Molecular optical imaging of therapeutic targets of cancer

Discovery of non-substrate, environmentally sensitive turn-on fluorescent probes for imaging HDAC8 in tumor cells and tissue slices

Histone deacetylase 8 (HDAC8) is overexpressed in multiple cancers and lack of effective chemical probes which could detect and visualize HDAC8 in tumor cells and tissues remains unsolved. In this work, three novel turn-on HDAC8 fluorescent probes 17-19 derived from solvatochromic fluorophore 4-sulfamonyl-7-aminobenzoxadiazole (SBD) conjugating with a potent HDAC8 inhibitor PCI-34051 (IC₅₀ = 10 nM) as the recognition group were fabricated. The probes exhibited much stronger fluorescence when they transfer from hydrophilic environment (Φ < 8%) to hydrophobic environment (Φ > 46%). Compared with PCI-34051 (K_D = 9.16 × 10^-6 M), probes 17 (K_D = 5.37 × 10^-6 M), 18 (K_D = 3.57 × 10^-6 M) and 19 (K_D = 8.89 × 10^-6 M) possessed slightly better affinity for HDAC8. Probe 19 was selected for cell imaging and it showed significantly enhanced fluorescence only after binding into the cavity of HDAC8 in SH-SY5Y and MDA-MB-231 tumor cells. Co-localization results demonstrated that HDAC8 is expressed in cytoplasm and nucleus. Furthermore, probe 19 was successfully utilized to distinguish the expression level of HDAC8 in SH-SY5Y tumor and normal tissue slices.

Organelle-Partitioned Sugar-Rhodamine Diad for In Vivo Tumor Imaging

Current tumor imaging agents are often limited by their liability to dissipate from tumor tissues. As cell sugar sorting enables exogenous sugars to be delivered into predetermined subcellular locations, we synthesized sialic acid (Sia) derivatives with rhodamine-X conjugated at C-9 (^ROXSia), which hitchhikes cell sialic acid sorting to target tumor cell lysosomes, exhibiting pH-independent long-term probe retention in lysosomes. ^ROXSia gives selective, bright, and endured fluorescence signals in subcutaneous tumors and orthotopic tumors in mice models. These results indicate the potential of ^ROXSia as a lysosome-targeted optical agent for fluorescence-guided tumor resection.

Antibody-based imaging strategies for cancer

Although mainly developed for preclinical research and therapeutic use, antibodies have high antigen specificity, which can be used as a courier to selectively deliver a diagnostic probe or therapeutic agent to cancer. It is generally accepted that the optimal antigen for imaging will depend on both the expression in the tumor relative to normal tissue and the homogeneity of expression throughout the tumor mass and between patients. For the purpose of diagnostic imaging, novel antibodies can be developed to target antigens for disease detection, or current FDA-approved antibodies can be repurposed with the covalent addition of an imaging probe. Reuse of therapeutic antibodies for diagnostic purposes reduces translational costs since the safety profile of the antibody is well defined and the agent is already available under conditions suitable for human use. In this review, we will explore a wide range of antibodies and imaging modalities that are being translated to the clinic for cancer identification and surgical treatment.

A carbohydrate-grafted nanovesicle with activatable optical and acoustic contrasts for dual modality high performance tumor imaging

Activatable molecular systems enabling precise tumor localization are valuable for complete tumor resection. Herein, we report sialic acid-capped polymeric nanovesicles encapsulating the near infrared profluorophore (pNIR@P@SA) for lysosome activation based dual modality tumor imaging. The probe features surface-anchored sialic acid for tumor targeting and a core of near infrared profluorophore (pNIR) which undergoes lysosomal acidity triggered isomerization to give optical and optoacoustic signals upon cell internalization. Imaging studies reveal high-efficiency uptake and signal activation of pNIR@P@SA in subcutaneous tumors and millimeter-sized liver tumor foci in mice. The high tumor-to-healthy organ signal contrasts and discernment of tiny liver tumors from normal liver tissues validate the potential of pNIR@P@SA for high performance optical and optoacoustic imaging guided tumor resection.

