Posted on Jul 26, 2023

POETIC study reveals further insights into Ki67

Ki67 has long been recognised as a valuable prognostic marker in breast cancer, with potential as a predictive marker to guide treatment decisions. A new analysis of a landmark study has revealed more insights into Ki67 and how it relates to other clinico-pathological features. The findings serve as a reminder of the importance of Ki67, but also highlights one of the many challenges of scoring Ki67 using immunohistochemistry.

The PeriOperative Endocrine Therapy for Individualised Care (POETIC) trial The POETIC trial recruited 4,480 post-menopausal women with ER+ primary breast cancer between 2008 and 2014. The participants were randomised to either aromatase inhibitor (AI) therapy for 14 days before and after surgery, or no treatment. In the study, Ki67 was measured in samples collected at diagnosis (baseline) and at surgery. The investigators used a standardised central immunohistochemistry (IHC) protocol and digital image analysis to count the percentage of Ki67-positive cells (referred to in this article as Ki67-positivity).

Results published in 2020 from the POETIC study showed that the pre-surgery AI therapy (either anastrozole or letrozole) did not reduce risk of recurrence. However, the study did show that Ki67-positivity of less than 10% at either baseline or at two-weeks was associated with a low risk of recurrence.

How does Ki67-positivity relate to other tumour features?

The new analysis of data from the POETIC trial, published April 2023, aimed to better understand how Ki67 measurement at baseline and surgery – and the change between them – relates to other features of the tumour.

The analysis confirms several findings from the earlier POETIC data and other studies. Ki67 at baseline had a strong relationship with HER2 status, so as with the original POETIC study, the analysis was split into HER2-negative and HER2-positive groups.

Among the HER2-negative group, Ki67-positivity was associated with a range of clinico-pathological factors after multivariate analysis, including PR-status, tumour grade, tumour size, histological type (ductal v lobular), and nodal status. In the HER2-positive group, only tumour grade was associated with Ki67-positivity after multivariate analysis.

When comparing Ki67 between diagnosis and surgery two weeks later (ΔKi67 or delta-Ki67), AI therapy reduced Ki67-positivity, as expected. The median relative reduction was -79% and -54% for HER2-negative and HER2-positive tumours, respectively. PR-status and tumour grade also influenced the magnitude of ΔKi67.

Ki67 values are affected by how the biopsy is taken

The authors highlight an interesting finding they discovered looking at the control group of patients with HER2-negative tumours, who received no treatment between diagnosis (baseline) and surgery (at two weeks).

While the baseline sample was taken via core needle biopsy, the two-week sample at surgery was taken either by core biopsy or by excision biopsy.

As expected, Ki67-positivity did not change when the second sample was taken by core biopsy (i.e. in the same way to the baseline sample). However, when the second sample was taken via excision biopsy, Ki67-posivity dropped by nearly 20%.

The authors recognise this drop is an artefact of the sample type, which could affect similar ‘window of opportunity’ trials in the future. They write: “in the absence of a control arm, a presurgical study in which excision specimens are used as the on-treatment sample may artifactually enhance the apparent antiproliferative impact of a treatment.” If used as the basis of risk-adapted treatment, this could mean some patients might receive less intensive therapy than they need.

They give a few possible explanations as to why Ki67-positivity is lower from excision biopsies than core-cut biopsies. One is that excision specimens are fixed by formalin more slowly than core-cuts, partly because of their size, and partly because there are sometimes delays in putting excision biopsies in formalin during surgery. This delay in fixation may affect the integrity of nuclei of cells from excision biopsies, resulting in lower Ki67 positivity.

To avoid this artefact, the authors recommend that surgical samples should be taken by core biopsy to match the method used at diagnosis.

Could quantitative PCR be the alternative to IHC for Ki67 scoring?

At Cerca Biotech, we believe this artefact adds to the large catalogue of pre-analytical variables which affect IHC scoring of Ki67 (and other markers).

However, a quantitative PCR-based system, like MammaTyper® offers a standardised method between laboratories, and between samples from the same patient. This could be hugely beneficial to clinical trials, where the variability of IHC Ki67 scoring may prevent proper assessment of benefit from treatments. Not to mention that qPCR methods are far simpler to carry out and more reliable than IHC methods.

Could quantitative PCR help you with scoring Ki67 in breast tumours? Get in touch to find out how you can test MammaTyper® for yourself: