First-in-human study of <sup>225</sup>actinium mti-201 (<sup>225</sup>Ac-MTI-201) in metastatic uveal melanoma (UM).
Authors: Nikhil I. Khushalani, Ghassan El-Haddad, Kenneth L. Gage, Mikalai Budzevich, Michael J Schell, Eduardo G Moros, Christopher Tichacek, William R Gibbons, Rebecca E Rogers, Michael L Nickels, Leticia Tetteh, Zeynep Eroglu, Lilit Karapetyan, Andrew Scott Brohl, Joseph Markowitz, Ahmad A. Tarhini, Karen Hayes, Lori Hazlehurst, Mark McLaughlin, David L Morse
Journal: Journal of Clinical Oncology (2024)

TPS9612 Background: Prognosis in metastatic uveal melanoma (UM) has historically been poor approximating 1 year from diagnosis. Tebentafusp is the only approved systemic agent for HLA-A*02:01 positive metastatic UM, and novel therapies are needed. The melanocortin-1 receptor (MC1R) receptor is highly expressed in UM with limited expression in normal tissues. Actinium-225 ( 225 Ac) is an alpha particle emitting radionuclide ideal for targeted ligand binding due to high linear energy transfer and limited free path (&lt;100 µm) in tissue. We developed a novel MC1R targeted radiopharmaceutical 225 Ac-MTI-201, and demonstrated high biostability, affinity, MC1R-specific cytotoxicity with defined dosimetry and pharmacokinetics in pre-clinical studies (1). Murine efficacy studies of UM showed significant delay of tumor growth and improved survival compared to controls. Methods: This is a single institution first-in-human phase I study (NCT05496686) of 225 Ac-MTI-201 in metastatic UM. The primary objective is to determine the safety of a single intravenous dose of 225 Ac-MTI-201 relative to radioactivity dose. Secondary endpoints include 1) pharmacokinetics (PK) and clearance of 225 Ac-MTI-201, 2) objective response rate and duration, progression-free survival, and overall survival in metastatic UM. Salient eligibility criteria include: 1) age ≥18 years; 2) histologically confirmed metastatic UM with progression after at least 1 prior line of therapy; 3) measurable disease per Response Evaluation Criteria in Solid Tumors version 1.1; 4) Eastern Cooperative Oncology Group performance status of ≤1; 5) adequate marrow, renal and hepatic function; 6) no more than 25% of bone marrow treated with prior radiotherapy. Patients will receive a single dose of 225 Ac-MTI-201 under appropriate guidelines and precautions for administration of radiopharmaceuticals. Pre-injection and post-injection PK sampling at defined intervals are undertaken. There are 12 dose levels for 225 Ac-MTI-201 being investigated ranging from 4.7 microcurie (µCi) to 1327 µCi. Dose escalation is done using a modified continual re-assessment method with a cohort size of one based on dose limiting toxicity (DLT) assessment within 28 days of drug administration using CTCAE v5.0. Definitions of DLT include G4 neutropenia or febrile neutropenia, G4 thrombocytopenia or G3 thrombocytopenia with clinically significant bleeding, &gt; G3 non-hematological toxicity with selected exceptions, laboratory abnormalities that satisfy Hy’s law, or any death not related to underlying disease or extraneous cause. Response assessments are undertaken with cross-sectional imaging every 8 weeks in year 1, and every 12 weeks in year 2. This study is currently open for enrollment. Dose levels 1-4 have been completed without DLT. 1. Tafreshi, J Nucl Med 2019. Clinical trial information: NCT05496686 .