Biological effects of CDP-choline loaded long circulating liposomes on rat cerebral post-ischemic reperfusion

Cerivastatin Nanoliposome as a Potential Disease Modifying Approach for the Treatment of Pulmonary Arterial Hypertension

In this study we investigated nanoliposome as an approach to tailoring the pharmacology of cerivastatin as a disease-modifying drug for pulmonary arterial hypertension (PAH). Cerivastatin encapsulated liposomes with an average diameter of 98 ± 27 nm were generated by a thin film and freeze-thaw process. The nanoliposomes demonstrated sustained drug-release kinetics in vitro and inhibited proliferation of pulmonary artery (PA) smooth muscle cells with significantly less cellular cytotoxicity as compared with free cerivastatin. When delivered by inhalation to a rat model of monocrotaline-induced PAH, cerivastatin significantly reduced PA pressure from 55.13 ± 9.82 to 35.56 ± 6.59 mm Hg (P < 0.001) and diminished PA wall thickening. Echocardiography showed that cerivastatin significantly reduced right ventricle thickening (monocrotaline: 0.34 ± 0.02 cm vs. cerivastatin: 0.26 ± 0.02 cm; P < 0.001) and increased PA acceleration time (monocrotaline: 13.98 ± 1.14 milliseconds vs. cerivastatin: 21.07 ± 2.80 milliseconds; P < 0.001). Nanoliposomal cerivastatin was equally effective or slightly better than cerivastatin in reducing PA pressure (monocrotaline: 67.06 ± 13.64 mm Hg; cerivastatin: 46.31 ± 7.64 mm Hg vs. liposomal cerivastatin: 37.32 ± 9.50 mm Hg) and improving parameters of right ventricular function as measured by increasing PA acceleration time (monocrotaline: 24.68 ± 3.92 milliseconds; cerivastatin: 32.59 ± 6.10 milliseconds vs. liposomal cerivastatin: 34.96 ± 7.51 milliseconds). More importantly, the rate and magnitude of toxic cerivastatin metabolite lactone generation from the intratracheally administered nanoliposomes was significantly lower as compared with intravenously administered free cerivastatin. These studies show that nanoliposome encapsulation improved in vitro and in vivo pharmacologic and safety profile of cerivastatin and may represent a safer approach as a disease-modifying therapy for PAH.

Liposomes for brain delivery

INTRODUCTION

Development of drug delivery systems for brain delivery is one of the most challenging research topics in pharmaceutical areas, mainly due to the presence of the blood-brain barrier (BBB), which separates the blood from the cerebral parenchyma thus limiting the brain uptake of the majority of therapeutic agents. Among the several carriers, which have been studied to overcome this problem, liposomes have gained increasing attention as promising strategies for brain-targeted drug delivery. The most advantageous features of liposomes are their ability to incorporate and deliver large amounts of drug and the possibility to decorate their surface with different ligands.

AREAS COVERED

The purpose of this review is to explore the different approaches studied to transport and deliver therapeutics and imaging agents to the brain by using liposomes. In the first part of the review, particular attention is paid to describe the anatomy of the BBB and different physiological transport mechanisms available for drug permeation. In the second part, the different strategies for the delivery of a drug to the brain using liposomes are reviewed for each transport mechanism.

EXPERT OPINION

Over the last decade, there have been significant developments concerning liposomal brain delivery systems conjugated with selected ligands with high specificity and low immunogenicity. An universally useful liposomal formulation for brain targeting does not exist but liposome design must be modulated by the appropriate choice of the specific homing device and transport mechanism.

CDP-choline liposomes provide significant reduction in infarction over free CDP-choline in stroke

Cytidine-5'-diphosphocholine (CDP-choline, Citicoline, Somazina) is in clinical use (intravenous administration) for stroke treatment in Europe and Japan, while USA phase III stroke clinical trials (oral administration) were disappointing. Others showed that CDP-choline liposomes significantly increased brain uptake over the free drug in cerebral ischemia models. Liposomes were formulated as DPPC, DPPS, cholesterol, GM(1) ganglioside; 7/4/7/1.57 molar ratio or 35.8/20.4/35.8/8.0 mol%. GM(1) ganglioside confers long-circulating properties to the liposomes by suppressing phagocytosis. CDP-choline liposomes deliver the agent intact to the brain, circumventing the rate-limiting, cytidine triphosphate:phosphocholine cytidylyltransferase in phosphatidylcholine synthesis. Our data show that CDP-choline liposomes significantly ( P < 0.01) decreased cerebral infarction (by 62%) compared to the equivalent dose of free CDP-choline (by 26%) after 1 h focal cerebral ischemia and 24 h reperfusion in spontaneously hypertensive rats. Beneficial effects of CDP-choline liposomes in stroke may derive from a synergistic effect between the phospholipid components of the liposomes and the encapsulated CDP-choline.

