Cell-dependent differential cellular uptake of PNA, peptides, and PNA-peptide conjugates

Triplex-mediated gene modification

Gene targeting with DNA-binding molecules such as triplex-forming oligonucleotides or peptide nucleic acids can be utilized to direct mutagenesis or induce recombination site-specifically. In this chapter, several detailed protocols are described for the design and use of triplex-forming molecules to bind and mediate gene modification at specific chromosomal targets. Target site identification, binding molecule design, as well as various methods to test binding and assess gene modification are described.

On The Biomedical Promise of Cell Penetrating Peptides: Limits Versus Prospects

The cell membrane poses a substantial hurdle to the use of pharmacologically active biomacromolecules that are not per se actively translocated into cells. An appealing approach to deliver such molecules involves tethering or complexing them with so-called cell penetrating peptides (CPPs) that are able to cross the plasma membrane of mammalian cells. The CPP approach is currently a major avenue in engineering delivery systems that are hoped to mediate the non-invasive import of problematic cargos into cells. The large number of different cargo molecules that have been efficiently delivered by CPPs ranges from small molecules to proteins and even liposomes and particles. With respect to the involved mechanism(s) there is increasing evidence for endocytosis as a major route of entry. Moreover, in terms of intracellular trafficking, current data argues for the transport to acidic early endosomal compartments with cytosolic release mediated via retrograde delivery through the Golgi apparatus and the endoplasmic reticulum. The focus of this review is to revisit the performance of cell penetrating peptides for drug delivery. To this aim we cover both accomplishments and failures and report on new prospects of the CPP approach. Besides a selection of successful case histories of CPPs we also review the limitations of CPP mediated translocation. In particular, we comment on the impact of (i) metabolic degradation, (ii) the cell line and cellular differentiation state dependent uptake of CPPs, as well as (iii) the regulation of their endocytic traffic by Rho-family GTPases. Further on, we aim at the identification of promising niches for CPP application in drug delivery. In this context, as inspired by current literature, we focus on three principal areas: (i) the delivery of antineoplastic agents, (ii) the delivery of CPPs as antimicrobials, and (iii) the potential of CPPs to target inflammatory tissues.

Enhanced delivery of cell-penetrating peptide-peptide nucleic acid conjugates by endosomal disruption

Improvement of cellular uptake and cellular localization is still one of the main obstacles to the development of antisense-antigene therapeutics, including peptide nucleic acid (PNA). Cell-penetrating peptides (CPPs) such as Tat peptide and polyarginine have been widely used to improve the cellular uptake of PNA and other antisense agents. Cellular uptake of most CPP conjugates occurs mainly through endocytotic pathways, and most CPP conjugate is retained in the endosomal compartments of the cell. Several methods to induce endosome disruption have been shown to improve the bioavailability of CPP conjugates to the cytosol and/or nucleus by facilitating escape from the endosomal compartments. Here we describe protocols for the delivery of CPP-PNA conjugates to adherent cultured cells using photodynamic treatment (photochemical internalization), Ca2+ treatment or chloroquine treatment to potentiate the antisense effects of CPP-PNA conjugates through increased release of CPP conjugates into the cytoplasm. This protocol, consisting of CPP-mediated delivery assisted by an endosome-disruption agent, allows the delivery of the CPP-PNA conjugates to the nucleus and/or cytosol of cultured cells. The endosome-disruption treatment improves the nuclear antisense effects of CPP-PNA conjugates by up to two orders of magnitude using 24-hour delivery.

Efficient splicing correction by PNA conjugation to an R6-Penetratin delivery peptide

Sequence-specific interference with the nuclear pre-mRNA splicing machinery has received increased attention as an analytical tool and for development of therapeutics. It requires sequence-specific and high affinity binding of RNaseH-incompetent DNA mimics to pre-mRNA. Peptide nucleic acids (PNA) or phosphoramidate morpholino oligonucleotides (PMO) are particularly suited as steric block oligonucleotides in this respect. However, splicing correction by PNA or PMO conjugated to cell penetrating peptides (CPP), such as Tat or Penetratin, has required high concentrations (5-10 microM) of such conjugates, unless an endosomolytic agent was added to increase escape from endocytic vesicles. We have focused on the modification of existing CPPs to search for peptides able to deliver more efficiently splice correcting PNA or PMO to the nucleus in the absence of endosomolytic agents. We describe here R6-Penetratin (in which arginine-residues were added to the N-terminus of Penetratin) as the most active of all CPPs tested so far in a splicing correction assay in which masking of a cryptic splice site allows expression of a luciferase reporter gene. Efficient and sequence-specific correction occurs at 1 muM concentration of the R6Pen-PNA705 conjugate as monitored by luciferase luminescence and by RT-PCR. Some aspects of the R6Pen-PNA705 structure-function relationship have also been evaluated.

Lipoplex and peptide-based strategies for the delivery of steric-block oligonucleotides

Synthetic oligonucleotides offer interesting prospects for the control of gene expression but clinical applications have been severely limited by their poor bioavailability. Cationic lipids have been widely used for the delivery of charged oligonucleotide (ON) analogues but most of the commercial formulations are toxic and poorly stable in the presence of serum proteins. We have developed a DOGS/DOPE liposome formulation named DLS (for delivery liposomal system), that allows for the efficient nuclear delivery of negatively charged antisense ON analogues as monitored by fluorescence microscopy and by their ability to correct deficient pre-mRNA splicing, even in serum-supplemented cell culture. Uncharged DNA mimics such as peptide nucleic acids (PNA), or phosphorodiamidate morpholino (PMO) ON are particularly interesting for their high metabolic stability and affinity for complementary RNA targets but they cannot be delivered with cationic lipids. Cell penetrating peptides (CPP), such as Tat or penetratin, have been used widely as conjugates for the delivery of various biomolecules and might be appropriate for neutral ON analogues. However, entrapment within endocytic vesicles severely limits the efficiency of PNA delivery by CPPs in the absence of endosomolytic drugs, such as chloroquine. The conjugation of new arginine-rich CPPs to PNA allows efficient nuclear delivery in the absence of chloroquine as monitored in a splicing correction assay. Both strategies have their advantages but DLS-mediated delivery remains more efficient than CPP delivery for the nuclear targeting of splice correcting ON analogues in vitro.

