Somatostatin Receptor Subtype-2 Targeting System for Specific Delivery of Temozolomide

Temozolomide (TMZ) is a DNA alkylating agent that produces objective responses in patients with neuroendocrine tumors (NETs) when the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT) is inactivated. At high doses, TMZ therapy exhausts MGMT activity but also produces dose-limiting toxicities. To reduce off-target effects, we converted the clinically approved radiotracer 68Ga-DOTA-TOC into a peptide-drug conjugate (PDC) for targeted delivery of TMZ to somatostatin receptor subtype-2 (SSTR2)-positive tumor cells. We used an integrated radiolabeling strategy for direct quantitative assessment of receptor binding, pharmacokinetics, and tissue biodistribution. In vitro studies revealed selective binding to SSTR2-positive cells with high affinity (5.98 ± 0.96 nmol/L), internalization, receptor-dependent DNA damage, cytotoxicity, and MGMT depletion. Imaging and biodistribution analysis showed preferential accumulation of the PDC in receptor-positive tumors and high renal clearance. This study identified a trackable SSTR2-targeting system for TMZ delivery and utilizes a modular design that could be broadly applied in PDC development.

A translational blueprint for developing intraoperative imaging agents via radiopharmaceutical-guided drug design

Cancer imaging is a rapidly evolving field due to the discovery of novel molecular targets and the availability of corresponding techniques to detect them with high precision, accuracy, and sensitivity. Nuclear medicine is the most widely used molecular imaging modality and has a growing toolkit of clinically used radiopharmaceuticals that enable whole-body tumor visualization, staging, and treatment monitoring for a variety of tumors in a non-invasive manner. The need for similar imaging capabilities in the operating room has led to the emergence of fluorescence-guided surgery (FGS) as a powerful technique that gives surgeons unprecedented ability to distinguish tumors from healthy tissues. While a variety of strategies have been used to develop contrast agents for FGS, the use of radiopharmaceuticals as models brings exceptional translational potential and has increasingly been explored. Here, we review strategies used to convert clinically used radiopharmaceuticals into fluorescent and multimodal counterparts. Unique preclinical and clinical capabilities stemming from radiopharmaceutical-based agent design are also discussed to illustrate the advantages of this approach.

Combined multiphoton microscopy and somatostatin receptor type 2 imaging of pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (PNETs) are a rare but increasingly more prevalent cancer with heterogeneous clinical and pathological presentation. Surgery is the preferred treatment for all hormone-expressing PNETs and any PNET greater than 2 cm, but difficulties arise when tumors are multifocal, metastatic, or small in size due to lack of effective surgical localization. Existing techniques such as intraoperative ultrasound provide poor contrast and resolution, resulting in low sensitivity for such tumors. Somatostatin receptor type 2 (SSTR2) is commonly overexpressed in PNETs and presents an avenue for targeted tumor localization. SSTR2 is often used for pre-operative imaging and therapeutic treatment, with recent studies demonstrating that somatostatin receptor imaging (SRI) can be applied in radioguided surgery to aid in removal of metastatic lymph nodes and achieving negative surgical margins. However not all PNETs express SSTR2, indicating labeled SRI could benefit from using a supplemental label-free technique such as multiphoton microscopy (MPM), which has proven useful in improving the accuracy of diagnosing more common exocrine pancreatic cancers. Our work tests the suitability of combined SRI and MPM for localizing PNETs by imaging and comparing samples of PNETs and normal pancreatic tissue. Specimens were labeled with a novel SSTR2-targeted contrast agent and imaged using fluorescence microscopy, and subsequently imaged using MPM to collect four autofluorescent channels and second harmonic generation. Our results show that a combination of both SRI and MPM provides enhanced contrast and sensitivity for localizing diseased tissue, suggesting that this approach could be a valuable clinical tool for surgical localization and treatment of PNETs.

