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Drug and gene targeting to the brain with molecular trojan horses

Nature Reviews Drug Discovery volume 1, pages 131–139 (2002)Cite this article

Key Points

  • The brains of all vertebrates are protected from substances in the blood by the blood–brain barrier (BBB). Although the BBB maintains brain function under normal conditions, it excludes the penetration into the brain of >98% of small-molecule drugs and virtually 100% of large-molecule drugs or gene medicines.

  • One solution to the BBB drug- and gene-delivery problem is chimeric peptide technology. In the formulation of a chimeric peptide, a drug that is normally not transported across the BBB is conjugated or fused to a BBB transport vector. The vector is an endogenous peptide or peptidomimetic monoclonal antibody that undergoes receptor-mediated transcytosis (RMT) through the BBB in vivo.

  • BBB transport vectors act as molecular Trojan Horses, which ferry the drug or gene across the BBB on the endogenous-peptide RMT system. The drug or gene can be attached to the Trojan Horse by either genetic engineering or avidin–biotin technology.

  • Neurotrophins are strongly neuroprotective if injected directly into the brain immediately after brain ischaemia, but are inactive following intravenous administration in the absence of BBB disruption because these large-molecule drugs do not cross the BBB. Neurotrophin chimeric peptides cause complete neuroprotection in the hippocampus after transient global brain ischaemia, and a 70% reduction in stroke volume in regional ischaemia after delayed intravenous administration, because these molecules have been enabled to cross the BBB.

  • Peptide radiopharmaceuticals could allow the early detection of brain diseases, such as brain cancer or Alzheimer's disease, should these molecules be made transportable through the BBB. The epidermal growth factor (EGF) receptor is overexpressed in brain cancer. An experimental human brain cancer could not be imaged with an EGF peptide radiopharmaceutical, because the peptide does not cross the BBB. However, the brain cancer was imaged with an EGF chimeric peptide that was able to cross the blood–tumour barrier after intravenous administration.

  • The imaging of gene expression in vivo is possible with antisense radiopharmaceuticals if these highly charged agents are able to cross both the BBB and the brain cell membrane in vivo. The conjugation of a sequence-specific antisense radiopharmaceutical to a molecular Trojan Horse has been shown to enable the imaging of a target gene in the brain in vivo after intravenous administration.

  • Widespread expression of a therapeutic gene throughout the central nervous system is required to treat many intractable neurological diseases. This is now possible with brain gene-targeting technology, using a combination of molecular Trojan Horses, non-viral gene medicines and tissue-specific gene promoters.

  • The BBB is the rate-limiting factor in the translation of progress in the molecular neurosciences into clinically effective neurotherapeutics.

Abstract

Getting drugs and genes into the brain is a tall order. This is because the presence of the blood–brain barrier prevents many molecules from crossing into the brain. Overcoming this problem will have a profound effect on the treatment of many neurological disorders, allowing larger water-soluble molecules to pass into the brain. Transport vectors, such as endogenous peptides, modified proteins or peptidomimetic monoclonal antibodies, are one way of tricking the brain into allowing these molecules to pass. This article will review such molecular Trojan Horses, and the progress that has been made in the delivery of drugs and genes to the brain.

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Figure 1: Four-cell model of the brain microvasculature.
Figure 2: Structures of delivery vehicles for crossing the BBB.
Figure 3: Imaging gene expression in the brain in vivo.
Figure 4: Non-viral, non-invasive gene targeting to the brain.

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References

  1. Pardridge, W. M. Brain Drug Targeting: the Future of Brain Drug Development (Cambridge Univ. Press, Cambridge, 2001).

    Book  Google Scholar 

  2. Cserr, H. F., Fenstermacher, J. D. & Rall, D. P. Comparative aspects of brain barrier systems for nonelectrolytes. Am. J. Physiol. 234, R52–R60 (1978).