Solid silica nanoparticles: applications in molecular imaging

Silica and silica-based nanoparticles have been widely used for therapeutic and diagnostic applications in cancer mainly through delivery of drugs, genes and contrast agents. Development of synthesis methods has provided the possibility of fabricating silica nanoparticles with different sizes in nanometer ranges as well as silica-based multimodal nanoparticles with many innovative properties and intriguing applications in biomedicine. The surface of silica particles facilitates different methods of surface modifications and allows conjugation of various biomolecules such as proteins and nucleic acids. In this review, different methods of fabrication of silica and silica-based nanoparticles, their surface modification and the application of these nanoparticles in molecular imaging are discussed. Overall, the aim of this review is to address the development of silica and silica-based multifunctional nanoparticles that are introduced mainly for molecular imaging applications using optical, magnetic (MRI), X-ray (computed tomography) and multimodal imaging techniques.

A targetable acid-responsive micellar system for signal activation based high performance surgical resolution of tumors

High-performance illumination of subcutaneous tumor and liver tumor foci at sub-millimeter levels was achieved with lectin-targeted glyco-micelles which become fluorescent upon internalization into tumor lysosomes.

Preferential accumulation of 5-aminolevulinic acid-induced protoporphyrin IX in breast cancer: a comprehensive study on six breast cell lines with varying phenotypes

We describe the potential of 5-aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) fluorescence as a source of contrast for margin detection in commonly diagnosed breast cancer subtypes. Fluorescence intensity of PpIX in untreated and ALA-treated normal mammary epithelial and breast cancer cell lines of varying estrogen receptor expression were quantitatively imaged with confocal microscopy. Percentage change in fluorescence intensity integrated over 610-700 nm (attributed to PpIX) of posttreated compared to pretreated cells showed statistically significant differences between four breast cancer and two normal mammary epithelial cell lines. However, a direct comparison of post-treatment PpIX fluorescence intensities showed no differences between breast cancer and normal mammary epithelial cell lines due to confounding effects by endogenous fluorescence from flavin adenine dinucleotide (FAD). Clinically, it is impractical to obtain pre- and post-treatment images. Thus, spectral imaging was demonstrated as a means to remove the effects of endogenous FAD fluorescence allowing for discrimination between post-treatment PpIX fluorescence of four breast cancer and two normal mammary epithelial cell lines. Fluorescence spectral imaging of ALA-treated breast cancer cells showed preferential PpIX accumulation regardless of malignant phenotype and suggests a useful contrast mechanism for discrimination of residual cancer at the surface of breast tumor margins.

Detecting epidermal growth factor receptor tumor activity in vivo during cetuximab therapy of murine gliomas

RATIONALE AND OBJECTIVES

Noninvasive molecular imaging of glioma tumor receptor activity was assessed with diagnostic in vivo fluorescence monitoring during targeted therapy. The study goals were to assess the range of use for treatment monitoring and stratification of tumor types using epidermal growth factor (EGF) receptor (EGFR) status with administration of fluorescently labeled EGF and determine its utility for tumor detection compared to magnetic resonance imaging (MRI).

MATERIALS AND METHODS

EGFR+ and EGFR- glioma tumor lines (human glioma [U251-GFP] and rat gliosarcoma [9L-GFP], respectively) were used to assess these goals, having a 20-fold difference between their EGF uptakes.

RESULTS

Treatment with cetuximab in the EGFR+ tumor-bearing animals led to decreased EGF tumor uptake, whereas for the EGFR- tumors, no change in fluorescence signal followed treatment. This diagnostic difference in EGFR expression could be used to stratify the tumor-bearing animals into groups of potential responders and nonresponders, and receiver-operating characteristic curve analysis revealed an area under the curve (AUC) of 0.92 in separating these tumors. The nonlocalized growth pattern of U251-GFP tumors resulted in detection difficulty on standard MRI, but high EGFR expression made them detectable by fluorescence imaging (AUC = 1.0). The EGFR+ U251-GFP tumor-bearing animals could be noninvasively stratified into treated and untreated groups on the basis of fluorescence intensity difference (P = .035, AUC = 0.90).

CONCLUSIONS

EGFR expression was tracked in vivo with fluorescence and determined to be of use for the stratification of EGFR+ and EGFR- tumors, the detection of EGFR+ tumors, and monitoring of molecular therapy.

