Exploitation of intracellular pH gradients in the cellular delivery of macromolecules

In vitro and in vivo measurement of pH and thiols by EPR-based techniques

In vitro and in vivo measurements of pH and thiols provide critical information on physiology and pathophysiology of living organisms, particularly related to oxidative stress. Stable nitroxides of imidazoline and imidazolidine types provide the unique possibility of measuring local values of pH and glutathione content in various biological systems, including in vivo studies. The basis for these applications is the observation of specific chemical reactions of these nitroxides with protons or thiols, followed by significant changes in the electron paramagnetic resonance (EPR) spectra of these probes, measured by low-frequency EPR techniques. The applications of some newly developed pH and SH probes in model systems of pharmacological interest, biological fluids, tissues, and cells as well as in vivo studies in isolated hearts and in the gut of living animals are discussed.

pH-Dependent Modification of Lipophilicity of Porphyrin-type Photosensitizers

Structural modifications of photosensitizers (changes in protonation, ionic state and aggregation state) under different environmental conditions should be precisely determined to understand the interaction of the photosensitizers with biological systems. In the present study partition coefficients of hematoporphyrin IX (HpIX), disulfonated meso-tetraphenylporphine, meso-tetra(3-hydroxyphenyl)porphine (mTHPP) and meso-tetra(3-hydroxyphenyl)chlorin in the 1-octanol-phosphate buffer system were determined in the pH region 4.0-8.0. Only the partition coefficients of HpIX and mTHPP were found to be pH dependent. Computer processing of fluorimetric titration data was applied to estimate pKa values of the imino nitrogens of mTHPP. Monoprotonated species of mTHPP seem to be unstable or nonexistent. The possibility that both imino nitrogens of this dye are protonated according to a common pKa is proposed. The pKa value of the imino nitrogens of mTHPP was found to be 2.99 +/- 0.04 after the application of a model taking aggregation of the drug into account. The contributions of various aqueous ionic species of mTHPP as functions of pH were calculated and compared with partition coefficients.

Chemical approaches to triggerable lipid vesicles for drug and gene delivery

Effective drug delivery requires the precise spatial and temporal delivery of therapeutic agents to the target site. To this end, a variety of chemical and physicochemical approaches have been devised to create lipid vesicles (liposomes) that can be triggered to release their contents in a controlled fashion. The triggers include changes in pH, redox potential, temperature, or the level of specific enzymes. We review the chemistries that have recently been applied to exploit the pH and redox potential triggers so as to release vesicle contents in the appropriate biological location.

Cytosolic delivery of macromolecules: I. Synthesis and characterization of pH-sensitive acyloxyalkylimidazoles

A series of 1-(acyloxyalkyl)imidazoles (AAI) were synthesized by nucleophilic substitution of chloroalkyl esters of fatty acids with imidazole. The former was prepared from fatty acid chloride and an aldehyde. When incorporated into liposomes, these lipids show an apparent pK(a) value ranging from 5.12 for 1-(palmitoyloxymethyl)imidazole (PMI) to 5.29 for 1-[(alpha-myristoyloxy)ethyl]imidazole (alpha-MEI) as determined by a fluorescence assay. When the imidazole moiety was protonated, the lipids were surface-active, as demonstrated by hemolytic activity towards red blood cells. As expected, AAI were hydrolyzed in serum as well as in cell homogenate. They were significantly less toxic than biochemically stable N-dodecylimidazole (NDI) towards Chinese hamster ovary (CHO) and RAW 264.7 (RAW) cells as determined by MTT assay. When fed to RAW cells, fluorescein-labeled oligonucleotides encapsulated in liposomes containing 20 mol% 1-(stearoyloxymethyl)imidazole (SMI) resulted in punctate as well as partially diffuse fluorescence. In a functional assay involving down-regulation of luciferase in CV-1 cells, neutral liposomes containing imidazole lipids showed suboptimal delivery of antisense phosphorothioate oligomers. Taken together, the results suggest that AAI are of potential use in developing nontoxic, pH-sensitive liposomes. However, these liposomal formulations need to be optimized to achieve higher concentrations of pH-sensitive detergents within the endosome to facilitate efficient cytosolic release of liposome-entrapped contents.

