Design, synthesis, and transfection biology of novel cationic glycolipids for use in liposomal gene delivery

Common co-lipids, in synergy, impart high gene transfer properties to transfection-incompetent cationic lipids

Efficacious cationic transfection lipids usually need either DOPE or cholesterol as co-lipid to deliver DNA inside the cell cytoplasm in non-viral gene delivery. If both of these co-lipids fail in imparting gene transfer properties, the cationic lipids are usually considered to be transfection inefficient. Herein, using both the reporter gene assay in CHO, COS-1 and HepG2 cells and the whole cell histochemical X-gal staining assay in representative CHO cells, we demonstrate that common co-lipids DOPE, Cholesterol and DOPC, when act in synergy, are capable of imparting improved gene transfer properties to a novel series of cationic lipids (1-5). Contrastingly, lipids 1-5 became essentially transfection-incompetent when used in combination with each of the pure co-lipid components alone.

A QSAR-modeling perspective on cationic transfection lipids. 1. Predicting efficiency and understanding mechanisms

BACKGROUND

As gene therapy using viral vectors involves clinical risks, limited DNA-carrying capacity, and manufacturing problems, non-viral vectors, including cationic lipids, have been investigated. Unfortunately, these agents have significantly lower transfectional ability and, due to the complexity of the transfectional pathway, no general schemes exist for correlating cationic lipid chemistry with transfectional efficacy.

METHODS

Quantitative structure-activity relationship (QSAR) analyses were carried out on sets of routinely used, experimental, and unsuccessful cationic lipid vectors taken from the literature. This approach described the amphipathic character, basicity, headgroup size, lipophilicity and shape of cationic lipids using numerical parameters. Compounds were plotted onto various parameter diagrams, and correlations were sought between numerical parameters and transfectional efficiency.

RESULTS

Transfectionally effective cationic lipids fell into restricted zones in various parameter spaces, indicating that amphipathic character, lipid shape and lipophilicity were generally significant factors, whilst basicity and headgroup size were only important for certain compounds. The data supported the general significance of membrane mixing followed by induction of membrane curvature, and the more limited role of osmotic shock, as mechanisms of membrane disruption. QSAR descriptions of effective lipids permitted detailed chemical guidelines for optimizing cationic lipid structure to be given. Limitations of the approach and models are discussed.

CONCLUSIONS

QSAR modeling indicated that induction of membrane curvature and osmotic shock are important mechanisms for membrane disruption by cationic lipids. The models also allowed specification of chemically detailed guidelines for selection or design of optimal cationic lipids.

Distance of Hydroxyl Functionality from the Quaternized Center Influence DNA Binding and in Vitro Gene Delivery Efficacies of Cationic Lipids with Hydroxyalkyl Headgroups

In vitro gene delivery efficacies of cationic amphiphiles 1-7 (Scheme 1) were measured by both the reporter gene expression assays in CHO, COS-1, HepG2, and MCF7 cells and by the whole cell histochemical X-gal staining of representative Chinese hamster ovary cells. Our results demonstrated that in vitro gene delivery efficiencies of cationic lipids with hydroxyalkyl headgroups are adversely affected by increased covalent distances between the hydroxyl functionality and the cationic centers. Findings in the DNase I protection experiments and transmission electron microscopic study support the notion that such compromised gene delivery efficacies may originate from poor lipid-DNA binding interactions and significantly increased lipoplex nanosizes.

On the disulfide-linker strategy for designing efficacious cationic transfection lipids: an unexpected transfection profile

Herein, employing a previously reported disulfide-linker strategy, we have designed and synthesized a novel cationic lipid 2 with a disulfide-linker and its non-disulfide control analog lipid 1. The relative efficacies of lipids 1 and 2 in transfecting CHO, COS-1 and MCF-7 cells were measured using both reporter gene and whole cell histochemical staining assays. In stark contrast to the expectation based on the disulfide-linker strategy, the control non-disulfide cationic lipid 1 showed phenomenally superior in vitro transfection efficacies to its essentially transfection incompetent disulfide counterpart lipid 2. Results in DNase I protection experiments and the electrophoretic gel patterns in the presence of glutathione, taken together, are consistent with the notion that the success of the disulfide-linker strategy may depend more critically on the DNase I sensitivity of the lipoplexes than on the efficient DNA release induced by intracellular glutathione pool.