Apoptosis: Future Targets for Neuroprotective Strategies

Focal permanent or transient cerebral artery occlusion produces massive cell death in the central core of the infarction, whereas in the peripheral zone (penumbra) nerve cells are subjected to various determining survival and death signals. Cell death in the core of the infarction and in the adult brain is usually considered a passive phenomenon, although events largely depend on the partial or complete disruption of crucial metabolic pathways. Cell death in the penumbra is currently considered an active process largely dependent on the activation of cell death programs leading to apoptosis. Yet cell death in the penumbra includes apoptosis, necrosis, intermediate and other forms of cell death. A rather simplistic view implies poor prospects regarding cell survival in the core of the infarction and therapeutic expectations in the control of cell death and cell survival in the penumbra. However, the capacity for neuroprotection depends on multiple factors, primarily the use of the appropriate agent, at the appropriate time and during the appropriate interval. Understanding the mechanisms commanding cell death and survival area is as important as delimiting the therapeutic time window and the facility of a drug to effectively impact on specific targets. Moreover, the detrimental effects of homeostasis and the activation of multiple pathways with opposing signals following ischemic stroke indicate that better outcome probably does not depend on a single compound but on several drugs acting in combination at the optimal time in a particular patient.

Ischemic cell death in brain neurons

This review is directed at understanding how neuronal death occurs in two distinct insults, global ischemia and focal ischemia. These are the two principal rodent models for human disease. Cell death occurs by a necrotic pathway characterized by either ischemic/homogenizing cell change or edematous cell change. Death also occurs via an apoptotic-like pathway that is characterized, minimally, by DNA laddering and a dependence on caspase activity and, optimally, by those properties, additional characteristic protein and phospholipid changes, and morphological attributes of apoptosis. Death may also occur by autophagocytosis. The cell death process has four major stages. The first, the induction stage, includes several changes initiated by ischemia and reperfusion that are very likely to play major roles in cell death. These include inhibition (and subsequent reactivation) of electron transport, decreased ATP, decreased pH, increased cell Ca(2+), release of glutamate, increased arachidonic acid, and also gene activation leading to cytokine synthesis, synthesis of enzymes involved in free radical production, and accumulation of leukocytes. These changes lead to the activation of five damaging events, termed perpetrators. These are the damaging actions of free radicals and their product peroxynitrite, the actions of the Ca(2+)-dependent protease calpain, the activity of phospholipases, the activity of poly-ADPribose polymerase (PARP), and the activation of the apoptotic pathway. The second stage of cell death involves the long-term changes in macromolecules or key metabolites that are caused by the perpetrators. The third stage of cell death involves long-term damaging effects of these macromolecular and metabolite changes, and of some of the induction processes, on critical cell functions and structures that lead to the defined end stages of cell damage. These targeted functions and structures include the plasmalemma, the mitochondria, the cytoskeleton, protein synthesis, and kinase activities. The fourth stage is the progression to the morphological and biochemical end stages of cell death. Of these four stages, the last two are the least well understood. Quite little is known of how the perpetrators affect the structures and functions and whether and how each of these changes contribute to cell death. According to this description, the key step in ischemic cell death is adequate activation of the perpetrators, and thus a major unifying thread of the review is a consideration of how the changes occurring during and after ischemia, including gene activation and synthesis of new proteins, conspire to produce damaging levels of free radicals and peroxynitrite, to activate calpain and other Ca(2+)-driven processes that are damaging, and to initiate the apoptotic process. Although it is not fully established for all cases, the major driving force for the necrotic cell death process, and very possibly the other processes, appears to be the generation of free radicals and peroxynitrite. Effects of a large number of damaging changes can be explained on the basis of their ability to generate free radicals in early or late stages of damage. Several important issues are defined for future study. These include determining the triggers for apoptosis and autophagocytosis and establishing greater confidence in most of the cellular changes that are hypothesized to be involved in cell death. A very important outstanding issue is identifying the critical functional and structural changes caused by the perpetrators of cell death. These changes are responsible for cell death, and their identity and mechanisms of action are almost completely unknown.

Survival rate improvement in a rat ischemia model by long circulating liposomes containing cytidine-5I-diphosphate choline

Unilamellar liposomes made up of DPPC-DPPS-Chol (7:4:7 molar ratio) and ganglioside GM1 8% mol were used to deliver cytidine-5I-diphosphate choline (CDP-choline) to the brain. The liposomal suspension consisted of unilamellar vesicles with a mean size of 50 nm and a very narrow size distribution. The therapeutic effectiveness of CDP-choline-loaded liposomes was investigated by an in vivo model of cerebral ischemia on Wistar rats (320-350 g). The animals were made ischemic to different extents (5, 15 and 30 min) by bilateral clamping of the common carotid arteries. The effect of free and liposomally encapsulated CDP-choline on the survival rate of post-ischemic reperfused rats was evaluated. The liposome formulation was much more active against ischemic injury than the free CDP-choline, ensuring a noticeable improvement of the survival rate with regards to the free drug ranging from 45% to 100% as a function of the duration of the ischemic event.