Targeting CDK4 and CDK6: From Discovery to Therapy

Abstract

Biochemical and genetic characterization of D-type cyclins, their cyclin D-dependent kinases (CDK4 and CDK6), and the polypeptide CDK4/6 inhibitor p16(INK4)over two decades ago revealed how mammalian cells regulate entry into the DNA synthetic (S) phase of the cell-division cycle in a retinoblastoma protein-dependent manner. These investigations provided proof-of-principle that CDK4/6 inhibitors, particularly when combined with coinhibition of allied mitogen-dependent signal transduction pathways, might prove valuable in cancer therapy. FDA approval of the CDK4/6 inhibitor palbociclib used with the aromatase inhibitor letrozole for breast cancer treatment highlights long-sought success. The newest findings herald clinical trials targeting other cancers.

Significance: Rapidly emerging data with selective inhibitors of CDK4/6 have validated these cell-cycle kinases as anticancer drug targets, corroborating longstanding preclinical predictions. This review addresses the discovery of these CDKs and their regulators, as well as translation of CDK4/6 biology to positive clinical outcomes and development of rational combinatorial therapies.

Figure 1. The Cell Cycle

The four phases of the mitotic cell division cycle are indicated in the inner circle, including mitosis (M phase), the cellular DNA synthesis (S) phase, and their separation by two gap (G) phases, the first (G1) between M and S phase and the second (G2) between S and M phase. The levels of total CDK activity are lowest in early G1 phase and progressively increase under the agency of different cyclin-CDK complexes, reaching maximal net CDK activity as cells enter mitosis. States of RB phosphorylation (P) throughout the cell cycle are schematized. RB is dephosphorylated in M phase (green arrow) and progressively rephosphorylated in G1, first by cyclin D-dependent CDK4/6 and later by cyclin E-dependent CDK2. RB becomes fully phosphorylated in late G1 (red arrow), resulting in inactivation of its proliferation suppressive function and triggering the cell's subsequent entry into S phase. The point in the cell cycle (sometimes called “the restriction point”) at which RB becomes fully phosphorylated temporally corresponds to a late G1 phase transition when cells lose their marked dependency on extracellular mitogens, and commit to enter S phase and complete the cycle. During the S and G2 phases, RB phosphorylation is maintained by the progressive activation of other CDKs including cyclin A-CDK2 and cyclins A/B-CDK1. Degradation of cyclins A and B in mitosis results in the collapse of CDK activity and restores the G1 state. INK4 proteins (the prototype p16 ^INK4A is shown) specifically inhibit the cyclin D-dependent kinases to inhibit RB phosphorylation and arrest cells in G1 phase. Arrested cells can return to a non-cycling but reversible quiescent state (G₀) after mitogen withdrawal in which D-type cyclins are usually degraded or, in response to particular stress conditions, can undergo durable cell cycle arrest (senescence). Quiescent cells re-stimulated with mitogens restore cyclin D synthesis and reenter the cell cycle in early G1, whereas senescent cells are refractory to mitogen restimulation and resist oncogenic challenge. Asynchronously dividing cells maintain mitogen-dependent cyclin D synthesis and have a contracted G1 phase when compared to quiescent cells reentering the division cycle [for pertinent detailed reviews, see Refs (4, 16, 41, 65)].