A conversation with Dr. Raphael Guzman, Professor of Neurosurgery and Neurosciences at University Hospital Basel; Dr. Miroslaw Janowski, Associate Professor of Radiology at the Johns Hopkins University School of Medicine; and Dr. Piotr Walczak, Associate Professor of Radiology at Johns Hopkins University.

Dr. Raphael Guzman, Dr. Miroslaw Janowski, Dr. Piotr Walczak

From left, Dr. Raphael Guzman, Dr. Miroslaw Janowski, and Dr. Piotr Walczak.

Interviewed by Dr. Kaustubh Limaye (@kaustubhslimaye), Assistant Professor of Neurology in the division of Cerebrovascular Diseases at the University of Iowa.

They will be discussing the paper “Intra-Arterial Delivery of Cell Therapies for Stroke,” published in the May issue of Stroke. The article is part of a Focused Update in Cerebrovascular Disease centered on stem cells and cell-based therapies.

Dr. Limaye: I read your review in Stroke that deals with intra-arterial cell therapy for stroke recovery with great interest and enthusiasm. Can you summarize the important points of your article for our readers?

Authors: The concept of using stem cells as a strategy for treatment of stroke is a few decades old. There were hundreds of animal studies, dozens of clinical trials, and we are still far from effective therapy. Perhaps it is time to take a step back and think whether something has to be changed with the approach to developing cell therapy for stroke. One problem is that we scientists and clinicians have been overly optimistic or even ignorant. The typical approach for research on stem cell-based therapies was: Let’s inject some cells into the brain lesion and hope for the best. The challenge was immense as the stem cells of choice after transplantation are required to find their way migrating to the target, survive, proliferate, appropriately differentiate and exert therapeutic effect. We see first-hand that such a simplistic approach was not constructive. While open label preclinical studies were optimistic, practically all rigorous clinical trials failed to demonstrate satisfactory therapeutic effects. This status quo has hurt the field of stem cell therapy for stroke as scientists, grant reviewers and funding agencies gradually lose enthusiasm and abandon the concept of brain regeneration after stroke with stem cells, stifling any further progress. It is urgent that we work towards reversing that trend, and our strategy is to shift from a “do all” approach to addressing very basic challenges in a systematic manner. After identifying promising and highly potent sources of stem cells, which is now largely accomplished, the next step is to develop methods for effective and safe delivery of stem cells to the site of brain injury.

Cell delivery to the site of stroke is at the center of this review article. Our groups have worked on this for nearly two decades, and after exploring all possible gateways to the brain, including intraparenchymal, intraventricular, intravenous and intra-arterial, we identified the last one as the most promising for stroke. The advantages of intra-arterial cell delivery are detailed in the article, but the most important point that we would like to emphasize here is the need for identifying a balance between the desired high cell engraftment and maintaining sufficient patency of microcirculation. The most effective solution would be the fast diapedesis so cells are quickly cleared from cerebral microcirculation keeping the microvessels patent, and then get to the brain parenchyma.

There are interesting safety patterns that we describe in this review, and, for example, injection of mononuclear cells was never associated with any side effects or complications regardless of cell dose. This excellent safety profile was a main driver for structuring clinical cell therapy protocols, and interestingly all clinical intra-arterial cell therapy trials were done with the use of mononuclear cells. Cellular imaging studies indicate that these small size cells perfuse uninterrupted through cerebral capillaries and are captured in peripheral organs. Rationale for intra-arterial injection in this case is rather questionable. On the other hand, large size mesenchymal stem cells were linked to microembolic complications especially when larger quantities of cells are injected. Ironically, it is more justified to consider these complication-prone cell types for intra-arterial injection because these have a chance for endothelial capture at the first pass so taking the clear advantage of intra-arterial route. Obviously, safe dosing of cells is critical.

Since capture of cells by cerebral endothelium at the first pass without entering circulation is the most important prerequisite for successful intra-arterial injection, we worked extensively to develop tools for enhancing adherent properties of stem cells by either genetic cell engineering inducing expression of adhesion molecules or sorting mixed population of stem cells to enrich for cells highly expressing these molecules. With these strategies, it is possible to improve cerebral endothelial capture of small size stem cells as well, which otherwise would not be mechanically trapped.

Appreciating the importance of balancing the engraftment efficacy, assuring precision of cell delivery and preventing excessive engraftment, we were one of the first advocates for using noninvasive imaging for monitoring intra-arterial cell delivery. We learned that due to complexity of cerebral vascularization, there is high patient to patient variability of cell engraftment, and without interventional MRI reproducible and precise delivery of stem cells to the brain is not possible.

Dr. Limaye: I agree with you; intra-arterial delivery does have some strong reasons favoring use, including being much less invasive than intraparenchymal implantation, being easily repeatable, and good distribution in the affected brain regions. What are the important limitations for IA cell therapy delivery as compared to other modalities of cell delivery?

