(P 347) INCIDENCE AND TIMING OF RECURRENCE AFTER ADJUVANT PALBOCICLIB IN COMPLETELY RESECTED RETROPERITONEAL LIPOSARCOMA

Objective: Retroperitoneal liposarcomas (RPS) are rare tumors often diagnosed at an advanced stage. While aggressive surgical resection is the cornerstone of treatment for these patients and can be curative, recurrent disease will still occur in 50% of patients at 5 years. As such, research is focused on preventing recurrence or metastatic disease. Our institution has previously demonstrated that Palbociclib, an oral CDK4/6 inhibitor, is well tolerated in the adjuvant setting and may delay the need for additional treatments (e.g. chemotherapy, surgery, radiation) by two years. However, it is unclear which patients derive the greatest oncologic benefit from Palbociclib and how long patients should remain on Palbociclib to prevent or delay recurrence. We sought to evaluate recurrence patterns in patients with high risk retroperitoneal liposarcomas who completed a course of adjuvant Palbociclib without disease recurrence.

Methods: Patients with no evidence of disease (NED) after resection of their RPS and treated with adjuvant Palbociclib were identified from a prospectively maintained institutional database. Beginning in 2017, high risk patients (e.g. high grade, multifocal disease) were offered adjuvant Palbociclib after complete macroscopic resection (R0/R1) for a period of up to two years. Patients included in this study had NED at the completion of their adjuvant Palbociclib therapy. Time to recurrence was defined as time between completing Palbociclib therapy and recurrence noted on imaging.

Results: Of the thirty-four patients who received adjuvant Palbociclib since 2017, eight met the inclusion criteria of this study. Average age of the cohort was 70y (range: 49-78) and the majority of patients were male (n=6). Most (n=7) of these patients were treated with adjuvant Palbociclib after resection of their primary tumor while one patient received it after resection of a recurrent RPS. Five patients developed a recurrence after therapy cessation. Time to recurrence after completing Palbociclib was within 4 months for most patients (n=4) and 23 months for one patient. The average time on Palbociclib for the patients who recurred was 16.4 months (range: 5-24 months) and for patients who did not recur was 16.3 months (range: 12-23 months). All patients completed at least 12 months of Palbociclib except for one who had to stop therapy after 5 months due to development of bullous pemphigoid. Among the patients with recurrent disease, 3 had de-differentiated RPS and 2 had mixed well and de-differentiated RPS. Among the patients who did not recur, 2 had de-differentiated RPS and 1 had mixed well and de-differentiated RPS. In the cohort of patients who recurred, 3 had FNCLCC grade 3 tumors and 2 had FNCLCC grade 2 tumors. In the cohort of patients currently NED, 2 had FNCLCC grade 2 tumors and 1 had a FNCLCC grade 3 tumor. The three patients without recurrence at the time of this study currently have a disease-free interval of 16, 25, and 33 months since stopping Palbociclib.

Conclusion: Half of the patients in this cohort developed short interval recurrence after completing adjuvant Palbociclib. The optimal duration of adjuvant Palbociclib therapy for patients after RPS resection is not defined nor is how to identify which patients will gain the most oncologic benefit. Studies with larger patient cohorts and genomic analysis of tumors may lend additional insight and is planned for future studies.