Vesicle Assay or Cell-Based Assay? — Choosing a Format for ABC Transporter Work
Both formats give you P-gp and BCRP data. One isolates the transporter, the other keeps it in a living barrier. Passive permeability decides which one will actually answer your question.
What each format measures
Inverted membrane vesicles
Membrane fragments from cells overexpressing one transporter, sealed with the intracellular face outward. In that orientation the transporter pumps substrate into the vesicle.
You measure accumulation inside the vesicle, ATP-dependently. One transporter, no metabolism, no other transport routes.
Polarised cell monolayer
Caco-2, MDCK-MDR1 or a comparable transfected line grown to a confluent monolayer on a permeable support, with an apical and a basolateral compartment.
You measure transport across the barrier in both directions and calculate the efflux ratio.
The efflux ratio, briefly
Apparent permeability is measured basolateral-to-apical and apical-to-basolateral. The ratio of the two is the efflux ratio. A ratio meaningfully above unity that falls when a specific inhibitor is added indicates active efflux — that is the standard evidence for substrate status.
Side by side
| Membrane vesicles | Cell monolayer | |
|---|---|---|
| Readout | Accumulation inside the vesicle | Efflux ratio across the barrier |
| Transporters present | One | The transfected one plus endogenous |
| Attribution of effect | Unambiguous | Requires inhibitor controls |
| Metabolism | None | Possible |
| Passive permeability | Works against you | Part of the measurement |
| Inhibition (IC50) | Well suited | Possible, less clean |
| Substrate assessment | Only for low-permeability compounds | Standard approach |
| Preparation time | Assay-ready | Days of culture to a qualified monolayer |
| Throughput | Higher | Lower |
The deciding factor: passive permeability
This single property determines whether the vesicular format can answer a substrate question at all.
Vesicles are small, and their membrane is a lipid bilayer. A lipophilic, highly permeable compound that the transporter pumps into a vesicle will diffuse straight back out before you can capture and measure it. The compound is a genuine substrate; the assay reports nothing.
A cell monolayer does not have this problem, because the readout is a ratio. Passive permeability contributes to both directions and largely cancels — what remains visible is the directional component contributed by active efflux.
What the cell-based format costs you
Physiological relevance is bought with interpretability.
- Other transporters are present. Caco-2 cells express a range of endogenous transporters. An efflux ratio tells you that something is exporting your compound — not necessarily which protein.
- Metabolism happens. Living cells metabolise. What appears in the receiver compartment may not be what you added.
- Monolayer integrity has to be proven. Every plate needs a barrier check — transepithelial resistance or a paracellular marker — or the numbers mean nothing.
- Time. Caco-2 monolayers need extended culture before they are usable. Vesicles are ready when you take them out of the freezer.
- Non-specific binding. Lipophilic compounds adsorb to plastic and to the insert membrane, which shows up as apparent loss.
What the vesicular format costs you
- No physiological context. An isolated transporter in an inside-out membrane fragment is not a cell. The number is clean, but it is a mechanistic number.
- No polarity. Nothing about apical versus basolateral behaviour can be observed.
- Substrate assessment is restricted to compounds the vesicle can retain.
- Uptake transporters are out of scope. OATP, OAT and OCT move substrate across an intact membrane along gradients — that requires cells. See our page on ciPTEC for renal uptake transporters.
How to combine them
In most well-designed cascades the question is not which one, but in what order.
| Question | Format |
|---|---|
| Does my compound inhibit P-gp or BCRP, and how strongly? | Vesicles |
| IC50 for a DDI submission | Vesicles |
| Is my compound a P-gp substrate? | Cell monolayer |
| Substrate status, low-permeability compound | Vesicles are viable |
| BSEP and hepatotoxicity risk | Vesicles — see BSEP and DILI |
| Intestinal absorption modelling | Caco-2 |
| Blood–brain barrier penetration | MDCK-MDR1 |
| Renal uptake transporters | Cell-based — ciPTEC |
| Which transporter is responsible for an observed effect? | Vesicles, one transporter at a time |
A common and defensible sequence: screen inhibition in vesicles, establish substrate status in a monolayer, and return to vesicles to attribute any signal to a specific transporter.
What regulators accept
ICH M12 (2023) harmonises FDA and EMA requirements for transporter-mediated drug interactions and expects P-gp and BCRP assessment for new molecular entities. It does not mandate a format.
In practice, submitted IC50 values come predominantly from vesicular assays, while substrate determinations come from cell-based systems. That split follows the strengths of each format rather than any rule — and a package containing both is rarely questioned on methodology.
Frequently asked questions
Can I determine substrate status in a vesicular assay?
Only for compounds with low to moderate passive permeability. A highly permeable compound diffuses back out of the vesicle before it can be measured, and the assay returns a false negative.
Why is the efflux ratio not affected by passive permeability?
Because it is a ratio of two directions. Passive diffusion contributes to both and largely cancels out. What remains is the directional component from active efflux.
Which cell line for which barrier?
Caco-2 for intestinal absorption, MDCK-MDR1 for blood–brain barrier questions and for cleaner P-gp attribution. Caco-2 is more physiological; MDCK-MDR1 is easier to interpret because the transfected transporter dominates.
Can I get an IC50 from a cell-based assay?
Yes, but attribution is harder because several transporters may contribute. For a defensible single-transporter IC50, vesicles are the cleaner route.
Why can uptake transporters not be studied in vesicles?
OATP, OAT and OCT move substrate across an intact membrane driven by electrochemical gradients rather than direct ATP hydrolysis. Inverted vesicles are built for ATP-dependent efflux transporters. Uptake work needs cells.
Do the two formats give comparable numbers?
Not directly. They measure different things — accumulation versus directional flux — under different conditions. Compare within a format, not across.
Which format is faster?
Vesicles, by a wide margin. They are assay-ready on thawing, whereas a qualified Caco-2 monolayer takes days of culture before the first compound can be tested.
Related
We supply both sides of this comparison
Vesicle kits for efflux transporters, ciPTEC for renal uptake — so we have no reason to push you toward one format. Describe the question and we will say which one answers it.
Request a quote
English 
Deutsch