Isaac Garcia-Murillas The Institute of Cancer Research, London

ctDNA dynamics and clonal

selection with palbociclib and

fulvestrant for breast cancer

Targeting advanced breast cancer

Pernas et al 2018, Therapeutic Advances in Medical Oncology

CDK4/6 inhibitors: a new standard of care in ER+

advanced breast cancer

CDK4/6 inhibitors in ER+ breast cancer

Inhibition of CDK4/6-cyclin D complexes

Effect G1 arrest

Mechanisms of resistance are unknown

Pre-clinical data

CCNE1 amplification

CDK6 amplification

RB1 mutation

Clinical data

Case series of n= 3 for RB1 mutation

Yang et al 2017, Oncogene

Herrera-Abreu et al 2016, Cancer Res

Condorelli et al 2017, Ann Oncol

O’Leary et al 2016, Nat Rev Clin Onc

*

* As of May 2018

CDK4/6 inhibitors in the clinic

PALOMA-3 Study Design

Progression Free Survival in the PALOMA-3 trial

Cristofanilli et al 2016, Lancet Oncology

Fulvestrant plus palbociclib associated with

significant and consistent improvement in

PFS irrespective of the degree of endocrine

resistance, hormone-receptor expression

level, and PIK3CA mutational status

Overall Survival in the PALOMA-3 trial

Turner et al 2018, NEJM

In patients with hormone-receptor–positive, HER2-negative advanced breast cancer with sensitivity to previous endocrine

therapy, treatment with palbociclib–fulvestrant resulted in longer OS but differences in OS in the entire trial group were not

significant

Aims: To identify genomic mechanisms of response/resistance to palbociclib and

endocrine therapy in hormone receptor positive, HER2- advanced breast cancer

patients

Aims

Mechanisms of CDK4/6 inhibitor resistance have been described pre-clinically, with limited evidence from

clinical samples

Early changes in circulating tumour DNA (ctDNA) level may provide early response prediction, but the

impact of tumor heterogeneity is unknown

Elucidation might inform future treatment strategies to address response/resistance to palbociclib plus

fulvestrant as this is now a standard of care for advanced estrogen receptor positive breast cancer

Approach

Paired exome sequencing of plasma

n= 14

(all palbociclib + fulvestrant)

Paired targeted sequencing of plasma

n= 195

(both treatment arms)

Paired sequencing for copy number and tumour purity

n= 154

(both treatment arms)

Multiplex ddPCR PIK3CA and ESR1

n= 455 (B), 73 (C1D15) PIK3CA

n= 445 (B), 65 (C1D15) ESR1

(both treatment arms)

ctDNA dynamics to assess for early

response prediction

Clonal evolution to identify biomarkers of resistance

O’Leary et al 2018, Cancer Discovery O’Leary et al 2018, Nat Comms

Early plasma dynamics differ between PIK3CA and

ESR1 mutations

n= 65, median ratio for mutant 0.022, median ratio for wild-type 0.21

n =73, median ratio mutant 0.076, median ratio for wild-type 0.54

Early PIK3CA ctDNA dynamics predict PFS on

palbociclib arm more strongly than ESR1

ESR1 mutations may be subclonal with distinct

response to therapy

Summed AF of ESR1 mutations lower than PIK3CA AF

in patients with both mutations suggest ESR1 is

predominantly sub clonal (n =35) Loss of ESR1 mutant clone suggests contrasting

clonal dynamics of a sub clonal ESR1 mutation

(n=25)

** analysis limited by an incomplete analysis of the

ESR1 gene

Identifying genomic mechanisms of resistance to

palbociclib and fulvestrant

Mutations at D1 and EoT (both arms)

New RB1 mutations at EoT

Palbociclib + Fulvestrant: 4.7% of patients

Placebo + Fulvestrant: 0% of patients

New mutations acquired on treatment in 30.8% of patients

O’Leary et al 2018, Cancer Discovery

RB1 mutations are acquired and likely subclonal

Time on treatment and acquired mutations

Subclonal changes on treatment

Different patterns of mutation acquisition and loss

No changes in copy number observed

ESR1 Y537S might be selected by fulvestrant

- ESR1 mutations (selected by prior hormone therapy) are often subclonal

- ESR1 ctDNA dynamics offers limited prediction of clinical outcome

- PIK3CA early ctDNA dynamics might provide a robust biomarker for CDK4/6 inhibitors

- Driver mutations are acquired frequently in both treatment groups including PIK3CA mutations and

other oncogenic mutations at low frequency (ERBB2 and FGFR2/3)

- RB1 mutations are acquired in palbociclib arm albeit at a low frequency

- Substantial subclonal evolution is observed on treatment

- ESR1 Y537S selected in both arms, likely by fulvestrant, suggesting resistance mechanisms to

CDK4/6i-endocrine combinations may arise in parallel

- Parallel evolution of resistance to fulvestrant and CDK4/6 inhibitors may inform future treatment

strategies

Conclusions

Ben O’Leary

Ros Cutts

Sarah Hrebien

Matthew Beaney

Alex Pearson

Nicholas Turner

Molecular Oncology

The Institute of Cancer Research

James Morden

Lucy Kilburn

Judith Bliss

CTSU

The Institute of Cancer Research

Xin Huang

John Jiang

Yuan Liu

Cynthia Huang Bartlett

Pfizer

Fabrice André

Massimo Cristofanilli

Sherene Loi

Sibylle Loibl

International Collaborators

Patients and their families in the PALOMA-3 study

All investigators, research coordinators, and site staff

This study was sponsored by Pfizer Inc.

Fulvestrant was provided by AstraZeneca.

Acknowledgements