Stealth Liposomes: Local Chemotherapy

One of the key problems in the treatment of serious diseases such as cancer is that many potent drugs are very poisonous and not only kill the diseased cells but also healthy ones. In the beginning of the twentieth century, the father of modern medical chemistry, Paul Erlich, envisioned the perfect drug as a bullet that automatically targets and selectively kills diseased cells without damaging healthy tissue. The magic bullet contains a number of features. First of all, it contains the drug or another related compound, a so-called pro-drug, which can be turned into a drug by an appropriate mechanism. The drug is attached to a carrier, which can transport the drug to the carrier. Finally the carrier may contain some kind of homing device that can search for the target and target the site where the drug is supposed to act.

: Magic Bullet schematic illustration

One such transport system could be based on liposomes, which is ideal for this purpose for several reasons. They are made of biocompatible, nontoxic, and biodegradable materials; they have a aqueous lumen that can contain hydrophilic compounds; they are composed of a lipid bilayer that can accommodate hydrophobic or amphiphilic drugs. They can be made in different sizes and they can be modified with specific chemical groups at the liposome surface that can act as homing devices and thereby target specific cells. Despite these advantages liposome based drug delivery also presents problems. Liposomes injected into the blood stream quickly become captured and degraded by the immune system. When this happens, the drug is released in the blood where is becomes degraded or, even worse, may damage or kill the red blood cells. Conventional liposomes therefore, seldom make it to other sites in the body than the liver and the spleen.

: Illustration og a Stealth liposome. a fraction of the lipids present have a polyethylene glycol polymer bound to their headgroups. This polymer binds a lot of water creating a water cloud around the liposome, which hides the liposome from the immune system

In order to overcome this problem liposomes have to become invisible to the immune system. Special lipids coupled to a water-soluble polymer through the lipid head group, a so-called lipopolymer, have to be used. A certain fraction of these lipopolymers are added to the lipid mixture, which is used to make the liposomes. The polymer part is dissolved in the aqueous environment; therefore the immune system only detects these stealth liposomes as harmless water. Besides being invisible to the immune system they also exhibit longer circulation times in the blood due to increased stability.
In addition to overcome the problem with the immune system another problem has also solved when working with liposome based drug delivery. Liposomes are quite sufficient at encapsulating anti-cancer drugs such as Cisplatin and Doxorubicin; therefore liposomes do not necessarily deliver more drugs to the tumours than through the application of the free drug.
One solution to this problem is to design the lipid mixture, which the liposomes are made of, to become leaky a few degrees above the body temperature hence letting the encapsulated material flow out. By tuning the lipid composition to become leaky at a certain temperature above the body temperature it is possible to heat the tumour locally by either microwave, ultrasound or radio-frequency radiation resulting in a very fast release of the anti-cancer drug, typically a million times faster than from conventional liposomes.

Even though the release of anti-cancer drugs is very efficient with this strategy it is necessary to know which area to heat. The position and the size of the diseased tissue, therefore, have to be known beforehand.It is known that several variants of secretory phopholipase A2 are over expressed in malignant tumours and sometimes occur in a concentration that maybe ten times larger than in healthy tissue. Likewise tissue infested with tumors tend to have leaky capillaries. Phopholipase A2 is an enzyme that catalyzes the hydrolysis of phospholipids into lysolipids and free fatty acids.

: Phospholipase A2 rendered as New Cartoon, PDB file (left). The hydrolysis of the etherlipid Plasmalogen catalysed by phospholipase A2 resulting in a lyso-etherlipid at the top and stearic acid at the bottom(right).

In 1999 a Danish pharmacist by the name Kent Jørgensen realized that it should be possible to use the phospholipase-induced drug release mechanism described above not only to release drugs but, but also produce a potent drug at the site of the disease. The idea is amazing simple and is illustrated below.

: Stealth liposomes travelling into the leaky capillaries associated with cancer rich tissue can pass through the capillary wall and venture out into the cancer tissue.

The trick is to use liposomes made of lipids that upon hydrolysis via the phospholipase lead to products that themselves are drugs. Compounds that can be turned into drugs but are not drugs themselves are called prodrugs. The prodrug in this case is a lipid in which the fatty acid chain in the first position is bound to the glycerol backbone by an ether bond and in the second position by and ester bond. After the hydrolysis catalysed by phospholipase A2 the products are a lyso-ether lipid and a free fatty acid.

: Schematic illustration of the binding of phospholipase A2 to the membrane, and the resulting products of the reaction.

The lyso-ether lipid is an extremely potent anti-cancer drug that so far has found limited use by conventional chemotherapy because it kills red blood cells. However in its masked prodrug form as a part of a lipid, it turns out to be completely harmless. Hence the prodrug can be incorporated into long-circulating stealth liposomes that, upon accumulation in the capillaries of porous cancer tissue is broken down by phospholipase A2. The drug is therefore produced exactly where it is needed, in fact, without prior knowledge of the localization of the tumour. Obviously the above described drug delivery system can be loaded with anti cancer agents like Cisplatin or Doxorubicin, so the tumour is treated with both the anti-cancer agent and the anti-cancer lyso-ether lipid [AEL].