model. The in vivo tumor targeting of ML-800 was evaluated by intravenous injection of probe into nude mice bearing bilateral subcutaneous tumors of A375 with low number of MC1R receptors and A375/MC1R with high expression of MC1Rs. Fluorescence imaging showed that the agent has higher uptake values in A375/MC1R tumors than those in A375M tumors (P, 0.05), demonstrating differentiation of probe retention in tumors with different levels of expression. In conclusion, the imaging probe designed in this study demonstrates the potential for the development of agents that can deliver imaging contrast and therapy to melanoma metastases that express MC1R. Since radiopeptides have proven their usefulness for diagnostic imaging and radiotherapy, in the future, this ligand could be developed as a targeted delivery vehicle for non-invasive nuclear imaging to detect regional lymph node involvement, or delivery of radiotherapy. Furthermore, by attachment to the MC1R ligand with a cleavable linker, cytotoxins may also be targeted to tumor cells for receptor mediated endocytosis and intracellular release.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4557. doi:1538-7445.AM2012-4557

Development of melanoma-targeted polymer micelles by conjugation of a melanocortin 1 receptor (MC1R) specific ligand

The incidence of malignant melanoma is rising faster than that of any other cancer in the United States. Because of its high expression on the surface of melanomas, MC1R has been investigated as a target for selective imaging and therapeutic agents against melanoma. Eight ligands were screened against cell lines engineered to overexpress MC1R, MC4R, or MC5R. Of these, compound 1 (4-phenylbutyryl-His-dPhe-Arg-Trp-NH(2)) exhibited high (0.2 nM) binding affinity for MC1R and low (high nanomolar) affinities for MC4R and MC5R. Functionalization of the ligand at the C-terminus with an alkyne for use in Cu-catalyzed click chemistry was shown not to affect the binding affinity. Finally, formation of the targeted polymer, as well as the targeted micelle formulation, also resulted in constructs with low nanomolar binding affinity.

Noninvasive detection of breast cancer lymph node metastasis using carbonic anhydrases IX and XII targeted imaging probes

PURPOSE

To develop targeted molecular imaging probes for the noninvasive detection of breast cancer lymph node metastasis.

EXPERIMENTAL DESIGN

Six cell surface or secreted markers were identified by expression profiling and from the literature as being highly expressed in breast cancer lymph node metastases. Two of these markers were cell surface carbonic anhydrase isozymes (CAIX and/or CAXII) and were validated for protein expression by immunohistochemistry of patient tissue samples on a breast cancer tissue microarray containing 47 normal breast tissue samples, 42 ductal carcinoma in situ, 43 invasive ductal carcinomas without metastasis, 46 invasive ductal carcinomas with metastasis, and 49 lymph node macrometastases of breast carcinoma. Targeted probes were developed by conjugation of CAIX- and CAXII-specific monoclonal antibodies to a near-infrared fluorescent dye.

RESULTS

Together, these two markers were expressed in 100% of the lymph node metastases surveyed. Selectivity of the imaging probes were confirmed by intravenous injection into nude mice-bearing mammary fat pad tumors of marker-expressing cells and nonexpressing cells or by preinjection of unlabeled antibody. Imaging of lymph node metastases showed that peritumorally injected probes detected nodes harboring metastatic tumor cells. As few as 1,000 cells were detected, as determined by implanting, under ultrasound guidance, a range in number of CAIX- and CAXII-expressing cells into the axillary lymph nodes.

CONCLUSION

These imaging probes have potential for noninvasive staging of breast cancer in the clinic and elimination of unneeded surgery, which is costly and associated with morbidities.

A sucrose-derived scaffold for multimerization of bioactive peptides

A spherical molecular scaffold bearing eight terminal alkyne groups was synthesized in one step from sucrose. One or more copies of a tetrapeptide azide, either N(3)(CH(2))(5)(CO)-His-DPhe-Arg-Trp-NH(2) (MSH4) or N(3)(CH(2))(5)(CO)-Trp-Met-Asp-Phe-NH(2) (CCK4), were attached to the scaffold via the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. Competitive binding assays using Eu-labeled probes based on the superpotent ligands Ser-Tyr-Ser-Nle-Glu-His-DPhe-Arg-Trp-Gly-Lys-Pro-Val-NH(2) (NDP-α-MSH) and Asp-Tyr-Met-Gly-Trp-Met-Asp-Phe-NH(2) (CCK8) were used to study the interactions of monovalent and multivalent MSH4 and CCK4 constructs with Hek293 cells engineered to overexpress MC4R and CCK2R. All of the monovalent and multivalent MSH4 constructs exhibited binding comparable to that of the parental ligand, suggesting that either the ligand spacing was inappropriate for multivalent binding, or MSH4 is too weak a binder for a second 'anchoring' binding event to occur before the monovalently-bound construct is released from the cell surface. In contrast with this behavior, monovalent CCK4 constructs were significantly less potent than the parental ligand, while multivalent CCK4 constructs were as or more potent than the parental ligand. These results are suggestive of multivalent binding, which may be due to increased residence times for monovalently bound CCK4 constructs on the cell surface relative to MSH4 constructs, the greater residence time being necessary for the establishment of multivalent binding.

