The use of Pep: Trans vectors for the delivery of drugs into the central nervous system
Introduction
Brain delivery is one of the major challenges for the neuropharmaceutical industry since increasing the number of hydrophilic therapeutic agents, such as anticancer drugs, antibiotics, and antiviral drugs are frequently unable to cross the blood–brain barrier (BBB). The BBB poses a formidable obstacle when attempting to deliver drugs to the brain. As new drugs for neurological disorders are discovered, new delivery techniques will have to be developed in concert to overcome this transport barrier. While researchers have devised many ingenious approaches that avoid, disrupt or exploit the BBB's specialized transport mechanisms, many of these continue to have significant drawbacks.
Non-invasive methods that exploit the formation of chimeric peptide or protein-drug conjugates as carriers have been developed. Such methods rely on the presence of specific receptor-mediated transport systems in the BBB, for example insulin, transferrin and melanotransferrin coupling of a non-transportable drug (peptide or protein) to an anti-receptor antibody or other receptor-specific molecule, results in a chimeric construct that can undergo receptor-mediated transcytosis [1], [2], [3]. Drug carriers such as liposomes [4] and nanoparticles [5], [6] have also been used for brain delivery. Despite these developments, there is still a need to develop non-invasive methods which promote the passage of inherently non-penetrating drugs through the intact brain blood vessel endothelium.
Recently, we have shown that small peptide vectors, derived from natural peptides called protegrins, can be used to enhance brain uptake of doxorubicin, penicillin and dalargin [7], [8], [9]. The potential of this approach as an effective delivery system for transporting drugs across the BBB has been demonstrated in a number of animal models. The mechanism by which these vectorized drugs cross into the brain has been shown to be an adsorptive-mediated endocytosis process [10].
This review will address the use and mechanism of transport of cell-penetrating peptides for the delivery of molecules across complex physiological barriers, with a focus on the use of SynB vectors for the potential transport of drugs across the BBB.
Section snippets
Cell-penetrating peptides
The initial discovery of cell-penetrating peptides (CPPs) originated from the unexpected observation that certain proteins or part of their domains can translocate across the plasma membrane. This was first shown for the HIV Tat transactivator and for the homeodomain of the transcription factor Antennapedia. This observation has since expanded to include other peptides that share this property even if they do not derive from proteins.
Penetratin, a short peptide segment, corresponding to the
Brain delivery
As described earlier, the BBB poses a formidable obstacle to drug therapy for the CNS. The fact that a peptide vector is internalized inside the cell does not guarantee that it will cross the BBB. The BBB is more complex than a simple cell layer; it comprises a specialized endothelium (compared with that of other blood vessels) associated with pericytes and astrocyte foot processes, which together elaborate a cellular barrier containing an efficient system of tight junctions. Despite this
Mechanism of brain uptake
We have shown by in situ brain perfusion studies, a technique allowing a first-pass BBB exposure, that the internalization of Dox-SynB is a saturable process [10]. The measured Km, which was in the range of 4–9 μM, compares well with the values observed for substrates reported to be taken up by adsorptive-mediated endocytosis. Furthermore, no difference in brain uptake was seen between doxorubicin linked to l-SynB vectors, indicating that a stereospecific receptor is not a requirement for its
Conclusion
Biological membranes pose a formidable obstacle to therapy with large or hydrophilic drugs. As new drugs for neurological and other disorders are discovered, new delivery techniques will have to be developed in concert to overcome this transport obstacle. As rapid advances in cell and molecular biology lead to a proliferation of potent molecules that cannot be effectively delivered into cells and brain by conventional means, continuing refinement of the new delivery methods will be essential to
Discussion
Smith
Could you tell me a little bit more about the linkers used in the formation of your agents and are they critical for the drug then to be cleaved off from the peptide and which are good linkers and which are not? Also, I was fascinated in your presentation by the demonstration of saturable BBB transport for your Pep:Trans drug system. Would cationic compounds in the plasma, or cationic drugs, possibly compete in some mechanism with this, or might these compounds bind to a limited extent to
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