Spacer-Arm Modulated Gene Delivery Efficacy of Novel Cationic Glycolipids: Design, Synthesis, and in Vitro Transfection Biology

Design, syntheses and relative in vitro gene delivery efficacies of six novel cationic glycolipids 1-6 containing open-form galactosyl units in CHO, COS-1, MCF-7 and A549 cells are described. The results of the present structure-activity investigation convincingly demonstrate that the in vitro gene delivery efficacies of galactosylated cationic glycolipids are strikingly dependent on the absence of a spacer-arm between the open-form galactose and the positively charged nitrogen atom in their headgroup region. While the cationic glycolipids 1-3 with no headgroup spacer unit between the positively charged nitrogen and galactose showed high in vitro gene transfer efficacies in all four cells (lipids 1 and 2 with myristyl and palmityl tails, respectively, being the most efficacious), lipids 4-6 with five-carbon spacer units between the quaternized nitrogen and galactose heads were essentially transfection incompetent. The transfection inhibiting role of the five-carbon spacer unit in the headgroup region of the present novel class of cationic lipids was demonstrated by both beta-galactosidase reporter gene expression and histochemical X-gal staining assays. Results of MTT assay-based cell viability measurements in representative MCF7 cells show that cell viabilities of lipoplexes (lipid:DNA complexes) prepared from all the lipids 1-6 are remarkably high. Thus, possibilities of differential cellular cytotoxicities playing any key role behind the strikingly contrasting transfection properties of lipids 1-3 with no spacer and lipids 4-6 with a spacer unit in the headgroup regions was ruled out. Electrophoresis gel patterns in DNase I sensitivity assays are consistent with more free DNA (accessible to DNase I) being present in lipoplexes of lipids 4-6 than in lipoplexes of lipids 1-3. Thus, the results of our DNase I protection experiments support the notion that enhanced degradation of DNA associated with lipoplexes of lipids 4-6 may play an important role in abolishing their in vitro gene transfer efficacies.

On the Gene Delivery Efficacies of pH-Sensitive Cationic Lipids via Endosomal Protonation

In an effort to probe the importance of endosomal protonation in pH-sensitive, cationic, lipid-mediated, non-viral gene delivery, we have designed and synthesized a novel cholesterol-based, endosomal pH-sensitive, histidylated, cationic amphiphile (lipid 1), its less pH-sensitive counterpart with an electron-deficient, tosylated histidine head group (lipid 2) as well as a third new cholesterol-based, cationic lipid containing no histidine head group (lipid 3). For all the novel liposomes and lipoplexes, we evaluated hysicochemical characteristics, including lipid:DNA interactions, global surface charge, and sizes. As anticipated, lipid 2 showed lower efficacies than lipid 1 for the transfection of 293T7 cells with the cytoplasmic gene expression vector pT7Luc at lipid:DNA mole ratios of 3.6:1 and 1.8:1; both lipids were greatly inhibited in the presence of Bafilomycin A1. This demonstrates the involvement of imidazole ring protonation in the endosomal escape of DNA. Conversely, endosome escape of DNA with lipid 3 seemed to be independent of endosome acidification. However, with nuclear gene expression systems in 293T7, HepG2, and HeLa cells, the transfection efficacies of lipid 2 at a lipid:DNA mole ratio of 3.6:1 were found to be either equal to or somewhat lower than those of lipids 1 and 3. Interestingly, at a lipid:DNA mole ratio of 1.8:1, lipids 2 and 3 were remarkably more transfection efficient than lipid 1 in both HepG2 and HeLa cells. Mechanistic implications of such contrasting relative transfection profiles are delineated.

Enhanced Intravenous Transgene Expression in Mouse Lung Using Cyclic-Head Cationic Lipids

Herein, we report enhanced intravenous mouse lung transfection using novel cyclic-head-group analogs of usually open-head cationic transfection lipids. Design and synthesis of the new cyclic-head lipid N,N-di-n-tetradecyl-3,4-dihydroxy-pyrrolidinium chloride (lipid 1) and its higher alkyl-chain analogs (lipids 2-4) and relative in vitro and in vivo gene transfer efficacies of cyclic-head lipids 1-4 to their corresponding open-head analogs [lipid 5, namely N,N-di-n-tetradecyl-N,N-(2-hydroxyethyl)ammonium chloride and its higher alkyl-chain analogs, lipids 6-8] have been described. In stark contrast to comparable in vitro transfection efficacies of both the cyclic- and open-head lipids, lipids 1-4 with cyclic heads were found to be significantly more efficient (by 5- to 11-fold) in transfecting mouse lung than their corresponding open-head analogs (5-8) upon intravenous administration. The cyclic-head lipid 3 with di-stearyl hydrophobic tail was found to be the most promising for future applications.

