Background: Leptomeningeal metastases (LM) represent an aggressive form of advanced cancer with few durable therapeutic options. One of the principal barriers in treating LM is the paucity of knowledge on cancer cell survival and proliferation within the nutrient-sparse cerebrospinal fluid (CSF). Single-cell RNA sequencing of patient-derived CSF has identified that cancer cells in the spinal fluid employ the single iron-binding transporter and receptor system, lipocalin-2/SLC22A17, to gather sparse iron to sustain their metabolic needs. This phenotype is recapitulated in preclinical mouse models of LM. Depletion of CSF iron via intracisternal administration of deferoxamine, a parenteral iron chelator, dramatically reduced LM growth and significantly prolonged survival in preclinical models. Exploiting LM iron dependency using intrathecal deferoxamine (IT-DFO) represents a novel therapeutic approach for patients with LM. Methods: This is a prospective, open-label, single center phase Ia dose escalation study of IT-DFO in patients with LM from any solid tumor malignancy to determine the maximum tolerated dose (MTD) and recommended phase II dose (RP2D), followed by a phase Ib dose expansion of IT-DFO at the RP2D in patients with LM from non-small-cell lung cancer (NSCLC). Eligibility criteria include newly diagnosed or recurrent LM identified by magnetic resonance imaging (MRI), positive CSF cytology, and/or elevated CSF circulating tumor cells (CTCs), age ≥ 18 years, Karnofsky Performance Status ≥ 60, and life expectancy ≥ 8 weeks. All patients will receive IT-DFO in 28-day cycles at a frequency of twice weekly (cycle 1), once weekly (cycle 2), and once every two weeks (cycle 3+). Patients will be monitored for LM progression by neurological examination, neuraxial MRI, and CSF cytology as per modified Response Assessment in Neuro-Oncology LM criteria. Phase Ia will involve a modified accelerated titration over 9 dosing cohorts (IT-DFO dose range 10mg to 495mg) with monitoring for dose-limiting toxicities until the MTD is reached. Phase Ib will further explore the safety of IT-DFO at the RP2D in 20 patients with NSCLC LM. Secondary objectives include the determination of pharmacokinetic and pharmacodynamic properties of IT-DFO and its metabolite, ferrioxamine, in the CSF and serum (phase Ia/Ib), and efficacy outcomes (phase Ib) including LM-objective response rate, LM-clinical benefit rate, LM-duration of response, LM-progression-free survival, and overall survival. Exploratory analyses will prospectively correlate CSF CTC enumeration with treatment response and characterize the impact of IT-DFO on cancer cell metabolism, resistance pathways, and the CSF immune microenvironment. Clinical trial information: NCT05184816.