Development of Promitil®, a lipidic prodrug of mitomycin c in PEGylated liposomes: From bench to bedside.

Several liposome products have been approved for the treatment of cancer. In all of them, the active agents are encapsulated in the liposome water phase passively or by transmembrane ion gradients. An alternative approach in liposomal drug delivery consists of chemically modifying drugs to form lipophilic prodrugs with strong association to the liposomal bilayer. Based on this approach, we synthesized a mitomycin c-derived lipidic prodrug (MLP) which is entrapped in the bilayer of PEGylated liposomes (PL-MLP, Promitil®), and activated by thiolytic cleavage. PL-MLP is stable in plasma with thiolytic activation of MLP occurring exclusively in tissues and is more effective and less toxic than conventional chemotherapy in various tumor models. PL-MLP has completed phase I clinical development where it has shown a favorable safety profile and a 3-fold reduction in toxicity as compared to free mitomycin c. Clinical and pharmacokinetic studies in patients with advanced colo-rectal carcinoma have indicated a significant rate of disease stabilization (39%) in this chemo-refractory population and significant prolongation of median survival in patients attaining stable disease (13.9 months) versus progressive disease patients (6.35 months). The pharmacokinetics of MLP was typically stealth with long T½ (~1 day), slow clearance and small volume of distribution. Interestingly, a longer T½, and slower clearance were both correlated with disease stabilization and longer survival. This association of pharmacokinetic parameters with patient outcome suggests that arrest of tumor growth is related to the enhanced tumor localization of long-circulating liposomes and highlights the importance of personalized pharmacokinetic evaluation in the clinical use of nanomedicines. Another important area where PL-MLP may have an added value is in chemoradiotherapy, where it has shown a strong radiosensitizing effect in animal models based on a unique mechanism of enhanced prodrug activation and encouraging results in early human testing.

Therapeutic efficacy of a lipid-based prodrug of mitomycin C in pegylated liposomes: studies with human gastro-entero-pancreatic ectopic tumor models.

BACKGROUND: A mitomycin-C lipid-based prodrug (MLP) formulated in pegylated liposomes (PL-MLP) was previously reported to have significant antitumor activity and reduced toxicity in mouse tumor models (Clin Cancer Res 12:1913-20, 2006). MLP is activated by thiolysis releasing mitomycin-C (MMC) which rapidly dissociates from liposomes. The purpose of this study was to examine the plasma stability, pharmacokinetics, and antitumor activity of PL-MLP in mouse models of human gastroentero-pancreatic tumors. METHODS: MLP was incorporated with almost 100% efficiency in pegylated liposomes composed of hydrogenated phosphatidylcholine, with or without cholesterol (Chol). Mean vesicle size was 45-65 nm for liposome preparations downsized by homogenization, and 80-100 nm when downsized by extrusion, the latter displaying narrower polydispersity. MLP to phospholipid mole ratio was 5% (~20 µg MMC-equivalents/µmol). Therapeutic studies were carried out in the N87 gastric carcinoma (Ca), HCT15 colon Ca, and Panc-1 pancreatic Ca models implanted s.c. in CD1 nude mice. Treatment was administered i.v. in mice with established tumors. RESULTS: PL-MLP was very stable when incubated in plasma, and whole blood with a maximum of 5% release and activation to free MMC after 24 h. In the presence of a strong reducing agent (dithiotreitol), MLP was almost entirely activated to free MMC. Pharmacokinetic studies revealed major differences in plasma clearance between free MMC and PL-MLP. The longest half-lives were observed for extruded and Chol-containing preparations. Using a liposome radiolabel, it was found that the plasma levels of liposomes and prodrug were nearly superimposable confirming the absence of drug leakage in circulation. In vivo prodrug activation was significantly increased by co-injection of a large dose of a biocompatible reducing agent, N-acetylcysteine. PL-MLP was significantly more effective in delaying tumor growth and resulted in more tumor regressions than irinotecan in the N87 and HCT15 models, and than gemcitabine in the Panc-1 model. PL-MLP was ~3-fold less toxic than free MMC at MMC-equivalent doses, and displayed mild myelosuppression at therapeutic doses. CONCLUSIONS: Delivery of MLP in pegylated liposomes is more effective than conventional chemotherapy in the treatment of gastroentero-pancreatic ectopic tumor models, and may represent an effective tool for treatment of these malignancies in the clinical setting with improved safety over free MMC. Reducing agents offer a tool for controlling in vivo prodrug release.