High-Contrast Detection of Somatostatin Receptor Subtype-2 for Fluorescence-Guided Surgery

Dye design can influence the ability of fluorescently labeled imaging agents to generate tumor contrast and has become an area of significant interest in the field of fluorescence-guided surgery (FGS). Here, we show that the charge-balanced near-infrared fluorescent (NIRF) dye FNIR-Tag enhances the imaging properties of a fluorescently labeled somatostatin analogue. In vitro studies showed that the optimized fluorescent conjugate MMC(FNIR-Tag)-TOC bound primarily via somatostatin receptor subtype-2 (SSTR2), whereas its negatively charged counterpart with IRDye 800CW had higher off-target binding. NIRF imaging in cell line- and patient-derived xenograft models revealed markedly higher tumor contrast with MMC(FNIR-Tag)-TOC, which was attributed to increased tumor specificity. Ex vivo staining of surgical biospecimens from primary and metastatic tumors, as well as involved lymph nodes, demonstrated binding to human tumors. Finally, in an orthotopic tumor model, a simulated clinical workflow highlighted our unique ability to use standard preoperative nuclear imaging for selecting patients likely to benefit from SSTR2-targeted FGS. Our findings demonstrate the translational potential of MMC(FNIR-Tag)-TOC for intraoperative imaging and suggest broad utility for using FNIR-Tag in fluorescent probe development.

Characterization of Peptides Targeting Metastatic Tumor Cells as Probes for Cancer Detection and Vehicles for Therapy Delivery

Metastasis is the leading cause of cancer-related deaths, and metastatic cancers remain largely incurable due to chemoresistance. Biomarkers of metastatic cells are lacking, and probes that could be used to detect and target metastases would be highly valuable. Here we hypothesize that metastatic cancer cells express cell-surface receptors that can be harnessed for identification of molecules homing to metastases. Screening a combinatorial library in a mouse mammary tumor model of spontaneous metastasis identified cyclic peptides with tropism for cancer cells disseminated to the lungs. Two lead peptides, CLRHSSKIC and CRAGVGRGC, bound murine and human cells derived from breast carcinoma and melanoma in culture and were selective for metastatic cells in vivo. In mice, peptide CRAGVGRGC radiolabeled with ⁶⁷Ga for biodistribution analysis demonstrated selective probe homing to lung metastases. Moreover, systemic administration of ⁶⁸Ga-labeled CRAGVGRGC enabled noninvasive imaging of lung metastases in mice by PET. A CRAGVGRGC-derived peptide induced apoptosis upon cell internalization in vitro and suppressed metastatic burden in vivo. Colocalization of CLRHSSKIC and CRAGVGRGC with N-cadherin⁺/E-cadherin^- cells indicated that both peptides are selective for cancer cells that have undergone the epithelial-to-mesenchymal transition. We conclude that CRAGVGRGC is useful as a probe to facilitate the development of imaging modalities and therapies targeting metastases. SIGNIFICANCE: This study identifies new molecules that bind metastatic cells and demonstrates their application as noninvasive imaging probes and vehicles for cytotoxic therapy delivery in preclinical cancer models.

Abstract LB120: A novel peptide-drug conjugate induces DNA damage in SSTR2-expressing cells

