Fighting cancer the small way
A combination of nanotechnology, biology and mathematics may represent the greatest challenge to drug treatment resistance, one of the biggest underlying causes of mortality in cancer. Khai Trung Le talks to Professors Mohammad Kohandel and Aaron Goldman about their collaborative approach.
‘Resistance to therapy is being recognised now more than ever as the biggest underlying cause of mortality in cancer,’ begins Professor Aaron Goldman, cancer biologist with the Harvard Medical School, USA. Previous research into cancer biology revealed developing resistance to treatment, with cells changing their behaviour in what is now recognised as preferred trajectories from the pressure of therapy.
‘It’s particularly true in aggressive malignancies like triple-negative breast cancer. There isn’t a very good cohort of therapeutic regimens for this, particularly when a patient has relapsed. They become extremely refractoried to most therapeutics available,’ Goldman added.
Pinpointing the cause of resistance to move towards a solution calls for a collaborative approach, and a joint University of Waterloo, Canada, and Harvard research team has combined nanotechnology, biology and mathematics to devise a combination drug nanoparticle capable of overcoming therapy resistance by containing and simultaneously releasing both the resistance inhibitor and cancer treatment at the same time.
Professor Mohammad Kohandel, Associate Professor of Applied Mathematics at the University of Waterloo and Goldman, who have previously collaborated on multiple projects, sought to evaluate resistance mechanisms in the context of computation biology. Goldman said, ‘There is huge complexity of cell phenotypes within a tumour, and in the dynamics of cancer cell behaviour under pressure. Because of the heterogeneity of the tumour, the only way we could dissect that was with a computational approach.’
Taxanes – anti-tubular compounds and one of the first drugs used in chemotherapy – were determined to be a significant factor in the cells developing resistance, as they ‘apply themselves and drive the resistance – but we can inhibit that resistance programme using an AKT inhibitor,’ said Goldman. ‘Because we identified the two important drugs, and the knowledge that the resistance is a deterministic process rather than stochastic, meant we recognised that the two drugs need to be together in the same cell at the same time.’
Kohandel added, ‘That’s impossible using free drug administration, so they had to double-up by constructing the two-in-one nanoparticle. Without
the nanotechnology approach, the construction of such an effective therapeutic strategy would have been impossible.’
Goldman continued, ‘Through our previous work, we can take taxanes, conjugate them with a cholesterol and use those cholesterol/taxane conjugates to self-assemble into a single nanoparticle through supramolecular chemistry. They fit together seamlessly, like Tetris or puzzle pieces, because of the hydrophobic interactions to assemble a single, bi-layered nanoparticle.’
The nanoparticles are capable of self-assembly, will endocytose into cells and, via enzymatic indigestion through the leaky vasculature of tumours, release the drugs into active forms and operate simultaneously. ‘That was when we found better efficacy in terms of preventing resistance, therefore enhancing the anti-tumour outcome,’ said Goldman.
Path of most resistance
It was previously believed that only privileged cancer cells – in the sense that they have certain inherent properties that allow them to give rise to new tumours and resist therapy – were able to overcome therapy, and did so in an unpredictable manner. ‘The hypothesis has been developed over several decades,’ Goldman stated. ‘It has largely been believed that a cancer stem cell is a very small population of cells within a tumour that repopulates it and enables growth and proliferation, as well as constitute the inherent population of cells.’
He continued, ‘The very first thing we did was put together a model that incorporated our evidence from experimental biology, the changes in cell phenotype that we had observed under pressure of chemotherapy to fit together a system of biological effects, and used the maths to validate which direction the cells were going in.’ This helped ascertain whether cell behaviour was stochastic or underwent a defined process to acquire a new phenotype that resembles an inherently resistant cell, promoting resistance mechanisms.
From this, Kohandel was able to determine the significance of the ‘commonly overactive kinase PI3K/AKT, a pathway which is associated with each step of deterministic programming. Drug resistance is deterministic under therapy pressure – the resistance programme is pre-determined to arrive under pressure of chemotherapy – and PI3K/AKT drives the synergetic features of resistance.’