Heterobifunctional PEGs: efficient synthetic strategies and useful conjugation methodologies.

The desire to develop nanoparticle and liposomal formulations as drug carriers capitalizing on active transport mechanisms requires constant development of novel heterobifunctional polyethyleneglycol (PEG) constructs. Such constructs should be capable of sequentially reacting with extracellular binding ligands and structural components of nanoparticles and/or liposomes. This paper describes two syntheses of heterobifunctional PEGs useful for tethering small molecule ligands to synthetic lysine-bearing polymers.

PEGylated iminodiacetic acid zinc complex stabilizes cationic RNA-bearing nanoparticles.

Self-assembling nanoparticles comprising cationic polymers are of interest for the delivery of oligonucleotide-based therapeutics. Unfortunately, exposure of the nanoparticle cationic surface to plasma and plasma proteins compromises particle stability and circulating half-life. Herein, we report that improved nanoparticle stability can be achieved through temporary grafting of PEG to the nanoparticle surface. Grafting is induced through zinc complexation between PEG-IDA and the exposed polyhistidylated polylysine (H-K) cationic polymer of pre-formed nanoparticles.

Improved therapeutic activity of folate-targeted liposomal doxorubicin in folate receptor-expressing tumor models.

PURPOSE: The folate receptor (FR) is overexpressed in a broad spectrum of malignant tumors and represents an attractive target for selective delivery of anti-cancer agents to FR-expressing tumors. Targeting liposomes to the FR has been proposed as a way to enhance the effects of liposome-based chemotherapy. METHODS: Folate-polyethylene glycol-distearoyl-phosphatidyl-ethanolamine conjugate was inserted into pegylated liposomal doxorubicin (PLD). The therapeutic activity of folate-targeted (FT-PLD) and non-targeted (PLD) pegylated liposomal doxorubicin was tested in two human tumor models (KB, KB-V) and in one mouse ascitic tumor model (FR-expressing J6456) by the i.v. systemic route in all models, and by the i.p. intracavitary route in the ascitic tumor model only. RESULTS: Consistent with previous studies, PLD was clearly superior to free doxorubicin in all tumor models. When targeted and non-targeted liposome formulations were compared, FT-PLD was more effective than PLD in the KB and KB-V xenograft models, and in the J6456 intra-cavitary therapy model. The therapeutic effect was dose-dependent in the KB model and schedule-dependent in the J6456 intra-cavitary therapy model. In some experiments, toxic deaths aggravated by folate-depleted diet were a major confounding factor. In a non-FR expressing J6456 model, FT-PLD was as active as PLD indicating that its activity is not limited to FR-expressing tumors. CONCLUSION: Folate-targeting confers a significant albeit modest therapeutic improvement to PLD in FR-expressing tumor models, which appears particularly valuable in intracavitary therapy. The potential clinical added value of this approach has yet to be determined.

Thiolytically cleavable dithiobenzyl urethane-linked polymer-protein conjugates as macromolecular prodrugs: reversible PEGylation of proteins.