    CAS  PubMed  Google Scholar 

  3. Mollgard, K. & Saunders, N. R. Complex tight junctions of epithelial and of endothelial cells in early fetal brain. J. Neurocytol. 4, 453–468 (1975).

    Article  Google Scholar 

  4. Brightman, M. W. Morphology of blood–brain interfaces. Exp. Eye Res. 25 (Suppl.), 1–25 (1977).This paper reviews the classical electron-microscopic histochemistry that showed that the anatomical basis of the BBB is the capillary endothelial-cell tight junction.

    Article  Google Scholar 

  5. Pardridge, W. M. Peptide Drug Delivery to the Brain (Raven, New York, 1991).

    Google Scholar 

  6. Pardridge, W. M., Yang, J., Buciak, J. & Tourtellotte, W. W. Human brain microvascular DR antigen. J. Neurosci. Res. 23, 337–341 (1989).

    Article  CAS  Google Scholar 

  7. Paulson, O. B. & Newman, E. A. Does the release of potassium from astrocyte endfeet regulate cerebral blood flow? Science 237, 896–898 (1987).

    Article  CAS  Google Scholar 

  8. Pardridge, W. M. CNS drug design based on principles of blood–brain barrier transport. J. Neurochem. 70, 1781–1792 (1998).

    Article  CAS  Google Scholar 

  9. Zhang, Y. & Pardridge, W. M. Mediated efflux of IgG molecules from brain to blood across the blood–brain barrier. J. Neuroimmunol. 114, 168–172 (2001).

    Article  CAS  Google Scholar 

  10. Skarlatos, S., Yoshikawa, T. & Pardridge, W. M. Transport of [125I]-transferrin through the rat blood–brain barrier in vivo. Brain Res. 683, 164–171 (1995).

    Article  CAS  Google Scholar 

  11. Zhang, Y. & Pardridge, W. M. Rapid transferrin efflux from brain to blood across the blood–brain barrier. J. Neurochem. 76, 1597–1600 (2001).

    Article  CAS  Google Scholar 

  12. Green, N. M. Avidin. Adv. Prot. Chem. 29, 85–133 (1975).

    CAS  Google Scholar 

  13. Belayev, L., Busto, R., Zhao, W. & Ginsberg, M. D. Quantitative evaluation of blood–brain barrier permeability following middle cerebral artery occlusion in rats. Brain Res. 739, 88–96 (1996). This investigation shows that the BBB is intact in the first 4–6 hours after experimental regional ischaemia, similar to other forms of acute brain injury. Neuroprotective agents will therefore not be effective unless they can cross the BBB.

    Article  CAS  Google Scholar 

  14. Albayrak, S., Zhao, Q., Siesjo, B. K. & Smith, M. L. Effect of transient focal ischemia on blood–brain barrier permeability in the rat: correlation to cell injury. Acta Neuropathol. (Berl.) 94, 158–163 (1997).

    Article  CAS  Google Scholar 

  15. Kaplan, B. et al. Temporal thresholds for neocortical infarction in rats subjected to reversible focal cerebral ischemia. Stroke 22, 1032–1039 (1991).

    Article  CAS  Google Scholar 

  16. Wu, D. & Pardridge, W. M. Neuroprotection with non-invasive neurotrophin delivery to brain. Proc. Natl Acad. Sci. USA 96, 254–259 (1999).

    Article  CAS  Google Scholar 

  17. Zhang, Y. & Pardridge, W. M. Conjugation of brain-derived neurotrophic factor to a blood–brain barrier drug targeting system enables neuroprotection in regional brain ischemia following intravenous injection of the neurotrophin. Brain Res. 889, 49–56 (2001).

    Article  CAS  Google Scholar 

  18. Zhang, Y. & Pardridge, W. M. Neuroprotection in transient focal brain ischemia following delayed, intravenous administration of BDNF conjugated to a blood–brain barrier drug targeting system. Stroke 32, 1378–1384 (2001).

