Depletion Strategies: Cross-Fertilization of Scientific Concepts Across Therapeutic Areas
Written by Dr Rhiannon Jenkinson, PhD (CSO)
Many of the clients we work with at Nexus BioQuest are developing therapeutics to rid the body of disease-causing, pathogenic cells with the aim of driving a cure in the treatment of cancer, autoimmunity, inflammation and transplantation. In this blog I’ll touch on why depletion approaches are appealing to so many companies, how this strategy for the treatment of various diseases could be beneficial, and give some examples of depletion approaches which have been or are in use to treat patients in the clinic today. Finally, I’ll present thoughts about how we can model depletion using human in vitro systems and why doing so is important for progressing development of new therapeutics.
Why Depletion Is a Major Therapeutic Strategy
Firstly, it is important to acknowledge that depletion is by no means a new concept for treating disease, but that advancements in science have changed the variety and sophistication of the therapeutics being developed as medicines. Therapeutics that drive depletion can be used to treat multiple different diseases, for example, in the field of oncology to remove tumour cells or in the fields of transplantation and autoimmunity to remove pathogenic (disease-causing) immune cell subsets. The first FDA approved antibody, muromonab (OKT3) was a T cell depleting antibody used to treat transplantation patients, with the aim of removing T cells which were involved in transplant rejection. As a side note, while the OKT3 antibody is no longer in use in the clinic, it is familiar to almost all immunologists and still used as a tool to drive polyclonal T cell activation in vitro, via engagement of CD3 and driving T cell receptor (TCR) signalling.
So why the need for novel depletion approaches and new drugs, and the explosion of interest in developing therapeutics that drive depletion of target cells? Simplistically, this comes down to a few factors:
1) Identification of new therapeutic targets leading to depletion of specific cell types.
2) Increasing design and selectivity of novel therapeutics, with decreased off-target effects and increased tolerability for the patient.
3) Driving complete depletion of the pathogenic cells to prevent disease recurrence; any residual disease-causing cells will have the ability to reinitiate disease, so ‘deep’ depletion is thought to be key to therapeutic success and ultimately cure.
The Rise of Next-Generation Depletion Technologies
Many of us consider depletion strategies to include:
1) Therapeutics that target direct depletion of the target cell. Examples include cytotoxic drugs and antibody-drug conjugates (ADC).
2) Therapeutic antibody or other large molecule, to target immune effector cell function to deplete the target cell, for example via ADCC (antibody-dependent cellular cytotoxicity, or ADCP (antibody-dependent cellular phagocytosis), or TCR engager (to target T cell killing of the target).
3) Cellular therapies, such as CAR-T, TCR-T, γδ cell therapies, all of which are engineered to recognise and kill cells expressing a specific target molecule.
In addition to these modalities, therapeutics that enhance immune cell effector functions could also be considered as depleting therapeutics; as the net aim of these drugs, mostly within the oncology field, is to drive effector cell depletion of tumour cells. Examples of such targets, which indirectly drive depletion, are diverse but include antibodies that block checkpoint inhibitors (e.g. PD-1/PD-L1), small molecules and cytokines which enhance T cell, NK cell or macrophage function, therapeutics that make the tumour a better or less ‘hidden’ target and tumour vaccines. What all of these different drug strategies have in common is that they aim to mobilise the immune system to recognize and kill and thereby drive tumour cell depletion.
Why Complete Depletion Remains Challenging?
This raises the question of “what are the major hurdles that these therapies face”? We already mentioned selectivity and this comes as no surprise as it’s key for any drug being developed; higher selectivity means less potential side effects as it means only the cells expressing the therapeutic target will be deleted. The challenge is therefore to try and minimise on-target/off pathogenic cell deletion which results in normal tissues which express the same target as the pathogenic cell being killed. To do this we need to either find targets that are only expressed by the pathogenic cell (e.g. in the case of oncology expressed by tumour cell but not by normal cells) or to get clever with therapeutic design.
