Through the Palace Walls: Blood-Brain Barrier Transport Technologies
This article was originally published in Start Up
Executive Summary
Delivering compounds across the blood-brain barrier remains the rate limiting step in brain-drug development, yet the problem generally gets ignored until it's too late. Several biotechs are bringing novel strategies to finding a solution, focusing especially on brain cancers. Success may open the floodgates of CNS drug development.
Transport Technologies
Delivering compounds across the blood-brain barrier remains the rate limiting step in brain-drug development, yet the problem generally gets ignored until it’s too late. Several biotechs are bringing novel strategies to finding a solution, focusing especially on brain cancers. Success may open the floodgates of CNS drug development.
Marc Wortman
The blood-brain barrier gets no respect. It should. Fewer than 2% of drugs have the requisite properties to get across the BBB. That’s still better than biopharmaceuticals’ success rate. Without assistance, no large molecule drug can penetrate the brain at all—keeping many of the most promising central nervous system therapeutics from ever reaching their targets. And it’s certainly worth a little extra effort: virtually every one of the very few drugs that do succeed in penetrating the BBB with enough active molecules to affect the palace of the soul becomes a blockbuster product.
But many known disease targets remain undruggable, even when potentially beneficial molecules exist. According to one industry executive interviewed by START-UP, most drug designers ignore BBB issues until they prove insurmountable, or simply discard otherwise promising molecules that can’t penetrate the BBB. The lack of focus on such a fundamental R&D problem led him to quip: "Nobody in the industry even knows how to spell BBB."
That ignorance has helped spark a cottage industry. Seeing an opportunity in opening more doorways to the brain, a few young companies have focused on developing new brain drug delivery strategies. Success is far from certain, but advances in BBB science could help crack the pharmaceutical industry’s toughest nut of all: if the brain suddenly became a far more accessible target, those holding the keys to the palace would be positioned to exploit an unprecedented opportunity. (See Sidebar: More Than a Castle Wall: Confronting the BBB.)
Axel Unterbeck, PhD, former president and CSO of Memory Pharmaceuticals Corp. (acquired last year by Roche), and now an executive-in-residence at the VC Oxford Bioscience Partners, spent much of his scientific career seeking therapies for Alzheimer’s disease. [See Deal] He says getting brain therapeutics to their target "is the key problem for CNS pharmacology. Many programs fail because they can’t be tweaked to get across the blood-brain barrier. It’s a huge opportunity." Christopher Starr, PhD, CEO and co-founder of Raptor Pharmaceutical Corp., which is pursuing novel vehicles for delivering drugs across the BBB, illustrates the scope of the problem and opportunity should a solution be found. "There are hundreds and hundreds of potentially efficacious drugs sitting on benches around the world and not being used because they don’t cross the blood-brain barrier," says Starr.
Not every molecule fails at this difficult task, but unfortunately, the few drugs presently able to reach inside the brain seem poorly suited to producing an effect on major neurodegenerative pathways, such as the enzyme beta-secretase in the amyloid pathway implicated in Alzheimer’s disease. Traumatic conditions such as stroke, genetic disorders such as Huntington’s disease and enzymatic deficiencies, and brain cancers are likewise difficult to affect. While treatment for many psychiatric disorders has improved substantially, patients still have significant side-effects and many experience only partial or no improvement.
Emerging Strategies
A handful of new BBB-crossing strategies have emerged, however. They may enable many more molecules of all sizes and types to get inside the brain, including presently barred biological drugs such as DNA-modulating agents, enzyme-replacement therapies, monoclonal antibodies, recombinant proteins, and oligonucleotide therapeutics such as small interfering RNA. These agents appear more likely to prove effective in treating many presently untreatable or difficult to treat CNS conditions. Some experimental BBB technologies employ mechanical means to dilate pores in the BBB temporarily. Others mask molecules within formulations that enable their diffusion through the BBB. Many of the most promising technology platforms, though, exploit the BBB’s array of endogenous selective receptors and their associated transport systems. Engineered protein vehicles that bind with selective surface receptors for transport through the endothelial wall could potentially deliver a therapeutic cargo, such as chemotherapeutic agents, to the doorstep of every neuron. Moreover, the re-engineering strategy tied to such delivery technologies creates a New Chemical Entity in the combination of vehicle and cargo that provides composition of matter and other valuable intellectual property protection while it reduces risk by incorporating already well-known drugs.
