The past decade has seen a significant positive evolution in the field of immuno-oncology (IO). The introduction of checkpoint inhibition therapies and adoptive cell transfer therapy (ACT) resulted in some excellent improvements in five-year survival rates in patients with hematologic cancers, as well as in certain patients with solid tumors.

The first checkpoint-inhibiting antibody therapies have been approved by the U.S. Food and Drug Administration (FDA) and European Medicines Agency (EMA) and are regularly used in patients with late-stage solid tumors. More recently, new ACT approaches with CAR T-cells were approved for use in certain hematologic cancers (“liquid tumors”).

Looking globally at the oncology space, about seven percent of tumors are of hematological nature, whereas 93 percent of all tumors are classified as solid tumors. The five-year survival breakthrough that has been achieved with ACT has been more spectacular (from some five percent to more than 80 percent) in liquid tumors than the five-year survival gains that have been reported in solid tumors with checkpoint inhibitors in solid tumors (from some five percent to about 20 percent).

The ACT approach alone has not yet resulted in substantive and lasting improvements in solid tumors, and today the role of checkpoint inhibitors in most hematologic cancers remains unclear, with promising findings in some hematologic malignancies, such as Hodgkin lymphoma, but safety concerns in others, specifically multiple myeloma.

As a result, scientists and physicians alike have started to explore if there are technologies available that could become the nexus between these two new treatment approaches in oncology. Indeed, the combinatorial approach(es) of CAR T and immune-checkpoint blockade therapies has shown tremendous potential in multiple preclinical models and is being applied clinically.

A combination of ACT and checkpoint inhibition therapies, however, comes at a significant cost. Considering the production cost of both checkpointinhibiting antibodies and the cost related to CAR T engineering and expansion, and the resulting high price of these therapies, trying to combine those two may face significant financial hurdles for broad use in the broader oncology space.

Also, considering the intrinsic safety profile of these two approaches individually, the combination therapy may come with additional or more pronounced adverse effect issues—for example, more severe versions of cytokine release syndrome.

Consequently, there is a major interest to find alternatives that can enhance the immune effector cell capability of ACT without the need for additional antibody therapy—for example, by incorporation of checkpoint blockade into CAR T-cells, or other approaches to improve the ACT immune cell phenotype. Considering the risk that may occur in combinatorial strategies due to T cell overactivation, permanent genetic engineering approaches are questionable. Approaches whereby immune cell phenotype can be improved more easily and more cost effectively, include RNAi technology.

When unwanted proteins are excessively produced in tumor cells or in immune effector cells, one can also use RNA interference (RNAi) to knock down the production of such proteins. Unfortunately, for use of conventional RNAi, uptake through electroporation, liposomes or viral vectors is a necessity, as spontaneous uptake of conventional RNAi without the use of such aids is not satisfactory.

Considering the complexity of using these delivery techniques in a GMP cell manufacturing setting, and the negative impact on cell viability of these approaches, other solutions that are more compatible with established GMP cell manufacturing are required.

Indeed, using those techniques in a laboratory setting is one thing; but translating it to the bedside and the patient with reproducible production methods and consistent quality of cGMP material in higher quantities turns out to be problematic, and has resulted in RNAi technology not being widely utilized for ACT manufacturing. The development of self-delivering RNAi (sd-rxRNA®) addresses both delivery and manufacturing concerns.

sd-rxRNA was first produced on a laboratory production scale about 10 years ago. A sd-rxRNA compound has three unique chemical features in combination that allow it to penetrate virtually any cell without the aid of electroporation, liposomes or viral vector constructs. These three key features are:

(1) an asymmetric double-stranded oligonucleotide with a longer guide strand and a shorter passenger strand;
(2) a compound with 10 to 15 base pairs. The single-stranded overhang of the longer guide strand is protected against enzymatic degradation in the body with small chemical groups (phosphorothioates); and
(3) at the head of the compound, a cholesterol molecule (or another lipid molecule) is incorporated into the compound structure.

The resulting sd-rxRNA molecule is a compound that has the potency and selectivity of an RNAi oligonucleotide and the pharmacokinetic and pharmacodynamic behavior of an antisense oligonucleotide. This means that these sd-rxRNA compounds do not require a delivery vehicle, or electroporation, to penetrate into tissues and cells. Therefore they can be easily used in ACT manufacturing in contrast to conventional RNAi oligonucleotides.

In addition, in contrast to the use of delivery vehicles or electroporation, the use of sd-rxRNA has virtually no negative effect on cell viability.

The sd-rxRNA compounds have a much lower production cost of that of checkpoint antibodies or a therapeutic batch of allogeneic or autologous immune effector cells through a rapid expansion protocol. The fact that sd-rxRNA does not require the use of other delivery systems as compared to other RNAi approaches reduces the cost as compared to such other approaches.

In addition, since cell viability is not negatively affected by sd-rxRNA, its use does not complicate or prolong the rapid expansion processes used for ACT.

The use of these sd-rxRNA compounds also includes the possibility of being able to co-transfect the same cell batch of T cells with several sd-rxRNA compounds targeting different proteins. It has been demonstrated that this can be done without negatively impacting the degree of gene silencing of each sd-rxRNA compound, or any other negative effect, including cell viability.

As such, the technology can be elegantly used to target multiple proteins during the rapid expansion protocol or prior to administration to the patient to get immune cells with the optimal phenotype and function. For
example, it has been shown that sd-rxRNA checkpoint inhibitors can enhance the tumor-killing effect of TILs and can weaponize NK cells. Additional targets can be used to prevent cell exhaustion and improve cell persistence. 

As such, self-delivering RNAi has the potential to become a nexus between IO and ACT due to its ability to: 

(1) weaponize immune cells through checkpoint inhibition, singly or in combination, 
(2) increase the persistence and reduce cell exhaustion of T cells, NK cells and other immune cells; and
(3) reduce tumor microenvironment barriers resulting in better access for immune effector cells to attack tumor cells.

As is the case with other forms of successful cancer therapies, a successful immunological approach to cancer therapy will likely require the use of a combination of various new approaches. However, considering the  current individual costs of antibodies and cell therapy, the cost-benefit of a combination therapy using these two modalities will become questionable.

The sd-rxRNA platform may become a way to prevent such cost escalation, without having to sacrifice the concept of combing multiple immunooncology approaches (e.g. cellbased therapies and checkpoint blockade).

About the Author: Gerrit Dispersyn, Dr.Med.Sc., is CEO of Phio Pharmaceuticals, a biotechnology company developing the next generation of immuno-oncology therapeutics based on its proprietary self-delivering RNAi (sd-rxRNA®) platform.