As the basis for safe use in over 1,000 worldwide clinical trials targeting all broad disease indications, [1] mesenchymal stem/stromal cells (MSCs) have nevertheless been likened to a double-edged sword. [2, 3, 4] Despite their undisputed talents for the resolution of infection and tissue regeneration, [5, 6] these cells exhibit pleiotropic activities toward various cancers during various stages of tumor progression. This blog article will briefly explain how clinical investigators are sharpening MSCs to be powerful “unidirectional” blades in the armamentarium against cancer, a scourge responsible for one-sixth of all deaths.

The potential for MSCs to heal severe side effects of cancer treatments is well known. This initiative accounts for approximately 80% of publicly posted cancer-related MSC (or MSC-exosome) clinical trials we’ve identified. Hematopoietic (stem) cell transplantation (HCT/HSCT) is necessary to reconstitute the patient immune system after hematolymphoid depletion following irradiation and/or chemotherapy to treat blood cancers and other conditions. However, the associated risk of graft-versus-host disease (GvHD) is high (~50%), and fatal for a significant percentage of sufferers (~%7). MSCs are now approved to treat GvHD outside of the USA, and on the threshold of FDA approval after compelling trial data. [7] In addition to GvHD, MSCs are considered in known trials for radiation — or chemotherapy-induced oral mucosal lesions, cachexia, cardiomyopathy, xerostomia, renal failure, loss of male genitourinary function, and pelvic radiation disease.

Although MSCs are best known as faithful cellular paramedics at sites of wounding, as unwitting henchmen, they’re also caught at the scene of the crime of malignancy. Cancer bears an uncanny similarity to a lesion that experiences aberrant and perpetual “healing,” a danger signal to which MSCs cling and attempt to remedy. [8, 9] Tumors are also greedy for oxygen [10] and access to growth factors, so their MSC collaborators signal for recruitment or possibly trans-differentiation [11] of endothelial cells for angiogenesis and new vascular nets. Cancers acquire a parasitic agenda to expand without immune surveillance, where local MSCs and cancer stroma [12] secrete immunosuppressive factors to cloak the tolerogenic microenvironment. When this cellular niche of a growing carcinoma becomes too large even for sufficient oxygen penetration, MSCs contribute to invasiveness by facilitating epithelial-to-mesenchymal transition (EMT), [13] and then, recolonization in a bone metastasis, a “welcoming” paracrine milieu that is abundant with nominal MSC secretome activities.

And yet(!!), the very same abilities that MSCs use for wound healing and tumors could be readily exploited by cell therapy developers with a few modest tweaks. Even native MSCs are observed to suppress AKT activation in some contexts, an anti-apoptotic effect that would otherwise confer chemotherapy resistance. [14]  Still, other unique MSC preparations have been observed to autonomously secrete TRAIL, [15] which is selectively toxic to tumor cells. In other contexts, MSCs are known to engage TGF-b signaling amongst their neighbors, [12] which suppresses epithelial (and carcinoma) cell growth. Most importantly, the well-known tropism of MSCs to biodistribute and/or activate in sites of injury (or tumors) is what might make them best suited as targetable and controlled drug delivery systems. [16] “Set a thief to catch a thief” might not be the best fitting idiom here, but it does reflect the subtlety of medicine’s recently won knowledge of the tumor microenvironment. That is, while MSCs are quite at home next to cancer, they’re a cell type with just enough tactical finesse to bypass a tumor’s security grid.

MSCs’ other major advantage is that they’re readily engineerable and comparatively easy to expand across 10-20+ population doublings. [17, 18, 19, 20] The scalable process development technology dose large patient cohorts for late-stage MSC trials and large indication populations has already been quietly developed and optimized across the last decade. [21] This is not merely true for live MSCs as the primary material for injected doses, but also their secretome products such as exosomes (a.k.a. extracellular vesicles or EVs). [22, 23, 24, 25, 26] The technology and bioprocess to genetically modify MSCs transiently or stably, virally or non-virally, has been largely worked out with a variety of GMP-ready tools. [27, 28] Future feats of this technology are projected to expand towards larger and more complex multi-gene programs—and multi-program systems—for synergistic effectors and finer-tuned controls. [29, 30]

MSCs are thus customizable with a combination of chemical, physical, and genetic tools to swing directly at cancers. In humans, trials have already tested (or will soon administer) various concepts of MSCs as carriers for oncolytic viruses (e.g., NCT03896568, NCT02068794, NCT02079324, NCT01844661). Similarly, trials that use genetically modified MSCs to express immune stimulatory or cytotoxic gene products (e.g., NCT03298763, NCT02530047) or exosomes carrying k-Ras oncogene-specific siRNA (e.g., NCT03608631) may be underway.