New thiolytically cleavable dithiobenzyl (DTB) urethane-linked conjugates of methoxypoly(ethylene glycol) (mPEG) and a model protein, lysozyme, were prepared and thoroughly characterized. In contrast to our earlier communication [Zalipsky, et al. (1999) Bioconjugate Chem. 10, 703], in the current study we used a more sterically hindered form of para-DTB urethane linkage containing a methyl group on the alpha-carbon to the disulfide moiety. The new reagent for covalent attachment of mPEG-DTB to amino groups of proteins was synthesized via a seven-step process. As a result of PEG conjugation, the lysozyme was shown to completely lose its bacterial cell wall-lysing activity. However, activity was almost fully restored upon cysteine-mediated cleavage of the PEG component. The conjugate decomposition process was monitored by RP-HPLC and by ion spray LC-MS, which showed the formation of the p-mercaptobenzyl urethane-lysozyme intermediate, and ultimately its conversion to the unmodified lysozyme as the sole protein component. Pharmacokinetic evaluation of (125)I-labeled cleavable and noncleavable PEG-lysozyme given intravenously in rats revealed similar clearance patterns; both cleared in a significantly slower manner compared to that of the native protein. However, subcutaneous administration of the same conjugates showed a significantly larger AUC of the cleavable conjugate, indicating that some cleavage of the DTB urethane may have occurred. Although the DTB-linked PEG-lysozyme exhibited almost the same plasma clearance as the noncleavable counterpart, hinting that methyl-DTB linkage might be stable in the bloodstream, SDS-PAGE examination of the conjugate incubated in plasma showed decomposition at least partially mediated by albumin. These results suggest the potential of PEG-DTB-proteins as macromolecular prodrugs capable of generating fully active native proteins under in vivo conditions.

Targeted binding of PLA microparticles with lipid-PEG-tethered ligands.

Solid core polymeric particles are an attractive delivery vehicle as they can efficiently encapsulate drugs of different physical and chemical characteristics. However, the effective targeting of such particles for therapeutic purposes has been somewhat elusive. Here, we report novel polymeric particles comprised of poly(lactic acid) (PLA) with incorporated poly(ethylene glycol)-lipids (PEG-lipids). Particles are characterized for morphology, surface charge, and composition with field-emission scanning electron microscopy (FESEM), zeta potential measurements, and proton nuclear magnetic resonance ((1)H NMR) spectroscopy, respectively. The surface densities of PEG lipids determined by (1)H NMR and particle size distributions are consistent with scaling theory for adsorption of chains onto a surface. We observe significant binding of liganded PEG-lipid tethers when the molecular weight is greater than the unliganded PEG-lipids for significant binding events. Importantly, the binding is not completely lost when the unliganded PEG molecular weight is greater than the liganded PEG-lipid tether. We observe a similar trend for the lower affinity ligand (thioctic acid), but the degree of binding is significantly lower than the high affinity ligand (biotin). This novel technique used to fabricate these liganded particles combined with the lipid bilayer binding studies provides a platform for systematic optimization of particle binding.

Antitumor activity of new liposomal prodrug of mitomycin C in multidrug resistant solid tumor: insights of the mechanism of action.

The antitumor activity of a novel thiolytically cleavable lipid-based prodrug of mitomycin C (MMC) delivered by STEALTH liposomes (SL) was studied in drug resistant human ovarian carcinoma A2780/AD model and compared with free MMC and both free and SL forms of an established anticancer drug--doxorubicin (DOX). It was found that SL-prodrug (SL-pMMC) possessed enhanced antitumor activity when compared with the parent MMC, free DOX, and SL-DOX. An observance of the high antitumor efficiency of SL-pMMC was a result of its preferential accumulation in the tumor by the enhanced permeability and retention (EPR) effect, suppression of multidrug resistance (MDR) associated with P-glycoprotein and MRP drug efflux pumps, activation of caspase-dependent apoptosis signaling pathways and suppression of antiapoptotic cellular defense by increasing the BAX/BCL2 ratio. Consequently, the described SL-pMMC formulations can be considered good candidates for the chemotherapy of multidrug resistant tumors.

Pros and cons of the liposome platform in cancer drug targeting.