Reports from the first clinical studies utilizing such strategies have begun to demonstrate their potential therapeutic benefits for several presently undruggable targets. Among companies with clinical studies underway, in April, AngioChem Inc. reported data from two clinical trials showing that its EPiC (Engineered Peptide Compound) vehicle could facilitate transfer of paclitaxel across the BBB to treat recurrent malignant glioblastoma and advanced solid tumors and brain metastases. That was followed by TransMolecular Inc.’s June report at the annual meeting of the American Society of Clinical Oncology (ASCO) that showed tumor uptake of its isotope-loaded tumor-targeting vehicle in patients with recurrent glioblastoma and other cancers.
Because they cannot be reached by systemic chemotherapies, brain tumors remain among the most difficult cancers to treat. They’re also highly attractive as proof-of-concept indications because they don’t have the clinical complexity of many other brain disorders and, as highly lethal diseases and orphan indications, offer valuable regulatory advantages.
Several other companies are advancing their own versions of so-called molecular Trojan horse technologies—that is, conjugates of a BBB-crossing delivery vehicle and a therapeutic payload able to bind with BBB surface receptors. One of the field’s biotech pioneers, ArmaGen Technologies Inc., announced in April that it will begin a clinical trial in acute ischemic stroke later this year. The study will test an engineered fusion protein that combines neurotrophin and an engineered monoclonal antibody. According to the company its fused protein should work as a neuroprotectant and could have multiple clinical indications. The company also claims that its various fusion proteins in development could deliver virtually any therapeutic entity systemically to the brain.
Perhaps the strongest evidence to date, though, that BBB-crossing technologies have begun to generate wider interest came in June 2009 when Raptor Pharmaceuticals announced the industry’s first-ever deal to explore a BBB transport technology in conjunction with a Big Pharma partner. [See Deal] In return for unspecified milestones and royalties, Roche will now fund Raptor’s studies of its NeuroTrans technology, a BBB chaperone based on a derivative of the receptor-associated protein (RAP), in conjunction with select Roche molecules in numerous indications.
Hoping that there are hundreds of drugs waiting to be delivered to the brain, Raptor’s Starr says, the company will offer a "FedEx approach" to other pharmaceutical "content partners," delivering their drugs to the disease site. As a co-founder of BioMarin Pharmaceutical Inc., while serving as that company’s SVP and CSO, he started looking for ways to transport enzymes across the BBB. BioMarin acquired a transport technology, a natural human blood protein known as p97, through its 2002 purchase of a Vancouver-based startup, Synapse Technologies. [See Deal] That program didn’t work out, but it led Todd Zankel, PhD, the then Senior Director of Research at BioMarin and now CSO and co-founder at Raptor, to look at RAP. The RAP-based transporter technology was eventually spun out by Starr and Zankel to seed Raptor in 2006, based on BioMarin experiments showing that RAP crossed the BBB and can be attached to other molecules. "This is an enabling technology platform," he says. "If it works to transport one drug, it’ll work for others."
Learning BBB Science
The Raptor-Roche deal, though, is just a small glint of light in a field that remains anything but robust. Despite their promise, BBB strategies have attracted few disclosed deals. William Pardridge, MD, founder, chairman and CSO of ArmaGen and director of UCLA’s Blood-Brain Barrier Research Laboratory, says that Big Pharma doesn’t yet understand the value of BBB science and technology. "Industry’s ability in the field is nil," he says. "They engineer protein drugs [unable to cross the BBB] and back-end the blood-brain barrier problem and brain-drug delivery." That approach ensures failure for most CNS drug programs, he believes. "All the low hanging fruit [of diseases treatable by the very few small molecule drugs that cross the BBB] has been picked." At this point, he asserts, only drugs specifically designed to cross the BBB to reach presently undruggable targets will change that situation. "The industry has to learn to front-end engineering molecules to cross the blood-brain barrier, but nobody in any company has training and expertise in the blood-brain barrier," he says.
According to Raptor’s Starr, understanding BBB science remains enormously challenging. First, scientists must identify a receptor or receptor class of interest—as a possible transport mechanism—and "they’re exquisitely selective and complex to understand," he explains. Once a likely transporter can be found, a binding ligand must also be identified or engineered as a possible vehicle to carry a therapeutic agent into the brain. Even then, he points out that many receptors are found on endothelial walls throughout the body causing any therapeutic bound to a ligand to be removed from the blood quickly, leaving only a small amount for entry into the brain. The pharmacology there can prove particularly thorny.
