Cancer and the need for combined approaches to therapy

For long-term survival of cancer, treatments that target multiple cancer vulnerabilities and adapt to changes within the tumor may be required.

Activating the patients own immune system is an imoportant part of such approach. The immune system is a powerful, flexible, and adaptive component for generating long-term remissions. However, chances of successful treatment may increase further if other aspects of cancer progression, like cancer stem cells, metastatic behaviour and  reduced tumor cell apoptosis are be targeted simultaneously. 

Blocking AhR holds the promise of a non-toxic therapy that targets multiple crucial tumor progressing processes at the same time.

AhR, hijacked to progress and sustain cancer

The aryl hydrocarbon receptor (AhR) is normally present in many epithelial cells where it can be activated many different ligands. In normal tissues, AhR activity is involved in (amongst others) managing inflammation and metabolizing exogenous toxins. For example, Poly Aromatic hydrocarbonsand (PAHs) are known activators of AhR.

In cancer tissue however, AhR is hijacked to act as direct mediator of tumor progression and a powerful inhibitor of tumor-specific immune responses. In the tumor microenvironment AhR seems to be mostly activated by tryptophan catabolites that are produced via IDO and TDO, the expression of which is upregulated in a positive feedback loop by AhR activation.

If activated, AhR translocates to the cell nucleus, where it causes the expression of many genes that are important to cancer progression.

Abnormally high levels of AhR are produced in many solid tumors, where AhR is constantly activated causing suppression of anti-tumor immune responses.  

Apart from reducing the response of the immune system to cancer, AhR activation causes tumor cells to migrate, invade local tissue and metastasize to distant organs. The AhR also plays a role in producing “cancer stem cells”, a type of tumor cell that is highly resistant to chemotherapies and is thought to be responsible for lethal relapse and metastasis even years after “successful” chemo- or radiotherapy.  

Activation of AhR in tumor cells

AhR is continuously activated in the tumor microenvironment by tryptophan catabolites like kynurenine. Tryptophan catabolites are produced from tryptophan via the enzymes IDO and TDO. Tryptophan catabolites are upregulated via a positive AhR - IDO/TDO feedback loop in tumors, which further stimulates AhR activation. 

Activation of AhR in components of the immune system

Many cells involved in immunity express AhR. Examples are macrophages, dendritic cells and T cells. Activation of AhR in immune cells causes reduced anti-tumor immunity. 

HP163, attacking cancer via multiple avenues 

Hercules has developed HP163, a small molecule inhibitor of the AhR. By blocking AhR, Hercules' inhibitors not only boosts anti-tumor immunity, they also suppress metastasis and the formation of chemo-resistant cancer stem cells. As such, HP163 has the potential to attack multiple vulnerabilities in the cancer microenvironment at the same time.

AhR inhibitors compared to IDO inhibitors

The enzymes IDO and TDO cause the production of tryptophan catabolites like kynurenine, which are endogenous agonists of AhR in tumors. 
The development of IDO inhibitors as novel anti-cancer treatment is receiving much attention. IDO inhibitors reduce the levels of tryptophan catabolites., which reduces AhR activation. However, apart from IDO, TDO is another enzyme that catabolyzes tryptophan to kynurenine. In many cancers TDO is equally, or more important for the production of tryptophan catabolytes than IDO. Consequently, IDO inhibitors are expected to work suboptimally in tumors where IDO is present at significant levels.
Serotonin, another strong agonist of AhR is catabolized from tryptophan via Tryptophan Hydroxylase (TPH). Serotonin is an important AhR agonist in many solid tumors where AhR is also highly expressed.
Hercules’ approach to block AhR has the advantage that it is blocking the agonistic effects of tryptophan catabolites that are produced via both IDO, TDO and TPH.
This may cause AhR inhibitors to have higher anti-cancer efficacy compared to IDO inhibitors and a specific clinical benefit in cancers that highly express AhR and where TDO or TPH are highly expressed.