The “route cause” of methotrexate-induced brain structure changes in a juvenile mouse model: Comparison of systemic and CNS-targeted chemotherapy

Acute lymphoblastic leukemia (ALL) is the most common childhood cancer and while five-year survival rates exceed 90 %, survivors display neurocognitive deficits. Magnetic resonance imaging (MRI) measurements indicate smaller volume across the brain in survivors compared to typically developing peers. Methotrexate (MTX) is the backbone of ALL chemotherapy and is delivered via various administration routes including systemic and central nervous system (CNS) targeted routes. The relative toxicities between routes have not been systematically compared. Our study aims to compare brain volume changes after systemic and CNS-targeted MTX treatment using MRI in a juvenile mouse model. MTX treatment was delivered at postnatal day 17 (P17) and P19 either via an intrathecal (IT) or intravenous (IV) injection, resulting in four total groups for the study: IV MTX (n = 14), IV saline (n = 16), IT MTX (n = 54), and IT saline (n = 51). MRI was performed pre-treatment at P14 and longitudinally after treatment at P24, P42, and P63. IT MTX was probed at a range of doses (0.5–5.0 mg/kg). Volumes of 183 segmented brain structures were compared between groups. Whole brain volume decreased after IT MTX (5.0 mg/kg) and IV MTX at P24. The number of structures significantly affected after IT MTX was highly dependent on dose. Comparison of systemic and intrathecal delivery routes revealed that systemic MTX had a wider impact on brain morphology than did IT MTX treatment, particularly at clinically relevant doses of IT MTX. This finding provides important insight into the mechanisms that likely underlie MTX-induced neurotoxicity and focuses potential interventions on systemic toxicity.