Kierkegaard observed that one of humanity’s enduring tensions is that while life can only be understood backwards, it must be lived forwards. It’s a truth medicine knows intimately: in the treatment that worked until it didn’t, the resistance that arrived without warning, the moment a doctor has to tell a patient that the drug that was helping has stopped. Not because anyone made a mistake, but because the critical knowledge that would have mattered arrived too late, if at all.

A recent paper from the National Cancer Institute is, in a small but meaningful way, science’s pursuit of that elusive foresight: an understanding that emerges early enough, for once, to change what happens next.

The Elegant Idea

For decades, chemotherapy has worked by brute force, flooding the body with toxins designed to kill rapidly dividing cells. The problem is that rapid division isn’t unique to cancer. Hair follicle cells, gut lining cells and immune cells also divide rapidly, which is why patients lose hair, lose energy and become susceptible to infection. Chemotherapy targets a behavior, but the drug has no way to tell a healthy cell from a cancerous one.

Antibody-drug conjugates (ADCs) change that. Instead of targeting what cancer cells do, they target what cancer cells are. Cancer cells tend to display certain proteins on their surface in far greater numbers than healthy cells do. The antibody is engineered to seek out those proteins specifically. It navigates to its target, binds and waits for the cell to do what cells routinely do: pull it inside. Once there, the cell’s own digestive machinery (the lysosome) breaks down the chemical tether holding the toxin to the antibody, releasing the toxin to kill the cell from within. More than a dozen ADCs have received FDA approval in recent years, and the field is evolving fast.

What the Cell Does Next

But cancer cells don’t simply accept their fate. Even when an ADC delivers its payload perfectly—the antibody finds its target, the cell pulls it inside, the lysosome cuts the tether—a pump embedded in the cell membrane can grab the released toxin and throw it back out before it causes damage.

The delivery worked. The package got ejected anyway.

These pumps—ATP-binding cassette transporters, or more plainly, efflux pumps—are a normal feature of cell biology. Their job is cellular housekeeping, clearing out unwanted or toxic substances before they cause damage. Under the pressure of drug treatment, cancer cells do what life has always done under pressure: the ones best equipped to survive do. The same mechanism that has shaped living things for billions of years now works against the treatment. Not all cancer cells are identical, and the ones that happen to produce more pumps survive while others don’t, gradually shifting the tumor toward resistance.