Design and Development of Fluorescent Vemurafenib Analogs for In Vivo Imaging

A cyanine-based NIR fluorescent Vemurafenib analog to probe BRAFV600E in cancer cells

BRAF represents one of the most frequently mutated protein kinase genes and BRAF^V600E mutation may be found in many types of cancer, including hairy cell leukemia (HCL), anaplastic thyroid cancer (ATC), colorectal cancer and melanoma. Herein, a fluorescent probe, based on the structure of the highly specific BRAF^V600E inhibitor Vemurafenib (Vem, 1) and featuring the NIR fluorophore cyanine-5 (Cy5), was straightforwardly synthesized and characterized (Vem-L-Cy5, 3), showing promising spectroscopic properties. Biological validation in BRAF^V600E-mutated cancer cells evidenced the ability of 3 to penetrate inside the cells, specifically binding to its elective target BRAF^V600E with high affinity, and inhibiting MEK phosphorylation and cell growth with a potency comparable to that of native Vem 1. Taken together, these data highlight Vem-L-Cy5 3 as a useful tool to probe BRAF^V600E mutation in cancer cells, and suitable to acquire precious insights for future developments of more informed BRAF inhibitors-centered therapeutic strategies.

Fluorescent kinase inhibitors as probes in cancer

Fluorescent dyes attached to kinase inhibitors (KIs) can be used to probe kinases in vitro, in cells, and in vivo. Ideal characteristics of the dyes vary with their intended applications. Fluorophores used in vitro may inform on kinase active site environments, hence the dyes used should be small and have minimal impact on modes of binding. These probes may have short wavelength emissions since blue fluorophores are perfectly adequate in this context. Thus, for instance, KI fragments that mimic nucleobases may be modified to be fluorescent with minimal perturbation to the kinase inhibitor structure. However, progressively larger dyes, that emit at longer wavelengths, are required for cellular and in vivo work. In cells, it is necessary to have emissions above autofluorescence of biomolecules, and near infrared dyes are needed to enable excitation and observation through tissue in vivo. This review is organized to describe probes intended for applications in vitro, in cells, then in vivo. The readers will observe that the probes featured tend to become larger and responsive to the near infared end of the spectrum as the review progresses. Readers may also be surprised to realize that relatively few dyes have been used for fluorophore-kinase inhibitor conjugates, and the area is open for innovations in the types of fluorophores used.

SMALL MOLECULE IMAGING AGENT FOR MUTANT KRAS G12C

Multiple potent covalent inhibitors for mutant KRAS G12C have been described and some are in clinical trials. These small molecule inhibitors potentially allow for companion imaging probe development, thereby expanding the chemical biology toolkit to investigate mutant KRAS biology. Herein, we synthesized and tested a series of fluorescent companion imaging drugs (CID) for KRAS G12C, using two scaffolds, ARS-1323 and AMG-510. We created four fluorescent derivatives of each by attaching BODIPY dyes. We found that two fluorescent derivatives (BODIPY FL and BODIPY TMR) of ARS-1323 bind mutant KRAS and can be used for biochemical binding screens. Unfortunately, these drugs could not be used as direct imaging agents in cells, likely because of non-specific membrane labeling. To circumvent this challenge, we then used a two step procedure in cancer cells where an ARS-1323 alkyne is used for target binding followed by fluorescence imaging after in situ click chemsitry with picolyl azide Alexa Fluor 647. We show that this approach can be used to image mutant KRAS G12C directly in cells. Given the current lack of mutant KRAS G12C specific antibodies, these reagents could be useful for specific fluorescence imaging.

Design and synthesis of novel fluorescently labeled analogs of vemurafenib targeting MKK4

The mitogen-activated protein kinase kinase 4 (MKK4) plays a key role in liver regeneration and is under investigation as a target for stimulating hepatocytes to increased proliferation. Therefore, new small molecules inhibiting MKK4 may represent a promising approach for treating acute and chronic liver diseases. Fluorescently labeled compounds are useful tools for high-throughput screenings of large compound libraries. Here we utilized the azaindole-based scaffold of FDA-approved BRAF inhibitor vemurafenib 1, which displays off-target activity on MKK4, as a starting point in our fluorescent compound design. Chemical variation of the scaffold and optimization led to a selection of fluorescent 5-TAMRA derivatives which possess high binding affinities on MKK4. Compound 45 represents a suitable tool compound for Fluorescence polarization assays to identify new small-molecule inhibitors of MKK4.

Imaging of Tie2 with a Fluorescently Labeled Small Molecule Affinity Ligand

The receptor tyrosine kinase inhibitor, Tie2, has significant roles in endothelial signaling and angiogenesis and is relevant in the pathophysiology of several diseases. However, there are relatively few small molecule probes available to study Tie2, making the evaluation of its activity in vivo difficult. Recently, it was discovered that the small molecule rebastinib (DCC-2036) is a potent Tie2 inhibitor. We hypothesized that fluorescent derivatives of rebastinib could be used as imaging agents for Tie2. On the basis of crystallography structures, we synthesized three fluorescent derivatives, which we then evaluated in both in vitro and in vivo assays. We found that the Rebastinib-BODIPY TMR (Reb-TMR) derivative has superior imaging characteristics in vitro, and we successfully labeled endothelial cells in vivo. We propose that this probe could be further used in in vivo applications for studying the role of Tie2 in disease.

