The body's cleaning crew
Part 1 described nanoparticles carrying drugs to tumours. There is a problem that comes before targeting even starts: the body is extremely good at removing foreign particles from the blood.
A system of immune cells, concentrated in the liver and spleen and often called the reticuloendothelial system or mononuclear phagocyte system, constantly swallows particles that look foreign. Most injected nanoparticles end up there, not in the tumour.
Chapter 7 of Peptide Nano-Chemistry and Nanotechnology describes several ways researchers try to get past this cleaning crew. They are some of the most inventive ideas in the book.
Strategy 1: an invisibility coat of PEG
The oldest approach is to coat particles with PEG (polyethylene glycol), a water-loving polymer. PEG attracts a shell of water around the particle, making it harder for blood proteins to stick and flag it for removal. The chapter notes PEG coatings improve circulation time, accumulation and cellular uptake.
PEG coating is used in approved medicines, including a long-established liposomal form of the chemotherapy drug doxorubicin. The chapter also notes PLGA and PEG are polymers already approved for medical use by the US Food and Drug Administration, which makes them practical starting points.
The downside of PEG, not covered in the chapter, is that some people develop antibodies against it, which can speed clearance on repeat doses. Nothing is perfectly invisible.
Strategy 2: dress up as a red blood cell
This is the idea I find most striking. Your own red blood cells circulate for about 120 days without being attacked, because their surfaces carry "don't eat me" signals that the immune system recognises as self.
So why not borrow them?
The chapter describes researchers who took the membranes from red blood cells, stripped out the contents, and wrapped them around drug nanocrystals. The result is a particle that looks, to the immune system, like a small red blood cell.
To add targeting, they inserted the tumour-targeting peptide cRGD into the borrowed membrane, using a very strong natural binding pair called avidin and biotin as the attachment method. The chapter reports the resulting particles showed good tumour accumulation and therapeutic effect in models of glioma (a brain tumour) and in tumours grown under the skin.
The chapter explains the general appeal of cell membrane coatings: the natural sugars and proteins on the membrane surface help particles avoid the reticuloendothelial system, stay in circulation longer, and reduce immune recognition.
Strategy 3: use the body's own messengers
Cells naturally release tiny membrane-bound packages called extracellular vesicles, between about 50 and 1000 nanometres in diameter, that carry proteins and genetic material between cells. They are the body's own delivery parcels.
The chapter describes researchers loading extracellular vesicles with the drug methotrexate and decorating their surfaces with peptides designed to help them reach brain tissue, for the treatment of glioma.
Strategy 4: build the carrier from a body protein
Another approach is to make the carrier from a protein the body already tolerates. The chapter describes nanoparticles made from human serum albumin, the most abundant protein in blood, carrying 5-fluorouracil and coated with a targeting peptide. It also describes particles made from genetically engineered elastin-like peptides carrying paclitaxel and decorated with iRGD.
Albumin is not a hypothetical choice. An albumin-bound form of paclitaxel is an approved cancer medicine. Using the body's own materials is one of the more promising routes to acceptance by both the immune system and regulators.
Strategy 5: get across the brain's wall
Brain tumours face an extra barrier: the blood-brain barrier, a tightly sealed lining of blood vessels that keeps most drugs out of the brain. Many of the glioma examples in the book use the peptide Angiopep, which binds a receptor that transports molecules across this barrier.
The chapter describes hollow porous nanospheres (covalent organic frameworks) designed for cascade targeting across the blood-brain barrier to glioma, reporting a significantly longer median survival in the animal model.
Hydrogels: stay put instead of travelling
Chapter 9 offers a completely different answer to the clearance problem: do not send the drug through the blood at all. Place a drug-loaded peptide hydrogel directly at the site, for example into a tumour cavity after surgery, and let it release the drug locally. The chapter describes peptide gels loaded with doxorubicin, curcumin, 5-fluorouracil and paclitaxel performing better than the free drugs in the studies reviewed. A table in the book's Chapter 8 also lists a peptide material designed for sustained drug release to prevent local recurrence after tumour surgery.
The honest picture
These strategies are clever and several have real-world precedents: PEGylated liposomes and albumin-bound paclitaxel are approved medicines. Cell membrane coating and extracellular vesicles are active research areas with serious investment.
But the examples in this chapter are from animal models. Mouse immune systems differ from human ones. Manufacturing membrane-coated particles consistently at scale, from cells that must be sourced safely, is a major challenge. And every added layer of complexity is another thing regulators must evaluate.
Why this matters to you
This part of the book shows a genuinely important idea: the best way to get past the body's defences is often to look like part of the body. Borrowing red cell membranes, using albumin, riding in the body's own vesicles. It is biomimicry at the nanoscale.
It also shows why the question "how long does it stay in the blood?" matters so much for any injected product. A molecule that is cleared in minutes cannot do much, however well it is designed. That is the same reason, in a different context, that the GLP-1 drugs discussed elsewhere on this blog carry fatty chains that bind albumin.
This series explains biomedical research for general readers. Most approaches described are at the laboratory or animal study stage and are not available treatments. Nothing here is medical advice. Anyone affected by cancer should rely on their oncology team.
