biomarkersBiomarker

Ki-67 index (proliferation by IHC)

aka Ki-67, Ki67, Ki-67 index, Ki-67 labelling index, Ki-67 score, MIB-1 index, Ki-67 >= 20%, Ki-67 ≥20%, proliferation index

Ki-67 is the percentage of tumour cells that are dividing. In neuroendocrine tumours it sets the grade; in breast cancer a 20 percent cut-off was briefly a condition of adjuvant abemaciclib, then dropped from the label in 2023.

The Ki-67 labelling index is the percentage of tumour cell nuclei staining with the MIB-1 antibody. In gastroenteropancreatic neuroendocrine tumours the WHO classification grades G1 (< 3 percent), G2 (3 to 20 percent) and G3 (> 20 percent). In breast cancer the October 2021 FDA approval of adjuvant abemaciclib (monarchE cohort 1) required node-positive, high-risk, HR-positive HER2-negative disease with Ki-67 >= 20 percent by the Ki-67 IHC MIB-1 pharmDx (Dako Omnis) companion diagnostic; the March 2023 label expansion removed the Ki-67 requirement, and the current label describes the assay only in the trial narrative. The International Ki-67 in Breast Cancer Working Group considers scores of 5 percent or less and 30 percent or more reproducible and the range between them not.

Part ofKi-67 (MKI67)· MKI67 All readouts

Ki-67 (MKI67): what it is and how drugs act on it · animated schematic, not to scale

Target · the protein and the cell it sits on
Drug · antibody, small molecule, cell or radioligand
Effect · signal, damage or kill

In plain words · Ki-67 is a protein present only in cells that are dividing, so the share of tumour cells that stain for it is a direct read of how fast the cancer is growing.

IHC score No approval threshold

What it means for a patient

Ki-67 tells you how fast the cancer is growing. In a neuroendocrine tumour it sets the grade, which steers treatment. In early breast cancer a high Ki-67 (20 percent or more) was once required for adjuvant abemaciclib; it no longer is, but the number still helps your team judge how much benefit chemotherapy or a CDK4/6 inhibitor is likely to add.

Written only from the label or guideline text cited on this page. Not medical advice; your own report and the reading your team gives it come first.

Scoring rule

Percentage of tumour cell nuclei staining for Ki-67 (MIB-1), counted in the invasive tumour; in monarchE cohort 2 a score of 20 percent or more was required, measured centrally with the Ki-67 IHC MIB-1 pharmDx.

“Breast tumor samples were tested at central sites using the Ki-67 IHC MIB-1 pharmDx (Dako Omnis) assay to establish if the Ki-67 score was ≥20%.”
Verzenio prescribing information (clinical studies section)

Thresholds used in approvals

No approval uses this readout as a threshold. It is defined by WHO Classification of Tumours, Digestive System Tumours, 5th edition (neuroendocrine grading by Ki-67).

1 historic or withdrawn threshold

Threshold	Drug	Cancer	Regulator	Source
Ki-67 >= 20% (2021 adjuvant approval, requirement removed March 2023)historic The current label no longer restricts adjuvant abemaciclib by Ki-67; the threshold is kept as the historic 2021 approval condition.	Abemaciclib	High-risk early HR-positive breast cancer	FDA	label

Drugs whose label names it

Abemaciclib

Tests that measure it

Ki-67 IHC (MIB-1)

Trials that name this readout

Matched on the name and aliases of the readout in the title, setting and summary of each trial; a match is a mention, not proof the readout was an entry criterion.