Structural requirements for cellular uptake and antisense activity of peptide nucleic acids conjugated with various peptides

Peptide nucleic acids (PNAs) have shown great promise as potential antisense drugs; however, poor cellular delivery limits their applications. Improved delivery into mammalian cells and enhanced biological activity of PNAs have been achieved by coupling to cell-penetrating peptides (CPPs). Structural requirements for the shuttling ability of these peptides as well as structural properties of the conjugates such as the linker type and peptide position remained controversial, so far. In the present study an 18mer PNA targeted to the cryptic splice site of a mutated beta-globin intron 2, which had been inserted into a luciferase reporter gene coding sequence, was coupled to various peptides. As the peptide lead we used the cell-penetrating alpha-helical amphipathic peptide KLAL KLAL KAL KAAL KLA-NH2 [model amphipathic peptide (MAP)] which was varied with respect to charge and structure-forming properties. Furthermore, the linkage and the localization of the attached peptide (C- vs N-terminal) were modified. Positive charge as well as helicity and amphipathicity of the KLA peptide was all required for efficient dose-dependent correction of aberrant splicing. The highest antisense effect was reached within 4 h without any transfection agent. Stably linked conjugates were also efficient in correction of aberrant splicing, suggesting that a cleavable disulfide bond between CPP and PNA is clearly not essential. Moreover, the placement of the attached peptide turned out to be crucial for attaining antisense activity. Coadministration of endosome disrupting agents such as chloroquine or Ca2+ significantly increased the splicing correction efficiency of some conjugates, indicating the predominant portion to be sequestered in vesicular compartments.

Internalization of novel non-viral vector TAT-streptavidin into human cells

Background

The cell-penetrating peptide derived from the Human immunodeficiency virus-1 transactivator protein Tat possesses the capacity to promote the effective uptake of various cargo molecules across the plasma membrane in vitro and in vivo. The objective of this study was to characterize the uptake and delivery mechanisms of a novel streptavidin fusion construct, TAT_47–57-streptavidin (TAT-SA, 60 kD). SA represents a potentially useful TAT-fusion partner due to its ability to perform as a versatile intracellular delivery vector for a wide array of biotinylated molecules or cargoes.

Results

By confocal and immunoelectron microscopy the majority of internalized TAT-SA was shown to accumulate in perinuclear vesicles in both cancer and non-cancer cell lines. The uptake studies in living cells with various fluorescent endocytic markers and inhibiting agents suggested that TAT-SA is internalized into cells efficiently, using both clathrin-mediated endocytosis and lipid-raft-mediated macropinocytosis. When endosomal release of TAT-SA was enhanced through the incorporation of a biotinylated, pH-responsive polymer poly(propylacrylic acid) (PPAA), nuclear localization of TAT-SA and TAT-SA bound to biotin was markedly improved. Additionally, no significant cytotoxicity was detected in the TAT-SA constructs.

Conclusion

This study demonstrates that TAT-SA-PPAA is a potential non-viral vector to be utilized in protein therapeutics to deliver biotinylated molecules both into cytoplasm and nucleus of human cells.

RNA targeting with peptide conjugates of oligonucleotides, siRNA and PNA

Towards the development of oligonucleotide analogues and siRNA as drugs, one potential alternative to the use of liposomal transfection agents is the covalent conjugation of a cell-penetrating peptide (CPP), with the intention of imparting on the oligonucleotide or siRNA an enhanced ability to enter mammalian cells and reach the appropriate RNA target. We have developed robust methods for the chemical synthesis of disulfide-linked conjugates of oligonucleotide analogues, siRNA and peptide nucleic acids (PNA) with a range of cationic and other CPPs. In a HeLa cell assay with integrated plasmid reporters of Tat-dependent trans-activation at the TAR RNA target in the cell nucleus, we were unable to obtain steric block inhibition of gene expression for conjugates of CPPs with a 12-mer oligonucleotide mixmer of 2'-O-methyl and locked nucleic acids units. By contrast, we were able to obtain some reductions in expression of P38alpha MAP kinase mRNA in HeLa cells using microM concentrations of Penetratin or Tat peptides conjugated to the 3'-end of the sense strand of siRNA. However, the most promising results to date have been with a 16-mer PNA conjugated to the CPP Transportan or a double CPP R(6)-Penetratin, where we have demonstrated Tat-dependent trans-activation inhibition in HeLa cells. Results to date suggest the possibility of development of CPP-PNA conjugates as anti-HIV agents as well as other potential applications involving nuclear cell delivery, such as the redirection of splicing.

Intravesical antisense therapy for cystitis using TAT-peptide nucleic acid conjugates

The present study investigated the potential of intravesical instillation for localized reduction of NGF (nerve growth factor) expression in the urinary bladder. Overexpression of NGF has been linked to the pathogenesis of interstitial cystitis (IC). A minimum free energy algorithm was used to predict suitable regions in mRNA of rat betaNGF, which can be targeted for an antisense approach. The candidate antisense oligos were evaluated for their ability to reduce NGF expression in vitro by cotransfecting HEK293 cells with NGF cDNA. A single oligonucleotide ODN sequence was chosen for testing in an acute cystitis model in rat induced by cyclophosphamide. Overexpression of NGF is known to mediate inflammation of bladder in this model. For improved stability, antisense ODN was replaced with antisense peptide nucleic acid (PNA) and its penetration into bladder was facilitated by tethering TAT peptide sequence. Rat bladders were instilled with either antisense or its scrambled control prior to cystitis induction. Cystometrograms performed on rats under urethane anaesthesia exhibited bladder contraction frequency that was significantly decreased in the antisense treated rats than rats treated with the control. NGF immunoreactivity was also decreased in the urothelium of the antisense treated bladders. Our findings demonstrate the feasibility of using TAT-PNA conjugates for intravesical antisense therapy.

Cellular delivery of polyheteroaromate-peptide nucleic acid conjugates mediated by cationic lipids

In the search of facile and efficient methods for PNA cellular delivery, we have tested a series of PNA conjugates based on (hetero) aromatic, lipophilic compounds such as 9-aminoacridine, benzimidazoles, carbazole, anthraquinone, porphyrine, psoralen, pyrene, and phenyl-bis-benzimidazole ("Hoechst"). These chemically modified PNAs were delivered to cultured pLuc705HeLa cells mediated by cationic liposomes (LipofectAMINE or LiofectAMINE2000), and their nuclear delivery was inferred from induced luciferase activity as a consequence of pre-mRNA splicing correction by the antisense-PNA. PNAs modified with 9-aminoacridine, "Hoechst", or acetyl-"Hoechst" showed highest antisense activities (while unmodified PNA failed to show any significant antisense activity). In particular, bis-acridine-conjugated PNA showed nearly 60% splicing correction at 250 nM concentration in combination with LipofectAMINE2000. Interestingly, relative differences between the derivatives were observed when LipofectAMINE was used as compared to LipofectAMINE2000, but in general the latter yielded the higher antisense activity. The most active modifications of these PNA constructs were further tested for antisense down-regulation of luciferase in p53R cells in order to evaluate the cytoplasmic activity (uptake) of the PNAs. A dose-dependent down regulation of luciferase was demonstrated also in this system. The PNA conjugated to acetyl-Hoechst caused a reduction of luciferase activity to less than 40% of the control at a concentration of 1 muM. These results indicate that conjugation of (hetero) polyaromatic compounds to PNA can dramatically improve liposome-mediated cellular delivery both to cytoplasm as well as to the nucleus. However, no clear structure/activity relations are apparent from the present results, except that both 9-aminoacridine and "Hoechst" are also nucleic acid binding ligands.