Introductory statement: To overcome enzyme-mediated resistance mechanisms associated with the DNA alkylating agent, temozolomide (TMZ), we seek to develop a peptide-drug conjugate (PDC) for selective drug delivery in cells that express the somatostatin receptor subtype-2 (SSTR2). Material and methods: The clinically used somatostatin analog, 68Ga-DOTA-TOC, was used as a model for development of the SSTR2-targeted PDC. Synthesis was performed by (i) replacing DOTA with a multimodality chelator (MMC), (ii) attaching a modified TMZ analog to MMC, and (iii) conjugating the payload moiety to TOC on solid-phase. The resulting product, MMC(TMZ)-TOC, was radiolabeled with 67Ga using cation exchange chromatography methods optimized for 68Ga. Retention of SSTR2 binding was examined in cell lines with different expression level of SSTR2 with and without blocking doses of octreotide. A membrane acid wash was performed to evaluate internalization efficiency. The cytotoxicity of MMC(TMZ)-TOC was tested in IMR-32 cells that endogenously express SSTR2, and potency was compared to free-TMZ. An alkaline comet assay was then performed to assess the DNA-damaging properties of the PDC in the presence and absence of SSTR2 blocking. Unpublished results: MMC(TMZ)-TOC was efficiently produced with chemical purity >90% as shown by high-performance liquid chromatography (HPLC). Radiochemical purity following 67Ga labeling was >95% and indicates the feasibility of using the MMC to directly label the drug conjugates. Cell-based experiments showed that specific binding of the 67Ga-labeled PDC was similar to 67Ga-DOTA-TOC and correlated with SSTR2 expression. In HCT116-SSTR2 cells that overexpress SSTR2, 14.8±4.8% of 67Ga-MMC(TMZ)-TOC and 17.0±4.2% of 67Ga-DOTA-TOC were taken up by cells. Blocking the excess octreotide reduced binding to near background levels, illustrating receptor-mediated uptake of the PDC. Acid-washing demonstrated internalization of 67Ga-MMC(TMZ)-TOC after receptor-binding, indicating retention of the agonist properties of TOC after payload conjugation. Results from the cell cytotoxicity study demonstrated that the PDC inhibited cell growth in a dose-dependent manner that was similar to free-TMZ, with the IC50 values of 81.6 and 75.6 µM for free-TMZ and MMC(TMZ)-TOC, respectively. The comet assay revealed that MMC(TMZ)-TOC was effective in causing DNA damage in cells that express SSTR2, as shown by reduced cytotoxic effects when co-incubated with octreotide. Conclusions: We showed that a clinically used SSTR2-targeted radiopharmaceutical can be converted into a PDC that retains receptor-binding and internalization properties. We also showed that the cytotoxic effects of TMZ are maintained by the PDC. Based on these results, in vivo evaluation of MMC(TMZ)-TOC is warranted to assess its potential as a targeted therapy for NETs.

Citation Format: Solmaz AghaAmiri, Sukhen C. Ghosh, Lea Stitzlein, Servando Hernandez Vargas, Bruno Perlatti, Daniel M. Halperin, Ali Azhdarinia. A novel peptide-drug conjugate induces DNA damage in SSTR2-expressing cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB120.

Receptor-Targeted Fluorescence-Guided Surgery With Low Molecular Weight Agents

Cancer surgery remains the primary treatment option for most solid tumors and can be curative if all malignant cells are removed. Surgeons have historically relied on visual and tactile cues to maximize tumor resection, but clinical data suggest that relapse occurs partially due to incomplete cancer removal. As a result, the introduction of technologies that enhance the ability to visualize tumors in the operating room represents a pressing need. Such technologies have the potential to revolutionize the surgical standard-of-care by enabling real-time detection of surgical margins, subclinical residual disease, lymph node metastases and synchronous/metachronous tumors. Fluorescence-guided surgery (FGS) in the near-infrared (NIRF) spectrum has shown tremendous promise as an intraoperative imaging modality. An increasing number of clinical studies have demonstrated that tumor-selective FGS agents can improve the predictive value of fluorescence over non-targeted dyes. Whereas NIRF-labeled macromolecules (i.e., antibodies) spearheaded the widespread clinical translation of tumor-selective FGS drugs, peptides and small-molecules are emerging as valuable alternatives. Here, we first review the state-of-the-art of promising low molecular weight agents that are in clinical development for FGS; we then discuss the significance, application and constraints of emerging tumor-selective FGS technologies.

Development of a drug-device combination for fluorescence-guided surgery in neuroendocrine tumors

SIGNIFICANCE

The use of cancer-targeted contrast agents in fluorescence-guided surgery (FGS) has the potential to improve intraoperative visualization of tumors and surgical margins. However, evaluation of their translational potential is challenging.

