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Drug delivery


you have cancer and you want to use an antibody that recognises the cancer, well, now, instead of building a big plant to make the antibody, you can just make the messenger RNA that codes for the antibody, put that into an LNP and have your liver make it,” he explains. “And in many ways, it’ll make a better version, because the ones that are made in these big plants are quite often a bit immunogenic as a result.” This is what Cullis calls the “third generation” of pharmaceuticals. Thanks to LNPs, these “smart medicines” can both deliver small molecules more specifically to where they’re needed, and enable the therapeutic use of macromolecules like mRNA, siRNA and DNA, which would otherwise be destroyed by the body’s defence mechanisms.


“We desperately need to use larger RNA and DNA-based molecules, because then you can make use of your understanding of how cells actually work to silence, express or edit a gene.”


Pieter Cullis


“The reason we use small molecules so much is because they’re the only molecules that can get into cells to affect what’s going on inside,” he explains. “But we desperately need to use larger RNA and DNA-based molecules, because then you can make use of your understanding of how cells actually work to silence, express or edit a gene.” In the case of the mRNA Covid-19 vaccines, LNPs transfect synthetic RNA ‘instructions’ for producing the SARS-CoV-2 spike protein into cells, which respond by building the protein and displaying it on their surface. Other immune cells then recognise the foreign antigen and mount an adaptive immune response, preparing the body to fend off future infection. Outsourcing so much work to the recipient saves a huge amount of time: within months of the SARS-CoV-2 virus being sequenced, multiple mRNA vaccines were in the clinic. “As soon as you know what protein it is that you want to express, or silence – or whatever, for that matter – then your medicine can effectively be ready in a couple of months,” continues Cullis. “All you have to do is synthesise the messenger RNA. The encapsulation procedure in these nanoparticles is very straightforward. And then you’ve got your potential very targeted therapeutic. It completely short-circuits your average small molecule development time, which is often 15 years and can cost a billion dollars.” Developing the right LNPs, however, has been an even longer and more arduous journey than that.


Walk through walls Langer, a kind of nanotechnology genealogist on top of everything else, traces the LNP family tree back


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to Alec Bangham’s discovery and development of liposomes (originally ‘banghasomes’) for delivering small molecule drugs in the 1960s. A decade later, Langer himself took on the challenge of (and the derision for) using similarly tiny (polymer) particles to safely carry, and slowly release larger proteins and nucleic acids in order to test angiogenesis inhibitors for treating cancer. To many experts, what he was attempting was closer to magic than science. Getting a large molecule through a nanoparticle shell was like “walking through a brick wall”.


“I always tell the joke that I found 200 different ways to get it to not work, which is true,” he says. “And when I first did it, people were very sceptical. I got my first nine grants rejected and no chemical engineering department in the country would hire me.” Slowly, though, the world came to appreciate what Langer, incessantly tinkering with the tortuosity and porosity of his polymers, had made possible. The next step on the similarly tortuous LNP pathway was to find a way to smuggle these large molecule delivery systems past the immune system they were designed to support, which Langer and others managed to do in the 1990s, by adding polyethylene glycol (PEG) to some of the lipids in nanoparticles. Pegylated lipids are one of the four ingredients in the LNPs used in today’s mRNA vaccines, keeping particles regular through the formulation process and preventing them from aggregating in storage, as well as disguising them from the immune system.


Cullis, who began encasing cancer drugs in liposomes in the 1980s and has often been dubbed the ‘godfather’ of LNPs, enters Langer’s tale in the following decade, when one of his companies, Inex Pharmaceuticals, made a breakthrough on the next ingredient. One of the reasons nucleic acids, which carry a negative electric charge, need to be incorporated into delivery vehicles is that they can’t cross our cells’ similarly negatively charged lipid membranes. You may have spotted a problem there. There are no negatively charged lipids in nature, so synthetic, positively charged ones needed to be engineered before LNPs could even pick up their payloads. This was first done in the 1980s, but these ‘cationic’ lipids didn’t so much cross cell membranes as destroy them. So, through the late 1990s and early 2000s, Inex and its sister company Protiva Therapeutics developed ‘ionisable’ lipids that were positively charged at a low pH, but were neutral in the blood.


Nothing comes easy on the nanoscale, so, of course, the first versions of these ‘ionisable cationic lipids’ were still toxic, but just as Langer had done in the 1970s, Inex and others strenuously tweaked, modified and optimised them, casting hundreds aside in the process. Their work eventually resulted in the LNP formulation used in Alnylam’s Onpattro, which became


World Pharmaceutical Frontiers / www.worldpharmaceuticals.net


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