Molecular imaging has both research and clinical applications. One clear research application has been in the development of gene therapies where molecular imaging can confirm the success of gene transfection, determine whether transfection is localised to the target organ and assess whether the level of transgene expression is sufficient for therapeutic effect. Molecular imaging has also gained an important role in the development of drugs that have specific molecular targets (e.g. an anticancer drug that aims to block VEGF activity). In the early stages of drug development, molecular imaging can confirm the mechanism of action (i.e. proof-of-principle) and assess the duration and magnitude of effect for specific drug dosages. In later phase trials, molecular imaging can be used as an early marker of response. Levels of expression of specific genes before therapy may also determine the likelihood of subsequent response. Thus molecular
imaging could be used in drug trials to identify subpopulations of patients enriched for response, increasing the probability of demonstrating drug efficacy.
The use of molecular imaging as a response marker is gaining in interest in clinical practice, particularly in oncology where the limitations of a purely structural approach are becoming apparent. A slow tumour response, residual non-malignant masses and measurement errors are significant constraints on using size criteria to assess tumour response to therapy. Molecular processes that can provide an early marker of tumour response are those associated with glucose metabolism, angiogenesis and apoptosis (programmed cell death). In lymphoma, for example, assessing the glucose metabolic response to therapy with FDG-PET has greater accuracy than CT, and can identify non-responders earlier in the course of treatment. Angiogenesis imaging in lymphoma with contrast-enhanced CT or MRI, or conventional scintigraphy with T1 or Tc-MIBI, may emerge as less expensive and more widely available alternatives to FDG-PET that may be particularly appropriate in assessing response to anti-angiogenesis therapy. Preliminary work in follicular lymphoma suggests that the degree of apoptosis, imaged with Tc-annexin V uptake 24h after the last fraction of radiotherapy, can predict the ultimate clinical response.
The ability of molecular imaging to select patients for specific therapy and to assess therapeutic response is also gaining recognition in clinical practice. A clear illustration of the application of molecular imaging in selecting therapy is the use of In-octreotide to confirm tumour expression of somatostatin receptors before somatostatin therapy. Potentially, such approaches can be highly cost-effective by enabling the cost and morbidity of treatment to be avoided in those patients with a low likelihood of response. For example, Tc-MIBI could be used to identify patients with non-small cell lung cancers that express pgp-mediated multidrug resistance (MDR) and so fail to respond to chemotherapy. Using currently available data, it has been estimated that this approach could potentially save £964 per patient, with no loss in life expectancy. If extrapolated to a national level, the total saving in the UK would be in excess of £1.5 million per year.
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