P-glycoprotein mediated efflux limits substrate and drug uptake in a preclinical brain metastases of breast cancer model

Abstract

The blood–brain barrier (BBB) is a specialized vascular interface that restricts the entry of many compounds into brain. This is accomplished through the sealing of vascular endothelial cells together with tight junction proteins to prevent paracellular diffusion. In addition, the BBB has a high degree of expression of numerous efflux transporters which actively extrude compounds back into blood. However, when a metastatic lesion develops in brain the vasculature is typically compromised with increases in passive permeability (blood-tumor barrier; BTB). What is not well documented is to what degree active efflux retains function at the BTB despite the changes observed in passive permeability. In addition, there have been previous reports documenting both increased and decreased expression of P-glycoprotein (P-gp) in lesion vasculature. Herein, we simultaneously administer a passive diffusion marker (14C-AIB) and a tracer subject to P-gp efflux (rhodamine 123) into a murine preclinical model of brain metastases of breast cancer. We observed that the metastatic lesions had similar expression (p > 0.05; n = 756–1214 vessels evaluated) at the BBB and the BTB. Moreover, tissue distribution of R123 was not significantly (p > 0.05) different between normal brain and the metastatic lesion. It is possible that the similar expression of P-gp on the BBB and the BTB contribute to this phenomenon. Additionally we observed P-gp expression at the metastatic cancer cells adjacent to the vasculature which may also contribute to reduced R123 uptake into the lesion. The data suggest that despite the disrupted integrity of the BTB, efflux mechanisms appear to be intact, and may be functionally comparable to the normal BBB. The BTB is a significant hurdle to delivering drugs to brain metastasis.

The Translational Significance of the Neurovascular Unit

“…These studies are now reaching clinical trials, an exciting landmark in targeted CNS therapeutics. Other groups are investigating inhibition of efflux mechanisms to maintain drug levels in the NVU (95,96). Successful modulation of these endogenous transport systems will prove critical to drug delivery in the NVU.…”

Emerging Strategies for Treating Brain Metastases from Breast Cancer

“…While preclinical data indicate that p-glycoprotein (p-gp) mediated efflux kinetics are similar between normal BBB and BTB (Adkins et al, 2013), clinical data suggest p-gp expression in metastatic brain tumors is similar to that of primary, extracranial tumors and decreased compared to primary brain tumors (Gerstner and Fine, 2007). In a murine model, a non-p-gp substrate HER2/EGFR kinase inhibitor displayed modest but significantly better control of brain tumors compared with lapatinib (Nakayama et al, 2013), supporting the role of active drug efflux on pharmacokinetics and efficacy.…”

Effects on P-Glycoprotein Expression after Blood-Brain Barrier Disruption Using Focused Ultrasound and Microbubbles

“…In the case of epilepsy and neurodegenerative disorders such as Amyotrophic Lateral Sclerosis (ALS), studies have suggested that Pgp expression may be elevated [2,3], potentially further restricting the delivery of drugs and resulting in less therapeutic benefits [4]. Additionally, with brain tumors, Pgp can be overexpressed in both the semi-permeable “blood-tumor barrier” (BTB) but also in the plasma membrane of tumor cells [5,6]. Overexpression of this protein and other efflux pumps are linked to multi-drug resistance against several anticancer drugs [7] and can result in tumors developing cross resistance to other therapeutics.…”