The Basics of PD-1

Several clinical trials for lung cancer (as well as other cancers) are pursuing therapies based on the PD-1 pathway of the immune system. These trials can usually be found on by searching with keywords such as PD-1, PDL-1, or PD-L1. Sometimes these therapies are referred to as anti-PD-1 or anti-PD-L1.

PD-1 (PD stands for Programmed Death) is part of an immune system “checkpoint” pathway that, among other functions, helps turn tumor suppression on or off. PD-1 is actually a protein expressed on the surface of certain cells in our immune system; it is NOT a mutation, but rather something inherent in everyone’s immune system. The cells of interest in these trials are activated T cells, but PD-1 is expressed on other types of cells, too. The PD-1 protein is a receptor, which means another molecule can bind to it.

PD-L1 is a protein of the surface of some (but not all) tumor cells. It is a ligand of PD-1 (hence the “L” in its name), which means it binds to the PD-1 protein. When PD-L1 binds to PD-1, it tells the immune system to ignore the tumor cells. PD-1 has one other known ligand (surprisingly named PDL-2).

PD-1 and PD-L1 therapies aim to blockade the PD-1 pathway so the immune system can better attack cancer tumors. The drugs used are designer molecules that bind to part of the PD-1 pathway and block its activity. Some drugs bind to PDL-1 so it can’t bind to PD-1. Other drugs bind to PD-1 to prevent ligands from binding to it. Both approaches aim for the same effect: keep the PD-1 pathway from telling the immune system to ignore tumor cells.

Not everyone responds to PD-1 pathway therapies. Early trial results show lung cancer patients had response rates on the order of 10% to 18%. Researchers are studying whether biomarkers — proteins such as PD-L1 on the surface of immune system or tumor cells — might indicate which patients will respond well to PD-1 therapies. That is why some trials (but not all) require a biopsy for testing before accepting the patient into the trial.

Since part of the immune response for suppressing tumors involves inflammation, participants in trials based on the PD-1 pathway often find their tumors will grow somewhat when they first start the therapy. A few lung cancer trial patients experienced serious or fatal pneumonitis, a lung inflammation.

PD-1 therapies are promising enough that at least four drug companies (Bristol-Myers Squibb, Roche/Genentech, Medimmune and Merck) are pursuing them in lung cancer trials. Because they modify the immune system, the hope is that these drugs will continue working longer than targeted therapies do.

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Edited 2013-09-12 12:53 PM PDT to add information.