"Receptors lining the blood-brain barrier are not designed to deliver a high volume of a circulating protein," he says. "If you can’t deliver a large amount a drug, you need the compound to be extremely active at low concentrations." Promising earlier studies had pointed to transferrin receptors as a likely delivery system, but there’s so much competition at the receptor level with transferrin already circulating in the blood that it can’t be engineered as a transporter because any transferrin-drug conjugate competes with natural transferrin at receptor binding sites. Starr says his company chose RAP because as a natural human protein it binds to transport receptors on the BBB, but is not found in the blood and thus should not compete with other blood-borne proteins. He thinks eventually there will "probably be multiple solutions for different conditions and different drugs." For now, Roche will collaborate with Raptor to see which molecules may work with NeuroTrans. While hopeful, Starr admits, "A lot of people are not convinced the blood-brain barrier problem can be solved."
Big Pharma’s lack of commitment to resolving the BBB challenge has undermined investor confidence in pursuing new BBB delivery technologies as well. "If you mention blood-brain barrier," Pardridge says, "[VCs] freeze up. They see it as flushing dollars down the toilet. That’s too bad because a successful blood-brain barrier technology will bring huge returns." He founded ArmaGen in 2004 to commercialize his UCLA laboratory’s findings, but has yet to raise any outside equity financing. He’s relied instead on NIH, SBIR, and U.S. Army grants to advance twelve different fusion proteins able to deliver a variety of molecules, including virtually any protein therapeutic, to brain disease targets in rhesus monkey studies. "There’s so much industry resistance that we’ve had to bring our technology along slowly," he says. ArmaGen scientists have succeeded in creating novel fusion genes that encode for fusion proteins with dual functions: first, to cross the BBB on one of the endogenous BBB-receptor-mediated transport systems; and then, to bind to the neuronal or glial receptor in the brain to trigger the desired pharmacological effect.
Pardridge is hopeful that data from ArmaGen’s forthcoming first clinical
trial will open the floodgates of investor and Big Pharma partnership interest.
The company has managed to demonstrate preclinical proof of concept and safety
for its lead molecular Trojan horse, AGT-190, and is in
Blasting Open the Walls
While the pharmacological delivery methods offer some of the most versatile and enticing possibilities for bringing novel therapeutics to the brain, a small number of academic laboratories and companies have explored mechanical means as a quick, noninvasive way to temporarily disrupt the BBB. The goal is to open passages through the BBB large enough to allow therapeutic entities to enter the brain. This might prove useful in cancer treatment, but appears most likely to offer therapeutic benefit for acute, highly destructive injuries, such as acute ischemic stroke, traumatic brain injury (TBI) and other severe brain trauma. Experimental devices for opening up the BBB without surgery include electrical stimulation and image-guided focused ultrasound.
Among the very few device companies in the BBB space, the Israeli firm BrainsGate Ltd. is developing an electrode implanted in the roof of the mouth to deliver electrical stimulation to the Spheno-Palatine Ganglion. This is known to increase cerebral vasodilation and augment perfusion. Increasing perfusion of the brain following acute ischemic stroke is believed to improve outcomes by enabling more blood to enter the areas suffering from reduced blood supply, and help save brain tissue. The company recently launched its first full-scale multicenter clinical trials of its Ischemic Stroke System (ISS), to deliver mild electrical stimulation in the 24-hour window following stroke. Unlike the cash-strapped biopharmaceutical ventures, BrainsGate has successfully raised more than $50 million from outside investors to date, including participation by potential partners or acquirers Boston Scientific Corp. and Johnson & Johnson Development Corporation, Johnson & Johnson’s venture arm. [See Deal] BrainsGate has also carried out pilot drug delivery studies in humans showing it can deliver more intense electrical stimulation through its device to open BBB cell junctions long enough to allow IV paclitaxel to pass through into cerebrospinal fluid.
There is skepticism about these mechanical approaches. Pardridge for one doubts such drug delivery strategies can succeed, and warns that the BBB exists to protect the brain from toxic assaults. Opening up the otherwise closed BBB pores results in "chronic neuropathological changes," he explains. "Any strategy opening up the blood-brain barrier sends albumin into the brain. This will have toxic effects because albumin kills astrocytes." However, particularly in acute stroke, brain trauma and spine breaks, such risks may be countered by its benefit to the patient, though Pardridge doubts this will prove true.
