More than 12,000 people were newly diagnosed with breast cancer over a five-year period between 2016 and 2020, according to the Singapore Cancer Registry, with yearly numbers on the rise. More than nine out of 10 cases are detected at an early stage.

At least 30 to 40 per cent of early breast cancer (EBC) cases face a high risk of recurring, according to Professor Lee Soo Chin, head & senior consultant, Department of Haematology-Oncology, National University Cancer Institute, Singapore.

To reduce the risk of the cancer coming back, doctors in Singapore have in recent times been able to prescribe a drug known as abemaciclib to treat selected high-risk EBC cases. The drug had already been used for about two to three years here in advanced breast cancer patients, but approval was subsequently extended to certain EBC patients. The cases are determined to be high risk by the patients’ doctors, based on the cancer’s stage and aggressiveness.

“High-risk EBC patients are traditionally given adjuvant chemotherapy as well as at least five years adjuvant anti-hormonal therapy in an attempt to eradicate micrometastases and reduce risk of recurrence,” says Prof Lee, who is also with the National University of Singapore’s Yong Loo Lin School of Medicine.

Adjuvant therapy is any additional treatment given to a patient to reduce the risk of cancer returning after surgery or initial treatment. For EBC, adjuvant therapy can include radiotherapy, chemotherapy, anti-hormonal therapy and immunotherapy, depending on the type and stage of the cancer.

Micrometastases are small clusters of cancer cells that have migrated from the original tumour to another part of the body.

Abemaciclib is indicated for adjuvant treatment of adult patients with hormone receptor (HR) positive, human epidermal growth factor receptor 2 (HER2) negative, node-positive early breast cancer at high risk of recurrence. In hormone receptor positive breast cancer, the cells use oestrogen or progesterone, or both, to grow and multiply. HER2-negative means that the cancerous cells do not contain high levels of a protein known as HER2.

Abemaciclib works by blocking the proteins that stimulate breast cancer cells to divide and grow.

“Abemaciclib is given in addition to adjuvant anti-hormonal therapy to further reduce the risk of the cancer recurring in high-risk patients. It does not replace other conventional adjuvant therapy, such as chemotherapy, radiotherapy or anti-hormonal therapy,” says Prof Lee.

The drug is given orally together with standard anti-hormonal therapy in patients with hormone receptor positive, HER2 negative EBC. Abemaciclib is prescribed for two years while standard adjuvant anti-hormonal therapy is given for at least five years.

“A large, randomised trial has shown that patients with high-risk early breast cancer who were treated with adjuvant anti-hormonal therapy and abemaciclib were less likely to have cancer relapse at four years compared to those who were treated with adjuvant anti-hormonal therapy alone,” says Prof Lee.

The relapse rate was 14.2 per cent at four years for those on both anti-hormonal therapy and abemaciclib, and a higher 20.6 per cent for patients receiving just anti-hormonal therapy.

The monarchE trial was an international study of 5,637 patients from 603 sites in 38 countries, including multiple countries in Asia, with participation from Singapore. Prof Lee said the trial results are applicable here, with 20 per cent of the patients in the study hailing from Asia.

Main side effect of the drug

“Up to 30 to 40 per cent of patients on abemaciclib could suffer from diarrhoea, the main side effect of the drug,” she says. This can be managed with anti-diarrhoea medications.

“The drug can also cause low blood counts and minor liver enzyme abnormalities, with more monitoring required at least during the first few months of treatment,” according to Prof Lee. Low white blood cell counts may cause serious infections that can be life-threatening if not closely monitored. Patients on abemaciclib would need more consultations and blood tests in the initial few months of treatment than if they were on anti-hormonal therapy alone.

Abemaciclib is currently available for the Medication Assistance Fund (MAF) subsidy in the treatment of advanced breast cancer. However, for EBC, it is not on the Cancer Drug List at the moment, so patients are unable to draw on Medishield Life or Medisave to help fund the cost of the drug.

The drug was approved by the US Food and Drug Administration in October 2021 to treat high-risk EBC, and is accepted as an option for such patients by international cancer treatment guidelines. The drug has also been approved by the Health Sciences Authority for high-risk EBC in August 2022.

Before beginning to take abemaciclib or a course of any new medication, “patients should understand their individual risk of cancer relapse as well as the potential side effects of the drug, then make an informed decision about whether it is worthwhile for them to take the drug”, advises Prof Lee.

They should speak to their doctors or healthcare professionals about the treatment option for more information.

The drug, which can only be prescribed by oncologists, is available from public health institutions and private oncology clinics.

Details of the Cancer Drug List in this article are accurate at the time of publication.