Abstract PO3-18-05: Pharmacokinetics (PK) of lasofoxifene (LAS) monotherapy and combined with abemaciclib (Abema)

Abstract Introduction: LAS is a selective estrogen receptor modulator (SERM)/tissue-selective estrogen agonist/antagonist studied in women with osteoporosis and vulvar-vaginal atrophy, and most recently, metastatic breast cancer (mBC) with ESR1 mutations. LAS data in healthy postmenopausal women and young men show linear PK and no differences by age or race (Gardner. J Clin Pharmacol 2006;46:52-58; Prakash. Drug Metab Dispos 2008;26:1218-26; Data on file). Also, no PK differences were found between Caucasian and Japanese patients (Jhee. AACR-JCA 2022, abstract A83; Fountaine. J Clin Pharmacol 2006;46:693-99). Here we report select PK data of LAS (5 mg/day) from women with mBC in the ELAINE 1 and ELAINE 2 oncology trials to indirectly investigate possible drug-drug interactions between LAS and Abema. Methods: Patients with mBC and ESR1 mutations who progressed on endocrine therapy and CDK4/6 inhibitors received LAS 5 mg/day as monotherapy (ELAINE 1) or combined with Abema (provided by Eli Lilly and Co) 150 mg BID (ELAINE 2). Blood was collected prior to daily dosing at weeks 2, 4, 8, and every 4 weeks up to the final visit in ELAINE 1, and visits during the first 8 weeks in ELAINE 2, to determine the plasma trough steady-state concentrations (Css) of LAS, Abema, and two active Abema metabolites (M2 and M20). LAS was assayed using validated high-performance liquid chromatography/tandem mass spectrometry (LC/MS/MS; detection 0.025 to 25.0 ng/mL); levels below the lower limit of quantification (LLOQ) were reported as 0 ng/mL. Abema and its M2 and M20 metabolites were measured by validated Turbo Ion Spray LC/MS/MS (detection 1.00 to 500 ng/mL); values below the LLOQ were reported as LLOQ ng/mL. Mean trough Css at each visit were calculated for LAS (ELAINE 1 and 2), and Abema and its M2 and M20 metabolites (ELAINE 2). LAS levels from ELAINE 1 and 2 were compared, while Abema, M2 and M20 plasma levels in ELAINE 2 were compared with previously reported levels. Such indirect comparisons may help determine if Abema altered the metabolism of LAS, or if LAS altered that of Abema, M2, or M20. The relationships of LAS and Abema levels with progression-free survival (PFS) were also explored. Results: Across all time points for subjects who had PK samples assessed, the LAS mean trough Css was 33.6 ng/mL (CV%, 39.8) in ELAINE 1 (n=40) and 30.9 ng/mL (CV 50.5%) in ELAINE 2 (n=29); in both studies, LAS concentrations were consistent across visits. LAS levels in ELAINE 2 were similar to those in ELAINE 1 with no evidence of accumulation. Mean (CV%) Abema plasma concentrations at weeks 2, 4, 6 and 8 were 199 (102%), 107 (130%), 82 (271%), and 94.3 (223%) ng/mL, respectively. Minimum and maximums of the mean trough Css for LAS, Abema, M2, and M20 are shown in Table 1. No relationships between LAS or Abema levels and antitumor activity measured by PFS were seen (data to be presented). Conclusion: The steady-state PK of 5 mg LAS in the ELAINE 1 and 2 studies suggest that the addition of abemaciclib to LAS did not result in an obvious or significant LAS-Abema, drug-drug interaction. Abema PK data from ELAINE 2 were consistent with published data (European Medicines Agency, Verzenio). These results support the continued development of LAS alone and combined with Abema for treating ER+/HER2-, ESR1-mutated mBC that progressed on endocrine therapy and CDK4/6 inhibitors. Table. Mean trough, steady-state concentrations of lasofoxifene and abemaciclib. XX- XX aData from patients who received oral LAS (5 mg/day) plus Abema (150 mg BID). Abema, abemaciclib; LAS, lasofoxifene; M2 and M20 are Abema metabolites. Citation Format: Senthil Damodaran, Paul V. Plourde, Simon Jenkins, Phil Mayer, David J Portman, Matthew Goetz. Pharmacokinetics (PK) of lasofoxifene (LAS) monotherapy and combined with abemaciclib (Abema) abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-18-05.