February 1, 2010 (Vol. 30, No. 3)

Nina Flanagan

Technological Advances Provide Benefits in Drug Discovery, Dosing, and Epigenetics

In vivo imaging technology is advancing rapidly and expanding to include not only drug discovery and development but also diagnostics. Current efforts range from developing novel brain-imaging agents to further understanding the molecular basis of human behavior to engineering antibody fragments for cancer imaging biomarkers.

There is also a growing trend to combine imaging modalities, such as MRI with PET or PET with CT, allowing for additional studies. Furthermore, pharmaceutical companies are working to develop imaging agents with therapeutics. CHI’s “High Content Analysis” conference and Molecular Medicine’s “Tri Conference” both provide a glimpse into the future of this field and the promise it holds to provide a better understanding of disease.

Although in vivo imaging continues to evolve rapidly, it still harbors challenges. “The ability to see within live animals or humans with these technologies is very powerful. Driving toward real applications for studying disease or monitoring drug therapies is costly and the science and technology intensive,” stated Matt Silva, Ph.D., head of the imaging group at Millenium: The Takeda Oncology Company.

Dr. Silva is developing assays that are sensitive and accurately read-out both disease and responses to drug therapy for preclinical and translational imaging. Efforts also include evaluating new technology with the goal of developing robust applications to benefit internal research.

One of the imaging procedures that his group is currently working with is dynamic contrast enhanced magnetic resonance. This is a method to monitor blood flow and permeability within a tumor, and it is used with drugs that target vasculature.

“You can assess very early whether a patient is responding to treatment. For early-phase clinical trials, it provides information on whether the mechanism of action that’s expected results in a functional change in the tumor. For novel targets, that’s a very critical question,” he said.

In a normal experiment using this method, a scan provides a static image—with no information about how the signal changed as a function of time. According to Dr. Silva, his group has been performing kinetic analysis via rapid scanning and then extracting from the data hemodynamic parameters that reflect the microenvironment of the tumor. These changes are monitored when administering therapy. This has been shown to work in highly perfused tumors like breast tumors.

A potential challenge with this method includes signal to noise—there has to be enough contrast agent to penetrate tissues for an accurate model of perfusion. “Since tumors are heterogeneous, imaging is one of the only ways to model that heterogeneity.”

Another key area of research involves inflammation and oncology. Bone topology analysis provides information on bone erosion during disease (arthritis) and bone cancer. “We created an algorithm in 3-D that allows us to visualize bone erosion and to quantify it. What we’re trying to do preclinically is to demonstrate that an internal drug is having an effect on this model.”

According to Dr. Silva, his team has also used this algorithm to study dose and scheduling as well as provide comparison against control drugs. Another potential application is to analyze tumor structure. “We’re not sure whether this matters, but the way cancer invades tissues may be related to malignancy.” Overall, Dr. Silva summarized that his group is doing preclinical research to see if any of these imaging techniques will add value in early clinical trials. They are also focusing on molecular biology to create probes to help understand more about tumor microenvironments.

Tracking Transplanted Cells

Recent progress in cell therapy has increased demand for real-time detection of transplanted cells, which provides critical information on cell behavior in vivo to determine optimal dosage and delivery routes. Celsense is addressing these demands with its fluorine-based imaging reagents (Cell-Sense, V-Sense) and imaging software (Voxel Tracker™).

“Our approach is MRI-based, doesn’t use any ionizing radiation, and provides good contrast with no depth-limitations,” explained Eric Ahrens, Ph.D., founder and CSO. This also allows for longitudinal studies days or weeks after administration of the imaging agent.

Over the past few decades, metal-ion based contrast agents have been used to tag cells. However, Dr. Ahrens says these probes lacked specificity, making it difficult to interpret where the cells were located, and often required prescans before and after cell implantation to examine differences in contrast in that region.

Cell-Sense uses Fluorine-19, which is nontoxic and readily enters a wide variety of cells in culture (including immune and stem cells) tagging them ex vivo without requiring transfection agents. The labeled cells are inoculated into the subject and imaged with MRI.

Images of the Fluorine-19 labeled cells and proton (background anatomy) channels are acquired in the same imaging session. A composite image is constructed using Voxel Tracker software, showing the regions containing the labeled cells. Since there is no fluorine in the body, this provides unambiguous detection. Cell-Sense is now manufactured under GMP conditions suitable for human use, and the company has opened a Drug Master File with the FDA for use in clinical trials.

Another key use for Cell-Sense is to allow the tagged cells to circulate and then assay the biodistribution without histology, which “is very often a bottleneck in preclinical studies,” according to Dr. Ahrens. A panel of tissues is run through a conventional liquid NMR, and the number of tagged cells in the tissue sample calculated.

“This is a high-throughput screening technique for visualizing biodistribution of the cells throughout the organism.” A dual-mode version of Cell-Sense is also available that can be detected by MRI and conventional fluorescence-based techniques such as confocal microscopy or flow cytometry. This is useful for validation studies.

V-Sense, a fluorine-based reagent for direct in situ labeling of monocytes and macrophages, enables visualization of hot spots of inflammation. Preclinical applications include autoimmune disease, cancer, coronary artery disease, infectious diseases, injury, and organ/tissue transplant rejection.

In vivo MRI of 19F-labeled dendritic cells in a mouse (Celsense)

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