    Article  CAS  Google Scholar 

  19. Hefti, F. Pharmacology of neurotrophic factors. Annu. Rev. Pharmacol. Toxicol. 37, 239–267 (1997).

    Article  CAS  Google Scholar 

  20. Beck, T., Lindholm, D., Castren, E. & Wree, A. Brain-derived neurotrophic factor protects against ischemic cell damage in rat hippocampus. J. Cereb. Blood Flow Metab. 14, 689–692 (1994).

    Article  CAS  Google Scholar 

  21. Yamashita, K., Wiessner, C., Lindholm, D., Thoenen, H. & Hossmann, K. Post-occlusion treatment with BDNF reduces infarct size in a model of permanent occlusion of the middle cerebral artery in rat. Metab. Brain Dis. 12, 271–280 (1997).

    CAS  PubMed  Google Scholar 

  22. Wong, A. J. et al. Increased expression of the epidermal growth factor receptor gene in malignant gliomas is invariably associated with gene amplification. Proc. Natl Acad. Sci. USA 84, 6899–6903 (1987).

    Article  CAS  Google Scholar 

  23. Nishikawa, R. et al. A mutant epidermal growth factor receptor common in human glioma confers enhanced tumorigenicity. Proc. Natl Acad. Sci. USA 91, 7727–7731 (1994).

    Article  CAS  Google Scholar 

  24. Kurihara, A., Deguchi, Y. & Pardridge, W. M. Epidermal growth factor radiopharmaceuticals: 111In chelation, conjugation to a blood–brain barrier delivery vector via a biotin–polyethylene linker, pharmacokinetics, and in vivo imaging of experimental brain tumors. Bioconjug. Chem. 10, 502–511 (1999).

    Article  CAS  Google Scholar 

  25. Kurihara, A. & Pardridge, W. M. Imaging brain tumors by targeting peptide radiopharmaceuticals through the blood–brain barrier. Cancer Res. 59, 6159–6163 (1999).

    CAS  PubMed  Google Scholar 

  26. Deguchi, Y., Kurihara, A. & Pardridge, W. M. Retention of biologic activity of human epidermal growth factor following conjugation to a blood–brain barrier drug delivery vector via an extended polyethyleneglycol linker. Bioconjug. Chem. 10, 32–37 (1999).

    Article  CAS  Google Scholar 

  27. Morris, J. C. et al. Cerebral amyloid deposition and diffuse plaques in 'normal' aging: evidence for presymptomatic and very mild Alzheimer's disease. Neurology 47, 707–719 (1996).

    Article  Google Scholar 

  28. Saito, Y., Buciak, J., Yang, J. & Pardridge, W. M. Vector-mediated delivery of [125I]-labeled β-amyloid peptide Aβ1–40 through the blood–brain barrier and binding to Alzheimer's Disease amyloid of the Aβ1–40/vector complex. Proc. Natl Acad. Sci. USA 92, 10227–10231 (1995).

    Article  CAS  Google Scholar 

  29. Kurihara, A. & Pardridge, W. M. Aβ1–40 radiopharmaceuticals for brain amyloid imaging, (111)In chelation, conjugation to poly(ethlyeneglycol)–biotin linkers, and autoradiography with Alzheimer's disease brain sections. Bioconjug. Chem. 11, 380–386 (2000).

    Article  CAS  Google Scholar 

  30. Lee, H. J., Zhang, Y., Zhu, C., Duff, K. & Pardridge, W. M. Imaging brain amyloid of Alzheimer's disease in vivo in transgenic mice with an Aβ peptide radiopharmaceutical. J. Cereb. Blood Flow Metab. 22, 222–231 (2001).

    Google Scholar 

  31. Brown, D. et al. Effect of phosphorothioate of oligodeoxynucleotides of specific protein binding. J. Biol. Chem. 269, 26801–26805 (1994).

