Today, patients need to wait two to three months before finding out if the drug palbociclib, used to treat breast cancer, is effective in treatment. But a new study may have found a blood test that could predict the effectiveness of the drug in a matter of weeks.

In March 2017, the U.S. Food and Drug Administration approved palbociclib for metastatic breast cancer in postmenopausal women in conjunction with Letrozole. Pfizer manufactures the drug under the brand name Ibrance. In November 2017, the National Health Service (NHS) of England approved the cancer drug for women with previously untreated advanced breast cancer.

Scientists from the Institute of Cancer Research, London, and The Royal Marsden NHS Foundation Trust published the study in the journal Nature Communications, cautioning that the results need further replicating before transitioning into clinical usage. The research received funding from the charity Breast Cancer Now and the pharmaceutical company Pfizer. 

"Our new study found that a blood test for cancer DNA in the first two weeks of treatment indicated whether the drug was likely to be effective," explained Prof. Nicholas Turner, senior author, and Professor of Molecular Oncology at The Institute of Cancer Research in London. Having an early indication of how likely a treatment is to work might allow us to adapt treatment – switching some patients to an alternative drug that is more likely to benefit them."

Unlike the current scan-based test to learn about the drug effectiveness, the new method operates with a blood test. It searches the bloodstream for circulating tumor fragments of DNA shed by the breast cancer. The study compared the amount of a gene PIK3CA detected in a blood test before treatment and 15 days after starting treatment. Of the 73 female patients who had the PIK3CA mutation, 52 received palbociclib.

The median progression-free survival, which is the length of time the patient survived and cancer did not worsen, showed a difference between the two groups. The patients who had a small decrease in PIK3CA circulating DNA at 15 days had a median progression-free survival of only 4.1 months, while the patients who had a large decrease in PIK3CA had a median progression-free survival of 11.2 months.

The former group, with the help of the identified blood test, could potentially learn sooner about the ineffectiveness of the drug and seek out alternative treatments.

"It is exciting to see that using advances in diagnostic techniques, such as genetic tests for circulating tumor DNA, we may be able to more accurately define groups of patients and help us deliver the right treatment to the right patient sooner," said Nathan Richardson, head of Molecular and Cellular Medicine at the MRC.

"This study provides early evidence that might help us understand sooner when a drug is successfully treating breast cancer, and if not, it can be discontinued and better approaches pursued."