Checkpoint Inhibitors for Cancer Treatment

One of the major issues preventing effective treatment of many cancers concerns the reduced ability of the body to defend itself against attack as some cancer cells have the capacity to deactivate the T lymphocytes (T cells) of the immune system.

Under normal circumstances, T cells are not triggered to attack healthy cells because healthy human cells carry a protein known as PD-L1. When a checkpoint (PD-1) on the T cell can bind to the PD-L1 of a normal cell, the immune system essentially recognizes the normal cell as being non-pathogenic. Yet some cancer cells also carry large amounts of PD-L1 and so evade attack from the immune system simply because the cells are not recognized as malignant.

Immune checkpoint blockers are monoclonal antibodies which disable either the PD-1 of the T cell or the PD-L1 protein on other cells preventing the binding from taking place.  This leaves the immune system free to attack the pathogenic cell.

Another protein which normally resides in healthy cells but which can also be carried by certain cancer cells is CTLA-4. More recently an immune checkpoint blocker to prevent this protein binding to immune cells has been developed.

Immune checkpoint inhibitors are an important advance in cancer treatments, and there are several drugs currently available which target either the PD-1 checkpoints or the PD-L1 proteins. The most commonly used are:

 

PD-1

Pembrolizumab (Keytruda)

Nivolumab (Opdivo)

 

PD-L1

Atezolizumab (Tecentriq)

 

CTLA-4

Ipilimumab (Yervoy)

 

Currently, these treatments are generally employed only against cancers which are known to carry the PD-L1 and CTLA-4 binding proteins, namely advanced melanoma, which is unresectable or metastatic, and non-small cell lung cancer.   The medicines have met with some success on these cancers.. Trials are also underway to find out if they work on other types of cancers.

Yet there are negative issues arising from these immunotherapy treatments, which in essence prevent protein binding not only to pathogenic cells but also to healthy ones. Sometimes the checkpoint inhibitors become a double-edged sword as the T-cells attack and disrupt the normal function of healthy organs and tissues.

Unfortunately though, it is difficult to predict not only how severe such issues will be, but also how frequently they will arise. And, although there are few, if any deaths, recorded as arising directly from the toxicity of these drugs, some feel there is evidence that fatalities from severe side-effects are more frequent. The manufacturers of each of the above drugs warn both clinicians and patients – from the home page of their websites –  that there is a risk of a positive feedback loop forming in the immune system, causing the immune system to attack healthy organs leading to severe problems including death.

See also: http://jamanetwork.com/journals/jamaoncology/fullarticle/2174768