Chemotheraputic drug uptake and the role of blood-tumor barrier in brain metastases of breast cancer

Brain is increasingly seen as a primary site of metastasis in breast cancer. The incidence of brain metastases is increasing each year with better systemic therapies and increasing control achieved over primary cancer. In brain metastases, tumor cells seed brain, forming micrometastases, which grow along blood vessels. These co-opted vessels are believed to have an altered bloodbrain barrier (BBB). The degree of change or the extent to which it is compromised is still widely discussed. While one theory states that the BBB in tumors, the Blood tumor barrier (BTB) is intact causing an impediment to drug delivery, the contrary hypothesis is that, the drugs cross the BTB and reach the tumor but are ineffective due to various intrinsic factors in the tumor. In this study, we measured blood-tumor barrier (BTB) permeability and uptake of the widely used anticancer drug doxorubicin using two new brain-seeking breast cancer metastatic models; the human cancer cell line, MDA-MB-231 HH2 and mouse mammary cancer cell line 4T1-Br5, in immune compromised mice. Our results suggest that - 75% or three in four tumors differ appreciably from normal brain in doxorubicin concentration with greater than 10 fold uptake compared to normal brain. However, these elevated concentrations were only 2% of that in peripheral tissues. This low uptake into brain was apparent even compared to peripheral metastases in heart, kidney and liver. Brain metastases concentration was less than 2% suggesting a substantial role of the BTB in limiting doxorubicin uptake into tumors and possibly limiting effect. This exclusionary role of the BTB was further confirmed by an estimation of Kp- the tissue to plasma partitioning. Partitioning of doxorubicin was lowest into brain and brain metastases; up to 200 times lower than most peripheral tissues and choroid plexuses, a non-barrier region in the brain. While most previous studies compared in vitro free cytotoxic concentrations to in vivo total tissue concentrations as a measure of efficacy, we measured both free and total levels of drug in vitro and in vivo for accurate comparisons. These results reveal that both free and total levels of drugs in tissue fall far below (200x less) those required for cytotoxicity. Anticancer drugs are extensively transported by efflux transporters at the BBB. Analysis for efflux transport suggested greater than two fold effect of P-gp, minimal role up to 1.5 fold BCRP and surprisingly our results suggest a substantial 15 fold increase for MRP 7, a whole new transporter for doxorubicin, suggesting that MRP's might also have a substantial role along with P-gp. Finally, we studied brain metastases uptake of Doxil and a novel drug Vorinostat. These two drugs showed slightly improved, but restricted delivery to brain metastases. Our results suggest the formidable role of the BTB in limiting anticancer drug uptake in brain metastases. The results raise new questions about drug delivery to brain metastases which lie behind a partially altered BTB, different from the normal BBB. In addition, there might be a role of more than one transporters in this limiting role. In addition to P-gp and BCRP, MRP's also might have a substantial role. There is a need for development of better CNS targeted drugs the modulating the role of transporters at the BBB may be crucial to this purpose.