Photochemically induced gene silencing using PNA-peptide conjugates

The potential for exploration of peptide nucleic acid (PNA) as an experimental and therapeutic regulator of gene expression has been hampered by a poor delivery and a lack of site-specific targeting. In the present study, we have developed an efficient strategy for nuclear delivery of PNA by combining cationically charged PNA-peptide conjugates and photochemical internalization (PCI) technology. When using the S100A4 gene as a model system, a consistent downregulation to around 10% remaining protein signal was obtained in three selected cell lines. Furthermore, a dose-dependent and time-dependent inhibition of the S100A4 protein was demonstrated. A main benefit of the strategy proposed is the possibility of site-specific targeting.

Corneal Toxicity of Cell-Penetrating Peptides That InhibitHerpes simplexVirus Entry

Cell-penetrating peptides (CPPs) inhibit Herpes simplex virus entry at low micromolar concentrations and may be useful either as prophylactic or therapeutic agents for herpetic keratitis. The aim of this study was to assess the in vitro and in vivo toxicity of three CPPs-EB, TAT-C, and HOM (penetratin)-for the cornea. Incubation of primary (HK320) or immortalized (THK320) human keratocytes with the EB peptide (up to 100 microM), bHOMd (up to 200 microM), or TAT-C (up to 400 microM) resulted in no evidence of toxicity using a formazan dye-reduction assay. Similar results were obtained with a human trabecular meshwork cell line (TM-1), primary human foreskin fibroblasts (DP-9), Vero, and HeLa cells with EB and TATC. The bHOMd peptide showed some toxicity in Vero and HeLa cells, with CC50 values of 70 and 93 microM, respectively. The EB peptide did not inhibit macromolecular synthesis in Vero cells at concentrations below 150 microM, although cell proliferation was blocked at concentrations of EB above 50 microM. In vivo toxicity was assessed by applying peptides in Dulbecco's modified Eagle's medium to the cornea 4 times daily for 7 d. At concentrations 1000 times the IC50 values, the EB and bHOM peptides showed no toxicity, whereas TAT-C caused some mild eyelid swelling. Some slight epithelial cell sloughing was seen with the bKLA peptide in vivo. These results suggest that these CPPs-and EB in particular-have a favorable toxicity profile, and that further development is warranted.

Evaluation of cell-penetrating peptides (CPPs) as vehicles for intracellular delivery of antisense peptide nucleic acid (PNA)

Cell-penetrating peptides (CPPs) are characterized by their ability to be internalized in mammalian cells. To investigate the relative potency of CPPs as carriers of medicinally relevant cargo, a positive read-out assay based on the ability of a peptide nucleic acid (PNA) oligomer to promote correct expression of a recombinant luciferase gene was employed. Seven different CPPs were included in the study: Transportan, oligo-arginine (R7-9), pTat, Penetratin, KFF, SynB3, and NLS. The CPP-PNA conjugates were synthesized by different conjugation chemistries: continuous synthesis, maleimide coupling, and ester or disulfide linkage. Under serum-free conditions PNA-SS-Transportan-amide (ortho)-PNA was found to be the most potent conjugate, resulting in maximum luciferase signal at a concentration of 1-2 microM. (D-Arg)9-PNA showed optimal efficacy at 5 microM but gave rise to only one-third of the luciferase signal obtained with the Transportan conjugate. The pTat- and KFF-PNA conjugates showed significantly lower efficacy. The penetratin-, SynB3-. and NLS-PNA conjugates showed only minimal or no activity. Serum was found to have a drastic negative impact on CPP-driven cellular uptake. PNA-SS-Transportan-acid (ortho) and (D-Arg)9-PNA were least sensitive to the presence of serum. Both the chemical nature and, in the case of Transportan, the position of the peptide PNA coupling were found to have a major impact on the transport capacity of the peptides. However, no simple relationship between linker type and antisense activity of the conjugates could be deduced from the data.

Enhanced antisense effect of modified PNAs delivered through functional PMMA microspheres

Peptide nucleic acids (PNA) are very promising antisense agents, but their in vivo application is often hampered by their low bioavailability, mainly due to their limited uptake through cellular and nuclear membranes. However, PNA chemical synthesis easily allows modification with functional structures able to improve the intrinsically low permeability and great interest is arising in finding specific and efficient delivery protocols. Polymeric core-shell microspheres with anionic functional groups on the surface were tested for their ability to reversibly bind lysine modified PNA sequences, whose antisense activity against COX-2 mRNA was already demonstrated in murine macrophages.

Synthesis of cell-permeable peptide nucleic acids and characterization of their hybridization and uptake properties

Guanidine-based peptide nucleic acid (GPNA) monomers and oligomers containing all four natural (adenine (A), cytosine (C), guanine (G), and thymine (T)) and two unnatural (2-thiouracil (sU) and 2,6-diaminopurine (D)) nucleobases have been synthesized. Thermal denaturation study showed that GPNA oligomers containing alternate D-backbone configuration bind sequence-specifically to DNA and, when incubated with mammalian cells, localized specifically to the endoplasmic reticulum (ER).

Evaluation of transfection protocols for unmodified and modified peptide nucleic acid (PNA) oligomers

We have compared the efficacy of different transfection protocols reported for peptide nucleic acid (PNA) oligomers. A precise evaluation of uptake efficacy was achieved by using a positive readout assay based on the ability of a PNA oligomer to correct aberrant splicing of a recombinant luciferase gene. The study comprised transfection of PNA conjugated to acridine, adamantyl, decanoic acid, and porphyrine (acr-PNA, ada-PNA, deca-PNA, and por-RNA, respectively) and unmodified PNA partially hybridized to a DNA oligomer (PNA/DNA cotransfection). Furthermore, the effect of conjugation to a nuclear localization signal (NLS) was evaluated as part of the PNA/DNA cotransfection protocol. Transfection of the tested PNAs was systematically optimized. PNA/DNA cotransfection was found to produce the highest luciferase activity, but only after careful selection of the DNA oligonucleotide. Both a cationic lipid, Lipofectamine, and a nonliposomal cationic polymer, polyethylenimine (PEI, ExGen 500), were efficient transfection reagents for the PNA/DNA complex. However, Lipofectamine, in contrast to PEI, showed severe side effects, such as cytotoxicity. acr-PNA, ada-PNA, and por-PNA were transfectable with efficacies between 5 and 10 times lower than that seen with PNA/DNA cotransfection. Conjugation of PNA to NLS had no effect on PNA/DNA cotransfection efficacy. An important lesson from the study was the finding that because of uncontrollable biologic variations, even optimal transfection conditions differed to a certain extend from experiment to experiment in an unpredictable way.