Unbiased discovery of in vivo imaging probes through in vitro profiling of nanoparticle libraries

In vivo imaging reveals how proteins and cells function as part of complex regulatory networks in intact organisms, and thereby contributes to a systems-level understanding of biological processes. However, the development of novel in vivo imaging probes remains challenging. Most probes are directed against a limited number of pre-specified protein targets; cell-based screens for imaging probes have shown promise, but raise concerns over whether in vitro surrogate cell models recapitulate in vivo phenotypes. Here, we rapidly profile the in vitro binding of nanoparticle imaging probes in multiple samples of defined target vs. background cell types, using primary cell isolates. This approach selects for nanoparticles that show desired targeting effects across all tested members of a class of cells, and decreases the likelihood that an idiosyncratic cell line will unduly skew screening results. To adjust for multiple hypothesis testing, we use permutation methods to identify nanoparticles that best differentiate between the target and background cell classes. (This approach is conceptually analogous to one used for high-dimensionality datasets of genome-wide gene expression, e.g. to identify gene expression signatures that discriminate subclasses of cancer.) We apply this approach to the identification of nanoparticle imaging probes that bind endothelial cells, and validate our in vitro findings in human arterial samples, and by in vivo intravital microscopy in mice. Overall, this work presents a generalizable approach to the unbiased discovery of in vivo imaging probes, and may guide the further development of novel endothelial imaging probes.

Multifunctional Nanoparticles as Biocompatible Targeted Probes for Human Cancer Diagnosis and Therapy

The use of nanoparticles in biological application has been rapidly advancing toward practical applications in human cancer diagnosis and therapy. Upon linking the nanoparticles with biomolecules, they can be used to locate cancerous area as well as for traceable drug delivery with high affinity and specificity. In this review, we discuss the engineering of multifunctional nanoparticle probes and their use in bioimaging and nanomedicine.

Multifunctional Particles: Magnetic Nanocrystals and Gold Nanorods Coated with Fluorescent Dye-Doped Silica Shells

Multifunctional colloidal core-shell nanoparticles of magnetic nanocrystals (of iron oxide or FePt) or gold nanorods encapsulated in silica shells doped with the fluorescent dye, Tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate (Rubpy) were synthesized. The as-prepared magnetic nanocrystals are initially hydrophobic and were coated with silica using a microemulsion approach, while the as-prepared gold nanorods are hydrophilic and were coated with silica using a Stöber-type of process. Each approach yielded monodisperse nanoparticles with uniform fluorescent dye-doped silica shells. These colloidal heterostructures have the potential to be used as dual-purpose tags-exhibiting a fluorescent signal that could be combined with either dark-field optical contrast (in the case of the gold nanorods), or enhanced contrast in magnetic resonance images (in the case of magnetic nanocrystal cores). The optical and magnetic properties of the fluorescent silica-coated gold nanorods and magnetic nanocrystals are reported.

Multimodality imaging of the HER-kinase axis in cancer

The human epidermal growth factor receptor (HER) family of receptor tyrosine kinases controls critical pathways involved in epithelial cell differentiation, growth, division, and motility. Alterations and disruptions in the function of the HER-kinase axis can lead to malignancy. Many therapeutic agents targeting the HER-kinase axis are approved for clinical use or are in preclinical/clinical development. The ability to quantitatively image the HER-kinase axis in a noninvasive manner can aid in lesion detection, patient stratification, new drug development/validation, dose optimization, and treatment monitoring. This review summarizes the current status in multimodality imaging of the HER-kinase axis using PET, SPECT, optical, and MR imaging. The targeting ligands used include small-molecule tyrosine kinase inhibitors, peptides, proteins, antibodies, and engineered antibody fragments. EGFR and HER2 imaging have been well documented in the past, and imaging of HER3, HER4, HER heterodimers, and HER-kinase mutants deserves significant research effort in the future. Successful development of new HER-kinase-targeted imaging agents with optimal in vivo stability, targeting efficacy, and desirable pharmacokinetics for clinical translation will enable maximum benefit in cancer patient management.