and other cellular proteins and promotes cell
cycle progression.
Cyclin D1–CDK4/6 directly phosphorylates,
stabilizes, and activates the transcription fac-
tor FOXM1. This promotes cell cycle progres-
sion and protects cancer cells from entering
senescence ( 35 ). Cyclin D–CDK4 also phos-
phorylates and inactivates SMAD3, which
mediates transforming growth factor–b(TGF-b)
antiproliferative response. CDK4/6-dependent
phosphorylation of SMAD3 inhibits its tran-
scriptional activity and disables the ability of
TGF-bto induce cell cycle arrest ( 36 ). FZR1/
CDH1, an adaptor protein of the APC complex,
is another phosphorylation substrate of CDK4.
Depletion of CDH1 in human cancer cells par-
tially rescued the proliferative block upon
CDK4/6 inhibition, and it cooperated with RB1
depletion in restoring full proliferation ( 37 ).
Cyclin D–CDK4/6 also phosphorylates and
inactivates TSC2, a negative regulator of
mTORC1, thereby resulting in mTORC1 acti-
vation. Conversely, inhibition of CDK4/6 led
to decreased mTORC1 activity and reduced
protein synthesis in cells representing differ-
ent human tumor types. It was proposed that
through TSC2 phosphorylation, activation of
cyclin D–CDK4/6couplescellgrowthwithcell
division ( 38 ). Consistent with this, the anti-
proliferative effect of CDK4/6 inhibition was
reduced in cells lacking TSC2 ( 38 ).
MEP50, a co-regulatory factor of protein
arginine-methyltransferase 5 (PRMT5), is
phosphorylated by cyclin D1–CDK4. Through
this mechanism, cyclin D1–CDK4/6 increases
the catalytic activity of PRMT5/MEP50 ( 39 ).
It was proposed that deregulation of cyclin
D1–CDK4 kinase in tumor cells, by increasing
PRMT5/MEP50 activity, reduces the expression
of CUL4, a component of the E3 ubiquitin-
ligase complex, and stabilizes CUL4 targets
such as CDT1 ( 39 ). In addition, by stimulating
PRMT5/MEP50-dependent arginine methyla-
tion of p53, cyclin D–CDK4/6 suppresses the
expression of key antiproliferative and pro-
apoptotic p53 target genes ( 40 ). Another studyproposed that PRMT5 regulates splicing of
the transcript encoding MDM4, a negative
regulator of p53. CDK4/6 inhibition reduced
PRMT5 activity and altered the pre-mRNA
splicing of MDM4, leading to decreased levels
of MDM4 protein and resulting in p53 acti-
vation. This, in turn, up-regulated the expres-
sion of a p53 target, p21CIP1,thatblockscell
cycle progression ( 41 ).
During oncogenic transformation of hema-
topoietic cells, chromatin-bound CDK6 phos-
phorylates the transcription factors NFY and
SP1 and induces the expression of p53 antag-
onists such as PRMT5, PPM1D, and MDM4
( 42 ). Also, in acute myeloid leukemia cells ex-
pressing constitutively activated FLT3, CDK6
binds the promoter region of theFLT3gene
as well as the promoter of PIM1 pro-oncogenic
kinase and stimulates their expression. Treat-
ment of FLT3-mutant leukemic cells with a
CDK4/6 inhibitor decreased FLT3 and PIM1
expression and triggered cell cycle arrest and
apoptosis ( 43 ). The relevance of these variousFasslet al.,Science 375 , eabc1495 (2022) 14 January 2022 3 of 19
Table 1. Currently available CDK4/6 inhibitors.This table lists major inhibitors of CDK4 and CDK6, half-maximal inhibitory concentration (IC 50 ) for different
cyclin D–CDK4/6 complexes (if known), other known targets, and the stage of clinical development.Ki, inhibitory constant.Name of compound IC 50 Other known targets Stage of clinical development
Palbociclib (PD-0332991) D1-CDK4, 11 nM;
D2-CDK6, 15 nM;
D3-CDK4, 9 nMFDA-approved for HR+/HER2–advanced
breast cancer in combination with
endocrine therapy; phase 2/3 trials
............................................................................................................................................................................................................................................................................................................................................for several other tumor types
Ribociclib (LEE011) D1-CDK4, 10 nM;
D3-CDK6, 39 nMFDA-approved for HR+/HER2–advanced
breast cancer in combination with
endocrine therapy; phase 2/3 trials
............................................................................................................................................................................................................................................................................................................................................for several other tumor types
Abemaciclib (LY2835219) D1-CDK4, 0.6 to 2 nM;
D3-CDK6, 8 nMCyclin T1–CDK9, PIM1, HIPK2, CDKL5,
CAMK2A, CAMK2D, CAMK2G,
GSK3a/b, and (at higher doses)
cyclin E/A–CDK2 and cyclin B–CDK1FDA-approved for early (adjuvant) and
advanced HR+/HER2–breast cancer in
combination with endocrine therapy;
FDA-approved as monotherapy in advanced
HR+/HER2–breast cancer; phase 2/3 trials
............................................................................................................................................................................................................................................................................................................................................for several other tumor types
Trilaciclib (G1T28) D1-CDK4, 1 nM;
D3-CDK6, 4 nMFDA-approved for small-cell lung cancer
to reduce chemotherapy-induced bone
marrow suppression; phase 2/3 trials
............................................................................................................................................................................................................................................................................................................................................for other solid tumors
Lerociclib (G1T38) D1-CDK4, 1 nM;
D3-CDK6, 2 nMPhase 1/2 trials for HR+/HER2–advanced
breast cancer and EGFR-mutant
............................................................................................................................................................................................................................................................................................................................................non–small-cell lung cancer
SHR6390 CDK4, 12 nM;
CDK6, 10 nMPhase 1/2/3 trials for HR+/HER2–advanced
............................................................................................................................................................................................................................................................................................................................................breast cancer and other solid tumors
PF-06873600 CDK4, 0.13 nM (Ki),
CDK6, 0.16 nM (Ki)CDK2, 0.09 nM (Ki) Phase 2 trials for HR+/HER2–advanced
............................................................................................................................................................................................................................................................................................................................................breast cancer and other solid tumors
FCN-437 D1-CDK4, 3.3 nM;
D3-CDK6, 13.7 nMPhase 1/2 trials for HR+/HER2–advanced
............................................................................................................................................................................................................................................................................................................................................breast cancer and other solid tumors
Birociclib (XZP-3287) Not reported Phase 1/2 trials for HR+/HER2–advanced
............................................................................................................................................................................................................................................................................................................................................breast cancer and other solid tumors
HS-10342 Not reported Phase 1/2 trials for HR+/HER2–advanced
............................................................................................................................................................................................................................................................................................................................................breast cancer and other solid tumors
CS3002............................................................................................................................................................................................................................................................................................................................................Not reported Phase 1 trial for solid tumorsRESEARCH | REVIEW