Dana-Farber Cancer Institute, Boston, MA; 2 City of Hope National Medical Center, Duarte, CA; 3 Roswell Park Cancer Institute, Buffalo, NY; 4

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1 Results from Ongoing Phase 2 Trial of SL-401 As Consolidation Therapy in Patients with Acute Myeloid Leukemia (AML) in Remission with High Relapse Risk Including Minimal Residual Disease (MRD) Andrew A. Lane, MD, PhD 1, Kendra Sweet, MD 6, Eunice Wang, MD 3, William Donnellan, MD 4, Roland B. Walter, MD, PhD 5, Anthony Stein, MD 2, David Rizzieri, MD 7, Hetty E. Carraway, MD 8, Ioannis Mantzaris, MD 9, Thomas Prebet, MD, PhD 10, Michael Maris, MD 11, Stefan Faderl, MD 12, Dale Bixby, MD 13, Janice Chen, PhD 14, Ross Lindsay, PhD 14, Shay Shemesh, MA 14, Chris Brooks, PhD 14, Richard M. Stone, MD 1, Hagop M. Kantarjian, MD 15, Elias Jabbour, MD 15, and Marina Konopleva, MD, PhD 15 1 Dana-Farber Cancer Institute, Boston, MA; 2 City of Hope National Medical Center, Duarte, CA; 3 Roswell Park Cancer Institute, Buffalo, NY; 4 Sarah Cannon Research Institute, Nashville, TN; 5 Fred Hutchinson Cancer Research Center, Seattle, WA; 6 Moffitt Cancer Center, Tampa, FL; 7 Duke University School of Medicine, Durham, NC; 8 Cleveland Clinic, Cleveland, OH; 9 Montefiore Medical Center, Bronx, NY; 10 Yale School of Medicine, New Haven, CT; 11 Colorado Blood Cancer Institute, Denver CO; 12 Hackensack University Medical Center, Hackensack, NJ; 13 University of Michigan Health System, Ann Arbor, MI; 14 Stemline Therapeutics, Inc., New York, NY; 15 The University of Texas MD Anderson Cancer Center, Houston, TX

2 MRD: Emerging Predictor of Relapse Risk in AML Patients in CR Despite high CR rates with combination chemotherapy in first-line AML, the large majority of patients ultimately relapse Minimal residual disease (MRD) positivity is an emerging prognostic for relapse and poor outcome AML in CR MRD(+) MRD(-) MRD has been shown to be a poor prognostic factor in a number of other hematologic malignancies including CML and ALL Techniques to reliably quantify MRD are being refined and optimized Freeman, S et al. JCO 31(32),

3 MRD Assessment MRD assessment tools 1) Flow cytometry Broadly applicable (>90% AML), ID of LAIPs, high sensitivity (0.01%-0.001% [10 6 events]) 2) Cytogenetics / FISH Widely available, economical, lower sensitivity (0.5-5%) 3) Real-time quantitative RT-PCR (RQ-PCR) Not all AMLs have a suitable target (30-60%), highly sensitive (0.01%-0.01%) 4) Somatic mutations Can be extremely sensitive (0.001% or lower), may require prior sequencing, some mutations do not track well with blasts (e.g., underlying clonal hematopoiesis) MRD assessment in this study Flow cytometry standardization for consistency and reproducibility, with other techniques supportive Centralize analyses for uniformity 3

4 MRD Assessment Flow Cytometry Standardization Decreased CD45 Characteristic CD34 and CD38 Subset of decreased CD15 Increased CD33 and HLA-DR Increased CD123 Grimwade and Freeman Blood (2014) 124(23); Ouyang, J. AJCP (2016); 145:769; Wood, BL Methods Mol Biol (2013) 999:

5 MRD is Enriched for CD123 + AML Stem Cells MRD is enriched for AML stem cells and CD123+ MRD correlates with AML stem cell %, which correlates with poor disease free survival (DFS) van Rhenen, A. Clin Can Res 2005 *Jorgensen, J. L. Clin Lymphoma Myeloma Leuk, 2011; Konopleva, M. (unpublished) 5 Vergez, F. Haematologica 2011

6 SL-401 Targets IL-3Rα (CD123); Clinically Active in CD123 + Cancer Truncated diphtheria toxin payload SL-401 IL-3 IL-3R/CD123 Cancer cell CD123 expressed on multiple hematologic cancers: AML Myeloproliferative neoplasms Blastic plasmacytoid dendritic cell neoplasm (BPDCN) Myeloma SL-401: Targeted Therapy Directed to IL-3Rα (CD123) Others: MDS, CML, Hodgkin s disease and certain NHLs Design: - Stage 1 dose escalation (completed) - Stage 2 expansion (ongoing) Clinical activity: Phase 2 Trial in BPDCN Abstract #342, Sunday 10:45am - High overall response rates, including multiple complete responses, in first-line and relapsed/refractory Side effect profile: tolerable and predictable - Most common side effects: transient transaminase elevation and hypoalbuminemia; not dose limiting - Capillary leak syndrome pre-emptively managed with vigilance of albumin and body weight, largely in 1 st cycle, has prevented severe cases since implementation - No cumulative side effects with multiple consecutive cycles 6