Hydroxyproline-based DNA mimics provide an efficient gene silencing in vitro and in vivo

To be effective, antisense molecules should be stable in biological fluids, non-toxic, form stable and specific duplexes with target RNAs and readily penetrate through cell membranes without non-specific effects on cell function. We report herein that negatively charged DNA mimics representing chiral analogues of peptide nucleic acids with a constrained trans-4-hydroxy-N-acetylpyrrolidine-2-phosphonate backbone (pHypNAs) meet these criteria. To demonstrate this, we compared silencing potency of these compounds with that of previously evaluated as efficient gene knockdown molecules hetero-oligomers consisting of alternating phosphono-PNA monomers and PNA-like monomers based on trans-4-hydroxy-L-proline (HypNA-pPNAs). Antisense potential of pHypNA mimics was confirmed in a cell-free translation assay with firefly luciferase as well as in a living cell assay with green fluorescent protein. In both cases, the pHypNA antisense oligomers provided a specific knockdown of a target protein production. Confocal microscopy showed that pHypNAs, when transfected into living cells, demonstrated efficient cellular uptake with distribution in the cytosol and nucleus. Also, the high potency of pHypNAs for down-regulation of Ras-like GTPase Ras-dva in Xenopus embryos was demonstrated in comparison with phosphorodiamidate morpholino oligomers. Therefore, our data suggest that pHypNAs are novel antisense agents with potential widespread in vitro and in vivo applications in basic research involving live cells and intact organisms.

RNA targeting using peptide nucleic acid

The efforts towards peptide nucleic acid (PNA) drug discovery using cellular RNAs as molecular targets is briefly reviewed, with special emphasis on recent developments. Special attention is given to cellular delivery in vivo bioavailability and the possibilities of using PNA oligomers to (re)direct alternative splicing of pre-messenger (m)RNA.

Medium-sized peptides as built in carriers for biologically active compounds

A growing number of oligopeptides of natural and/or synthetic origin have been described and considered as targeting structures for delivery bioactive compounds into various cell types. This review will outline the discovery of peptide sequences and the corresponding mid-sized oligopeptides with membrane translocating properties and also summarize de novo designed structures possessing similar features. Conjugates and chimera constructs derived from these sequences with covalently attached bioactive peptide, epitope, oligonucleotide, PNA, drug, reporter molecule will be reviewed. A brief note will refer to the present understanding on the uptake mechanism at the end of each section.

Photochemically enhanced cellular delivery of cell penetrating peptide-PNA conjugates

Recent studies have shown that endosomal release is a major rate-limiting step for cellular delivery via a variety of cationic cell penetrating peptides. Thus, methods and/or protocols for effective release of endosomally entrapped drugs are highly warranted. Photochemical internalization (PCI) has previously been proposed for this purpose. Here, we demonstrate an enhancement of up to two orders of magnitude of the antisense effects (cytosolic/nuclear) of peptide nucleic acid-peptide conjugates (Tat, Arg7, KLA) in HeLa cells by a PCI approach using the photosensitizer AlPc2a. These results emphasize the importance of endosomal release for cellular activity of this type of drug delivery and also raise hope that methods like PCI which have applications for in vivo use may also enhance the bioavailability and in vivo efficacy of these types of conjugates.

Calcium ions effectively enhance the effect of antisense peptide nucleic acids conjugated to cationic tat and oligoarginine peptides

Cell-penetrating peptides have been widely used to improve cellular delivery of a variety of proteins and antisense agents. However, recent studies indicate that such cationic peptides are predominantly entering cells via an endosomal pathway. We now show that the nuclear antisense effect in HeLa cells of a variety of peptide nucleic acid (PNA) peptide conjugates is significantly enhanced by addition of 6 mM Ca(2+) (as well as by the lysosomotrophic agent chloroquine). In particular, the antisense activities of Tat(48-60) and heptaarginine-conjugated PNAs were increased 44-fold and 8.5-fold, respectively. Evidence is presented that the mechanism involves endosomal release. The present results show that Ca(2+) can be used as an effective enhancer for in vitro cellular delivery of cationic peptide-conjugated PNA oligomers, and also emphasize the significance of the endosomal escape route for such peptides.

Biological activity and biotechnological aspects of peptide nucleic acid

During the latest decades a number of different nucleic acid analogs containing natural nucleobases on a modified backbone have been synthesized. An example of this is peptide nucleic acid (PNA), a DNA mimic with a noncyclic peptide-like backbone, which was first synthesized in 1991. Owing to its flexible and neutral backbone PNA displays very good hybridization properties also at low-ion concentrations and has subsequently attracted large interest both in biotechnology and biomedicine. Numerous modifications have been made, which could be of value for particular settings. However, the original PNA does so far perform well in many diverse applications. The high biostability makes it interesting for in vivo use, although the very limited diffusion over lipid membranes requires further modifications in order to make it suitable for treatment in eukaryotic cells. The possibility to use this nucleic acid analog for gene regulation and gene editing is discussed. Peptide nucleic acid is now also used for specific genetic detection in a number of diagnostic techniques, as well as for site-specific labeling and hybridization of functional molecules to both DNA and RNA, areas that are also discussed in this chapter.

Endosome trapping limits the efficiency of splicing correction by PNA-oligolysine conjugates

Splicing correction by steric-blocking oligonucleotides (ON) might lead to important clinical applications but requires efficient delivery to cell nuclei. The conjugation of short oligolysine tails has been used to deliver a correcting peptide nucleic acid (PNA) sequence in a positive readout assay in which ON hybridization to the cryptic splice site is strictly required for the expression of a luciferase reporter gene. We have investigated the mechanism of cellular uptake and the efficiency of a (Lys)(8)-PNA-Lys construction in this model system. Cell uptake is temperature-dependent and leads to sequestration of the conjugate in cytoplasmic vesicles in keeping with an endocytic mechanism of internalization. Accordingly a significant and sequence-specific splicing correction is achieved only in the presence of endosome-disrupting agents as chloroquine or 0.5 M sucrose. These endosome-disrupting agents do not affect the activity of free PNA, and do not increase (Lys)(8)-PNA-Lys uptake.

Structure-activity relationship study on a simple cationic peptide motif for cellular delivery of antisense peptide nucleic acid

Improving cellular uptake and biodistribution remains one of the major obstacles for a successful and broad application of peptide nucleic acids (PNAs) as antisense therapeutics. Recently, we reported the identification and functional characterization of an antisense PNA, which redirects splicing of murine CD40 pre-mRNA. In this context, it was discovered that a simple octa(l-lysine) peptide covalently linked to the PNA is capable of promoting free uptake of the conjugate into BCL1 cells as well as primary murine macrophages. On the basis of this peptide motif, the present study aimed at identifying the structural features, which define effective peptide carriers for cellular delivery of PNA. While the structure-activity relationship study revealed some clear correlations, only a few modifications actually led to an overall improvement as compared to the parent octa(l-lysine) conjugate. In a preliminary PK/tissue distribution study in healthy mice, the parent conjugate exhibited relatively broad tissue distribution and only modest elimination via excretion within the time frame of the study.

