Which drug is classified as a CDK4/6 inhibitor?

Palbociclib, known commercially as Ibrance, is classified as a CDK4/6 inhibitor. This class of drugs targets cyclin-dependent kinases 4 and 6, which are critical for cell cycle progression in cancer cells. By inhibiting these kinases, Palbociclib effectively halts the cell cycle in the G1 phase, thereby preventing cancer cells from proliferating. This mechanism is particularly beneficial in the treatment of hormone receptor-positive breast cancer where the proliferation of cancer cells can be driven by estrogen.

The other options represent different classes of medications. Everolimus is a mTOR inhibitor, which works by blocking a pathway that cancer cells use to grow and divide, but it does not specifically target CDK4/6. Fulvestrant is an estrogen receptor antagonist, used to block the effects of estrogen on tumors, without direct action on CDK4/6. Exemestane is an aromatase inhibitor that reduces estrogen levels in the body, also not affecting CDK4/6 directly. Hence, Palbociclib stands out as the sole CDK4/6 inhibitor among the choices provided.