BODIPYS and aza-BODIPY derivatives as promising fluorophores for in vivo molecular imaging and theranostic applications

Since their discovery in 1968, the BODIPYs dyes (4,4-difluoro-4-bora-3a, 4a diaza-s-indacene) have found an exponentially increasing number of applications in a large variety of scientific fields. In particular, studies reporting bioapplications of BODIPYs have increased dramatically. However, most of the time, only in vitro investigations have been reported. The in vivo potential of BODIPYs and aza-BODIPYs is more recent, but considering the number of in vivo studies with BODIPY and aza-BODIPY which have been reported in the last five years, we can now affirm that this family of fluorophores can be considered important as cyanine dyes for future in vivo and even clinical applications. This review aims to present representative examples of recent in vivo applications of BODIPYs or aza-BODIPYs, and to highlight the potential of these dyes for optical molecular imaging.

Fluorescence anisotropy imaging in drug discovery

Non-invasive measurement of drug-target engagement can provide critical insights in the molecular pharmacology of small molecule drugs. Fluorescence polarization/fluorescence anisotropy measurements are commonly employed in protein/cell screening assays. However, the expansion of such measurements to the in vivo setting has proven difficult until recently. With the advent of high-resolution fluorescence anisotropy microscopy it is now possible to perform kinetic measurements of intracellular drug distribution and target engagement in commonly used mouse models. In this review we discuss the background, current advances and future perspectives in intravital fluorescence anisotropy measurements to derive pharmacokinetic and pharmacodynamic measurements in single cells and whole organs.

Single-Cell Intravital Microscopy of Trastuzumab Quantifies Heterogeneous in vivo Kinetics

Cell-to-cell heterogeneity can substantially impact drug response, especially for monoclonal antibody (mAb) therapies that may exhibit variability in both delivery (pharmacokinetics) and action (pharmacodynamics) within solid tumors. However, it has traditionally been difficult to examine the kinetics of mAb delivery at a single-cell level and in a manner that enables controlled dissection of target-dependent and -independent behaviors. To address this issue, here we developed an in vivo confocal (intravital) microscopy approach to study single-cell mAb pharmacology in a mosaic xenograft comprising a mixture of cancer cells with variable expression of the receptor HER2. As a proof-of-principle, we applied this model to trastuzumab therapy, a HER2-targeted mAb widely used for treating breast and gastric cancer patients. Trastuzumab accumulated to a higher degree in HER2-over expressing tumor cells compared to HER2-low tumor cells (~5:1 ratio at 24 h after administration) but importantly, the majority actually accumulated in tumor-associated phagocytes. For example, 24 h after IV administration over 50% of tumoral trastuzumab was found in phagocytes whereas at 48 h it was >80%. Altogether, these results reveal the dynamics of how phagocytes influence mAb behavior in vivo, and demonstrate an application of intravital microscopy for quantitative single-cell measurement of mAb distribution and retention in tumors with heterogeneous target expression. © 2019 International Society for Advancement of Cytometry.

Peptide-modified vemurafenib-loaded liposomes for targeted inhibition of melanoma via the skin

Vemurafenib is a chemotherapeutic drug recently approved by the FDA to treat melanoma. Because the drug is usually delivered orally, the route of administration readily causes damage to major organs with limited antitumor efficacy and bioavailability. In this study, we developed a peptide-modified vemurafenib-loaded liposome for the targeted inhibition of subcutaneous melanoma via the skin. First, the peptide-modified vemurafenib-loaded liposomes (Vem-TD-Lip) were prepared and characterized with respect to the size, shape and charge; the loading efficiency of vemurafenib; and the stability. Then, the intracellular uptake of these liposomes, their limited cytotoxicity, the selective inhibition of melanoma cells harboring BRAF mutations, and the liposome permeation ability were confirmed through in vitro experiments. Finally, the safety and antitumor activity of Vem-TD-Lip were evaluated in vivo. The results showed that transdermal delivery of Vem-TD-Lip effectively targeted and inhibited subcutaneous melanoma in male mice, the administration of Vem-TD-Lip through skin was better than that through oral administration and intravenous injection in terms of reducing damage to major organs and enhancing antitumor efficacy, and the peptide TD significantly enhanced the delivery of Vem-TD-Lip across the skin. This work provides a new strategy for delivering vemurafenib to target and inhibit subcutaneous melanoma.

Measurement of drug-target engagement in live cells by two-photon fluorescence anisotropy imaging

The ability to directly image and quantify drug-target engagement and drug distribution with subcellular resolution in live cells and whole organisms is a prerequisite to establishing accurate models of the kinetics and dynamics of drug action. Such methods would thus have far-reaching applications in drug development and molecular pharmacology. We recently presented one such technique based on fluorescence anisotropy, a spectroscopic method based on polarization light analysis and capable of measuring the binding interaction between molecules. Our technique allows the direct characterization of target engagement of fluorescently labeled drugs, using fluorophores with a fluorescence lifetime larger than the rotational correlation of the bound complex. Here we describe an optimized protocol for simultaneous dual-channel two-photon fluorescence anisotropy microscopy acquisition to perform drug-target measurements. We also provide the necessary software to implement stream processing to visualize images and to calculate quantitative parameters. The assembly and characterization part of the protocol can be implemented in 1 d. Sample preparation, characterization and imaging of drug binding can be completed in 2 d. Although currently adapted to an Olympus FV1000MPE microscope, the protocol can be extended to other commercial or custom-built microscopes.