A mammaglobin-A targeting agent for noninvasive detection of breast cancer metastasis in lymph nodes

Pathologic axillary lymph node (ALN) status is an important prognostic factor for staging breast cancer. Currently, status is determined by histopathology following surgical excision of sentinel lymph node(s), which is an invasive, time consuming, and costly procedure with potential morbidity to the patient. Here, we describe an imaging platform for noninvasive assessment of ALN status, eliminating the need for surgical examination of patients to rule out nodal involvement. A targeted imaging probe (MamAb-680) was developed by conjugation of a mammaglobin-A-specific monoclonal antibody to a near-infrared fluorescent dye. Using DNA and tissue microarray, mammaglobin-A was validated as a cell-surface target that is expressed in ALN-positive patient samples but is not expressed in normal lymph nodes. In vivo selectivity was determined by i.v. injection of MamAb-680 into mice with mammaglobin-A-positive and -negative mammary fat pad (MFP) tumors; and by peritumoral MFP injection of the targeted imaging probe in mice with spontaneous ALN metastases. Fluorescence imaging showed that probe was only retained in positive tumors and metastases. As few as 1,000 cells that endogenously express mammaglobin-A were detected in ALN, indicating high sensitivity of this method. Translation of this approach offers considerable potential as a noninvasive clinical strategy to stage breast cancer.

Molecular and Functional Imaging of Breast Cancer

Background

Significant efforts have been directed toward developing and enhancing imaging methods for the early detection, diagnosis, and characterization of small breast tumors. Molecular and functional imaging sets the stage for enhancement of current methodology.

Methods

Current imaging modalities are described based on the molecular characteristics of normal and malignant tissue. New molecular imaging methods that have the potential for clinical use are also discussed.

Results:

Dynamic contrast-enhanced magnetic resonance imaging is more sensitive than mammography in BRCA1 carriers. It is used in screening and in the early evaluation of neoadjuvant therapy. Positron emission mammography is 91% sensitive and 93% specific in detecting primary breast cancers. Sentinel node scintigraphy is a key component of axillary lymph node evaluation. Other imaging modalities being studied include Tc99m sestamibi, radiolabeled thymidine or uridine, estrogen receptor imaging, magnetic resonance spectroscopy, and diffusion magnetic resonance imaging.

Conclusions

Molecular and functional imaging of the breast will likely alter clinical practice in diagnosing and staging primary breast cancer and assessing response to therapy since it will provide earlier information regarding the underlying biology of individual breast cancers, tumor stage, potential treatment strategies, and biomarkers for early evaluation of treatment effects.

Identification of novel pancreatic adenocarcinoma cell-surface targets by gene expression profiling and tissue microarray

Pancreatic cancer has a high mortality rate, which is generally related to the initial diagnosis coming at late stage disease combined with a lack of effective treatment options. Novel agents that selectively detect pancreatic cancer have potential for use in the molecular imaging of cancer, allowing for non-invasive determination of tumor therapeutic response and molecular characterization of the disease. Such agents may also be used for the targeted delivery of therapy to tumor cells while decreasing systemic effects. Using complementary assays of mRNA expression profiling to determine elevated expression in pancreatic cancer tissues relative to normal pancreas tissues, and validation of protein expression by immunohistochemistry on tissue microarray, we have identified cell-surface targets with potential for imaging and therapeutic agent development. Expression profiles of 2177 cell-surface genes for 28 pancreatic tumor specimens and 4 normal pancreas tissue samples were evaluated. Expression in normal tissues was evaluated using array data from 103 samples representing 28 organ sites as well as mining published data. One-hundred seventy unique targets were highly expressed in 2 or more of the pancreatic tumor specimens and were not expressed in the normal pancreas samples. Two targets (TLR2 and ABCC3) were further validated for protein expression by tissue microarray (TMA) based immunohistochemistry. These validated targets have potential for the development of diagnostic imaging and therapeutic agents for pancreatic cancer.