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The natural history of hormone receptor–positive breast cancer tends to be more favorable than other subtypes such as human epidermal growth factor receptor 2–amplified and triple-negative. In addition, the natural dependence on steroid hormone signaling has allowed for therapeutic targeting of this pathway and significant improvements in survival while maintaining quality of life: the two main goals in management of the disease. The sequential use of endocrine agents including the selective estrogen receptor modulators (tamoxifen), aromatase inhibitors (letrozole, anastrozole, and exemestane) and the selective estrogen receptor degrader fulvestrant has been the backbone of management for years. In the past decade, the introduction of molecularly targeted agents against intracellular targets such as mammalian target of rapamycin (everolimus), cyclin-dependent kinases 4 and 6 (palbociclib, ribociclib, and abemaciclib), and phosphatidylinositol 3-kinase (alpelisib) has offered patients effective nonchemotherapy-based options, which are improving outcomes. Although knowledge gaps still exist in regard to the optimal sequencing of these new regimens, they have expanded our repertoire of options for patients and have shifted the need for cytotoxic chemotherapy and its associated complications to later lines. Still, formatting a plan for these patients includes taking into account traditional prognostic factors such as menopausal status, previous treatments, disease-free interval for those patients with early breast cancer that has recurred, and tumor burden. To assist in developing this treatment plan, we will review the current data with systemic agents in the management of these patients.

Purpose: The CDK 4/6 inhibitor palbociclib rapidly and reversibly inhibits the cell cycle. The goal of this study was to exploit the cell cycle through intermittent, alternating dosing with palbociclib/paclitaxel to enhance efficacy. We determined the combination dose-limiting toxicity (DLT) in patients with Rb protein-expressing, advanced breast cancer. Patients and Methods: This open-label, phase I trial (NCT01320592) enrolled patients to sequential cohorts of palbociclib orally dosed intermittently between days 1 and 19 of a 28-day cycle alternating with weekly paclitaxel. Dose escalation proceeded in a standard 3 þ 3 design. Ten additional patients received the combination at the recommended phase II dose (RP2D). Those who reached response plateau !6 cycles could continue on palbociclib alone on a 3 week on/1 week off schedule at one dose level above their combination dose. Results: Twenty-seven patients enrolled. Although there was only 1 DLT (grade 3 alanine aminotransferase/aspartate aminotransferase at 125 mg), neutropenia (NTP) requiring dose modification in cycle 1 (C1) resulted in an RP2D of 75 mg palbociclib/80 mg/m 2 paclitaxel. During C1, the most common adverse event was NTP, occurring in 15 patients (55.6%); grade 1 or 2 nausea and peripheral neuropathy were also observed in 8 patients each (29.6%). The clinical benefit rate was 55% at the RP2D; benefit was observed across all receptor subtypes. Conclusions: Alternating sequential palbociclib/ paclitaxel in patients with Rb þ advanced breast cancer is feasible and safe, without evidence of additive toxicity. This represents a new application for CDK 4/6 inhibitors in Rb þ breast cancer regardless of subtype; efficacy trials are warranted.

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