Treatment patterns and outcomes in pacritinib-treated patients with MF and higher platelet count: MY-PAC study.

6575 Background: Pacritinib (PAC), a JAK1-sparing JAK2/IRAK1/ACVR1 inhibitor has shown spleen volume and symptom improvement in clinical trials of in patients (pts) with myelofibrosis (MF) with similar benefits in the real-world. This analysis evaluated real-world outcomes in pts with MF and platelet counts (PLT) ≥50 x10 9 /L at PAC initiation. Methods: This was a multisite chart review of pts with MF treated with PAC for ≥1 month between 06/01/2022 and 07/31/2024 with ≥6 months of follow-up through 01/31/2025. Pts were followed from PAC initiation (index) until the earliest of end of the study, the end of data availability for the pt, or death. Evaluated data included Pt characteristics, treatment patterns, palpable spleen length (total spleen length minus 10cm) and spleen size category based on palpation or palpable spleen length below costal margin (not palpable/minimally palpable 20 cm palpable), PLT, hemoglobin (Hb), MF-related symptoms, and overall survival through Day 180. Results for pts with PTL ≥50 x10 9 /L at index are described using counts and percentages, medians and interquartile range (IQR), and survival probabilities. Results: Of 169 pts with MF treated with PAC, 27% (45/169) had PLT ≥50 x 10 9 /L at index. Median age was 67 years (IQR: 62-74) at diagnosis and median time from diagnosis to index was 6 months (IQR: 1.3-14.3). The median duration of follow-up from index was 7.6 months (IQR: 6.7-11.0) and 82% of pts (n=37) were still on PAC at end of follow-up. Of 12 pts with spleen length measured by ultrasound at index and Day 180, median spleen length reduction (SLR) was 39% (IQR: 30.4-48.3) with 83% (10/12) achieving SLR ≥25% by Day 180. Among those with categorical spleen size based on palpation (n=5) or ultrasound (n=12), 53% (9/17) achieved a reduction in spleen size category. No pts had a worsening in spleen size category, and spleen size category remained stable for 47% (8/17) of pts. At index, all but one pt had ≥1 MF-related symptom. By Day 180, 80% (35/44) experienced a reduction in the number of symptoms, with a median reduction of 67% (IQR: 33-75). Median PLT at index was 72 x 10 9 /L (IQR: 53-110) and from index to Day 180, median increase in PLT was 14% (n=38, IQR: 0-30). Among pts with PLT ≤100 x10 9 /L at index 20% (6/30) achieved an increase of ≥30x10 9 /L. At index, median Hb was 8.8 g/dL (IQR: 8.0-9.7). Among 29 pts with Hb