A Phase 1a/b Dose Escalation Study of the MYC Repressor Apto-253 in Patients with Relapsed or Refractory AML or High-Risk MDS

INTRODUCTION: APTO-253 represses expression of the MYC oncogene by targeting a conserved G-quadruplex structure in its promoter, down-regulates MYC mRNA and protein levels and induces apoptosis in AML cell lines and marrow samples from patients with AML, MDS, and MPN in vitro. After injection, a large fraction of APTO-253 binds iron and transforms to the Fe(253) ₃ complex which retains full activity. APTO-253 has been granted orphan drug designation for AML by the US FDA and is being studied in a Phase 1a/b clinical trial in patients with relapsed or refractory AML (R/R AML) or high-risk myelodysplasias (high-risk MDS) (NCT02267863).

AIMS: Primary objectives are to determine the safety and tolerability of APTO-253, MTD, dose limiting toxicities (DLT), and the RP2D. Key secondary objectives are to assess the pharmacokinetic (PK) profile, pharmacodynamic (PD) activity, and preliminary evidence of antitumor activity.

METHODS: Eligible patients have R/R AML or high-risk MDS for which either standard treatment has failed, is no longer effective, or can no longer be administered safely. Treatment- emergent adverse events (TEAEs) and tumor responses are evaluated using International Working Group criteria. APTO-253 is administered by IV infusion once weekly on days 1, 8, 15, and 22 of each 28-day cycle; ascending dose cohorts were enrolled at a starting dose of 20 mg/m ² with planned escalation to 403 mg/m ².

RESULTS: As of June 7, 2021, a total of 18 patients (median age 64.0 years, 16 AML and 2 high-risk MDS) with a median of 2.5 prior treatments (range of 1 - 9) have been treated with APTO-253 at doses of 20 (n=1), 40 (n=1), 66 (n=4), 100 (n=4) and 150 mg/m ² (n=8). Most patients were RBC (87.5% of AML and 100% of MDS) and/or platelet (75% of AML and 50% MDS) transfusion-dependent. No DLTs or drug-related serious adverse events have been reported. Only 1 patient had a drug-related TEAE of grade 3 or greater (fatigue, Grade 3, probably related). Preliminary PK analysis (Figure 1) showed that serum levels of APTO-253 were dose proportional. C _max and AUC _0-72h for C1D1 dosing were 0.06, 0.02, 0.36 ± 0.37, 0.44 ± 0.41 and 0.72 ± 0.70 µM and 0.11, 0.15, 3.98 ± 1.77, 4.79 ± 0.87 and 2.51 ± 1.73 µM*h for dose levels of 20, 40, 66, 100 and 150 mg/m ², respectively. Plasma levels for Fe(253) ₃ were significantly higher than those for the APTO-253 monomer. For example, C _max and AUC _0-72h of Fe(253) ₃ for C1D1 dosing of patients in Cohort 150 mg/m ² were 2- and 20- fold higher than the ATPO-253 monomer at 15.09 ± 0.42 µM and 51.52 ± 28.26 µM*h, respectively. Following dosing at 150 mg/m ², serum concentrations of Fe(253) ₃ were above 0.5 µM for > 48 h, which approaches the therapeutic range based on in vitro studies.

CONCLUSIONS: APTO-253 has been well-tolerated at doses of 20, 40, 66, 100 and 150 mg/m ² over multiple cycles and escalated to 210 mg/m ² (Cohort 6). PK analysis revealed that APTO-253 is rapidly transformed to and co-exists with the Fe(253) ₃ in serum from R/R AML and high-risk MDS patients. Enrollment of patients at the 210 mg/m ² dose level is ongoing and updated clinical data will be presented at the meeting.

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