Anatomy must also be considered. For example, B cell-depleting therapies for autoimmune diseases are an area of intense current interest, particularly in indications such as systemic lupus erythematosus (SLE) and systemic sclerosis. Depleting B cells is a desired clinical outcome as the autoantibodies produced by pathogenic B cells are responsible for driving autoimmune damage of normal tissue, and the accepted dogma is that, if these cells can be fully depleted, this should result in long lasting absence of disease. Therapeutic antibodies such as rituximab (targeting CD20) can be used to deplete B cells through ADCC, ADCP and CDC mechanisms and the dominant mechanism in the body is most likely driven by anatomy. For rituximab or similar antibodies to deplete, they need to get into the tissues where pathogenic B cells are present, this could be blood, bone marrow, secondary lymphoid tissue or site of inflammation e.g. the kidney. This is not the end of the story, as now the depleting antibody is bound to its target B cell, they are reliant on effector cells e.g. NK cells, neutrophils (ADCC), macrophage (ADCP) or, complement (CDC) to be present to drive depletion of the pathogenic autoantibody-producing B cells.
So, we have a few barriers to complete depletion of the autoantibody-producing pathogenic B cells: a good target expressed across B cell subsets, access of the therapeutic to the appropriate anatomical sites, and the presence of functional effector immune cells at those sites that can mediate the depletion.
Cellular therapies and the Pursuit of Deep Depletion
This is where it’s worth mentioning the ‘new kid on the block’ cell therapies, such as CD19 CAR-T (developed and licensed to treat B cell cancers), which are now being explored as a way of treating autoimmune disease. So why this approach? The idea is that CAR-T cells effectively are the antibody and effector cell combined into one, as the patient’s T cells have been engineered to express the anti-CD19 antibody as a receptor at the cell surface; once this is engaged the T cell mediates killing of the CD19-expressing cell. Indeed, early clinical studies indicate that the CD19-CAR-T may drive deeper depletion than antibody therapies such as rituximab. Why could this be? CAR-T may home better into peripheral tissue and secondary lymphoid tissue resulting in deeper depletion making it less likely to leave residual pathogenic B cells that can re-emerge. The CAR-T cells may also form memory cells that remain in the body, so any re-emerging pathogenic B cells are depleted. In contrast, the rituximab-type antibodies would need to be re-injected to maintain levels in the body. One of the other key differences between depleting antibodies and CAR-T therapies is that patients receiving CAR-T infusion are typically pre-conditioned by treatment with lymphodepleting agents (e.g. to allow the CAR-T to engraft. While on the surface this may appear to be irrelevant it will completely change the immune landscape in terms of cytokine responses and also the T cell repertoire, including CD4 T cell subsets (including T follicular helper cells) that will help the pathogenic B cells differentiate to become antibody producing cells. By removing this subset this has the real potential to affect the quality and ability of any re-emerging pathogenic B cells to differentiate into autoantigen-producing cells. Future studies where conditioning is reduced or not included may alter clinical outcomes if this hypothesis is true.
Modelling Depletion in Human In Vitro Systems
Given that novel therapeutic strategies are growing in complexity and because of the known differences in the immune systems of mouse versus man, there is a strong need to be able to test the ability of therapeutics to drive depletion in in vitro models created with human cells. While the full human anatomy is hard to model, different relevant cellular systems can be modelled in the tissue culture dish. For example, in terms of oncology, immune-mediated killing of tumour cell multicellular aggregates can be measured using imaging to measure depletion or killing of different cell subsets. The secondary lymphoid structures can be modelled using tonsil reaggregates and measurement of selectivity and on/off targets assessed using normal primary cells from different tissues or by using organoids.
How Nexus BioQuest Supports Depletion Programmes
At Nexus BioQuest we specialise in creating bespoke tissue culture models using human cells to answer specific questions related to efficacy, mode of action and specificity (on/off target). With the push away from animal models and increasing acceptance of in vitro models by the regulatory bodies for drug development, it is an exciting time for innovative in vitro solutions to help develop drugs which either target the immune system or use the immune system as a tool to treat disease. Specialist CROs like Nexus BioQuest play a key role in helping advance in vitro solutions.