7 Trial Design and Eligibility Criteria (NCT ) Lead-in (Stage 1) - Completed (n=9) AML in CR Either MRD(+) or MRD(-) - Local analysis of MRD SL-401 daily IV infusion for 5 days every 3 weeks - Dose escalation: 7, 9, or 12 µg/kg/day Primary endpoint: safety and tolerability AML in CR MRD(+) only Expansion (Stage 2) - Open - Centralized analysis of MRD SL-401 daily IV infusion for 5 days every 3 weeks - At highest tested dose in Stage 1 (12 µg/kg/day); MTD not reached Secondary endpoints: Change in MRD, survival Select inclusion criteria AML, achieved first or second CR or CRi within 6 months prior to enrollment Not considered immediate candidates for allogeneic SCT High-risk disease including taml, saml, adverse karyotype, FLT3-ITD, or MRD+ (Stage 1); MRD+ (Stage2) Age 18; ECOG PS 0-2 Adequate organ function including: LVEF LLN, creatinine 1.5mg/dL, albumin 3.2 g/dl, bilirubin 1.5 mg/dl, AST/ALT 2.5x ULN 7

8 Demographics and Baseline Characteristics Age, years Median [range] 63 [51 78] Gender [n, (%)] Male 6 (67) MRD status [n, (%)] MRD(+) 4 (44) 12 µg/kg/day 1 (11) 9 µg/kg/day 2 (22) 7 µg/kg/day 1 (11) MRD(-) 5 (56) 12 µg/kg/day 2 (22) 9 µg/kg/day 1 (11) 7 µg/kg/day 2 (22) Follow-up time on study, months Median [range] 2.48 [ ] Remission [n, (%)] 1 st CR 8 (89) 2 nd CR 1 (11) Line of Therapy [n, (%)] 1 st line Idarubicin + cytarabine 2 (22) Daunorubicin + cytarabine 5 (56) Guadecitabine + idarubicin 1 (11) G-CLAM 1 (11) 2 nd line Cladribine, cytarabine, mitoxantrone, daunorubicin 1 (11) 8

9 SL-401 Dose Escalation and Safety Most common adverse events ( 10% treatment-related adverse events, TRAEs) 7 μg/kg/day (n=3; 3 evaluable) 9 μg/kg/day (n=3; 2 evaluable) 12 μg/kg/day (n=3; 3 evaluable) Adverse Event All Grades (% of total patients) Grade 3 (% of total patients) All Grades (% of total patients) Grade 3 (% of total patients) All Grades (% of total patients) Grade 3 (% of total patients) TRAEs All AEs TRAEs All AEs TRAEs All AEs TRAEs All AEs TRAEs All AEs TRAEs All AEs Transaminase elevation Thrombocytopenia Hypoalbuminemia Headache Nausea Dose escalation (3x3 design) Cohort 1 (7 μg/kg/day): n=3 patients treated, no DLT Cohort 2 (9 μg/kg/day): n=3 patients treated, no DLT Cohort 3 (12 μg/kg/day): n=3 patients treated, no DLT Dose and schedule determined for expansion (Stage 2) 12 μg/kg/day (Highest tested dose; MTD not reached) 9

10 Best MRD Response and Outcomes MRD (+) MRD decrease 12 μg/kg/day (-) Individual patients 9 μg/kg/day (-) (-) (+) (+) 7 μg/kg/day (-) (-) (+) Stem cell transplant (SCT) Relapse-free; bridged to SCT, ongoing Relapse-free; off drug due to AE, unrelated to drug Relapse-free; off drug due to 6 cycle max. for Stage 1 Progression AE, drug-related Months 10

11 MRD Reduction in Patient Treated with SL-401 (12 µg/kg/day) 65-year old female with AML, went into CR following daunorubicin + cytarabine induction MRD(+) by local analysis received 4 cycles of SL-401 (12 μg/kg/day) Reduction in marrow aberrant CD123 + population with monocytic markers (similar to diagnosis) Underwent successful allogeneic stem cell transplant Aberrant population (CD45+/CD14+/CD56 dim /CD123+/CD33+) Screen 0.6% CD123 End of Cycle % CD45 CD14 CD14 11

12 Summary and Conclusions AML in CR has unacceptably high relapse rates (70-80%) MRD positivity is an emerging predictor of relapse and poor outcomes; techniques to quantify MRD are being refined and optimized MRD overexpresses CD123, the target for SL-401 Initial dose escalation clinical results, with SL-401 in AML patients in CR, indicate a tolerable safety profile with preliminary signs of activity Phase 2 expansion currently open for AML patients in CR - MRD(+) only - MRD centrally analyzed for uniformity - Patients receive SL-401 at the highest tested dose (12 μg/kg/day) from dose escalation stage Expansion data updates expected throughout

13 We would like to thank: Acknowledgements Investigators, co-investigators, and study teams at each participating center: This study is sponsored by Stemline Therapeutics, Inc. 13