Delivery of Neuropeptides from the Periphery to the Brain: Studies with Enkephalin

Abstract

Many peptides with the potential of therapeutic action for brain disorders are not in clinical use because they are unable to cross the blood-brain barrier (BBB) following peripheral administration. We have developed two potential strategies for the delivery of peptides to the brain and demonstrated their feasibility with enkephalins. In the first approach, designated induced reversible lipophilization, Leu/Met Enkephalins were converted to 9-fluorenylmethoxycarbonyl (Fmoc) derived lipophilic prodrug analogues, which undergo slow, spontaneous hydrolysis under physiological conditions, generating the native agonists. In contrast to Enkephalin, Fmoc-Met-Enkephalin was found to facilitate an analgesic effect following intraperitoneal administration in mice. Fmoc-Leu-Enkephalin was not analgesic. In the second approach, Enkephalin was linked to BBB transport vectors through an Fmoc based linker spacer, forming conjugates that slowly release Enkephalin under physiological conditions. A pronounced antinociceptive response was thus obtained following intraperitoneal administration of either cationized-human serum albumin-Fmoc-Enkephalin or polyethylene glycol(5)-Fmoc-Enkephalin. Derivatives of Enkephalin covalently linked to the same BBB-transport vectors through a stable (nonreversible) chemical bond were not analgesic. In summary, we have demonstrated that lipophilicity can be conferred to hydrophilic peptides to a degree permitting the permeation of the BBB by passive diffusion, without the drawback of agonist inactivation, which is often caused by irreversible derivatization. Similarly, in the second strategy, the conjugation to BBB-permeable vectors overcomes the obstacle of peptide inactivation by releasing the active form in the central nervous system.

Converting Peptides into Drug Leads by Lipidation

“…Fmoc-Metenkephalin exhibited increased BBB permeability, enhanced analgesic activity, and controlled release following spontaneous chemical hydrolysis in vivo [149]. Replacing the octanoyl group of human ghrelin with 1-adamantaneacetyl or 8-bromooctanoyl maintained its binding affinity to receptor hGHSR1a [46].…”

Blood–brain transfer and antinociception of linear and cyclic N-methyl-guanidine and thiourea-enkephalins

“…Based upon multiple time regression analysis, a constant unidirectional clearance, without detectable efflux, into the mouse brain was observed for both cyclic enkephalin analogs, 125 I-CycNMe (0.31 ± 0.12 L/g min) and 125 ICycS (0.11 ± 0.08 L/g min), but was more pronounced for 125 I-LinNMe (1.64 ± 0.88 L/g min). Labeled LinS was transported bidirectionally [4,8,12,20,25,28,47,54,62,63], with a high initial influx rate (K 1 of 3.46 ± 0.49 L/g min). The high initial influx for 125 I-LinS can be explained by the blood-to-brain transport of the peptide by a saturable transport system, which leveled off rapidly, due to efflux (k out of 0.07 ± 0.02 min −1 ), to a net flux into the brain.…”

PEGylated Cationic Serum Albumin for Boosting Retroviral Gene Transfer

“…Cationic HSA (cHSA), for instance, can be prepared by modifying nHSA with ethylenediamine (in excess), which allows capping of negatively charged carboxyl groups with positively charged primary amines . cHSA has been studied as a gene delivery carrier to facilitate efficient plasmid transfection into several cell types, and is even found to enhance passage through the blood–brain barrier significantly . However, cHSA has the potential to elicit immunogenicity, which could significantly limit its clinical applications .…”

“…[13] cHSA has been studied as ag ene delivery carriert of acilitatee fficient plasmid transfection into several cell types, [13,16] andi se ven found to enhance passage through the blood-brain barrier significantly. [17] However, cHSA has the potentialt oe licit immunogenicity,w hich could significantly limit its clinicala pplications. [18] To overcome this drawback, poly(ethylene glycol) (PEG) chains have been introduced at the cHSA surface.…”