Rather than injecting tumor cells orthotopically, we injected MDA-MB-231Br cells intracardially and waited 32–35 days for mice to develop neurological symptoms before injecting either 14C-irinotecan or 14C-NKTR-102 intravenously to collect brains at respective plasma SN38 Cmax times (2-h for conventional irinotecan, and 6-h for NKTR-102). Here, NKTR is linked to neoplasm.