I read somewhere that scientists are trying to create nanobots carrying functional mitochondria to search out cancer cells and restart apoptosis. Instead of trying to create nanobots with a fully functioning mitochondria, would it be easier to create nanobots that search and destroy cells with broken mitochondria or excess glucocytosis instead?
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The theory is that nanobots that destroy cancer cells could leave hunks of necrotic tissue in your body.
Ideally, your body would re-absorb this and heal the wound over time, but this is not always viable. Stomach cancer for example can be difficult to treat right now because destroying the cancer risks leaving a hole that would spill bacteria and stomach acid into your abdominal cavity killing the patient from septic shock much faster than the cancer would have on its own.
Currently we work around this when we use radiation and chemotherapy by applying many-many doses over a long period of time to kill some of the cancer with each treatment, then give your body time to heal before the next dose. Destructive nanobots could do the same thing, but it would still take a long time to work which may make it inadequate for treating late-stage or highly aggressive cancer.
Repairative nanobots don't need to be scheduled out. One dose could halt the spread of cancer by repairing all the damaged cells at once because you would not need to wait for healing. In the process, the repaired cells could also be marked for selective removal later with a slower, controlled wave of destructive bots if need be.
It's not so much a question of if destructive nanobots are easier to make. The point of the research is that using bots that can repair the cancerous cells may be able to return the tissue to normal function without causing any unnecessary trauma and open up new options for treatment.
As a side note: Modern medicinal Nanobots are not actually robots at all. They are engineered viruses; so, once they are in your blood, they don't just swim where you want them to go and do their thing, they will spread through your body, with the potential to do things in unexpected places. Another possible advantage of using them to carry a set amount of mitochondria, is that you limit them to the point they expend their payload whereas a destructive one may lie dormant in your system and cause unforeseen issues in the future.
would it be easier to create nanobots that search and destroy cells with broken mitochondria or excess glucocytosis instead?
No, I don't think it would be easier.
Probing a cell for being cancerous can be done from the outside of the cell itself, via the surface proteins. This makes easy to give the nanobot an appropriate receptor, that attaches it only to cancerous cells.
Probing a broken mithocondrion requires entering the cell and test one by one all of them. This is more difficult, because it would require an active motion of the nanobot into a dynamic environment.
Probing a cell for excess glucocytosis is even more difficult: it requires the nanobot, or someone for it, to check the input and output of all the cells and then asses if they are within glucose budget or not. Yes, you could attach the nanobot to a glucose molecule, but that would be not much different from a normal chemotherapy targeting replicating cells: you would have casualties also among healthy cells.
What you are describing behaves more like a phage than the most modern immunotherapies. You are correct, according to what I've read and listened to in lectures, that one approach to treating cancer is by activating the bodies immune response to start recognizing the cancer cells as damaged. As has been inaccurately described in other answers, cancer is not a single mutation.
Cancer's life cycle is a sequence of mutations that cause it to hide from the body's immune system, grow wildly, then sometimes become mobile and move around the body (metastasize), find a new home, and start growing again.
The way it was explained to me was that each change in behavior is the result of a series of mutations. That is why recognizing which cells are mutated in a bad way and are malignant is difficult. Since we have random harmless mutations and dark codings in our DNA. We pick sequences from viruses and bacteria we encounter over our lives. They don't always cause us harm or benefit.
So playing one of these cells is not like the other on a genetic level is hard. If a nanobot killed cells based on just 'it is different' and not 'it is malignant' then we'd be reduced to pudding.
But, phages are natural critters that attack and eat specific bacteria -- each phage species eats a very select set of bacteria. And, ignore the others. They were a big topic of research before the development of antibiotics. The west stopped looking into them since we had the wealth to research new antibiotics. The Soviets didn't have that money so they stuck with phages. Now, in the time of drug resistance bacteria, phages are of renewed interest since they don't hurt people and they like to each thing bad for us. A recent book on the topic details one couple's encounter with an obscure disease. The husbands'life was saved in the last moments with phages.
Anyway, if your nanobots were designed with the discriminating sense of your typical phage, just going after a specific set of properties and chemical signatures, then as a story it would be believable. I think though killing the cells could generate a lot of debris in the blood, in the case of widely spread cancer, and could overwhelm the liver and kidneys to filter it out. I liken it to being struck by lightning and living. You often die later because of necrosis breaking down the cells and destroying your liver (or kidneys, or both.) But, if you are smart and go to a hospital, they put you on dialysis and you live.
Cancer is essentially a malfunction of the body’s cellular regeneration system. An error if you will, in the accuracy/fidelity of replicating the same DNA pattern. This happens sometimes completely at random at sometimes with the help of damage to the DNA from toxic substances, radiation or just degradation from old age. Each cancer therefore has a random and unique genome that in theory a properly programmed nanobot patrolling the bloodstream might be able to differentiate from a digitally stored profile of the patient’s original genome.
Depending on how successful the bots are at identifying cancers in the body, you might then be able to flag the cancers with chemical markers in a way that the immune system can then recognize and attack. That would at least get rid of the tumors but it won’t necessarily solve the fact that your DNA replication is getting errors. And it’s not going to be helpful if the bots misdiagnose healthy tissue which might be unavoidable since the body is continuously regenerating itself and it’s using your DNA as the blueprint for that.