Coating of liposomes with polyethylene-glycol (PEG) by incorporation in the liposome bilayer of PEG-derivatized lipids results in inhibition of liposome uptake by the reticulo-endothelial system and significant prolongation of liposome residence time in the blood stream. Parallel developments in drug loading technology have improved the efficiency and stability of drug entrapment in liposomes, particularly with regard to cationic amphiphiles such as anthracyclines. An example of this new generation of liposomes is a formulation of pegylated liposomal doxorubicin known as Doxil or Caelyx, whose clinical pharmacokinetic profile is characterized by slow plasma clearance and small volume of distribution. A hallmark of these long-circulating liposomal drug carriers is their enhanced accumulation in tumors. The mechanism underlying this passive targeting effect is the phenomenon known as enhanced permeability and retention (EPR) which has been described in a broad variety of experimental tumor types. Further to the passive targeting effect, the liposome drug delivery platform offers the possibility of grafting tumor-specific ligands on the liposome membrane for active targeting to tumor cells, and potentially intracellular drug delivery. The pros and cons of the liposome platform in cancer targeting are discussed vis-à-vis nontargeted drugs, using as an example a liposome drug delivery system targeted to the folate receptor.

Reduced toxicity and superior therapeutic activity of a mitomycin C lipid-based prodrug incorporated in pegylated liposomes.

PURPOSE: A lipid-based prodrug of mitomycin C [MMC; 2,3-(distearoyloxy)propane-1-dithio-4'-benzyloxycarbonyl-MMC] was designed for liposome formulation. The purpose of this study was to examine the in vitro cytotoxicity, pharmacokinetics, in vivo toxicity, and in vivo antitumor activity of this new lipid-based prodrug formulated in polyethylene glycol-coated (pegylated) liposomes. EXPERIMENTAL DESIGN: MMC was released from the MMC lipid-based prodrug (MLP) by thiolytic-induced cleavage with a variety of thiol-containing reducing agents. MLP was incorporated with nearly 100% efficiency in cholesterol-free pegylated liposomes with hydrogenated phosphatidylcholine as the main component and a mean vesicle size of approximately 90 nm. This formulation was used for in vitro and in vivo tests in rodents. RESULTS: In vitro, the cytotoxic activity of pegylated liposomal MLP (PL-MLP) was drastically reduced compared with free MMC. However, in the presence of reducing agents, such as cysteine or N-acetyl-cysteine, its activity increased to nearly comparable levels to those of free MMC. Intravenous administration of PL-MLP in rats resulted in a slow clearance indicating stable prodrug retention in liposomes and long circulation time kinetics, with a pharmacokinetic profile substantially different from that of free MMC. In vivo, PL-MLP was approximately 3-fold less toxic than free MMC. The therapeutic index and absolute antitumor efficacy of PL-MLP were superior to that of free MMC in the three tumor models tested. In addition, PL-MLP was significantly more active than a formulation of doxorubicin in pegylated liposomes (DOXIL) in the M109R tumor model, a mouse tumor cell line with a multidrug-resistant phenotype. CONCLUSIONS: Delivery of MLP in pegylated liposomes is a potential approach for effective treatment of multidrug-resistant tumors while significantly buffering the toxicity of MMC.

Electrostatics of PEGylated micelles and liposomes containing charged and neutral lipopolymers.

The electrostatics of large unilamellar vesicles (LUVs) of various lipid compositions were determined and correlated with steric stabilization. The compositional variables studied include (a) degree of saturation, comparing the unsaturated egg phosphatidylcholine (EPC) and the fully hydrogenated soy phosphatidylcholine (HSPC) as liposome-forming lipids; (b) the effect of 40 mol % cholesterol; (c) the effect of mole % of three methyl poly(ethylene glycol) (mPEG)-lipids (the negatively charged mPEG-distearoyl phosphoethanolamine (DSPE) and two uncharged lipopolymers, mPEG-distearoyl glycerol (DSG) and mPEG-oxycarbonyl-3-amino-1,2-propanediol distearoyl ester (DS)); and (d) the negatively charged phosphatidyl glycerol (PG). The lipid phases were as follows: liquid disordered (LD) for the EPC-containing LUV, solid ordered (SO) for the HSPC-containing LUV, and liquid ordered (LO) for either of those LUV with the addition of 40 mol % cholesterol. The LUV zeta potential and electrical surface potential (psi(0)) were determined. It was found that progressive addition of mPEG(2k)-DSPE to liposomes increases negative surface potential and reduces surface pH to a similar extent as the addition of PG. However, due to the "hidden charge effect", zeta potential was more negative for liposomes containing PG than for those containing mPEG(2k)-DSPE. Replacing mPEG-DSPE with mPEG(2k)-DS or mPEG-DSG had no effect on surface pH and surface potential, and zeta potential was approximately zero. Addition of low concentrations of cationic peptides (protamine sulfate and melittin) to PG- or mPEG-DSPE-containing liposomes neutralized the liposome negative surface potential to a similar extent. However, only in liposomes containing PG, did liposome aggregation occur. Replacing the negatively charged lipopolymer mPEG-DSPE with the neutral lipopolymers mPEG-DS or mPEG-DSG eliminates or reduces such interactions. The relevance of this effect to the liposome performance in vivo is discussed.

