Date: 12th November 2020
Glioblastoma (GBM) is the most aggressive form of brain cancer, and due to complications of therapeutics having to pass the ‘impenetrable’ blood-brain-barrier, limited clinical progress has been made over recent years. Now, scientists have been inspired by nature and engineered a biomimetic, synthetic protein nanoparticle (SPNP) equipped with a cell-penetrating, tumour-targeting peptide, which delivered an inhibitor of STAT3 to tumour sites, resulting in tumour regression, long-term survival and immunological memory against further GBM.
GBM currently accounts for just under 50% of diagnosed brain cancer, and has a poor prognosis with less than 5% of patients surviving 5 years following diagnosis. There is currently an urgent need for new strategies to target therapeutics to the tumour site which involves crossing the brain-brain-barrier (BBB), a barrier which only a few natural proteins and viral particulates can transport through.
Scientists led by Joerg Lahann and Maria Castro from the University of Michigan, US, have developed a therapy around human serum albumin (HSA), which is present in blood and is one of the few molecules that can cross the BBB. They have engineered a GBM-targeting synthetic protein nanoparticle (SPNP) comprised of polymerised HSA and attached a STAT3 inhibitor, which turns an immunologically ‘cold’ tumour into a ‘hot’ one. The SPNP could transport its cargo across the BBB, home to the solid-tumour, and when combined with the standard of care, ionised radiation, resulted in tumour regression and long-term survival in nearly 90% of GBM-bearing mice.
Previous work had shown that co-delivery of the cell-penetrating peptide iRGD could increase tumour targeting of nanoparticles. Therefore, the team used iRGD as a targeting ligand for their SPNP, the cargo was a siRNA against Signal Transducer and Activation of Transcription 3 factor (STAT3i) which would downregulate STAT3. STAT3 is central in several signalling pathways and is associated with GBM progression, promoting cell proliferation and metastasis, inhibiting apoptosis and allows tumours to evade the immune system.
In vitro analysis of the STAT3i-SPNP showed uptake by the cells was increased 5-fold by the presence of iRGD, that the siRNA was active as demonstrated by STAT3 downregulation. The SPNPs showed gradual release of the drug, with 96% of encapsulated siRNA released over a 21 day period.
To really test the SPNPs the team wanted a more clinically relevant model, they chose an intracranial aggressive GBM mouse model. In general, intravenous injection are one of the least invasive way of administering drugs however, up until now this method hindered drug delivery to the brain as they have to cross the BBB.
To evaluate if systemic delivery of SPNPs could home to brain tumours, the team administered multiple doses of STAT3i-SPNPs into the tail vein of mice, starting seven days after glioma cell implantation. They found that whilst the control mice had a median survival time of 28 days, this was extended to 41 days with the treatment of the loaded SPNPs.
The team were encourage by these results however, they wanted to optimise yet further. So they combined the novel nanomedicine with the current standard of care – focused radiotherapy (IR). They used the multi-dose SPNP regime with a repetitive, two-week focused radiotherapy regime and found that now seven out of the eight mice reached long-term survival (100 days) and appeared completely tumour-free.
There were no signs of malignant, invasive tumour cells and there were also no signs of inflammation. Furthermore, they concluded that the combo therapy did not cause any systemic toxiticity. Importantly, they also recorded a >50% reduction in total STAT3 protein present in the brain of GBM bearing mice treated with STAT3i-SPNP.
From the previous experiment there was evidence that STAT3i-SPNP could selectively decrease the immune suppressive M2 macrophage subpopulation, and that there was an activation of dendritic cells – which are responsible for the initiation of adaptive immune responses. As glioblastoma is so aggressive and infiltrates readily, recurrence is very high in these patients. With this in mind, the group wanted to explore whether the nanomedicine offered any immunity to GBM.
So they performed a tumour rechallenge study, implanting tumours into mice previously cured by the combo therapy. Amazingly, despite not receiving any additional treatment, all rechallenged mice survived to a second long-term survival point of 90 days. They showed no adverse effects and tumour cells were absent from the brain, suggesting the adaptive immune system could protect against secondary tumours.
Conclusions and future applications
This work here demonstrated the ability to deliver therapeutic drugs systemically, or intravenously, that can cross the BBB to target tumours in the brain. By combining the new nanomedicine with traditional care, the team have shown GBM tumour regression and long-term survival in nearly 90% of treated mice. Furthermore, the SPNPs prime the immune system to develop anti-GBM immunological memory, in this case in all of the mice.
The efficacy of the treatment was totally unexpected due to the aggressive nature of GBM, and the team were expecting some level of tumour growth in the rechallenged mice. Such positive findings will likely accelerate the therapy toward clinical implementation, and the team are hoping it will offer new hope of a novel clinical therapy for treating GBM.
The SPNP system also offers flexibility, and could be used to deliver other small-molecule drugs and therapies to other ‘undruggable’ solid-based tumours. The team will be developing and testing further therapeutic applications.
Whilst this may be the one of the first systemic administration of a therapeutic drug to cross the BBB and treat tumours in the brain, there have been other recent advances in transportation across this barrier. We have seen formulated nanoparticles that delivered drugs across the BBB to treat HIV-infected macrophages. An enzyme delivery system, which encapsulated cargo in a nanoparticle, and targeted the brain was recently used in mice to support the treatment of Krabbe Disease. Scientists have also engineered ultrasound-controllable drug carriers that are dependent on ultrasound for both targeting and uncaging allowing focal delivery of drugs, which can cross the BBB.
With respect to GBM we reported just last month that scientists have engineered antibody-cytokine protein fusions, called immunocytokines, which have striking single-agent, anti-cancer activity for glioblastoma in mouse models and are currently in a pilot trial in human patients. This therapy also saw a reduction in tumour size from systemic administration, and cured several of the mice, albeit in lower levels than seen here. Furthermore, they also observed a robust adaptive immune response, which resulted in all mice surviving and showing no signs of tumour after a rechallenge. It might interesting to reinvestigate this work with respect to including the current standard of care, IR, which here increased the efficacy of SPNPs.
Although, this study is still in its infancy, and its safety and efficacy has yet to be tested thoroughly with respect to human use, it together with some of the other new technologies that we’ve mentioned are enabling us to cross this once impenetrable BBB and target such aggressive and devastating diseases that were once deemed untreatable.
For more information please see the press release from the University of Michigan
Gregory, J. V., P. Kadiyala, R. Doherty, M. Cadena, S. Habeel, E. Ruoslahti, P. R. Lowenstein, M. G. Castro and J. Lahann (2020). “Systemic brain tumor delivery of synthetic protein nanoparticles for glioblastoma therapy.” Nature Communications 11(1): 5687.