Question

You are designing different liposomes for drug delivery through the blood stream and for skin application as a topical lotion. Describe your selection of lipids for each application that would ensure that liposomes remain in a semi-fluid state between liquid-ordered and liquid-disordered states.

Answer 1

From the drug delivery point of view, Liposomes are one of the safest nanocontainer when compared to other nanomaterials (e.g. quantum dots, nanowires etc.). Thus, it is widely used in several drug delivery system such as gene and cancer drug delivery, skin drug delivery and other clinical purpose.

Liposomes have been studied for a long time. Following these studies, liposomes containing neutral and negatively or positively charged phospholipids along with cholesterol have been successfully prepared. A Few of these formulations have been approved for use in humans and others have reached Phase I/II/ III clinical trials. It is remarkable to mention that DaunoXome (liposomes containing anticancer drug Doxorubicin Nexster pharmaceuticals) and Doxil/Caylex (PEG-liposomes containing anticancer drug Doxorubicin, Sequus pharmaceuticals) were the two honourable liposome based formulations approved for patients in clinics.

Other primary examples of clinically approved liposome based products are AmBisome (Gilead Sciences , FosterCity, USA) which is used to deliver the antifungal drug amphotericin B, Myocet (Elan Pharmaceuticals, USA) which is used to deliver anticancer drug doxorubicin (Alberts et al., 2004). Currently Daunoxome is considered as a stable liposomal formulation as this is not cleared by MPS osponization and available in ready- toinject formulation. A number of formulations have also been prepared by combining sphingomyelin phospholipids and conventional liposomes. For instance novel liposomal formulation vincristine (Marqibo Hana biosciences, USA) has been approved as orphan medical products in US and Europe and used to treat uveal melanoma. Furthermore, liposomal formulations such as INX-0125 (liposomal vinorelbine) and INX-0076 (topotecan) are also used as potential therapeutic agent. Clinical data have shown that incorporation of OSI-211 in the acid aqueous core of the vesicle can increases stability of the active compound as well as improve its tumour toxicity. Therefore, OSI-211 (Lurtotecan, OSI Pharmaceuticals,Inc., NY) has been successfully used to treat ovarian cancers and B-cell lymphoma. A large number of MLVs from Neo Pharma Inc. have been used clinically and these include liposome formulations such as LEM-ETU (mitoxantrone-entrapped), LEPETU (paclitaxel-loaded) and LESN38 (irinotecanencapsulated). Recently, DepoCyt (Pacira Pharmaceuticals, San Diego, CA)- a liposomal formulation entrapping cytarabin has been approved for neoplasicmeningitis and lymphomatousmeningitis and given by intrathecal route. Other two MLV liposome formulations possessing saturated phospholipids such as dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) have been approved for clinical use. For instance Nyotran® (Aronex Pharmaceuticals, The Woodlands, TX, USA) for treating antifungal infection and Aroplatin® (Antigenics Inc., Lexington, MA, USA) for treating B-cell lymphoma. Another remarkable liposomal formulations is made by Aronex Pharmaceuticals named as Atragen® which contains the active ingredients tretinoin, dimyristoylphosphatidylcholine (DMPC), soybean oil and is very effective against retinoid-responsive cancers and acute promyelocytic leukemia.

A lipophilic anthracycline antibiotic i.e. Annamycin liposomes is primarily investigated by MD Anderson Cancer Institute. This formulation has reached Phase II clinical trials and is being investigated as potential tool in the treatment of doxorubicin resistant breast cancer. Another potential drug candidate which entered Phase II clinical trial is EndoTAG™-1 (MediGene A.G., Martinsried, Germany). This is effective against pancreatic cancers. Apart from this, Visudyne® was approved by FDA in 2000 and for the first time age related macular degeneration was successfully treated with these liposomes. Other clinically approved liposomes are DepoDur® (containing morphine) which is used for postoperative pain after surgery; Definity® (containing octafluoropropane) for ultrasound imaging; Abelcet® (containing amphotericin- B) for fungal infection and Mepact® (containing Muramyl dipeptide) for osteosarcoma. During 2009, Octocogalfa (containing plasma factor VIII) has been approved for the treatment of haemophilia A. In conclusion, to date a few number of liposome based formulations are clinically approved while other liposome based formulations are in the queue awaiting clinical trials, even though patient treatment can be improved by considering two aspects i.e. drug targeting potential and triggering mechanism of liposomes.