    CAS  PubMed  Google Scholar 

  32. Cossum, P. A. et al. Disposition of the 14C-labeled phosphorothioate oligonucleotide ISIS 2015 after intravenous administration to rats. J. Pharmacol. Exp. Ther. 267, 1181–1190 (1993).

    CAS  PubMed  Google Scholar 

  33. Wu, D., Boado, R. J. & Pardridge, W. M. Pharmacokinetics and blood–brain barrier transport of [3H]-biotinylated phosphorothioate oligodeoxynucleotide conjugated to a vector-mediated drug delivery system. J. Pharmacol. Exp. Ther. 276, 206–211 (1996).

    CAS  PubMed  Google Scholar 

  34. Pardridge, W. M., Boado, R. J. & Kang, Y.-S. Vector-mediated delivery of a polyamide ('peptide') nucleic acid analogue through the blood–brain barrier in vivo. Proc. Natl Acad. Sci. USA 92, 5592–5596 (1995).

    Article  CAS  Google Scholar 

  35. Hanvey, J. C. et al. Antisense and antigene properties of peptide nucleic acids. Science 258, 1481–1486 (1992).

    Article  CAS  Google Scholar 

  36. Shi, N., Boado, R. J. & Pardridge, W. M. Antisense imaging of gene expression in the brain in vivo. Proc. Natl Acad. Sci. USA 97, 14709–14714 (2000).

    Article  CAS  Google Scholar 

  37. Kajiwara, K. et al. Humoral immune responses to adenovirus vectors in the brain. J. Neuroimmunol. 103, 8–15 (2000).

    Article  CAS  Google Scholar 

  38. Herrlinger, U. et al. Pre-existing herpes simplex virus 1 (HSV-1) immunity decreases but does not abolish, gene transfer to experimental brain tumors by a HSV-1 vector. Gene Ther. 5, 809–819 (1998).

    Article  CAS  Google Scholar 

  39. Driesse, M. J. et al. Intracerebral injection of adenovirus harboring the HSVtk gene combined with ganciclovir administration: toxicity study in nonhuman primates. Gene Ther. 5, 1122–1129 (1998).

    Article  CAS  Google Scholar 

  40. Lawrence, M. S. et al. Inflammatory responses and their impact on β-galactosidase transgene expression following adenovirus vector delivery to the primate caudate nucleus. Gene Ther. 6, 1368–1379 (1999).A single injection of adenovirus into the primate brain causes local inflammation that leads to demyelination.

    Article  CAS  Google Scholar 

  41. Kramm, C. M. et al. Herpes vector-mediated delivery of marker genes to disseminated central nervous system tumors. Hum Gene Ther 7, 291–300 (1996).

    Article  CAS  Google Scholar 

  42. McMenamin, M. M. et al. A γ34.5 mutant of herpes simplex 1 causes severe inflammation in the brain. Neuroscience 83, 1225–1237 (1998).Even replication-deficient strains of the herpes simplex virus are toxic to the brain, causing local inflammation, increased antigen-presenting cells and perivascular lymphocyte cuffing.

    Article  CAS  Google Scholar 

  43. Hofland, H. E. J. et al. In vivo gene transfer by intravenous administration of stable cationic lipid/DNA complex. Pharm. Res. 14, 742–749 (1997).

    Article  CAS  Google Scholar 

  44. Plank, C., Tang, M. X., Wolfe, A. R. & Szoka, F. C. Jr. Branched cationic peptides for gene delivery: role of type and number of cationic residues in formation and in vitro activity of DNA polyplexes. Hum. Gene Ther. 10, 319–332 (1999).

    Article  CAS  Google Scholar 

  45. Osaka, G. et al. Pharmacokinetics, tissue distribution, and expression efficiency of plasmid [33P]DNA following intravenous administration of DNA/cationic lipid complexes in mice: use of a novel radionuclide approach. J. Pharm. Sci. 85, 612–618 (1996).