Sneaking Through the Walls
Another way through the BBB is to slip past its defenses. German nano-encapsulation company Capsulution Pharma AG (the resulting entity after the 2008 merger between Capsulution NanoScience AG and NanoDel Technologies GMBH) has announced its intention to begin clinical studies later this year of its first product, doxorubicin loaded into nanoparticles for systemic treatment of glioblastoma. The cytotoxic doxorubicin is widely used in chemotherapeutic regimens, but does not cross the BBB. The biologically degradable PBCA nanoparticles (polybutyl-cyanoacrylate) enable passage of pharmaceutical drugs across the BBB. Capsulution’s nano-strategy could permit the doxorubicin to slide through the BBB and also keep molecules from being pumped back into the bloodstream by efflux transporters.
Masking a therapeutic payload within a BBB-transiting robe could get
numerous otherwise non-penetrating drugs into the brain. Enough companies need
help in this technologically complex engineering work that Genzyme Corp.’s
Genzyme Pharmaceuticals has partnered with
Engineering new formulations of existing drugs could also enable the otherwise non-penetrating agents to leverage the BBB’s endogenous transporters. Serendipitously discovered BBB-transiting drugs provided insights that drug designers are now trying to mimic. Levodopa, or L-Dopa, the generic Parkinson’s disease treatment, Pfizer Inc.’s gabapentin (Neurontin) for epilepsy and neuropathic pain off-label, and its antiseizure medication pregabalin (Lyrica) have structures sufficiently similar to endogenous amino acids that get transported successfully across the BBB in sufficient quantities to modulate neurotransmitters. Such structural models have begun to open a pathway for engineering versions of drugs that could also be carried into the brain.
One of the two broad classes of endogenous BBB transporters, carrier-mediated transporters (CMTs) are membrane proteins which selectively take up water-soluble small molecule nutrients like glucose and amino acids. L-Dopa, the major treatment for Parkinson’s disease for forty years, is an amino acid that gets carried across the BBB by the large neutral amino acid (LAT-1) CMT system. Once L-Dopa enters the brain, an enzyme converts it to dopamine, its active form. Companies have modeled this prodrug process to design other small molecule drugs that CMTs might carry into the brain where they could be converted into therapeutic forms.
XenoPort Inc. has pursued a prodrug strategy by identifying nutrient CMTs in the intestinal lining which can increase the bioavailability of drugs transported by CMTs into the bloodstream. In January, together with GlaxoSmithKline PLC, the company filed an NDA for XP13512 (Solzira), the company’s most advanced product candidate, as a potential treatment for restless leg syndrome. [See Deal] Solzira is a novel prodrug formulation of gabapentin, which gets absorbed rapidly via CMTs through the intestinal lining into the bloodstream, from where it reaches the CNS. A Phase II study of the compound with GSK in painful diabetic neuropathy, though, recently failed to meet primary endpoints. The drug is also being studied in post-herpetic neuralgia and migraine prophylaxis.
Another XenoPort program, XP21279, is a novel prodrug form of levodopa and is now in Phase I clinical development as a potential treatment for Parkinson's disease. Once absorbed in the GI tract via natural nutrient transporter mechanisms, XP21279 is rapidly converted to L-Dopa by the body's endogenous enzymes. Because XP21279 is designed to be absorbed from the colon, it can be formulated for sustained release. This should result in delivery of more L-Dopa across the BBB and improve the effectiveness and duration of dosages, potentially overcoming the on-off and yo-yoing effects that afflict Parkinson’s disease patients on current L-Dopa regimens.
Employing its own mimetic strategy, to-BBB technologies BV, a Dutch company, utilizes the endogenous ligand glutathione to target drug-loaded liposomes to the brain. The company has tested its G-Technology (glutathione-PEG-liposomes) together with the antiviral drug ribavirin in animal models of lethal viral encephalitis. Ribavirin normally does not reach the brain in pharmaceutically active amounts quickly enough to knock down the virus. But according to-BBB, its technology can deliver the antiviral more quickly to the brain by inducing endocytosis in the same way as the brain-penetrating bacterial toxin that causes diphtheria does. Diphtheria toxin binds to a membrane protein called EGF-like growth factor to induce endocytosis. to-BBB conjugates a nontoxic mimic of this toxin to a liposome full of the antiviral. Animal studies showed high brain concentrations of ribavirin and a significant increase in survival, paralleled with a reduction of clinical symptoms in infected animals.