Present and future of cell-penetrating peptide mediated delivery systems: “Is the Trojan horse too wild to go only to Troy?”

During the last decade, small peptides (10 to 15 amino acids) derived from the HIV-1 Tat protein and from the drosophila Antennapedia homeodomain have been used to internalize various types of molecules into the cells. The way these peptides enter cells is still under investigation and the object of strong controversy. The main discussions rely on whether these peptides are internalized or not in an energy-independent fashion, and, depending on the situation, whether they follow one pathway instead of another. At present, we find in the literature a very large number of data with, at times, some contradictory results. Indeed the diversity of employed peptide sequences, the cell type used, the attachment or not of a cargo molecule, the chemical nature of this cargo itself, and the followed protocol during the experimental process do not simplify the comparison and hence final conclusions about the mechanism of cell entry. However, one common feature emerges with these cell-penetrating peptides: most of them do not show any cell specificity. Despite their demonstrated efficiency in delivering biologically active molecules in in vitro experiments, their use for a therapeutic application in vivo has been the object of a relatively little number of studies, probably because of the quite important amounts of CPP-cargo that needs to be prepared for an accurate and complete in vivo study, but more likely, because of the massive spreading of the cargo all around the body. However, it appears from recent studies that an increased targeting ability of these CPPs is possible, making the use of CPP mediated delivery compatible with an in vivo therapeutic approach.

Cell-penetrating peptide conjugates of peptide nucleic acids (PNA) as inhibitors of HIV-1 Tat-dependent trans-activation in cells

The trans-activation response (TAR) RNA stem–loop that occurs at the 5′ end of HIV RNA transcripts is an important antiviral target and is the site of interaction of the HIV-1 Tat protein together with host cellular factors. Oligonucleotides and their analogues targeted to TAR are potential antiviral candidates. We have investigated a range of cell penetrating peptide (CPP) conjugates of a 16mer peptide nucleic acid (PNA) analogue targeted to the apical stem–loop of TAR and show that disulfide-linked PNA conjugates of two types of CPP (Transportan or a novel chimeric peptide R₆-Penetratin) exhibit dose-dependent inhibition of Tat-dependent trans-activation in a HeLa cell assay when incubated for 24 h. Activity is reached within 6 h if the lysosomotropic reagent chloroquine is co-administered. Fluorescein-labelled stably-linked conjugates of Tat, Transportan or Transportan TP10 with PNA were inactive when delivered alone, but attained trans-activation inhibition in the presence of chloroquine. Confocal microscopy showed that such fluorescently labelled CPP–PNA conjugates were sequestered in endosomal or membrane-bound compartments of HeLa cells, which varied in appearance depending on the CPP type. Co-administration of chloroquine was seen in some cases to release fluorescence from such compartments into the nucleus, but with different patterns depending on the CPP. The results show that CPP–PNA conjugates of different types can inhibit Tat-dependent trans-activation in HeLa cells and have potential for development as antiviral agents. Endosomal or membrane release is a major factor limiting nuclear delivery and trans-activation inhibition.

Comparison of basic peptides- and lipid-based strategies for the delivery of splice correcting oligonucleotides

Expression of alternatively spliced mRNA variants at specific stages of development or in specific cells and tissues contributes to the functional diversity of the human genome. Aberrations in alternative splicing were found as a cause or a contributing factor to the development, progression, or maintenance of numerous diseases. The use of antisense oligonucleotides (ON) to modify aberrant expression patterns of alternatively spliced mRNAs is a novel means of potentially controlling such diseases. Oligonucleotides can be designed to repair genetic mutations, to modify genomic sequences in order to compensate for gene deletions, or to modify RNA processing in order to improve the effects of the underlying gene mutation. Steric block ON approach have proven to be effective in experimental model for various diseases. Here, we describe our experience in investigating two strategies for ON delivery: ON conjugation with basic peptides and lipid-based particulate system (lipoplex). Basic peptides or Cell Penetrating Peptides (CPP) such as the TAT-derived peptide appear to circumvent many problems associated with ON and drug delivery. This strategy may represent the next paradigm in our ability to modulate cell function and offers a unique avenue for the treatment of disease. Lipoplexes result from the intimate interaction of ON with cationic lipids leading to ON carrying particles able to be taken up by cells and to release ON in the cytoplasm. We have used as an experimental model the correction of a splicing alteration of the mutated beta-globin intron causing thalassemia. Data on cell penetration and efficacy of correction of specific steric block ON delivered either by basic peptides or lipoplex are described. A comparison of the properties of both delivery systems is made respective to the use of this new class of therapeutic molecules.

Cell-penetrating peptides: A comparative membrane toxicity study

Cell-penetrating peptides (CPPs) constitute a new class of delivery vectors with high pharmaceutical potential. However, the abilities of these peptides to translocate through cell membranes can be accompanied by toxic effects resulting from membrane perturbation at higher peptide concentrations. Therefore, we investigated membrane toxicity of five peptides with well-documented cell-penetrating properties, pAntp(43-58), pTAT(48-60), pVEC(615-632), model amphipathic peptide (MAP), and transportan 10, on two human cancer cell lines, K562 (erythroleukemia) and MDA-MB-231 (breast cancer), as well as on immortalized aortic endothelial cells. We studied the effects of these five peptides on the leakage of lactate dehydrogenase and on the fluorescence of plasma membrane potentiometric dye bis-oxonol. In all cell lines, pAntp(43-58), pTAT(48-60), and pVEC(615-632) induced either no leakage or low leakage of lactate dehydrogenase, accompanied by modest changes in bis-oxonol fluorescence. MAP and transportan 10 caused significant leakage; in K562 and MDA-MB-231 cells, 40% of total lactate dehydrogenase leaked out during 10 min exposure to 10 microM of transportan 10 and MAP, accompanied by a significant increase in bis-oxonol fluorescence. However, none of the CPPs tested had a hemolytic effect on bovine erythrocytes comparable to mastoparan 7. The toxicity profiles presented in the current study are of importance when selecting CPPs for different applications.