    Article  CAS  Google Scholar 

  46. Shi, N. & Pardridge, W. M. Non-invasive gene targeting to the brain. Proc. Natl Acad. Sci. USA 97, 7567–7572 (2000).

    Article  CAS  Google Scholar 

  47. Shi, N., Boado, R. J. & Pardridge, W. M. Receptor-mediated gene targeting to tissues in the rat in vivo. Pharm. Res. 18, 1091–1095 (2001).

    Article  CAS  Google Scholar 

  48. Cruz, M. T., Simoes, S., Pires, P. P. C., Nir, S. & Lima, M. C. Kinetic analysis of the initial steps involved in lipoplex–cell interactions: effect of various factors that influence transfection activity. Biochim. Biophys. Acta 1510, 136–151 (2001).

    Article  Google Scholar 

  49. Zhang, Y., Lee, H. J., Boado, R. J. & Pardridge, W. M. Receptor-mediated delivery of an antisense gene to human brain cancer cells. J. Gene Med. (in the press).

  50. Papahadjopoulos, D. et al. Sterically stabilized liposomes: improvements in pharmacokinetics and antitumor therapeutic efficacy. Proc. Natl Acad. Sci. USA 88, 11460–11464 (1991).This classical study shows that the conjugation of PEG to the surface of liposomes greatly prolongs the blood residence time and optimizes the pharmacokinetics.

    Article  CAS  Google Scholar 

  51. Shi, N., Zhang, Y., Boado, R. J., Zhu, C. & Pardridge, W. M. Brain-specific expression of an exogenous gene following intravenous administration. Proc. Natl Acad. Sci. USA 98, 12754–12759 (2001).

    Article  CAS  Google Scholar 

  52. Lee, H. J., Engelhardt, B., Lesley, J., Bickel, U. & Pardridge, W. M. Targeting rat anti-mouse transferrin receptor monoclonal antibodies through the blood–brain barrier. J. Pharmacol. Exp. Ther. 292, 1048–1052 (2000).

    CAS  PubMed  Google Scholar 

  53. Pardridge, W. M., Kang, Y.-S., Buciak, J. L. & Yang, J. Human insulin receptor monoclonal antibody undergoes high affinity binding to human brain capillaries in vitro and rapid transcytosis through the blood–brain barrier in vivo in the primate. Pharm. Res. 12, 807–816 (1995).

    Article  CAS  Google Scholar 

  54. Coloma, M. J. et al. Transport across the primate blood–brain barrier of a genetically engineered chimeric monoclonal antibody to the human insulin receptor. Pharm. Res. 17, 266–274 (2000). This study describes the genetic engineering of a brain transport vector and shows targeting to the primate brain in vivo . The genetically engineered vector could be used to target drugs or genes to the human brain.

    Article  CAS  Google Scholar 

  55. Pardridge et al. Blood–brain barrier: interface between internal medicine and the brain. Ann. Intern. Med. 105, 82–95 (1986).

    Article  CAS  Google Scholar 

  56. Duvernoy, H., Delon, S. & Vannson, J. L. The vascularization of the human cerebellar cortex. Brain Res. Bull. 11, 419–480 (1983).

    Article  CAS  Google Scholar 

  57. Cornford, E. M., Hyman, S., Cornford, M. E. & Caron, M. J. GLUT1 glucose transporter activity in human brain injury. J. Neurotrauma 13, 523–536 (1996).

    Article  CAS  Google Scholar 

  58. Brownlees, J. & Williams, C. H. Peptidases, peptides, and the mammalian blood–brain barrier. J. Neurochem. 60, 793–803 (1993).

    Article  CAS  Google Scholar 

  59. Tsuji, A. & Tamai, I. Carrier-mediated or specialized transport of drugs across the blood–brain barrier. Adv. Drug Deliv. Rev. 36, 277–290 (1999).