Three important features of the PI3K/AKT kinase – cell size, DNA content and CD44 expression – were factored into the modelling and helped identify that all of the resistance came from the inherent population of cancer cells, gained through a specific pathway to become hybrid cancer cells.
The smallest benefits
The team claims that using nanoparticles confers a number of advantages over free drug administration, including formulation, managing the rate of drug metabolism to increase efficiency, and abusing the enhanced permeability and retention effect of tumour vasculatures.
‘Taxanes are classically very difficult to formulate to provide to a patient, because they are extremely hydrophobic. You have to use very strong chemicals in order to get them into a patient, and it can lead to a lot of toxicities that have nothing to do with the drug itself,’ said Goldman. ‘Using nanoparticles in the context of the taxanes overcomes some of those toxicities.'
Goldman also specified the advantages regarding distribution of the AKT inhibitor, which is metabolised almost immediately after entering the blood stream, experiencing forced glucuronidation – a biotransformation reaction in which glucuronide acts as a conjugation molecule and binds to a substrate, because of chemical moieties within the blood stream. ‘By formulating into nanoparticles, we’re able to protect the sites of metabolism on the drug that enable it to be more efficacious and bio-available.’
Goldman believes the ability to formulate nanoparticles in different ways will allow them to overcome challenges in the client. However, there is one particular advantage free drugs hold over nanoparticles – ‘Getting free drugs past regulations with the US Food and Drug Administration and European Medicines Agency. There is a higher bar to challenge because the nanoparticle contains several extra layers of formulation to get it up to par. But, classically, a nanoparticle is more efficacious than having them in free drug form.’
Stock the medicine chest together
Goldman was keen to see further interest in mathematics within biology and oncology, inadvertently recalling the driving premise behind Isaac Asimov’s Foundation trilogy. ‘Mathematics can help describe ecological behaviour, between populations of animals in the wild. You can build what are essentially game theories and understand how interactions between animals influence the ecology of that small niche in a region of the world.
‘It’s analogous in cancer. You have all of these different players and they’re basically trying to survive, pushing out or using other tumour cells to their advantage. We’re exploring that right now, how can we understand this, and the best way is taking some evidence from biology, some from experiments, and applying them to different computational models and simulations.’
Goldman believes that a stronger drive towards interdisciplinary collaboration will help support future cancer research, noting, ‘You’re going to be noticing the need for more seamless integration of scientific disciplines. We wouldn’t be asking the right questions if we didn’t have someone who was well versed in mathematics sitting with an oncologist, because we simply weren’t appreciating the interdisciplinary nature that’s required to study cancer.’
The combination of mathematics and biology continues to entice some, although Kohandel stated that the integration of nanotechnology remains an unexplored area. ‘Many people have tried to explore combination therapy, and some have tried nanotechnology, but to my knowledge no one else has the same adaptive structure we are working on.’
He continued, ‘Our Waterloo group is perhaps the only one worldwide to have established a mathematical medicine wet lab within a mathematics department. Combination is a unique aspect of our work, as is looking at eliminating cells that enable drug resistance through nanotechnology.
‘I think we are on the brink of reaping the rewards of the rapidly developing synergy between mathematics and the biomedical sciences.’
Taxane - A type of drug commonly used in cancer treatment. Taxanes restrict cell growth by preventing cell division/mitosis.
AKT inhibitor – Used to disrupt the key node of the PI3K/AKT signalling network, AKT inhibitors prevent the phosphorylation – turning protein enzymes on or off – of AKT substrates that control cellular processes including mitosis.
Endocytosis – The process by which materials move into cells. By entering the tumour cells, the Waterloo/Harvard nanoparticle will be able to deliver both treatment and AKT inhibitor simultaneously.
Moiety – In organic chemistry, moiety is a term used to describe part of a molecule.
Around 30% of breast cancer patients suffer recurrent or relapse disease, with the emergence of refractory tumours associated with multiple resistance mechanisms and factors. Cancer cells have the ability to develop resistance to multiple therapies, and these are often based on tumour-drug interactions and what is increasingly observed as late toxic effects in treatment regimen stages. These include drug inactivity, drug target alteration, DNA damage repair and cell death inhibition, as well as inherent tumour cell heterogeneity.