Sym021 will be given to patients in escalating dose cohorts; each patient will be given one fixed dose level. There will be a minimum of 3 patients per dose cohort, assuming approximately 3 dose cohorts to be evaluated to establish the MTD and/or RP2D dose. Any cohort will be expanded to 6 patients in the event of a Cycle 1 DLT in any of the initial 3 patients. A minimum of 6 patients are to be treated at the MTD and/or RP2D dose; expansion of this cohort (or any other) up to 12 patients may be considered to further evaluate tolerability.