Drug targeting potential of liposomes

The requirement for drug targeting emerges when drug enters into blood after intravenous injection, then most of the drugs is wasted by distribution to other tissues and organs. A small amount of drug reaches to the disease affected area and remaining of the drug causes drug toxicity. In order to increase the availability of drugs to the disease affected area and to diminish the undesirable side effects of drugs, drug targeting through liposomes is essential . Drug targeting through liposomes has successfully been achieved by the use of a variety of ligands such as antibodies, hormones, peptides and tagged antibodies. This tagged ligand easily determines the exact receptor site and this mechanism pushes liposomes to saturate at such target sites. In this way, more drugs become available to the target site and distribution of liposomes into the RES region is minimised or precluded. The bio- distribution of lipid vesicles into RES region is effectively altered by the inclusion of protein or carbohydrates molecule (which show specific affinity towards a particular tissue or organ) into the lipid bilayer. Drug targeting via liposomes is very important tool. However, from clinical point of view, success stories from these types of liposomes formulations are less frequent. Therefore, it is imperative to focus on another aspect of drug delivery i.e. triggering of liposomes.

Triggering of liposomes

In a bid to increase the availability and diminish the rate of clearance of liposomes, a concept of sterically-stabilized liposomes has come forward. For instance-polyethylene glycol conjugated to distearoylphosphatidylethanolamine (PEG-DSPE). Liposome formulation containing sterically stabilized liposomes exhibit a prolonged circulation time in blood and reduced uptake by the liver and spleen and improved accumulation in implanted tumours. Even though, the therapeutic window of these liposomes is very low due to the slow release of drug from the liposomes. To overcome this drawback, “intelligent” liposomes, which possess energy –activated release mechanism that speeds up the drug release rates and increases the therapeutic efficacy, have been developed. These “intelligent” liposomes not only improve the efficacy of drug but also increase intracellular stability and activity of drug. Based on the type of stimulus, the “intelligent” liposomes, also termed as functional liposomes, are classified into internally and externally controllable vesicles. Internal stimuli are caused by internal triggers such as activation by pH, redox potential and enzymes whereas external stimuli are caused by external triggers such as light, heat, ultrasound, electromagnetic fields or ionizing radiation. All triggerable liposomes follow the same mechanism and drug release depends on structural alteration caused by the disruption of the lipid bilayer via a stimulus at the weakest point. Internal triggers possess some limitations. The pH-sensitive liposomes are easily cleared out by receptor mediated endocytosis and that is why they need to be coated with PEGylation material. Moreover, all internal triggers (pH, redox potential and enzymes) are difficult to control because the reaction occurs in complex biological systems like an entire organism. It is very hard to find the disease specific internal triggers for certain diseases. As a result external triggers are more beneficial than internal triggers for cancer treatment. External triggers can at time focus on certain body areas. So if the one site of target cell is involved in pathogenesis (for instance- stem cells from tumour) then other site of the target cell can easily get targeted by external triggers (for instance –stem cells from bone marrow). External triggers also possess several limitations. Temperature sensitive liposomes cause hyperthermia along with major inflammation. Ultrasound triggered release causes tissue damage whereas electromagnetically and magnetically controllable liposomes require smart design and neutron capturing principles from the tissue which is very difficult. Among all of the triggers, light triggered release is versatile and has distinctive advantages over other systems for the controlled drug release. The major drawback associated with light is tissue penetration depth which could be solved by use of near infra-red (NIR) irradiation. A large number of light triggered liposomes have been approved for clinical trials (e.g. Visudyne).