1005 Background: Metastatic breast cancer (MBC) is generally incurable and the majority of patients with MBC will remain on treatment indefinitely. Patients usually begin each new treatment at the Recommended Starting Dose (RSD) on the FDA-approved label based on results from clinical trials. However, patients’ ability to tolerate the RSD in the real-world may differ from the clinical trial setting. While the importance of patient reported outcomes is recognized, understanding tolerability from the patient’s perspective is lacking and patients’ willingness to discuss individualized doses for MBC therapy has not been evaluated. Methods: Patient advocates from the Patient-Centered Dosing Initiative distributed a confidential online survey to patients with MBC via social media groups, organizational newsletters, and online support forums. The survey was developed by patients and medical oncologists to ascertain the prevalence and impact of patients’ treatment-related side effects, quality of patient-physician communication, management of side effects, and interest in alternative approaches to the RSD when a new treatment is initiated or adverse side effects are experienced. Results: 1,221 patients with MBC completed the survey within 15 days. The median number of lines of MBC therapy was 2.5 (range 1 - ≥5) and 46% (n = 564) of patients received their MBC diagnosis within two years of taking the survey. 86% (n = 1,051) reported experiencing at least one significant treatment-related side effect, and of these, 20% (n = 213) visited the Emergency Room/hospital and 43% (n = 454) missed at least one treatment. 98% (n = 1,026) of patients with side effects discussed them with their doctors and 82% (n = 838) were helped by their physicians. The most common (non-exclusive) mitigation strategies were dosage reductions (66%, n = 556) and prescription medications (59%, n = 494). Of the 556 patients given a dosage reduction, 83% (n = 459) reported feeling better. Notably, 92% (n = 1,127) of patients expressed willingness to discuss alternative dosing options with their physicians based upon their personal characteristics and individual preferences. Conclusions: Given that 86% of patients with MBC experienced at least one significant treatment-related side effect and 83% improved after dosage reduction, innovative dosage-related strategies are warranted to sustain Quality of Life. Patient-physician discussions in which the patient’s physical attributes and circumstances are periodically assessed may determine the right dose for the patient upon treatment initiation and afterwards, and the vast majority of patients would be receptive to such discussions.

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Background: MBC is generally incurable and most patients will remain on treatment indefinitely. Medical oncologists commonly prescribe the Recommended Starting Dose (RSD) from the FDA-approved label based upon clinical trial results, with lower doses typically administered after patients’ experience treatment-related toxicity. However, patient tolerance for the RSD in the real-world setting differs from that of clinical trial participants. We previously reported results from a survey of patients with MBC that revealed patients’ willingness to discuss individualized dosing with medical oncologists (Loeser, ASCO 2021). We now report results from a corresponding survey of medical oncologists. Methods: Patient advocates and medical oncologists from the Patient-Centered Dosing Initiative disseminated a confidential, 28-question online survey to US medical oncologists via professional organizations, social media, and email lists. The survey’s objectives were to understand oncologists’ perspectives regarding the prevalence and severity of treatment-related toxicity, increase recognition of the impact of dosage-related mitigation strategies on clinical outcomes, and determine oncologists’ willingness to discuss flexible dosing approaches when patients with MBC begin a new systemic treatment or experience side effects. Results: 131 medical oncologists responded to the survey within the allotted period (4/26 -5/24, 2021). 12 were excluded from further analysis because they were not US-based medical oncologists or lacked experience treating patients with MBC. Of the 119 eligible participants, 85% specialized in breast cancer, 66% worked in an academic setting, and 82% had practiced for > 5 years. Overall, physicians estimated that 47% of patients reported a distressing treatment-related side effect, 15% visited the ER/hospital, and 37% required a break from treatment. The most common mitigation strategies (96% each) were prescribing supportive medication, offering suggestions for palliation, and lowering the dose. Dose reduction was effective, with 74% of doctors reporting patient improvement thereafter. 87% (n=104) of physicians indicated that they had started patients at a lower dose for various reasons, and 60% noted that these patients felt better than patients commencing the same therapy on the RSD. The drugs/classes most commonly prescribed at lower initial doses were capecitabine (91%), taxanes (60%), and palbociclib (50%). Notably, the majority (85%, n=101) of oncologists surveyed do not believe that the RSD is always more effective than a lower dose, and 97% (n=115) would be willing to discuss drug dosing options with patients based on their unique personal attributes. Conclusion: Medical oncologists reported that a high percentage of patients experience distressing toxicity, a need for acute care, and dose interruptions with standard delivery of therapy for MBC. Many elect to decrease the starting dose for select drugs, and almost all are open to discussing lower initial and successive doses. We conclude that these patient-centered dosing discussions should be part of routine care and may consequently improve QOL for patients with MBC. Citation Format: Anne L Loeser, Aditya Bardia, Mark E. Burkard, Kevin M Kalinsky, Jeffrey Peppercorn, Hope S. Rugo, Martha Carlson, Janice Cowden, Julia Maues, Sheila McGlown, Maryam Lustberg. Patient-centered dosing: Oncologists’ perspectives about treatment-related side effects and individualized dosing for patients with metastatic breast cancer (MBC) [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-10-09.