Loss of the tumour suppressor PTEN was a frequent cause of resistance to the investigational drug BYL719, which blocks the activity of a protein called PI3K-alpha, in a small sample of women with breast cancer that progressed after initially responding to BYL719 treatment, according to results presented at the AACR Annual Meeting.

“Disruption of a signalling pathway called the PI3K pathway is thought to have an important role in driving a number cancer types, including breast cancer,” said Pau Castel, a graduate student in the Human Oncology and Pathogenesis Program at Memorial Sloan Kettering Cancer Center in New York.

“Several drugs that target PI3K proteins are currently being tested in clinical trials.

One of these drugs, BYL719, which targets PI3K-alpha, has shown promising results for patients with breast cancer harbouring mutations in the gene that encodes PI3K-alpha, PIK3CA.

“Unfortunately, as for other targeted therapies, most patients treated with BYL719 eventually relapse as their breast tumours become resistant to the effects of the drug,” Castel continued.

“We have found strong clinical evidence that a common way in which breast tumours become resistant to BYL719 is through loss of PTEN.

We also found using a mouse model derived from these tumours that treating mice with both BYL719 and a PI3K-beta inhibitor overcame BYL719 resistance.

“These data suggest that assessment of PTEN levels could help clinicians select patients most likely to respond to BYL719 and determine which patients may benefit from the addition of a PI3K-beta inhibitor to their treatment regimen,” Castel added.

Castel and colleagues started their study by analysing tumour samples from a patient with breast cancer who had relapsed after initially responding to treatment with BYL719 through a clinical trial.

They performed sequencing of samples of the patient’s tumour obtained prior to BYL719 treatment and samples of two metastatic tumours obtained after relapse.

One of these metastatic tumours, obtained from the patient’s lung, was resistant to BYL719 and one remained sensitive to the drug. The researchers identified loss of the PTEN gene and mutation of the PTEN gene only in the metastatic lung tumour.

Analysis of metastatic tumour samples from an additional 14 sites showed that loss of PTEN via numerous mechanisms had occurred in all samples resistant to BYL719 but that PTEN remained intact in all samples still responding to the drug.

The researchers then used cells from the metastatic lung tumour to generate a patient-derived mouse model.

They found that the model recapitulated the behaviour of the original metastatic lung tumour and was resistant to BYL719.

Adding a PI3K-beta inhibitor to BYL719 led to tumour regression, as did monotherapy with a pan-PI3K inhibitor.

“Our study has provided evidence that combining PI3K-alpha and -beta inhibitors or the use of a pan-PI3K inhibitor may be effective in patients who escape PI3K inhibition via loss of PTEN function,” said Castel.

“These results stress the importance of dynamic therapy, meaning that we have to be ready to adjust treatment as tumours evolve and drug resistance emerges.”

According to Castel, analysis of samples from nine other patients enrolled in the first-in-human clinical trial testing BYL719 as a treatment for breast cancer showed that two of these patients who acquired resistance to the drug had tumours lacking PTEN.

“This suggests that PTEN loss is a relatively common event in patients with breast cancer that becomes resistant to BYL719 and we are in the process of trying to obtain more samples to analyse to confirm this,” said Castel. 

Source: AACR