Cytosolic delivery of macromolecules. II. Mechanistic studies with pH-sensitive morpholine lipids

Drug carriers containing weak acids or bases can promote cytosolic delivery of macromolecules by exploiting the acidic pH of the endosome. We have prepared two pH-sensitive mono-stearoyl derivatives of morpholine, one with a (2-hydroxy) propylene (ML1) linker and the other, an ethylene (ML2) linker. The pK(a) values of lipids ML1 and ML2, when incorporated into liposomes, are 6.12 and 5.91, respectively. Both lipids disrupt human erythrocytes at pH equal to or below their pK(a) but show no such activity at pH 7.4. Confocal microscopy studies suggest partial endosome-to-cytosol transfer of fluorescent dextran (MW 10 kDa) encapsulated in liposomes that contained 20 mol% of morpholine lipids. Interestingly, co-incubation of morpholine lipids in free or micellar form (without liposomal incorporation) with dextran resulted in efficient cytosolic delivery. Upon acidification to the endosomal pH, liposomes containing ML1 revealed: (a). leakage of entrapped solute that is independent of solute size; (b). lack of liposomal collapse into micelles as evidenced by photon correlation spectroscopy and UV light scattering; and (c). minimal inter-bilayer interactions as shown in a fluorescence resonance energy transfer assay. These observations are consistent with progressive intravesicular reorganization of lipids into stable liposomes of smaller size, but of more homogeneous distribution, upon acidification. The results emphasize a need to manipulate liposomal formulations containing ML1 such that ML1 will promote catastrophic collapse of liposomes to mixed micelles upon exposure to acidic pH. It is only then that micelle-mediated permeabilization of the endosomal membrane will lead to efficient cytosolic delivery of macromolecules originally loaded in liposomes.

Low-pH-sensitive PEG-stabilized plasmid-lipid nanoparticles: preparation and characterization

The acid-labile poly(ethyleneglycol)-diorthoester-distearoylglycerol lipid (POD), was used with a cationic lipid-phosphatidylethanolamine mixture to prepare stabilized plasmid-lipid nanoparticles (POD SPLP) that could mediate gene transfer in vitro by a pH triggered escape from the endosome. Nanoparticles of 60 nm diameter were prepared at pH 8.5 using a detergent dialysis method. The DNA encapsulation efficiency in the nanoparticles was optimal between 10 and 13 mol % ratio of cationic lipid and at a POD content of 20 mol %. The apparent zeta potential of the nanoparticles at 1 mM salt and pH 7.5 was positive, indicating cationic lipid on the external surface. However, the external layer of the nanoparticles was depleted in the cationic component compared to the starting mole ratio. Low pH sensitivity of the POD SPLP was characterized by a lag phase followed by a rapid collapse; at pH 5.3 the nanoparticles collapsed in 100 min. Nanoparticles prepared from a pH-insensitive PEG-lipid, PEG-distearoylglycerol had similar physicochemical characteristics as the POD SPLP but did not collapse at low pH. The POD SPLP had up to 3 orders of magnitude greater gene transfer activity than did the pH-insensitive nanoparticles. Both the pH-sensitive and pH-insensitive nanoparticles were internalized to a qualitatively similar extent in a punctate pattern into cultured cells within 2 h of incubation with the cells; thus, increased gene transfer of the POD SPLP was due to a more rapid escape from the endosome rather than to greater cell association of these nanoparticles. These results suggest that the pH-sensitive stabilized plasmid-lipid nanoparticles may be a useful component of a synthetic vector for parenterally administered gene therapy.

Design strategies to improve soluble macromolecular delivery constructs

Macromolecular therapeutics provide numerous benefits for the delivery of cytotoxic or poorly soluble drugs in vivo. However, these constructs often encounter barriers for drug delivery on both the systemic and subcellular level. Many soluble polymer carriers have been designed to surmount specific physiological barriers individually, but less work has been dedicated to designing an all-encompassing construct that addresses multiple therapeutic barriers at once. Incorporation of multiple agents already individually known to increase effectiveness into one carrier could further improve current drug delivery technology. Recent developments in subcellular delivery of therapeutic agents in soluble macromolecular carriers are discussed in the context of the future possibility for the design of an all-encompassing soluble multi-functional drug delivery vehicle.

Endosomolysis by masking of a membrane-active agent (EMMA) for cytoplasmic release of macromolecules

Endosomolysis, a critical barrier to efficient delivery of macromolecules such as nucleic acids, has been breached using a novel approach: endosomolysis by masking of a membrane-active agent (EMMA). To demonstrate the concept of EMMA, a cationic membrane-active peptide, melittin, was reversibly inhibited using a maleic anhydride derivative. At neutral pH, the lysines of melittin are covalently acylated with the anhydride, thereby inhibiting melittin's membrane disruption activity. Under acidic conditions such as those present within endosomes, the amide bond of the maleamate is cleaved, thus unmasking melittin. The active melittin can then disrupt the endosomal membrane resulting in release of biologically active molecules into the cytoplasm. This approach avoids cellular toxicity by restricting melittin's activity until it reaches the endosomal compartment. The utility of this approach was demonstrated by delivery phosphorodiamidate morpholino oligonucleotides (PMOs).