They grow up so fast.

The CDK4/6 inhibitor drug class grew annual global revenue 15.1% to $14.6 billion in 2025, up from $12.7 billion the previous year. Additional approvals and plans to explore novel combinations, including with next-generation CDK inhibitors capable of reversing or slowing resistance to the pioneering molecules, suggest the mechanism will remain a critical component of the evolving standard of care for HR+/HER2- breast cancers.

Pfizer's Ibrance (palbociclib) was the first CDK4/6 inhibitor to launch in 2015. It immediately became one of the fastest drug launches of all time, eclipsing $2 billion in annual revenue in just its second year on the market. Humira, Keytruda, and Ozempic didn't even achieve that. The brand crossed $5 billion in annual revenue before a single competitor reached blockbuster status (>$1 billion in annual revenue).

The immediate success demonstrated the urgent need for better treatment options in HR+/HER2- breast cancer. Large commercial opportunities also breed healthy competition. It didn't take long for newcomers to challenge Ibrance's dominance.

Eli Lilly's Verzenio (abemaciclib) and Novartis' Kisqali (ribociclib) launched in late 2017. Although they didn't enjoy the same swift initial ramp as Pfizer's prized asset, improved tolerability profiles and later approvals spurred significant growth spurts years after approval.

• Verzenio grew year-over-year revenue by $1.11 billion in 2022, added another $1.38 billion in 2023, and $1.44 billion in 2024. That slowed to $416 million in 2025.
• Kisqali grew year-over-year revenue by $849 million in 2023, added another $953 million in 2024, and surged with $1.75 million in 2025.

Despite the success of CDK4/6 inhibitors, doctors and scientists see room for improvement. 

Cyclin-dependent kinases (CDK) play important roles in driving cell growth, but are especially active in certain types of breast tumors. Therefore, a CDK inhibitor works by blocking  CDK enzymes to stop cell growth. The drug class is combined with hormone therapy to disrupt estrogen-driven cell growth in HR+/HER2- breast cancer in both first-line plus (1L+) and even adjuvant settings. 

But broad inhibition has other drawbacks. CDK4/6 inhibitors reduce white blood cell counts (neutropenia) by whacking CDK6 and cause gastrointestinal side effects such as diarrhea. In their respective pivotal studies in combination with fulvestrant, Ibrance triggered neutropenia in 83% of patients (66% grade 3+), Kisqali in 69% of patients (53% grade 3+), and  Verzenio in 46% of patients (27% grade 3+). The latter is more selective for CDK4 than CDK6, which gives it an improved tolerability profile. However, Eli Lilly's prized oncology asset triggers the highest rates of diarrhea (86% all grades, compared to 24% for Ibrance and 29% for Kisqali) and has the highest discontinuation rate (16% vs. 4% vs. 9%) among the drug class.

Those clinical observations have kicked off a new race to develop more selective CDK inhibitors. Scientists believe CDK4 inhibitors could have much-improved tolerability profiles by sparing CDK6. In early studies, Pfizer's CDK4 inhibitor atirmociclib led to neutropenia in 36% of patients (11% grade 3+), diarrhea in 19%, and had a discontinuation rate of only 3%.

Meanwhile, CDK2 inhibitors are being evaluated for their potential to overcome resistance mechanisms in patients who previously received CDK4/6 inhibitors, while Roche (via China's Regor Therapeutics) and others are exploring CDK2/4 inhibitors.