HOW MEDICINE RELIABLY REACHES ITS TARGET

With liposome-based RNA therapies helping to create new segments in the pharmaceutical market for advanced drug delivery, Evonik acquired Transferra Nanosciences, a leading contract development and manufacturing company for liposomal active ingredient formulations, in 2016. This Canadian company, which is based in Vancouver and is now called Evonik Vancouver Laboratories, has also developed a technology that enables liposomes to be produced with extreme precision in a narrow, well-defined range of sizes.

One option for the production of liposomal active ingredient formulations involves a special extrusion process. First of all, an ethanolic solution of lipids, which are molecules that also occur naturally as the building blocks of cell membranes, are mixed with an aqueous phase in which the active pharmaceutical ingredient is present. During the mixing, tiny structures arise in which the lipids create a bilayer. Lipids are long molecules that have one hydrophilic (water-attracting) end and one hydrophobic (with little affinity to water) end. In aqueous systems, they line up parallel to one another and form a ball filled with water. The membrane that forms the outer shell of the liposome is a lipid bilayer, which is a double layer of lipids that is similar to the membrane surrounding a cell. This is one reason why researchers were fascinated by the discovery of liposomes. During the production of the formulation, medically active ingredients that are not soluble in water accumulate preferentially in the lipid bilayer, while water-soluble ingredients accumulate in the interior of the sphere.

The lipid structures with their active ingredient loading are then forced through a membrane with pores of a specific diameter in the range of one hundred nanometers, i.e. one hundred times smaller than the diameter of many of the cells that comprise human tissue. The liposomes form when they pass through the membrane. As a result, the original dispersion of liposomes becomes more homogeneous and the size distribution grows narrower. After the extrusion processes, the liposomes produced are all of similar size—around the same diameter as the pores in the membrane.

“The advantage of this process is that it can easily be scaled to clinical and commercial production,” says Randl. A broad range of batch sizes, from a few milliliters to hundreds of liters, can be produced using the extrusion process at Evonik Vancouver Laboratories. To illustrate, a batch of 100 liters can theoretically provide up to 10,000 ten-milliliter doses, which is the standard dose of the medication Doxil. Thanks to the use of membranes with different pore sizes, the process can produce liposomes with diameters ranging from 50 to 200 nanometers.

RNA strands are copies of individual genes (refer to the information box). Similar to the punched cards that were used with early computers to store data, the sequence of their nucleobases is evident in the interior of the cell. They can determine which amino acids are linked together, and which proteins (which essentially control the cell’s metabolism) are formed.

In cancer cells, the genome has mutated so that the individual genes are permanently issuing instructions to the cell that it is to divide, and thus grow a tumor. Likewise, in hereditary diseases genes with defects are no longer transcribed, resulting in metabolic or other disorders. Because RNA strands carry the instructions for these processes, scientists recognize that RNA interference technologies may provide solutions and enable causal treatment of the disease. If RNA strands with nucleobase sequences that are complementary to those of the cell’s own RNA are introduced, the cellular RNA is inhibited. These RNA strands, which are known as siRNA (small interfering RNA), can suppress the action of the genes transcribed by the RNA.

Liposomes can be used to transport siRNA into the cells. The liposomes provide the siRNA with a kind of protective coating that enables them to be transported without being noticed by the body’s immune system. The liposomes can reach their target cells because the blood vessels in tumor tissue are not properly lined up, as in other tissue, but have gaps. The liposomes can thus diffuse out of the bloodstream into the pathological tissue and make their way further into the cells.

Various pharmaceutical companies have developed siRNA that can destroy carcinogenic liver cells or reduce the uncontrolled cellular division of tumor cells. “Some RNA liposome preparations are already in phase III clinical studies,” says Engel. It is therefore likely that therapies of this type will become part of the range available for treating patients in the coming years. The first liposomal RNA medication was recently approved by the US pharmaceuticals authority. Onpattro^® will in the future be used for the treatment of familial amyloid polyneuropathy, a hereditary disease which leads to symptoms including signs of paralysis and muscle loss in the extremities. With the active ingredient molecule inhibiting the abnormal production of the protein that is responsible for the syndrome, the therapy promises to substantially improve the quality of life for affected patients.

Similarly to the RNA process, liposomes can also be used to transport genes, which are strands of DNA, into a cell. That makes lipid vesicles of interest to those seeking to create new types of gene therapy. Randl therefore expects to see considerable growth in the market for RNA and DNA therapies over the coming decade.

However, the first generation of RNA and DNA therapies has not yet been fully exploited within this market segment. Many research institutes around the world are working on a further improvement that is known as active drug targeting. Here, transport systems, such as liposomes, are augmented by the addition of other molecules which bind to receptors on, for example, tumor cells. The objective is to prevent liposomes from getting lost on the way through the bloodstream, and to deliver them in complete form to the target tissue. In the future, the use of such precise drug targeting in precision medicine will open up new treatment options that are far beyond what is possible today.

Illustrations: Science Photo Library/Alfred Pasieka, Science Photo Library/Carlos Clarivan