Tumor cell targeting of liposome-entrapped drugs with phospholipid-anchored folic acid-PEG conjugates.

Targeting of liposomes with phospholipid-anchored folate conjugates is an attractive approach to deliver chemotherapeutic agents to folate receptor (FR) expressing tumors. The use of polyethylene glycol (PEG)-coated liposomes with folate attached to the outer end of a small fraction of phospholipid-anchored PEG molecules appears to be the most appropriate way to combine long-circulating properties critical for liposome deposition in tumors and binding of liposomes to FR on tumor cells. Although a number of important formulation parameters remain to be optimized, there are indications, at least in one ascitic tumor model, that folate targeting shifts intra-tumor distribution of liposomes to the cellular compartment. In vitro, folate targeting enhances the cytotoxicity of liposomal drugs against FR-expressing tumor cells. In vivo, the therapeutic data are still fragmentary and appear to be formulation- and tumor model-dependent. Further studies are required to determine whether folate targeting can confer a clear advantage in efficacy and/or toxicity to liposomal drugs.

Pharmaco attributes of dioleoylphosphatidylethanolamine/cholesterylhemisuccinate liposomes containing different types of cleavable lipopolymers.

Various amounts of one of three different types of cleavable methoxy polyethylene glycol (mPEG)-phospholipids or of a non-cleavable counterpart (mPEG-DSPE) were included into pH-sensitive liposome formulations containing dioleoylphosphatidylethanolamine (DOPE) and cholesterylhemisuccinate (CHEMS) at a 6:4 molar ratio, and the effect on plasma clearance and contents release rates was determined. The cleavable lipopolymers were all based on a distearoylphosphatidyl lipid anchor, which was linked to mPEG via dithiodipropionateaminoethanol (mPEG-DTP-DSPE), dithio-3-hexanol (mPEG-DTH-DSPA), or Gly-Phe-Leu-Gly-aminoethanol (mPEG-GFLG-DSPE) linkers. In contrast to the first-generation thiolytically cleavable lipopolymer, mPEG-DTP-DSPE, the second generation conjugates contained a hindered disulfide or enzymatically cleavable tetrapeptide, respectively, as the points of scission. In the absence of mPEG-lipid, DOPE/CHEMS liposomes had rapid clearance half-lives. As the mol% of mPEG-lipid in the liposomes increased, the rate of clearance of DOPE/CHEMS liposomes in mice decreased. Zeta-potential measurements showed that decreased clearance was correlated with a decrease in the apparent surface charge of the liposomes, which approached neutrality as the content of mPEG-lipids increased to above 15 mol%. At these levels, liposomes containing mPEG-DTP-DSPE were cleared from blood circulation faster than liposomes containing other, less vulnerable lipopolymers. Liposomes with the peptide-linked lipopolymer exhibited the slowest clearance. The presence of either cleavable or non-cleavable mPEG-lipids at concentrations of 5 mol% or higher in the DOPE/CHEMS liposomes inhibited the release of doxorubicin from these liposomes in response to acid pH.

