In this short article, we highlight recent advancements in optical preclinical imaging and assess the current state of the art in the field. In addition, and by referencing work done using the latest system from Bruker, we identify new potential for demonstrating and measuring biological processes in small animal models
In drug discovery and development, preclinical in vivo molecular imaging is considered to be an essential tool. It provides researchers with clear visibility of cellular changes at a molecular level. Imaging approaches such as optical positron emission tomography (PET) and single-photon emission computed tomography (SPECT) bring high specificity and wide applicability, enabling numerous molecular events to be monitored and key markers to be identified.
These techniques feed our understanding of disease progression, as well as revealing the mode of action and pharmacokinetics of potential therapeutics.
Considering small animal optical imaging systems in particular, combining multiple imaging modalities in a single instrument gives access to valuable information about physiological and disease mechanisms in the preclinical setting. For example, five imaging modalities, including bioluminescence, multispectral VIS-NIR fluorescence, direct radioisotopic imaging, Cerenkov radiation and high-speed digital X-ray, are provided as standard within the latest Bruker system (Xtreme II), supplying functional images that allow for the coregistration of molecular events with tissue or organ morphology.
In the early 2000s (Figure 1), commercial multimodal small animal imaging systems became affordable for the pharma research community. Even early systems combined luminescent, fluorescent and/or radioisotopic imaging with X-ray functionality to deliver a more inclusive representation of biological processes. Data accessibility and ease of use was subsequently improved with the introduction of automation, alongside other software developments, allowing labs to increase throughput and operate more efficiently.
Figure 1: Timeline depicting the development of multimodal imaging systems from the early 2000s up until the present day
More recent advances in system technologies, such as geometrical magnification and back-illumination for multimodal X-ray and optical imaging are improving the resolution and sensitivity that researchers are able to work with, and enabling them to successfully track cells and monitor physiological changes and disease development — all fundamental sources of data in a preclinical setting.
Individual modalities carry unique functionality and, when used in combination, provide researchers with better insight, awareness and understanding of a range of biological processes. Bioluminescence (BLI), and fluorescence (FLI) are the two most commonly used modalities: BLI detects luciferase light emission from engineered cells and is used to monitor tumour cells, infections, disease progression and response to therapy, making it ideally suited to preclinical, longitudinal animal studies. Understanding disease progression can also be enhanced by DRI, as the increased uptake of radio isotopes can indicate specific molecular events.
Alternatively, FLI quantifies the light emission of excited fluorophores on exogenous probes, offering insight into specific biological processes, molecular events and tissue vascularity. Similar to BLI, but only recently implemented in preclinical imaging, CLI is effective in monitoring therapeutic efficacy by detecting light emitted from radioactive isotopes.
Finally, X-ray is most usefully employed to provide exact anatomical orientation for molecular imaging. Resulting images from all modalities can be layered using intelligent software to ensure accurate coregistration, ultimately giving researchers a greater understanding of molecular mechanisms and interactions of interest, twinned with specific anatomical reference.
The development of the latest multimodal instruments is set to transform research practice further, and one such system combines all five imaging modalities, which are applied consecutively. The advanced technology and tools that have continued to emerge are addressing issues associated with detection limits and cross-modal functionality; enhanced camera capabilities deliver new levels of sensitivity, speed and flexibility, and the latest cross-platform software allows simple, fast and automated transfer between techniques.
Researchers have the tools to uncover new biological mechanisms in disease and to inform best practice in treatment and monitoring. The introduction of system-compatible multimodal animal beds also guarantees accuracy during cross-platform studies, ensuring exact positioning with little need to disturb the animal and facilitating the precise layering of imagery to improve insight in the investigative phase that follows.
With five imaging modalities in one instrument, the Xtreme II allows researchers to leverage the individual strength of each while combining them to obtain both anatomical and functional information
Optical imaging technology combinations are being most effectively applied in preclinical oncology research, strengthening the options for in vivo visualisation of cancer-related processes such as tumour progression and the mode of action of cancer therapeutics with time. BLI is suited to longitudinal studies addressing tumour growth, as there is a strong positive correlation between the BLI signal and actual tumour burden (confirmed by tumour histology).
Interestingly, direct radioisotopic imaging (DRI) can be used for the study of malignant tumour development. The accumulation of mutations, possibly related to apoptosis regulation, cell adhesion molecule expression and angiogenic/metastatic potential, are thought to produce unique molecular markers that are visualised in preclinical in vivo studies by the use of probes.
Optical in vivo imaging can also be important in the study of therapeutic response; again, BLI is engaged in the evaluation of novel therapeutic agents, whereas FLI has been widely used to track the biodistribution of traditional drug compounds and candidate drug delivery vehicles. The technique is making strong ground, impacting significantly on the areas it touches, and promises further potential as the preclinical research field continues to move forward.
Preclinical research is under more pressure than ever to accurately inform drug development decisions and assess delivery choices as the pharmaceutical industry continues to drive down costs and looks to minimise time-to-market for new products. At the forefront of this work is imaging, a set of tools that is delivering increasingly valuable insights into disease mechanisms and progression, and informing therapeutic development in a number of clinical areas, particularly progressive diseases.
Technological advances are assisting the translation of preclinical research into the clinical situation, facilitating accurate analyses and access to useful data for higher test throughput. Looking ahead, these innovations will continue to place more powerful data in the hands of researchers, ultimately accelerating drug development and informing clinical best practice.