THE STANDARD

Rodents have been the workhorse

Drug developers have long tested brain penetration in mice and rats, measuring how much of a drug ends up in brain tissue compared with blood. These studies show the whole living system at work.

In vivo tests show the full picture, but they come with limits.

THE LIMITS

Four reasons to want alternatives

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Species differences

Transporters and efflux pumps differ between rodents and humans in amount and in which drugs they recognize

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Time and cost

Studies take weeks to months and use many animals per compound

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Hard to see inside

It is difficult to change one variable, such as flow or one cell type, and watch single cells

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Ethics

Researchers aim to reduce, refine, and replace animal use, known as the 3Rs

An animal is a whole system, which is also what makes it hard to dissect.

A REPLACEMENT'S JOB

What a better model must do

Human cells Carry the human versions of transporters, receptors, and pumps

Control Change one variable at a time: flow, cell type, or drug

Throughput Test many compounds quickly and cheaply

Realism Behave enough like the living barrier to be trusted

Real, controllable, scalable: very few models manage all three.

THE LADDER

From dish to organism

Each step up the ladder adds realism and cost. Chips aim for a sweet spot: human cells, controlled flow, and far less effort than an animal study.

A chip sits between a dish and a living body.

POLICY IS SHIFTING

Regulators are opening the door

In 2022 the U.S. FDA Modernization Act 2.0 removed the requirement that new drugs be tested in animals first, allowing cell-based assays, organ-chips, and computer models to substitute where appropriate.

Alternatives still have to be validated, which is why good measurements matter.

UNIT 6 STUDY COMPLETE

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Ready for the Fab Challenge?

You've weighed what animal studies offer, where they fall short, and what a replacement has to do.