Imaging tissue derived biomarkers

Imaging tissue derived biomarkers

1. Biomarkers defintion

   • Biomarkers can be described as measurable and quantifiable indicators of normal physiological function, disease states or of the body's response to therapeutics.
   • The Biomarkers Definition Working Group (2001) classified five different types of biomarker based on function
     • diagnostic
     • Prognostic
     • Predictive
     • Staging
     • Surrogate endpoints
   • Biomarkers are present in bodily fluids
     • Blood and blood fractions (serum, plasma, buffy coat)
     • Urine
     • saliva
     • Tissues
   • National Cancer Institute defines a tumour marker as a substance that may be found in tumour tissue or released from a tumour into the blood or other body fluids
   • Imaging derived biomarkers
     • Aim: to predict metastatic spread of tumours
       • which causes the vast majority of cancer related death
     • the primary tumour is examined for biomarkers which could predict the metastatic event occurring many years later

2. Overview

   • starting point for imaging process: Tissue microarrays
     • take small pieces of tumour tissue from individual patients and look for expression of markers across hundreds of tumour specimens simultaneously
   • In vivo imaging used to monitor tumour growth or the response of tumour to treatment
   • New imaging technologies are increasingly being used to understand the in vivo behaviour of cancers
   • Omic approaches provide and overview of biomarkers in cancer
     • large bottleneck in converting these candidate biomarkers into clinical use
   • Imaging has the ability to take this omic-generated information further, allowing us to see the actual activity and location of the biomarkers in vivo.

3. Antibody based proteomics (in vitro)

   • forefront of omic technologies
   • Antibodies are useful tools for quantifying protein expression and detecting antigens for biomarkers
   • using bioinformatics, genes of interest are identified and then antibodies are used to profile tissue
     • s is then combined with vision approaches to develop an assay
   • Antibodies form the basis of tissue microarrays which area platform for high throughput pathology
     • sections of FFPE taken
       • looked at under microscope to confirm tissue is cancerous
         • additional staining is performed to see if the tissue is expressing particular proteins
           • Tiny cores are taken from all the tumour samples
             • put on master block
               • RNA expression analysis is examined with respect to its morphology
     • Digital pathology used to help this system
       • major advance from the highly subjective and time consuming method of viewing slides under a microscope
       • takes a slide, scans it and creates a digital image
       • uses computer based vision approach rather than relying on pathologist to manually examine it

4. moves being made toward an in vivo approach

   • advances in clinical imaging are improving how cancer is understood at a systems level and enable doctors to not only locate a tumour but examine its biological processes
   • Imaging systems grouped by
     • energy used to derive the visual information
       • X-rays
       • positrons
       • photons
       • sound waves
     • spatial resolution obtained
       • macroscopic
         • widespread use of macroscopic imaging systems that provide anatomical and physiological information
           • Computed Tomography (CT)
           • Magnetic resonance imaging (MRI)
           • ultrasound
       • microscopic
     • type of information obtained
       • anatomical
       • physiological
       • cellular
       • molecular
         • systems that obtain molecular information are only beginning to be used clinically
           • positron emission tomography (PET)
           • single-photon emission CT (SPECT)
           • fluorescence based imaging

5. nuclear imaging

   • based on administration and detection of decaying radioisotopes in vivo.
   • These radioisotopes combine with biologically active compounds to form radiopharmaceuticals which target specific molecular events
   • Decaying radioisotopes emit a positron or gamma ray which produce high-energy photons
     • can be detected using PET or SPECT
       • PET
         • a contrast agent or tracer based imaging method
         • most commonly used tracer is 18F-FDG (flurodeoxyglucose)
           • reflects glucose uptake and metabolism
         • disadvantage:
           • PET tracer agents typically have a short half life
             • because of this there is a requirement for scanners to be located close to a cyclotron

6. optical imaging

   • based on photons travelling through tissue and their interactions
   • fluorescence based
     • fluorescence refers to the property of certain molecules to absorb light at a specific wavelength and emit light of a longer wavelength after a brief interval
     • discovered in the 19th century
       • first application was in 1924 when the autofluorescence of endogenous porphyrins were observed in tumour illuminated with ultraviolet light
     • macroscopic
       • systems rely on photographic principles to collect images in low light
     • microscopic
       • used to examine the activity of cells in biological settings including tumours
       • possible to analyse multiple cell types at the same time and in solid tissues
       • Examples
         • Multiphoton microscopy imaging systems
           • yield 3D information from light emitted by fluorescently labelled objects
         • Intravital multiphoton microscopy
           • derives quantitative parameters of intravascular and interstitial cell migration
           • cells investigated in lymph nodes, cranial bone marrow, and organs harbouring orthotopic cancers
     • Tomographic fluorescence systems
       • reconstruct 3D maps of fluorochromes on the basis of algorithms
       • enable fluorescent proteins or genetically modified cells to be tracked in vivo
   • non-fluorescence based

7. In vivo techniques can also be applied to the anti-cancer drug development process

   • Photodynamic therapy is a treatment where a fluorescent photosensitiser (e.g. ADPM) is administered to the patient
   • The drug then preferentially accumulates in tumour cells where it can be illuminated with a light so that it becomes toxic to targeted malignant cells
   • this process has the advantage of only harming toxic cells so it has a low mutagenic potential
   • Example: phototrin