AIM

We examined the utility of a somatostatin receptor subtype-2 (SSTR2)-targeted fluorescent agent in combination with a benchtop near-infrared fluorescence (NIRF) imaging system to visualize mouse xenografts under conditions that simulate the clinical FGS workflow for open surgical procedures.

APPROACH

The dual-labeled somatostatin analog, Ga67-MMC(IR800)-TOC, was injected into mice (n  =  24) implanted with SSTR2-expressing tumors and imaged with the customized OnLume NIRF imaging system (Madison, Wisconsin). In vivo and ex vivo imaging were performed under ambient light. The optimal dose (0.2, 0.5, and 2 nmol) and imaging time point (3, 24, 48, and 72 h) were determined using contrast-to-noise ratio (CNR) as the image quality parameter. Video captures of tumor resections were obtained to provide an FGS readout that is representative of clinical utility. Finally, a log-transformed linear regression model was fitted to assess congruence between fluorescence readouts and the underlying drug distribution.

RESULTS

The drug-device combination provided high in vivo and ex vivo contrast (CNRs  >  3, except lung at 3 h) at all time points with the optimal dose of 2 nmol. The optimal imaging time point was 24-h post-injection, where CNRs  >  6.5 were achieved in tissues of interest (i.e., pancreas, small intestine, stomach, and lung). Intraoperative FGS showed excellent utility for examination of the tumor cavity pre- and post-resection. The relationship between fluorescence readouts and gamma counts was linear and strongly correlated (n  =  334, R2  =  0.71; r  =  0.84; P  <  0.0001).

CONCLUSION

The innovative OnLume NIRF imaging system enhanced the evaluation of Ga67-MMC(IR800)-TOC in tumor models. These components comprise a promising drug-device combination for FGS in patients with SSTR2-expressing tumors.

Arenicolins: C-Glycosylated Depsides from Penicillium arenicola

During investigation of the secondary metabolism of four strains of Penicillium arenicola, two new depsides, arenicolins A (1) and B (2), were isolated and characterized. Their structures were established mainly by analysis of NMR and HRMS data and by comparison with known compounds. These depsides incorporate intriguing structural features, including dual alkyl side chains and a C-glycosyl unit, with 1 also containing an acylated 2-hydroxymethyl-4,5,6-trihydroxycyclohexenone moiety. Although the arenicolins were produced by all strains tested, arenicolin A (1) was obtained using only one of five medium compositions employed, while arenicolin B (2) was produced in all media tested. Neither compound showed antibacterial or antifungal activity, but 1 exhibited cytotoxicity toward mammalian cell lines, including colorectal carcinoma (HCT-116), neuroblastoma (IMR-32), and ductal carcinoma (BT-474), with IC50 values of 7.3, 6.0, and 9.7 μM, respectively.

Abstract P1-02-01: HER2/neu antibody-fluorophore conjugate for intraoperative detection of HER2-positive breast cancer

Background: Although advances in neoadjuvant and adjuvant treatments have led to enhanced survival in patients with HER2 positive breast cancer, intraoperative detection of residual disease and tumor margins (including HER2 positive DCIS) remain challenging. Real-time fluorescence imaging is being used during surgery to improve tumor visualization and identify margins. However, the lack of an approved intraoperative imaging agent that is specific for HER2 expressing cancers limits the utility of fluorescence-guided surgery (FGS) in this patient population. We examined the potential of using clinically approved anti-HER2 monoclonal antibodies (mAbs), trastuzumab and pertuzumab, for the development of fluorescent immunoconjugates dual-labeled with radioactivity to enable quantitative assessment.

Methods: Trastuzumab, and pertuzumab were conjugated to the near-infrared (NIR) fluorophore, IR800CW, and radiometal chelator, deferoxamine (DFO), to permit dual labeling with 89Zr (positron emitting radionuclide). To confirm retention of HER2 specificity and functionality, flow cytometry analysis of the dual-labeled immunoconjugates was performed in HER2+ (BT474, SKBR3), HER2 moderate (MCF7), and HER2− (MDA231) cell lines. Using the same cell lines, radioactive uptake studies were employed to quantitatively measure cellular uptake study using 89Zr with and without blocking doses of unlabeled mAb. In vivo optical imaging of each immunoconjugate and IgG controls was performed in BT474 and MCF7 xenografts in BALB/c (nu/nu) mice 48 hours post injection using the BrukerXtreme system.