In vivo fate of folate-targeted polyethylene-glycol liposomes in tumor-bearing mice.

PURPOSE: To compare the in vivo tissue distribution of folate-targeted liposomes (FTLs) injected i.v. in mice bearing folate receptor (FR)-overexpressing tumors (mouse M109 and human KB carcinomas, and mouse J6456 lymphoma) to that of nontargeted liposomes (NTLs) of similar composition. EXPERIMENTAL DESIGN: A small fraction of a folate-polyethylene-glycol (PEG)-distearoyl-phosphatidylethanolamine conjugate was incorporated in FTLs. Both FTLs and NTLs were PEGylated with a PEG-distearoyl-phosphatidylethanolamine conjugate to prolong circulation time. Liposomes were labeled with [(3)H]cholesterol hexadecyl ether with or without doxorubicin loading. Liposome levels in plasma, tissues, or ascites were assessed by the number of [(3)H] counts. For doxorubicin-loaded formulations, we also determined the tissue doxorubicin levels by fluorimetry. To estimate the amount of liposomes directly associated with tumor cells in vivo, we determined the [(3)H]radiolabel counts in washed pellets of ascitic tumor cells using the ascitic J6456 lymphoma RESULTS: FTLs retained the folate ligand in vivo, as demonstrated by their ability to bind ex vivo to FR-expressing cells after prolonged circulation and extravasation into malignant ascitic fluid. In comparison with NTLs, FTLs were cleared faster from circulation as a result of greater liver uptake. Despite the lower plasma levels, tumor levels of FTL-injected mice were not significantly different from those of NTL-injected mice. When NTLs and FTLs were loaded with doxorubicin, liver uptake decreased because of liver blockade, and uptake by spleen and tumor increased. When tumor-to-tissue liposome uptake ratios were analyzed, the targeting profile of FTLs was characterized by higher tumor:skin, and tumor:kidney ratios but lower tumor:liver ratio than NTLs. After a concomitant dose of free folic acid, FTLs (but not NTLs) plasma clearance and liver uptake were inhibited, indicating that accelerated clearance was mediated by the folate ligand. Surprisingly tumor uptake was not significantly affected by a codose of folic acid. In the J6456 ascitic tumor model, tumor cell-associated liposome levels were significantly greater for FTL-injected mice than for NTL-injected mice, despite slightly higher levels of the latter in whole ascites. CONCLUSIONS: Whereas folate targeting does not enhance overall liposome deposition in tumors, the targeting profile of tumor versus other tissues is substantially different and intratumor liposome distribution in ascitic tumors is affected favorably with a selective shift toward liposome association with FR-expressing cells.

Interaction of differently designed immunoliposomes with colon cancer cells and Kupffer cells. An in vitro comparison.

PURPOSE: Evaluate the effectiveness of distal-end coupling of a tumor-specific antibody to liposomal polyethylene glycol (PEG) chains to improve target binding and reduce interference by macrophage uptake. METHODS: Monoclonal antibody CC52, specific for CC531 rat colon carcinoma, was coupled to the bilayer of PEG-liposomes (type I) or to the distal end of bilayer-anchored PEG-chains (type II). Uptake of both (radiolabeled)liposome types by CC531 cells and rat liver macrophages was determined. RESULTS: With increasing antibody density, both immunoliposome types showed increased binding to target cells, but type II liposomes displayed better target recognition than type I. Uptake by macrophages increased with antibody density for both liposome types. Lowest uptake by macrophages was found for type II liposomes at low antibody densities. Unexpectedly, not only for type I but also for type II liposomes, in which the antibody is coupled via its Fc moiety, uptake by macrophages was inhibited by aggregated IgG, indicating involvement of Fc receptors. Also polyinosinic acid, an inhibitor of scavenger receptors, reduced uptake of type II liposomes. CONCLUSION: Although distal end coupling of antibodies to bilayer-anchored PEG chains in liposomes through the Fc moiety enhances target cell binding, it does not prevent the recognition by Fc receptors on macrophages.