Overcoming the Lack of Oral Availability of Cyclic Hexapeptides: Design of a Selective and Orally Available Ligand for the Integrin αvβ3

Abstract

A highly systematic approach for the development of both orally bioavailable and bioactive cyclic N-methylated hexapeptides as high affinity ligands for the integrin αvβ3 is based on two concepts: a) screening of systematically designed libraries with spatial diversity and b) masking of the peptide charge with a lipophilic protecting group. The key steps of the method are 1) initial design of a combinatorial library of N-methylated analogues of the stem peptide cyclo(d-Ala-Ala ); 2) selection of cyclic peptides with the highest intestinal permeability; 3) design of sublibraries with the bioactive RGD sequence in all possible positions; 4) selection of the best ligands for RGD-recognizing integrin subtypes; 5) fine-tuning of the affinity and selectivity by additional Ala to Xaa substitutions; 6) protection of the charged functional groups according to the prodrug concept to regain intestinal and oral permeability; 7) proof of biological effects in mice after oral administration.

Oral aktive Peptide: Gibt es ein Patentrezept?

“…Dieswurde erstvor KurzembeimA ustausch vonAla mit Asp oderA rg in ein zuvor permeables, N-methyliertes cyclo-Hexaalanin-Peptid beobachtet, [115] undfr üherbei lipophilen und permeablenP eptiden,b ei denenL eu durch Asp oderL ys ersetzt wurde. Dieswurde erstvor KurzembeimA ustausch vonAla mit Asp oderA rg in ein zuvor permeables, N-methyliertes cyclo-Hexaalanin-Peptid beobachtet, [115] undfr üherbei lipophilen und permeablenP eptiden,b ei denenL eu durch Asp oderL ys ersetzt wurde.…”

“…Einei nteressante Beobachtung bezüglichdes Einflussesvon Lysmachte die GruppeumBorchardt, welche berichtete, dass der Einbau vonLys in einT ripeptid die Permeabilitätverhindert,wohingegendie selbe Substitutionin einH exapeptid dieP ermeabilitäti mV ergleichz um Derivat, dasdas neutrale Asnenthält,nicht beeinflusst. [115] Bei diesem Ansatzw erdend ie positiven (Arg) undn egativen (Asp) Ladungen einesbioaktiven Peptidsdurch Esterase-labile [119] und lipophile (abspaltbare)R este abgeschirmt. geben an,d asse inep olareS eitenkette beid er Permeabilität [118] toleriert wird.…”

“…[94] Hickeyetal. [115] Zusätzlich wurde eine Verschiebung desA ufnahmemechanismusv on passivparazellulärzutranszellulärbeobachtet, nachdem es in seine lipophile Prodrug-Form überführt wurde. Dieser Befund deckt sich mit demf ast allgegenwärtigen Ty rinden permeablen, lipophilen Peptiden, die vonLokey et aluntersuchtwurden.…”

Orally Active Peptides: Is There a Magic Bullet?

“…[94] Hickey et al claimed that one polar side chain is tolerated for permeability [118] and this finding fits to the omnipresent Ty ri nt he permeable lipophilic peptides examined by the Lokey group. [115] In that approach, the positive (Arg) and negative (Asp) charges of ab ioactive peptide were shielded by an esterase-labile [119] and lipophilic pro-moiety. [115] In that approach, the positive (Arg) and negative (Asp) charges of ab ioactive peptide were shielded by an esterase-labile [119] and lipophilic pro-moiety.…”

“…[86] On the other hand, reports indicate the participation of at ransporter, suggested by different uptake rates of cyclic hexaalanine enantiomers,which indicates the participation of achiral structural element in their transport and different flux ratios in Caco-2 membranes. [126] As tudy of af urther refunctional- hiding side-chain polarity i) Thr is apolar amino acid that is well-tolerated in bioavailable peptides [99a,116] ii)replacementofThr by Ser led to decreased permeability and, thus, the b-methyl group seems to orient the hydroxy group to form an intramolecular hydrogen bond and to shield the polarity iii)2-pyridinealanine can hide the side-chain polarity in amanner similar to Thr, [155] but attempts to modify permeable cyclic peptides resulted in lower permeability [150] [99a, 116] exocyclic peptide bond i) introducedbyaziridine-aldehyde-based macrocyclization, followed by nucleophilic attack by isocyanide and hydrolysis [118] ii)used to optimize the hydrogen bonding network, thereby reducing the polarity and flexibility of ap eptide iii)has aturn-inducing effect [107b] [118] lipophilic prodrug charge masking (LPCM) approach i) shields charges through esterase-labile protecting groups that enable uptake of the peptide [119] ii)prodrug concepts for cyclizing peptides, which release alinear peptide after cleavage, have been developed [156] iii)positive and negative charges can be protectedbyenzymatically labile groups, such as in arecently publishedc yclic peptide (24) iv) the biologically active compound is releasedi nto the blood stream and can bind to surface receptors, such as integrins; [115] this concept was adopted for cyclic peptides derived from dabigatran [157] [ 115,156] lipidation i) increases lipophilicity through aliphaticc hains ii)increases plasma stability iii)binds to serum albumin, prolongingthe duration of systemic circulation [158] iv) forms self-assembling oligomeric macromolecules, enhances enzymatic degradation [159] v) its effect on bioavailability strongly depends on the fatty acid chain length [160] vi)t he modification with amyristoyl group (n = 12) improved the oral bioavailability of the peptide c(MyD 4-4) by af actor of > 50 to F = 47 AE 16 % [161] [162] glycosylation i) affects the backbone structure of peptides [163] ii)improves half-life against enzymatic degradation [164] iii)glycosylated sandostatin (d(+)-maltose) shows a10-fold greater oral effect than the nonglycosylated derivative [165] iv) glycosylated endomorphin-1 (27)s hows a700-fold increase in memb...…”