This was largely due to the following: (i) the hydrophilicity of the radiolabeled analogues—as reflected in the “measured” the radiotracer distribution (LogD) in octanol/PBS—which stands in contrast to the structure-based estimate of LogP, which was the rationale for initiating the study; (ii) the presence of a modest BTB in intracranial U87-CXCR4 gliomas and an intact BBB/BTB in the intracranial PCNSL animal model. The gene discussed is CXCR4; the disease is central nervous system cancer.