With in vivo proof-of-concept, to-BBB is pursuing the preclinical
development of brain-targeted chemotherapy, a disease-modifying approach to
treat Alzheimer’s disease, and biological agents for treating the neurological
symptoms of lysosomal storage diseases. To-BBB has existing partnerships for a
related BBB transport technology with
Such an approach may attract companies seeking novel ways of delivering siRNAs to the brain. RNAi therapeutics hold enticing potential in a plethora of presently undruggable brain disorders. According to Dinah Sah, PhD, VP Research, CNS and Oncology at Alnylam Pharmaceuticals Inc., the company is focused on direct delivery of RNAi to the brain in a program to develop an RNAi therapeutic for Huntington’s disease with pump manufacturer Medtronic Inc. [See Deal] Other RNAi-focused companies, among them SanoGene Therapeutics Inc., have also looked to local delivery, such as leaving behind therapeutic molecules following surgery as is now done typically with chemotherapies following removal of a tumor. But "there are also perhaps opportunities to utilize systemic RNAi," remarks Sah. Experiments with animal models of glioblastoma were the first to show systemic delivery of RNAi to the brain was possible. The study used pegylated immunoliposomes of a plasmid encoding a siRNA targeting the epidermal growth factor receptor. The pegylated immunoliposomes acted to prolong half-life in the circulation; an associated monoclonal antibody binding the transferrin receptor enabled crossing the blood-brain barrier; while monoclonal antibodies to the insulin receptor resulted in uptake by glioma cells once the RNAi entered the brain. Other BBB technology-focused companies have advanced those techniques further and appear ready to bring RNAi and other biological therapeutics to some of the medical world’s most intractable problems.
Molecular Trojan Horses
Receptor-mediated transporters (RMT), the other broad class of endogenous BBB transporters, will likely prove key to hopes for opening the brain to RNAi therapies and the other promising classes of biological drugs. RMTs carry endogenous large molecules across the BBB, most prominent among them being insulin via the insulin receptor and transferrin via its own receptor. Several companies have been studying molecular Trojan horses in the form of peptides, such as insulin-receptor-specific monoclonal antibodies which can fuse to protein agents and which mimic peptides in binding to RMT systems. According to ArmaGen’s Pardridge, "Hundreds of drugs could be applied to fusion-protein technologies." Such molecular Trojan horses could potentially deliver even the largest biological drugs, as well as siRNAs, growth factors, and enzyme-replacement therapeutics, systemically to the brain. "There is almost an endless number of therapeutic proteins that could be fused to create new chemical entities," he contends. "Some won’t work because of the aggregating properties of the drugs in manufacturing," but he projects that will be around 10% of biological drugs.
AngioChem hopes that its data from anti-cancer drugs based on its RMT technology, called EPiC, mark the beginning of a new era in BBB understanding. The company reported in April on the first brain cancer patients receiving ANG1005, a taxane derivative that crosses the BBB via the LRP-1 (low-density lipoprotein receptor-related protein-1) receptor. LRP-1 receptors are highly expressed on the blood-brain barrier. According to AngioChem CEO Jean-Paul Castaigne, MD, the company’s EPiC platform opens up a wealth of opportunities—and several unidentified partners have already allied with AngioChem to explore them. He points to possible fusion drugs that include leptin for obesity—presently not able to reach appetite-triggering brain targets—monoclonal antibodies for Alzheimer’s disease, and RNAi therapies for a variety of inherited genetic disorders. AngioChem is working with nonexclusive partners funding the latter two projects, he says. "The partnerships are specific to one product in both cases, not to the platform, and it is for neurodegenerative diseases in both cases."
Reaching the Undruggable Targets
Certainly with barely more than a handful of brain diseases currently amenable to the 2% of BBB-crossing small molecule drugs, there’s an enormous need to figure out better ways to deliver other therapeutics into the brain, especially biologics. Most current brain treatments require opening the cranial or spinal cavity via surgery or placing an intrathecal pump to deliver an agent to targeted CNS sites. For instance, in the standard glioblastoma treatment, the neurosurgeon removes or debulks the tumor and then leaves behind a polymer wafer soaked with the chemotherapeutic agent carmustine (Gliadel, BiCNU). Given the highly invasive spread of glioma tentacles into vital surrounding tissue, complete removal of tumor cells is virtually impossible and median survival from the cancer remains less than a year. Despite additions to oncologists’ drug arsenal—the blockbuster temozolomide (Schering-Plough Corp.’s Temodar and Temodal) and the FDA’s approvable notification in May for Roche’s bevacizumab (Avastin), projected to add $500 million in revenue—the five-year patient survival rate has been stuck at less than 3% for 30 years. (See Sidebar, "Blocking Potential Brain Metastases at the Blood Brain Barrier.")
