Abstract 4416: Plasma thymidine kinase activity in patients with luminal metastatic breast cancer treated with Palbociclib within the phase II TREnd trial

Abstract

Abstract Background: The CDK4/6 inhibitor palbociclib (P) is approved for the treatment of luminal metastatic breast cancer (MBC) in combination with endocrine therapy (ET). It leads to reduced phosphorylation of the Rb protein resulting in a decrease in E2F activity and eventual cell cycle arrest. Thymidine kinase 1 is a well-known cancer proliferation marker downstream of the E2F pathway, whose activity can be measured in plasma samples as readout of tumor proliferation. Circulating thymidine kinase activity (TKa) is a prognostic marker in MBC patients (pts) treated with ET, both when measured at baseline and during treatment. TKa has been previously shown to decrease in pts treated with neoadjuvant P+ET for 15 days, which was attributed as pharmacodynamic change on P treatment. The predictive value of TKa changes during treatment with P, as well as the dynamics of TKa changes on P-containing treatments are not yet comprehensively defined. Here we investigate the role of plasma TKa measured at different timepoints in a cohort from the TREnd study (NCT02549430). Methods: TREnd was a randomized phase II trial that allocated 115 pts with moderately pre-treated luminal MBC to receive single-agent P or P plus the same ET agent that was received in the prior line of therapy. Plasma samples were collected at baseline (T0; n=45), at day 1 of cycle 2 (T1; n=45) and at disease progression (T2; n= 36) from 46 consenting pts. TKa was measured with DiviTum®, a refined ELISA-based assay. Patients were dichotomized as high/low at T0 based on an optimal cut-off (260 Du/L) determined by maximally selected rank statistics, and on the median value at T2 (250 Du/L). Dynamic changes between T0 and T1 were deemed meaningful if >10% of T1 or T0, whichever was greatest. Clinical outcome was estimated using the Kaplan-Meier method. Results: Median TKa (mTKa) at T0 was 73 Du/L (range 20-4302). As expected, P-containing treatment reduced mTKa levels at T1 (37 Du/L, range 20-4504). Conversely, at disease progression, TKa increased compared to T0 (mTKa at T2, 250 Du/L, range 20-3653). Median time to progression (mTTP) in pts with low TKa at T0 (n= 33) was 8.5 months, compared to 5.6 months in pts with high TKa (n= 12). Interestingly, pts with an increase in TKa at T1 (n=9) had a mTTP of only 3.1 months compared to pts with stable/reduced TKa (N=35), who showed a mTTP of 9 months. Considering the potential significance of TKa measured at disease progression, pts with high levels at T2 (n=18) had a worse outcome on subsequent post-study treatment (both chemotherapy and ET) compared to those with lower levels (n=18) (mTTP at T2, 2.9 vs 8.9 months, respectively). Conclusions: These data suggest for the first time that TKa may be a useful prognostic biomarker for non-invasive monitoring of MBC in the context of treatment with P. These results warrant further investigation in larger sample sets. Citation Format: Luca Malorni, Chiara Biagioni, Francesca De Luca, Martina Bonechi, Giuseppe Curigliano, Alessandro M. Minisini, Erica Moretti, Mattias Bergqvist, Karin Mattsson, Emanuela Risi, Ilenia Migliaccio, Stefania Vitale, Stefano Gabellini, Amelia McCartney, Irene De Santo, Francesca Galardi, Giulia Boccalini, Matteo Benelli, Lorenzo Rossi, Laura Biganzoli, Angelo Di Leo. Plasma thymidine kinase activity in patients with luminal metastatic breast cancer treated with Palbociclib within the phase II TREnd trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4416.