A versatile drug carrier, poly(β-l-malic acid) (PMLA), a natural polymer obtained from the slime mold Physarum polycephalum25,26, is used to deliver covalently conjugated CTLA-4 and PD-1 antibodies (a-CTLA-4 and a-PD-1) to brain tumor cells, which results in local immune system activation and prolonged survival of intracranial GBM GL261-bearing mice. Here, CTLA4 is linked to glioblastoma.