Authors: Since we are using the vascular tree for cell delivery, blood vessels need to be perfused. Luckily, in most of the stroke cases we observe either spontaneous reperfusion or reperfusion is achieved via intravenous thrombolysis or intra-arterial mechanical thrombectomy; however, in rare occasions when the stroke area is not perfused, intra-arterial delivery is less justified. Nevertheless, a patent collateral circulation might be sufficient for cell delivery in the stroke penumbra. Another important issue, which was mentioned above, is a risk of microembolism. Excessive engraftment is clearly a significant risk; thus, cell dosing should be adjusted for each individual cell type preferable with a feedback of imaging during interventional procedures.

Dr. Limaye: Has device development for endovascular stroke therapy helped in this?

Authors: Endovascular mechanical thrombectomy has revolutionized treatment of acute stroke, and due to excellent efficacy, it triggered unprecedented worldwide interest and investment in expanding interventional suites. With these, there is a dramatic increase in the number of patients that can benefit from this procedure. This situation has created excellent opportunities for intra-arterial stem cell delivery. A large number of patients after reperfusion who often have some degree of brain injury may benefit from cell therapy delivered very early after the insult. Also, the very fact that stroke patients undergo catheter placement for the thrombectomy procedure introduces a unique opportunity for introducing adjuvant stem cell treatment almost immediately after reperfusion is established. The increasing access to the interventional suite may allow for widespread use of intra-arterial cell therapies also in patients, which does not qualify for thrombectomy, because of lack of thrombus.

Dr. Limaye: In your opinion, what are the ideal cell type and size to be used for IA therapy post stroke and why?

Authors: This is a very difficult question. There are many cell types that were shown efficacious in the context of stroke. Repair mechanisms are also diverse; neural stem cells, for example, might work through replacing damaged neurons or glia but also yield trophic support to endogenous repair mechanisms. Mesenchymal stem cells are thought to provide trophic and immunomodulatory effects. Patients could likely benefit from many types of stem cells, but regardless of the cell type used, it is critical that we understand how to deliver the cells to the site of action at the right time but fast whenever they are needed, safely and efficiently.

Dr. Limaye: What imaging modalities are optimal to monitor and guide IA cell therapy and in assessing post procedure efficacy?

Authors: Currently, MRI of cells labeled with iron oxide nanoparticles is by far the most frequently used for cell tracking including intra-arterial cell delivery. The advantage of this modality is that it is clinically applicable; contrast provided by iron oxide is very strong, allowing detection of single or small numbers of cells, but the disadvantage is ambiguous signal during long-term tracking. Due to that ambiguity utility of MRI cell tracking is limited to the interventional procedure and monitoring cell biodistribution. For long-term tracking, the most reliable methods are based on the reporter gene. An example of an excellent reporter system is firefly luciferase for bioluminescence imaging. This technique works well in mice, but because of light absorption and scattering, there are no perspectives for translating it into large animals or humans. There is significant effort with developing reporter genes for PET or MRI, but currently reporters suitable for tracking transplanted stem cells are not available.

Dr. Limaye: Learning from previous clinical trials in cell therapy, and specifically IA delivery, what would be a few things that you would suggest changing for future trials?

Authors: There are not so many clinical trials with the intra-arterial delivery of stem cells, and the majority of them are small, phase I/II. They have shown to be safe, but in the only randomized clinical trial, they were not shown effective, and no information about the cell destination was acquired. However, small size mononuclear cells were used so the cells may actually flow though arteries unstopped, providing no advantage of intra-arterial route. Thus, any next trial with mononuclear cells or any cell-based trial with small size cells would benefit from labeling and imaging during or after intra-arterial administration in real-time using MR imaging. However, such a neurointervention would be quite a new approach, and would require very thorough and prudent planning.  There are also highly awaited clinical trials with larger cells, such as mesenchymal stem cells, which were shown to be highly effective in animal models, but overdosing was leading to serious complications. Thus, the small safety clinical trial without cell labeling would be very useful before going to more complex settings.

Dr. Limaye: Do you anticipate a large-scale clinical trial of IA cell therapy for stroke soon? What specific factors have limited such an endeavor?

Authors: Large-scale clinical trials need substantial investment, so they are typically sponsored by the for-profit companies, and it is difficult to predict the mood of investors. While phase I/II clinical trials primarily assess safety, there is also collected information about efficacy, and might be the outcomes were not sufficiently spectacular to bring investment to this field. However, the results could be suboptimal due to limited knowledge about the cell destination, as well as the lack of use of large cells. Thus, we still have homework to do in terms of technological progress in cell engineering, and advanced imaging of the intervention, so the procedure is performed precisely enough to give a definite answer about the efficacy.

Dr. Limaye: Would you like to add anything to what we have already discussed?

Authors: Intra-arterial route of cell delivery is potentially very powerful, but must be deployed in a correct fashion, and as for now still needs a lot of development, which can be achieved only through continuous back-and-forth between laboratory and clinic.

Dr. Limaye: Thank you all for your time and for discussing what we all hope to be another modality in the armamentarium of neuroendovascular surgeons around the world to help stroke patients.

Authors: Thank you very much for giving us an opportunity to bring closer the intra-arterial route to the scientific and clinical community. Better understanding will surely lead to higher impact preclinical research and better designed clinical trials.