01Start by locating the target

A target may be outside a cell, accessible at its surface or located in an intracellular compartment. These locations pose different access questions. A cell-surface receptor interaction does not automatically require the ligand to enter the cytosol; an intracellular target raises a different requirement.

Write down the proposed target location before reading a delivery claim. Then ask which compartment the measurement actually observes. This prevents a generic phrase such as enters cells from standing in for the more precise question of whether the molecule reaches the target.

02Distinguish added quantity from local exposure

In whole-organism pharmacology, distribution concerns movement between the bloodstream and tissues. Molecular properties, protein binding and tissue conditions can influence that distribution. A concentration measured in blood is therefore not automatically the concentration at every target site.IUPHAR — Drug distribution (opens in a new tab)

The same reading discipline applies to a simplified assay. A nominal amount added to the surrounding medium is one recorded input. The amount available at a particular target is a different quantity that may need its own evidence. Do not fill that second field merely by copying the first.

An experiment can still be valuable without measuring every compartment. Its conclusion simply needs to remain compatible with what was measured and what was assumed about exposure.

03Cell association is not the same as cytosolic access

Teo and colleagues developed the SLEEQ assay to examine cytosolic delivery. In their tested peptide–protein systems, increased cell association could account for enhanced cytosolic delivery without an improvement in endosomal escape efficiency. The study distinguishes steps that a broad uptake claim might combine.Teo and colleagues — Quantifying cytosolic delivery and endosomal escape (opens in a new tab)

This matters because material can be associated with a cell or contained within a membrane-bound compartment without being available throughout the cytosol. A positive whole-cell signal should therefore be interpreted according to the method's ability to resolve location.Teo and colleagues — Quantifying cytosolic delivery and endosomal escape (opens in a new tab)

The finding is not a universal ranking of cell-penetrating peptides. Its value for a reader is the separate accounting of association, uptake and escape in a defined system, with a defined cargo and readout.

04A missing response leaves several questions open

Imagine a hypothetical peptide that binds a purified target but gives no detectable response in intact cells. Possibilities to investigate include access to the target, integrity of the material, target availability and whether the selected endpoint captures the expected event. The two experiments do not isolate those possibilities by themselves.

Conversely, a response in cells does not automatically prove the proposed intracellular interaction. The study still needs evidence connecting the measured effect to the target. Exposure evidence and mechanism evidence support different links in the explanation.

05Summarise delivery at the level actually demonstrated

• ✓Identify the target compartment and molecular form studied.
• ✓Distinguish nominal input from measured local exposure.
• ✓Name the delivery endpoint: association, uptake, escape or target engagement.
• ✓Keep the cargo, cell model and sampling time attached to the finding.

This vocabulary makes delivery studies easier to compare without turning them into administration guidance. It also prevents a result from one peptide–cargo combination from becoming an unsupported claim about every form of that peptide or a catalogue product.