Chimeric antigen receptor (CAR) T cells benefit patients with treatment resistant B-cell leukemia, B-cell lymphoma and multiple myeloma, raising hopes that CAR T cells could be used to treat “solid” cancers, including breast cancer, lung cancer, pancreatic cancer, brain cancer, and sarcomas. Studies show that CAR T cells can work against solid cancers in mouse models, but they also reveal challenges that must be overcome for treating human patients. 

One major challenge relates to the fact that the protein targets that CAR T cells recognize in leukemia/lymphoma/myeloma are not expressed on vital normal tissues, whereas CAR T-cell targets for solid cancers are generally expressed on vital tissues as well, albeit at lower levels. Therefore, CAR T cells targeting solid cancers pose greater risk to harming normal tissue.

A related challenge relates to greater suppression and evasion of immune responses by solid cancers, which requires more potent CAR T cells. These competing issues (“increased risk for toxicity combined with a need for greater potency”) have led to limited progress for solid tumors.

The Mackall lab recently developed a new CAR T-cell platform that both increases potency AND increases safety. 

SNIP-CARs allow “remote control” of CAR T cells using a drug administered as a pill. SNIP-CARs contain a molecule (called a protease) that continuously “snips” the CAR molecule in half, preventing its function unless a drug is present to inhibit the “snipping protease.” Drugs that inhibit the protease are FDA-approved and well-tolerated. SNIP-CARs are “OFF” at baseline but are able to be activated (they still require the tumor target to be activated) in the presence of the drug.

In mouse models where standard CAR T cells killed the animals due to toxicity, stopping the drug after the animals became ill allowed complete recovery. Surprisingly, in settings where toxicity was not an issue, SNIP-CARs plus daily dosing of the drug resulted in greater tumor control than seen with standard CAR T cells. This is due to the variations in drug levels by drug metabolism, which provided the CAR T cells with periods of activation followed by periods of “rest.” Finally, in situations where tumor and normal vital tissue shared the target and standard CAR T cells killed the animals, a lower dose of the drug allowed the SNIP-CAR T cells to attack the tumor but ignore the normal tissue. 

This proposal generates the necessary processes, procedures and materials needed to test SNIP-CARs in patients with solid cancers whose solid cancers are not effectively treated with standard therapies. The proposal will amplify ACGT investment by leveraging substantial infrastructure in place at Stanford University to greatly accelerate clinical testing of a cutting-edge cancer gene therapy platform for patients with critical unmet need.

Outcomes for osteosarcoma have not improved in more than two decades. The addition of “first generation” cytotoxic chemotherapy to surgery improved outcomes in the late 1980s, but additional gains have not been made since, despite increasing the doses of drugs given and the number of chemotherapy agents administered.  

Clearly, advances in osteosarcoma require novel therapeutics capable of killing chemoresistant cancer cells. Evidence from many different disease settings demonstrates that immunotherapy can kill chemoresistant cancer. The challenge lies in developing a way to activate and harness the impressive power of human T cells to recognize osteosarcoma as foreign.  

Gene therapy has provided new approaches to direct human T cells to recognize human cancers by engineering “chimeric antigen receptors” (CARs), which incorporate the binding portion of an antibody and the signaling portion of a T cell. CARs have worked well in acute leukemia and have activity in lymphoma, but have not yet mediated impressive effects on solid tumors.  

My laboratory is working diligently to improve CARs for solid tumors. We have focused on engineering CAR T cells to recognize GD2, a fatty sugar (glycolipid) uniquely overexpressed on cancer cells, including osteosarcoma. We have already conducted a clinical trial of GD2-CAR in patients with osteosarcoma and neuroblastoma. In this study, we saw that the GD2-CAR T cells expanded after transfer, but did not persist beyond 1-2 months.  

Successful immunotherapy in other solid cancers demonstrates that T cells must persist for several months if they are to have meaningful clinical effects. This project incorporates a series of improvement to the GD2-CAR therapy aimed at rendering GD2-CAR T cells more persistent, and thus more potent, in patients with osteosarcoma and neuroblastoma. Studies conducted in my laboratory demonstrate the modifying the GD2-CAR itself increases persistence and potency. In addition, modifying the way in which the T cells are grown, prior to transfer, enhances GD2-CAR persistence and potency. We also discovered that co-administration of a drug that depleted immune suppressive populations can render the GD2-CAR T cells more potent.  

This protocol builds upon all of these advances, incorporating them into one trial, which will deliver a state-of-the-art, next generation gene therapy trial for the treatment of osteosarcoma and neuroblastoma. True clinical advances rarely result from one scientific breakthrough. Rather, effective cancer treatments most often result from iterative improvements that begin with careful observations from clinical trials, scientific innovation to overcome challenges, then retesting in clinical trials. The GD2-Persist Trial incorporates a series of iterative improvements that build upon careful clinical studies and innovative science, thus leveraging the power bedside-to-bench-to-bedside iteration to improve osteosarcoma therapy.       

This Alliance for Cancer Gene Therapy Research Fellow is funded in part by Swim Across America and Wendy Walk. 