Lysine-based peptide nucleic acids (PNAs) with strong chiral constraint: control of helix handedness and DNA binding by chirality

Two enantiomeric chiral PNAs bearing three adjacent D- or L-lysine-based residues in the middle of the strand ("chiral box" PNAs, sequence H-GTAGA(Lys)T(Lys)C(Lys)ACT-NH2) have been used as models in order to comprehensively study the effects of the stereogenic centers on PNA conformation and on PNA binding properties to complementary PNA and DNA strands. The binding properties of the two enantiomeric PNAs and of their homologous achiral PNA have been extensively studied by UV and CD spectroscopy and by mass spectrometry, both in the antiparallel and in the parallel mode with complementary PNA and DNA strands. In the antiparallel PNA:PNA duplexes, L-Lys PNA were found to form left-handed, and D-Lys PNA right handed helices, while in parallel duplexes, the reversed helicities were observed. Correspondingly, the preferred mode of binding and the best mismatch recognition of the D-Lys containing PNA with (right handed) DNA was found to be in the antiparallel orientation, while that of L-Lys PNA was found to be in the parallel mode. A rationale which correlates the preferred handedness of the PNA-PNA duplexes to the directionality of the binding to complementary DNA duplexes has been devised according to structural data and considering the "retro-inverso" concept widely used for peptides.

Free uptake of cell-penetrating peptides by fission yeast

An increasing number of peptides translocate the plasma membrane of mammalian cells promising new avenues for drug delivery. However, only a few examples are known to penetrate the fungal cell wall. We compared the capacity of different fluorophore-labelled peptides to translocate into fission yeast and human cells and determined their intracellular distribution. Most of the 20 peptides tested were able to enter human cells, but only one, transportan 10 (TP10), efficiently penetrated fission yeast and was distributed uniformly inside the cells. The results show that the fungal cell wall may reduce, but does not block peptide uptake.

Mechanism of PDX-1 protein transduction

PDX-1 plays a central role in differentiation of insulin-producing cells. We previously reported that exogenous PDX-1 protein can permeate cells and induce insulin gene expression in progenitor cells. These data suggest a strategy for facilitating differentiation into insulin-producing cells. Here we show the mechanism of PDX-1 protein transduction. Initially, a punctate cytoplasmic distribution of PDX-1 protein transduction domain (PTD), which co-localized with an endosomal marker, was observed in treated cells. However, homogeneous distribution of PDX-1-PTD was observed in some cells, indicating that PDX-1 is transduced by endocytosis and then released. The experiments using inhibitors suggested that the PDX-1 is transported through the Golgi complex and to the endoplasmic reticulum. Moreover, we observed in real-time PDX-1-PTD release from endosomes. These data suggest that mechanism of transduction of PDX-1 protein is by endocytosis and subsequent release from the endosome homogeneously in cytoplasm and nuclei, and that PDX-1 protein transduction could be a valuable strategy for facilitating differentiation of progenitor cells into insulin-producing cells.

Synthesis and cellular uptake of cell delivering PNA-peptide conjugates

The synthesis and cellular uptake of fluorescently labelled PNA-peptide conjugates is described; Dde/Mmt protected PNA monomers, fully orthogonal to Fmoc chemistry, were used to develop a flexible strategy to give Peptide Nucleic Acids conjugated to tri- and hepta-arginine and the short basic Tat(48-57) peptide as examples of cellular penetrating peptides, thereby allowing efficient cellular delivery of PNA into cells.

Metabolic cleavage of cell-penetrating peptides in contact with epithelial models: human calcitonin (hCT)-derived peptides, Tat(47-57) and penetratin(43-58)

We assessed the metabolic degradation kinetics and cleavage patterns of some selected CPP (cell-penetrating peptides) after incubation with confluent epithelial models. Synthesis of N-terminal CF [5(6)-carboxyfluorescein]-labelled CPP, namely hCT (human calcitonin)-derived sequences, Tat(47-57) and penetratin(43-58), was through Fmoc (fluoren-9-ylmethoxycarbonyl) chemistry. Metabolic degradation kinetics of the tested CPP in contact with three cell-cultured epithelial models, MDCK (Madin-Darby canine kidney), Calu-3 and TR146, was evaluated by reversed-phase HPLC. Identification of the resulting metabolites of CF-hCT(9-32) was through reversed-phase HPLC fractionation and peak allocation by MALDI-TOF-MS (matrix-assisted laser-desorption ionization-time-of-flight mass spectrometry) or direct MALDI-TOF-MS of incubates. Levels of proteolytic activity varied highly between the investigated epithelial models and the CPP. The Calu-3 model exhibited the highest proteolytic activity. The patterns of metabolic cleavage of hCT(9-32) were similar in all three models. Initial cleavage of this peptide occurred at the N-terminal domain, possibly by endopeptidase activity yielding both the N- and the C-terminal counterparts. Further metabolic degradation was by aminopeptidase, endopeptidase and/or carboxypeptidase activities. In conclusion, when in contact with epithelial models, the studied CPP were subject to efficient metabolism, a prerequisite of cargo release on the one hand, but with potential for premature cleavage and loss of the cargo as well on the other. The results, particularly on hCT(9-32), may be used as a template to suggest structural modifications towards improved CPP performance.

Intracellular uptake and inhibition of gene expression by PNAs and PNA-peptide conjugates

Peptide nucleic acids (PNAs) offer a distinct option for silencing gene expression in mammalian cells. However, the full value of PNAs has not been realized, and the rules governing the recognition of cellular targets by PNAs remain obscure. Here we examine the uptake of PNAs and PNA-peptide conjugates by immortal and primary human cells and compare peptide-mediated and DNA/lipid-mediated delivery strategies. We find that both peptide-mediated and lipid-mediated delivery strategies promote entry of PNA and PNA-peptide conjugates into cells. Confocal microscopy reveals a punctate distribution of PNA and PNA-peptide conjugates regardless of the delivery strategy used. Peptide D(AAKK)(4) and a peptide containing a nuclear localization sequence (NLS) promote the spontaneous delivery of antisense PNAs into cultured cells. The PNA-D(AAKK)(4) conjugate inhibits expression of human caveolin 1 (hCav-1) in both HeLa and primary endothelial cells. DNA/lipid-mediated delivery requires less PNA, while peptide-mediated delivery is simpler and is less toxic to primary cells. The ability of PNA-peptide conjugates to enter primary and immortal human cells and inhibit gene expression supports the use of PNAs as antisense agents for investigating the roles of proteins in cells. Both DNA/lipid-mediated and peptide-mediated delivery strategies are efficient, but the compartmentalized localization of PNAs suggests that improving the cellular distribution may lead to increased efficacy.

Delivery of bioactive molecules into the cell: the Trojan horse approach

In recent years, vast amounts of data on the mechanisms of neural de- and regeneration have accumulated. However, only in disproportionally few cases has this led to efficient therapies for human patients. Part of the problem is to deliver cell death-averting genes or gene products across the blood-brain barrier (BBB) and cellular membranes. The discovery of Antennapedia (Antp)-mediated transduction of heterologous proteins into cells in 1992 and other "Trojan horse peptides" raised hopes that often-frustrating attempts to deliver proteins would now be history. The demonstration that proteins fused to the Tat protein transduction domain (PTD) are capable of crossing the BBB may revolutionize molecular research and neurobiological therapy. However, it was only recently that PTD-mediated delivery of proteins with therapeutic potential has been achieved in models of neural degeneration in nerve trauma and ischemia. Several groups have published the first positive results using protein transduction domains for the delivery of therapeutic proteins in relevant animal models of human neurological disorders. Here, we give an extensive review of peptide-mediated protein transduction from its early beginnings to new advances, discuss their application, with particular focus on a critical evaluation of the limitations of the method, as well as alternative approaches. Besides applications in neurobiology, a large number of reports using PTD in other systems are included as well. Because each protein requires an individual purification scheme that yields sufficient quantities of soluble, transducible material, the neurobiologist will benefit from the experiences of other researchers in the growing field of protein transduction.

