Molecular imaging is increasingly allowing physicians to:
- characterize a tumor’s biological properties and make the selection of a therapeutic regimen based on those characteristics as well as the unique biologic characteristics of the patient
- determine a patient’s response to specific drugs and accurately assess the effectiveness of a treatment regimen, early in the course of treatment
- adapt treatment plans quickly in response to changes in cellular activity.
Novel radiotracers using in molecular and nuclear imaging are being developed to image critical cancer processes including:
- cell death
- tumor proliferation, for example, F-18 fluorothymidine (FLT) looks at cellular growth and proliferation
- angiogenesis, the growth of new blood vessels
- hypoxia, or decreased oxygen supply, for example, F-18 fluoromisonidazole (FMISO) is a marker of tumor hypoxia, being tested in clinical trials.
These agents can also be used to select and evaluate targeted therapies, such as those designed to target hypoxia, angiogenesis, chemotherapeutic resistance and others.
Molecular Imaging Biomarker Use in Oncology
Molecular imaging will likely become established as a tool for measuring biomarkers, various substances present int he body that indicate a disease state. Trials using a variety of molecular imaging techniques to validate this function are underway.
Potential uses of molecular imaging biomarkers include:
- surrogate endpoints—biomarker intended as a substitute for a clinical endpoint, such as tumor shrinkage.
- prognostic biomarkers—used prior to treatment to predict negative and positive results, e.g., hypoxic status of tumor.
- predictive biomarkers—used to assess the effect of drug treatment, e.g., FDG-PET in lymphoma.
The potential benefits of imaging biomarkers in clinical trials include:
- determining if a patient’s tumor is likely to respond to specific treatments
- assessing after one or two treatments if a tumor is dying, even if it is not shrinking in size
- determining which patients are at high risk of tumor recurrence
- evaluating whether an experimental therapy is effective for tumor treatment.
Reporter-probe pairs are also being developed for molecular-genetic imaging. A reporter gene is a gene whose product can be readily detected and either fused to the gene of interest or replace by it. The main applications for these reporters include monitoring gene expression levels, investigating dynamic molecular interactions between proteins, studying cellular interactions, tracking cells in normal and abnormal development or in cell transplantation therapy and monitoring gene replacement therapy. Optical reporter genes are the most commonly used and widely developed for imaging. A new emerging class of reporter genes encodes for proteins with an affinity for radioisotopes or positron emitter probes.
Finally, agents are also being developed that can be used concurrently as diagnostic and therapeutic agents, an emerging field called ‘theranostics.’
Antigen-Specific Therapy
As the fields of molecular biology and genomics advance, tumor properties and pathways will be revealed, leading to a set of new cancer specific targets. Molecular radiotherapies can be developed to target tumor cells based on these advances.
- Targeting CD20 in non-Hodgkin’s lymphoma is an example of antigen-specific therapy.
- Molecular radiotherapy has therapeutic potential for other types of cancers including bone metastases, prostate, metastatic melanoma, ovarian, leukemia, high-grade brain tumors, metastatic colorectal cancer and neoplastic meningitis.
Nanomedicine
Cancer cells have unique properties that can be exploited by nanoparticles. Guided by and/or in conjunction with molecular imaging technologies, nanoparticles can be targeted at cancer cells to:
- detect and monitor disease
- deliver drugs, such as chemotherapy and gene therapy
- treat through ablation.