The lack of effectiveness of existing treatments and the lethal nature of the disease makes glioblastoma and metastatic brain cancer the first target of choice for many companies seeking new ways to get past the BBB. "Glioblastoma is a big target for developers," says Raptor CEO Starr. "If you extend life just a few weeks, you’ve got an approvable drug, and you can get a foothold in that way for demonstrating proof-of-concept for a blood-brain barrier delivery strategy."
That’s why Castaigne expects AngioChem to partner further clinical development of ANG1005 soon. The company also has doxorubicin and etoposide derivatives which he says are IND-ready. "We are in partnering discussions with all of them," he says. He points out that Avastin was approved in May as a second-line treatment for brain cancer based on a 26% response rate and four months of median duration of response. "We have observed clinical benefit and response at sub-optimal doses," he says, "so we believe the opportunity for ANG1005 is significant. The Avastin approval only underscores this opportunity. Furthermore, Avastin has a different and complementary mechanism of action, therefore ANG1005 will most likely not have to demonstrate superiority."
TransMolecular will likewise aim to partner its TM601, the peptide chlorotoxin, in brain cancer. The compound is derived from the unlikely source of scorpion venom, though chlorotoxin by itself is not toxic, and is remarkable for its multiple properties as both a vehicle for delivering payloads across the BBB and also—at high doses—its own anti-angiogenic potential. According to TransMolecular’s CEO Robert Radie, the company is ready to launch a Phase III trial of radiolabeled chlorotoxin (131I-TM601) in newly diagnosed glioblastoma. Presently the drug is being delivered locally via a catheter post-surgery but is also being studied for systemic delivery. Radiolabeled chlorotoxin seems to have highly specific uptake by annexin-A2 receptors, which tumor cells overexpress. Radie wants to position TM601 at low doses as a tumor-targeting platform which will deliver a partner’s payload to its site. "We can get a new chemical entity protection for anything conjugated with it," he says.
After years of failures of CNS drugs because of their inability to cross the BBB, Big Pharma remains tentative at best about venturing into the brain with Trojan horse drugs. However, the rising numbers of promising technologies that seem about to open the palace walls may soon begin to bring a wealth of new therapies to bear on the brain. Says ArmaGen’s Pardridge: "The limiting factor is all political. The technology is there."
SIDEBAR: More Than a Castle Wall: Confronting the BBB
The BBB is the dense endothelial wall surrounding the 100 billion brain capillaries—that’s more than 400 miles of capillary containing about a quarter of the body’s blood by volume. The microvasculature perfuses every neuron with circulating nutrients and other molecules necessary for normal continuous brain function, including glucose, oxygen, lipoproteins, and transferrin, as well as cytokines and hormones such as insulin, growth factors, and other blood-borne proteins and peptides. Unlike peripheral tissue capillaries which have porous endothelial walls, the endothelia surrounding brain capillaries may be the most highly impenetrable biological barrier in nature. High-resistance tight junctions fuse the endothelial cells together to keep out toxins, and especially bacteria and other infective agents, which cannot be reached by antibodies should they infect the brain, and otherwise protect the brain from insult and injury. Typically molecules larger than about 500 daltons have great difficulty penetrating the BBB.
But it’s not just size that matters: only a lipid-soluble molecule and one that is not polar won’t get stuck in the BBB’s tight junctions. Even electric current has a tough time getting across. Beyond the physiochemical barriers, many molecules that might otherwise get through the BBB have an affinity for promiscuously arrayed P-glycoprotein efflux pumps on the BBB’s luminal membrane that return them immediately to the bloodstream. Adding to the difficulty of sorting out ways past these many physiochemical barriers and biological sentries, rodent disease models do not represent the human BBB system adequately; often a drug that penetrates the rodent brain fails to do so in man, increasing the uncertainty of predictions based on preclinical studies.