The myriad options of oncology uses for MSCs and their secreted exosome/EVs have barely been scratched, however, and yet most have a strong foundation in translation-oriented published literature. In the diagram and “tactical gear” catalog (below), we mention “only” ten unique mechanism concepts that could be incorporated into MSC-EVs, alone. With combination therapies in mind to achieve maximum efficacy, there are 720 3-part permutations of these ten broad ideas that could be uniquely mixed and matched…to say nothing of any variants within each. In short, the MSC cellular platform as a bioproduction host or as the unit of cell therapy, provides innovative investigators with an entire armamentarium full of shiny blades to contend with the cancer foe.

At RoosterBio, we hope this will inspire you (or your friends in advanced therapy development) to consider that MSCs and their EVs/exosomes aren’t just for healing. With a little extra preparation (see schematic, below), a few of these lead candidates just might prove to be outstanding “cancer commandos,” ready to kick some cancer… assays.

Sharpening the Therapeutic Blade of an MSC Sword

A Programmable Anti-Oncology Pipeline

 

Multiple, combinatorial shots on goal to target cancer with an MSC cellular platform (above). Customizable cellular engineering “inputs” can modify MSCs and their secretomes via small molecules, transduced viral vectors, transfected small RNAs and/or miRNAs, mRNA or DNA, or upstream priming or physical treatments. These inputs converge on the modality of MSCs as a delivery system for programmable medicines, which can be used as a bioproduction cell platform to generate exosomes (i.e., extracellular vesicles; EVs) or cell ghosts. The modality may also involve whole MSCs classically injected as live cells—or used as adjunctive cells in concert with another cell type. Representative mechanisms thereby enabled for multipartite biologic therapy concepts are listed below (1-14), along with example related references:

Engineered Exosomes:

	Multi-valent targeted exosomes with selectivity for tumors: cell-specific internalization or retention in tumor microenvironment. [31, 32]
	Ligand-displaying exosomes to elicit anti-tumor signal transduction in microenvironment with superior potency and more favorable window of efficacy. [33]
	Exosomes displaying multi-valent molecular decoys to block immune checkpoint signaling and/or pro-angiogenic signals. [34]
	Exosomes displaying tumor antigen vaccines externally (for antibody response) and/or internally (processing for T cell response). [35, 36]
	Exosomes containing native or engineered transcriptional activators or repressors (including dCas9 fusions) as internalized cargo for selective tumor cell killing. [37, 38, 39]
	Exosomes cargoed with small RNAs (e.g., miRNAs or anti-miRs) to interfere with gene expression signatures related to radio-resistance, immunosuppression, other Hallmarks of Cancer. [40, 41]
	Exosomes cargoed with mRNAs to express potent inducers of apoptosis, immune surveillance, or other molecular antagonists of the Hallmarks of Cancer. [42]
	Exosomes cargoed with small reporter molecules that facilitate contrast enhancement of concentrated tumor regions for radiation therapy, or real-time monitoring of an oncology regimen. [43, 44]
	Exosomes cargoed with chemotherapy drugs to achieve superior potency and/or reduced side effects. [45]
	Exosomes encapsulating viral gene therapy vectors to achieve “cloaking” effect to minimize anti-capsid neutralizing antibodies (nAbs) and allow repeat dosing. [46]

Cell Ghosts:

MSCs physically processed into enucleated “cell ghosts” that retain beneficial features of both exosomes/EVs and live cells, including larger particle volume to load more (or larger-sized) engineered cargo molecules (here, #1-10). [47, 48, 49]

Engineered Live Cells:

	Live injected MSCs for retention in tumor microenvironment with programmed secretome factors released as a localized, in vivo cellular “bioreactor,” to affect the multi-cellular milieu and block metastasis. [50, 51]
	MSCs injected in tumor microenvironment carrying oncolytic virus as cytotoxic “trojan horse,” enhanced by transgene co-expressed pro-drug convertase. [52, 53]
	MSCs reprogrammed and polarized into pro-immunogenic antigen presenting cells (APCs), pulsed with tumor antigens for cross presentation to T cells for vaccine effect. [54]

 

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