San Antonio, Texas—Data being presented in a poster at the 2019 San Antonio Breast Cancer Symposium suggest that measuring PI3K-involvement in the hyperactive S1P signaling of breast cancer cells can help identify patients who could benefit from PI3K inhibitor therapy.

“Less than 20% of PI3KCA mutated late stage breast cancer patients achieved an objective response in a Phase III clinical trial with alpelisib, a recently approved PIK3CA inhibitor. This suggests biological factors other than PIK3CA status, such as aberrant GPCR-linked signaling, may be important to measure when identifying patients eligible for PI3K inhibitors,” explained Lance Laing, PhD, Chief Science Officer and Co-Founder, Celcuity, Minneapolis, Minnesota, and colleagues.

Thus, the CELx PI3K Signaling Function (CELx PI3K) Test was created to diagnose breast tumors that have PI3K-involved hyperactive signaling by measuring ex vivo live tumor cell response in real-time to a specific S1P agonist and PI3K antagonists.

The CELx PI3K assay works by measuring GPCR-initiated signaling activity and the involvement of the PI3K node in transducing this activity in live tumor cells via an impedance biosensor.

Dr Laing et al conducted a study with the purpose of characterizing the involvement of the PI3K node in hyperactive S1P-initiated signaling in breast cancer cells and cell lines and evaluating whether PI3K-involved hyperactive S1P signaling is limited to breast cancer cells with PI3KCA mutations.

They collected a panel of 18 HER2-/PIK3CA wild-type (WT) tumor cells and 5 PIK3CA-mutated breast tumor cell lines from patients with breast cancer before measuring real-time live cell response to an S1P agonist, alpelisib, a gamma-specific PI3K antagonist (IPI-549), and taselisib via an xCELLigence RTCA impedance biosensor.

Using these responses, the investigators quantified SIP-initiated signaling, as well as PI3K isoform participation in S1P-initiated signaling. They also used a previously determined cutpoint to identify abnormal signaling activity levels.

Ultimately, abnormal S1P-initiated signaling activity levels were found in 5 of 18 PIK3CA WT primary patient breast tumor cells and 2 of 5 PIK3CA-mutated breast tumor cell lines (BT20 and HCC1954). In the 5 primary breast tumor cells, the mean reduction of abnormal SIP signaling was twice as high with IPI-549 as with alpelisib (60% vs 30%, respectively).

Furthermore, the mean S1P signaling reduction in the 2 PIK3CA-mutated cell lines with abnormal S1P-initiated signaling was not significant with IPI-549 and alpelisib (0% and 28%, respectively) but was 82% with taselisib.

“The CELx PI3K test found hyperactive S1P-initiated signaling involves the PI3K node in PI3KCA mutated and WT breast tumor cells. The abnormal levels of PI3K-involved signaling in PIK3CA WT patient breast cancer tumors were comparable in magnitude to levels found in well-characterized PI3KCA mutated breast tumor cell lines. Unlike the PIK3CA mutated cells lines, the bulk of the PIK3-involved signaling in the PIK3CA WT patient tumor cells was associated with the PI3K-gamma isoform rather than the PI3K-alpha isoform,” Dr Laing and his co-investigators reported.

Findings from the CELx PI3K test demonstrated the superior reduction of S1P signaling by taselisib compared with alpelisib (74% vs 21%, respectively). These results reflected data from xenograft studies that showed that taselisib induced HCC1954 tumor reduction whereas alpelisib yielded no significant effect.

“This study thus suggests that measurement of PI3K-involvement in hyperactive S1P signaling in live patient breast cancer cells may provide a means to identify breast cancer patients who may benefit from treatment with PI3K inhibitors,” the investigators concluded.—Hina Porcelli

Laing L, Khan S, MacNeil I, et al. Sub-group of PIK3CA WT breast cancer patients have hyperactive S1P signaling tumor cells responsive to PIK3-inhibitors: Functional signaling profile test identifies new patient group who may benefit from PIK3-targeted therapies. Presented at: the 2019 San Antonio Breast Cancer Symposium; December 10-14, 2019; San Antonio, TX. Abstract P1-09-07.