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The implication is that, as Becky mentioned, "large molecule" drugs do not pass the blood-brain barrier (BBB). The BBB is an anatomic structure consisting of endothelial vessel cells, astrocytes and pericytes with tight junctions and a number of carrier proteins, controls and limits the passage of molecules to the brain.
In the presence of an intact BBB, only "small lipophilic molecules" (molecular weight [MW] >400 Da) can cross the BBB by diffusion, while the passage of other molecules is regulated by the carrier proteins.
Apart from a few drugs, such as temozolomide, melphalan, carmustine and irinotecan, chemotherapeutic agents, which are "large hydrophilic molecules," are unable to cross the BBB, which is furthermore characterized by a high concentration of multidrug resistance efflux pumps, which may be another cause of the low concentration of drugs reaching the site of action.
As soon as the drugs enter the cancer cells, the P-glycoprotein (PGP) pumps start pumping the drugs out. PGP is a transmembrane efflux pump - it pumps harmful things from the inside of the cell to the outside of the cell. The presence of PGP signals that patients will not respond well to chemotherapy.
The reasons for the association between treatment of breast cancer with a taxane-containing chemotherapy regimen and an increased incidence of CNS involvement is that the concentration in the CNS is very low after intravenous administration.
Taxanes are unable to to penetrate the BBB, the concentration of radiolabeled taxane in the cerebrospinal fluid is found to be significantly lower than in other organs, and thus undetectable in the brain, in the spinal cord or in any other site of the CNS.
Also, taxanes are exported from the PGP and other ATP-binding cassette transporters placed at the luminal membrane of brain capillaries, as an explanation for the low concentrations of taxanes in the CNS.
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