Synthesis, cellular uptake and HIV-1 Tat-dependent trans-activation inhibition activity of oligonucleotide analogues disulphide-conjugated to cell-penetrating peptides

Oligonucleotides composed of 2′-O-methyl and locked nucleic acid residues complementary to HIV-1 trans-activation responsive element TAR block Tat-dependent trans-activation in a HeLa cell assay when delivered by cationic lipids. We describe an improved procedure for synthesis and purification under highly denaturing conditions of 5′-disulphide-linked conjugates of 3′-fluorescein labelled oligonucleotides with a range of cell-penetrating peptides and investigate their abilities to enter HeLa cells and block trans-activation. Free uptake of 12mer OMe/LNA oligonucleotide conjugates to Tat (48–58), Penetratin and R₉F₂ was observed in cytosolic compartments of HeLa cells. Uptake of the Tat conjugate was enhanced by N-terminal addition of four Lys or Arg residues or a second Tat peptide. None of the conjugates entered the nucleus or inhibited trans-activation when freely delivered, but inhibition was obtained in the presence of cationic lipids. Nuclear exclusion was seen for free delivery of Tat (48–58), Penetratin and R₉ conjugates of 16mer phosphorothioate OMe oligonucleotide. Uptake into human fibroblast cytosolic compartments was seen for Tat, Penetratin, R₉F₂ and Transportan conjugates. Large enhancements of HeLa cell uptake into cytosolic compartments were seen when free Tat peptide was added to Tat conjugate of 12mer OMe/LNA oligonucleotide or Penetratin peptide to Penetratin conjugate of the same oligonucleotide.

Studies on the cellular uptake of substance P and lysine-rich, KLA-derived model peptides

In the last decade many peptides have been shown to be internalized into various cell types by different, poorly characterized mechanisms. This review focuses on uptake studies with substance P (SP) aimed at unravelling the mechanism of peptide-induced mast cell degranulation, and on the characterization of the cellular uptake of designed KLA-derived model peptides. Studies on structure-activity relationships and receptor autoradiography failed to detect specific peptide receptors for the undecapeptide SP on mast cells. In view of these findings, a direct interaction of cationic peptides with heterotrimeric G proteins without the participation of a receptor has been proposed. Such a process would require insertion into and translocation of peptides across the plasma membrane. In order to clarify whether a transport of cationic peptides into rat peritoneal mast cells is possible, transport studies were performed by confocal laser scanning microscopy (CLSM) using fluorescence-labeled Arg(3),Orn(7)-SP and its D-amino acid analog, all-D-Arg(3),Orn(7)-SP, as well as by electron microscopic autoradiography using (3)H-labelled SP and (125)I-labelled all-D-SP. The results obtained by CLSM directly showed translocation of SP peptides into pertussis toxin-treated cells. Kinetic experiments indicated that the translocation process was rapid, occurring within a few seconds. Mast cell degranulation induced by analog of magainin 2 amide, neuropeptide Y and the model peptide acetyl-KLALKLALKALKAALKLA-amide was also found to be very fast, pointing to an extensive translocation of the peptides. In order to learn more about structural requirements for the cellular uptake of peptides, the translocation behavior of a set of systematically modified KLA-based model peptides has been studied in detail. By two different protocols for determining the amount of internalized peptide, evidence was found that the structure of the peptides only marginally affects their uptake, whereas the efflux of cationic, amphipathic peptides is strikingly diminished, thus allowing their enrichment within the cells. Although the mechanism of cellular uptake, consisting of energy-dependent and -independent contributions, is not well understood, KLA-derived peptides have been shown to deliver various cargos (PNAs, peptides) into cells. The results obtained with SP- and KLA-derived peptides are discussed in the context of the current literature.

PNA Technology

Peptide nucleic acids (PNA) are deoxyribonucleic acid (DNA) mimics with a pseudopeptide backbone. PNA is an extremely good structural mimic of DNA (or of ribonucleic acid [RNA]), and PNA oligomers are able to form very stable duplex structures with Watson-Crick complementary DNA and RNA (or PNA) oligomers, and they can also bind to targets in duplex DNA by helix invasion. Therefore, these molecules are of interest in many areas of chemistry, biology, and medicine, including drug discovery, genetic diagnostics, molecular recognition, and the origin of life. Recent progress in studies of PNA properties and applications is reviewed.

Cellular uptake of antisense morpholino oligomers conjugated to arginine-rich peptides

Although the sequence specificity, biostability, and low toxicity of PMO (phosphorodiamidate morpholino oligomers) make them good antisense agents to study gene function, their limited ability to cross cell membranes limits their use in cell culture. In this paper we show that conjugation to arginine-rich peptides significantly enhanced the cellular uptake of PMO. The factors that affect the conjugate's cellular uptake and its antisense activity toward a targeted mRNA were investigated. Factors studied include the number of arginines in the peptide, the choice of cross-linker, the peptide conjugation position, the length of the PMO, and the cell culture conditions. Delivery of PMO to the cell nucleus and cytosol required conjugation rather than complexation of peptides to PMO. R(9)F(2)C was best suited to deliver a PMO to its target RNA resulting in the strongest antisense effect. By simply adding the R(9)F(2)C-PMO conjugate into the cell culture medium at low microM concentration, missplicing of pre-mRNA was corrected. This particular peptide-conjugated PMO was more effective than the PMO conjugated to the transmembrane transport peptides of HIV-1 Tat protein, Drosophila antennapedia protein, or to peptides with fewer arginines. Length of PMO did not affect a peptide's delivery efficacy, but all other factors were important. R(9)F(2)C peptide provided a simple and efficient delivery of PMO to a RNA target. Conjugation of peptide to PMO enhances the opportunities to evaluate gene functions in cell cultures.

