A2.4 Drug resistance

Topic

Drug resistance is the leading cause of cancer-related death: almost all advanced tumors eventually develop resistance to all available treatments, and it is this drug resistance that explains why a treatment that works initially may later cease to be effective.

This drug resistance involves three distinct layers acting on the tumor simultaneously. The first is genetic resistance: within the tumor, there are—either present before treatment begins or emerging later under the pressure exerted by the treatment itself—subpopulations of cells (clones) that already possess a mutation rendering them resistant. The second is a form of drug resistance that does not depend on mutations but rather on physical barriers within the microenvironment surrounding the tumor cells: the amount of drug that actually penetrates the tumor; whether the tumor cell possesses a membrane pump capable of expelling the drug before it can act; and the acidity level of the tumor microenvironment, which can alter the drug's mode of action. The third is phenotypic resistance: without any change to the genetic material, the tumor cell itself shifts its state—for example, by adopting behavior similar to that of a cancer stem cell—and this state change allows it to evade the treatment's effects.

Of these three layers, genetic resistance is the most predictable: the Coldman-Goldie model demonstrates that the presence of resistant clones—even before treatment begins—is virtually inevitable once the tumor cell count exceeds a range of approximately one million to one hundred million cells; this is roughly the size at which a tumor becomes clinically detectable. Below this cell count, the existence of resistant clones is far less likely; above it, they are almost invariably present. Therefore, when a tumor whose cell count has already surpassed that threshold is treated with a single drug, the therapy rarely succeeds in curing the patient: since resistant clones are almost invariably already present within such a large tumor, they survive the single-drug regimen and eventually repopulate the tumor.

The physics of this system suggests three strategies for addressing drug resistance. The first is combination therapy: while the probability of a clone resistant to a single drug existing is already low, the likelihood of a clone resistant to multiple distinct drugs existing simultaneously is far lower, as these probabilities are multiplicative. The second is adjuvant therapy, which involves treating the tumor while the cell count is still low—before it approaches the threshold where the emergence of resistant clones becomes nearly inevitable. The third is adaptive therapy; rather than attempting to eliminate all clones at once, this approach actively manages the mix of distinct clones within the tumor as treatment progresses. All three strategies share the same goal: to reduce the likelihood that drug resistance—with its three layers—will ultimately defeat the treatment.

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