    Article  CAS  Google Scholar 

  60. Takasawa, K., Terasaki, T., Suzuki, H. & Sugiyama, Y. In vivo evidence for carrier-mediated efflux transport of 3′-azido-3′-deoxythymidine and 2′,3′-dideoxyinosine across the blood–brain barrier via a probenecid-sensitive transport system. J. Pharmacol. Exp. Ther. 281, 369–375 (1997).

    CAS  PubMed  Google Scholar 

  61. Pardridge, W. M. Receptor-mediated peptide transport through the blood–brain barrier. Endocr. Rev. 7, 314–330 (1986).

    Article  CAS  Google Scholar 

  62. Krewson, C. E., Klarman, M. L. & Saltzman, W. M. Distribution of nerve growth factor following direct delivery to brain interstitium. Brain Res. 680, 196–206 (1995).

    Article  CAS  Google Scholar 

  63. Yan, Q. et al. Distribution of intracerebral ventricularly administered neurotrophins in rat brain and its correlation with Trk receptor expression. Exp. Neurol. 127, 23–36 (1994).

    Article  CAS  Google Scholar 

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Author information

Authors and Affiliations

  1. Department of Medicine, UCLA School of Medicine, Los Angeles, 90024, California, USA

    William M. Pardridge

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  1. William M. Pardridge
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DATABASES

LocusLink

BDNF

CNT2

EGF

EGFR

GFAP

GLUT1

insulin

insulin receptor

LAT1

leptin receptor

α2-macroglobulin

MCT1

NGF

OATP2

scavenger receptor

TF

TFR

Tfr (mouse)

Tfr (rat)

TRKB

OMIM

Alzheimer's disease

Parkinson's disease

FURTHER INFORMATION

Alliance for Aging Research

American Heart Association

BT-CGAP

Blood–brain barrier

Glossary

PEPTIDOMIMETIC

A molecule that binds to a blood–brain barrier (BBB) receptor and initiates receptor-mediated transcytosis (RMT) across the BBB. A peptidomimetic monoclonal antibody binds to an extracellular projecting epitope on the receptor to trigger RMT in a manner similar to the endogenous ligand for the receptor.

ENDOTHELIUM

Consists of cells of mesenchymal origin that comprise the innermost cellular lining of the capillary.

TRANSCYTOSIS

Transport across the endothelial barrier by movement through the endothelial cytoplasm (the transcellular pathway), in contrast to transport across the endothelial barrier by movement through the inter-endothelial junctional spaces (the paracellular pathway).

PINOCYTOSIS

Transcytosis across the endothelial cell through endosomal vesicles and/or a tubulovesicular network within the endothelial cell.

MICROVASCULATURE

The capillary network that perfuses the brain and delivers to the brain circulating nutrients and oxygen.

PERICYTE

A multifunctional cell that shares the basement membrane of the microvasculature with the endothelium, and sits on the brain side of the capillary.

EXOFACIAL

Part of a plasma-membrane receptor that projects outwards into the extracellular space.

NEUROTROPHIN

A protein produced within the brain that modulates neuronal function and structure.

CEREBRAL INFARCTION

Loss of brain tissue subsequent to the transient or permanent loss of circulation and/or oxygen delivery to that region of the brain; also known as a stroke.

XENOBIOTIC

A drug or naturally occuring alkaloid that has pharmacological properties.

EPENDYMAL SURFACE

The surface of the brain that is lined by the ependymal epithelium, which is in contact with the cerebrospinal fluid.

AMYLOID

A silk-like protein aggregate that consists of a peptide that has a high degree of β-pleated-sheet secondary structure, which enables the aggregation of the peptide into plaques that deposit in the extracellular space of the brain.

RETICULO-ENDOTHELIAL

A system of phagocytic cells that line the microvasculature of the liver and spleen, and which remove foreign substances or particles from the blood.

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Pardridge, W. Drug and gene targeting to the brain with molecular trojan horses. Nat Rev Drug Discov 1, 131–139 (2002). https://doi.org/10.1038/nrd725

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