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An increasing number of studies demonstrate the ability of peptides to cross the blood-brain barrier (BBB), opening perspectives for a new class of therapeutics for central nervous system diseases. However, information on the BBB transport of peptides suffer from a wide variety in used methods and experimental set-up. Therefore, it is currently difficult, if not impossible, to classify peptides according to their BBB influx characteristics. To allow direct comparison of BBB influx results of peptides, we introduce a classification method and unified response for BBB influx transport of peptides. First, the results of BBB influx response types (i.e. Kin (MTR), Kin (Perfusion), Pin vitro and Pin vivo), which quantitatively express brain influx, were classified into five classes of BBB influx magnitude based on the distribution of these results for the individual response types. Then, these classes were converted to a BBBin-response, representing a scaled value ranging from zero (no influx) to ten (high influx), independent from the BBB influx response type from which it was derived. This unified response can immediately be applied for new BBB influx results of peptides and represents a ballpark figure for BBB influx and allows direct comparison and ranking of peptides independent of the response type.

Peptides are able to cross the blood-brain barrier (BBB) through various mechanisms, opening new diagnostic and therapeutic avenues. However, their BBB transport data are scattered in the literature over different disciplines, using different methodologies reporting different influx or efflux aspects. Therefore, a comprehensive BBB peptide database (Brainpeps) was constructed to collect the BBB data available in the literature. Brainpeps currently contains BBB transport information with positive as well as negative results. The database is a useful tool to prioritize peptide choices for evaluating different BBB responses or studying quantitative structure-property (BBB behaviour) relationships of peptides. Because a multitude of methods have been used to assess the BBB behaviour of compounds, we classified these methods and their responses. Moreover, the relationships between the different BBB transport methods have been clarified and visualized.