Macromolecular drugs inefficiently cross membranes to reach their cytosolic targets. They require drug delivery vectors to facilitate their translocation across the plasma membrane or escape from endosomes. Optimization of these vectors has however been hindered by the difficulty to accurately measure cytosolic arrival. We have developed an exceptionally sensitive and robust assay for the relative or absolute quantification of this step. The assay is based on benzylguanine and biotin modifications on a drug delivery vector of interest, which allow, respectively, for selective covalent capture in the cytosol with a SNAP‐tag fusion protein and for quantification at picomolar sensitivity. The assay was validated by determining the absolute numbers of cytosolic molecules for two drug delivery vectors: the B‐subunit of Shiga toxin and the cell‐penetrating peptide TAT. We expect this assay to favor delivery vector optimization and the understanding of the enigmatic translocation process.

This symposium is the third PSL (Paris Sciences & Lettres) Chemical Biology meeting (2016, 2019, 2023) held at Institut Curie. This initiative originally started at Institut de Chimie des Substances Naturelles (ICSN) in Gif‐sur‐Yvette (2013, 2014), under the directorship of Professor Max Malacria, with a strong focus on chemistry. It was then continued at the Institut Curie (2015) covering a larger scope, before becoming the official PSL Chemical Biology meeting. This latest edition was postponed twice for the reasons that we know. This has given us the opportunity to invite additional speakers of great standing. This year, Institut Curie hosted around 300 participants, including 220 on site and over 80 online. The pandemic has had, at least, the virtue of promoting online meetings, which we came to realize is not perfect but has its own merits. In particular, it enables those with restricted time and resources to take part in events and meetings, which can now accommodate unlimited participants. We apologize to all those who could not attend in person this time due to space limitation at Institut Curie.