Laurent LecanuPharmD, PhD

Associate Professor

“Challenges in targeting the brain”

__________________________________________________________ L’Institut de recherche du Centre universitaire de santé McGill The Research Institute of the McGill University Health Centre

CRS International Meeting, July 15-18 2012, Quebec, CA

THE NEXT FRONTIER

The Brain

The blood-brain barrier

• Lack of knowledge of the gate defense

• Lack of knowledge of what is beyond the gate

• Wrong equipmentA bridge too far?

Lack of capacity to accurately plan/develop and to achieve goal.

• First described by Ehrlich in 1885 and then by Goldman in 1913

• The blood-brain barrier exists within 600 km of capillaries

• 1 km per cm3 of brain tissue• 20 m2 surface

What is the blood-brain barrier?

What is the blood-brain barrier?

• Physical, chemical and metabolic barrier• The blood-brain barrier is restrictive for some

compounds owing to efflux mechanisms, absence of permeation and limited pinocytosis.

• Its role is to protect the brain against overexposure and toxins

• The blood-brain barrier (BBB) segregates the circulating blood from interstitial fluid in the brain, and restricts drug permeability into the brain.

Brain barriers/blood exchanges

The various barriers that one can find in the brain (broken lines), representing the blood-brain barrier (BBB), the blood-cerebrospinal fluid (CSF)-barrier, the brain-CSF-barrier, and the blood-spinal barrier. The BBB has the largest surface area, and is, therefore, considered to be the most important influx barrier for solutes to enter the brain. Also shown are the paths of fluid movement (solid arrows) between cerebral intracellular fluid (ICF), interstitial fluid (ISF), CSF, blood, and lymphatics. Thick arrows represent major paths of fluid movement under normal conditions. Thin arrows represent minor paths of fluid movement under normal conditions.

From Kola and Landis, Nature Reviews, 2004

Proportion of drug effectively release on the market

High failure rate

WHY?

The blood-brain barrier

Repelling transportersMetabolism

Non permissive barrierUnique structure

Like the liver, the BBB should be considered as an organ on its own

• In human, the brain is the only organ that is completely sealed and isolated from the rest of the body.– Steroid– Cholesterol– Neuronal, endocrine,

immunological functions

The brain is a body within the body

Brain capillaries structureBrain capillary endothelial cellsPericytesAstrocytes

• Restrictive physicochemical characteristics that limit passive diffusion• Lack of capillary wall fenestration• High efflux capacity• Metabolism within the endothelial cell• Uptake transport

Transporters• ABC family: P-glycoprotein (oncology, virology)

• Influx: LAT-1 (PD, epilepsy), GLUT1, Oatp1A2 (thyroxin, prostaglandin, steroids), SVCT2 (VitC)– Phenylalanine derivatives of valproic acid (Peura et al., 2011); Tyrosine conjugated drug (Gynther et al., 2008)– Glut1 and glycosylated peptides, LMWH and D-Glu derivatives (Guo et al., 2005)– Targeting glioma cells with SVCT2-nanocarrier (Salamaso et al., 2009); Ascorbic and 2-bromoascorbic acid conjugates with neuroactive

molecules (Manfredini et al. 2004)

• Efflux: ASCT2, EEAT, Oatp2• And many more…

Many polymorphisms, ethnic-specific

Sex, aging

Heterogeneous repartition, regio-specificity

Blood-brain barrier and neuropathologies

• Neurodegenerative diseases• Stroke• Traumatic brain injury• Gliomas• Viral and parasitic infections

Still to be fully characterized

Nanoparticles• Dendrimers (Beg et al., 2011)

• Lipidic nanostructures (Bondi et al., 2012)

• Sialic acid and glycopeptides conjugated NPs (Tosi et al., 2010 J Controlled Release)

• Smart nanovehicle (SNV) (Agyare et al., 2008 Pharm Res)

• Poly(n-butylcyano-acrylate) NPs coated with Tween®80 (Wilson et al., 2008 Brain Res)

• Biotinylated-pegylated NPs (Pulkkinen et al., 2008 Eur J Pharm Biopharm)

• Other polymers: albumin, dextran, chitosan, polylactic acid Not necessarily devoid of toxicity

Yet clinical benefit over other platforms to be validated

BBB-targeting delivery systems

• Immunoglobulin tethered to the NPs• Apo A-I, E3, B100, MMP-200 fragment covalently

attached to the NPs

Miscellanous

• Nanoemulsion for intranasal delivery (Kumar et al., 2009 PDA J Pharm Sci Technol)

• Focused ultra-sound (Alonso et al., 2010 J Cereb Blood Flow Metab)

Mechanism of diffusion still to unveil

Facts

• >98% of small molecules do not cross the BBB• ~100% of larger molecules (growth factors,

peptides, biotech…) do not cross the BBB

Conceptual Problem• None existing entity until it

is too late• Either the BBB dimension is

not integrated or integrated too late

• R&D CNS budget, 99% for drug design and 1% for BBB crossing

Lack of Basic Knowledge• No college/university in

North-America has a program that emphasizes the importance of the BBB.

• No pharmaceutical company has a program aiming at BBB (drug or delivery system)

We know it crosses, we do not know how and we do know why!

Caprospinol Brain tissue

CSF

Lecanu, Yao, Teper, Yao, Greeson and Papadopoulos, 2004Tillement, Lecanu, Yao, Greeson and Papadopoulos, 2006Lecanu, Tillement, Rammouz, Tillement, Greeson and Papadopoulos, 2009Lecanu, Rammouz, McCourty, Sidahmed, Greeson and Papaddopoulos, 2010Tillement, Lecanu and Papadopoulos, 2011Papadopoulos and Lecanu, 2012

LogP=7.5

The FDA's Nanotechnology Task Force released a report that recommends the agency consider developing guidance and taking other steps to address the benefits and risks of drugs and medical devices using nanotechnology. The Task Force was initiated by Commissioner von Eschenbach in 2006. The Task Force reports that nanoscale materials potentially could be used in most product types regulated by FDA and that those materials present challenges similar to those posed by products using other emerging technologies. The challenges, however, may be complicated by the fact that properties relevant to product safety and effectiveness may change as size varies within the nanoscale.

Crossing the BBB, then what?

• Crossing where?• To go where?• Brain parenchyma structure• Bound versus unbound fraction• Plasma versus brain tissue macromolecules

What’s left to be done? Complete characterization of the various transporters and drug interacting BBB

components

Mapping

Drug design

Initiate, develop and implement drug design based on transporter specificity

Initiate, develop and implement drug-targeting technology programs based on

transporter specificity

Not mentioning…

• Tissue diffusion• In vivo– Invasive (catheterism, microdialysis…)– Non-invasive (Combining various technology,

NMR/PET/MRI/CT)• in vitro model– In particular, assays and HTS that includes pericyte cell type

• Imaging• PK modelization, translational pharmacokinetic