Results: In vitro results confirmed retention of receptor mediated binding on both flow cytometry and radioactive analyses. As shown in Table 1, uptake of each immunoconjugate correlated with the degree of HER2 expression. Importantly, the addition of unlabeled mAb led to a 79-93% reduction in binding to cells with the highest HER2 expression (BT474 and SKBR3). In MDA231 cells binding of the immunoconjugates was similar to the IgG control and indicates low non-specific binding. In vivo and ex vivo analysis of HER2+ and HER2 moderate xenografts demonstrated prominent tumor uptake with minimal signal in non-clearance organs. As a result, tumor-to-muscle, tumor-to-fat pad, and tumor-to-lung ratios were &gt;5, indicating excellent contrast in key regions where surgical excisions are performed clinically.

Table 1: Uptake of 89Zr-mAb-IR800 in breast cancer cell linesCell Lines% Uptake of 89Zr-PertuzumabPertuzumab Block (Unlabeled)% Uptake of 89Zr-TrastuzumabTrastuzumab Block (Unlabeled)% Uptake of 89Zr-IgGBT47449.2±14.83.7±0.036.9±5.84.6±2.61.3±0SKBR337.9±1.77.9±0.942.8±3.52.9±1.73.5±0.4MCF721.7±2.63.2±0.44.0±0.12.5±0.11.3±0.7MDA2311.9±0.40.8±0.31.0±0.31.9±0.51.3±0.7

Conclusion: This study demonstrates the potential utility of HER2-targeted FGS to improve tumor visualization, increase the likelihood of obtaining clean surgical margins, and ultimately reduce morbidity in patients with HER2 breast cancer.

Citation Format: Jo Simien, Julie Voss, A.M. Thompson, S.C. Ghosh, S. AghaAmiri, A. Azhdarinia, H.S. Tran-Cao. HER2/neu antibody-fluorophore conjugate for intraoperative detection of HER2-positive breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P1-02-01.

A proof-of-concept methodology to validate the in situ visualization of residual disease using cancer-targeted molecular agents in fluorescence-guided surgery

Introduction

The clinical need for improved intraoperative tumor visualization has led to the development of targeted contrast agents for fluorescence-guided surgery (FGS). A key characteristic of these agents is their high tumor specificity, which could enable detection of residual lesions that would likely be missed by visual inspection. Here, we examine the utility of a promising somatostatin receptor subtype-2 (SSTR2)-targeted fluorescent agent for detecting residual disease in mouse xenografts using FGS and post-operative histopathological validation.

Methods

Mice (n=2) implanted with SSTR2 overexpressing tumors were injected with 2 nmol of the dual-labeled somatostatin analog, ⁶⁷Ga-MMC(IR800)-TOC, and tumors were resected 48 h post-injection using traditional white light reflectance and palpation. Tumors underwent gamma counting and histopathology analysis. The wide-field FGS imaging platform (OnLume) was used to evaluate residual disease in situ under ambient light representative of an operating room.

Results

The tumor was resected with grossly negative margins using conventional inspection and palpation; however, additional in situ residual disease was found in the tumor cavity using FGS imaging. In situ fluorescent tumor contrast-to-noise ratios (CNRs) were 3.0 and 5.2. Agent accumulation was 7.72 and 8.20 %ID/g in tumors and 0.27 and 0.20 %ID/g in muscle. Fluorescence pixel values and gamma counts were highly correlated (r = 0.95, P < 0.048). H&E and IHC staining confirmed cancer positivity and SSTR2-overexpression, respectively.

Conclusion

Our findings demonstrate that the use of clinically relevant fluorescence imaging instrumentation enhances the evaluation of promising FGS agents for in situ visualization of residual disease.