Doublet and Triplet Combinations of Eribulin, Fulvestrant, and Palbociclib in Preclinical Breast Cancer Models

Background/Aim: This study investigated in vivo synergism between eribulin and palbociclib in a breast cancer patient-derived xenograft (PDX) model, with expanded scope to include fulvestrant as a third drug. Materials and Methods: Eribulin plus palbociclib combinations were tested in vitro in six cell lines each of estrogen receptor positive and triple-negative breast cancer, and in vivo in the OD-BRE-0192 PDX model using weekly eribulin plus 5×/week or 7×/week palbociclib (holiday or noholiday schedules, respectively). When included as a third drug, fulvestrant was dosed weekly. Results: In vitro, combining palbociclib with eribulin led to increased eribulin IC50s in 11 of 12 cell lines, suggesting that the drugs antagonized each other due to mutual exclusion of the mitotic and G1/S cell cycle block points for eribulin and palbociclib. An in vivo study in the OD-BRE-0192 PDX model compared weekly eribulin plus either palbociclib holiday or no-holiday schedules to gauge the importance of post-palbociclib cell cycle synchronization. Results showed no advantage of holiday over no-holiday schedules, arguing that differing pharmacokinetics of the drugs were sufficient to overcome cell cycle-based mechanistic antagonism. In vivo comparisons of doublet and triplet combinations of eribulin, palbociclib, and fulvestrant showed that all three doublets were superior to individual monotherapies, and that the triplet combination was markedly superior to all three doublets, being the only group to show tumor regression in 100% of the mice. Conclusion: Results show complex synergistic interactions between eribulin, fulvestrant, and palbociclib, and point to a particularly robust synergy when combining all three drugs.