Design, Syntheses and In Vitro Gene Delivery Efficacies of Novel Mono-, Di- and Trilysinated Cationic Lipids: A Structure−Activity Investigation

Structure-activity investigation including design, syntheses, and evaluation of relative in vitro gene delivery efficacies of a novel series of cationic amphiphiles (1-10) containing mono-, di-, and trilysine headgroups are described in CHO, COS-1, and HepG2 cells. Several interesting and rather unexpected transfection profiles were observed. In general, lipid 1 with the myristyl tail used in combination with DOPE as colipid exhibited superior transfection properties compared to (a) the monolysinated analogues with longer hydrocarbon tails (lipids 2-4), (b) the dilysine (lipids 5-7) and the trilysine headgroup analogues (lipids 8-10), and (c) commercially available LipofectAmine with multiple positive charges in its polar region. As a preliminary estimate of the relative DNA-compacting efficacies of these new lysinated cationic lipids, the hydrodynamic diameters of representative lipoplexes were measured using dynamic laser light scattering technique. Our lipoplex size data are consistent with the notion that covalent grafting of an increasing number of positively charged functional groups in the headgroup region of cationic lipids need not necessarily result in more compacted lipoplexes. Both gel retardation and DNase I sensitivity assays indicated similar lipid/DNA binding interactions for all the novel mono-, di-, and trilysinated cationic lipids. MTT-assay-based cell viability results clearly demonstrate that the overall lower transfection properties of trilysine analogues (8-10) compared to their mono- (1-4) and dilysinated (5-7) counterparts are unlikely to originate from differential toxicity related effects. Taken together, the present findings support the notion that caution needs to be exercised in ensuring enhanced gene delivery efficacies of cationic lipids through covalent grafting of multiple lysine functionalities in the headgroup region.

Single additional methylene group in the head-group region imparts high gene transfer efficacy to a transfection-incompetent cationic lipid

In combination with equimolar 1,2-dioleoyl-L-alpha-glycero-3-phosphatidyl ethanolamine, a novel cholesterol-based cationic lipid with beta-alanine head-group (2) has been demonstrated to be strikingly more efficacious (10-24-fold) in transfecting CHO, COS-1 and HepG2 cells than its glycine analog (1) containing just one less methylene unit in its head-group region. Syntheses, characterizations and in vitro transfection biology of lipids 1 and 2 are described. Present findings demonstrate that even truly minor structural alterations, such as inclusion of just one additional methylene functionality in the polar head-group region, can convert an essentially transfection-incompetent cholesterol-based cationic amphiphile to a remarkably efficient cationic transfection lipid.

Synthesis of poly(beta-amino ester)s optimized for highly effective gene delivery

Several families of synthetic polymers, including degradable poly(beta-amino ester)s, have been previously shown to effectively mediate gene transfer. However, the combined impact of potentially significant factors-such as polymer molecular weight, polymer chain end-group, and polymer/DNA ratio-on different gene transfer properties has yet to be systematically investigated. The elucidation of these relationships may aid in the design of nonviral vectors with greatly enhanced transfection properties. To examine these factors, two distinct poly(beta-amino ester) structures, Poly-1 and Poly-2, were generated by adding 1,4-butanediol diacrylate and 1,6-hexanediol diacrylate, respectively, to 1-aminobutanol. Twelve unique versions of each structure were synthesized by varying amine/diacrylate stoichiometric ratios, resulting in polymers with either amine or acrylate end-groups and with molecular weights ranging from 3350 to 18000. Using high throughput methods, all polymers were tested in quadruplicate at nine different polymer/DNA ratios ranging from 10:1 w/w to 150:1 w/w. Through the optimization of molecular weight, polymer chain end-group, and polymer/DNA ratio, these polymers successfully mediated gene transfer at levels that surpassed both PEI and Lipofectamine 2000 in vitro.

Single histidine residue in head-group region is sufficient to impart remarkable gene transfection properties to cationic lipids: evidence for histidine-mediated membrane fusion at acidic pH

