Nikolaos Zafiropoulos

Kallikreins are proteolytic enzymes that constitute a subfamily of serine proteases. Novel kallikrein genes were cloned recently, and it was shown that the human kallikrein family contains 15 genes tandemly aligned on chromosomal locus 19q13.3-q13.4. Based on their altered expression in tumor cells, kallikreins may be involved in the pathogenesis and/or progression of cancer. Evidence is presented that certain kallikreins may be exploited as diagnostic cancer biomarkers. Although the function(s) of novel kallikreins is currently unknown, increasing evidence suggests that kallikreins may participate in regulatory enzymatic cascade(s). Elucidation of the function of novel kallikreins largely depends on the availability of active recombinant proteins. Here, the zymogen for kallikrein 13 was overexpressed in Pichia pastoris and biochemically characterized. It was shown that the kallikrein 13 zymogen displays intrinsic catalytic activity leading to autoactivation. A clipped form of kallikrein 13 was identified, indicating autocatalytic cleavage at the internal bond R114-S115. Mature kallikrein 13 displays trypsin-like activity with restricted specificity on synthetic and protein substrates. Combinatorial P1-Lys libraries of tetrapeptide fluorogenic substrates were synthesized and used for the profiling of the P2 specificity of selected kallikreins. Interestingly, it was shown that human kallikrein 13, similarly to PSA, could specifically cleave human plasminogen to generate angiostatin-like fragments, suggesting that specific kallikreins may have antiangiogenic actions. An understanding of the physiology of human kallikreins is emerging with potential clinical applications.

In order for a medicinal product to get marketing authorization in Europe, it has to demonstrate a positive benefit-risk balance. Although this has been the cornerstone of the evaluation process, there is no standardised methodology that is used in this context. Recognising the need for a structured approach that can enhance the transparency and consistency in assessing the benefit-risk balance, the European Medicines Agency (EMA) began a three-year project in early 2009. This project consists of five consecutive work packages. The first four work packages form a research phase that aims to develop and test tools and methods for balancing benefits and risks of medicinal products. The fifth work package is intended for training and initial implementation. Currently, the project has completed its research phase, which recognised the usefulness of two levels of gradation. The first level is a qualitative approach, mainly consisting of a table listing the key effects of the benefit-risk balance and their uncertainty in a common and concise format. The second level, recommended for more complex situations, is a quantitative method that utilises Multi-Criteria Decision Analysis (MCDA) to derive a numerical value for the benefit-risk balance. However, the implementation of MCDA in the assessment of a medicine poses some practical challenges that remain to be addressed in the last work package of the project.