The role of molecular imaging in modern drug development Lídia Cunha 1 , Krisztián Szigeti 2 , Domokos Mathé 3 , Luís F. Metello 1, 4 1 Nuclear Medicine Department, High Institute for Allied Health Technologies, Polytechnic Institute of Porto (ESTSP.IPP), Vila Nova de Gaia 4400-330, Portugal 2 Nanobiotechnology & In Vivo Imaging Center, Semmelweis University, Budapest H-1094, Hungary 3 CROmed Ltd, H-1047 Budapest Baross u. 91-95, Budapest, Hungary 4 IsoPor, SA, Porto, Portugal Abstract Drug development represents a highly complex, inefficient and costly process. Over the past decade, the widespread use of nuclear imaging, owing to its functional and molecular nature, has proven to be a determinant in improving the efficiency in selecting the candidate drugs that should either be abandoned or moved forward into clinical trials. This helps not only with the development of safer and effective drugs but also with the shortening of time-to-market. The modern concept and future trends concerning molecular imaging will assumedly be hybrid or multimodality imaging, including combinations between high sensitivity and functional (molecular) modalities with high spatial resolution and morphological techniques. Introduction The discovery and development of new drugs is perhaps one of the most challenging activities driven by the scientific community and simultaneously one of the most expensive and time consuming. On average, the cycle of bringing a new drug to market can last between 5 and 10 years (with around 8 years being typical), costing more than US$1 billion. Drug development is a rather inefficient process, because only one in 5000 tested compounds will reach the market 3 and 4. Owing to the dimension of the involved investment, all stakeholders (pharmaceutical companies, physicians and patients) want effective drugs to be developed and become widely available at a reasonable cost. To achieve this, mechanisms of disease need to be well understood and the compounds to be tested need to be well characterized in the early phases of research, those that generally are less costly. In this review we will provide an overview of: (i) the main steps of drug development and its related challenges; as well as (ii) the role and potential of molecular imaging and how it might contribute to improving the overall efficiency of the process. Stages of drug development