Presence of endosome-disrupting multiple histidine functionalities in the molecular architecture of cationic polymers, such as polylysine, has previously been demonstrated to significantly enhance their in vitro gene delivery efficiencies. Towards harnessing improved transfection property through covalent grafting of endosome-disrupting single histidine functionality in the molecular structure of cationic lipids, herein, we report on the design, the synthesis and the transfection efficiency of two novel nonglycerol-based histidylated cationic amphiphiles. We found that L-histidine-(N,N-di-n-hexadecylamine)ethylamide (lipid 1) and L-histidine-(N,N-di-n-hexadecylamine,-N-methyl)ethylamide (lipid 2) in combination with cholesterol gave efficient transfections into various cell lines. The transfection efficiency of Chol/lipid 1 lipoplexes into HepG2 cells was two order of magnitude higher than that of FuGENE(TM)6 and DC-Chol lipoplexes, whereas it was similar into A549, 293T7 and HeLa cells. A better efficiency was obtained with Chol/lipid 2 lipoplexes when using the cytosolic luciferase expression vector (pT7Luc) under the control of the bacterial T7 promoter. Membrane fusion activity measurements using fluorescence resonance energy transfer (FRET) technique showed that the histidine head-groups of Chol/lipid 1 liposomes mediated membrane fusion in the pH range 5-7. In addition, the transgene expression results using the T7Luc expression vector convincingly support the endosome-disrupting role of the presently described mono-histidylated cationic transfection lipids and the release of DNA into the cytosol. We conclude that covalent grafting of a single histidine amino acid residue to suitable twin-chain hydrophobic compounds is able to impart remarkable transfection properties on the resulting mono-histidylated cationic amphiphile, presumably via the endosome-disrupting characteristics of the histidine functionalities.

Novel galactosylated polyamine bolaamphiphiles for gene delivery

We describe the synthesis of a series of alpha-galacto-omega-polyamine double-chain bolaamphiphiles (Gal-CL) and report on the gene transfer mediated with lipoplexes they form either when used in conjunction with DOPE or with pcTG90:DOPE. Lipofection was investigated with human HepG2 and murine BNL-CL2 hepatocytes expressing the asialoglycoprotein (ASGP) receptor which displays a high affinity for galactosyl residues, and with A549 cells which do not express ASGP. Our results show that cationic N/P = 5 and 2.5 Gal-CL lipoplexes constitute very efficient nonspecific gene transfer systems. Lipofection experiments performed in the presence of asialofetuin (a high affinity ligand of ASGP) led us to evidence also the involvement of a specific receptor-mediated endocytosis pathway for the transfection of the ASGP(+) HepG2 or BNL-CL2 hepatocytes with some Gal-CL formulations. This work suggests that targetable lipopolyamines presenting a single galactose residue appear as promising synthetic vectors for specific gene delivery to ASGP(+) cells.

Structural changes in DNA mediated by cationic lipids alter in vitro transcriptional activity at low charge ratios

Lipid/DNA complexes or Lipoplexes have been characterized by various biochemical and biophysical methods to understand the physical basis of transfection. Here we have addressed the effect of cationic liposomes, 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), on transcription of DNA templates in vitro. Transcriptional activity of DNA-dependent RNA polymerase at DNA templates complexed with the cationic lipid varied as a function of charge ratio of lipid/DNA. At low charge ratios of 0.3:1 lipid/DNA and up to 1:1, we observed stimulation in transcription, while at higher charge ratios of lipid/DNA 3:1, complete inhibition in the activity occurred. Cetyl tri-methyl ammonium bromide (CTAB), a cationic detergent, and polyethylenimine (PEI), a cationic polymer, also bring about similar changes although to a lesser extent. The stimulation in transcription motivated us to probe into the molecular nature of the lipid/DNA interactions by absorbance spectroscopy and circular dichroism (CD). Upon interaction with lipids, hyperchromicity and susceptibility to micrococcal nuclease has increased, which suggests that the DNA was partially denatured. On complexation with the cationic lipid (DOTAP), the magnitude of the positive band in CD spectra decreased, accompanied with a red shift, as a function of charge ratio. Results from spectroscopic and enzyme assays suggest that at low charge ratios DNA may be partially unwound.

Glycosidation of alkylamino-alkan-1-ol. A simple and convenient synthesis of glycosylated cationic lipids

Starting from long chain alkylamino-alkan-1-ol a series of amino glycolipids were synthesized. This procedure allows a short and convenient preparation of glycosylated cationic lipids for gene delivery.

Novel non-glycerol-based cytofectins with lactic acid-derived head groups

We report herein the design, synthesis, and transfection biology of a novel series of non-glycerol-based cationic lipids with lactic acid-derived head groups The synthetic procedure adopted herein for preparing 1-hydroxy-prop-2-yl head-group-based monocationic transfection lipids 1-7 is fairly straightforward and potentially applicable in designing other cationic lipids with lactic acid-derived head groups. A striking anchor-length dependency was observed in NIH3T3 cells in the sense that except lipid 4, all the other lipids were essentially transfection-inefficient. Ethidium bromide assay for the lipid:DNA interactions is consistent with the general observation that significant lipid:DNA interactions do not guarantee on improved transfection efficiency cationic lipid mediated gene delivery. Given its remarkable transfection properties and low cellular toxicity, lipid 4 is likely to find future use in the area of liposomal gene delivery.