Brain tumors are the most common solid tumor in children and represent the leading cause of death from childhood cancer. Diffuse intrinsic pontine gliomas (DIPG) are a highly aggressive pediatric brain tumor of the brain stem, with a five-year survival rate of less than 1% and median survival of only 9 months. While significant improvement in survival has been achieved in treating other forms of brain cancer, the outcome for children with DIPG has remained poor, and has not changed in over three decades.  

The major challenge in the treatment of DIPG is its extremely invasive nature and delicate anatomical location in the brain stem, which precludes surgical removal. Previous research has shown that transplanted neural stem cells (NSC) possess remarkable tropic migratory capacity toward adult brain tumors, but the use of NSCs in clinics is severely limited by the ethical and technical challenges to obtain these cells in human. Furthermore, current approaches rely on viral-based vectors for delivery of therapeutic genes, which face safety concerns for broad clinical applications. While gene therapy targeting tumor apoptosis has been shown to be effective in eradicating adult brain tumors, pediatric brain tumors including DIPG, easily gain drug resistance to apoptosis inducing-based gene therapy alone.  

Through working at the interface of biology, material science, bioengineering and medicine, this Alliance for Cancer Gene Therapy funded research will develop a novel treatment regimen to enhance targeting and eradication of disseminated DIPG tumor by directing addressing the current critical bottlenecks in the field of cancer gene therapy.  

To overcome the critical barrier of cell availability by employing adipose-derived stem cells (ADSCs), an abundant and easily accessible autologous stem cells source as drug delivery vehicle for targeting DIPG cells in vivo.  

Unlike the conventional, viral vector-based cancer gene therapy, the proposed strategy employs non-viral gene delivery using biodegradable polymeric vectors, a platform well established in the applicant’s laboratory.  

To overcome drug resistance commonly observed in treating pediatric brain tumors, this approach employs a combined therapy by co-delivering non-viral engineered stem cells with nanoparticles containing chemotherapeutic drugs, which has been found to enhance the responsiveness of pediatric brain tumor cells to gene therapy.  

The outcomes of the proposed interdisciplinary approach may advance care for DIPG in ways that would not be possible using conventional treatment paradigms. This will lead to improved survival for patients of DIPG, one of the most deadly forms of pediatric brain cancer and may substantially reduce the associated socioeconomical burden on our society. While the proposed work will initially focus on DIPG as a model disease, the proposed strategy to enhance targeting and treating cancer metastasis may be adapted for treating a broad range of other cancer types as well. 

This Alliance for Cancer Gene Therapy Research Fellow is funded in part by Swim Across America. 

Vaccine-strain measles virus is a promising platform for the development of genetically programmable anti-cancer therapies. Measles vaccine virus has a natural preference for binding and infecting many types of tumor cells. With an impeccable safety record from decades of clinical use, the measles vaccine virus has already entered clinical trials for several cancers. 

Recently, genetic modifications to the measles vaccine virus that enhance its ability to replicate in cells have shown further improvements in tumor killing in preclinical testing. As this therapeutic virus platform is modified to be more lethal to cancer cells, however, it becomes important to be able to limit its replication to particular times and places in order retain its low toxicity to normal cells.  

We propose to develop two mechanisms for improving measles vaccine virus control and specificity. The first mechanism allows administration of an orally available nontoxic drug to rapidly shut off virus protein production. The second mechanism renders virus protein production dependent on a specific molecular abnormality commonly found in cancer cells. Both will be based on encoding in the virus genome rationally engineered protein switches to control the function of viral replication proteins. In preliminary work, we have successfully created prototypical control elements for both mechanisms.  

We propose to test the ability of these control elements to function in the context of the whole virus in order to control replication in time and by tumor type. If successful, the proposed work will establish a generalizable method by which replication of measles vaccine virus, and other RNA-based viruses, can be programmed for increased tumor specificity and safety.

Mantle cell lymphoma carries the worst prognosis of any lymphoma. Treatment includes high-dose chemotherapy followed by stem cell transplant, which eliminates most of the lymphoma, but temporarily wipes out a patient’s immune system. Even with this aggressive therapy, residual lymphoma cells remain and cause the patients to recur within a few years. Novel approaches are needed to eliminate those residual lymphoma cells.  

Gene therapy is amongst the most promising approaches being studied for the elimination of cancer, but a primary obstacle is the difficulty in delivering the therapeutic gene to a sufficient proportion of cancer cells. Our approach of using a gene-engineered lymphoma vaccine induces the patient’s immune system to eliminate residual lymphoma cells, so that widespread gene-delivery is not needed.  

Earlier trials of such an approach have demonstrated its feasibility and some impressive initial results. The innovative aspects of our study are that it utilizes a novel, non-viral gene transfer technology and takes advantage of the unique immunologic setting of patients who have undergone stem cell transplantation. Prior to stem cell transplant, the patient’s immune system accepts lymphoma, allowing it to progress. After a patient’s immune system is wiped clean during stem cell transplant, giving an anti-lymphoma vaccine may cause it to be re-established in such a way that it is no longer accepting of lymphoma and can eliminate those few remaining cells.