Protein Cargo Delivery Properties of Cell-Penetrating Peptides. A Comparative Study

Application of cell-penetrating peptides for delivering various hydrophilic macromolecules with biological function into cells has gained much attention in recent years. We compared the protein transduction efficiency of four cell-penetrating peptides: penetratin, Tat peptide, transportan, and pVEC and studied the effects of various medium parameters on the uptake. Depletion of cellular energy and lowering of temperature strongly impaired the internalization of protein complexed with cell-penetrating peptides, confirming the endocytotic mechanism of peptide-mediated protein cellular transduction. Peptide-induced protein association with HeLa cells decreased 3-6-fold in energy-depleted cells. Inhibition of clathrin-dependent endocytosis by the hyperosmolar medium decreased the uptake of peptide-avidin complexes 1.5-3-fold and the removal of cholesterol from the plasma membrane 1.2-2-fold, suggesting that both clathrin-dependent and independent endocytosis were involved in peptide-induced cellular delivery of avidin. However, even under conditions of cellular energy depletion, ceasing of cellular traffic, and partial depolarization of plasma membrane, peptide-protein complexes associated with HeLa cells, as observed by FACS analysis and spectrofluorimetry. Among the studied peptides, pTat and transportan revealed higher protein transduction efficiency than penetratin or pVEC.

Chances and pitfalls of cell penetrating peptides for cellular drug delivery

Over the past decade, several classes and/or prototypes of cell penetrating peptides (CPP) have been identified and investigated in multiple aspects. CPP represent peptides, which show the ability to cross the plasma membrane of mammalian cells, and may thus give rise to the intracellular delivery of problematic therapeutic cargos, such as peptides, proteins, oligonucleotides, plasmids and even nanometer-sized particles, which otherwise cannot cross the plasma membrane. Most of the currently recognized CPP are of cationic nature and derived from viral, insect or mammalian proteins endowed with membrane translocation properties. The exact mechanisms underlying the translocation of CPP across the cellular membrane are still poorly understood. However, several similarities in translocation can be found. Early studies on CPP translocation mechanisms tended to suggest that the internalization of these peptides was neither significantly inhibited by low temperature, depletion of the cellular adenosine triphosphate (ATP) pool, nor by inhibitors of endocytosis. Moreover, chemical modification of the peptide sequence, such as the synthesis of retro-, enantio- or retroenantio-analogs, appeared not to affect the internalization properties. Therefore, translocation was concluded to result from direct, physical transfer through the lipid bilayer of the cell membrane. Later studies, however, showed convincing evidence for the involvement of endocytosis as the dominating mechanism for cellular internalization. In addition to describing the general properties of the commonly recognized classes of CPP, in this review we will also point out some limitations and typical pitfalls of CPP as carriers for therapeutics. In particular we will comment on emerging discrepancies with the current dogma, on cell-to-cell variability, biological barrier permeability, metabolic fate, toxicity and immunogenicity of CPP.

Recent advances in the development of peptide nucleic acid as a gene-targeted drug

Peptide nucleic acid (PNA) is a non-ionic mimic of DNA that binds to complementary DNA and RNA sequences with high affinity and selectivity. Targeting of single-stranded RNA leads to antisense effects, whereas PNAs directed toward double-stranded DNA exhibit antigene properties. Recent advances in cell uptake and in antisense and antigene effects in biological systems are summarised in this review. In addition to traditional targets, namely genomic DNA and messenger RNA, applications for PNA as a bacteriocidal antibiotic, for regulating splice site selection and as a telomerase inhibitor are described.

Synthesis of radiometal-labeled and fluorescent cell-permeating peptide-PNA conjugates for targeting the bcl-2 proto-oncogene

The B-cell lymphoma/leukemia-2 (bcl-2) proto-oncogene has been associated with the transformation of benign lesions to malignancy, disease progression, poor prognosis, reduced survival, and development of resistance to radiation and chemotherapy in many types of cancer. The objective of this work was to synthesize an antisense peptide nucleic acid (PNA) complementary to the first six codons of the bcl-2 open reading frame, conjugated to a membrane-permeating peptide for intracellular delivery, and modified with a bifunctional chelating agent for targeting imaging and therapeutic radiometals to tumors overexpressing bcl-2. Four peptide-PNA constructs were synthesized by a combination of manual and automated stepwise elongation techniques, including bcl-2 antisense conjugates and nonsense conjugates with no complementarity to any known mammalian gene or DNA sequence. The PNA sequences were synthesized manually by solid-phase 9-fluorenylmethoxycarbonyl (Fmoc) techniques. Then a fully protected lysine monomer, modified with 1,4,7,10-tetraazacyclododecane-N,N',N'',N'"-tetraacetic acid (DOTA) for radiometal chelation, was coupled manually to each PNA sequence. Synthesis of the DOTA-PNA conjugates was followed by automated elongation with a peptide sequence (PTD-4-glycine, PTD-4-G), known to mediate cellular internalization of impermeable effector molecules, or its retro-inverso analogue (ri-PTD-4-G). Preparation of the four conjugates required an innovative synthetic strategy, using mild acid conditions to generate hydrophobic, partially deprotected intermediates. These intermediates were purified by semipreparative reversed-phase HPLC and completely deprotected to yield pure peptide-PNA conjugates in 6% to 9% overall yield. Using modifications of this synthetic strategy, the ri-PTD-4-G conjugate of bcl-2 antisense PNA was prepared using a lysine derivative of tetramethylrhodamine (TMR) for fluorescence microscopy. Plasma stability studies showed that (111)In-DOTA-labeled ri-PTD-4-G-anti-bcl-2 PNA was stable for 168 h at 37 degrees C, unlike the conjugate containing the parent peptide sequence. Scanning confocal fluorescence microscopy of TMR-labeled ri-PTD-4-G-anti-bcl-2 PNA in Raji lymphoma cells demonstrated that the retro-inverso peptide was active in membrane permeation and mediated cellular internalization of the antisense PNA into the cytoplasm, where high concentrations of bcl-2 mRNA are expected to be present.

Combination of a new generation of PNAs with a peptide-based carrier enables efficient targeting of cell cycle progression

The design of potent systems for the delivery of charged and noncharged molecules that target genes of interest remains a challenge. We describe a novel technology that combines a new generation of peptide nucleic acids (PNAs), or HypNA-pPNAs, with a new noncovalent peptide-based delivery system, Pep-2, which promotes efficient delivery of PNAs into several cell lines. We have validated the potential of this technology by showing that Pep2-mediated delivery of an antisense HypNA-pPNA chimera directed specifically against cyclin B1 induces rapid and robust downregulation of its protein levels and efficiently blocks cell cycle progression of several cell lines, as well as proliferation of cells derived from a breast cancer. Pep-2-based delivery system was shown to be 100-fold more efficient in delivering HypNA-pPNAs than classical cationic lipid-based methods. Whereas Pep-2 is essential for improving the bioavailability of PNAs and HypNA-pPNAs, the latter contribute significantly to the efficiency and specificity of the biological response. We have found that Pep-2/HypNA-pPNA strategy promotes potent antisense effects, which are approximately 25-fold greater than with classical antisense oligonucleotide directed specifically against the same cyclin B1 target. Taken together, these data demonstrate that peptide-mediated delivery of HypNA-pPNAs constitutes a very promising technology for therapeutic applications.